Copyright 2009
Definition
The anterior cruciate ligament (ACL) is the predominant restraint to anterior tibial displacement during activities. The ligament accepts 75 % of anterior force at full extension. The deep portion of the MCL is a major secondary restraint to anterior translation. The menisci also provide resistance to anterior translation of the tibia (particularly the medial meniscus).
The ACL has a unique intra-articular and extrasynovial (compared to the intrasynovial location of the PCL) which makes healing after a rupture difficult. The femoral attachment of the ACL is on posterior part of medial surface of lateral condyle while the tibial attachment is in a fossa in front of & lateral to anterior tibial spine. Its tibial insertion measures 11 mm in width to 17 mm in AP direction location.
The ACL is taut in full knee extension, and tends to externally rotate tibia. It has a role not only in preventing anterior translation of the tibia on the femur, but also in control the rotation of the tibia on the femur. It is composed of two principal parts: small anteromedial band and a larger bukly posterolateral portion. The anteromedial bundle is tight in flexion and the posterolateral bundle is tight in extension.
It is supplied by the middle geniculate artery and the nerve supply is posteriorly from the tibial nerve. It serves as a proprioceptive structure for the knee to help the body in gauging position of the leg in space.
It has a unique intra-articular and extrasynovial place within the joint. Both of these factors make healing torn ACLs very difficult. Other ligaments that are extra-articular such as the LCL and MCL heal much more easily since they are not bathed in synovial fluid.
Common diseases include tears to the ACL and much less commonly, a congenitally absent ACL. In order to play cutting sports without damaging the knee, it is critical to have a functioning, intact ACL. Otherwise the excessive translation of the tibia on the femur leads to damage of the menisci and surrounding cartilage, eventually leading to degenrative joint disease.
Commonly used diagnostic procedures include a good history and physical examination and an MRI. Over 50% of patients with a hemaarthrosis of the knee after a traumatic event have an ACL tear. A Lachman test is performed by stabilizing the femur and anteriorly translating the tibia on the femur. If there is a soft endpoint, this is a positive test. The Lachman test as well as MRIs are about 95% sensitive for detecting ACL injuries.
It is usually treated with reconstruction procedures in the young active patient. Older patients (older the 40-50 years) who aren’t interested in getting back to cutting sports can be treated non-operatively if they are asymptomatic. Reconstruction is performed with either hamstring or bone-patellar-bone autograft or equivalent allograft. Repair has been tried in the past, but due to the high failre rates was abandoned in favor of reconstructing the ligament.
References
http://www.wheelessonline.com/ortho/anatomy_of_acl
uhrad.com – Musculoskeletal Imaging Teaching Files
Case Eight – Disruption of the Anterior Cruciate Mechanism
uhrad.com – Musculoskeletal Imaging Teaching Files
Case Nine – Bucket Handle Tear
uhrad.com – Musculoskeletal Imaging Teaching Files
Case Eighty Two – Tear of Anterior Cruciate Ligament and Nondisplaced Lateral Tibial Plateau Fracture