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    Mobile-Bearing Knees: The Answer to Osteolysis

    The mobile-bearing knee design is a successful concept developed over the last 3 decades to meet the increased demands of younger, more active patients undergoing total knee arthroplasty.

    Authors

    Charlie C. Yang, MD, and Douglas A. Dennis, MD

    Introduction

    While early total knee arthroplasty (TKA designs) were generally reserved for older and sedentary patients with debilitating pain and functional loss, the excellent 10- to 15-year outcomes with TKA [1-7] have encouraged many surgeons to consider TKA for younger patients who have increased activity requirements, performance expectations, and longevity expectations.

    To meet these demands, future TKA designs must reduce articular bearing surface wear and subsequent osteolysis while maintaining the excellent long-term fixation typically obtained in properly aligned and balanced TKAs currently in use today. One successful concept developed over the last three decades to meet the increased demands is the use of mobile-bearing knee implants.

    Advantages of Mobile-Bearing Knees

    Premature polyethylene wear and secondary osteolysis are a major cause of TKA failure and have been attributed to numerous factors, including:

    • Poor surgical technique
    • Reduced polyethylene thickness [8,9]
    • Poor locking mechanisms of modular fixed-bearing tibial components [10-12]
    • Gamma irradiation sterilization in the presence of oxygen [13-15]
    • Use of low-conformity implant designs

    Use of mobile-bearing implants potentially reduces polyethylene wear by providing increased implant conformity in the sagittal and coronal planes, thereby reducing polyethylene contact pressure and cross shear stresses [16].

    • By increasing sagittal plane conformity in mobile-bearing knees, in vivo fluoroscopic analyses have demonstrated improved control of anterior-posterior translation with reduced paradoxical anterior femoral translation, particularly when tested during gait [17].
    • The increased coronal plane conformity typically present in mobile-bearing knees increases contact area and lessens increased contact stresses which are present if femoral condylar lift-off occurs [8,18,19].

    The increased conformity and reduction in contact stresses in mobile-bearing designs has been shown to substantially lower polyethylene wear in numerous evaluations [20-23]. McEwen et al [22] noted a more than four-fold reduction in wear in knee simulator testing of a rotating platform TKA vs. a fixed-bearing design with identical femoral component geometry when tested under “high kinematic” conditions typically found in the younger, more active patient (Figure 1).

    Figure 1. Histogram of a high kinematic knee simulator analysis demonstrating polyethylene wear (mg) per million cycles in the mobile vs. fixed bearing PFC Sigma TKA (Depuy, Inc.). (With permission, reference 35)

    To avoid high polyethylene stresses typically observed with highly conforming fixed-bearing implants, rotational bearing mobility must be maintained. Two in vivo fluoroscopic kinematic studies of rotating platform TKA detected bearing mobility in all subjects during a deep knee bend maneuver; this was maintained at a 5-year follow-up [24,25].

    Most rotation occurred at the polyethylene bearing-tibial tray interface, with the bearing typically “following” the rotation of the femoral component. Rotation of the rotating platform polyethylene insert with the femoral component, independent of the rotation of the firmly fixed tibial tray, reduces torsional stresses transmitted to the fixation interface [26] and creates the potential for self-alignment of the polyethylene bearing with the femoral component. This is supported by excellent long-term clinical results in numerous studies of mobile bearing TKA which report revision rates for aseptic loosening to be as low as 0-0.2% [2,27].

    Advantages of bearing self-alignment include:

    • Maintenance of large centrally located surface contact areas at the femoral-tibial articulation during both flexion-extension and axial rotation of the knee [21]
    • Facilitation of central patellar tracking [28]
    • Reduction of stresses transmitted to posterior cruciate substituting tibial posts

    In fixed-bearing knees, the tibial tubercle is lateralized if substantial internal rotation of the tibial component relative to the femoral component is present, which enhances the risk of patellar subluxation. A rotating platform design permits greater self-correction of component rotational malalignment, allowing better centralization of the extensor mechanism.

    This is supported by a review of lateral retinacular release rates in 1,318 consecutive TKAs (378 fixed bearing; 940 rotating platform) performed by the senior author. The incidence of lateral retinacular release was 14.3% in patients implanted with a fixed-bearing knee versus 5.3% in the rotating platform group (p < 0.001) [29].

    Bearing rotation lessens rotational impingement and wear on posterior cruciate stabilizing posts, which has been a problem reported in fixed-bearing implants [30]. Nakayama et al [31] measured contact area and polyethylene stresses on stabilizing posts of multiple fixed and mobile-bearing TKA designs. The femoral and tibial components were in ideal alignment, with the tibial component internally rotated 10° relative to the femoral component. When components were not in ideal alignment, the highest contact area and lowest post stresses were observed in mobile bearing implants.

