Anterior cruciate ligament
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The anterior cruciate ligament (ACL) is one of the two cruciate ligaments that stabilize the knee joint.
The ACL arises from the anteromedial aspect of the intercondylar area on the tibial plateau and passes upwards and backwards to attach to the posteromedial aspect of the lateral femoral condyle. The ACL tibial footprint substantially overlaps the anterior root lateral meniscus footprint 6.
The ACL measures 31-38 mm in length and 10-12 mm in width, with the anteromedial bundle (6-7 mm) slightly thicker than the posterolateral bundle (5-6 mm) 5. However, it is considered to be the weaker of the two cruciate ligaments 8. Like the posterior cruciate ligament, the ACL is intracapsular but extrasynovial having its own synovial membrane 5.
The ACL consists of two components named for their tibial attachment sites 4,5:
- anteromedial bundle (AMB)
- attaches from the roof of the intercondylar notch
- posterolateral bundle (PLB)
- more vertically oriented, and slightly shorter
- attaches from the wall of the intercondylar notch
- tibial nerve: proprioception 5
- single bundle (~25%) 5
- third intermediate bundle 5,7
- anomalous insertion of the medial meniscus
The ACL functions to prevent posterior translation of the femur on the tibia (or anterior displacement of the tibia) during flexion-extension of the knee. The anteromedial bundle is responsible for the posterior translation of the femur at 30 degrees flexion, and the posterolateral bundle resists hyperextension and prevents posterior translation of the femur in extension 1, 2.
Unlike other ligaments or tendons, the anterior cruciate ligament normally has a heterogeneous appearance and the anteromedial and posterolateral bundles are defined by surrounding high-intensity structures 1.
The ACL Blumensaat line angle is normally ≤15º. It is calculated by drawing a line parallel to the roof of the intercondylar notch of the femur (Blumensaat line) and one parallel to the distal portion of the anterior cruciate ligament. The angle is calculated at the intersection of these two lines. An angle of >15º indicates an abnormal course and probable anterior cruciate ligament tear 3.
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