FRONTAL PLANE ROTATION OF A TOTAL STABILISED TOTAL KNEE REPLACEMENT UNDER SEVERE VARUS LOADING

INTRODUCTION During activities of daily living (ADL), varus moments are experienced in the knee, which can result in frontal plane rotation, or liftoff, of the lateral femoral condyle with respect to the tibial plateau. An understanding of this rotation is valuable as it could potentially lead to contact between the femoral component and polyethylene post of a total knee replacement (TKR). Therefore, the purpose of this study was 1) to assess how much frontal plane rotation was achieved due to varus moments imposed on a total stabilized (TS) TKR from the stair ascent activity, and 2) to determine whether a TS TKR could withstand the contact stresses imposed by the varus loading for 1 million cycles without the post fracturing or plastically deforming. METHODS A PS femoral component paired with a TS polyethylene insert and baseplate (Triathlon, Stryker, Mahwah, NJ) were aligned on a multi-axis testing system (MTS Systems Corp, Eden Prairie, MN) (Figure 1). Size 1 components were used as they represented the worst-case size for testing. The femoral component was fixed at 60 degrees of flexion, representing an angle of peak varus moment during stair ascent [1]. The peak varus moment used in this study was determined by scaling the data from [Orthoload.com][1] for a 136 kg patient body weight (3 SD above average TKR patient body weight) [1, 2]. In order to evaluate the frontal plane rotation achieved due to the varus moment with minimal influence from other loads, an FEA model of the physical test setup was used to determine the lowest joint compressive load that would allow testing to be stable. Given this, testing was completed with a constant joint compressive load of 1500 N (33% of that reported by [Orthoload.com][1]) while sinusoidally applying a varus moment from 5Nm to 54.5Nm [1, 2]. The loads were applied to three samples for 1 million cycles to represent the number of stair ascent cycles experienced over 20 years [3]. Lastly, a validation test was run on a component with the polyethylene post notched at the medial distal aspect. The post fractured during testing indicating that the test could induce the clinical failure mode of interest. RESULTS The average frontal plane rotation achieved for the three components was 16.19 degrees +/− 0.86. Contact was noted between the femoral component and the polyethylene post at the medial distal aspect and lateral proximal aspect of the post, as demonstrated with the FEA prior to testing (Figure 2). At the completion of testing, all components were noted to be intact with no plastic deformation present. DISCUSSION The frontal plane rotation achieved due to the applied varus moment imposed on TS TKR was found to be 16 degrees, on average. This was investigated with minimal influence from a joint compressive load and without the influence of the collateral ligaments. Given this, the data from this study is useful in determining the amount of restraint achieved by the components themselves and understanding the impact on knee kinematics following implantation of a TS TKR. For figures, please contact authors directly [1]: http://Orthoload.com