Aetiology
ACL injury occurs when the forces placed upon the ligament exceed its ability to handle those forces. Alternatively, the bony insertion of the ACL may be weaker than the ligament and may avulse from the underlying bone. The ligament may remain intact or may partially tear. Avulsion is most common in children.[7][19]
Pathophysiology
The ACL originates from the posteromedial aspect of the intercondylar notch on the lateral femoral condyle. It inserts broadly onto the articular surface of the tibia, medial to the attachment of the anterior horn of the lateral meniscus.
The ACL prevents excessive anterior translation of the tibia on the femur and also acts to minimise tibial rotation and resist valgus and varus forces.[3][20] The ACL receives a rich blood supply, primarily from the middle geniculate artery, so when the ACL is ruptured, a haemarthrosis usually develops rapidly. However, despite its intra-articular location, the ACL is actually extrasynovial. Due to the poor intrinsic healing properties of the ACL, a torn ACL will not heal on its own. Over time, the damaged fibres may scar down to the posterior cruciate ligament or to the intercondylar notch. This may result in confusing findings on physical examination but rarely if ever results in functional stability.[21]
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