The current status of lumbar total disc replacement

Orthop Clin N Am 35 (2004) 33 – 42

The current status of lumbar total disc replacement Russel C. Huang, MDa,*, Harvinder S. Sandhu, MDb a

Orthopaedic Spine Surgery, University Hospitals of Cleveland – Case Western Reserve University, 11100 Euclid Avenue, BHC 5128, Cleveland, OH 44106, USA b Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021, USA

Arthrodesis remains the current gold standard treatment for degenerative disc disease that is unresponsive to prolonged nonsurgical modalities. Total disc replacement (TDR) is a rapidly developing technology that may one day offer relief to patients with degenerative disc disease (DDD) while avoiding the arthrodesis-related complications of pseudarthrosis, iliac crest donor site pain, and adjacent level degeneration. Furthermore, TDR may permit patients to return to normal daily activities more quickly. The purpose of this article is to review the anatomy, physiology, and biomechanics of the intervertebral disc (IVD) in health and disease and to review the preliminary clinical and radiographic results of TDR. Short- and long-term complications of TDR are also reviewed.

Anatomy and physiology of the intervertebral disc The IVD develops from notochordal remnants (nucleus pulposus) and perichordal mesenchyme (annulus fibrosus) during the first trimester. At birth, the intervertebral disc is a vascular structure, receiving vessels that penetrate the annulus and the cartilaginous vertebral endplates to supply the annulus and nucleus pulposus. Because of gradual involution of these vessels, by skeletal maturity only the peripheral fibers of the annulus receive a vascular supply. The metabolic needs of the nucleus are therefore met by diffusion. Paralleling the vascular supply, the nucleus and inner annulus are aneural. Branches of the sinuvertebral nerve of Luschka, however, innervate the

* Corresponding author. E-mail address: [email protected] (R.C. Huang).

posterior annulus and posterior longitudinal ligament, and the density of innervation has been shown to increase in DDD [1]. In addition, intraosseous innervation of the vertebral endplates may contribute to back pain in DDD [2,3]. Because the sinuvertebral nerve may ascend or descend one to two levels before reaching its target tissue, pain from DDD may not localize precisely [4]. The IVD is composed of an outer annulus fibrosus and a gelatinous nucleus pulposus. The two regions are not sharply demarcated but are separated by a transitional zone. The annulus is composed of concentric lamellae of fibrocartilaginous and fibrous connective tissue. There are approximately 12 lamellae, and each lamella is characterized by a parallel arrangement of fibers oriented at 65° relative to the longitudinal axis of the spine. Alternating lamellae are oriented in opposite directions [5]. The primary component of the annulus is type I collagen that is maintained by cells that resemble fibroblasts. The nucleus pulposus, which is located in the posterocentral area of the disc space, is composed primarily of proteoglycans that maintain disc turgor through their ability to attract water, which comprises 80% – 90% of the normal disc. Interspersed throughout the matrix is a meshwork of type II collagen fibers that is randomly oriented in contrast to the highly organized annulus. Disc cells derived from notochord and resembling reticulocytes or chondrocytes are responsible for synthesizing and maintaining the proteoglycan matrix of the nucleus. The disc turgor that results from the hydrophilicity of the nuclear proteoglycans is essential for the biomechanic function of the disc. There is significant variation of disc morphology in different regions of the spine. In the lumbar spine, the anterior disc is thicker than the posterior disc, which is responsible for most of the global lumbar lordosis.

