Abstract
Introduction
Knee joint instability, which is a primary reason for TKA revision surgeries, is typically caused by deficiency in the knee ligaments [1, 2]. Managing ligament deficiency and restoring joint stability continues to be one of the greatest challenges for revision surgeries [3]. To treat such patients, revision TKA implants frequently incorporate a constrained post and cam mechanism to provide enhanced varus-valgus constraint to supplement the function of the collateral ligaments. The aim of this study was to evaluate knee kinematics during a weight bearing deep knee bend for both a primary TKA system and its complimentary revision system. The hypothesis of the study was that the revision tibial insert would demonstrate improved knee stability, in the form of a reduced range of motion under out-of-plane loading, when compared to the primary system
Methods
Eight cadaveric knees (age: 59±10 years, BMI 23.3±3.5) were implanted with an ATTUNE™ revision femoral component and a primary posterior stabilized tibial component. Each knee was mounted and aligned into the Kansas Knee Simulator (Fig. 1) [4]. A deep knee bend was performed between 10° and 110° flexion with no out-of-plane loading. Additional deep knee bends were performed with constant 6Nm external and 6Nm internal torques about the tibial long axis, and with 40N medial and 40N lateral loads applied at the ankle sled. The 40N medial and 40N lateral loads produce approximately 15Nm adduction and abduction moments at the knee, respectively. The primary tibial insets were then replaced with revision tibial inserts from the same TKA system and the deep knee bend cycles were repeated. The revision tibial inserts included a larger tibial post intended to constrain the varus-valgus rotation of the knee. The change in knee kinematics of the revision tibial insert compared to the primary insert was calculated and student t-tests were performed to identify significant differences between the two tibial insert types for each loading condition.
Results
The baseline deep knee bend with no out-of-plane loads showed no statistical difference in kinematics between the primary and the revision tibial inserts. The revision tibial insert demonstrated a significant reduction in varus-valgus range-of-motion compared to the primary tibia for the deep knee bends with adduction and abduction moments (Fig. 2). The deviation in the internal-external rotation for internal-external torque cycles were significantly smaller for the revision compare to the primary tibial inserts (Fig. 3)
Discussion
The primary and revision implants have the same tibial plateau geometries; therefore, it was expected that they have similar tibiofemoral kinematics for the baseline deep knee bend. The variations in tibiofemoral kinematics in the cycles with out-of-sagittal plane loads between the two inserts were primarily due to the differences in their intercondylar box and post geometry. The larger post in the revision implants resulted in tighter fit between the post and cam which restricted the knee joint motion. Increased conformity of the TKA revision system successfully reduced deviation in varus-valgus and internal-external rotations from baseline kinematics which may be desirable for patients with instability due to ligaments deficiency.
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