In vivo elongation of the anterior cruciate ligament and posterior cruciate ligament during knee flexion

Background: Most knowledge regarding cruciate ligament function is based on in vitro experiments. Purpose: To investigate the in vivo elongation of the functional bundles of the anterior cruciate ligament and posterior cruciate ligament during weightbearing flexion. Hypothesis: The biomechanical role of functional bundles of the anterior cruciate ligament and posterior cruciate ligament under in vivo loading is different from that measured in cadavers. Study Design: In vivo biomechanical study Methods: Elongation of the anterior cruciate ligament and posterior cruciate ligament was measured during a quasi-static lunge using imaging and 3-dimensional computer-modeling techniques. Results: The anterior-medial bundle of the anterior cruciate ligament had a relatively constant length from full extension to 90degrees of flexion. The posterior-lateral bundle of the anterior cruciate ligament decreased in length with flexion. Both bundles of the posterior cruciate ligament had increased lengths with flexion. Conclusion: The data did not demonstrate the reciprocal function of the 2 bundles of the anterior cruciate ligament or the posterior cruciate ligament with flexion observed in previous studies. Instead, the data suggest that there is a reciprocal function between the anterior cruciate ligament and posterior cruciate ligament with flexion. The anterior cruciate ligament plays a more important role in low-flexion angles, whereas the posterior cruciate ligament plays a more important role in high flexion. Clinical Relevance: Understanding the biomechanical role of the knee ligaments in vivo is essential to reproduce the structural behavior of the ligament after injury (especially for 2-bundle reconstructions) and thus improve surgical outcomes.