Quantified kinematics of the injury to the posterior cruciate ligament: a computer-aided design simulation study

Objective. To quantify the kinematics of the injury to the posterior cruciate and the other major knee ligaments as a function of the knee flexion angle at the moment of impact. Design. Computer-aided design modelling was used to investigate the strain response of all major knee ligaments during antero-posterior abnormal tibio-femoral translation at 0–90° knee flexion. Background. It is generally believed that the likelihood of injury to the posterior cruciate ligament following anterior impact is higher in the flexed knee. However, there are no kinematical studies to quantify this clinical observation or investigate the role of the other knee ligaments in the above situation. Methods. Computer calculations of the individual ligament strain were plotted against the magnitude of posterior tibial translation. Additionally, the strain rate for each ligament (defined as the ligament strain produced per mm of posterior tibial linear translation) was calculated as the slope of the strain–displacement curve for all tested degrees of knee flexion. Results. The posterior cruciate ligament has been shown to be the primary restraint to posterior tibial translation in all degrees of knee flexion. However, at 90° of knee flexion the strain rate of the posterior cruciate ligament is approximately half that in the fully extended knee and the posterior cruciate ligament is the only ligament to resist posterior tibial translation. Conclusions. The strain behaviour of the posterior cruciate ligament during injury is highly dependent on the knee flexion during the moment of impact. Forced posterior tibial translation in the 90° flexed knee may result in isolated posterior cruciate ligament deficit rather than a complex ligament disruption. The strain rate of a ligament as introduced in the present study is a quantified parameter related to the resistance that the ligament imposes to an abnormal joint movement.