The Clinical Performance of UHMWPE in the Spine

14  The Clinical Performance of UHMWPE in the Spine Marta L. Villarraga, PhD and Steven M. Kurtz, PhD Exponent, Inc.; Drexel University, Philadelphia, PA, USA

14.1 Introduction In recent years, UHMWPE has been used in implants to treat chronic neck and back pain that represents leading causes of disability in the United States and around the world. Neck and back pain may manifest acutely or, more uncommonly, as a chronic condition. Acute back pain typically resolves within weeks, but it is also possible for low-grade symptoms to persist for years following an initial episode [1]. Although less common, chronic, and intractable back pain can lead to serious, permanent disability [2]. A complex combination of anatomical, biomechanical, genetic, and psychosocial factors may potentially contribute to chronic neck and back pain, greatly complicating diagnosis and treatment of the condition [2]. As a result, patients presenting with neck or back pain are usually initially treated by conservative therapies that include antidepressants, massage, exercise, spinal manipulation, combined physical and cognitive behavior training, and corticosteroid injections. When degenerative disc disease is the underlying cause of pain, conservative therapy may not alleviate the patient’s symptoms. In the United States, fusion is the most common surgical procedure for chronic low back pain for patients who do not respond to conservative treatment [1]. In 2005, an estimated 228,000 Americans underwent lumbar spine fusion procedures for intractable lower back pain, and the annual number of lumbar fusions in the United States is projected to increase to 6.5 million by 2030 [3]. However, the clinical success rate of lumbar fusion is variable, and reportedly ranges between 16% and 95% [4]. Furthermore, fusion of a painful degenerated disc has been associated with subsequent degeneration at adjacent intervertebral discs (sometimes referred to as “adjacent segment disease” by clinicians) [5,6]. Starting in the 1960s, total disc arthroplasty was conceived as an alternative treatment to fusion, in an effort to avert adjacent segment degeneration in the lumbar spine. Researchers adapted successful

UHMWPE Biomaterials Handbook Copyright © 2016 Elsevier Inc. All rights reserved.

b­iomaterials and design principles from hip and knee replacements for total disc replacements (TDR). The first such design, the CHARITÉ artificial disc, consists of two metallic endplates that are fixed to the superior and inferior vertebral bodies of the lumbar spine. A mobile, biconvex ultrahigh molecular weight polyethylene (hereafter, UHMWPE) core articulates against the concave bearing surfaces of the superior and inferior endplates. The CHARITÉ originated in Europe in the 1980s and was approved by the FDA for use in the United States in 2004. Additional lumbar TDRs, including the ProDisc®-L, Mobidisc®, activL®, and Baguera® L were developed within the past decade based on similar design principles and utilize similar biomaterials. The ProDisc-L was approved by the FDA in 2006; the activL® is used in Europe and Asia, and recently approved in the United States; the Mobidisc, though currently available in Europe and Asia, is no longer enrolling in the US clinical trial, as the trial was terminated [7]. One of the historical driving factors for the development of disc replacements has been growing dissatisfaction with fusion as a long-term treatment, especially for younger patients. TDR surgery is a promising alternative treatment for advanced degenerative disc disease. The central aims of disc arthroplasty are to restore pain free motion, as well as the load-carrying capacity of a diseased functional spinal unit. Therefore, disc replacement occupies a unique and important position at one end of the pathological spectrum in the treatment continuum spanning early degenerative disc disease, with spinal fusion at the other end. Proponents of TDR have hypothesized that by maintaining the mobility of the treated disc, long-term adjacent segment degeneration may be forestalled or possibly averted. The state of knowledge in TDRs is evolving rapidly. Here we provide an overview of TDR designs incorporating metal-on-UHMWPE articulation. This chapter reviews the clinical and biomechanical studies of TDRs containing UHMWPE, as well as recent findings from retrieval analyses.

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14.2  The CHARITÉ Artificial Disc Total disc arthroplasty can trace its clinical history back to the early 1960s, when Fernström first implanted stainless steel spheres to replace the intervertebral disc [8]. From these modest beginnings, developments in TDR technology have progressed at a slow but exponentially increasing pace. The plethora of patents described by Szpalski et al. attest to engineering creativity over the past 40 years [9]. However, only a small fraction of the innovations described in the body of patent literature has been validated by biomechanical and animal testing, and an even smaller number of implant designs have ever been used clinically in humans. In this section, we focus on the CHARITÉ artificial disc. Earlier artificial disc designs, including ­ Fernström’s spheres, from the 1960s, and the ­AcroFlex artificial disc, which evolved starting in the 1980s, fell out of clinical use soon after their introduction. The early design concepts of Fernström’s spheres and the AcroFlex have been re-engineered in the twenty-first century and may yet prove to be viable with the proper choice of biomaterials, surgical instrumentation, and patient indications. In contrast, the CHARITÉ artificial disc has been used continuously since its development in the 1980s up to the present day, with only evolutionary changes to its design and biomaterials over the past 20 years. The historical in vivo performance of the CHARITÉ thus provides important insight into the complex and demanding environment in which contemporary disc replacements incorporating UHMWPE must function. With the CHARITÉ, the designers sought to replicate the kinematics, but not the structure, of a vertebral disc. The CHARITÉ design, formally known as the SB CHARITÉ artificial disc by its inventors, consists of two metallic endplates fixed to the adjacent vertebral bodies that articulate against a central core fabricated from UHMWPE (Figure 14.1). This artificial disc was invented by Kurt Shellnack, MD, and Karin BüttnerJanz, MD, PhD, who were affiliated with the Charité Center for Musculoskeletal Surgery at the Medical University of Berlin at the time (Universitätsmedizin Berlin, http://www.charite.de) [10]. In the CHARITÉ design, the polyethylene core has two domed (convex) surfaces that articulate against the concave metallic endplates. In full flexion or extension, the core is (theoretically) free to translate and rotate, so that the polyethylene rim contacts the metallic rims of the endplates (Figure 14.2). This artificial disc is implanted by an anterior approach.

Figure 14.1  CHARITÉ artificial disc. (A) Photograph of the SB I and SB II CHARITÉ; (B) Photograph of the SB III CHARITÉ with uncoated endplates; (C)  Anterioposterior and lateral radiographs of the SB III C ­ HARITÉ implanted in the lumbosacral spine. Reproduced with permission from Ref. [11].

Figure 14.2 Kinematics of the CHARITÉ artificial disc. The metallic endplates are designed to contact the rim of the UHMWPE core in flexion and extension. In neutral alignment, load is transferred through the central, domed portion of UHMWPE core. Reproduced with permission from Ref. [12].

An excellent summary of the design can be found in a recent monograph [12]. The CHARITÉ has gone through four distinct phases in its design history. In this part of the chapter, we review the details about the major phases of its design, and explain how the CHARITÉ helped to launch the field of modern total disc arthroplasty. In addition to the historical significance of this design, we also briefly highlight some of the controversies that have been reported surrounding its use.

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14.2.1  SB CHARITÉ I and II

14.2.2  SB CHARITÉ III

The first two iterations of the design (i.e., the SB CHARITÉ I and II) occurred between 1984 and 1985 at the Charité Hospital. In this phase of its history, the implants were not commercially available, and were implanted in a total of 49 patients [10]. The smooth endplates were fashioned from nonforged stainless steel, and only fixed to the vertebral endplates by a number of protruding teeth. A circumferential radiographic wire marker was also included in the polyethylene core. In the SB I design, the endplates were circular and shaped like bottle caps. In the SB II design, the endplates were expanded with lateral wings to provide more coverage of the vertebral endplates (Figure 14.1A). The CHARITÉ’s designers adopted the “Low Friction Principle” of polymer-on-metal articulation, employed by Sir John Charnley when creating the first successful hip replacement design [13]. As was covered in earlier chapters of this book, ­Charnley’s hip design evolved to become the dominant joint replacement technology in orthopedics during the 1970s, due largely to the outstanding frictional and fatigue properties of UHMWPE. Up until 1997, all the CHARITÉ cores were fabricated from compression-­ molded GUR 412 resin, packaged in air, and gamma sterilized in air. Only retrospective clinical data are available regarding the SB CHARITÉ I and II designs [12,14]. McAfee evaluated the early “suboptimal” results of the Type I and II designs that he attributed to suboptimal surgical technique (e.g., undersizing, malpositioning), rather than to deficiencies in the design [15]. Most recently, investigators from the Charité hospital have reviewed the performance of 15 patients with the Type I design and 22 patients with the Type II design after an average follow-up of 18.2 and 17.5 years for the two groups, respectively [14] (Table 14.1). Fair to excellent results were found in 87% of patients with Type I implants and in 68% of patients with SB II implants. The higher incidence of poorer results with the Type II was due to stainless steel endplate fractures that occurred in 7/22 patients. No endplate fractures were reported by the authors in the SB I design. Other long-term complications included subsidence, dislocation, and persisting pain (Table 14.1). Interestingly, Putzier et al. reported that heterotopic ossification (HO), leading to spontaneous fusion of the treated level, occurred in 12/15 (80%) of Type I patients and 11/22 (50%) of Type II patients [14].

The CHARITÉ artificial disc was commercialized by Waldemar Link GmbH & Co. in 1987, resulting in the SB CHARITÉ III (i.e., third iteration design) [12]. As an orthopedic implant manufacturer, Waldemar Link made major changes in the design of the metallic endplates, including changing the material from stainless steel to CoCr alloy. Specifically, Waldemar Link GmbH & Co. employed a proprietary CoCr alloy, VACUCAST® (0.20–0.25% C, 28.0–30.0% Cr, 5.5–6.5% Mo, max. 0.5% Ni, max. 0.5% Fe, 0.4–1.0% Si, 0.4–1.0% Mn, with the remainder 57.1–69% Co) [16]. The backside of the CoCr was “satin finished” by corundum blasting, and fixation was achieved with six sharp teeth [16]. Since the redesign of the endplates by Link, there have been no further reports of component fractures [12,14]. The geometry and properties of the UHMWPE core remained essentially unchanged by Link at that time. Unlike previous iterations of the design that were only used at the inventors’ institution, this version was launched internationally beginning with France and the Netherlands in 1989 and the United Kingdom in 1990. The clinical performance of the CHARITÉ has been documented in a number of published studies, the majority of which pertain to the Type III design (Table 14.2). Taken together, these studies demonstrate that, in at least some patients, the CHARITÉ TDR has the potential to preserve motion, result in good to excellent clinical outcomes, and survive longterm implantation in the human body. However, these largely retrospective cohort studies also demonstrate broad variations in their complication rates and clinical success rates, raising concerns about the repeatability and reproducibility of the procedure. The largest, and most comprehensive body of clinical data about the CHARITÉ was collected for the prospective, randomized trial undertaken for the FDA’s Investigational Device Exemption (IDE) study in the United States (Table 14.2). At national meetings, 5-year results from the prospective IDE study have been presented [17] and most recently reported in the peer-reviewed literature by Guyer et al. [18].

14.2.3  Recent Generations of the CHARITÉ Artificial Disc There have been a number of changes in the CHARITÉ made by Link, in the late 1990s, and subsequently by DePuy Spine, in 2004. All of these

Table 14.1 Long-Term Clinical Performance of the SB CHARITÉ (I, II, and III) Fair to Excellent Patient Outcome (%)

Segmental Fusion for Subsidence

Segmental Fusion for Dislocation

Segmental Fusion for Persisting Pain

Arthrodesis or Spontaneous Fusion

13/15 (87%)

2/15

0/15

1/15

12/15 (80%)

45

15/22 (68%)

1/22

0/22

0/22

11/22 (50%)

43

12/16 (75%)

0/16

1/16

0/16

9/16 (56%)

40/53 (75%)

3/53

1/53

1/53

32/53 (60%)

CHARITÉ Design Type

Treatment Period (years)

Average Follow-Up (years)

Number of Patients

Number of TDRs Implanted

Average Age

SB I

1984–1985

18.2

15

16

44

SB II

1985–1987

17.5

22

25

SB III

1987–1989

16.1

16

22

Total

1984–1989

53

63

No significant difference in long-term clinical performance was noted between the three CHARITÉ designs. Adapted from Ref. [14].

Table 14.2 Clinical Details of Primary, Peer-Reviewed Studies Involving the SB CHARITÉ (I, II, and III)

Zeegers et al. [22]

Sott and Harrison [23]

Lemaire et al. [24]

Putzier et al. [14]

FDA IDE Study, McAfee et al. [25]a

Hcoh

Pcoh

Hcoh

Hcoh

Hcoh

RCT

Hcoh

Hcoh

RCT

SB I, II, III

SB III

SB III

SB III

SB III

SB I, II, III

SB III

SB III

SB III

SB III

43

43

36

43

48

40

44

40

36

46

40

26–59

25–59

27–44

24–59

31–61

24–51

30–59

19–60

23–50

27–73

19–60

No. of patients

62

93

46

50

14

100

53

205

106

160

90b

No. of arthroplasties

76

139

56

75

15

147

63

205

106

226

90

Ave follow-up in years

1.3

1.0

3.2

2.0

4.0

11.3

17.3

2.0

13.2

6.6

5.0

81%

?

63%

70%

10/14

90%

75%

57.1%c

82% (87/106)

68%

57.8%

17/50

1/14

5/100

12/53

11/205

11/106

12/226

7/90 (8%)

2/100

32/53

2/205

3/106

21/100

44/53

68/205

44/106

41/123

0

BüttnerJanz et al. [19]

Griffith et al. [20]

Cinotti et al. [21]

Study type

Hcoh

Hcoh

Type of prosthesis

SB I, II, III

Average age Age range

Good or excellent Secondary surgery Arthrodesis or spontaneous fusion Complications

29/76

55/139

8/46

3/50

2/15

David [26]

Ross [27]

FDA IDE Study, Guyer et al. [18]

Motion on flexion– extension radiographs in degrees (range) Average

No change from 2-year time periodd 5

?

14.2

9 (2–17)

?

10.3 (0–16)

?

7.1 (6.6–7.7)

10.1e (Continued)

Table 14.2 Clinical Details of Primary, Peer-Reviewed Studies Involving the SB CHARITÉ (I, II, and III) (cont.)

BüttnerJanz et al. [19]

Griffith et al. [20]

Cinotti et al. [21]

Zeegers et al. [22]

Sott and Harrison [23]

Lemaire et al. [24]

Putzier et al. [14]

FDA IDE Study, McAfee et al. [25]a

David [26]

Ross [27]

L3–L4

–

12.0 (10–16)

L4–L5

16

9.6 (0–15)

12.2

4.0 (0–18)

L5–S1

9

9.2 (0–14)

9.4

4.7 (0–16)

FDA IDE Study, Guyer et al. [18]

1.4 (0–6)

Hcoh, historical cohort study; Pcoh, prospective cohort study; RCT, randomized controlled trial. a Five-year follow-up of this series have been reported in conference presentations [17] and most recently in the peer-reviewed literature by Guyer et al. [18]. b This is the report on 5-year follow-up on patients eligible and available at this time period; of the 14 initial sites, 6 declined participation reducing the total number of eligible CHARITÉ patients to 90. c Overall clinical success was defined as patients having more than 25% improvement in ODI score at 24 months compared with the preoperative score, no device failure, no major complications, and no neurological deterioration [28]. d Two-year study reported by McAfee et al. [25]. e Average mobility was calculated from the reported data [29].

