CORR Insights®: Bearing Dislocation and Progression of Osteoarthritis After Mobile-bearing Unicompartmental Knee Arthroplasty Vary Between Asian and Western Patients: A Meta-analysis

Abstract

Where Are We Now? Unicondlyar knee arthroplasty (UKA) implants are generally nonconstrained, allowing for more-normal rollback and rotation of the knee during knee flexion. Mobile-bearing UKA implants provide a fully conforming bearing surface between the femoral component and tibial insert that allows AP movement and rotation between the tibial insert and tibial baseplate. By contrast, fixed-bearing UKA implants permit AP and rotational movement between the femoral component and relatively flat tibial bearing surface. The fully conforming nature of mobile-bearing UKA implants results in greater ultra-high-molecular-weight polyethylene (UHMWPE) contact area and lower contact stresses than fixed-bearing UKA, which has been associated with low UHMWPE wear in vivo [6, 12]. And although mobile-bearing UKAs have demonstrated excellent long-term survivorship [10], they also have a risk of bearing dislocation, which can result in revision surgery [10, 17]. Prior to the mid-1990s, UHMWPE implants were sterilized by gamma irradiation in air, resulting in oxidative degradation and decreased wear resistance of the polymer [14], and during this time, studies reported excellent long-term survivorship and low wear for mobile-bearing UKA implants [4, 12]. But after gamma irradiation in air sterilization was abandoned as a solution to polyethylene oxidation [9], alternative sterilization methods emerged, including ethylene oxide, gas plasma, and gamma irradiation in an inert atmosphere. Crosslinking, which is currently used in most hip and many knee implants [15], can reduce UHMWPE wear. The benefit of mobile-bearing UKA in reducing UHMWPE wear compared to fixed-bearing implants may be less important as improvements have been developed in UHMWPE sterilization and processing. Mobile-bearing TKAs are also associated with lower bone-implant interface stresses compared to relatively conforming fixed-bearing implants [2]. Cementless mobile-bearing UKA has been used successfully, which may be related to the reduced bone-implant interface stresses of these implants [16]. But since most of the currently available fixed-bearing UKA implants have relatively flat nonconforming surfaces, we may not find clinically relevant differences in interface stresses between the two implant types. Indeed, both mobile- and fixed-bearing UKA implants include gap balancing, computer navigation, and robotics, and currently available implant materials include more wear-resistant UHMWPE and abrasive-resistant counterface surfaces than what has been available in the past. In the current study, Ro and colleagues [13] demonstrated that the causes for revision of mobile- and fixed-bearing UKA differ between Asian and Western patient populations. The authors note that Western patients have a higher risk of revision because of lateral compartment osteoarthritis (OA) progression than do Asian patients after medial UKA, perhaps suggesting that patient selection criteria may be more important for Western than Asian patients in deciding between UKA and TKA. The authors also suggest that the greater flexion activities in Asian patients may result in a greater risk of bearing dislocation. In this context, patients with greater knee flexion activities may have a more-favorable risk-benefit ratio with use of fixed- rather than mobile-bearing UKA. Lateral mobile-bearing UKA implants have been shown to have a relatively high risk of dislocation in clinical studies, which is consistent with the greater AP excursion and rollback in the lateral compared to the medial tibiofemoral compartment and supports the findings in the current study that more knee motion is associated with greater risk of bearing dislocation [17]. Where Do We Need To Go? When gamma irradiated in air UHMWPE was used in total joint arthroplasty, highly conforming mobile bearings offered an advantage compared to fixed bearings in reducing risk of delamination wear-related failures [8]. However, delamination wear has been