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What is the function of the Anterior Cruciate Ligament

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What is the function of the Anterior Cruciate Ligament (ACL) in the knee?  How is the ACL torn? Along those lines, why might it be the case (as we suspect) that skiing-related ACL tears occur disproportionately after 2pm?  How is an ACL tear detected on exam? 

The main function of the ACL is restraint of anteroposterior translation of the tibia relative to the femur. It also acts as a secondary restraint to tibial rotation and valgus or varus stress.



The ACL courses from the anterior tibia to posterior femur at the knee, as shown:


(This sagittal view points out that the ACL is fairly vertical–not the most advantageous orientation to prevent anterior translation. Yet if the ligament were more horizontal (as I drew it below, with artistic license) the knee would not flex and extend normally)  


The mechanisms of injury is typically a sudden deceleration or rotational maneuver with a force that sends the tibia one way and femur another (typically because the foot is planted and the body spins).


Not all such forces that exceed the strength of the ligament lead to a tear: often, the secondary restraints (the hamstrings mostly) can help resist a tear. When the secondary restraints are overwhelmed, the ligament can be exposed to forces it cannot bear.


(Why would the hamstrings be overwhelmed? Well, for one thing the knee may be extending at the very second of injury, a phase of motion where the hamstrings do not fire; alternatively, they could be too tired. While skiing, for example, at 10am your still-powerful hamstrings protect the ACL; at 2pm, when the tibia starts to subluxate, the hamstrings just acquiesce and let the bone subluxate, ultimately tearing the ligament.)


An ACL tear is suspected first by history. A "pop" heard by the patient, immediate pain and swelling after a twist are typical features.
To test for an ACL tear on physical exam one can use the anterior drawer and Lachman tests. The Lachman test is the gold standard because it is thought to isolate the ACL and not involve other stabilizing structures. 

The Lachman test is performed by p attempting to produce anterior translation of the tibia. An intact ACL limits anterior translation and provides a distinct endpoint. Increased translation compared to the uninjured knee and a vague endpoint suggest ACL injury.

6. The right hand then pulls the tibia foward to place the ACL under tension.


The anterior drawer is performed with the patient lying supine and the knee flexed at 90 degrees. The proximal tibia is gripped with both hands and pulled anteriorly, checking for anterior translation. Often the clinician sits on the foot while performing the test to provide stability. The test is positive if there is anterior translation.

In brief, if there is a good story on history; effusion; what you think is maybe a positive lachman or drawer then get MRI.



Knee aspiration can be a very important step in the initial management:



Here's why:

  1. If there is too much fluid you cannot sense laxity–the pressure of the fluid "stabilizes" the knee
  2. The pressure of the fluid hurts! It is simply kind and humane to relieve it
  3. BLOOD (as seen above) usually indicates a significant injury (MRI worthy to be sure!); if not an ACL then perhaps a chondral fracture, meniscal tear or patellar dislocation.

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