Analyzing anterior knee laxity with isolated fiber bundles of anterior cruciate ligament

Anterior cruciate ligament or ACL is damaged frequently during strenuous activities like in sports. Following injury and treatment, not only a significant percentage of patients are not able to return to their pre-injury level activity, but also they continue to have knee related complications in medium to long term period. Therefore, there is a need for better understanding of the knee. In the present study, anterior laxity of the knee that is related to the function of ACL, is analyzed during passive flexion of the joint with intact and isolated deficiency of selected fibers of the ligament. A mathematical model of the knee was developed in the sagittal plane with ligaments represented as bundles of nonlinear elastic fibers. A laxity test with 90N and 180N anteriorly directed external force on the tibia was simulated at several flexion angles. Anatomical parameters and material properties were obtained from previous studies on cadaver knees. The results from model calculations showed agreement with experimental observations on cadaver knees. An anterior force translated the tibia anterior to the femur non-Iinearly throughout the flexion range. Isolated deficiency of antero-medial fibers of the ACL resulted in increased translations for all flexion positions and at both 90N and 180N laxity test. Isolated deficiency of the postero-lateral fibers for 90N test resulted in increased translations only near full extension but for 180N it resulted in increased translations for early flexion and high flexion positions of the joint. This suggests that the postero-lateral bundle may be required for situations involving large anterior tibial translations. The analysis has relevance to ACL-reconstruction and ACL rehabilitation.