Mobile-bearing unicompartmental knee arthroplasty: Rationale and surgical technique

The rationale for a mobile-bearing articulation

Mobile-bearing unicompartmental components were introduced in an attempt to increase the longevity of the prosthesis by maximizing the articular contact area, thereby reducing stress on the polyethylene and minimizing wear. They might also reduce the rate of aseptic loosening in a conforming articulation. Stress across an articulation is related to the difference between the radius of curvature of the two articulating components. A round-on-flat articulation will promote high stresses. If the radii of curvature are identical, stress is minimized. It is also ideal if high contact can be maintained throughout the functional range of motion and be capable of accommodating to the normal anterior–posterior and rotational kinematics innate to each patient’s individual knee. Although a flat-on-flat articulation with an identical (infinite) radius of curvature would fulfill the aforementioned criteria and provide minimal stress when precisely articulated, it could not maintain congruent contact throughout the range of motion and edge-loading would be inevitable. A round-on-round articulation with identical radii is ideal.

Fixed-bearing articulations with high conformity have been introduced in the past in an attempt to lower polyethylene stresses. As noted previously, the typical wear pattern of a varus osteoarthritic knee is anterior and peripheral and the replaced knee wants to return to this wear pattern ( Fig. 7.1 ). When conforming fixed bearings were introduced to try to control this wear pattern, they suffered a higher-than-normal incidence of loosening because of the kinematic conflict that occurred ( Fig. 7.2 ). It has been concluded, therefore, that fixed-bearing unicompartmental knee arthroplasty (UKA) articulations must be nonconforming, that is, round-on-relatively-flat. Maximally conforming UKAs must be mobile-bearing. They can provide maximum conformity on the topside of the articulation via the identical radius of curvature between the femoral component and the topside of the mobile-bearing insert while allowing adjustment to the anteromedial wear pattern via a flat-on-flat second articulation between the undersurface of the insert and the polished topside of the modular metallic tibial tray ( Fig. 7.3 ). Other benefits of mobile-bearing metal-backed tibial components include that their modularity allows for a decreased inventory and for the potential for an isolated insert exchange if needed in the future. They also provide for a conservative composite component thickness of as little as 6 mm and for potential cementless fixation via the metallic undersurface. Early results employing cementless fixation in a mobile-bearing design have been favorable.