Function of the crocodilian anterior cruciate ligaments.

The anterior cruciate ligament (ACL) is an important knee stabilizer that prevents the anterior subluxation of the tibia. Extant crocodiles have two ACLs, the ACL major and minor, yet their functional roles are unclear. We here examined in-situ forces within the ACL major and minor in saltwater crocodiles (Crocodylus porosus) with a 6-degree-of-freedom robotic testing system under the following loading conditions: (a) 30 N anterior tibial load at 150°, 120°, and 90° knee extension; (b) 1 Nm internal/external torque at 150° and 120° knee extension; (c) 30 N anterior tibial load +1 Nm internal/external torque at 150° and 120° knee extension. The In-situ force in the ACL minor was significantly higher than that of the ACL major in response to anterior tibial load at 90° knee extension, and anterior tibial load + external torque at both 150° and 120° knee extension. Meanwhile, the force in the ACL major was significantly higher than that of the ACL minor in response to internal torque at 120° knee extension, and anterior tibial load + internal torque at 150° knee extension. The present results showed that the ACL minor and major of saltwater crocodiles have different functions. In response to anterior tibial load + internal/external torques, either of two ACLs reacted to opposing directions of knee rotation. These suggest that two ACLs are essential for walking with long axis rotation of the knee in crocodiles.