14:  The Clinical Performance of UHMWPE in the Spine

changes have been made on a rolling basis, and therefore, it is difficult to appreciate at the present time what effect these will have on the long-term clinical performance. For example, in 1997, Link changed its packaging of the UHMWPE core that was initially air permeable (gamma air), to two polymeric pouches in a nitrogen/ vacuum process (N-VAC®) (Figure 14.3) (Serhan, personal communication, 2005). The UHMWPE material used in the cores was changed to compressionmolded GUR 1020, converted by Poly Hi Solidur. At the FDA Panel meeting in 2004, it was revealed that the polymeric packaging was also air permeable, resulting in measurable oxidation of the UHMWPE during shelf storage [30]. On the basis of recently published retrieval analyses, the in vivo oxidation behavior of UHMWPE sterilized in N-VAC packaging appears to be indistinguishable from those that were gamma sterilized in air [31]. All of the UHMWPE cores used in the United States IDE trial were sterilized in Link’s first generation, N-VAC packaging.

Figure 14.3  First-generation, polymeric barrier packaging of the CHARITÉ UHMWPE core by Link, employed between 1998 and 2004. Reproduced with permission from Ref. [11].

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The changes to the CHARITÉ by DePuy Spine (Raynhnam, MA) include changing the packaging of the UHMWPE to a foil pouch (Gamma Vacuum Foil, GVF). This most recent packaging change was initially implemented in April 2004 and launched worldwide in October 2004 (Serhan, personal communication, 2005). Traceable retrieval studies will be necessary to establish the natural history of CHARITÉ polyethylene components stored within firstgeneration (N-VAC) and second-generation (GVF) barrier packaging. Other changes to the CHARITÉ, as documented in PMA supplements, include coating on the endplates and additional endplate sizes (2006, 2007) [32]; change of trade name to IN MOTION with inclusion of additional cores with modified outer rim [33], and modified tolerance for dimensions [34]. The CHARITÉ was withdrawn from the United States/Outside ­United States (US/OUS) market after the DePuy-Synthes merger in 2012 [7].

14.2.4  Bioengineering Studies of the CHARITÉ Although much of the literature about the CHARITÉ is related to clinical outcomes, there is an increasing number of studies related to the bioengineering aspects of TDR that have appeared in the literature [35–45]. These bioengineering studies include both cadaveric studies, as well as finiteelement studies. Among the cadaveric studies, the work of ­Cunningham is perhaps the best known, as the data was made available to the FDA during the IDE study, and has also been published in the peer-reviewed scientific literature [35–37]. Cunningham’s research, therefore, is highly instructive as to the scope and quality of biomechanical testing that is needed to address questions by regulatory bodies, such as the FDA, for the evaluation of TDR technologies. More recently, O’Leary et al. have also published biomechanical data, collected with an in vitro cadaveric model, that characterizes the motion patterns for the CHARITÉ [38]. Cunningham’s work has been summarized in a recent review article [35], and included both in vitro cadaveric spine testing and animal studies. ­ Cunningham’s research analyzed the biomechanics and biocompatibility of wear debris for the ­CHARITÉ. For the in vitro biomechanical studies, Cunningham studied control (untreated) lumbar spines, as well as lumbar spines implanted with the CHARITÉ artificial disc as compared to the BAK

224

cage and the AcroFlex artificial disc [35,36]. The lumbar spines were tested with the application of pure moments (±8 Nm) in flexion–extension, lateral bending, and axial rotation. It is noteworthy that Cunningham’s tests did not include axial preload, and thus quantify the maximum theoretical flexibility of an unloaded spine. Motion was characterized using an OptoTrak 3020 motion system (Northern Digital Inc., Waterloo, Canada) as well as lateral plain radiographs. Under the applied moments, the CHARITÉ exhibited significantly greater axial rotation, but less lateral bending as compared to the intact spine. In absolute terms, the average axial rotation of the intact spine was measured to be 2.4° as opposed to 3.9° after treatment with the CHARITÉ [36]. There was no significant difference between the intact spine and the CHARITÉ in flexion–extension. Cunningham’s animal studies [35] evaluated the response to wear debris of neural tissue by applying 4 mg of particles onto the epidura of New Zealand white rabbits (n = 50, total). Five groups of rabbits were tested (n = 10, each): (1) sham operation (control); (2) stainless steel 316LVM; (3) titanium alloy (Ti-6Al-4V); (4) CoCr alloy; and (5) UHMWPE. The particle sizes ranged between 0.5 mm and 10 mm in diameter, and were verified to be endotoxin free prior to implantation. Animals were sacrificed at 3 and 6 months postoperatively. Even though there was evidence of a chronic inflammatory reaction for all of the particles, it appeared localized within the epidural tissue. CoCr particles, in particular, were shown to diffuse from the epidural layers into the cerebrospinal fluid, and into spinal cord itself. Although this study provides some insight into the biological response following a massive particle load delivered directly to the epidural layer of the cord, the likelihood of this scenario occurring in the clinical situation is not clear. O’Leary et al. [38] have recently studied the motion patterns of the CHARITÉ using an elegant in ­vitro biomechanical testing system that can simultaneously apply pure moments (up to 8 Nm in flexion and 6 Nm in extension) along with a 400N axial preload. The preload is also referred to as a “follower load,” because the orientation of the force is such that, by means of cables, it follows the curvature of the spine during flexion–extension testing. Motion was characterized using an OptoTrak 3020 motion system (Northern Digital Inc., Waterloo, Canada) and biaxial motion sensors. Motion of the UHMWPE core was tracked during flexion–extension using digital video-fluoroscopy. Five lumbar spines

UHMWPE Biomaterials Handbook

were used in the testing from patients ranging in age from 39 to 60. Four distinct motion patterns were observed for the UHMWPE core, when tested under compressive preload. With Motion Pattern 1, relative angular motion occurred predominantly between the superior endplate and the core, with little or no core translation (Figure 14.4A). In Motion Pattern 2, liftoff occurred by either the superior endplate from the core, or by the core from the inferior endplate (Figure 14.4B). In Motion Pattern 3, the UHMWPE core “locked” in plane, resulting entrapment, over a portion of the flexion–extension range (Figure 14.4C). In Motion Pattern 4, angular motion occurred between the core and both the superior and inferior endplates (Figure 14.4D). O’Leary et al. concluded that, “CHARITÉ TDR restored near normal quantity of flexion–extension range of motion (ROM) under a constant physiologic preload; however, the quality of segmental motion differed from the intact case over the flexion–extension range” [38]. Because some of the implants in this study may have been undersized or not optimally placed, the authors further concluded that positioning, changes in lordosis, and the magnitude of the preload would also likely influence the kinematics of the CHARITÉ [38]. Panjabi et al. evaluated the effect on operated and adjacent levels with CHARITÉ implantations versus simulated fusions using a follower load in a new cadaveric hybrid test method in flexion–extension and bilateral torsion [40]. Cadaveric lumbar spines (T12S1) first underwent nondestructive, unconstrained 3D testing with a maximum moment of ± 10 Nm using a flexibility test method in flexion–extension and bilateral torsion in the presence of a 400 N follower load, followed by hybrid testing using the intact total ROM as an input. The five evaluated groups included: (1) Disc at L5–S1; (2) Fusion at L5–S1; (3) Discs at L4–L5 and L5-S1; (4) Disc at L4–L5 and fusion at L5-S1; and (5) Fusions at L4–L5 and L5–S1. The one-level CHARITÉ did not show other-level effects during flexion–extension and torsion, in contrast to the one-level fusion. In the two-level CHARITÉ, the results were similar, except that in flexion–extension, there were small other-level effects. The finite-element studies that have included modeling the CHARITÉ have examined a range of biomechanical issues both on operated and adjacent segments [39,41–45]. Grauer et al. reported on a finite-element model of L3–S1 to compare the biomechanical effects of a two-level CHARITÉ versus a model including a one-level cage fusion and pedicle screw in addition to a one-level CHARITÉ [39]. The

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225

Figure 14.4  Four motion patterns of the UHMWPE Core in the CHARITÉ TDR observed during in vitro biomechanical testing. (A) With Motion Pattern 1, relative angular motion occurred predominantly between the superior endplate and the core, with little or no core translation. (B) In Motion Pattern 2, lift-off occurred by either the superior endplate from the core, or by the core from the inferior endplate. (C) In Motion Pattern 3, the UHMWPE Core “locked” in plane, resulting entrapment, over a portion of the flexion–extension range. (D) In Motion Pattern 4, angular motion occurred between the core and both the superior and interior endplates. Reproduced with permission from Ref. [11].

biomechanical changes at the L3–L4 level for both cases were similar in magnitude, although in the cage/CHARITÉ model it increased, whereas in the two-level CHARITÉ model it decreased. The model also showed changes in motion at the L4–L5 level to be larger for the cage/CHARITÉ model in comparison to the two-level CHARITÉ model (70% increase vs. 10% increase). Rohlman et al. reported on the use of a validated finite-element model to examine the effects of implant position (sagittal and frontal plane) and height of the mobile core (three different heights) of a CHARITÉ [41]. The model showed that the implant position and height markedly affected intersegmental rotation and facet joint forces, but hardly had an effect on intradiscal pressure of adjacent discs. Zander et al. used a validated finite-element

model to compare the effect of total disc arthroplasty using three different artificial discs (CHARITÉ, ProDisc-L, and activL®) on intervertebral rotations, location of helical axes of rotation, intradiscal pressures, and facet joint forces at operated and adjacent levels [45]. For all the artificial discs evaluated, the models showed a reduction in interverterbral rotation for flexion and increased for extension, lateral bending, and axial torsion at the implant level. The position of the helical axes was impacted, especially for lateral bending and axial torsion. Facet joint forces were increased for the model with the CHARITÉ during extension as impacted by the existence of the simulated anterior scar. The models only showed a minor effect on intradiscal pressures at adjacent levels. The model reported by Schmidt et al. compared

226

the effect of the CHARITÉ (unconstrained design) to that of a constrained design on ROM, facet joint forces, and instantaneous center of rotation (COR) [44]. Both models predicted small interimplant differences in both ROM and instantaneous COR, whereas they predicted higher facet joint forces for the CHARITÉ model. A subsequent study by Schmidt et al., using a validated L1–L5 model, considered the effect of multilevel implantation and implant positioning on spine kinematics and facet joint loads [43]. The model showed that the more artificial discs are implanted, the stronger the motion increases in flexion and extension in comparison to the intact state and that deviations form the optimal implant position lead to unfavorable kinematics, high facet joint forces, and lift-off. Rundell et al. reported on the use of a validated L4–S2 finite-element model to examine the sensitivity of CHARITÉ impingement to disc height distraction, spinal sagittal orientation, implant position, and lumbar lordosis [42]. The model showed sensitivity of CHARITÉ impingement to all the studied parameters.

14.2.5  Controversies Surrounding the CHARITÉ Artificial Disc The complete history of the CHARITÉ spans two decades and numerous iterations in design (1984– 2004). During this time period, it was implanted in 9000 patients, mostly in Europe. However, the longterm success of the CHARITÉ remains controversial. The broad range of outcomes for the CHARITÉ reported in the literature (Tables 14.1 and 14.2) raises questions about the probability of long-term success of motion preservation with this implant. In a systematic review of the artificial disc literature published in 2003, the authors concluded that “there is no evidence that disc arthroplasty reliably, reproducibly, and over longer periods of time fulfills the three primary aims of clinical efficacy, continued motion, and few adjacent segment degeneration problems” [29]. Putzier, writing in 2005 from the Charité hospital in Berlin, where the implant was first used clinically, stated that, “the long-term follow-up study demonstrates dissatisfying results after artificial disc replacement in the majority of the evaluated cases.... the CHARITÉ artificial disc replacement cannot guarantee long-term near to normal function of spinal motion segment in patients with moderate to severe [degenerative disc disease]” [14]. As a result, the long-term prognosis for patients with a CHARITÉ TDR remains uncertain.

UHMWPE Biomaterials Handbook

14.2.6  The Legacy of the CHARITÉ Artificial Disc Despite the reported controversies surrounding the CHARITÉ, it is nonetheless the icon for contemporary total disc arthroplasty. Other artificial disc designs are currently in clinical use in Europe, and may become available in the United States within the coming decade. However, these newer designs build upon the design philosophy established by the CHARITÉ that adapted the successful bearing concepts from hip and knee replacements for TDR. The CHARITÉ artificial disc is the first design to establish a history of clinical use spanning two decades, with 10-year outcomes available in, albeit limited to, retrospective studies. The CHARITÉ is a pioneering design for this reason, despite it no longer being available on the market for implantation [7].

