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Classification of Mobile-Bearing Knee Designs: Mobility and Constraint

Restoration of normal knee joint function through surgical reconstruction is dependent upon load-sharing between the implant and the surrounding soft-tissue structures. Mobile-bearing knee designs offer the advantage of maximally conforming geometry while diminishing constraint forces to fixation interfaces through plateau mobility. The degree of mobility afforded by these designs in the anterior-posterior, medial-lateral, and rotational directions defines the required interaction between the soft tissues and the design geometry to maintain a stable articulation.

This study characterizes nine contemporary mobile-bearing designs in terms of the force generated during a prescribed displacement. Among the designs evaluated, only the LCS Deep Dish Rotating Platform is available for clinical use in the United States.

A dynamic testing system capable of applying biaxial loads (Instron Testing Machine, model 1115; Instron, Canton, Massachusetts) was utilized to assess the intrinsic performance characteristics of nine non-hinged, mobile-bearing knee designs. Anterior, posterior, medial, lateral, and rotational constraints were determined for each total knee design under a compressive load consistent with normal walking gait1,2. A compressive load of four times body weight and 0° of flexion was chosen to represent a position of gait where maximum shear forces act in the posterior and lateral directions as well as in rotation1,2. Anterior and medial shear forces are presented at the same gait position for completeness. A body weight of 163 lb (74 kg), which corresponds to the average for a sixty-year-old, 5-ft, 8-in (172.7-cm) male subject, was used in this evaluation3.

### Anterior-Posterior and Medial-Lateral Shear Testing

Three tibial inserts were evaluated in each test direction for each system. Under the prescribed in vivo compressive load (652 lbf), shearing displacements were applied to the system until the implant subluxated (Fig. 1). Anterior, posterior, medial, and lateral subluxation was defined as the dislocation of the tibial component relative …