Failure in the initial years after total knee arthroplasty (TKA) was frequently secondary to aseptic loosening and often associated with malalignment, instability, or use of implants with excessive prosthetic constraint.66 Loosening rates have been minimized with improvements in surgical instrumentation and operative techniques (eg, intramedullary alignment guides, ligamentous balancing techniques and a better understanding of ideal alignment parameters) and the use of lower conformity prosthetic devices.19,25,61 However, low conformity TKA designs can result in reduced polyethylene contact area and premature polyethylene wear, with periprosthetic osteolysis as a predominant failure mode.63 The excellent 10 to 15 year TKA results of some clinical reports13,14,19,61 have encouraged many surgeons to perform TKA on younger patients who have increased activity requirements and longevity expectations, which further increases the risk of failure from premature polyethylene wear.18,28,36 One design innovation created to reduce polyethylene wear has been the introduction of mobile bearing TKA designs. Controversy surrounding polyethylene wear reduction with mobile bearing TKA has focused on the potential of enhanced polyethylene wear resulting from creation of a second articulating surface.49
We reviewed clinical and basic science studies focused on evaluating factors affecting polyethylene wear after TKA and discuss design and technical issues of mobile- bearing TKA designs reported to reduce wear.
Polyethylene Wear Factors
Reduced polyethylene wear after TKA requires use of meticulous surgical technique, avoidance of kinematic patterns known to accelerate polyethylene wear, and innovative prosthetic design improvements. Operative strategies include precise ligament balancing, reproducing anatomic extremity alignment, restoring the proper joint line level, and assuring symmetry and balance of the flexion and extension gaps. Proper attention to each criterion will encourage more uniform loading of the articular surface rather than placing excessive loads on either the medial or lateral aspects of the polyethylene surface.
In vivo fluoroscopic studies of multiple implant designs after TKA have shown kinematic variances from normal knees, including paradoxical anterior femoral translation during deep knee flexion,2,20,21 reverse axial rotational patterns,21,24,27,64 and femoral condylar lift-off.21,23,62,64 Paradoxical anterior femoral translation of the femoral component on the tibial polyethylene surface during deep flexion increases subsurface polyethylene shear forces and risks accelerated polyethylene wear.4,10 Femoral condylar lift-off creates excessive loads on both the polyethylene bearings, risking premature polyethylene wear, and increased load transmission to the subchondral bone,23,45 enhancing the chance of prosthetic loosening. These are amplified in TKA designs that have reduced conformity in the coronal plane (ie, flat-on-flat designs) because of edge loading of the prosthetic components.4,23,45
Design factors shown to reduce polyethylene wear include thicker polyethylene bearings,4,5 improvements in polyethylene locking mechanisms of modular tibial components,29,56,67 use of better sterilization techniques,3,9,69 TKA designs with increased articular surface conformity,13,15,19 and mobile-bearing TKA systems.13,15,50
Contact stress studies have demonstrated rapidly increasing polyethylene stresses with decreases in polyethylene thickness.4,5 Current recommendations are to utilize tibial polyethylene inserts with a minimum thickness of 8 mm to avoid rapid increases in polyethylene stresses associated with accelerated polyethylene wear.4
Wear on the inferior surface of modular fixed bearing polyethylene inserts (ie, backside wear) is common and contributes to microscopic polyethylene particle generation and osteolysis.29,56,67 The amount of debris released from this backside articulation has been estimated to be 2 to 100 times greater than the debris generated at the femorotibial articulation.56 These findings initiated design changes to improve the rigidity of modular locking mechanisms and minimize undersurface wear in the presence of some degree of unavoidable backside micromotion (eg, cobalt-chromium tibial trays and highly polished modular baseplate surfaces).
Gamma irradiation sterilization techniques in the presence of oxygen are associated with accelerated polyethylene wear ostensibly from increased oxidation of polyethylene and disruption of polyethylene polymer chains secondary to oxidative chain scission.3,9,69 Numerous polyethylene sterilization techniques have been introduced to reduce polyethylene oxidation and the associated polyethylene degradation and wear, including use of gamma irradiation in an inert environment (inert gas or vacuum), ethylene oxide, or gas plasma sterilization techniques.3,69 In an analysis of retrieved tibial polyethylene components, Williams et al69 demonstrated a reduction (p < 0.01) in polyethylene oxidation (0.009 versus 0.080 mm/year) and an 84% reduction (p < 0.01) in wear penetration rate (0.09 versus 0.55mm/year) when utilizing ethylene oxide sterilization compared to components sterilized with gamma irradiation in air.