14.3  Lumbar Disc Arthroplasty As summarized in Table 14.3, there are currently five lumbar TDR designs incorporating UHMWPE that have been in clinical use: CHARITÉ, ProDisc-L, Mobidisc, activL®, and Baguera L. In the previous section, we reviewed the clinical history and bioengineering studies related to the CHARITÉ. The four newer designs differ from the CHARITÉ in several respects, such as the amount of constraint in the bearing and the incorporation of keels into the endplates (Table 14.3). The amount and quality of available clinical data also varies among the designs (Table 14.3). Although long-term data are available for the ProDisc-L and the CHARITÉ, respectively, the reader should be aware that these long-term studies are limited to retrospective evaluations [14,24,26,49]. In this section, we summarize the design theory and available literature for the ProDisc-L, Mobidisc, activL®, and Baguera L artificial discs. Like the CHARITÉ, these devices are based on CoCr-on-UHMWPE or Ti-onUHMWPE articulations, and are similarly implanted by an anterior approach. However, the newer designs represent departures from the unconstrained, biconvex mobile-bearing philosophy embodied by the CHARITÉ. The four designs incorporate varying degrees of constraint, ranging from a ball-andsocket design of the ProDisc-L, to the sliding convex mobile-bearing designs of the Mobidisc and activL®, to the choice of being fixed or mobile for the UHMWPE bearing of the Baguera L. It is hoped that these newer designs will be able to overcome some of the

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227

Table 14.3  Summary of Contemporary Lumbar Artificial Disc Designs Incorporating Metal/UHMWPE Articulations Design

CHARITÉa

ProDisc-L

Mobidisc

activL®

Baguera Lb

Current manufacturer

DePuy Synthes (Raynham, MA; https://www. depuysynthes. com)

DePuy Synthes (Raynham, MA; https://www. depuysynthes. com)

LDR Spine (Troyes, France; http://www. ldrmedical. com/)

Aesculap AG (Tuttlingen, Germany; http://www. aesculap.com/)

Spineart (Geneva, Switzerland http://www. spineart.com)

Current regulatory statusc

FDA-approved but withdrawn from US/OUS market after DePuy Synthes merger (2012)

FDA-approved, available US/ OUS

Withdrawn US regulatory process; available OUS

FDA-approved, available US/ OUS

Available OUS

Number of components for surgeon assembly

Three

Three

Three

Three

Three

Number of articulating surfaces

Two

One

Two

Two

Two

Bearing design

Mobile bearing

Ball-and-socket

Mobile bearing

Mobile bearing

Mobile or fixed bearing

Rotational constraint

Unconstrained

Semiconstrained

Unconstrained

Unconstrained

Unconstrained and semiconstrained (referred to as mobile or fixed)

Bone/implant fixation

Teeth with and without CaP/Ti coating

Keel and Ti textured coating

Keel and Ti textured coating

Keel or teeth and Ti textured coating

Fins and Diamolithcoated Ti

Keel

No

Yes

Yes

Yesd

No

Published clinical studies

Yes (see Table 14.2)

Yes (see Table 14.4)

Yes (retrospective) [46]

Yes (prospective and retrospective) [47,48]

No

Longest published average follow-up

18.2 years [14]

8.7 years [49]

5.5 years [46]

3 years

None

2-Year FDA randomized controlled trial

Completed and published [25,28]

Completed and published (see Table 14.4)

Terminatedb (Currently in use in Europe and Asia)

Active – not recruitingb (currently in use in Europe and Asia)

Not registered

(Continued)

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Table 14.3  Summary of Contemporary Lumbar Artificial Disc Designs Incorporating Metal/UHMWPE Articulations (cont.) Design

CHARITÉa

ProDisc-L

Mobidisc

activL®

Baguera Lb

Control procedure in FDA randomized trial

Anterior interbody fusion with BAK cage

360° Fusion

N/A

CHARITÉ and ProDisc

N/A

The CHARITÉ is no longer available on the market [7] Information adapted from Veruva et al. (with permission) and for Baguera L from Spineart website (see: http://www.spineart.com/ product-platforms/motion/9/product/baguera®l/128). c Information extracted from Veruva et al. [7] d The activL® has endplates available with either a keel or teeth. a

b

long-term complications and controversies reported in studies of the CHARITÉ.

14.3.1 ProDisc-L 14.3.1.1  History and Clinical Experience: ProDisc-L Conceived in 1989 by Thierry Marnay, MD, from Montpellier in France, the ProDisc consists of two metallic endplates and a UHMWPE core (Figure 14.5) [50]. Unlike the CHARITÉ, the UHMWPE core in the ProDisc is firmly attached to the flat, inferior endplate by a locking mechanism (Figure 14.6). The domed (convex) surface of the core articulates against the concave, superior metallic endplate. There have been two versions of the ProDisc (I and II), but only the more recent version of the design (ProDisc II) has been commercially available. The ProDisc II was originally produced by Aesculap AG & Co. (Tuttlingen, Germany) [49], and then commercialized by Spinal Solutions. Synthes received FDA approval for marketing the ProDisc-L

Figure 14.5 Two CoCr alloy endplates and UHMWPE core of the ProDisc II design. Reproduced with permission from Ref. [11].

in the United States in August 2006. DePuy acquired ­Synthes in 2012, and thus the manufacturer of record changed to DePuy Synthes (West Chester, PA) and the lumbar version of this design is now referred to as the ProDisc-L.

Figure 14.6  (A) Two titanium alloy endplates and UHMWPE core of the ProDisc I design. (B) Two CoCr alloy endplates and UHMWPE core of the ProDisc II design. Reproduced with permission from Ref. [11].

14:  The Clinical Performance of UHMWPE in the Spine

Besides the CHARITÉ, the ProDisc has the second-­longest clinical follow-up of any currently available artificial disc design [49]. Marnay implanted 93 first-generation, ProDisc I artificial discs in 64 patients between March 1990 and September 1993 [49,51]. The endplates of the ProDisc I were fabricated from Ti alloy (Figure 14.6A). Each endplate featured two short, parallel keels for short-term fixation, and the back surfaces were plasma sprayed with titanium for bone ongrowth. The intermediate-term clinical results of the ProDisc I patients were published in 2005 [49]. The mean follow-up of this study, which included 55 (86%) patients from Marnay’s original cohort, was 8.7 years. Marnay demonstrated significant improvement using nonvalidated outcome scoring methods that confirmed his hypothesis that the ProDisc I was safe and effective, at least in his hands (Table 14.4). Procedure-­ related complications were observed in five cases (deep venous thrombosis, iliac vein laceration, transient retrograde ejaculation, and two incisional hernias), but none were related to implant. The investigators were not able to detect any measurable UHMWPE wear when comparing the core height immediately after surgery with the core height at the longest follow-up. Subsequent analysis of the range in clinical performance for this cohort by Huang et al. revealed an intriguing correlation between patient outcomes and the ROM of the operated levels (2005). Patients with more than 5° of flexion–­extension ROM had significantly less postoperative back pain and better clinical outcomes (as measured by postoperative Oswestry Disability Index (ODI) and Stauffer– Coventry Scores) than patients with less than 5° ROM. Marnay followed his patients until 1998 before making further modifications, which resulted in the current ProDisc-L design. Since 1999, more than 5000 ProDisc-L components have been implanted worldwide up to 2005 [50]. In the second-generation design, the endplates are fabricated from CoCr alloy that has improved tribological properties as compared with Ti alloy. In addition, a single, slightly taller keel is used. The textured, back surfaces of the endplates are plasma sprayed with titanium. The UHMWPE core is machined from compression molded GUR 1020, and is gamma sterilized in nitrogen. Foil packaging is currently used for its oxygenbarrier properties (Figure 14.7). The ProDisc-L has a unique locking detail that engages the UHMWPE core with the inferior endplate. The capture mechanism for the core consists of a sliding, tongue-in-groove along the left and right (laterally), with a raised, triangularprofiled ridge on the inferior surface of the UHMWPE.

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During implantation, the surgeon slides the tongues on the core backward into the grooved, capture mechanism of the inferior endplate until the anterior raised ridge of the UHMWPE snaps into a mating slot in the endplate, thereby preventing anterior ejection of the core (Figure 14.5). The reports of generally positive clinical outcomes of the ProDisc-L, which include a study with follow-up periods of up to 5 years for the FDA prospective, randomized clinical trial, are summarized in Table 14.4. In addition, published prospective clinical data for the ProDisc-L provide some evidence that the short-term and long-term outcomes of multilevel implantation are comparable to single-level disc replacement [60,61]. In general, the available clinical data for the ProDisc-L is stronger, scientifically speaking, than that for the CHARITÉ, which has historically been documented by mostly retrospective studies in Europe (Table 14.2). With the exception of the FDA trial, none of the CHARITÉ studies included validated outcome measures, such as the ODI. In contrast, the majority of available studies for the ProDisc-L are prospective, and employ validated outcome measures that can be compared with alternative spine procedures (Table 14.4). The prospective, randomized clinical trial for the ProDisc-L, with both single-level and double-level arms, began in the United States in October 2001 and completed its target enrollment by the end of 2003. The reference procedure was an anterior–posterior (360°) fusion: the control patients first underwent an anterior-fusion procedure, involving a femoral ring allograft. The patients were then flipped, and a posterior fusion was performed, consisting of bilateral intertransverse fixation with pedicle screws [62]. Although by no means obsolete, a 360° procedure represents a highly conservative fusion strategy and arguably a “worst-case” scenario with which to compare with disc replacement. Two of the 19 clinical sites involved in the pivotal trial published interim results, with follow-up periods of up to 18 months (Table 14.4) [50,54,62,63]. At this time point, there was a major difference in the complexity, and hence the operative time between the disc replacement and the 360° fusion (on average, 75.4 vs. 218.2 min, respectively) [62]. Disc replacement patients recovered faster than controls. Furthermore, the 2-year IDE data indicate that the final outcomes the ProDisc-L, as measured by visual analog scale (VAS) and Oswestry Disability Scores, are significantly better than 360° ­fusion [57]. Additional data on short-term and long-term complications with the ProDisc II has appeared in the

Table 14.4 Clinical Details of Primary, Peer-Reviewed Studies Involving the ProDisc I and -L Tropiano et al. [49,51]

Mayer et al. [52]

Tropiano et al. [53]

Zigler [54]

Bertagnoli et al. [55]

Bertagnoli et al. [56]

Delamarter et al. [50]

Zigler et al. [57]

Delamarter et al. [58]

Park et al. [59]

Zigler and Delamarter [60]

Study type

Hcoh

Pcoh

Pcoh

RCT

Pcoh

Pcoh

RCT

RCT

RCT

Hcoh

RCT

Type of prosthesis

ProDisc I

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

ProDisc-L

38

48 (median)

51 (median)

40

39

41.8

46.5

38.7

30–60

19–59

104

25

56

161

165 (148 available for follow-up)

35

161 (137 available for follow-up)

104

60

91

161

165

35

161

31 (median)

31 (median)

18

24

24

72 (mean)

72

24–45

25–41

Average age

46

44

45

25–65

25–65

28–67

No. of patients

55

34

53

No. of arthroplasties

78

37

68

Follow-up (months)

104

12

17

Age range

Range in followup

85–128

28

12

12–24

Good or excellent

75%

Initial oswestry score (%), average ± SD

N/A

19 ± 7

56 ± 8

Owestry score (%) at longest follow-up, average ± SD

18 ± 16

7.2 ± 9.6

14 ± 7

Secondary surgery

3/55

0/34

Arthrodesis or spontaneous fusion

1/55

Complications

10/55

27–70

60–94 53.4%a

58.8%b

71.4%

93.2%

54

65 (42–92)

31

63.4 ± 12.6

64.7

44.0 ± 12.7

63.4 ± 12.6

29

22 (0–48)

20

34.5 ± 24.8

52.4 ± 38.1

18.9 ± 16.4

34.2 ± 24.3

3/53

1/104

1/25

6/161

4/165

11/137

13/161

0/34

0/53

0/104

0/25

0/161

3/34

5/53

5/104

4/25

10/161

9 (2–18)

7

Decreased

1/165

2/161

Motion on flexion–extension radiographs in degrees (range) Average

4.0 (0–18)

7.7 ± 4.7

6.0 ± 4.5

L4–L5

10 (8–18)

10

7.8 ± 5.3

L5–S1

8 (2–12)

8

6.2 ± 4.1

Hcoh, historical cohort study; Pcoh, prospective cohort study; RCT, randomized controlled trial. a Overall clinical success was defined by FDA as patients having achieved ≥15% improvement in ODI, SF-36, radiographic success, and neurologic success, all without device failure [57]. b Overall statistical success was defined by FDA as having achieved ≥15 point improvement in ODI, improvement in SF-36 PCS compared with baseline, neurological status improved or maintained from baseline.

14:  The Clinical Performance of UHMWPE in the Spine

Figure 14.7 Outer polymeric pouch and inner foil barrier packaging of the ProDisc II UHMWPE core developed by Spine Solutions and currently employed by Synthes Spine. Reproduced with permission from Ref. [11].

­literature [55–57,64,65]. At the 2005 North American Spine Society meeting, for example, Bertagnoli reported an overall complication rate for the ProDisc at 3.0% (15/522 patients) [66]. Although relatively few in number (7/522, 1.4%), many of the complications reported for the ProDisc II, such as wound infections, retrograde ejaculation, epidural or retroperitonial haematoma, appear related to the anterior approach itself [66]. Device-related complications have been reported in 8/522 cases (1.6%), and include partial subsidence of the endplates, anterior migration, or partial ejection of the UHMWPE core, and L5 root/motion deficit [66]. Two cases of vertebral split fractures associated with the ProDisc have also been reported in the literature [64]. In both cases, the split fractures were caused by the surgeons when chiseling of the bone grooves to accommodate the keels. Thus, the vertebral split fractures, as well as the majority of the complications reported by Bertagnoli, are likely related to surgical technique. The cases of core ejection may be technique related, particularly if the ­surgeon

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fails to properly engage the capture mechanism during installation. Indeed, Bertagnoli opines “more than 90% of device-related complications are iatrogenic. Poor patient selection, improper implantation, and wrong sizing are the most common examples of surgical errors causing a higher risk of failure” [65]. In the IDE study [57], the 3.7% reoperation rate of the ProDisc-L at 2 years included dislodgement of the UHMWPE inlay due to “extreme trauma” (in two cases), and one case of technical error in which the surgeon inserted the UHMWPE inlay backward. In one case the inlay dislodged after surgery and was interpreted by the authors to have not been properly inserted. In another case, the entire TDR migrated anteriorly. All of these short-term clinical failures required immediate revision surgery. The 5-year results of the prospective, randomized, multicenter FDA study have been reported in the literature demonstrating that the ProDisc-L group had significantly better improvement in some of the rating scales [60]. At 5-years, the overall follow-up rate was 81.8% and study success demonstrated that TDR was noninferior to fusion with a 12.5% margin (p = 0.0099). Both ProDisc-L and fusion groups showed significant improvement on the ODI scale at 5 years compared to baseline (p, 0.0001). Although secondary surgeries at the index level were performed, these were only occurred for 8% of the ProDisc-L patients, whereas they occurred at a rate of 12% for the fusion patients. The segmental ROM following ProDisc-L implantation remained within normal range, although it decreased by approximately 0.5° in 3–5 years . VAS pain scores decreased from preoperative values by 48% in both groups at 5 years. Patient satisfaction remained high (77%) in both groups, while there was a higher percentage (82.5%) of ProDisc-L patients indicating they would have the surgery again as compared to fusion patients (68%). A separate study focused on reporting the radiographically demonstrated adjacent level changes from the prospective multicenter FDA study [67]. At year 5 following the index surgery, ProDisc-L patients maintained ROM and were associated with a significantly lower rate of changes in adjacent-level degeneration (DeltaALDs) than the patients treated with circumferential fusion.