0030-5898/04/$ – see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/S0030-5898(03)00103-2

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Biomechanics Functionally, the IVD has been compared with a pneumatic tire. The nucleus pulposus is primarily loaded in compression. With the appropriate turgor, axial compressive loads on the incompressible nucleus are redistributed radially and converted into tensile loads in the annulus fibrosus. If the turgor of the nucleus is insufficient, analogous to a flat tire, loading of the motion segment results in compressive loads in the annulus, which is poorly suited to withstand compression. Similarly, loading with inadequate turgor in the nucleus may lead to shearing forces in the transitional zone between the nucleus and annulus, leading to fatigue failure, fissures, and degeneration. The loss of disc hydration and turgor is one of the initiating steps in the degenerative cascade that culminates in diffuse disc and facet degeneration. The IVD is the primary load-bearing and stabilizing component of the functional spinal unit. Its ability to withstand repeated loading in compression, shear, and torsion over many decades while providing motion and stability to the spinal column is truly impressive. Physiologic loads in the lumbar spine are large. Lumbar intervertebral discs are subjected to 1.0 – 2.5 times body weight during normal walking [6]. The magnitude of compressive loads in the lumbar spine was estimated at 5300 – 6900 N (8 – 10 times body weight for a 70-kg person) during the lifting of 14 – 27-kg objects. Anteroposterior shear loads were estimated to range from 1100 – 1500 N during the same activities [7]. Another study estimated anteroposterior shear loads to be 700 N during trunk extension at 70% of maximal torque generation [8]. Approximately 80% of compressive loads pass through the anterior column of the lumbar spine, with the remainder being transmitted by the facets [9,10]. Degeneration of the IVD, however, may result in excessive load transfer to facet joints. The kinematics of the motion segment have been well described, especially in flexion – extension. The normal flexion – extension instantaneous axis of rotation (IAR) of a lumbar motion segment differs between different levels in the spine, differs between individuals, and migrates during intervertebral motion [11 – 13]. In general, the lumbar IAR lies slightly posterior to the midline of the vertebral body and slightly below the superior endplate of the inferior vertebra on sagittal imaging. L5 – S1 is a notable exception, because the IAR lies within the disc space instead of below the inferior endplate. Lumbar range of motion (ROM) is also highly variable between levels and individuals, such that precise ranges of normal motion are difficult to define [14]. Given the level-to-

level and person-to-person variation in ROM and IAR, it is unlikely that any TDR will be able to recreate individual kinematics precisely. It is likely, however, that the differing biomechanic properties of different implant designs will affect long-term outcomes. The posterior elements play an important role in the kinematics and loading of the lumbar spine. They are an integral part of the functional spinal unit and must be considered when treating disc disease. Facet joints resist excessive lumbar extension and experience high compressive loads in extension. In contrast, they experience minimal loads in flexion or compression loading of the spine. Even in the absence of flexion – extension rotation, pure anterior shear loads increase facet pressures because the facets resist anterior translation because of their plane of orientation [15]. In posterior shear loading, facets are unloaded, but the posterior ligaments (supraspinous, interspinous, facet capsule) resist posterior translation and are subjected to tensile loads [15,16]. Facet joint and posterior capsuloligamentous loads are clinically relevant because they are large [17] and the facets and posterior ligaments may be pain generators in the lumbar spine [18,19]. The extent to which a TDR influences facet loads and degeneration affects the clinical performance of the implant.

Pathophysiology of degenerative disc disease Given that the IVD bears some of the highest loads in the human body and is nearly avascular, it is not surprising that DDD is a common phenomenon in middle age and a universal condition in old age [20,21]. The pathophysiology of DDD is not entirely understood but is likely the result of the inability of the disc’s reparative capacity to keep pace with the micro- and macro-trauma that occurs with the activities of daily living. Classically, it is believed that degradation of the mechanical properties of the nucleus pulposus results in abnormal loading of the annulus and facets [22]. Over time, diffuse degeneration of the nucleus, annulus, and facets results. Inflammatory and degenerative changes may occur in the vertebral endplates also [23]. Buckwalter outlined several changes observed in the IVD with aging that include declining cell nutrition and viability, cell senescence, accumulation of degraded matrix molecules, and resulting fatigue failure of the matrix [24]. Gruber and Hanley have suggested that apoptosis may be responsible for the age-related depopulation of disc cells [25]. More recently, matrix metalloproteinases and aggrecanase have been implicated in degradation of the nucleus pulposus [26]. Although every individual is subject to these pro-

