A Midlevel Constrained Insert Reduces Coupled Axial Rotation but Not Coronal Midflexion Laxity Induced by Joint Line Elevation in Posterior Stabilized TKA: A computational study.
Academic Article
Overview
abstract
BACKGROUND: 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 midflexion laxity. We determined whether a mid-level constraint (MLC) insert reduced mid-flexion laxity after JLE. METHODS: Six computational knee models were developed using CT 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 (VV) ±10 Nm moments were applied at 30°, 45, and 60° of flexion. Coronal laxity (the sum of VV angulation) and coupled axial rotation (the sum of internal/external rotation) were measured and compared between insert models. RESULTS: 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=0.06) and 1.0° (p=0.07), 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=0.01) and 7.1° (p=0.02), respectively, with the MLC insert. Similar patterns were observed at 45° and 60° of flexion. CONCLUSIONS: 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 TKA as even MLC inserts may not mitigate coronal laxity.
