top of page


It seems like ACL tears are one of the more common injuries in the Colorado mountains. How do they occur and how would I know if I tore mine?

You are correct; ACL tears are a common knee ligament injury often occurring as a result of an athletic injury.

Anatomy of the ACL Tear and Mechanism of Injury

The ACL is one of the primary stabilizing ligaments of the knee. It originates from the back of the femur (thigh bone) and inserts on the front of the tibia (shin bone). It is about the size of your small finger but can withstand forces of up to 500 lbs. before it ruptures.

The ACL minimizes excessive forward movement and rotation of the tibia in relationship to the femur. A tear of the anterior cruciate ligament (ACL) results from over stretching of the ligament. It is the most commonly injured ligament in the knee.

How ACL Tears Occur

An injury to the ACL can occur as the result of a slow, twisting fall skiing or with a sudden deceleration in cutting and pivoting sports such as football, basketball or soccer. It is most often a non-contact injury. Women are three to five times more likely than male athletes to tear their ACL for a number of biomechanical reasons.

Complete and Partial ACL Tears

The vast majority of ACL tears are complete tears, but one can occasionally suffer only a partial tear. Most people report feeling their knee give way, hearing a pop in the knee, and then having immediate pain and swelling.

Pain is commonly located on the outside or lateral aspect of the knee. However, patients can also tear the medial collateral ligament (MCL) or medial meniscus, which can cause pain on the medial or inside of the knee.

What Does an ACL Tear Feel Like?

People can typically walk on the knee with pain after such an injury, but they may feel that their knee gives way or feels weak. Oftentimes, other structures, such as the meniscus cartilages, are also injured in the knee when the ACL tears. However, after four to six weeks, the knee can feel nearly normal and one can do non-athletic, everyday activities.

However, if a person without a functioning ACL then suddenly twists or rotates on the knee, they may experience a giving way episode of the knee. Over time, these repeated giving way episodes damage other structures in the knee such as the meniscus and articular cartilage, leading to premature knee arthritis.

Do ACL Tears Heal on Their Own?

Unfortunately, tears of the ACL do not heal. The ACL is bathed in synovial fluid. When it tears, the small blood vessels that once surrounded it also tear and there is bleeding within the knee. However, the surrounding synovial fluid quickly dilutes this blood and therefore the healing potential is greatly diminished.

In contrast, when the MCL tears, the bleeding that occurs is not washed away from the site of the tear by synovial fluid. In turn, this bleeding helps mobilize healing cells to the site of the MCL tear. These cells form collagen fibers, which act to bridge the torn MCL back to its native attachment site from which it tore away. Most MCL tears can therefore fully heal as long as the knee is protected in a knee brace so that the MCL does not heal in an elongated and lax position.

This article first appeared in the Summit Daily News in Dr. Cunningham's "Ask A Sports Medicine Doc."


Dr. Rick Cunningham is a Knee and Shoulder Sports Medicine Specialist with Vail-Summit Orthopaedics. He is a physician for the US Ski Team and Chief of Surgery at Vail Valley Medical Center. Do you have a sports medicine question you'd like him to answer in this column?

Visit his website at to submit your topic idea. For more information about Vail-Summit Orthopaedics, visit


bottom of page