The influence of contact alignment of the tibiofemoral joint of the prostheses in in vitro biomechanical testing

Objective. To investigate the influence of contact alignment of the tibiofemoral joint of the prostheses in in vitro biomechanical testing. Design. An experimental set-up was used to measure the total contact areas of the tibiofemoral joint of the prostheses subjected to a compressive load, and the malalignment situations were simulated. Background. The contact alignment of the femoral component related to the tibial component in the literature was not described clearly and the effect of malalignment on the testing method has not been reported well. Methods. Three commercial knee prostheses (Omnifit, Genesis, and AMK) were used for testing under a compression load (3000 N) at flexion 0° and 10°. After aligning the normal contact alignment, the simulated malalignment was done to evaluate the influence on the total contact areas in these conditions relative to the normal contact alignment. The simulated malalignment includes the medial–lateral translation (0.5 and 1 mm), anterior–posterior translation (2 and 4 mm) and internal–external rotation (1°, 3° and 5°) of the femoral component relative to the tibial component. Results. The ratios of total contact areas of malalignment relative to normal contact alignment ranged from 1.06 to 0.93 in medial–lateral malalignment in these three prostheses. In anterior–posterior malalignment, the ratios ranged from 0.69 to 0.79 in Omnifit, 0.93–0.96 in Genesis and 0.96–1.04 in AMK. In internal–external malrotation, the ratios ranged from 0.90 to 1.03 in these prostheses. Conclusions. This study proposes that contact alignment would affect the contact characteristics, especially in anterior–posterior alignment when high conformity knee prosthesis is tested. The contact alignment must be standardized in the procedure in in vitro biomechanical testing, which will be more objective to evaluate the contact characteristics of different knee prostheses.