Applications of UHMWPE in Total Ankle Replacements

Chapter 11

Applications of UHMWPE in Total Ankle Replacements Allyson Ianuzzi, PhD and Chimba Mkandawire, PhD, CAISS

11.1 Introduction 11.2 Anatomy 11.3 Ankle biomechanics 11.4 Total ankle replacement design 11.4.1 Early Designs

11.4.2 Results of Early Designs 11.4.3 Contemporary Designs 11.5 UHMWPE loading and wear   in total ankle replacements

11.1  Introduction The ankle joint was the last joint in which total joint replacement was attempted. For that reason, total ankle replacement (TAR) currently presents more problems than encountered in total hip or knee replacement. In the 1970s, several first-generation TAR designs were introduced. The main distinguishing features among the designs included whether they were fixed versus mobile bearings and whether they had congruent versus incongruent articulating surfaces. Commonalities of the designs included replacement or resurfacing of both the tibial and talar surfaces, the use of cement for bony fixation, and a metalon-ultrahigh molecular weight polyethylene (hereafter UHMWPE) bearing surface. Based on their clinical performance, many of the early designs were recommended only for older patients with low physical demands, or their use was abandoned entirely. Many lessons were learned from this first generation of TARs. For example, almost all devices currently are used without cement in clinical practice (although this convention constitutes off-label use in the United States because none of the FDA-cleared devices are cleared for cementless use). One of the more important lessons with respect to the UHMWPE components was highlighted by studies demonstrating high local stresses in the UHMWPE components with incongruent articulating surfaces. All contemporary designs incorporate congruent articulating surfaces. UHMWPE Biomaterials Handbook Copyright © 2009, Academic Press. Inc. All rights of reproduction in any form reserved.

11.6 Complications and retrieval analysis 11.7 Conclusion 11.8 Acknowledgments References

Debate remains as to whether a fixed or mobile bearing is superior. All devices that are currently FDA-cleared in the United States are fixed bearing devices. There are no FDA-approved mobile bearing devices, although approval is pending for one design (STAR). A query of the Nationwide Inpatient Sample (NIS) indicated that the estimated number of patients discharged from US hospitals after receiving TAR almost doubled between 1997 and 2000 (Figure 11.1), with an estimated 1229 patients discharged after TAR procedures in 2000. Overall the estimated number of patients discharged with TAR rose from 147 patients in 1993 to 870 patients in 2006. By comparison, the estimated number of patients discharged for ankle fusion has fluctuated from 1993 to 2006, where the estimated number of fusions decreased from 7037 patients in 1993 to 6010 patients in 1999, and then it again increased after 1999. These trends could reflect changes in TAR design, the number or types of designs available in the United States over the years, and different recommendations resulting from publications of clinical studies. The purpose of this book chapter is to provide a historical overview of the TAR. The anatomy and biomechanics of the ankle are briefly reviewed. An in-depth description is provided of early-generation and contemporary TAR designs, with a focus on the UHMWPE component of each design. UHMWPE wear in TAR is also considered. Lastly, available data related to clinical complications 153

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and retrieval analysis of the UHMWPE components are reviewed.

11.2  Anatomy The foot and ankle complex (Figure 11.2) comprises 26 bones within the foot, which are non-sesamoid bones (sesamoid bones vary in quantity and location from person to person), two bones of the lower leg, and approximately 109 ligament and fascicle groups spanning these 28 bones [1]. TAR

Fusion

Number discharges

8000

6000

4000

11.3  Ankle biomechanics

2000

0 1992

It is noted that accessory bones of the lower leg and foot are developmental abnormalities, which are excluded from this count. There are five functional joint groupings present, the ankle (talocrural joint), subtalar joint, midtarsal or transverse tarsal joint, tarsometatarsal joint, and metatarsophalangeal joints [2, 3]. Due to the detailed nature of the anatomy of this complex, we will only place attention on anatomy of the hindfoot and midfoot. The ankle comprises the articulation between the tibia, fibula and talus, and 10 ligaments and fascicles. The hindfoot (talus and calcaneus) contains the subtalar joint; there are three points of articulation between the inferior aspect (bottom) of the talus and the superior aspect (top) of the calcaneus. Approximately five ligaments span the subtalar joint; however, the cervical ligament (also the strongest of these five ligaments) has four distinct fascicles. There are approximately six ligaments and fascicles that originate on the distal tibia or fibula and insert on the calcaneus or midfoot. The midtarsal joint forms the boundary between the midfoot (navicular, cuboid) and hindfoot, where eight ligaments span.

1994

1996

1998 2000 Year

2002

2004

2006

Figure 11.1  Estimated number of discharges for ankle fusion and total ankle replacements (TAR) from 1993 to 2006.

The motions of the ankle can be described in three orthogonal axes, which include axial rotation (internal/external) in the transverse plane, inversion/eversion in the frontal plane, and dorsiflexion/plantarflexion in the sagittal plane (Figure 11.3). Motion along all three axes is pronation/ supination, where pronation is dorsiflexion, eversion and external rotation; and supination is plantarflexion, inversion,

Posterior view

Hindfoot

Midfoot

Forefoot

Figure 11.2  The foot and ankle complex. This disarticulated foot is divided into three groupings: the hindfoot, midfoot, and forefoot. Bone nomenclature: 1, fibula; 2, tibia; 3, talus; 4, calcaneus; 5, navicular; 6, cuboid; 7, cuneiforms; 8, metatarsals; 9, phalanges. The following joints are outlined with a solid black line: The ankle is the articulation of the tibia and fibula on the talus, the subtalar joint is the articulation between the calcaneus and talus, the midtarsal joint is between the hindfoot and midfoot, the tarsometatarsal joint is between the midfoot and forefoot, and the metatarsal phalangeal joints are between the metatarsals and the adjoining proximal phalange.

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and internal rotation. The sagittal plane range of motion required for walking is approximately 12 degrees of dorsiflexion and 15 degrees of plantarflexion [4]. The majority of this motion occurs at the talocrural joint. During inversion and eversion, the subtalar joint has a greater contribution, with motion at the talocrural joint occurring at the extremes of motion [4–6]. Axial rotation occurs at approximately equal proportions in the two joints [4, 5]. Total passive ankle range of motion within the sagittal plane in cadavers and living subjects is 50 to 70 degrees from maximum dorsiflexion to maximum plantarflexion [2, 7]. A maximum of 35 degrees is required for normal gait, with 10 degrees of dorsiflexion in early stance phase and

Figure 11.3  Motions of the ankle in the sagittal plane (top from left to right: neutral, dorsiflexion, plantarflexion), frontal plane (middle from left to right: neutral, eversion, inversion), and transverse plane (bottom from left to right: neutral, external rotation, internal rotation).

