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Anterior cruciate ligament

Introduction

The anterior cruciate ligament (ACL) is a cruciate ligament, one of the four major ligaments of the knee, and it is a vital ligament for proper movement. Its name is derived from its anterior insertion on the tibial plateau and the fact that it "crosses" the posterior cruciate ligament within the intercondylar notch ( Latin: crux, cruc-, cross). 

ACL injury is more likely to cause knee instability than injury to other knee ligaments. Injuries of the ACL range from mild such as small tears to severe when the ligament is completely torn. There are many ways the ACL can be torn; the most prevalent is when the knee is bent too much toward the back and when it goes too far to the side.

Tears in the anterior cruciate ligament often occur when the knee receives a direct impact from the front while the leg is in a stable position, for example a standing football player is tackled sideways when his feet are firmly planted. Torn ACL’s are most often related to high-impact sports or when the knee is forced to make sharp changes in movement and during abrupt stops from high speed. esearch has shown that women involved in sports are more likely to have ACL injuries than men. ACL tears can also happen in older individuals through slips and falls and are seen mostly in people over age 40 due to wear and tear of the ligaments.

Attachments

  • Broad tibial footprint lies at a point one third to one half the distance between the medial and lateral tibial spines, 5-7 mm anterior to the PCL.
  • On the femoral side, the attachment lies on the medial aspect of the lateral femoral condyle, just anterior to the posterior aspect of the intercondylar notch.

The ACL's femoral attachment is at the posteromedial aspect of the lateral femoral condyle. The tibial attachment is larger and more stable than its attachment to the femur; it is located medial to the insertion site of the anterior horn of the lateral meniscus, 15mm posterior to the anterior border of the tibial articular surface.

The primary blood supply is the middle geniculate artery which enters the ligament near its femoral attachment after entering the posterior capsule. There is collateral blood supply from the medial and lateral geniculate arteries. The innervation of the ACL is from the tibial nerve via the posterior articular branch, which is primarily vasomotor in function. Mechanoreceptors and possibly pain fibers are present as well.

Course

  • Ligament courses obliquely, running from the tibia anteriorly and medially to the femur posteriorly, superiorly, and laterally
  • An intercruciate ligament joins the ACL to the PCL. This intercruciate ligament may have some role in proprioception and coupling of the 2 ligaments.
  • The typical size of the ligament anywhere from 30 mm to 37 mm in length and an averages 40 mm squared in cross section at midsubstance.

Function

The ACL consists of two bundles named after their tibial insertion sites, the anteromedial (AM) bundle and the posterolateral (PL) bundle . The AM bundle tightens when the knee is in flexion. The PL bundle tightens when the knee is in extension and also tightens during internal and external rotation of the tibia.

Disability if injured

  • Functional instability when abruptly changing direction at high speed, ie cutting (typical)
  • Functional instability even at low speeds moving straight ahead (rare)
  • None (rare)

Diagnosis of injury

See Anterior cruciate ligament injuries of the knee

Treatment

See Anterior Cruciate Ligament Reconstruction

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