Torn ACL (cont.)

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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, running, jumping, 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 and allow the condyles and plateaus to maintain their anatomic relationship so that the knee can glide easily through its range of motion. The knee is a hinge joint, but there is also some rotation that occurs when it bends and straightens.

There are four thick bands of tissue, called ligaments, that stabilize the knee and keep its movement in one plane.

  • The medial collateral ligament (MCL) and the lateral collateral ligament (LCL) stabilize the sides of the knee preventing side to side buckling.
  • The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) form an X on the inside of the knee joint and prevent the knee from sliding back to front and front to back respectively.

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. A first-degree sprain is a ligament that is stretched but with no fibers torn, while a second-degree sprain is a partially torn ligament. A third-degree sprain is a completely torn ligament.

Picture of the anterior cruciate ligament (ACL)
Picture of the anterior cruciate ligament (ACL)
Medically Reviewed by a Doctor on 2/4/2015

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Torn ACL - Causes Question: Please discuss the cause of your torn ACL.
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