Increasing implant conformity (ie, a measure of the radius of curvature of two opposing articulating surfaces) also reduces contact stresses polyethylene wear.4-6 The higher the conformity of two aligned articular surfaces, the greater is the contact area, the less the subsurface polyethylene contact stress per unit area, resulting in a reduced potential for polyethylene wear. One analysis suggests contact stresses are reduced as contact area is increased at least until a contact area of 300 to 350 mm2 is reached59; increasing contact area beyond this level reduced contact stresses further but to a much lesser extent (Fig 1). Increasing coronal plane conformity is the most critical plane to reduce peak polyethylene stresses.4,45 This is particularly true in the presence of femoral condylar lift- off.4,26,45
The native knee is much more complicated than a simple hinge joint, exhibiting complex motion patterns involving coronal and sagittal plane translation and axial rotation. Biomechanical studies have shown highly conforming fixed-bearing TKA designs can be intolerant of higher rotational and anteroposterior (AP) translational kinematic motion patterns commonly encountered after TKA, with excessive polyethylene stresses frequently observed.16,26,45,47,65 Subsurface polyethylene stresses experienced in conditions of malalignment are substantially higher in highly conforming fixed-bearing TKA systems than in less conforming fixed or highly conforming mobile bearing TKA designs.16,47,65 To capitalize on the benefits of a highly conforming articular interface, alternative design concepts have been proposed such as mobile-bearing TKAs.
Theoretical Mobile Bearing Advantages
Mobile-bearing TKA designs offer the advantage of allowing increased implant conformity and contact area without dramatically increasing stresses transmitted to the polyethylene material or fixation interface. The incorporation of polyethylene bearing mobility minimizes the transfer of torsional stresses to the fixation interface that have been associated with failure of fixed bearing TKA implants.41 This is supported by excellent long-term (9-20 years) clinical results in several studies of mobile-bearing TKA with revision rates for aseptic loosening between 0 to 0.2%.12-15
By increasing sagittal plane conformity in mobile- bearing TKA, in vivo fluoroscopic analyses have demonstrated improved control of AP translation with reduced paradoxical anterior femoral translation, particularly when tested during gait.20 The increased coronal plane conformity typically present in mobile-bearing TKA increases the contact area and lessens the increased contact stresses which are present if femoral condylar lift-off occurs.4,26,40,45
The increased conformity and subsequent reduction in contact stresses in mobile-bearing designs substantially lower polyethylene wear in numerous studies.4,26,48,53,65 Greenwald33 demonstrated contact areas of mobile- bearing TKA during gait range from approximately 400 to 800 mm2, which keeps contact stresses at 14 MPa or less (Fig 2). These contact areas are substantially greater than is typically seen in most fixed bearing TKA designs (200- 250 mm2). Finite element evaluations demonstrating reduced polyethylene contact stresses as a direct result of increased contact area further support these findings.6,52,53 The advantage of this increase in contact area is reflected in knee simulator wear studies of fixed versus mobile- bearing TKA. McEwen et al48 noted over a four-fold reduction in wear in knee simulator testing of a rotating platform TKA versus a fixed-bearing design of identical femoral component geometry (Fig 3).
Rotational bearing mobility must be maintained to avoid high polyethylene stresses typically observed with highly conforming fixed-bearing TKA. To investigate the presence of bearing mobility, we conducted two in vivo fluoroscopic kinematic studies involving 39 patients implanted with four differing mobile-bearing TKA designs (PFC Sigma Posterior Stabilized Rotating Platform, LCS Rotating Platform, LCS Deep Dish Rotating Platform, and LCS Anterior-Posterior Glide TKA; DePuy Inc, Warsaw, IN).22,43 Because the polyethylene bearing is transparent during fluoroscopy, four metallic beads were inserted at known positions into each polyethylene bearing by the implant manufacturer. Motions at the various articulating interfaces (femoral component-polyethylene bearing, femoral component-tibial tray, and polyethylene bearing- tibial tray) were determined using a computer-assisted model-fitting algorithm.35 Polyethylene bearing mobility was detected in all patients during deep knee bending. The majority of axial rotation in these rotating platform designs occurred at the polyethylene bearing-tibial tray interface, with the polyethylene bearing typically following the rotation of the femoral component. This finding has also been observed in independent in vitro27 and in vivo studies30 of bearing mobility after TKA.