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cesses, additional environmental and genetic factors may predispose some to more severe disease. Recognized environmental risk factors include exposure to heavy repetitive lifting, prolonged sitting posture, and vibration [27]. Disc degeneration has been shown to be related to specific collagen IX alleles, an aggrecan gene polymorphism, matrix metalloproteinase-3 gene alleles, and vitamin D receptor alleles [28]. It has been estimated in identical twin studies that 60% of the variance in DDD could be accounted for by genetic factors [29]. Finally, surgical arthrodesis is a recognized cause of disc degeneration at adjacent levels. Theoretically, loss of segmental motion at fused levels leads to increased stresses at adjacent unfused levels. Furthermore, fusions that perturb sagittal alignment may result in compensatory mechanisms in the mobile spine that lead to degenerative changes. Although controlled studies are lacking, degenerative changes frequently appear adjacent to fused levels [30 – 32], and some investigators have noted an increased incidence of junctional degeneration adjacent to fusion masses with suboptimal sagittal alignment [33]. This implies that a TDR that does not restore normal sagittal alignment in addition to motion may not be protective against junctional degeneration. Although the precise relationship between TDR motion and adjacent level degeneration remains unknown, a statistical association between junctional degeneration and low prosthetic range of motion has been reported [34].

Treatment Nonsurgical therapies remain the first-line treatment for back pain and DDD. The vast majority of patients achieve acceptable results without surgery [35]. Nonsurgical modalities including physical therapy, massage, and manipulation have been shown to be effective in controlled trials [36]. A small percentage of patients, however, do not respond to nonsurgical modalities. In this chronically disabled patient population, surgery may be beneficial. In a randomized trial comparing nonsurgical treatment to fusion in patients with a minimum 2-year history of chronic back pain, surgery was significantly more effective in providing pain control and improvement of disability [37]. Intradiscal electrothermal treatment (IDET) is an invasive nonsurgical procedure that has been developed to treat refractory DDD. The procedure consists of percutaneous placement of a catheter tipped with a thermal resistive coil within the disc as close as possible to the inner margin of the posterior annulus. In theory, thermal treatment of the posterior annulus may provide

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pain relief by way of destruction of annular nociceptors and changes in the mechanical properties of the disc collagen [38,39]. Randomized studies are lacking; however, several investigators have reported encouraging short-term results after IDET [40,41]. Saal and Saal reported the results of 58 patients who had IDET with minimum 2-year follow-up. There were clinically and statistically significant improvements in Visual Analog Scores (VAS) and SF-36 scores. Ultimately, a randomized trial comparing IDET to nonsurgical treatment is needed to establish its efficacy. IDET may be a useful alternative for patients who have failed nonsurgical treatment but who wish to avoid the added morbidity of surgery. It is important to remember that although the procedure is minimally invasive, it is not without risk. Cauda equina syndrome, massive disc herniation, and vertebral osteonecrosis are rare complications of IDET [42 – 44]. Finally, the long-term effects of heating annular collagen on the disc’s biomechanic properties are unknown. Surgical arthrodesis is the current gold standard treatment for recalcitrant DDD. Theoretically, elimination of motion across the interspace and reduction of loads on disc tissues result in pain relief. Several investigators have reported acceptable clinical results in up to 80% of patients with DDD after circumferential fusion [45 – 47]. Although some investigators have advocated isolated posterolateral fusion for DDD [48], most believe that circumferential fusion provides better results in this challenging patient population by completely eliminating loading and motion across the anterior column. In fact, Barrick et al [49] demonstrated clinical improvement after anterior fusion in patients with persistent symptoms despite solid posterolateral fusion. The role of surgery in DDD has been controversial for several reasons. Most reports of the clinical results of fusion for DDD suffer from short follow-up periods and nonrandomized study design. The validity of methods for the diagnosis of DDD is controversial [50]. Finally, precise surgical indications, a key issue in this difficult patient population, have been elusive. In a randomized prospective trial comparing fusion to nonsurgical treatment in patients with chronic low back pain, Fritzell et al [37] demonstrated better results in the fusion group with regard to back pain and disability. Clinical improvement in the fusion group was moderate at best, however, with a 33% improvement in VAS back pain rating and 25% improvement in Oswestry Disability Questionnaire (ODQ) scores. There are several disadvantages inherent to arthrodesis that have prompted surgeons to seek nonfusion alternatives. In an extensive meta-analysis of the literature on lumbar fusion surgery, Turner et al