25 degrees of plantarflexion at push-off [8]. The average range of sagittal plane motion was 20–27 degrees, with approximately 10 degrees of plantarflexion in early stance and 14–25 degrees dorsiflexion during push-off [9]. Range of sagittal plane motion while ascending stairs was 37 degrees and 56 degrees for descending stairs [7]. Normal gait can be grouped by two phases (Figure 11.4): the swing phase (where the leg swings freely during walking) and stance phase (where the leg is in contact with the ground during walking). The stance phase can further be divided into five subcategories: heel strike or heel contact, foot flat or weight acceptance, midstance, heel-off or pushoff, and toe-off [2]. To describe stair ascent or stair descent (as well as abnormal gait), the nomenclature used for the stance phase becomes initial contact, loading response, midstance, terminal stance, and preswing. The maximum compressive load through the ankle during normal gait can be greater than five times body weight (compared to the compressive load of three times body weight found in the hip and knee) [9]. Stauffer pointed out that the surface contact area of a normal ankle is approximately 12 cm2 [9]. Maximum shear load was about 80% of body weight [9]. Weight transfer to the fibula can be approximately 17% of body weight [10, 11]. Compressive load studies of the ankle that utilized pressure sensitive film show average contact pressures of 1.84 MPa in the neutral ankle position; 2.16 MPa, 20 degrees of dorsiflexion; 2.14 MPa, 20 degrees of plantarflexion [12]. The Wagner study was performed with each cadaver specimen under 800 N of compression through the tibia and fibula, approximating 1  body weight [12]. McKinely and others performed similar compression studies, with a piezoresistive elastomer that measured compressive force. That study was performed with 600 N of compressive load (0.87 BW) through quasistatic gait orientations (heel strike, 10% stance phase, 60% stance phase, toe-off) and found a mean compressive load of 2.7 MPa [13]. These studies are conservative estimates of contact pressure because the primary extrinsic muscles of the lower leg were not modeled. Placing tension on the extrinsic leg muscles increases the compressive loads borne at the ankle.

Swing

Stance

Heel strike or contact

Foot flat or weight acceptance

Midstance

Heel-off or push-off

Toe-off

Figure 11.4  Schematic of normal gait, demonstrating the stance phase and swing phase.

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11.4  Total ankle replacement   design The goal of a TAR design should be to mimic the ankle joint as closely as possible. Specifically, a suitable range of motion should be available to allow for proper gait patterns and other activities of daily living. TAR designs are expected to have a reproducible surgical technique, minimal bone resection, rapid and adequate bone ingrowth, minimal constraint and replication of physiological ankle motion, and pain relief [14]. Relevant to the UHMWPE components, the design should also offer minimal complications and need for early revision, as well as long-term survivorship [14]. The contact area should be appropriate to distribute the load and avoid high contact stresses. In the next sections, these parameters are considered in discussing historical and modern TAR designs. The FDA currently has three classifications for TARs, two of which are Class II (metal/polymer or metal/composite semi-constrained cemented prosthesis) and one that is Class III (non-constrained cemented prosthesis). There are currently 20 different TAR designs worldwide that have been released for clinical use or are currently in development [14, 15]. FDA clearance (Class II) is currently limited to the Eclipse Total Ankle Implant (Kinetikos Medical, Inc.), Salto Talaris™ Total Ankle Prosthesis (Tornier), Agility Total Ankle Prosthesis (DePuy Orthopaedics), INBONE Total Ankle (INBONE Technologies, formerly Topez Orthopedics), and the Brigham Total Ankle Prosthesis (Howmedica Corp.). To date, there are no FDA-approved, non-constrained (Class III) TARs, although an FDA panel recommended approval of the STAR device in April 2007, with final approval of the device pending.

11.4.1  Early Designs The first total ankle replacement was designed in 1970 and implanted by Lord and Marotte [16]. The design contained a tibial component with a long stem that was similar to the femoral component of total hip replacements. The talar component was a UHMWPE replacement of almost the entire talar body. After 10 years, only 7 of 25 arthroplasties were considered to be satisfactory, and 12 of the implants failed. The Richard Smith ankle joint prosthesis (Figure 11.5), designed in 1972, was an unconstrained, ball-and-socket type design with metal tibial “socket” and UHMWPE talar “ball”[17]. It has been described as a multi-axis joint, where rotation of the tibia can occur about any of the major three axes and the rotational motion is not limited [18, 19]. In a clinical study of 24 Smith ankle replacements, with 18 replacements followed for an average of 7 years (implanted between 1975 and 1979), there was an improvement of range of motion in 10 ankles [17]. However, seven patients

Figure 11.5  Schematic of the Richard Smith ankle joint prosthesis.

had loosening of tibial components, with three patients revised at 4 years [17]. The authors concluded that although some patients were pleased, the Smith prosthesis could not be recommended [17]. The Imperial College of London Hospital (ICLH) implant was a two-part, semi-constrained prosthesis with a metal talar component and UHMWPE tibial component. It was designed in 1972, with subsequent design changes described in 1982 [20]. The design included a domeshaped metal talar component and a UHMWPE tibial insert with large lateral flanges for fibular articulation; the UHMWPE component was trimmed to size at the time of operation [21] (Figure 11.6). In a clinical study of 24 ankles implanted in 22 patients, the authors concluded that patients were improved compared to their pre-surgery conditions but that the prosthesis was not superior to arthrodesis [22]. After a clinical trial reported in 1982, the procedure was not recommended due to the high complication rates [21]. The ICLH and another prosthesis, the St. Georg [23], can be considered as the basis of the total ankle prostheses that were developed after 1972 [24]. The St. Georg prosthesis was nonconstrained, while the ICLH was semi-constrained [25]. While the ICLH was eventually abandoned, the design of the St. Georg evolved into the “Endo” system, which was a semiconstrained, two-part prosthesis [24]. The Newton ankle prosthesis (designed in 1973) was a two-part, nonconstrained prosthesis that had a UHMWPE tibial component with a flat proximal surface and a curved, cylindrical articulating surface [26, 27] (Figure 11.7). It has been described as a multiaxial joint that allows unrestricted motion about any of the three major axes [19]. The talar component comprised CoCr alloy and had a spherical articulating surface with a distal stem that was implanted into the talus. The result was an incongruent articulating surface that allowed dorsiflexion/plantarflexion, as well as slight inversion, eversion, and rotation [26, 27].

Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

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Figure 11.6  ICLH ankle prosthesis shown in anterior-posterior (left) and lateral (right) views (reproduced with permission from [21]).

Figure 11.7  Newton ankle prosthesis (from [26], with permission).

The Irvine ankle prosthesis (also called the Howmedica total ankle) was a two-part, nonconstrained prosthesis with a UHMWPE tibial component and metal talar component that was designed in 1975 [28, 29] (Figure 11.8). It has been described as a multi-axial joint, which allows tibial motion about any of the three major axes and did not restrict rotational motion [18, 19]. The device was cemented and permitted dorsiflexion/plantarflexion of 114 degrees and abduction/adduction of 40 degrees [29]. Initial clinical results were satisfactory [28]. A biomechanics study in cadaveric human ankle specimens indicated that it did not reproduce normal ankle motion because there was increased coupled motion and hysteresis in ankles with the prosthesis during plantarflexion/dorsiflexion and axial rotation [30]. Designed in 1976, the Thompson Parkridge Richards prosthesis (TPR; Smith & Nephew) was a cemented, semiconstrained device that utilized a convex talar component and UHMWPE tibial component with a concave articular surface [19, 31–36]. The radius of curvature of the tibial component was greater than that of the talar component, resulting in a line contact area of 0.30 cm2 [33]. The device utilized a “lip” on either side that restricted side-to-side movement of the talar component [33]. Patients had some improvement but “disappointing” clinical results due to

Figure 11.8  Schematic of the Irvine ankle prosthesis.

delayed wound healing and loosening [32]. Subsidence and loosening of the talar component were also observed [31]. Case studies have also been reported where complications of loosening led to revision [37].

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talus with a congruent UHMWPE tibial component [48, 49]. The correction of malalignment was difficult with this prosthesis, which resulted in undesirable varus/valgus forces [48]. As of a publication in 2001, the prosthesis was no longer manufactured [48].

11.4.2  Results of Early Designs

Figure 11.9  The New Jersey Cylindrical Replacement (reproduced from [79] with permission from Endotec, Inc.).

The PCA (also called the Scholz) prosthesis was initially designed in 1976 and implanted in 1984; it was the first porous coated, non-cemented design [25, 38]. This semi-constrained device had a UHMWPE tibial component with a metal-backed tray and a metal talar component. The New Jersey Cylindrical Replacement design was introduced in 1976 as a two-part prosthesis that had a metal tibial component and a UHMWPE talar dome [39] (Figure 11.9). The congruent spherical surfaces resulted in a contact area of 5.2 cm2 [39]. The prosthesis allowed a total of 65 degrees of motion, with 20 degrees of dorsiflexion and 45 degrees of plantarflexion [39]. This design was subsequently revised and evolved into the contemporary Buechel-Pappas design, which is described later in this chapter. The Mayo total ankle prosthesis was a two-part, constrained design that consisted of a UHMWPE tibial component and stainless steel talar component with congruent surfaces [40, 41]. The UHMWPE tibial component was concave, while the Vitallium talar component was convex [9]. The resulting articulating area was approximately 9 cm2 [7, 40]. The device theoretically allowed 30 degrees of ankle dorsiflexion/plantarflexion before impingement with bony structures [7, 9]. Clinical results have been described [7, 42–47]. Generally, there were improvements in ROM or functional motion for a large percentage of patients, with some patients experiencing no change or loss of motion [43]. Clinical problems included fracture of the medial malleolus without major long-term effects in the first clinical trials, infection, delayed healing, residual bony impingement, and loosening in younger, active patients [7, 9, 40, 43, 44]. The clinical studies reported that the device should be indicated for less active, older patients [7, 9, 43]. Overall, the procedure was not recommended based on poor short-term and long-term survival rates [46]. Designed in 1984, the Bath and Wessex prosthesis (Howmedica International) was a multi-axial, cemented two-part prosthesis [48, 49]. It had a metal dome for the

Ultimately, clinical results from early generation designs resulted in recommendations to discontinue use of TAR based on Mayo [43, 45, 46], Conaxial (Beck-Stefee) [50], Bath and Wessex [48], Newton [26], Waugh [18], Smith [18], ICLH [21], and Oregon [18]. Overall, indications for the initial TAR implants were limited due to the high complication rates and failure, with recommendations for use only in elderly patients with limited physical demands [19]. However, valuable lessons were learned from these designs. First, it was recognized that the use of bone cement required a larger resection of bone, which could result in subsidence of the metal components [4]. The community recognized the need to strike a balance between stability of the prosthesis with achieving larger range of motion [4]. Predominant clinical complications included loosening, persistent pain, and delayed wound healing [4]. In a clinical study of 21 patients with varying early-generation designs, signs of loosening (radiolucent lines) and decreased muscle strength occurred in 19/21 patients [18]. Based on objective analysis of gait and electromyography, patients who were implanted with early generation total ankle replacements did not function as well during activities of daily living as had been expected; ranges of motion were within normal limits, although motion patterns were “abnormal” with response to tibiopedal angles during certain aspects of the gait cycle [18] . The authors suggested that this may have been attributed to the weakness of calf and peroneal muscle groups on the side of the TAR. One of the more valuable lessons learned from early generation designs was related to the UHMWPE component. It was recognized that incongruent articulating surfaces allowed dosiflexion/plantarflexion and axial rotation but resulted in high local stresses on UHMWPE components that could result in high wear rates [4, 39]. Congruent designs were therefore determined to be needed for greater stability and resistance to wear [4].

11.4.3  Contemporary Designs Second-generation and contemporary designs offer improvement in TAR survival rate [36]. Currently, commonly used prostheses include [15]: STAR (Scandinavian Total Ankle Replacement, Waldemar Link), Buechel-Pappas Total Ankle Replacement (Endotec), TNK ankle (Japan Medical Materials), and Agility Total Ankle System (DePuy). Other TAR devices that currently have FDA clearance

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Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

Figure 11.10  Three generations of the ceramic TNK prosthesis with anterior (left) and lateral (right) views; reproduced from [54], with permission.

in the United States include [15]: INBONE Total Ankle (INBONE Technologies, formerly Topez Orthopedics), Salto Talaris™ Total Ankle (Tornier), and Eclipse Total Ankle (Integra Life Sciences Holdings). Although these devices can be used in the United States, little information is available related to the clinical performance of these devices.

