Fixed‐bearing medial unicompartmental knee arthroplasty restores neither the medial pivoting behavior nor the ligament forces of the intact knee in passive flexion

Abstract

Medial unicompartmental knee arthroplasty (UKA) is an accepted treatment for isolated medial osteoarthritis. However, using an improper thickness for the tibial component may contribute to early failure of the prosthesis or disease progression in the unreplaced lateral compartment. Little is known of the effect of insert thickness on both knee kinematics and ligament forces. Therefore, a computational model of the tibiofemoral joint was used to determine how non-conforming, fixed bearing medial UKA affects tibiofemoral kinematics, and tension in the medial collateral ligament (MCL) and the anterior cruciate ligament (ACL) during passive knee flexion. Fixed bearing medial UKA could not maintain the medial pivoting that occurred in the intact knee from 0° to 30° of passive flexion. Abnormal anterior-posterior (AP) translations of the femoral condyles relative to the tibia delayed coupled internal tibial rotation, which occurred in the intact knee from 0° to 30° of flexion, but occurred from 30° to 90° of flexion following UKA. Increasing or decreasing tibial insert thickness following medial UKA also failed to restore the medial pivoting behavior of the intact knee despite modulating MCL and ACL forces. Reduced AP constraint in non-conforming medial UKA relative to the intact knee leads to abnormal condylar translations regardless of insert thickness even with intact cruciate and collateral ligaments. This finding suggests that the conformity of the medial compartment as driven by the medial meniscus and articular morphology plays an important role in controlling AP condylar translations in the intact tibiofemoral joint during passive flexion. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1868-1875, 2018.

A geometric ratio to predict the flexion gap in total knee arthroplasty

“…The ability of the proposed MCL ratio to predict medial and lateral flexion gaps was evaluated using six computational models of the tibiofemoral joint following TKA. A previously published modeling framework that has been used to predict kinematics, contact forces, and ligament forces of the tibiofemoral joint in its native state and following TKA was utilized 18‐21 . Intraoperative clinical exams were simulated to evaluate the medial and lateral gaps in flexion, and the new geometric ratio was measured and related to these gaps.…”

“…All model components were incorporated into a multibody dynamics program that generated and solved the equations of motion (MSC software; Adams, Newport Beach, CA). All models integrated a compliant rigid body contact algorithm to account for the shape of the implant articular surfaces 20,31 . The contact force at the articulation of the femoral component and the tibial insert was modeled using a nonlinear function of the penetration depth.…”

“…A previously published modeling framework that has been used to predict kinematics, contact forces, and ligament forces of the tibiofemoral joint in its native state and following TKA was utilized. [18][19][20][21] Intraoperative clinical exams were simulated to evaluate the medial and lateral gaps in flexion, and the new geometric ratio was measured and related to these gaps. This study was approved by the institutional review board (IRB) of the Hospital for Special Surgery.…”

“…All models integrated a compliant rigid body contact algorithm to account for the shape of the implant articular surfaces. 20,31 The contact force at the articulation of the femoral component and the tibial insert was modeled using a nonlinear function of the penetration depth. The coefficients of this two-parameter power function was based on a compression test of the metallic femoral component and a polyethylene tibial insert as described in our previous work.…”

“…The coefficients of this two-parameter power function was based on a compression test of the metallic femoral component and a polyethylene tibial insert as described in our previous work. 20 The MCL ratio was defined as the proportion of the distances of the femoral insertion of the MCL anterior fiber to the posterior and to the distal cuts (Figure 3). Openings in the medial and lateral compartment (ie, gaps) in extension (0°of flexion) and at 90°of flexion were evaluated in response to valgus and varus moments of 10 Nm.…”