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[51] estimated that 32% of patients had an unsatisfactory outcome. The incidence of pseudarthrosis was 14%, and 9% of patients suffered from chronic iliac crest donor site pain. Furthermore, some postoperative protocols for lumbar fusion include bracing for a 3 – 6-month period, which can be a significant hardship for patients. In longer-term follow-up, adjacent level degeneration may compromise results. Prosthetic nucleus pulposus replacement (NPR) and total disc replacement have been developed to avoid the morbidity and complications associated with fusion. The goal of NPR is to restore the ability of the nucleus pulposus to redistribute compressive loads to the endplates and annulus. It may be performed in DDD with normal annulus and posterior element morphology. Annular defects or degeneration, posterior element disease, and disc space collapse (< 5 mm) are relative contraindications to prosthetic NPR. The nucleus replacement technology with the most extensive clinical history is the PDN (Prosthetic Disc Nucleus, RAYMEDICA Inc.; Bloomington, MN). Clinical trials of the PDN have been ongoing in Europe since 1996, and more than 1000 patients have been implanted with the PDN as of January 2003. The PDN is composed of a hydrogel pellet encased in a woven polyethylene jacket. Two PDN devices are inserted through a posterolateral or lateral annulotomy in a dehydrated state. The hydrogel imbibes water and theoretically may re-establish nuclear function by restoring disc turgor. Outcomes data from nonrandomized studies are preliminary but seem to be promising, although reoperation rates are high [52,53]. Patients have experienced statistically significant improvements in VAS pain scores and Oswestry disability scores at 2-year follow-up. Sustained increases in disc height have been observed at 2 years. The primary implant-related complication has been extrusion or migration. The rate of reoperation for reposition or explantation of the PDN was 26% from 1997 – 1998 and 12% from 1999 – 2001 [52]. Randomized trials are required to further elucidate the effectiveness of NPR. Furthermore, longer follow-up periods are required to ensure that any nuclear replacement is not susceptible to fatigue failure given the large magnitude of lumbar spine loading during activities of daily living.

Total disc replacement In TDR, the functions of the nucleus/annulus complex are assumed by the implant. An ideal TDR would perform all of the functions of the native disc, including preservation of physiologic range of motion, transmission of compressive loads across the disc

space, protection of the posterior elements from abnormal loads, and it would function for many decades. The first rudimentary disc prosthesis was implanted in humans by Fernstro¨m in the late 1950s, who placed a single ball bearing in the disc space of approximately 250 patients (Fig. 1) [54]. Subsidence of the implant into the endplates was common. Since then, several concepts have been investigated. One notable failure is the AcroFlex disc (DePuy AcroMed Inc.; Raynham, MA) that was constructed of a viscoelastic polyolefin rubber core sandwiched between two titanium endplates (Fig. 2). The rubber core suffered from high rates of fatigue failure; not surprising, given the large compression and shear loads in the lumbar spine [55]. TDRs in current clinical use are based on concepts borrowed from total joint arthroplasty of the hip and knee. Currently, two Food and Drug Administration (FDA) Investigational Device Exemption (IDE) trials are underway in the United States. The SB Charite´ (Waldemar Link; Hamburg, Germany) and the Prodisc (Spine Solutions; New York, NY) are constructed of cobalt-chromium endplates with metal-on-polyethylene bearings. The Prodisc is a constrained ball-andsocket design (Fig. 3), whereas the SB Charite´ has a less constrained articulation consisting of a free-floating biconvex polyethylene disc sandwiched between two concave endplates (Fig. 4). The difference in biomechanic constraint between the two implants is likely to affect long-term clinical outcomes, but currently it is unknown which implant is superior. Clinical results The SB Charite´ was first developed in the former German Democratic Republic in the early 1980s and has been used more extensively than any other TDR. Because of problems with fatigue fracture of the metallic endplates and implant dissociation, the SB Charite´ has been redesigned twice. The third generation SB Charite´ has been implanted in more than 5000 patients worldwide since 1987 and may be approved by the FDA for use in the United States in the near future. Satisfactory clinical outcomes have been reported in 63% – 85% of patients at 1 – 4-year follow-up [56 – 60]. Lemaire et al [59] published the longest follow-up of the Charite´ at 51 months. In this study of 105 patients, 79% had good and 6% had satisfactory results at final follow-up. Eighty-seven percent of patients returned to work. The mean flexion – extension ROM was 13° at L4 – 5 and 9.5° at L5 – S1. Clinical failures were attributed to inappropriate indication of patients with pre-existing contraindications (osteoporosis, facet arthrosis) and the development of de novo facet arthrosis. Implant failure, migration, or

