A Mid-Level Constrained Insert Reduces Coupled Axial Rotation but Not Coronal Mid-Flexion Laxity Induced by Joint Line Elevation in Posterior-Stabilized Total Knee Arthroplasty: A Computational Study.

Abstract

Surgeons may resect additional distal femur during primary posterior-stabilized (PS) total knee arthroplasty (TKA) to correct a flexion contracture. However, the resultant joint line elevation (JLE) increases mid-flexion laxity. We determined whether a mid-level constraint (MLC) insert reduced mid-flexion laxity after JLE. Six computational knee models were developed using computed tomography scans and average soft tissue properties yielding balanced extension gaps but with a 10° flexion contracture. Distal femoral resections of+2 and+4 mm were simulated with PS and MLC inserts. Varus-valgus ±10 Nm moments were applied at 30°, 45°, and 60° of flexion. Coronal laxity (the sum of varus-valgus angulation) and coupled axial rotation (the sum of internal/external rotation) were measured and compared between insert models. At 30° of flexion, coronal laxities with the PS insert at the+2 and+4 mm resections averaged 7.9° ± 0.6° and 11.3° ± 0.6°, respectively, and decreased by 0.8° (P= .06) and 1.0° (P= .07), respectively, with the MLC insert. PS rotational laxities at the+2 and+4 mm resections averaged 11.1° ± 3.9° and 12.5° ± 4.6°, respectively, and decreased by 5.6° (P= .01) and 7.1° (P= .02), respectively, with the MLC insert. Similar patterns were observed at 45° and 60° of flexion. With additional distal femoral resections to alleviate a flexion contracture, utilizing an MLC insert substantially reduced coupled axial rotation but had a minimal impact on coronal laxity compared to a PS insert. Efforts should be taken to avoid JLE in primary total knee arthroplasty as even MLC inserts may not mitigate coronal laxity.