14.3.1.2  Bioengineering Studies: ProDisc-L Limited test data, related to shock absorption, have been reported for the ProDisc-L in the scientific literature [68]. Dynamic testing at frequency ranges

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between 0 Hz and 100 Hz, as well as “shock loading” with 0.1 s durations, demonstrated that the ProDisc II exhibits negligible shock-absorption capacity, regardless of the fact that a UHMWPE core is included in the design. Biomechanical range-of-motion evaluations for the ProDisc II were presented at the 2003 meeting of the Orthopedic Research Society [69]. Lipman et al. tested six lumbar spines, with and without a ProDisc II. The specimens were mounted in a 6-degree of freedom spine testing apparatus, attached to a biaxial MTS load frame. The specimens were tested with ±10 Nm flexion–extension, lateral bending, and axial torsion, under applied compressive preloads of 600 N and 1200 N. The investigators found no significant difference in the ROM of the natural and treated lumbar spine when testing under these conditions. Other biomechanics evaluations have been reported on the use of lumbar cadaveric models to examine various questions in regards to the implantation of the ProDisc alone, in comparison to fusion, or in comparison to other TDRs. Panjabi et al. reported on intervertebral rotation changes expressed as adjacent level effects (ALEs) as seen in the fusion simulations in comparison to the cadaveric model implanted with ProDisc-L, which preserved physiologic motion at all spinal levels [70]. Rousseau et al. used a cadaveric spine model to examine the effect of semiconstrained (ProDisc) versus unconstrained (CHARITÉ) implant on facet joint forces and instantaneous axes of rotation (IAR) [71]. The facets were partially unloaded with the ProDisc, whereas the IAR did not match the geometric center of the UHMWPE insert. Wilke at al. examined the role of prosthesis design on segmental biomechanics was assessed by comparing the CHARITÉ and the ProDisc using a cadaveric lumbar spine (L2–L5) model [72]. Their study found that despite the different alterations in the ROM and segmental lordosis due to implant design, the noted differences were negligible compared to the overall increase in ROM and segmental lordosis with the implantation of a TDR as compared to the physiologic state. Wear testing for 5 million cycles of the ProDisc II in comparison to the CHARITÉ was presented at the 2006 ORS meeting [73]. Two different types of duty cycle were employed. In one set of simulations, a curvilinear motion of the endplates was prescribed in flexion–extension and axial rotation. In a second set of simulations, crossing path motion was prescribed in flexion–extension, lateral bending, and axial rotation. The load magnitude varied between 350 N and 1750 N; however, no further details about

UHMWPE Biomaterials Handbook

the test conditions (e.g., test temperature and fluid) were provided in the abstract. Simulator kinematics had a significant effect on wear rates, with crossing path motion producing a higher wear rate as compared with curvilinear motion. The wear rates of the ProDisc II and CHARITÉ artificial discs were found to be similar under both sets of simulator conditions. The effect of loading frequency on the wear rate of cyclic fatigue testing of seven ProDisc-L was reported by Kettler et al., where the 4 MC were applied at 1 Hz, and 8 MC were tested at 2 Hz, in an alternating sequence [74]. The mean wear rate was 5.6 ± 2.3 per MC at 1 Hz and 7.7 ± 1.6 per MC at 2 Hz, during the first 6 MC and 2.0 ± 0.6 and 4.1 ± 0.7 per MC during the second 6 MC. The results indicated that testing at 2 Hz increased the wear rate compared to 1 Hz. The authors noted that a loading frequency of 1 Hz is probably closer to physiology than 2 Hz. Similar to the CHARITÉ, a series of reports exist on the use of finite-element models to examine a range of biomechanical issues both on levels implanted with ProDisc-L as well as adjacent segments [45,75–77]. Rohlmann et al. used a validated 3D nonlinear finite-element model of the lumbar spine to evaluate the effect of ProDisc-L position on intersegmental rotation and found that in addition to position, implant height also impacted intersegmental rotation [76]. Another validated nonlinear finite-element model of the lumbar spine reported by Rundell et al. evaluated the effect of ProDisc-L implantation and positioning on facet joint forces and vertebral body strains [77]. The model found that after implantation of the disc, there was an increase in loading at the facets due to reduced rotational stiffness, as well as an increase in vertebral body strains during flexion. Additionally, posterior placement of the ProDisc-L resulted in more physiologic load transfer to the vertebral body. Rohlmann et al. reported on a probabilistic finite-element study to examine how intervertebral rotation, intradiscal pressure, and facet contact force were impacted by implant position, implant ball radius, presence of scar tissue, and gap size at the facet joints [75]. As reviewed previously, Zander et al. used a validated finite-element model to compare the effect of total disc arthroplasty using three different artificial discs (CHARITÉ, ProDisc-L, and activL®) on intervertebral rotations, location of helical axes of rotation, intradiscal pressures, and facet joint forces at operated and adjacent levels [45]. For all the artificial discs evaluated, the models showed a reduction in interverterbral rotation for flexion and increased for extension, lateral bending, and axial torsion at the implant level. The position of the helical axes was

14:  The Clinical Performance of UHMWPE in the Spine

233

impacted, especially for lateral bending and axial torsion. Facet joint forces were increased for the model with the ProDisc-L, during lateral bending and axial torsion. The models only showed a minor effect on intradiscal pressures at adjacent levels. In summary, there is a growing body of literature supporting the clinical safety and effectiveness for the ProDisc II. The 5-year results of the singlelevel, prospective randomized trial are now publicly available [60,67]. However, many aspects of the clinical evaluation, in particular long-term complication rates and in vivo wear rates, remain yet to be fully explored.

14.3.2 Mobidisc The Mobidisc may be considered a second-­ generation mobile-bearing design, and has a clinical track record of less than a decade. Developed by LDR spine (Troyes, France), clinical use of the Mobidisc began in November 2003. The prosthesis is available in Europe and Asia, but not in the United States. The IDE trial in the United States for the Mobidisc was terminated due to insufficient recruitment [7,78]. The Mobidisc consists of two polished cobalt chromium endplates and a UHMWPE core, which has a spherical domed superior (cranial) surface and a flat inferior (caudal) surface (Figure 14.8). Thus, the Mobidisc is a convex, sliding mobile-bearing design. The UHMWPE core has two lateral wings that articulate within lateral capture mechanisms in the inferior endplate. The UHMWPE is gamma-sterilized in a polymeric barrier package. The results of an ongoing clinical case series have been reported in a recent book chapter [79]. Steib et al. summarize one surgeon’s results of 149 patients that were enrolled and underwent surgery by one of eight surgeons in a prospective clinical case series. After 2 years, back pain improvement on the VAS averaged 60% and improvement on the ODI was 55%. The mobility of the affected level changed from 4° (range: 0–15°) preoperatively to 9.4° (0–18°) postoperatively. At the last follow-up, 76% of all prostheses had mobility. A total of 12 complications that resulted in three revision surgeries were reported. Ten cases of subsidence, one laterally malpositioned prosthesis, and one improperly sized prosthesis were reported. Additionally, a retrospective clinical study published by Lee et al. reviewed 18 consecutive patients operated on 21 levels with the Mobidisc and reported radiographic assessments (disc space height and

Figure 14.8  (A) Two CoCr alloy endplates and UHMWPE core of the Mobidisc; (B) Design of mobile UHMWPE core, illustrating the lateral wings; and (C) Design of inferior CoCr endplate, illustrating the stops which provide antirotational and translational constraint for the mobile core. Reprinted with permission from LDR spine.

ROM), as well as lower back and leg pain (with ODI) at a mean follow-up period of 5.5 ± 1.6 years [46]. This study reported 77.8% patient satisfaction with surgical outcomes, with a decrease in low back pain and increase in function as compared to preoperative states. Radiographic findings showed an improved disk space height and preservation of ROM as the COR index increased. The study also concluded that prosthesis positioning might be important given that clinical outcomes and preservation of ROM correlated with the COR index. Although clinical improvements were reported in these series of patients, the results of longer-term, prospective randomized controlled clinical studies have not yet been published.

14.3.3 activL® The activL® is another second-generation artificial disc, and was designed by Aesculap AG (Tuttlingen, Germany) after developing the ProDisc. Unlike the ProDisc that is a fixed-bearing design, the Activ-L is a mobile design consisting of two CoCr endplates and a mobile UHMWPE core (Figure 14.9). The UHMWPE core is flat on the inferior/caudal side to accommodate anterior–posterior translation, and convex on the superior/cranial side to allow rotation of the endplates with respect to one another. The core edge is geometrically constrained in a groove ­within

234

Figure 14.9  Two CoCr alloy endplates and UHMWPE core of the activL® design (Aesculap AG, Tuttlingen, Germany).

the inferior endplate (Figure 14.9). According to personal communications with the manufacturer, the UHMWPE core is gamma irradiation sterilized in a barrier package. The activL® received its CE Mark in 2005 and is currently used in Europe and Canada. It was recently approved by the FDA. An Investigational Device Exception clinical trial for the Food and Drug Administration was initiated in January 2007, and it is currently closed after having an estimated enrollment of 414 patients. The 24-month follow-up was expected to be completed by December 2012, with a final study completion date of July 2017 [80]. The IDE includes both the CHARITÉ and ProDisc as controls. A report by Yue and Mo provide an overview of the study plan for the prospective, randomized, multicenter, singlemasked clinical trial of the activL® [81]. There are two reports of studies documenting the clinical experience with activL®. A report by Nabhan et al. provides an overview of a prospective controlled study comparing lumbar spine disc replacement using the activL® versus fusion for monosegemental degenerative disc disease [82]. They report on evidence of significant segmental motion in the activL® group (n = 13) in comparison to a fusion group (n = 11) during a period up to 1 year after surgery. Separately, as reviewed by Lu et al., 32 patients were included as part of a prospective observational study, of which 30 patients, with a mean age of 45.1 years, were available for a mean follow-up of 28.8 months [47]. Overall the 1–3 year follow-up of this group of patients showed satisfactory clinical outcomes, with an intervertebral disc height at the index and adjacent segments maintained after surgery. The ROM at the index and upper segments showed a slight increase, whereas the ROM at the lower adjacent segments remained unchanged. The same group published a retrospective case series on the same group of patients

UHMWPE Biomaterials Handbook

in 2015 [48]. The long-term clinical performance of activL® still needs to be followed. Grupp et al. reported on the biotribological evaluation of the activL® to examine long-term wear and impingment [83,84]. Grupp et al. followed protocols outlined in ISO/FDIS 18912 (first 10 MC) and ASTM 2423 (next 5 MC) and reported gravimetric wear for the first 10 MC (ISO) as 2.7 ± 0.3 mg/MC and during the ASTM test period (10–15 MC) the gravimetric wear rate was 0.14 ± 0.06 mg/MC [83]. The authors proposed that the main explanation of divergence between the ISO- and ASTM-driven wear simulations was the multidirectional pattern of movement described in ISO resulting in cross-shear stress on the UHMWPE material. They also concluded that on the basis of observations on previous retrieved implants, it would be very unlikely to have an artificial lumbar disc loaded with a linear wear path as per ASTM 2423-05. Grupp et al. suggest that the clinical wear behavior of an artificial disc is more predictable using the ISO/FDIS 18192-1 method. A follow on study by Grupp et al. examined the effect of impingement on wear, contact pattern, metal-ion concentration, and particle release [84]. An average volumetric wear of 0.67 mm3/MC was measured by impingement under flexion, 0.21 mm3/MC under extension, 0.06 mm3/MC under lateral bending, and 1.44 mm3/MC under combined flexion bending. The metal size particles detected were in the range of 0.2–2 mm. The authors noted that their impingement wear simulation protocol appears to be well suited to predict in vivo impingement in regards to the contact pattern as compared to retrieved devices. In terms of biomechanics type evaluations, Ha et al. reports on the evaluation of adjacent level effects (ALEs) [85]. Ha et al. report on the assessment of ROM, disc pressure and facet strain using human lumbar (L2–S2) cadaveric spines implanted with activL® at the L4–L5 level. The activL® showed statistically significant decrease of ROM during rotation, increase of ROM during flexion and lateral bending at the index level and increase of ROM at the inferior segment during flexion. The disc pressure of the superior disc was comparable to baseline levels of the intact spine and the disc pressure of the inferior disc decreased in all loading modes (not significant). Facet strain decreased significantly at the index level during flexion and at the inferior level during rotation. Use of a validated finite-element model has been reported to examine the kinematics of three established TDRs, including the activL® and the effects on intervertebral rotations, location of helical axes of

14:  The Clinical Performance of UHMWPE in the Spine

rotation, intradiscal pressures, and facet joint forces at operated and adjacent levels [45]. As reviewed previously, for all the artificial discs evaluated, the models showed a reduction in interverterbral rotation for flexion and increased for extension, lateral bending, and axial torsion at the implant level. The position of the helical axes was impacted, especially for lateral bending and axial torsion. In the case of the model evaluated with the activL®, the facet joint contact forces were increased during lateral bending and axial torsion.

14.3.4  Baguera L The Baguera L is another second-generation artificial disc, and was designed by Spineart (Geneva, Switzerland). The Baguera L includes two titanium plates coated with DIAMOLITH® (each plate includes five fins for stability) and an UHMWPE nucleus. The shape of the inferior plate and the UHMWPE nucleus are designed to enable shock absorption. The UHMWPE component can be implanted as a mobile or fixed nucleus without changing the inferior or superior plates. The movement of the mobile UHMWPE nucleus is controlled with respect to rotation. The Baguera L is currently used outside the United States and there is no currently registered IDE [7].

14.4  Cervical Disc Arthroplasty In several key aspects, total disc arthroplasty is a more attractive technology for the cervical spine than for the lumbar spine. First of all, the biomechanical demands on a cervical prosthesis, which must support the weight of the head and the neck above it, are about an order of magnitude less stringent than in the lumbar region, which instead must support the weight of the entire upper torso. The lower loads of the cervical spine also permit a much broader range of design and biomaterial options, which would not otherwise be feasible in the lumbar spine. Second, the indications for cervical disc replacement, which include myelopathy and radiculopathy, are broader and more prevalent than for lumbar disc replacement [86]. That is, more patients undergoing cervical fusions today may be candidates for disc arthroplasty, provided certain other contraindications are not met, such as nonintact posterior elements and osteoporosis. In contrast, the list of contraindications in lumbar disc replacement is currently so extensive that it effectively precludes the vast majority of lum-

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bar fusion candidates from receiving an arthroplasty [87]. Third, the surgical access to the anterior cervical spine is easier, and less risky for the patient, than the access to the anterior lumbar spine. Furthermore, the ease of access for cervical disc replacements is preserved during revision, whereas in the lumbar spine, scar tissue, and adherent vessels greatly complicate the anterior revision process. For these main reasons, cervical disc arthroplasty appears to be a less risky proposition than lumbar arthroplasty, both in terms of the feasibility of available technology options for the implants, as well as in terms of the risk to the patient during the procedure. As noted previously, Fernström’s stainless steel sphere was the first cervical disc arthroplasty implanted back early 1960s [8]. Although Fernström’s 1966 publication acknowledges his use of the spherical prosthesis in the cervical spine, two South ­African surgeons, Hjalmar Reitz and Mauritius ­Joubert, are credited with publishing the first paper on cervical disc arthroplasty 2 years earlier [88]. Reitz and Joubert visited with Fernström in Sweden, and were “favourably impressed by his results” [88]. After returning home, they implanted 75 spherical, cervical disc replacements in 32 patients who suffered from severe headaches and neck-related arm pain. The surgeons reported generally positive outcomes for their patients who were followed for less than a year. However, Reitz and Joubert were optimistic that Fernström’s prosthesis “preserves the mobility of the intervertebral joints, thus obviating the inherent disadvantages of all fusion procedures” [88]. The surgeons acknowledged that longer-term follow-up would be necessary to confirm the viability of their technique. Fernström’s pioneering design, along with the experience of Reitz and Joubert, would provide inspiration for future generations of cervical disc replacement technologies. In this section, we summarize the design features and available literature for contemporary cervical metal-on-UHMWPE artificial discs. Many cervical disc replacements have been proposed over the years, but today seven contemporary designs have been documented in the peer-reviewed literature. A selection of these have currently either concluded clinical trials or have active clinical trials in the United States: ProDisc-C, PCM, Mobi-C®, SECURE®-C, and Discover (Table 14.5). Four of these designs (ProDisc-C, PCM, Mobi-C, and SECURE-C) are FDA approved. As detailed further, these designs differ from each other in several respects, including the philosophy as well as the constraint in the bearing (Table 14.5).