    Disadvantages of Mobile-Bearing TKA

    Concerns expressed with use of mobile-bearing implants include:

    • The need for a more exacting surgical technique
    • The occurrence of bearing instability [2,32,33]
    • The risk of enhanced polyethylene wear resulting from creation of a second articulating surface
    • The hypothesis that microparticulate wear debris created from the undersurface articulation of mobile-bearing TKA designs will be smaller and have greater potential to create osteolysis

    The surgical goals and techniques utilized for implantation of a mobile-bearing knee, such as soft-tissue balancing, creation of equal flexion and extension gaps, and precise component positioning, are no different than those utilized during implantation of fixed-bearing TKA systems. Extension and flexion gap balance is of particular importance during implantation of a mobile-bearing TKA because imbalance risks bearing dislocation.

    The authors have found that the use of some type of tensioning instrument, such as laminar spreaders, spacer blocks, and a specific gap-tensioning device, provides the most reproducible balance and tension of the extension and flexion gaps. They have performed over 3000 rotating platform TKA using this methodology without a bearing dislocation. With use of these modern tensioning techniques, bearing instability has been minimized with several recent evaluations reporting an incidence of 0-2.2% [2,28,34].

    Currently, backside polyethylene wear has not emerged as a clinically significant issue with use of rotating platform TKA designs. Studies examining the undersurface of retrieved rotating platform polyethylene inserts have reported minimal visual evidence of significant undersurface wear [34,35].

    Explanations for the lack of clinically significant backside wear include the decoupling of multidirectional motions occurring at the articular interfaces with rotating platform TKA designs [12] and the high contact area (typically >700mm²) present at the inferior mobile articulation,n [36] which has been shown to generate mean subsurface polyethylene stresses of less than 8 MPa when subjected to forces up to five times body weight (Figure 2) [32,36].

    Figure 2. Contact area analysis of the superior and inferior aspects of a rotating platform TKA demonstrating the high contact areas (mm2) present at the mobile (undersurface) interface throughout knee flexion. (With permission, reference 42)

    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 anterior-posterior translation, the inferior or tibial tray-polyethylene articulation experiences purely rotational (unidirectional) motion patterns. The polyethylene bearing primarily tracts with the femoral component. [19,24,25,33] This allows the superior articular surface to primarily experience flexion-extension (unidirectional) motion because rotation is occurring on the inferior aspect of the bearing.

    Pooley and Tabor [37] reported when polyethylene is subjected to unidirectional sliding, the molecules align along the direction of motion, lowering the coefficient of friction and reducing wear of the material. Conversely, when polyethylene is exposed to multidirectional wear patterns, increased cross-shear stresses are created, which enhance wear.

    Therefore, rotating platform TKA designs can reduce wear by decoupling multidirectional motions to unidirectional motion patterns occurring at two differing interfaces, reducing cross-shear stresses and wear at both interfaces.

    The fear that microparticulate debris created from a mobile-bearing TKA design will be smaller and more osteolytic is not supported, based on the recent analysis of Brown et al [38], who analyzed the number, size, and osteolytic potential of microparticulate debris created in fixed vs. rotating platform knees.

    No difference in particle size, and therefore no difference in osteolytic activity of the microparticulate debris, of fixed- vs. mobile-bearing knees was observed. The fixed-bearing group demonstrated a higher functional osteolytic potential because the magnitude of microparticulate debris created in fixed bearing TKA was over four times higher.

    Summary

    Bearing mobility in TKA reduces polyethylene wear by providing increased implant conformity in the sagittal and coronal planes, thereby reducing polyethylene contact pressure and cross shear stresses [16]. Bearing rotational freedom assists in maintaining alignment of the patellofemoral and femorotibial articulations throughout knee flexion.

    Self-alignment via polyethylene bearing rotation improves kinematics, lessens polyethylene surface stresses and minimizes stabilizing post impingement, increasing the potential for enhanced polyethylene longevity and a lower incidence of osteolysis.

    Author Information

    Charlie C. Yang, MD, is with Colorado Joint Replacement, Denver, Colorado. Douglas A. Dennis, MD, is with Colorado Joint Replacement, Denver, Colorado; the Department of Biomedical Engineering, University of Tennessee, Knoxville, Tennessee; the Department of Bioengineering, University of Denver, Denver, Colorado; and the Department of Orthopedics, University of Colorado School of Medicine, Denver, Colorado.

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