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.

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 mid-flexion laxity. We determined whether a mid-level constraint (MLC) insert reduced mid-flexion laxity after JLE. METHODS: 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. 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 = .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. CONCLUSION: 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.

Mid-flexion stability in the anteroposterior plane is achieved with a medial congruent insert in cruciate-retaining total knee arthroplasty for varus osteoarthritis.

PURPOSE: This study aimed to compare the intraoperative kinematics, especially for mid-flexion femorotibial anteroposterior (AP) stability, between newly developed medial congruent (MC) inserts and cruciate-retaining (CR) inserts in navigated cruciate-retaining total knee arthroplasty (CR-TKA). METHODS: Thirty consecutive patients with varus osteoarthritis undergoing CR-TKA using an image-free navigation system were enrolled. AP kinematics, the AP translation under manual maximum stress to the knee joint at 45° flexion, rotational kinematics, and varus-valgus laxity were evaluated using a navigation system and statistically compared between the MC and CR inserts. RESULTS: AP kinematic analysis showed that the femoral position with the CR insert was significantly anterior at a maximum extension to 45° flexion compared with the MC insert (p < 0.05). The amount of AP translation at 45° flexion with the MC insert was significantly smaller than that with the CR insert (p < 0.05). Rotational kinematics found that the tibial position at maximum extension was significantly externally rotated with the MC inserts than with the CR inserts. Varus-valgus laxity was comparable between the MC and CR inserts. CONCLUSION: The current results showed that greater mid-flexion AP stability was achieved with the MC inserts than with the CR inserts in CR-TKA. Intraoperative kinematics with the MC inserts more closely resembled those with preoperative conditions in CR-TKA. LEVEL OF EVIDENCE: III, prospective comparative study.

Motion type and knee articular conformity influenced mid-flexion stability of a single radius knee prosthesis.

PURPOSE: Single radius knee implants were introduced to reduce the level of paradoxical anterior femoral translation (AFT) during mid-flexion after total knee arthroplasty. Findings from clinical and experiment studies are inconsistent, which may be due to the different loading conditions and articular conformities of the knee implants studied. The aim of this study is to analyze how variations in these two factors affect the mid-flexion stability of a single radius knee prosthesis. METHODS: Six daily activities (walking, stair ascent, stair descent, sit-to-stand, pivot turn and crossover turn), and three articular conformity ratios (low, moderate and high) were considered. The resulting AFTs from the 18 finite element models were analyzed. RESULTS: For low conformity knees, the worst case activity (the greatest AFT) was sit-to-stand with an AFT of 6.2 mm, while for the moderate conformity the worst case was crossover turn and pivot turn. For high conformity, all activities produced a relatively small AFT ranging from 0 mm to 1.8 mm, which more closely resembles natural knee motion. In addition, no AFT was recorded during stair ascent for all three conformities (low, moderate, high). CONCLUSIONS: This study demonstrated that the amount of AFT is highly dependent on the activity being undertaken and the articular conformity of the knee prosthesis, and the worst case activity depends on the knee conformity. The clinical relevance of this study is that it offers valuable information towards the design of improved knee prostheses and selection of knee implants for clinical use. LEVEL OF EVIDENCE: II.