11.4.3.1  TNK Prosthesis The TNK prosthesis is unique in that it is a ceramic-onUHMWPE device that utilizes a UHMWPE tibial articulating surface [51] (Figure 11.10). The device is a ­cementless, fixed bearing with partially conforming ­ surfaces, having a concave ceramic talus that articulates with a flat UHMWPE tray [52]. The UHMWPE tray is secured to the ceramic tray on the tibial side [52]. Bony fixation methods include hydroxyapatite-coated beads and a tibial screw [52]. Concerns have been raised regarding the articulation and that it may transfer excessive shear and torque to the prosthesis-bone interface, which, along with the ceramicbone interface, will affect the long-term mechanical fixation [52]. In a clinical study of the TNK, radiolucent lines were prevalent although they did not spread, and there were no instances of talar component sinking [53]. Clinical results

also demonstrated a linear decrease in range of motion three years after operation, where range of motion after eight years equaled the range of motion measured preoperatively for both cemented and uncemented implants [51]. The inventors demonstrated that fewer cases of loosening and subsidence were observed in uncemented components compared to cemented components, although this study included a range of ceramic-on-UHMWPE and metal-on-UHMWPE devices [51, 54].

11.4.3.2  Agility Total Ankle Prosthesis The Agility Total Ankle Prosthesis (DePuy) is a semiconstrained, fixed-bearing prosthesis that was designed in 1984 and currently has 510(k) clearance from the FDA [14] (Figure 11.11). In the United States, it is the most commonly used TAR prosthesis and has more than 20 years of clinical experience [55]. The device is currently offered in six sizes of matching tibial and talar components [56]. The device consists of a titanium alloy tibial component, a size-matched and separate UHMWPE insert that slides and locks into the tibial component, and an onlay cobalt-chromium talar component [55, 57, 58]. The tibial articular surface is rotated 20 degrees externally to mimic physiologic ankle anatomy [14, 55]. The semi-constrained

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Figure 11.11  The Agility Total Ankle Prosthesis (left) and Agility LP Total Ankle Prosthesis (right) (courtesy of DePuy Orthopaedics).

design allows dorsiflexion/plantarflexion, as well as axial rotation and medial-lateral shift of the talar component within the tibial plane [57–59]. The Agility has unique design attributes that broaden the bony base for the tibial component; this includes an obliquely-rectangular tibial component and a requirement for tibiofibular syndesmosis fusion that allows weight transfer to the fibula [59]. Both the tibial and talar surfaces that interface with bone have porous ongrowth surfaces [57–59]. Due to the patterns of (and larger amounts of) bone resection that are required for the Agility compared to other TARs (e.g., Buchel-Pappas), the Agility is often a potential candidate for revision procedures [60, 61]. Although the Agility is indicated for cemented use only [56], it has been used as an uncemented device off-label in the United States. The Agility permits dorsiflexion/plantarflexion of 60 degrees [57, 59]. The UHMWPE insert of the Agility is sterilized using gas plasma and packaged using a double blister/Tyvek package, which the manufacturer claims “addresses poly insert oxidation and degradation issues” [56]. The resin is either Ticona GUR 1020 or GUR 1050. Thickness of the UHMWPE component varies from 3.73 to 4.7 mm, depending on implant size. Revision components offer thicker “plus 2 mm” UHMWPE inserts that increase the thickness to 5.73 to 6.7 mm [52, 57]. Before 1987, the talar component of the Agility was manufactured from titanium; this was changed to CoCr in 1987 in light of some studies revealing poor wear qualities of titanium against UHMWPE [62]. Complications related to the UHMWPE component have been reported, including bone resorption (lysis), component loosening, and excessive wear resulting in failure of the UHMWPE component. Radiolucent lines have been observed in a clinical study of 95 patients implanted with Agility TARs between 1984 and 1993, but these were not attributed to UHMWPE wear debris due to the short timecourse (two years) and radiographic appearance; the loosening was attributed to high shear stresses between the implant and lateral malleolus [57]. In a continuation study published in 2004 of the first Agility TAR patients with an average follow-up of nine years, 49/117 ankles showed

Figure 11.12  Fractured UHMWPE component from an Agility that was retrieved from a patient with a talar component that was malaligned in varus (Copyright © 2008 by the American Orthopaedic Foot and Ankle Society, Inc., originally published in Foot & Ankle International, 24(12):901–903, and reproduced here with permission [67]).

some evidence of mechanical lysis (7/49 progressive), and 18/117 ankles had expansile lysis of late onset (4/18 progressive) [63]. Other clinical studies have also reported lysis surrounding Agility TAR prostheses [62, 64–66]. UHMWPE wear and fracture can occur due to malpositioning, which can generate a region of excessive wear and potentially full-thickness catastrophic fracture of the UHMWPE component [67, 68] (Figure 11.12). Nicholson et al. reported a cadaver study utilizing 10 human ankle specimens, where ankles implanted with Agility TAR prostheses were axially and cyclically preconditioned and then loaded to 700 N for 10 cycles at a frequency of 1 Hz. During cyclic loading, the average contact pressure was 5.6  2.1 MPa distributed over 0.83 cm2, and the average peak pressure was 21.2  5.7 MPa. In a separate phase of the study, the authors observed that contact pressure was larger for smaller component sizes and increased with increasing axial load. The axial loads applied in this study represented approximately 1  body weight; forces of over 300% body weight could be expected during daily activities such as descending stairs. Therefore, it can be expected that mean peak pressures on the UHMWPE component would exceed those reported in this study.

Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

Cadaveric testing conducted by Valderrabano et al. compared range of motion [69] and talus rotation [70] for normal and fused ankles compared to those implanted with Agility, Hintegra, and STAR prostheses. Compared to the normal condition, the Agility did not show a significant difference in plantarflexion but significantly decreased dorsiflexion [69]. The Agility also showed a significant and substantial increase in inversion/eversion compared to normal ankles, with no significant differences for internal/ external rotation [69]. Valderrabano et al. concluded that the increase in overall inversion/eversion range of motion might be explained by the mismatch of size and congruency of the two components of the Agility, which could potentially lead to a tilt of the talus component and result in an overload of ankle ligaments and UHMWPE wear. Cadaveric ankles implanted with the Agility had decreased 60% talus rotation and 80% talus shift compared to normal ankles, while other three-component designs (i.e., Hintegra and STAR) did not differ in talus motion compared to normal ankles [70]. This restriction in talar motion may result in increased stresses within and around the prosthesis, possibly resulting in UHMWPE wear and loosening at the bone-prosthesis interfaces.

