Anatomic loads that are exerted on the tibial component can result in forces which exert compressive, tensile, and shear stresses in the modular connection between the articular surface and the tibial tray. Stability, as well as strength of the onnection, are important to the long-term function of the implant. Consequently, new modular knee designs should be subjected to rigorous analysis and testing to evaluate the modular connection stability.
Component tolerances and locking mechanism design are important to the stability of the interlock. Stability of the interlock is important when evaluating the ability of the design to limit relative motion and the generation of material debris. A static testing technique has been utilized to quantify the amount of relative motion that may occur between the articulating surface and the tibial tray. The method applies a transverse load to the locking mechanism in the plane of the articular surface/tibial tray interface while relative displacement versus load information is collected. With sufficiently low forces, multiple loadings may be applied to the same components for statistical analysis. Tests with displacements ranging from 0.13 to 0.56 mm have shown standard deviation ranges of 0.6 percent to 8.6 percent.
A plot of the force versus displacement data is useful in the analysis of the locking mechanism performance. Regions of movement, or sliding, between the components are indicated by a relatively low slope in the curve. A relatively high slope will indicate regions of stiffness of the mechanism. The results may then be used to compare the performance of the specimens with clinically successful designs.