Abstract
Introduction
Achieving proper ligament tension in knee flexion within posterior cruciate retaining (CR) total knee arthroplasty (TKA) has long been associated with clinical success. Ligament balance has been achieved through specific surgical technique steps. No prior study evaluated the possible effects of varying levels of posterior cruciate ligament (PCL) release on femorotibial contact location and PCL ligament strain. The purpose of this computational analysis was to determine what effect-varying levels of PCL release may have on the tibiofemoral kinematics and PCL strain.
Methods
A computational analysis was performed utilizing a musculoskeletal modeling system with ligaments modeled as non-linear elastic structures and ligament insertions. A single CR knee system with two different tibial insert designs was tested, a Guided Motion (GM) and an ultra-congruent, Deep Dished (DD) design. Varying levels of PCL release were simulated by setting the stiffness of both bundles of the PCL to a percentage, ranging from 0–100% in 25% increments. Tibiofemoral kinematics was evaluated by measuring the contact points estimated from the femoral condyle low points, and ligament strain of the anterior-lateral (AL) and posterior-medial (PM) bundles. The maximum PCL strain was determined for each bundle to evaluate the risk of PCL rupture based on the PCL failure strain.
Results
The femoral AP position of both medial and lateral condyles became more anterior as the PCL stiffness was reduced to simulate greater release in both GM and DD inserts. The effect of reduction in PCL stiffness on femoral AP position increased as the PCL stiffness became a smaller percentage of the intact stiffness. The DD insert had smaller changes in femoral AP position resulting from reduced PCL stiffness than the GM insert.
PCL strain in both bundles increased as PCL stiffness was reduced. The effect of reduction in PCL stiffness on PCL strain increased as the PCL stiffness became smaller. The DD insert had smaller changes in PCL strain resulting from reduced PCL stiffness than the GM insert. The model predicts that the AL bundle should not rupture for a 75% release of the PCL. The maximum PM bundle strain data indicates that the risk of PM bundle rupture is greater than AL bundle.
Discussion
Our findings suggest that a partial PCL release does have an impact on tibiofemoral kinematics and ligament tension throughout the knee flexion range of motion for varying implant designs. The effects of increased PCL release were: more anterior femoral position on the tibia and increased strain in both bundles of the PCL. The maximum strain data indicates that the AL bundle of the PCL should be able to safely withstand a 75% release, but the PM bundle of the PCL may be at risk of rupture after as little as a 25% release because its stiffness is lower than the AL bundle. Our findings indicate that though partial PCL release can be correlated with both knee kinematics and PCL strain, the relationship is rather dynamic and care should be taken when seeking to find optimal balance intra-operatively.