Rotation of the rotating platform polyethylene insert with the femoral component, independent of the rotation of the firmly fixed tibial tray, should reduce stresses transmitted to the fixation interface and create the potential for self-alignment of the polyethylene bearing with the femoral component. Self-alignment is advantageous for optimal TKA kinematics and for maintenance of acceptable polyethylene surface stresses and stresses exerted on posterior cruciate-substituting tibial posts. This self-aligning behavior with a highly conforming design has been shown to maintain large, centrally located surface contact areas at the femorotibial articulation during both flexion-extension and axial rotation of the knee,65 which is more difficult to achieve in fixed-bearing TKA designs. An additional advantage of the self-aligning feature of rotating platform TKA systems is the potential facilitation of central patellar tracking.15 In a fixed-bearing TKA, if substantial internal rotation of the tibial component relative to the femoral component is present, the tibial tubercle is lateralized, enhancing the risk of patellar subluxation. A rotating platform design, through bearing rotation, permits greater self- correction of component rotational malalignment, allowing better centralization of the extensor mechanism. This is supported by the in vivo fluoroscopic studies by Rees et al57 which report patellofemoral kinematics of mobile- bearing TKAs more closely resembled the normal knee than fixed-bearing TKAs. The benefit of improved patellar tracking, however, was not found in one randomized study of fixed versus mobile-bearing TKA.54
One in vivo fluoroscopic evaluation of more than 1000 TKAs incorporating 33 different fixed and mobile-bearing TKA designs demonstrated most experienced less than 10° of axial rotation with normal postoperative activities.24 However, in this large multicenter analysis, a number of subjects experienced either normal or reverse axial rotational magnitudes greater than 20°, which are beyond the rotational boundaries of most fixed-bearing TKA designs.24 This is an advantage for rotating platform TKA designs that can accommodate a wider range of axial rotation without creating excessive polyethylene stresses.
Studies examining the contribution of posterior cruciate-substituting (PS) polyethylene post wear to TKA failure have shown excessive axial rotation in PS fixed- bearing designs can predispose to premature polyethylene wear and compromise the integrity of the central post due to post impingement.17,44,55 The freedom of rotation present in rotating platform designs allow them to adapt to a greater range of axial rotation without creation of rotational impingement and wear on posterior cruciate- stabilizing posts. Nakayama et al51 measured contact area and polyethylene stresses on posterior cruciate- substituting posts of multiple fixed and mobile-bearing TKA designs with the femoral and tibial components in ideal alignment and with the tibial component internally rotated 10° relative to the femoral component. When the femoral and tibial components were not in ideal alignment, the highest contact area and lowest post stresses were observed in mobile bearing implants. This suggests rotational post impingement in PS TKA systems can be reduced in rotating platform designs due to post rotation with the femoral component intercondylar box rather than attempts to rotate against it.
The rotational freedom provided in mobile-bearing TKA assists in maintaining alignment of both the patellofemoral and femorotibial articulations throughout knee flexion. Self-alignment via polyethylene bearing rotation improves postoperative TKA kinematics, lessens polyethylene surface stresses and minimizes PS post impingement, increasing the potential for enhanced polyethylene longevity.