Table 14.5 Summary of Contemporary UHMWPE/Metal Cervical Artificial Disc Designsa

a

Design

ProDisc-C

PCM

Mobi-C

Activ-C

Discover

SECURE-C

Baguera C

Manufacturer

DePuy Synthes (Raynham, MA; https://www. depuysynthes. com)

Nuvasive (San Diego, CA; http://www. nuvasive. com/)

LDR Spine (Troyes, France, http://www. ldrmedical.com/)

Aesculap AG (Tuttlingen, Germany; http:// www.aesculap. com/)

DePuy Synthes (Raynham, MA; https://www. depuysynthes. com)

Globus Medical (Audobon, PA; http://www. globusmedical. com/)

Spineart (Geneva, Switzerland http:// www.spineart. com)

Current regulatory status

FDA-approved, available US/ OUS

FDA-approved, available US/ OUS

FDA-approved, available US/OUS

Available OUS

Available OUS; IDE FDA trial active (not recruiting)

FDA-approved, available US/ OUS

Available OUS

Number of components for surgeon assembly

Two

Two

Three

Three

Two

Three

Three

Number of articulating surfaces

One

One

Two

Two

One

Two

Two

Bearing design

Ball-and-socket

Surface replacement

Mobile bearing

Mobile bearing

Ball-and-socket

Sliding core (spherical superior; cyclindrical inferior)

Mobile bearing

Constraint

Semiconstrained

“Minimally” constrained

Semiconstrained in core translation

Semiconstrained in core translation

Semiconstrained

Selectively constrained

Guided mobile nucleus

Bone/implant fixation

Keel and Ti textured coating

CaP/Ti coating

Lateral self-retaining teeth and Ti textured coating

Lateral self retaining teeth and Ti textured coating

Teeth and Ti textured coating

Serrated keels and Ti plasma spray

Fins and Diamolith-coated Ti

Keel

Yes

No

No

No

No

Yes

No

Published clinical studies

Yes

No

Yes

Yes

Yes

Yes

Yes

Longest published average follow-up

5 Years prospective [60] 7.7 years retrospective [89]

None

7.5 Months

2 Years

26.4 Months

24 Months

Not reported

Information extracted from Veruva et al. [7].

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Figure 14.10 The ProDisc-C cervical disc replacement. Reproduced with permission from Ref. [11].

14.4.1 ProDisc®-C The ProDisc-C, produced by DePuy Synthes Spine (West Chester, PA), is a cervical version of the lumbar ProDisc II, and employs CoCr-on-UHMWPE articulation (Figure 14.10). In contrast to the lumbar design, in the ProDisc-C, the UHMWPE component is provided already inserted into the inferior endplate by the manufacturer. Bone-implant fixation is achieved by a combination of a keel and titanium plasma sprayed surface, similar to the ProDisc II. Naturally, the size and proportion of the keel and articulations are much smaller in the ProDisc-C as compared with the ­ProDisc-L but the technology underlying both designs is identical. Synthes completed a prospective randomized clinical trial with the ­ProDisc-C for the FDA, and received approval to market the device in the United States in December 2007. In one of the earliest published clinical followup studies, Bertagnoli et al. reported promising 1-year clinical follow-up for the ProDisc-C [90,91]. Starting in December 2002, the ProDisc-C study included 27 single-level patients who were implanted between C4–C5 and C6–C7. The average patient age was 49 years (range: 31–66 years). No devicerelated or procedure-related complications were reported. After 1 year, the patients’ average ROM had improved from 4.2° (preoperative ROM) to 10.2°. The patients’ neck disability index (NDI) also improved, on average, from 28.9 points (preoperatively) to 18.8 points at 1 year.

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Additional clinical reports on the ProDisc-C have now appeared in the peer-reviewed literature, including reports of the cohort of patients enrolled in the prospective randomized controlled trial used for FDA approval [92,93], as well as other smaller clinical trials of international patients (see Table 14.6) [92–100]. The FDA IDE study included comparison to a group with ACDF with a total of 103 patients (average age 42.1) receiving a ProDisc-C. These reports on the IDE study include updates at 2- and 5-year follow-up periods [92,93]. At the 5-year follow-up, the rate of reoperations reported for the ProDisc-C group was 2.9%, with the reasons reported as pain and/or adjacent level disease [95]. None of these reoperations were due to device failures. Additionally, the results at the 5-year period also showed that there is significant sparring of ALEs with TDR. The data also showed that the ProDisc-C patients had significantly greater improvement, maintained motion at the index level, and had significantly less pain (VAS frequency and intensity) [93]. The international clinical trials reported in the literature, report follow-up of patients up to 3 years [100]. Nabhan et al. reported in 2007 on their trial following 25 patients who had received ProDisc-C in comparison to ACDF, in which segmental motion as well as clinical symptoms (VAS neck and arm) were assessed [100]. This study showed that after 3 years, the ProDisc-C group maintained segmental motion and appeared to be preventing adjacent level degeneration, along with a significant reduction in neck and arm pain. Additional insights on the clinical performance of the ProDisc-C can be gained from the various retrospective analyses that have also examined the effects on ROM, adjacent segment motion, progression of facet arthrosis, rate of reoperation, evidence of HO, and various clinical outcomes (e.g., NDI, VAS) either in comparison to other TDRs or to ACDF [89,101– 106]. The extent of the follow-up period has been as high as 7.7 years. These retrospective analyses have also included patients from the IDE trial [102,107], whereby patients with greater disc height were found to benefit more in ROM from a TDR [102], but longer-term follow-up is still warranted. The longest follow-up period documented in a retrospective study is by Malham et al. with an average follow-up of 7.7 years in a group of 24 patients implanted with ProDisc-C [89]. The reported average VAS neck pain and VAS arm pain improved by 60 and 79%, respectively, compared to baseline. The outcome scores at the last follow-up were not statistically different than the reported outcomes at 2 years. No patient required revision at the index level or secondary surgery for

Table 14.6 Clinical Details of Primary, Peer-Reviewed Studies Involving the ProDisc-C

Study type

Mehren et al. [97]

Nabhan et al. [100]

Anakwenze et al. [94]

Murrey et al.a [92]

Vorsic et al. [98]

Yambin et al. [99]

Kelly et al. [96]

Delamarter et al. [95]

Zigler et al. [93]

Pcoh (Intl)

RCT (Intl)

RCT (FDA)

RCT (FDA)

Pcoh (Intl)

RCT (Intl)

Hcoh (FDA)

RCT (FDA)

RCT (FDA)

Average ageb

–

44

42.2 (TDR)

42.1

48.1

44

42.1

42.1

42.1

Comparison group

None

ACDF

ACDF

ACDF

ACDF

Byan disc

ACDF

ACDF

ACDF

No. of patientsc

54

25

89

103

40

26

100

103

103

No. of arthroplasties

77

–

89

103

50

26

100

103

103

Follow-up (months)

12

36

24

24

12

13.6

24

60

60

Outcomes tracked

HO, VAS, NDI

RSA VAS

Radiographic

VAS, NDI, SF-36, radiographic ROM

NDI VAS

FSU angle, Cobb angle

ROM

Second surgery

NDI, VAS, SF-36

Initial VAS score, average

Arm: 6.1 Neck: 5.4

Arm: 7.3 Neck: 6.0

Arm: 6.9 Neck: 6.7

–

–

–

Arm: 64 Neck: 73

VAS at longest follow-up,

Arm: 1.8 Neck: 2.4

Arm: 1.2 Neck: 1.7

Arm: reduction of 43 mm Neck: reduction of 46 mm

Arm: 3.3 Neck: 4.3

–

–

–

Arm: 18 Neck: 20

Conclusion

Improvement in clinical parameters

ROM at 3 years unchanged from 1 year

Equivalent restoration of sagittal balance

72.3% Success ProDisc-C

Pain relief and improved functional outcome

ProDisc can restore lordosis

No association between treatment and ROM

Significant sparring of adj level with TDR

TDR greater improvement in VAS-neck

Secondary surgery

–

None

–

2 (1.9%)

–

–

–

3 (2.9%)

2.9%

HO-Grade 0–IV

Grade 0: 26 (34%) Grade I: 6 (7.8%) Grade II: 30 (39%) Grade III: 8 (10.4%) Grade IV: 7 (9.1%)

–

–

–

–

–

–

–

–

–

–

3

–

–

–

–

–

–

–

2 Implant related; 1 implantation related

1 Abscess requiring TDR removal

–

–

–

–

Arthrodesis or spontaneous fusion Complications

–

Hcoh, historical cohort study; Pcoh, prospective cohort study; RCT, randomized controlled trial. a Overall clinical success was defined by FDA as patients having achieved >15% improvement in NDI, no revision, removal or re-operation, maintenance, or improvement in neurological evaluations. b Average age is for all patients unless stated otherwise. c Number of patients implanted with ProDisc-C.

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symptomatic ASD. In this series the mean ROM was 6.5° at 2 years, which was maintained at the last follow-up (7.7 years). Two review articles have included analyses of comparative outcomes of cervical TDR and ACDF and offer additional valuable perspectives on the clinical performance of the ProDisc-C out to 60 months, in comparison to other devices [108,109]. These review articles have focused on adjacent segment disease, overall clinical, and neurological success, either based analysis of the published data inclusive of other TDRs [109], or as a meta-analysis of existing IDE data by McAfee et al. [108]. In vitro biomechanical testing of the ProDisc-C was performed by DiAngelo et al. [110]. Six cervical spines (C2–C7) were tested sequentially in the intact state, following disc arthroplasty, and then after a simulated single-level fusion. The testing frame was a custom-built load frame attached to a servo motor load actuator (International Device Corp., Novato, CA) with a robotic controller (Adept, Inc., San Jose, CA), in line with a load cell. Custom fixtures were then used to drive the cervical spine in displacement control to produce flexion–extension, lateral bending, or axial rotation. The authors justify their displacement control method by suggesting that their procedure is more physiologic than applied pure bending moments, and further opine that the use of a follower load “restricts the spine from following its natural path.” They measured a significant difference in the extension ROM of the intact spine (28° ± 6°) as compared with the ProDisc-C (39° ± 2°), but in all other degrees of freedom, the flexibility of the intact spines and the disc replacement were not significantly different. The authors concluded that the ProDisc-C maintains the overall flexibility of the cervical spine, comparable to its intact state. The question of impacting the adjacent segments of the cervical spine following disc replacement continues to be explored using cadaveric models, by examining parameters such as disc pressures during motion testing [111], ROM [112], effect on facet joint contact pressure [113,114], or impact on postural compensation [115]. Laxer et al. reported that on the basis of their initial testing of four cadaveric specimens, their data indicated that adjacent level discs to those implanted with a ProDisc-C experience substantially lower pressure after a 2­ -level replacement when compared to a 2-level fusion [111]. Cho et al. reported on biomechanical testing of various combinations of treatments and concluded that 2-level ProDisc-C increased the entire C4-T1 ROM, 2-level ACDFs decreased it, whereas

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a hybrid ACDF/ProDisc-C operation did not alter the ROM [112]. The same study showed that in 2-level ­ProDisc-C samples, the combined ROM of the operated level increased, with changes at the adjacent levels compensating for the operative levels. B ­ auman et al. demonstrated in their cadaveric study that although implantation of ProDisc-C at the C5–C6 level increased angular rotations, it did not significantly alter the local facet pressure at the index level during flexion or extension [113], whereas Jaumard et al. demonstrated similar findings in regards to facet contact pressure in the posterior region of the cervical facet joints when exposing their cadaveric specimens to lateral bending and axial rotation [114]. More recently, Safavi-Abbasi et al. have reported on the evaluation of segmental postural compensation, neutral buckling, and the shift in instantaneous axis of rotation (IAR) following ProDisc-C implantation [115]. Their cadaveric study results indicate that ProDisc-C levels provided the path of least resistance through which the initial motion was more likely to occur, while the tendency to buckle under vertical load was greater with more arthroplasty levels. Additionally, their study showed that although it might be predicted that more severe shifts in IAR would lead to more severe buckling, their study did not show that correlation, indicating that ProDisc-C malpositioning should not necessarily predispose the spine to buckling. Similar to other TDRs, finite-element modeling has also been used to examine the biomechanical effects of ProDisc-C on ROM, bone–endplate interface stresses, and adjacent segment loading. Lin et al. reported on a comparative finite-element analysis of ProDisc-C with other TDRs and concluded that the rigidity of the core initially guarantees disc height maintenance but can show high contact stresses at the bone–endplate interfaces if improperly placed or undersized [116]. Lee et al. compared the biomechanics of the ProDisc-C with that of a mobile core TDR and showed that in the presence of a cervical disc replacement there was an increase in ROM, facet joint force, and ligament tension at the index levels [117]. In another comparative study, Mo et al. showed in a finite-element model that implantation of a ProDiscC resulted in hypermobility at the index level that led to an increase in ligament force and strain [118]. The effects of hybrid surgery (fusion and ProDisc-C) have also been examined by Zhao et al. using finiteelement models in comparison to 2-level fusion indicating a better ROM with the hybrid surgery simulation and less increase in adjacent segment intradiscal pressures [119]. The findings of these models provide

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UHMWPE Biomaterials Handbook

Figure 14.11  The PCM cervical disc replacement. (A) Implant photograph; (B) AP and lateral radiographs of the PCM implanted in the cervical spine. Reproduced with permission from Ref. [11].

valuable information on the biomechanical behavior on the ProDisc-C, but should be considered in light of the details presented regarding the validation of each individual finite-element model.

