A Computer Model of Mid-Flexion Instability in a Balanced Total Knee Arthroplasty.

BACKGROUND: Some patients have mid-flexion instability despite stability at 0° and 90° of flexion. This study aims to determine the effects of total knee arthroplasty (TKA) stability while changing femur implant size and position. METHODS: A computational analysis was performed simulating knee flexion of posterior stabilized (PS) and cruciate retaining (CR) TKA designs. Deviations from the ideal TKA implant position were simulated by adjusting tibiofemoral proximal-distal position and femur anterior-posterior position as well as implant size. Forces in ligaments connecting the femur and tibia were collected. Total tibiofemoral ligament load for mid-knee flexion of 15°-75° was analyzed vs proximal-distal implant position, implant size, implant design, and knee flexion for PS and CR knees. Posterior cruciate ligament load was also analyzed for CR knees. RESULTS: Total tibiofemoral ligament load was significantly reduced by a more proximal tibiofemoral and anterior femur position (P < .001). Implant size did not have a significant effect on tibiofemoral ligament load (P > .1). Implant design and knee flexion significantly influenced total tibiofemoral ligament load (P < .001), but the interactions with implant proximal-distal position were not significant (P > .2), indicating that implant proximal-distal position had a similar effect across the 15°-75° knee flexion range for both studied PS and CR implant designs. CONCLUSION: PS and CR TKA can be well-balanced at 0° and 90° knee flexion and have instability in mid-flexion. Elevating the joint line and shifting the femur anteriorly can cause the knee to be too loose in mid-flexion.

Effect of Varying Posterior Cruciate Ligament (PCL) Recessions on Kinematics and Ligament Strains with Cruciate Retaining Total Knee Prostheses.

INTRODUCTION: Proper ligament tension in flexion with posterior cruciate retaining (CR) total knee arthroplasty (TKA) has long been associated with clinical success. The purpose of this study was to determine the effect of varying levels of posterior cruciate ligament (PCL) release on the tibiofemoral kinematics and PCL strain. MATERIALS AND METHODS: A computational analysis was performed and varying levels of PCL release were simulated. Tibiofemoral kinematics was evaluated. The maximum PCL strain was determined for each bundle to evaluate the risk of rupture based on the failure strain. RESULTS: The femoral AP position shifted anteriorly as the PCL stiffness was reduced. PCL strain in both bundles increased as stiffness was reduced. The model predicts that the AL bundle should not rupture for a 75% release. Risk of PM bundle rupture is greater than AL bundle. DISCUSSION: Our findings suggest that a partial PCL release impacts tibiofemoral kinematics and ligament tension and strain. The relationship is dynamic and care should be taken when seeking optimal balance intra-operatively.