Fears Associated with Mobile Bearing TKA
Despite the many theoretical and demonstrated advantages of use of a mobile-bearing TKA, several concerns have been expressed with their use. These include the need for a more exacting surgical technique, bearing instability,7,13,38 a risk of enhanced polyethylene wear resulting from creation of a second articulating surface,39,49 and a concern the microparticulate wear debris created from the undersurface articulation of mobile bearing TKA designs will be smaller and have greater potential to create osteolysis.39,49
The surgical goals and techniques (alignment, bone re- sections, and ligamentous balancing) for implanting a mobile-bearing TKA are theoretically no different from preparations used for fixed-bearing TKA systems. Soft tissue balancing, creation of equal flexion and extension gaps, and precise component positioning are extremely important in fixed and mobile-bearing TKA systems. Extension and flexion gap balance is of particular importance in use of a mobile bearing TKA because imbalance risks bearing dislocation or spin out where the polyethylene bearing is no longer congruous with the femoral component. Gap balance can be achieved by several methods. We have found some type of tensioning instruments (eg, laminar spreaders, spacer blocks, or a specific gap tensioning device) provide the most reliable and reproducible balance and tension of the extension and flexion gaps. Specific gap tensioning devices provide an additional advantage of facilitating equalization of the flexion gap width to the previously established extension gap. These tensioning devices have been specifically designed to allow measurements (width and tension) obtained from a balanced extension gap to determine and direct flexion gap bone resections and femoral component rotation. Instability of mobile polyethylene bearings most commonly occurs at flexion magnitudes greater than 60° in which the flexion gap is either asymmetric or loosely tensioned. Bearing dislocation (meniscal bearings) or spin out (rotating platform) occurred more commonly in the early years of mobile-bearing TKA use when the importance of flexion/ex- tension gap balancing and femoral component rotation were less understood and emphasized.7,13 With the use of modern tensioning techniques, bearing instability has been minimized with several recent evaluations reporting an incidence of 0% to 2.2% in primary TKA up to 20 years postoperatively.13-15,38 Ideally, when implanting a mobile- bearing TKA, laxity of 1 to 2 mm medially and laterally with varus and valgus stress testing in flexion and extension is ideal.
The additional articulation at the undersurface of the polyethylene bearing has raised concerns about the generation of additional polyethylene particles and accelerated wear. As previously discussed, with fixed-bearing TKA systems, backside polyethylene motion against a metal surface that is not designed to accommodate motion (ie, relatively rough surface) results in substantial polyethylene wear and subsequent periprosthetic osteolysis if the modular locking mechanism is not rigid.29,56,67 In rotating platform systems, a rotating, yet flat polyethylene bearing is matched against a flat, highly polished, cobalt- chromium surface with low surface roughness. Despite theoretical concerns, backside polyethylene wear has not yet emerged as a clinically major issue in rotating platform designs. Studies examining the undersurface of retrieved rotating platform polyethylene inserts have reported minimal visual evidence of undersurface wear.38,39
One explanation for the lack of clinically important backside polyethylene wear is the decoupling of multidirectional motions occurring at the articular interfaces with rotating platform TKA designs.48 In fixed-bearing systems, all rotational, translational, and flexion-extension motion patterns are experienced at a single (superior) articular surface, resulting in multidirectional motion pathways. In rotating platform designs which allow no AP translation, the inferior, or tibial tray-polyethylene articulation, experiences purely rotational (reciprocal/unidirectional) motion patterns. Because the polyethylene bearing primarily tracts with the femoral component,22,27,30,43 the superior articular surface (femoral component- polyethylene bearing interface) primarily experiences flexion-extension (reciprocal/unidirectional) motion as rotation occurs on the inferior aspect of the bearing. It has been shown when high density polyethylene is subjected to unidirectional sliding, the molecules align along the direction of motion, which lowers the coefficient of friction, reducing wear of the material.46 Conversely, when polyethylene is exposed to multidirectional wear patterns, increased cross shear stresses are created which enhance wear of polyethylene.41,46,48 Additional laboratory studies have shown the multidirectional shear stresses typically experienced at the single polyethylene interface in fixed- bearing systems may contribute to the generation of four to ten times the polyethylene wear experienced at the unidirectional interfaces in rotating platform designs.41,48,50 Therefore, rotating platform TKA designs can reduce polyethylene wear by decoupling multidirectional motions to more monodirectional motion patterns at two differing interfaces, thus reducing cross-shear stresses and wear.
In contrast to a purely rotating platform TKA design, some mobile-bearing TKA systems permit rotation and AP translation to occur on the inferior aspect of the polyethylene bearing.1,42 In these designs, the inferior aspect of the polyethylene bearing is exposed to multidirectional motion patterns. Close followup evaluation of these mobile bearing designs is merited to see if premature failure due to backside wear occurs.