14.4.2 PCM The porous coated motion (PCM) artificial disc is manufactured by Nuvasive (San Diego, CA) (Figure 14.11). The PCM was originally developed by Cervitech, Inc. (Rockaway, NJ) and the technology was subsequently acquired by Nuvasive in 2009. Like the CHARITÉ, the PCM is a “minimally constrained” bearing design [120]. The “gliding surfaces” of the PCM are intended to accommodate smooth translations and rotations. Consequently, the PCM strongly depends upon the competency of the surrounding soft tissues for proper kinematics; in that respect it is more analogous to a cruciate retaining total knee replacement than to a total hip replacement. The inventors compare the PCM with a “surface replacement.” The PCM uses the same well-established bearing and coating technologies as the CHARITÉ; the reader is referred to the earlier section for a more

detailed description of the bearing materials and coating methods. The cephalad and caudal endplates of the PCM are CoCr alloy. The UHMWPE core is fixed to the caudal endplate, so that motion occurs between the concave, cephalad endplate and the convex UHMWPE surface. The outer surface of the endplates have a serrated profile, for primary fixation, and have the same Ti/CaP coating as the CHARITÉ for long-term bone ongrowth [120]. The porosity of the Ti plasma spray with the PCM is smaller than the CHARITÉ, to account for the finer trabecular architecture in the cervical spine as opposed to the lumbar spine [121]. The anterior profile of the PCM varies with the low profile and fixed designs. The low profile design does not protrude beyond the anterior margins of the vertebral column. With the fixed design, on the other hand, the anterior face of the endplates is flanged for supplemental fixation using cancellous bone screws. The anterior flange protrudes over the surface of the vertebral body. The low profile is the standard implant that has been used in the majority of cases, whereas the Fixed design is now only used in “extreme” cases, such as in revisions and adjacent to multilevel fusions (Wefers, personal communication, 2005).

14:  The Clinical Performance of UHMWPE in the Spine

For the PCM from a prospective, 1-year follow-up data are available, pilot study conducted by P ­ imenta and associates in Brazil. The clinical study began in December 2002 and included 52 patients who were implanted with 81 cervical discs, between C3–C4 and C7–T1. The average patient age was 45 years (range: 28–68 years). Two complications were reported, including one case of anterior migration and one case of heterotopic bone formation. By 1 year, the clinical success was good or excellent in 97% of the cases. The patients’ NDI had improved, on average, to 15 points (range: 40–0) following 1 year; preoperatively, the average NDI for the patients was 45 (range: 98–18). Interestingly, 10 of the procedures were revisions of previous anterior surgeries [121]. The 1-year results of the randomized, controlled clinical trial as part of the US IDE study have been reported by Phillips et al. demonstrating that treatment with the PCM at a single level achieved clinical outcomes that were at least equivalent to ACDF, while maintaining motion [122]. The primary endpoint, termed “overall success,” was defined as having at least 20% improvement in the NDI from the preoperative score; absence of reoperation, revision, or removal; maintenance or improvement in neurological status; and absence of radiographical or major complications during this period. At the 24-month time period, 189 PCM patients were evaluated (out of the 218 originally treated), who were on average 45.3 years old (18–68). NDI success was achieved by 83.4% in the PCM patients, compared to 81.5% in the ACDF group, 94.7% neurological success; the reported ROM in flexion–extension was 5.7 degrees, with a reported radiographical success of 98.9%; and 5.2% of PCM patients required a subsequent secondary surgical intervention. In terms of other clinical outcomes that were monitored, superior and inferior segment degeneration were reported at 17.1 and 23.1%, respectively. HO was reported as 15.9% Grade I, 17.6% Grade II, 3.3 Grade III, and 1.1% Grade IV. A separate publication by McAfee et al. reported on the incidence of dysphagia in the PCM patient population in this clinical trial patient group, reporting a lower incidence of dysphagia at 3 and 12 months compared to the ACDF group, with a resolution noted in 74% of the PCM patients who had noted an increase in severity at either the 6 week or 3 month follow-up visit [123]. McAfee et al. has also included analysis of PCM IDE clinical data as part of his meta-analysis of comparative outcomes from IDEs of four cervical arthroplasty devices, demonstrating superiority to ACDF in

241

overall success, neurological success, and survivorship outcomes at 24 months postoperatively [108]. The ROM of the PCM and role of the posterior longitudinal ligament (PLL) have been evaluated by McAfee and coworkers [124], using the flexibility testing protocol developed by Cunningham [35]. Using this protocol, cervical spine sections between C3 and C7 were tested from seven donors under applied pure moments in flexion, extension, left and right lateral bending, and left and right axial torsion. As described previously in the CHARITÉ section of this chapter, Cunningham’s protocol does not include axial preload, and therefore assesses the maximum theoretical flexibility of the cervical spine column. At C5–C6, no significant difference was observed between the flexion–extension ROM for the PCM versus the intact spine. In contrast, resecting the PLL had a significant effect on ROM in flexion–extension, axial rotation, and lateral bending, as compared with the intact spine. McAfee et al. also implanted the PCM into the C3–C4 levels of 12 goats, which were followed for 6 months [124]. No significant postoperative complications occurred, and no evidence of wear debris, osteolysis, or particle-related biological reactions was observed. In a recent study, Dmitriev et al. tested the hypothesis that the cervical disc replacement preserves the segmental kinematics and disc pressures of adjacent levels [125]. Ten cervical spines were obtained and treated at the C5–C6 level using the PCM disc replacement, allograft dowel, and allograft dowel with anterior cervical plate. The researchers focused their attention on the motion of adjacent segments, C4–C5 and C6–C7, in addition to the treated level. Disc pressures were monitored using miniature pressure transducers (width = 1.5 mm, height = 0.3 mm, Precision Measurement Co., Ann Arbor, MI) inserted into the C4–C5 and C6–C7 discs. The intact spine specimens were first loaded with pure moments (±5 Nm in flexion, extension, left/right lateral bending, and left/right axial rotation) to record the maximum ROM in each degree of the six degrees of freedom. The apparatus for applying the pure moments was similar to that used by Cunningham [35]. After each of the three interventions, the tests were rerun under displacement control at a rate of 3° for all six degrees of freedom, using the maximum ROM in that particular degree of freedom as the limit for the test. The investigators observed no significant difference in adjacent level disc pressures or segmental motions between the intact and ­PCM-treated spines. In general, the greatest difference between treatments was observed in flexion and extension testing. Both fusion treatments resulted

242

in higher disc pressures than the intact or TDR-­treated spines during flexion–extension testing. Although limited by the lack of axial preload during the testing, the available data from this study supports the hypothesis that cervical disc replacement preserves adjacent level disc pressures and segmental kinematics. In a more recent report by Cunningham et al., a biomechanical comparison was presented of single and two-level arthroplasty with the PCM disc versus ACDF and its effect on adjacent level spinal kinematics using a four functional spinal unit model (C4–T1) [126]. Testing was performed using a custom-­designed six-degree of freedom spine simulator configured with a motion analysis system and capable of applying pure, unconstrained rotational moments about three axes (x, y, and z), and also capable of applying unconstrained translations. This study showed that the operative level ROM was preserved on single- and two-level implanted PCM samples under all loading modes, with the distal adjacent level (C7–T1) demonstrating the greatest increase among the four levels in ROM in comparison to the intact condition after two-level arthrodesis. The PCM cervical disc system was approved by FDA on October 2012, with Nuvasive as the manufacturer of record. The device is indicated (as outlined in the regulatory approval letter on the FDA website for P100012) for use in skeletally mature patients for reconstruction of a degenerated cervical disc at one level from C3–C4 to C6–C7 resulting from a herniated nucleus pulposus, spondylosis, and/or visible loss of disc height.

14.4.3 Mobi-C The Mobi-C is manufactured by LDR spine (Troyes, France) (Figure 14.12). Like its lumbar

Figure 14.12  The Mobi-C cervical disc replacement. Reprinted with permission from LDR spine.

UHMWPE Biomaterials Handbook

analogue, the Mobi-C is a convex sliding mobilebearing design, and achieves short-term fixation via lateral teeth and a textured ongrowth surface. The Mobi-C was first implanted in Europe in N ­ ovember 2004, and since that time, over 4300 artificial discs have been implanted, according to the manufacturer. In October 2007, LDR spine announced that it had completed enrollment for its single level IDE study. The Mobi-C received FDA approval on ­August 2013. Early clinical reports on the use of the Mobi-C include various international retrospective studies. The earliest clinical report on the Mobi-C by Kim et al. in 2007 includes a group of 23 patients with a mean age of 43 years and a follow-up period of up to 6 months [127]. The favorable and ­radiological clinical outcomes reported included significant changes (p < 0.001) in VAS and preservation of the upper adjacent ROM. Park et al. reported in 2008 on a group of 21 Mobi-C patients who were compared to an ACDF group [128]. An improvement in mean NDI and extremity VAS was reported at the 12 month follow-up in both groups. The arthroplasty group had shorter postoperative recovery and also preserved motion in the operated levels. Guerin et al. reported on 2012 on a group of 40 patients who were evaluated in a prospective study in which SF-36, VAS, and NDI scores showed a significant improvement in clinical outcomes, as well as maintenance of the ROM and overall segmental cervical alignment [129]. A separate retrospective study by Park et al. reported in 2013 on a comparative assessment that included assessment of adjacent segment degeneration and ROM with four different cervical disc replacement, inclusive of the Mobi-C [130]. The incidence of adjacent segment degeneration with the Mobi-C was 14.2%, while all cervical discs reported preservation of sagittal ROM and increased superior adjacent segment kinematics. Zhang et al. reported in 2014 on a retrospective study in China including 65 patients implanted with Mobi-C and followed up to 36 months postoperatively [131]. This cohort showed that the Mobi-C was reliable by maintaining intervertebral height and ROM in the cervical spine and adjacent segments. These studies have generally commented that long-term follow-up will be needed to continue to assess the effectiveness and advantages of the procedure. The potential for HO has been a commonly reported occurrence among patients implanted with cervical arthroplasty and one that has been also reported for the Mobi-C. Guerin et al. followed 71 patients out to 21 months in a noncomparative prospective study to

14:  The Clinical Performance of UHMWPE in the Spine

determine the incidence of HO and investigate whether HO affects clinical outcomes and ROM [132]. In this cohort, HO was detectable in 27.7% of the segments, but HO did not appear to affect clinical outcomes. Lee et al. reported in 2012 on a retrospective clinical and radiologic study of 28 consecutive patients operated with Mobi-C followed up to 24 months [133]. At the last follow-up, 64.3% had HO (Grade 1: 6; Grade II: 8; Grade III: 3, and Grade IV: 1), concluding that there was a high incidence of HO with the possibility of spontaneous fusion. More recently, Park et al. reported in 2013 on a retrospective study including a group of 75 patients (85 implanted levels) with a minimum follow-up of 24 months [130]. Although clinical rating scores (NDI and VAS) decreased significantly over 24 months, and the procedure had a reported rate of 86.7% for overall success, HO was reported in 67 levels at 12 months and increasing to 80 levels at 24 months, with surgical technique noted as the most important factor affecting HO. Yi et al. also reported on the predisposing factors for HO after cervical disc replacement including three different types of devices (Mobi-C, Bryan, and ProDisc-C) in a group of 170 patients with a follow-up duration longer than 12 months [105]. The study found that HO occurred in 40% of the patients and in this group males, and patients with Mobi-C and ProDisc-C, were at greater risk of HO. More recently, various prospective studies have been published documenting both international cohorts and/or patients enrolled in the FDA IDE study comparing Mobi-C to ACDF, with follow-up periods ranging from 2 to 4 years (see Table 14.7). Beaurain et al. reported in 2009 on a group of 76 arthroplasty patients, as part of a multicenter prospective study with a follow-up period of 2 years [134]. Their study demonstrated at the efficacy and safety of the MobiC, with 85.5% of the segments being mobile at the 2-year follow-up time point. Davis et al. published two reports on the follow-up of the patients in the FDA IDE trial, one at 2 years [135], and another report at the 4-year period [136]. Data from the IDE study at the 4-year time point showed that patients experienced improved clinical outcomes with 2-level implantation of Mobi-C, including improvement in pain and function outcomes, as well as superiority in overall primary endpoint success, as well as reduced incidences of adjacent segment degeneration and subsequent surgeries. Hisey et al. reported on the single level FDA IDE study of the Mobi-C and also provided clinical outcome data at the 2-year and 4-year time periods [137,138]. At the 4-year time period, MobiC patients maintained motion and had significantly

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lower rates in reoperation and adjacent segment degeneration compared to ACDF, thus supporting the device’s safety and efficacy of TDR in appropriately selected patients. Zhang et al. reported in 2014 on an international randomized controlled trial comparing Mobi-C to ACDF with a follow-up out to 48 months [139]. Clinical outcomes in this study indicated that treatment with Mobi-C was reliable and radiographic data showed that ROM of the spine, FSU angle and treated and adjacent segments were better reconstructed and maintained in comparison to the ACDF group. These studies have generally commented that long-term studies will assist in monitoring the effectiveness and advantages of the procedure. A recent review by Alvin and Mroz provides an overview of 15 studies that investigated the use of Mobi-C published between 2007 and 2014. Their review concluded that one-level Mobi-C is noninferior, but not superior, to one-level ACDF for patients with cervical degenerative disc disease, while being associated with high rates of HO [140]. On the other hand, their review suggests that two-level Mobi-C may be superior to two-level ACDF, but more prospective studies are needed in the future. Many of the articles reviewed by Alvin and Mroz are also reviewed previously. Finite element models have also been used to examine the biomechanics of cervical segments implanted with Mobi-C. Lee et al. reported on the use of a validated C2–C7 model to compare the biomechanical changes in the cervical spine implanted with a fixed (ProDisc-C) core and a mobile core (Mobi-C) cervical disc replacement [117]. Their study showed an increase in ROM, facet joint contact force, and ligament tension at the implanted segments. Additionally the Mobi-C model showed higher increase in segmental motion, facet force, and ligament tension, but lower stress on the PE core than the fixed core model.