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Fig. 1. (A,B) Radiographs of Fernstrom ball.

dislocation was reported in 7% of patients in an early article [57]; however, this complication has not been reported in more recent articles. These data suggest that the SB Charite´ has clinical promise, but randomized trials (in progress in the United States) and longer follow-up periods are necessary to establish superiority over arthrodesis or nonsurgical treatment. The SB Charite´ is currently being evaluated in an FDA IDE trial comparing the disc replacement to anterior fusion

with stand-alone BAK cages. Enrollment in the study closed in December 2001, and the FDA is awaiting 2-year follow-up data. The Prodisc implant was developed in France and first implanted in 1990. The Prodisc was redesigned in

Fig. 2. Acroflex implant.

Fig. 3. Prodisc implant.

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least four other lumbar TDRs are currently in various stages of development. Of these, only the Maverick (Medtronic Sofamor Danek; Memphis, TN) has been implanted in humans (in Europe) and an FDA IDE is in the planning stages. The Maverick prosthesis is a metal-on-metal, ball-and-socket design (Fig. 5).

Short-term complications of total disc replacement

Fig. 4. SB Charite implant.

the late 1990s. The primary differences between the first and second generation implants are the change from titanium to cobalt chrome and the introduction of polyethylene liner modularity. In a report of 7 – 10-year follow-up of the first generation Prodisc implant, Marnay [61] reported good to excellent results in 78% of 70 patients. This is the longest reported follow-up of a contemporary TDR. Short-term studies on the second generation Prodisc have also reported encouraging results. Mayer et al [62] and Tropiano et al [63] found patients had clinically and statistically significant improvements in VAS and ODQ scores at 1-year follow-up with subjective patient satisfaction rates of 83% – 100%. It is not surprising that outcomes seem to deteriorate over time, given the progressive and diffuse nature of lumbar degenerative disease. Recently, early results from the first randomized prospective trial of the Prodisc and anterior/posterior lumbar fusion were reported by Zigler et al [64]. In this study of 39 patients (28 TDR, 11 fusion), estimated intraoperative blood loss, operative time, and hospital length of stay were significantly smaller in the TDR group. VAS and ODQ scores were not statistically different at 6 weeks or 6 months. ROM of the second generation Prodisc reported by Bertagnoli et al [26] at short-term follow-up was 10° at L4 – L5 and 9° at L5 – S1. At 9-year follow-up the mean ROM for the first generation Prodisc was reported by Huang et al [34] as 3.8°. The reason for this apparent time-dependent decrease in ROM is unknown but may be related to differences in surgical technique, implant characteristics, or patient age. In summary, the clinical results of the Prodisc implant have been similar to those of the SB Charite´, although longer follow-up has been reported for the Prodisc. An FDA IDE trial comparing the Prodisc second generation implant to 360° instrumented fusion began in the United States in October 2001. Consideration for release to the general market may be made in 2005. At