11.4.3.3  Scandinavian Total Ankle Replacement The Scandinavian Total Ankle Replacement (STAR) prosthesis is currently manufactured by Link Orthopaedics, but the design was to be acquired by Small Bone Innovations, Inc., in 2008. The device is a three-component, cylindrical, mobile bearing design with CrCoMo tibial and talar components and a mobile, congruent UHMWPE component [33, 71] (Figure 11.13). The contact area is 3.2 cm2 on the talar surface and 6.0 cm2 on the tibial surface [33]. The thickness of the UHMWPE component is 6 to 10 mm [71]. The superior surface of the UHMWPE component is flat, making planar contact with the tibial component to allow translation and medial/lateral and anterior/posterior sliding within the constraints of the surrounding tissue. Dorsiflexion and plantarflexion take place on the curved talar surface. There is a groove in the UHMWPE component that conforms to a crest on the metal talar component; its length is oriented in the anterior-posterior direction to prevent sliding of the prosthesis in the medial-lateral direction [71]. The talar component has wings to replace the medial and lateral talus facets, which protects sculpted surfaces of the talus to allow resection for correcting malalignment, prevents degenerative arthritis of the facets, and allows for retention of blood vessels and ligaments [71]. The UHMWPE components were initially gamma sterilized in air, while more recent components are sterilized with gamma irradiation in a nitrogen-vacuum [72, 73]. Outside of the United States, earlier versions of the metal components have hydroxyapatite coatings to encourage bony ingrowth, while more recent designs available outside the United States have

161

Figure 11.13  Scandinavian Total Ankle Replacement (STAR).

double-coated titanium plasma spray and hydroxyapatite surfaces [72]. Within the United States, CP-Ti plasma spray is available on the bony surfaces of both the tibial and talar components. Cadaver studies have demonstrated changes in ankle biomechanics in normal ankles versus those implanted with STAR devices [69]. There were no significant differences in inversion/eversion. Dorsiflexion and plantarflexion were significantly reduced in STAR ankles. Significant increases in range of motion were observed during internal tibial rotation, while significant decreases in exterior tibial rotation were observed in the STAR ankles [69]. Biomechanical and clinical studies have emphasized that proper sizing and positioning of the UHMWPE component is essential for function of the STAR device. In a biomechanical study of six cadaveric ankles, anterior liftoff of the UHMWPE component was observed as a result of anterior positioning of the talar component and/or undersizing the UHMWPE component [74]. Anterior positioning could have caused edge loading on the posterior side of the UHMWPE component, where UHMWPE loading is localized to an edge of the component [74]. On the other hand, undersized UHMWPE components could cause subfibular impingement that reduces contact pressure on the anterior edge of the UHMWPE component [74]. Complications related to the UHMWPE component have been discussed in clinical studies. Edge loading of the UHMWPE component creates the potential for increased UHMWPE wear and premature failure. In a study by Wood et al. that investigated 200 STAR ankle implantations, nine patients experienced edge loading of the UHMWPE component [75]. Seven of those patients exhibited preoperative varus or valgus deformity that was greater than 15 degrees [75]. In a case study, a patient developed edge loading of the UHMWPE component due to varus tilting of the talus [76].

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Figure 11.14  The Buechel-Pappas total ankle prosthesis (reproduced from www.endotec.com, with permission).

The patient in this study developed a wear debris cyst in the fibula (confirmed by histology) with subsequent fracture; the UHMWPE component was replaced, and at 3 months post revision the patient’s symptoms were improved [76]. Other clinical studies report UHMWPE fractures or revisions related to the UHMWPE component [31, 36, 77]; although these studies did not report a biomechanics rationale for the UHMWPE failure, exaggerated varus/valgus deformity and/or ligamentous instability resulting in accelerated wear could participate in the mechanism for failure of these components as well.

11.4.3.4  Buechel-Pappas The New Jersey Cylindrical Replacement (Figure 11.9) later evolved into a trunion device, which was the first occasion of a three-part mobile bearing system being used in the ankle joint [4]. This later developed into the New Jersey Low Contact Stress total ankle replacement and was described previously [78]. This three-component design was cylindrical and cementless with congruent surfaces, and it was used from 1981 onward by inventors [78] (Figure 11.14). The metal components are manufactured from titanium with a porous coating to promote bony ingrowth, and the mobile core is UHMWPE [79]. A nitride ceramic film is added to the titanium bearing surfaces to improve the wear characteristics [79]. The device enables axial rotation and sliding in mediolateral/anteroposterior directions without constraint at the superior bearing surface [78]. The superior bearing surface is flat and somewhat smaller than the articulating surface of the tibial component, while the inferior surface was congruent to the trochlear groove in the talar component, which allows dorsiflexion/ plantarflexion and eversion/inversion [78]. Wear on the superior surface was expected to be relatively low due to the fact that there was little secondary motion (axial rotation)

UHMWPE Biomaterials Handbook

compared to the primary motion (dorsiflexion/plantarflexion) and because the secondary bearing surface was 20% larger than the primary bearing surface [78]. The prosthesis provides 60 degrees dorsiflexion/plantarflexion with congruent contact and 30 degrees axial rotation with congruent contact [78]. Retention of the mobile UHMWPE bearing is achieved via compression of the collateral ligaments and adjacent tissues, as well as gravitational loads [78]. Failure modes of the New Jersey Low Contact Stress design included talar component subsidence, as well as lateral- and medial-bearing subluxation due to ligamentous instability or subsidence [80]. The device was modified to deepen the sulcus in the talar component, two fixation fins were added to the talar component to improve bony fixation, and the UHMWPE meniscal bearing was thickened [80]. Prior to the design change, the UHMWPE component was sterilized using gamma irradiation in air [81]. In the new design, the UHMWPE component is manufactured using 1150 UHMWPE powder, machined from extruded bar and sterilized using ethylene oxide. This design is used today and referred to as the Buechel-Pappas design (Endotec) [4]. Improvements in the design resulted in better survival rates and decrease in talar subsidence [4, 78, 81, 82]. Surgical technique, clinical results, and kinematics have been described extensively [81–84]. The performance of the UHMWPE component of the Buechel-Pappas design is dependent upon proper orientation of the articulating surfaces. Failure of the UHMWPE component has been observed in conjunction with varus or valgus deformity (caused by malleolar fracture or instability) that subsequently leads to edge loading of the UHMWPE component [80, 85, 86]. The existence of a preoperative deformity dramatically influenced the survival of both the Low Contact Stress and Buechel-Pappas designs, where TARs with varus or valgus deformity greater than 10 degrees had an overall survival rate of 48% at eight years, while ankles with neutral alignment preoperatively had an eight-year survival rate of 90% [85]. In one clinical study, osteolysis was suspected radiographically in 3/19 Buechel-Pappas patients at 4.7 years or more followup; it is unknown whether edge loading of the UHMWPE component or varus/valgus deformities caused accelerated wear and subsequent osteolysis in these patients [87].