Another explanation for minimal undersurface polyethylene wear is the high contact area (typically > 700 mm2) present at the inferior mobile articulation52 (Fig 4). This high contact area has been shown to generate peak sub- surface stresses of less than 17 MPa and mean subsurface stresses of less than 8 MPa at this articulation when subjected to forces up to five times bodyweight.47
The fear that microparticulate debris created from mobile-bearing TKA will be smaller and more osteolytic is not supported by the recent report by Brown et al.11 They analyzed the number and size, the osteolytic potential (individual reactivity of the debris created), and functional osteolytic potential (reactivity of the individual particles plus the actual number of particles created) of microparticulate debris created in fixed and rotating platform TKAs using a knee simulator evaluation. They observed no difference in particle size and no difference in biologic activity of the microparticulate debris of fixed versus mobile-bearing TKAs. The fixed-bearing TKA group demonstrated a higher functional osteolytic potential because the magnitude of microparticulate debris created in fixed bearing TKA was more than four times higher. Minoda et al49 performed an in vivo analysis of polyethylene wear particles retrieved from both fixed and mobile bearing TKA at a mean follow-up period of one year. They observed no statistical differences in particle number, shape, or size between the two groups.
Performing TKA in younger patients with increased life expectancies will undoubtedly amplify TKA failures due to excessive polyethylene wear. This necessitates evaluation of alternative technologies with potential to reduce long-term polyethylene wear such as the use of mobile bearing TKA. It is important to emphasize that currently available clinical studies of fixed and mobile bearing TKA over a period of use of 10 to 15 years have not yet demonstrated clinical superiority of one technology over the other.12-15,19,25,31,58 Actual comparative studies of fixed vs. mobile bearing TKA have yet to show significant clinical differences, but the follow-up duration of these investigations are limited to less than one decade.8,68 While the numerous basic scientific analyses discussed in this review provide substantial support for considering use of mobile bearing TKA in subjects with longer life expectancies and increased activity requirements, it will likely take more than 15 years of use to prove if these analyses are of clinical relevance.
Mobile-bearing TKA allows the incorporation of increased implant conformity without an associated increase in fixation interface stresses and resultant aseptic loosening. The increase in sagittal conformity creates more predictable and controlled AP motion during gait while increased coronal conformity prevents excessively high polyethylene stresses if femoral condylar lift-off occurs. The overall increase in conformity additionally increases surface contact area, decreases subsurface polyethylene stresses and should ultimately decrease polyethylene wear. Reduction in fixation stress despite increased conformity is supported by roentgenstereophotogrammetric analyses (RSA) which have shown reduced micromotion at the tibial component fixation interface.32,60
The rotating articulation is more forgiving of tibial component rotational malalignment and patient outliers who demonstrate excessive axial rotation after TKA. It facilitates some correction of patellar alignment through optimization of the Q-angle. Self-alignment of the polyethylene insert with the femoral component also minimizes medial and lateral tibial post wear in situations where a posterior cruciate substituting system has been utilized. While mobile-bearing TKA designs demonstrate a number of favorable features when compared to fixed- bearing systems, it is important to remember all mobile bearing systems are not the same. Differences exist both in condylar geometry and bearing mobility patterns. A purely rotating platform design has emerged as the most clinically successful among mobile bearing designs.13-15,37
Limitations of this review include the lack of long-term, randomized clinical studies comparing fixed and mobile bearing TKA. Additionally, clinical followup studies of sufficient duration (20-30 years) to answer the question if current TKA designs will meet the needs of the younger patient requiring TKA are not yet available. Lastly, it is important to emphasize that all mobile bearing TKA designs are not the same and may function differently in long-term evaluations. The majority of the available long- term results of mobile bearing TKA which have demonstrated excellent results have involved rotating platform designs which do not permit associated anteroposterior translation of the polyethylene bearing and, by definition, exhibit only unidirectional motion patterns on the inferior aspect of the bearing. Long-term analyses of mobile bearing implants which allow both rotation and anteroposterior translation are needed to see if the multidirectional motion patterns occurring on the inferior aspect of these bearings will result in accelerated polyethylene wear.
The kinematics of mobile-bearing TKAs are not perfect. There are still situations when femoral condylar lift- off and reverse rotational patterns occur27,64 and paradoxical anterior sliding during deep flexion can occur in non- stabilized designs.34 Additionally, paradoxical anterior translation of the polyethylene bearing in designs which permit anteroposterior translation has been reported.1 Future goals include the development of mobile-bearing TKA designs which create better control of bearing mobility patterns. Use of mobile bearing TKA designs should be reserved for those cases in which excellent balance of the flexion and extension gaps can be obtained to reduce the incidence of bearing instability. Recent introduction of new stabilized mobile bearing designs which provide for both posterior and collateral ligamentous stability may extend the indications of this technology.
The authors thank Kendall Slutzky for her help in manuscript preparation.
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