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Unicompartmental knee arthroplasty is an alternative to total knee arthroplasty for patients with unicompartmental tibiofemoral noninflammatory disease. Careful patient selection and newer instrumentation has reduced the progression of arthrosis in the other compartment and tibial loosening, leaving polyethylene wear as the predominant failure mechanism in more contemporary designs. Increased wear increases the debris volume at the bone-implant interface, and wear particles will generate osteolysis leading to component loosening and unreplaced compartment degeneration. The design-related factors that minimize wear include a polyethylene thickness of greater than 6 mm, a limited shelf age, and a design allowing large areas of contact mediolaterally and anteroposteriorly. Congruous mobile-bearing implants could play a substantial role in reducing wear if they are not associated with dislocation and nonreproducible surgical techniques. Important technical factors include accurate instrumentation avoiding component-to-component malposition and edge loading, allowing slight under-correction of the pre- operative deformity. The patient-related factors include a weight limit, a functional anterior cruciate ligament, and a correctable frontal deformity. Continued research including that related to cross-linking and sterilization methods is mandatory to improve polyethylene strength. A better understanding of kinematics and contact forces may provide long-term survival and patient satisfaction after unicompart- mental arthroplasty.Level of Evidence: Level V, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.