11.4.3.5  Salto Talaris™ and Salto™ The Salto Talaris™ Anatomic Ankle (Tornier, Edina, Minnesota, USA) is a metal-on-UHMWPE, semi-constrained cemented TAR (Figure 11.15). It has been in clinical use since 2006 [88]. The device consists of two mating components: a CoCr tibial base in association with a conforming UHMWPE insert and a CoCr talar resurfacing component [89]. The bony interface of the metal components are coated with titanium plasma spray, although the device is intended for cemented use only in the United States [89].

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Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

Figure 11.15  The Salto Talaris™ total ankle prosthesis (left) and Salto™ total ankle prosthesis (right), courtesy of Tornier, Inc.

The UHMWPE component is manufactured per ISO 5834-2 [89]. The UHMWPE resin is Type B 4150HP, and it is sterilized using gamma irradiation. Sterilization of the UHMWPE component in a vacuum with double-peel package was introduced at the first launch of the Salto™. The UHMWPE insert combined with the tibial component provides thicknesses of 8, 9, 10, and 11 mm, which is compatible with various levels of instability [88]. The insert is fixed to the tibial component before implantation. The polyethylene insert design allows 5 degrees of internalexternal rotation, 4 degrees of varus-valgus rotation, and 2 mm of anterior-posterior translation. The Salto™ implant (Tornier SAS, Saint Ismier, France) (Figure 11.15) was developed between 1994 and 1996 and has been used clinically since January 1997 [90]. It is distinguished from the Salto Talaris™ Anatomic Ankle in that the Salto™ is a three-component design (unconstrained, possible mobility between the plane top area of the liner and the bottom plane mirror polished tibial component), while the Salto Talaris™ is a two-component design (semiconstrained). The design consists of metal tibial and talar components with a mobile UHMWPE insert. The resin, sterilization, and packaging of the UHMWPE component, as well as the CoCr used in the metal components, are identical to that of the Salto Talaris™. The coating on the bony surface of the Salto is titanium with hydroxyapatite. The Salto™ was intended as an improvement to threecomponent mobile bearing prostheses to optimize the bearing surfaces, the conical articular surface, accuracy of positioning, and primary fixation of the implant while requiring minimal resurfacing of the bony surfaces [90]. The UHMWPE mobile bearing is available in thicknesses from 4–8 mm (insert only), and it maintains full congruency with the talar component in dorsiflexion and plantarflexion. The UHMWPE component also accommodates as much as 4 degrees varus and valgus deformity or mobility by keeping congruency to avoid edge loading [90]. Coronal plane stability is accomplished via a central, sagittal sulcus on the

Figure 11.16  Schematic of the HINTEGRA total ankle prosthesis.

talar component that conforms to a central ridge on the inferior surface of the UHMWPE component [14]. There is also a medial stop mechanism on the tibial component to avoid impingement with the internal malleolus [14]. While there are few clinical studies published at the time of this writing, one clinical study of the 98 first cases of the prosthesis (93 available at follow-up) showed no evidence of edge loading after a mean follow-up of 35 months (range: 24 to 68 months) [90]. Further clinical follow-up with this patient cohort is pending publication.

11.4.3.6  HINTEGRA The HINTEGRA prosthesis was first implanted in May 2000 [4]. It is a three-component device (Figure 11.16) and has a unique feature in that the tibial and talar components have screws to reduce the amount of bone resection required at the tibia and talus [4, 91]. The tibial articulating surface is flat, while the talar articulating surface is

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Figure 11.18  BOX prosthesis (reproduced from [4], with permission).

Figure 11.17  Mobility Total Ankle System (courtesy of DePuy Orthopaedics).

conically shaped; both surfaces conform to the UHMWPE bearing [91]. The prosthesis provides 50 degrees of congruent contact in dorsiflexion and plantarflexion and 50 degrees of congruent contact in axial rotation [91]. Motion is restricted by the natural soft tissue [91]. The device is also designed with a degree of inclination on the tibial component to provide resistance to the posterior load produced during walking [91]. A biomechanical analysis of cadaveric ankles indicated that ankles implanted with the HINTEGRA had significantly less dorsiflexion, while there was no difference in plantarflexion, inversion/eversion, or internal/external rotation [69]. Clinical results are limited, where one study of 122 HINTEGRA implantations followed for an average of 18.5 months demonstrated a 92% survival rate [91].

11.4.3.7  Mobility The Mobility Total Ankle System (DePuy) became commercially available in 2004 and is currently being compared to the Agility in a multi-center FDA trial [14] (Figure 11.17). The device is an unconstrained, three-component design with a UHMWPE mobile bearing [52]. The design of the tibial component resembles the Buechel-Pappas because of the conical stem that protrudes from the superior surface, although the sagittal plane dimensions of the Mobility’s tibial component are larger than those of the Buechel-Pappas and is tapered posteriorly to avoid overhang and soft tissue impingement [14]. The talar component can be inserted while retaining the medial and lateral aspects of the talus, similar to the Buechel-Pappas [14]. The inferior surface of the UHMWPE component has a ridge that conforms to a

groove on the talar component, and the superior surface is flat and conforms to the flat articulating surface of the tibial component. The constraint offered at the talar articulating surface is therefore dissipated by the tibial articulating surface. The UHMWPE component is manufactured from 1050 UHMWPE powder, which is machined from extruded bar and sterilized using gamma irradiation under vacuum (GVF) [92]. Mechanical and wear testing demonstrated that the Mobility compares favorably with the Buechel-Pappas implant [14]. Similar loads were required to cause dislocation of the UHMWPE component, although the Mobility dislocated less often and generated less UHMWPE wear [14, 92]. Little is known about the clinical performance of this device.

