Load-controlled knee simulators, representing the passive constraints and joint loads observed in the natural knee, have been developed to assess device-dependent kinematics and wear damage of total knee replacements (TKR) in a controlled mechanical environment. Using a finite element model (FEM) to represent the simulator, our objective in this study was to quantify the variations in kinematics, contact stresses, and contact areas that occur with variations in the ‘soft-tissue’ spring stiffness and coefficient of friction for a conforming knee design.

A finite element model was created of the Insall-Burstein Posterior-Stabilized II knee system. The model conditions corresponded with the International Standards Organisation (ISO) test protocol #14243-1 and consisted of the prescribed flexion angle, the axial compressive load, the anterior-posterior (AP) force, the internal-external (IE) moment, and linear springs mounted to provide AP and IE restraints. This setup has been validated as a reasonable equivalent system for this design in the Instron-Stanmore knee simulator. The linear spring constant was set at 7.24 N/mm and the coefficient of friction was 0.01; both values were then varied by an order of magnitude. The implant kinematics and the maximum contact stress and areas of contact over the loading cycle were determined.

Varying the spring constant by a factor of two changed the AP motions and IE rotations of the tibial insert by about 20%. The maximum contact stresses, occurring during peak loads and moments, varied by 40%, while the area of contact over the full cycle changed by 30%. Changing the coefficient of friction had little effect upon the dependent variables. Wear is a function of both stresses and kinematics. This study indicates that stresses in this design are more sensitive than kinematics to changes in ‘soft-tissue’ stiffness. Therefore, both must be considered to determine wear potential.