14.4.4 Activ®-C The Activ-C is produced by Aesculap AG (­Tuttlingen, Germany). The Activ-C is a convex sliding mobile-bearing design, similar to its lumbar analog. Fixation of the CoCr endplates is accomplished by three teeth, along with a Ti spray bone ongrowth surface. Few reports exist on the clinical use of the ­Activ-C. In 2011, McGonagle et al. reported on a prospective trial including 10 patients implanted with one, two, or three Activ-C who were followed up to 24 months

Table 14.7 Clinical Details of Primary, Peer-Reviewed Studies Involving the Mobi-C Beaurain et al. (Intl) [134]

Davis et al. (FDA)a[136]

Hisey et al. [137]

Hisey et al. [138]

Zhang et al. (Intl) [139]

Pcoh

RCT – 2 level

RCT – single level

RCT

RCT

Average age

43.9

45.3

43.3

–

44.8

Comparison group

None

ACDF

ACDF

ACDF

ACDF

76 (nine 2-level patients)

225

164

164

55

No. of arthroplasties

85

450 (2 level)

Follow-up (months)

24

48

24

48

48

Outcomes tracked

NDI VAS SF-36

NDI VAS SF-12 and PCS

NDI VAS SF-12

NDI VAS

NDI VAS

Initial VAS score, average

Neck: 45.3 Arm: 64.2

Neck: 70 Arm: 68

Neck: 70 Arm: 40

Neck: 70 Arm: 70

6.72

VAS at Longest Follow-up

Neck: 20.3 Arm: 22.6

Neck: improvement of 53 Arm: improvement of 56

Neck: improvement of 17.34 Arm: improvement of 13.6

Neck: 20 Arm: 18

1.5

Conclusion

72% Success rated 9.1% adjacent level deg changes

96.4% Satisfied patients 66% overall successe

95% Satisfied patients 73.7% overall successe

69.5% Overall clinical success

Secondary surgery

1.3% (arthrodesis below)

4% (stenosis, migration, poor endplate fixation, pain)

1.2%

3%

None

HO – Grade 0–IV

Grade 0: 32.9% Grade I: 11.8% Grade II: 43.3% Grade III: 3.9% Grade IV: 7.9%

Grade III: 10.7% Grade IV: 5.9% (segments)

5%

1/164

Total: 18/55 (33%) Grade I: 11/55 Grade II: 5/55 Grade III: 2/55

Study type b

No. of patientsc

Arthrodesis or spontaneous fusion

None

Complications

13.2%: dysphagia, upper limb sensitivity, local hematoma

4%

Radiculopathy

ROM

Preserved: 9° 85.5% segments mobile at 2 year

Maintained superior F/E and LB both inf/ superior

Higher than at baseline: 10.8° (F/E) and 5.4° (LB)

Pharyngeal discomfort

10°

Restored to pre-op level by 3 months: ∼12°

Hcoh – historical cohort study; Pcoh – prospective cohort study; RCT – randomized controlled trial. a Prior study at the 2-year follow-up was also published by Davis et al. [135]. b Average age is for all patients unless stated otherwise. c Number of patients are those implanted with Mobi-C. d Overall clinical success was defined by FDA as patients having achieved >15% improvement in NDI. e Overall success rate was determined from a composite endpoint of multiple conditions including, NDI, subsequent surgery, adverse events, neurologic function, and radiographic success.

14:  The Clinical Performance of UHMWPE in the Spine

[141]. The average age of the patients was 54 years and there were no reported complications from the procedure. At the follow-up period of 24 months, the VAS scores improved in comparison to baseline (VAS neck 1.4 vs. 5.9 baseline; VAS arm 2.6 vs. 5.4 baseline) and an improvement in function was also reported. Suchomel et al. reported in 2014 on a prospective, multicenter nonrandomized study with data reported on 175 patients followed up to 2 years, in which 103 patients received the standard Activ-C design, and 72 patients received the flat Activ-C design [142]. The results showed significant improvements in all three outcome scales used for clinical evaluation (NDI, VAS neck, VAS arm). The location of the COR differs slightly between the flat and standard designs, with the COR for the flat implant being ­located close to the midpoint of the upper endplate of the caudal vertebral body and more anterior compared to the standard implant. Comparison between the groups showed statistically significant differences in HO grading, with less severe HO in flat implants. Additionally, segmental angle measures showed a significant better lordotic alignment for both groups after surgery, with the degree of correction higher for the flat group, and a segmental ROM decrease in the standard group but maintained in the flat implant group.

14.4.5 Discover™ Discover cervical disc was developed by DePuy Synthes Spine (Rayham, MA) (Figure 14.13). This artificial disc is a fixed bearing, with a convex Ti alloy endplate articulating on a UHMWPE convex inferior core. The core is affixed to the inferior endplates. Bone fixation is achieved by a combination

Figure 14.13 Discover cervical disc replacement. Used with permission from DePuy Spine, Inc.

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of a textured Ti surface and teeth. Discover has been clinically available in Europe since 2007 [143]. The US IDE clinical trial comparing to ACDF, which was started on July 2006, is still listed as active, but is not recruiting as of early 2011 [144]. The earliest clinical report on Discover cervical disc is a prospective study by Greiner-Perth et al. including a cohort of 70 patients (101 devices) with a mean age of 44.3 years and a maximum follow-up of 12 months (clinical and radiological) [143]. Although study parameters such as NDI and VAS (arm/ neck) showed significant improvement at follow-up, the VAS arm score rose up again significantly. The segmental angle improved from 3.1° to 5.3° after 12 months. Three surgical revisions took place due to device translations and HO. The need for a longer follow-up period was recognized as a limitation to the study. Additional clinical reports on the clinical use of Discover cervical disc have been published, including prospective and retrospective studies evaluating its performance individually, or comparing its performance to that of ACDF, and other studies comparing its performance to other cervical disc ­replacements. These studies include mainly international (outside the United States) reports and followup periods usually in the 1–2-year range, and are reviewed further. Various authors have reported the clinical performance of studies including only the performance of Discover, with the majority being studies conducted outside the United States (OUS) [145–151]. The largest retrospective study, by Qi et al. reported on 125 patients implanted with Discover cervical disc and followed up to an average of 26.4 months. They identified significant risk factors for postoperative HO, which in this cohort was 27.92% at the surgical level [150]. Rihn et al. reported on a cohort of the United States. IDE trial (n = 243 patients, 304 levels) of Discover cervical disc with a follow-up out to 1 year to determine which radiographic parameters, if any, were predictive of clinical outcome [151]. They identified decreased preoperative disc height and excessive postoperative segmental lordosis as predictive of clinical outcomes. Comparisons of the clinical performance of Discover cervical disc to that of ACDF include various studies (prospective and retrospective) with the longest follow-up being 2 years [152–156]. The two studies with the largest cohorts and longest followup periods are those of Hou et al. and Skeppholm et al. [153,155]. Hou et al. included 149 patients implanted with Discover cervical disc for single or

246

two-level degenerative disease [153]. At a followup period of 22.1 months, Discover cervical disc group showed relatively well-maintained mobility and lower incidence of radiographic adjacent-level changes than the ACDF group. Skeppholm et al. reported in 2015 outcomes of Discover cervical disc from a randomized controlled multicenter trial, including three spine centers in Sweden with a followup of 2 years [155]. A total of 153 patients received Discover cervical disc and in comparison to ACDF, it did not result in better outcomes as measured with the NDI. Coric et al. reported on the comparison of three arthroplasty systems, inclusive of Discover cervical disc, against an ACDF group, and a planned followup out to 60 months [157]. At the 2-year follow-up period, all groups showed improvement from preoperative state. The combined arthroplasty group showed an improvement of angular ROM by 0.91°, as well as a 5.6% incidence of HO. The authors report that at this intermediate follow-up period the patients with cervical TDR are showing significantly better results, maintained motion at the treated level, and trended toward less adjacent segment disease. A biomechanical study by Phillips et al. characterized the kinematics of human cadaveric spines implanted with Discover cervical disc at one and two levels [158]. Their study showed that cervical TDR at two levels can provide near-normal mobility at both levels without destabilizing the implanted segments or impacting adjacent segment mobility, thus supporting the concept that single or multilevel cervical TDR may be advantageous as compared to fusion.

14.4.6 SECURE®-C The SECURE-C cervical artificial disc was developed by Globus Medical (Audobon, PA) and has been commercially available outside the United States since 2006. This artificial disc is described as having a selectively constrained design, with a spherical interface between the UHMWPE core and the superior endplate and a cylindrical interface between the UHMWPE core and the inferior endplate. The design is intended to allow ±15° flexion–extension, ±10° lateral bending, unconstrained axial rotation, and ±1.25 mm translation. Fixation of the CoCrMo endplates is accomplished via multiple serrated keels and a titanium plasma spray coating designed to allow bony ongrowth. Durability and wear testing data submitted with the PMA for FDA approval indicated that the average weight loss over 10 MC was

UHMWPE Biomaterials Handbook

2.57 ±1.21 mg/MC for the first round of testing, and 0.89 ± 0.3 mg/MC for the second round of testing. During the course of the clinical trial, the endplate keels were slightly modified for ease of manufacture and for clearer demarcation of the keel boundaries during radiographic imaging. The SECURE-C cervical artificial disc received FDA approval to market in the United States on September 2012. Clinical outcomes of the IDE study conducted for the SECURE-C are reported by Vaccaro et al. in 2013 [159]. The IDE study design included a comparison to ACDF, with a total of 240 patients receiving the SECURE-C at a single level, in both randomized (n = 151) and nonrandomized (n = 89) groups, with mean ages of 43.4 and 41.6 years, respectively. The follow-up reported at 24 months demonstrated overall success rate (superiority) of the randomized SECURE-C group over the randomized ACDF, and specifically clinically significant improvements in NDI, VAS, and SF-36. Additionally, 85.5% of the SECURE-C patients were considered a success in ROM. At 24 months, the percentage of patients having secondary surgery at the index level was lower for the combined SECURE-C group (2.5%) as compared to the ACDF group (9.7%), and the total number of patients having adverse events was also lower for the SECURE-C (5.5%) than for the ACDF (12.5%) group.

14.4.7 Baguera®C The Baguera C was designed by Spineart (Geneva, Switzerland). The Baguera C includes two titanium plates coated with DIAMOLITH (each plate includes fins for stability) and an UHMWPE nucleus. The core is described as a “guided mobile PE nucleus” that is designed to prevent excessive constraints on the facet joints and allows six degrees of freedom. The plates have a “sloping anatomical design” to optimize fit between the device and the disc space, thus maximizing endplate coverage. Spineart reports that the first surgery with a Baguera C took place in January 2007 in France and, as of January 15, 2015, 12,500 Baguera C cervical discs have been implanted worldwide [160,161]. In terms of clinical studies, a 2-year, prospective, clinical follow-up data registry and a retrospective clinical analysis has been sponsored by Spineart (study #16001) in Europe and is currently complete with 118 patients enrolled [162]. The primary objectives identified by Spineart in this study (#16001) were safety and effectiveness of the Baguera C.

14:  The Clinical Performance of UHMWPE in the Spine

The safety was monitored by analyzing all adverse events reported during the 2-year period, whereas the effectiveness was monitored on the basis of five parameters that included: functional improvement, neurological improvement, neck and arm pain, improvement in health-related quality of life (SF-36), and no subsequent surgery. The study included a total of 149 Baguera C cervical discs implanted in 118 patients at the C3–C4, C4–C5, C5–C6, and C6–C7 levels, with some patients receiving hybrid surgeries. The study concluded that the Baguera C cervical disc procedure is safe with a low complication rate and that it was an effective treatment at one or two levels, with less improvement when hybrid surgery was performed [163]. Spineart also identifies a subsequent study, designated as #16002, as including retrospective radiographic evaluation on the 2-year prospective registry data, with 96 patients included. The objectives include evaluating motion at the treated levels after 2-years following Baguera C cervical disc implantation as well as disc height restoration [164]. A case series including 49 patients implanted with 53 Baguera C cervical disc replacements between February and December 2007 was reported by Benmekbhi et al. in 2008 [165]. The average age of the patients was 38 years and the clinical assessment included VAS (neck and arm), NDI, and SF-36. There was a reduction in VAS arm (7–1.6), VAS neck (6–1.7), NDI (45–14.8) when comparing postoperative to preoperative levels, with the results being comparable to that of other prostheses previously used by the authors.

14.5  Wear and In Vivo Degradation of UHMWPE in the Spine Wear and in vivo degradation are key functional aspects of TDRs. The majority of in vivo clinical data for UHMWPE wear behavior and oxidation in TDRs has been obtained from retrieval studies of the historical CHARITÉ artificial disc, manufactured by Link between 1989 and 2004. Table 14.8 provides a summary of findings from retrieval studies of lumbar and cervical TDRs, including CHARITÉ, ProDisc-L, activL®, Mobidisc and ProDisc-C. In vivo wear data for second generation lumbar artificial discs, as well as for UHMWPE in most cervical disc replacement designs, is still somewhat limited in the peer reviewed literature. Based on the clinical experience of UHMWPE in total hip and knee replacements, the production of wear debris from artificial discs, as well as from

247

other motion preserving spine implants, is a clinical concern. Wear debris induced osteolysis has been implicated as a potential mechanism for late onset pain following the failure of stainless steel and titanium instrumented fusions [178]. Osteolysis has also been observed around certain TDR designs, such as the Acroflex artificial disc [179], and case studies of osteolysis around CHARITÉ disc replacements have also been reported [168,180,181]. According to recent conference presentations, the UHMWPE particle load around long-term implanted artificial discs may be comparable to total hip arthroplasty [182], and the periprosthetic particle concentration appears to be correlated with a local inflammatory response [183]. Although the occurrence of osteolysis with metal-on-polyethylene TDRs has thus far been relatively rare, the long-term wear behavior of artificial discs remains of clinical importance [168]. Analysis of wear and surface damage in long-term implanted CHARITÉ TDRs has been reported in a series of journal publications [184–186]. In the latest update of this multi-institutional series, 38 CHARITÉ components were retrieved with up to 16 years of implantation [185]. The components were revised for intractable pain and/or facet degeneration. Components were analyzed using optical microscopy, scanning electron microscopy, white light interferometry, and MicroCT. 43% (15/35) of components analyzed using MicroCT displayed one-sided wear patterns [185]. Significant correlations were observed between implantation time and penetration and penetration rate (Figure 14.14). The dome of the components typically exhibited burnishing that is consistent with the multidirectional wear observed in hip replacements, whereas the rim frequently showed evidence of radial and transverse cracking, often produced by impingement (Figure 14.15). The rim-­ damage modes of plastic deformation, delamination, and cracking were similar to those associated with knee components. Evidence of dome burnishing, as well as rim impingement, has also been noted in a conference poster summarizing a collection of five short-term ProDiscL prostheses, implanted up to 2.2 years [187]. The bearing surface of the ProDisc-L showed burnishing (3/5 implants), mild scratching, and pitting (3/5 implants). Impingement was noted in 3/5 components and associated with burnishing and plastic deformation. Similar observations of wear, surface damage, and impingement have been noted for the ProDisc-C [187]. The authors of the ProDisc-L retrieval study remarked that, “a potentially worrisome finding is the evidence of impingement. Whether caused by

248

Table 14.8  Summary of Findings from Published Studies of Retrieved Total Disc Replacements: Lumbar and Cervicala Type: Lumbar or Cervical

Mean Implantation Time (years)

Impingement

Periprosthetic Debris Polymeric Metallic

Device

Study

Lumbar

CHARITÉ

David et al. [166]

9.5

0/1

NR

NR

NR

NR

0/1

Lumbar

ProDisc-L

Steiber and Donald [167]

0.1

NR

NR

1/1

NR

NR

NR

Lumbar

CHARITÉ

Van Ooij et al. [168]

9.4

5/5

5/5

0/5

Y

N

1/5

Lumbar

CHARITÉ

Kurtz et al. [169]

8.5

34/48

NR

NR

NR

NR

1/48b

Lumbar

ProDisc-L

Choma et al. [170]

1.2

1/1

1/1

0/1

N

N

NR

Lumbar

activL Mobidisc

Austen et al. [171]

1.9

3/3

3/3

0/3

Y

N

NR

Lumbar

CHARITÉ

Punt et al. [172]

10.0

NR

21/22

0/22

Y

N

NR

Lumbar

ProDisc-L

Lebl et al. [173]

1.1

11/19

NR

NR

NR

NR

NR

Lumbar

CHARITÉ

Baxter et al. [174]

9.7

NR

11/11

0/11

Y

N

NR

Cervical

ProDisc-C

Pitzen et al. [175]

0.3

NR

1/1

1/1

N

N

NR

Cervical

ProDisc-C

Tumalian and Gluf [176]

1.3

NR

NR

NR

NR

NR

1/1

Cervical

ProDisc-C

Lebl et al. [177]

1.0

24/30

NR

NR

NR

NR

NR

®

Osteolysis

UHMWPE Biomaterials Handbook

NR, not reported; Y, yes; N, no. a Information partially extracted from Veruva et al. 2014 [7]. b This cohort includes retrievals from study performed by van Ooij et al. [168].

b

Inflammation Innate Adaptive

14:  The Clinical Performance of UHMWPE in the Spine

249

Figure 14.14  UHMWPE dome penetration and penetration rate versus implantation time for retrieved CHARITÉ artificial discs. Adapted from Ref. [185].