11.4.3.8  BOX The BOX prosthesis derived its name from its designers from the Instituti Ortopedici Rizzoli, Bologna, Italy, and the laboratory in Oxford, England [14] (Figure 11.18). The design is a three-component device with an UHMWPE mobile bearing. The metal tibial and talar components are manufactured from CoCr with titanium spray coating on the surface [14]. Similar to the Buechel-Pappas design, the BOX resurfaces only the superior dome of the talus, and the sides remain intact [14]. There is a spherical interface between the articular surface of the tibial component and UHMWPE component, allowing rotation about all three axes [93]. There is a concave sulcus in the talar component, and the UHMWPE inferior surface is fully conforming with a bi-concave surface [93]. The size of the UHMWPE component varies in 1-mm steps from 5 mm to 8 mm [93]. The UHMWPE component is manufactured from compression molded sheet from PUR 1020, complies with ISO 5834: Implants for surgery— Ultra-high molecular weight polyethylene Part 1 (Powder form) and Part 2 (Moulded forms), and is sterilized using gamma irradiation in the presence of nitrogen [94]. Similar to all other three-component, mobile bearing designs, the BOX prosthesis is dependent on ligamentous stability to prevent dislocation of the UHMWPE component [93]. Clinical results are currently limited to few patients and short-term follow-up periods [93, 95].

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Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

Table 11.1  Contact Pressures for Different TAR Components Device

Type of study

Axial load (N)

Location

Contact pressure (MPa)

Agility [98]

FEM

3330

Talar surface

Peak: 26–36

Agility [98]

FEM

3330

Center

Peak: 20–24

Agility [99]

Cadaver

  700

Not indicated

Average: 5.6 Peak: 21.2

BOX [96]

FEM

1600

Tibial surface

Average: 6.4 Peak: 10.3

BOX [96]

FEM

1600

Talar surface

Average: 10.3 Peak: 16.1

STAR [97]

FEM

3650

Edges

20

STAR [97]

FEM

3650

Superior/inferior surfaces

8–10

11.5  UHMWPE loading and wear in total ankle replacements Because the ankle joint can experience loading in excess of five times body weight, the TAR needs to tolerate these high loads while maintaining a small contact area. As reviewed by Reggiani et al. [96] and summarized in Table 11.1, contact pressures in TARs have been well studied in cadaveric and finite element studies. Peak contact stresses for ankle replacements are summarized in Table 11.1. In a finite element study of the BOX prosthesis, the tibial and talar articulating surfaces had mean contact pressures of 6.4 and 10.3 MPa, respectively, with peak values of 10.3 and 16.1 MPa, respectively [96]. In a finite element study, the STAR component had larger contact pressures at the anterior, posterior, and internal edges compared to the superior and inferior surfaces (20 MPa versus 8–10 MPa, respectively) under 3650 N of compressive load [97]. In a finite element study of the Agility prosthesis, peak pressures occurred along the talar component edges [98]. Contact pressures in this region ranged from 26 to 36 MPa, with contact pressures in the center ranging from 20 to 24 MPa (a reduction in contact pressure was observed by increasing the width of the talar component) [98]. It has been postulated that the dominant mode of wear for first-generation, incongruent TARs was related to high local stresses occurring approximately 1 mm below the surface [100]. The theory proposed that points of high stress initiate cracks below the surface, and coalescence of these cracks can produce pitting, delamination, and finally propagation of crack(s) throughout the thickness of the UHMWPE component [100]. TARs with congruent surfaces were intended to mitigate these failure mechanisms of the UHMWPE components. However, clinical research of total knee replacements does not support this theory.

Conforming and nonconforming total knee replacement designs experience peak stresses that may be sufficiently high to produce localized yielding and deformation of UHMWPE inserts [101]. However, these designs exhibit successful clinical performance [102, 103]. While data related to contact stress is useful, it is not the sole governing entity with respect to failure of UHMWPE components. Factors that influence the oxidation of the UHMWPE (e.g., sterilization, type, packaging), in combination with load distribution, affect its performance. Improvements in TAR UHMWPE component clinical performance likely occurred not only from the development of congruent designs that minimized contact stress but also improvements in UHMWPE sterilization and packaging (e.g., the STAR device, which was gamma sterilized in air and is currently sterilized in a nitrogen-vacuum). In light of the history of UHMWPE sterilization in other orthopedic devices, as well as cell culture studies that have been conducted, devices that are gamma sterilized in nitrogen (e.g., STAR, Mobility, BOX) benefit from some degree of crosslinking that occurs from the gamma irradiation, which could improve the wear properties of UHMWPE. However, the free radicals formed in the UHMWPE during irradiation increase the component’s susceptibility to in vivo oxidative degeneration, which subsequently alters the physical, chemical, and mechanical properties of UHMWPE. Barrier packaging and sterilization in an inert environment (e.g., nitrogen-vacuum sterilization of the STAR device, GVF packaging of the Mobility) address the risk of oxidative degeneration during shelf storage. Other sterilization methods (e.g., gas plasma, which is used in the Agility total ankle prosthesis) may improve UHMWPE wear properties without introducing free radicals that can lead to in vivo oxidation. While it has

