Effect of Tibial Tray Malalignment on Micromotion and Subsidence: A Preliminary Report

Malalignment of the tibial tray in total knee arthroplasty has been linked to several complications including tibial tray subsidence, leading to revision surgery. However, quantitative biomechanical evidence to directly support the mechanism of failure is not available. We developed a model to study tibial tray micromotion and subsidence in vitro under multiaxial physiological loading conditions for up to 100,000 cycles. In Phase I, we tested four cadaver knees and two surrogate bone models to determine whether the surrogate models could reproduce the fatigue damage induced by cyclic loading. In Phase II, we tested six cadaver knees in a pairwise manner under conditions representing either neutral or varus malalignment. The surrogate bone models did not reproduce the progressive damage that was seen in human cadaver specimens. The altered loading conditions used to represent varus tray alignment increased the cyclic strain at the start of the fatigue loading and increased the cyclic strain at the end of the fatigue loading as well as the subsidence of the tray. The increase in final cyclic strain was greater in the varus condition indicating reduction in stiffness due to bone damage. This in vitro testing represents the clinical reports of early tray migration that lead to eventual aseptic loosening. This study provides biomechanical evidence supporting the hypothesis that a varus tray alignment could increase the risk of failure.