Figure 14.15 Impingement damage to the rims of TDRs. (A) Burnishing, plastic deformation, and transverse crack (white arrow) – implanted in 2000 for 4.9 years (Maa008); (B) Transverse crack, rim fracture (white arrow) – implanted in 1998 for 6.5 years (Maa006); (C) Burnishing, plastic deformation (white arrow) – implanted in 1995 for 10.6  years (Maa018); (D) Rim fracture, transverse cracks (white arrow) – implanted in 1992 for 13.5  years (Maa014); (E) Burnishing, plastic deformation, and transverse crack (white arrow) – implanted in 1989 for 16 years (Maa015). From Ref. [77] with permission of Elsevier Science.

patients achieving a larger ROM that the implant is designed to accommodate or by component positioning that allows impingement at even smaller ROM, impingement can be problematic” [187]. Other clinical reports of analyses of retrieved ProDisc-L implants are also available (Table 14.8)

[170,173]. Choma et al. reported on the analysis of a single ProDisc-L retrieved 14 months postimplantation [170]. Upon examination of the implant, minimal wear was observed, along with oxidation, and periprosthetic tissue reaction. There was evidence of burnishing as a result of short-term impingement.

250

The tissue analysis revealed some early cell degeneration as well as evidence of polyethylene particles as identified by polarized light microscopy. In a prospective collection of explanted ProDisc-L implants collected over a 7-year period, Lebl et al. reported on 19 ProDisc-L devices from 18 patients (mean age 44.7 ± 2.9 years) that were evaluated after a mean length of implantation of 13.0 ± 3.9 months [173]. The explanted devices were found to have areas of bony ongrowth (9.6 ± 2.9%), burnishing on the metallic endplates (posteriorly), consistent with component impingement in extension, evidence of polyethylene deformation (mainly in AP plane), backside wear in 75%, and third-body wear in 33%. Although rim impingement has been observed in retrieved TDRs of different designs, the clinical consequences of chronic rim impingement remain poorly understood. In a study presented at the 2008 Spine Arthroplasty Society meeting [188], a retrieval collection of polyethylene mobile-bearing TDRs was analyzed to determine whether rim impingement adversely affected dome penetration. Implanted for 2–16 years (7.9 year ave), 28/40 (70%) of retrieved cores were classified as exhibiting chronic rim impingement based on observations of plastic deformation, burnishing, and/or fracture of the rim. Dome penetration was comparable in chronically impinged cores (ave: 0.3, range: 0.1–0.9 mm) as compared with nonimpinging cores (ave: 0.3, range: 0.1–0.5 mm). Rim penetration was significantly greater in chronically impinged cores (p < 0.05). Using linear regression, the dome penetration rate for cores with negligible impingement (0.036 mm/year, 95% CI: 0.012–0.061 mm/year) appeared slightly higher than in cores with chronic impingement (0.021 mm/year, 95% CI: 0.005–0.038 mm/year),

UHMWPE Biomaterials Handbook

however the difference was not significant. Thus, the results of this study did not support the hypothesis, that chronic rim impingement would be associated with greater dome penetration. However, the findings would suggest that dome wear and impingement are effectively decoupled phenomena, and may be studied independently of each other. In addition to impingement, rim damage observed in polyethylene TDR retrievals has also been associated with postirradiation oxidation [31,189]. Analysis of explanted CHARITÉ cores using F ­ ourier transform infrared spectroscopy has shown that the exposed rim experiences severe oxidation after ten or more years [31,189]. These findings appear consistent for TDRs that were gamma irradiated in air, as well as in first-generation polymeric barrier packaging [31,189]. However, the central dome appears to somewhat protected from in vivo oxidation due to contact with the metallic endplates (Figure 14.16). No correlation was observed between wear of the central dome and oxidation. These observations are similar to the in vivo oxidation patterns noted in artificial hips, which exhibit rim embrittlement after 10 years in vivo, but show reduced oxidation at the bearing surface where the femoral head contacts the polyethylene [190]. Unlike in hip replacement, the rim of a TDR core may be intended to support chronic loading for the lifetime of the patient. The findings of in vivo oxidation in gamma-sterilized polyethylene TDR components provide additional motivation for developing in vitro mechanical tests that incorporate accelerated aging, or some other oxidative challenge, to simulate changes in the bearing materials that may occur with long-term in vivo exposure. The published CHARITÉ retrieval literature provides crucial long-term in vivo wear data for validation

Figure 14.16  Rim versus dome oxidation (A) and oxidation potential (B) in polyethylene TDRs. Adapted from Ref. [188].

14:  The Clinical Performance of UHMWPE in the Spine

251

Figure 14.18  Comparison of wear tested surfaces of the CHARITÉ artificial disc. Figure 14.17  Commercially available spine wear simulator for cervical and lumbar artificial discs. Courtesy of Exponent.

of spine wear simulators as well as for in vitro biomechanical testing. Multistation, six-degree of freedom spine wear simulators are now commercially available (Figure 14.17). However, two independent sets of protocols for wear simulation of UHMWPE in the spine, fostered by ASTM International and ISO, have not yet been validated. A study presented at the 2008 Spineweek meeting was conducted to better correlate long-term clinical wear rates of the CHARITÉ with simulator wear rates [191]. It was hypothesized that the wear mechanisms of the retrievals would be more accurately simulated by ISO protocols with coupled motion as compared with ASTM-type protocols that resulted in linear motion. Researchers analyzed dome wear rate and surface morphology of 41 CHARITÉ (SBIII) explants from 35 patients (71% female). The cores were implanted for 7.5 year (range: 1.8–16.3 year). Twelve CHARITÉ wear-tested cores and six controls were also examined. Six cores were tested according to an ASTM-type protocol for 10 million cycles [192]; three additional cores were unloaded and soaked. Six cores were tested according to the ISO protocol for 2 million cycles with three loaded and soaked controls. All the cores in this study were produced by the same manufacturer (Link, ­Germany). The explanted cores typically exhibited burnishing or evidence of adhesive/abrasive dome wear, consistent with multidirectional motion. The wear rate of the explants, obtained by correlation of dome height with implantation time, was 0.023 mm/year. The ASTM-tested cores exhibited unidirectional abrasive wear at a rate of 0.007 mm/Mcycles. The ISOtested cores exhibited regional burnishing and wear at a rate of 0.124 mm/Mcycles (Figure 14.18). Thus, the ISO protocol generated wear-surface morphology that was closer to the retrievals than the ASTM-type protocol, and one million cycles of the ISO protocol

corresponded, on average, to about 5.6 years of clinical wear. The findings from this study further suggest that the ISO protocol provides a useful starting point for clinical validation of spine wear simulations incorporating lumbar polyethylene TDRs. Additional research is still needed so that wear simulations of CoCr alloy-on-UHMWPE, let alone other biomaterial combinations, can be considered validated for cervical or lumbar TDRs. As identified in Table 14.8, Austen et al. reports the evaluation of retrieved activL® and Mobidisc arthroplasties [193]. One patient had an activL® retrieved after 1.3 years, whereas the second patient had both an activL® and Mobidisc retrieved after 2.8 years. Both patients were revised for recurrent back and leg pain. Histological examination of the tissue near the implants revealed UHMWPE particles, with giant cells near the tissue surrounding the Mobidisc, and macrophages near the tissue sample of the activL®. Examination of the UHMWPE cores showed evidence of minor adhesive and abrasive wear with signs of impingement in both TDR designs. Longer-term followup is warranted to monitor wear of these TDRs. Although most of the literature on TDR retrieval analysis includes lumbar prostheses, few reports are now available evaluating cervical TDRs, with a focus on ProDisc-C samples, including a postmortem sample [175–177]. Lebl et al. report on the largest cohort of ProDisc-C retrievals, with a total of 30 ­ProDisc-C samples from 29 patients and a mean length of implantation of 1 year [177]. Burnishing, consistent with endplate impingement, was present in 80% of the samples, and most commonly present in the posterior quadrant. Backside wear was not observed on any of the disassembled implants and third body wear occurred in 12% (confirmed as the porous coated surface). Future retrieval analyses will provide additional insight on the predominant modes of wear on long-term metal on UHMWPE semiconstrained prostheses.

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14.6  Alternatives to UHMWPE in Disc Replacement The current state-of-the-art for UHMWPE in disc replacement is conventional, gamma-sterilized material in barrier packaging. None of the radiation cross-linking and stabilization technologies developed for UHMWPE in hip and knee replacement have yet emerged as clinical candidates for artificial discs. Due in part to controversies we discussed surrounding gamma-air-sterilized UHMWPE with the CHARITÉ, the commercial developers of new artificial disc designs in recent years have favored alternative bearing solutions, rather than developing novel UHMWPE formulations specifically for disc replacement. Commercial interest in product differentiation has also fostered a greater willingness to explore alternative bearings in disc replacement than has yet been witnessed in hip and knee arthroplasty. One technical justification for alternative bearings is to reduce wear and the biological response associated with conventional UHMWPE. Additional technical motivations for alternative bearings are to reduce the thickness of the components, as well as to improve the MRI compatibility of the implants. For all of these reasons, the field of motion preservation spine implants has witnessed an explosion of different designs with alternative bearing couples. Until now we have focused on designs incorporating UHMWPE that have reached clinical use. However, the reader should be aware that many more disc-replacement designs, as yet unproven and unvalidated, are currently under development or in various stages of animal or human clinical experimentation [11]. All of these disc replacements have the potential to exhibit wear and particulate debris release, at some length scale, however microscopic. For materials with long-standing use in total joint replacements,

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such as UHMWPE, there may be a risk of osteolysis, albeit in a small patient population. With metal-onmetal disc replacements, the long-term biological consequences of the metallic wear debris and their soluble corrosion products in the spine are also unknown at the present time. With other metallic and polymeric bearing materials, the risks associated with their long-term in vivo use remain unknown. In the lumbar spine, metal-on-metal (CoCr/CoCr) is the main alternative to UHMWPE as a bearing surface for TDRs [11]. The Maverick lumbar artificial disc (Medtronic Spinal and Biologics: Memphis, TN) is a semiconstrained, ball-in-socket articulation (Figure 14.19) [194]. The FlexiCore design (Stryker Spine: Allendale, NJ) is a constrained ball-in socket bearing [195]. The Kineflex design (Spinal Motion: Mountain View, CA) is mobile-bearing metal-onmetal lumbar artificial disc. All three metal-on-metal lumbar disc designs are used in Europe and Asia, with limited investigational use in the United States. The IDE for the Maverick lumbar artificial disc was completed, whereas the Kineflex and Flexicore TDRs, are not registered or have terminated their IDE, respectively [7]. In the cervical design, because of the relatively low service loads, a plethora of alternative bearings are currently being explored [11]. The Prestige artificial disc is a stainless steel ball-in-trough design, with a design history spanning over a decade (Figure 14.20) [196]. The Prestige was approved by the FDA in 2007. The Bryan artificial disc, consisting of polyurethane components articulating with titanium endplates, has been used in Europe since the late 1990s (Figure 14.21) [197], and was approved by the FDA in 2009. Other artificial disc designs employ polymeric and metalloceramic biomaterials without any clinical precedence as long-term orthopedic bearings. Although these novel biomaterials appear

Figure 14.19  The Maverick cervical disc replacement. Courtesy of Medtronic Spinal and Biologics. Reproduced with permission from Ref. [11].

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materials that have a clinical history spanning five decades of continuous use.

14.7  Many Unanswered Questions Remain

Figure 14.20  The Prestige cervical disc replacement. Courtesy of Medtronic Spinal and Biologics. Reproduced with permission from Ref. [11].

TDR is a new, promising field of spine implant technology that added treatment options for degenerative disc disease. Today, conventional UHMWPE is available in both cervical and lumbar disc arthroplasty, with more clinical interest in the cervical replacements. It is clear from both in vitro and clinical data that disc replacements can successfully preserve the motion of treated spinal level. Aside from patient satisfaction and the speed of recovery, there are modest clinical benefits with disc replacement that manifest in the short-term as compared with fusion. Furthermore, unlike fusion procedures, disc replacements may also need to be revised due to poor implantation technique or failure of the device. On the other hand, over the long-term, the primary benefit of disc replacement is expected to be the reduced incidence of adjacent segment degeneration, which will hopefully offset the new, and as yet poorly quantified risks associated with the technology. It will be many years, probably over a decade, before sufficient data has been generated to evaluate whether the promises inherent in TDR have been fulfilled, and in turn, whether the long-term benefits of the technology justify the additional risks inherent with their implantation and potential revision. The current technologies for disc replacement should be viewed, therefore, as still being in their infancy.

Acknowledgments Supported in part by NIH R01 AR47192. This chapter was not written with the financial support of any manufacturer of TDRs. The authors would like to thank Allyson Morris from DePuy Synthes for her assistance with the second edition version of this chapter.

Figure 14.21 The Bryan cervical disc replacement. Courtesy of Medtronic Spinal and Biologics. Reproduced with permission from Ref. [11].

to exhibit satisfactory short-term biocompatibility, their biological response over the long-term is also simply unknown. At least 10 years of clinical followup will be needed to confirm that these novel bearing combinations are comparable to historical bearing

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