Complications of TDR can be divided into those that are approach-related or implant-related. Approach-related complications include vascular injury, deep venous thrombosis, distal embolization of atheromatous plaques, and retrograde ejaculation. The reported rates of these complications have been similar to those reported for anterior lumbar interbody fusions and are clearly related to surgical technique and experience [57,60]. Implant-related complications during the perioperative period include implant dissociation, vertebral body fracture, implant malposition, infection, and postoperative radiculopathy. The incidence of anterior migration or dissociation of the free-floating polyethylene core of the SB Charite´ was 7% in early reports [57] but has become less common. These problems may present in the perioperative period or at long-term

Fig. 5. Maverick implant.

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follow-up [65]. Surgeon experience and the introduction of a hydroxyapatite coating to the polished SB Charite´ endplates may have decreased migration rates. Conversely, follow-up of the first generation Prodisc implant at 7 – 10 years did not reveal migration or dissociation [61]. The Prodisc endplates have keels to provide primary stability and a plasmapore ingrowth coating for long-term stability. Ironically, since the introduction of modularity to the polyethylene liner of the second generation Prodisc, there have been rare reports of anterior liner subluxation in the early postoperative period [62,64]. Because the implant is constrained and must bear significant anterior shear loads, it is not unlikely that fatigue failure of the liner’s locking mechanism will lead to cases of late liner subluxation in the future. Liner subluxation will lead to backside polyethylene wear and excessive facet loads. Redesign of the locking mechanism may be prudent. Vertebral body fracture during implantation or in the immediate postoperative period has been reported rarely [59,63]. The incidence of this complication can be reduced by avoiding surgery on osteopenic patients and excessive distraction. Treatment may require anterior/posterior fusion with removal of the prosthesis. Implant malposition in the coronal and sagittal planes has been reported by many investigators [56,59,63]. Although significant malposition may require reoperation for repositioning, minor degrees of malposition seem to be well tolerated and have not had statistically significant effects on outcome. Biomechanic and in vivo studies, however, have shown that anterior malposition reduces prosthetic ROM, and therefore surgeons should attempt to place the IAR of the implant close to the motion segment IAR [56,66]. Postoperative radiculopathy has been attributed to iatrogenic herniation of nucleus pulposus (HNP) [62] or nerve root traction resulting from disc space distraction in patients with prior lumbar surgery and postoperative epidural fibrosis [56,63]. Postimplantation radiculopathy tends to improve without surgery and there are no reports of cases that have required surgical intervention. Intraoperative HNP can be avoided by complete removal of the nucleus pulposus before implant insertion. The incidence of radiculopathy resulting from presumed nerve root traction is as high as 10% in patients with prior posterior lumbar surgery [63]. Although the prognosis for this radiculopathy is favorable, patients with a history of previous lumbar surgery who wish to undergo TDR should be apprised of this risk. Deep prosthetic infection is a serious concern in any arthroplasty procedure. It has not been reported after TDR.

