Anterior cruciate ligament fibres-effects of tibial translation during flexion at the knee

Anterior Cruciate Ligament (or ACL) of the knee is frequently injured during activities involving large relative translations of the bones. Contributions of the knee ligaments in stabilizing the joint have been studied. Separately, tightening or slackening of the ACL fibres during flexion has also been analyzed. However, there is a need to understand the role of different fibre bundles in the ligament during activities. In the present study, a planar mathematical model of the knee is used to analyze the patterns of changes in the ACL fibres during passive motion and because of anterior tibial translation, which stretch the ligament fibres. The joint ligaments were modeled as bundles of non-linear elastic fibres. The bones were frictionless and impenetrable. Anatomical data and material properties were obtained from literature. An anterior laxity test was simulated. During passive flexion over 0-120o range, the most anterior fibre of ACL remained isometric while other fibres slackened. Tibial translation during the simulated test stretched the fibres variably over the flexion range. For example, at 45o flexion, 130N anterior load on the tibia resulted in 6.1 mm tibial translation and stretched the anterior fibre by 11.5% and the intermediate fibre by nearly 1.2% while the posterior fibre remained slack. The model calculations showed reasonable agreement with the experimental observations from literature. The analysis suggests that injuries can affect different fibres of the ligament depending on the relative positions of bones at the time of injury. Such effects may be clinically significant and may help in explaining complex functional behavior of the ligament during activity and injury.