The effect of posterior cruciate ligament deficiency on knee laxity

Abstract

Partial posterior cruciate ligament (PCL) injuries often occur in athletes; however, limited information is available about the laxity of knees with different degrees of PCL injury. In this study, a three-dimensional finite element model of the human tibiofemoral joint with articular cartilage layers, menisci and four main ligaments was constructed to investigate the changes in the knee laxity for different PCL injuries. The simulation of PCL injury (slight, medium, severe, and rupture) was simplified by reducing the elastic modulus (25%, 50%, 75%, and 100%) of the intact PCL. A posterior tibial force (100 N, 200 N) was applied to the knee joint at 0°, 30°, 60°, and 90°o f flexion. The boundary conditions included constraining all degrees of freedom at the top surface of the femur and assuming non-constraint on the bottom of the tibia except for flexion-extension motion. Under 100 N and 200 N forces applied to the tibia at full extension in the intact PCL model, the maximum posterior tibial translations in the lateral compartment were 3.60 mm and 4.53 mm, coupled with tibial external rotation of 2.60° and 2.64°, respectively. The posterior tibial translation and tibial external rotation increase gradually with increasing knee flexion. The laxity responses of slight and medium PCL injury models were not noticeably different from those of the intact PCL model. In the severe PCL injury model, the external tibial rotation was noticeably decreased at 100 N, and was changed to internal rotation at 200 N. The PCL rupture caused obviously greater posterior tibial translation (6.77 mm and 9.68 mm under 100 N and 200 N drawer force, respectively) in the medial tibia, which, coupled with internal tibial rotation (-3.00° and -4.56° under 100 N and 200 N drawer force, respectively) at full extension. The posterior translation of the medial tibia and tibial internal rotation increased with increasing knee flexion. According to our study, the posterior drawer test does not necessarily indicate the existence of an isolated PCL injury, especially in partial cases. Furthermore, the magnitude of the loading affects the performance of mechanics in knees with PCL injuries, and internal tibial rotation is an important indication a knee with a PCL injury.