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Long-term challenges in total disc replacement Long-term implant-related complications include mechanical failure, osteolysis, and subsidence. The incidence of mechanical failure in the form of metallic endplate fracture was high in earlier TDR implants. Contemporary metallurgic technology as used in total hip and knee arthroplasty is likely to eliminate fatigue failure of well designed metallic endplates. Possible fatigue failure of the Prodisc polyethylene locking mechanism was discussed previously. Only one case of apparent polyethylene wear and osteolysis has been reported after TDR [65]. In this patient who had SB Charite´ implantation 13 years previously, apparent loss of height of the polyethylene core combined with gross loosening and cystic radiographic changes in the vertebral endplates are highly suggestive of osteolysis, although histologic confirmation was not possible because the patient refused revision surgery. Cinnotti et al [56] and Marnay (unpublished data) were unable to detect loss of polyethylene thickness by plain radiographs at 2- and 9-year follow-up, respectively. The rarity of reports of osteolysis after implantation of polyethylene bearing TDRs since the 1980s suggests that this will be a rare problem in TDR. The Maverick TDR, which has a metal-onmetal articulation, may represent a step forward if osteolysis or polyethylene failure should become a significant problem in the future. Implant subsidence has been reported, but its precise prevalence is difficult to define because of inconsistent reporting and measurement techniques in the literature. Cinnotti et al [56] reported a 9% incidence of subsidence of the SB Charite´ implant at 2-year follow-up but were unable to detect an effect of subsidence on outcome. In a review of 27 patients who reported with persistent pain at mean 57 months after SB Charite´ implantation, Van Ooij et al [65] found that 67% had subsided and believed that this was a significant contributor to poor outcomes. Of the patients with subsidence, 56% were implanted with undersized prostheses but 44% had appropriately sized implants. The mean age of patients with subsidence was 40 years, so it is unlikely that age-related osteopenia caused subsidence. It seems likely that biomechanic failure of the endplate (subsidence) presents a more significant challenge to long-term outcomes than mechanical failure of prostheses. Progressive facet arthrosis is a significant cause of failure after TDR. Van Ooij et al [65] identified significant facet arthrosis in 41% of their series of patients who failed TDR, and many investigators have pointed out the importance of facet integrity in longterm outcomes. Huang et al [67] have pointed out

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the importance of biomechanic constraint and neutralization of anterior shear forces in facet preservation. It is unknown at this time if this theoretic advantage of constrained implants will have an effect on long-term outcomes. Facet arthrosis that fails nonsurgical treatment can be treated with posterior fusion with implant retention. Like subsidence, the true prevalence of adjacent level degeneration is unknown. It may take the form of disc degeneration, facet arthrosis, instability, or stenosis. It is clear that the incidence of junctional degeneration is low at short-term follow-up. Cinnotti et al [56] found a 0% incidence of junctional degeneration at 2 years after SB Charite´ implantation. Bertagnoli et al [26] found that 5% of patients had progression of adjacent level degeneration at 3 month to 2-year follow-up. Huang et al [34] found that 24% of patients had adjacent level degeneration on plain radiographs 9 years after Prodisc implantation. There was a statistically significant difference between the mean motion of Prodiscs below normal discs (1.6°) and below degenerated junctional discs (4.7°, P < 0.035). It seems that inadequate motion may predispose to adjacent level degeneration. The clinical significance of junctional degeneration is controversial, as some investigators have been able to link radiographic junctional degeneration with outcomes [30], whereas others have not [32,68]. Symptomatic junctional degeneration may require fusion or TDR. Indications and contraindications TDR is currently an experimental procedure in the United States. If reported clinical outcomes in longerterm follow-up continue to be favorable, the prime indication for TDR would be a well motivated patient with severe degenerative disc disease on imaging and concordant discogram with negative adjacent levels, and who has failed prolonged nonsurgical treatment. Contraindications include facet arthrosis, central or lateral recess stenosis, HNP that cannot be decompressed by way of anterior approach, instability (spondylolisthesis, laterolisthesis, or postlaminectomy), fixed deformity (scoliosis), osteoporosis, or infection. Foraminal stenosis resulting from disc space collapse may be treated by TDR if distraction is adequate. The prevalence of one or more contraindications in the patient population of most degenerative spine surgeons is high. Analysis of a cohort of 100 consecutive patients who had lumbar surgery in the practice of one degenerative spine surgeon revealed that 95% of these patients had one or more contraindications to TDR, and the mean number of contraindications was 2.5 per patient [69].

Summary Total disc replacement is an exciting technology that may one day replace fusion as the gold standard treatment for DDD, but it is currently an experimental procedure in the United States. Promising short- and mid-term results have been reported for TDR, but longer follow-up and randomized trials comparing TDR to fusion and nonsurgical treatment are needed to fully define the role of TDR in the spine surgeon’s armamentarium. Short-term complication rates have been acceptably low, but in the long term the durability of TDR implants and the vertebral endplate will provide challenges. Finally, it is essential that practitioners understand that a limited subset of patients are good candidates for TDR and that indiscriminate application of this technology will result in poor outcomes.

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