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been demonstrated that sterilization via ethylene oxide (as is used in the contemporary Buechel-Pappas device) does not alter the physical, chemical, and mechanical properties of UHMWPE, ethylene oxide sterilization is not associated with improved wear rate that is observed with gamma irradiation (refer to Chapter 3). In a wear simulation study comparing the BuechelPappas device with the Mobility, Bell and colleagues utilized a knee wear simulator to evaluate wear in TAR prostheses [92]. The UHMWPE components for the Buechel-Pappas devices were manufactured from 1150 UHMWPE powder, machined from extruded bar, and sterilized using gas plasma. The Mobility UHMWPE insert was manufactured from 1050 UHMWPE powder, machined from extruded bar, and sterilized using gamma irradiation under vacuum (GVF). The components were cyclically loaded under 3.1 kN maximum load using a load profile that mimicked the gait cycle. After the first 5 million cycles (MC), where there was no anterior/ posterior displacement, the mean wear rate for the BuechelPappas device was (mean  95% confidence interval) 10.36  11.8 mm3/MC, and for the Mobility it was (mean  95% confidence interval) 3.38  10.0 mm3/MC. After introducing anterior/posterior displacement from 5 MC to 6 MC, wear rate significantly increased for the Buechel-Pappas TAR (16.4  17.4 mm3/MC) and the Mobility (10.4  14.7 mm3/MC). There was no significant difference in wear rate noted between the two designs at any time point, although this may have been due to the small sample sizes (n  3). The Mobility had wear scars that ranged from 78% to 86% of the surface area, while the wear area for the Buechel-Pappas device was 100% of the contact area. More backside wear occurred on the Mobility design compared to the Buechel-Pappas design. The authors conclude that the observed trends from this study may have been due to differences in UHMWPE formulation, where the 1150 UHMWPE in the BuechelPappas design may have had a slightly higher wear rate than the nonstearate containing 1050 UHMWPE in the Mobility. They also suggest that the larger contact area in the Buechel-Pappas design may be susceptible to greater wear via a mechanism of “lubrication starvation.” A similar study was conducted for the BOX prosthesis, where a knee wear simulator was used to evaluate the wear rate of the prosthesis [94]. Loading consisted of plantardorsiflexion being applied to the tibial component above, while anterior-posterior displacement and internal-external rotation were imposed on the talar component, while the support of the talar component was self-aligning along the medial-lateral displacement and varus/valgus rotation. The maximum axial load was 1600 N, and the load profile was designed to be similar to walking with 10 to 20 degrees of dorsi-plantarflexion, 2.6 to 7.7 degrees internal/ external rotation, and 0 to 8.45 mm of anterior-posterior displacement for 2 million cycles (MC) at 0.9 Hz. Tests

UHMWPE Biomaterials Handbook

were conducted in deionized water at 37°C. The linear penetration rate was 0.0178, 0.0081, and 0.0339 mm per MC for the three specimens. The average wear at 2 MC was 37.2 mm3 (range 15.0–63.3 mm3), corresponding to a wear rate of 18.6 mm3/MC (range: 7.5–31.7 mm3/MC). The variability in wear rate corresponded to the variability seen in wear patterns, where two components demonstrated predominant scratches and polishing in the anterior-posterior direction due to sliding, and the third demonstrated nearly no wear on the most anterior third of the tibial component. The differences in wear pattern may have been due to effects of device positioning or potential damage during the calibration phase. During an in vivo study of the size and shape of wear particles in TARs, synovial fluid was aspirated at least 6 months postoperatively from 15 patients with TAR implants and 11 additional patients with posterior stabilized total knee replacements [104]. The TAR designs included the Agility (n  4) and STAR (n  11). The two TAR designs generated UHMWPE wear in similar concentrations, size, and morphology. UHMWPE particles from TARs were significantly rounder compared to total knee replacement. There was also no significant difference between the concentration of UHMWPE particles in TAR patients compared to patients with total knee replacements. The authors acknowledge that the capacity of the ankle joint differs from the knee joint, which may influence the body’s response and potential for osteolysis in the presence of similar concentrations of UHMWPE particles. The few wear studies that have been conducted on TARs highlight the need for more of this type of testing, as well as the need for a standardized test method. The two studies that were published demonstrated comparable wear rates (mean values approximately 10 to 20 mm3/MC), though there was high interspecimen variability. The work of Bell et al. demonstrated how modifications to a loading profile can substantially affect the amount of wear that is generated [92]. A standardized testing protocol would allow for more direct comparisons of wear rates across different devices tested by different research groups. For example, the work of Bell et al. suggests that a larger contact surface does not necessarily result in a reduction in wear; repeatable experimental methods utilizing similar loading profiles, lubricants, and number of loading cycles would allow for a better evaluation of the effects on device design and UHMWPE type and sterilization method.

11.6  Complications and retrieval analysis Intraoperative complications for TARs include injury to neurovascular or soft tissue structures, malpositioning, improper sizing, excessive bone resection, and/or malleolar fractures [15]. Early postoperative complications

Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

include infection, impaired wound healing, swelling, stress fractures around the medial malleolus, and/or syndesmotic nonunions (applies to Agility only) [15]. Delayed postoperative complications include deep infection, periprosthetic radiolucencies, aseptic loosening/subsidence, periprosthetic fractures, UHMWPE wear with osteolysis, migration or fracture, heterotopic bone formation, syndesmotic nonunions, and reflex sympathetic dystrophy [15]. UHMWPE wear and aseptic loosening are among the most common complications, as well as wound healing problems and deep infection [15]. A major late complication is failure secondary to loosening [15]. For three-component devices, dislocation of the UHMWPE component may result in altered biomechanics of the prosthesis and accelerated generation of wear particles. As previously reviewed, UHMWPE wear particles in TAR patients were found in similar concentrations as knee replacement patients [104]. This may suggest that the occurrence of osteolysis in TARs could be expected to be similar to that in total knee replacements; however, the effect of the same concentration of UHMWPE particles on the relatively smaller ankle joint remains to be seen. Although the potential for dislocation and subsequent accelerated wear exists for three-component, mobile designs, the added degrees of freedom allows for more physiologic ranges of motion. Ligamentous stability and correction of varus/valgus deformities are important aspects of preventing dislocation of the UHMWPE component. In two-component (fixed-bearing) designs, the partial conformity at the single articulation increases stability but also increases contact stress and wear. These issues, along with considerations regarding the amount of bone resection required for a given design, should be considered when choosing which TAR is appropriate for a particular patient.

11.7  Conclusion Many lessons have been learned since first-generation TARs were implanted in the 1970s. Improvements to the first-generation designs have resulted in better loading conditions for the UHMWPE component, and changes to the sterilization methods and packaging have reduced the propensity of UHMWPE oxidative degeneration. Similar to other total joint replacements, a balance between stability and natural joint motion is desired. Improvements to TAR designs and surgical technique will continue to improve the survival and clinical performance of ankle replacements.

11.8  Acknowledgments This chapter was not written with the financial support of any manufacturer of total ankle replacements. However, some of the manufacturers whose products are mentioned

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in this review were contacted by the author and given the opportunity to verify the factual accuracy of the information related to their products. Thanks to Chris Espinosa and Michael Drzal of Exponent, Inc.’s Visual Communication Practice for their assistance with figures and photographs. The authors are especially grateful to Hina Patel and Alexis Cohen for their editorial assistance with this chapter.

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Chapter  |  11  Applications of UHMWPE in Total Ankle Replacements

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