The anterior cruciate ligament (ACL) is located in the middle of the knee joint and helps stabilize the knee in both front to back and rotational motions at the joint. A common injury to the ACL is a sprain or a tear. The ACL ligament is surgically reconstructed using a graft.

Source: https://www.orthoillustrated.com/animation/16-allinside-acl-reconstruction-with-arthrex-graftlink and https://www.youtube.com/watch?v=Typ1ZNwbRZE&feature=youtu.be.