Anterior Cruciate Ligament Tear (Torn ACL)

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What is the function of the knee joint?

The purpose of the knee joint is to bend and straighten (flex and extend), allowing the body to change positions. The ability to bend at the knee makes activities like walking and running or standing and sitting much easier and more efficient.

The thighbone (femur) and the shinbone (tibia) meet the kneecap (patella) to form the knee joint. The rounded ends of the femur or condyles line up with the flat tops of the tibia called the plateaus. There are a variety of structures that hold the knee joint stable to allow the condyles and plateaus to maintain their anatomic relationship so that the knee can glide easily through its range of motion.

There are four thick bands of tissue, called ligaments, that stabilize the knee:

  • The medial collateral ligament (MCL) and the lateral collateral ligament (LCL) stabilize the sides of the knee.


  • The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) form an X on the inside of the knee joint and prevent the top and bottom of the knee from sliding back and forth.

The major muscles of the thigh also act as stabilizers: the quadriceps in the front of the leg and the hamstrings in the back.

A sprain occurs when a ligament is injured and the fibers are either stretched or torn.

Picture of the anterior cruciate ligament (ACL)
Picture of the anterior cruciate ligament (ACL)

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Torn ACL - Causes Question: Please discuss the cause of your torn ACL.
Torn ACL - Signs and Symptoms Question: What were the signs and symptoms associated with your torn ACL?
Torn ACL - Diagnosis Question: Describe the events that led to a diagnosis of a torn ACL.
Torn ACL - Treatment Question: What types of treatment did you receive for your torn ACL? Did you have surgery?
Torn ACL - Recovery Question: What was the recovery process like after you tore your ACL? Did you have physical therapy?
Learn about rehabilitation and recovery time after surgery for a torn ACL.

Torn ACL & Surgery Recovery Time

Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: Melissa Conrad Stöppler, MD

Do people appreciate how amazing it is that Philip Rivers (quarterback for the San Diego Chargers) played on Sun., Jan. 20, 2008, just six days after having arthroscopic surgery? Aside from the fact that he likely had some cartilage debris cleaned out and the rehab time is measured in weeks, he still had a torn anterior cruciate ligament(ACL).

For regular people and pseudo-athletes, the days after arthroscopic knee surgery are spent reducing knee swelling and starting range-of-motion exercises. This process is hampered by a couple of physiologic barriers.

When the knee is invaded, the muscles around it tend to shut down involuntarily. The quadriceps muscle in the front of the thigh tends to get immediately weaker, and since it is one of the stabilizing muscles of the knee, it is a big deal when this muscle decides not to work. The next barrier has to do with hydraulics. The knee joint has its largest volume of fluid when it's flexed at 15 degrees. When there is even a little fluid, straightening it out combats the law of physics that says "you can't compress fluids."


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