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An anterior cruciate ligament (ACL) tear is an injury to one of the major ligaments stabilizing the knee. It occurs when the biomechanical limits of the anterior cruciate ligament have been exceeded. It is a common injury associated with sports that involve sudden stops, changes in direction or jumping. An anterior cruciate tear may be partial or complete.

The knee joint is a complex hinge joint connecting the thigh bone (femur), which is the longest bone in the body, to the second longest bone in the body, the shin bone (tibia). It is the largest joint in the body and acts in coordination with the pelvis, hip, upper leg, lower leg, ankle and foot to facilitate a complete range of lower body movements. The knee is a strong weight-bearing joint that allows the leg to flex, extend, and twist slightly from side to side.

There are four major ligaments in the knee. With help from the quadriceps muscles in the front of the thighs and the hamstrings in the back of the thighs, these ligaments act to stabilize the knee and keep it moving in the proper direction. The anterior cruciate ligament is one of these four ligaments. Located in the center of the knee it attaches the front of the tibia to the back of the femur. The ACL prevents hyperextension of the knee and keeps the tibia from sliding forward in relation to the femur.

The ACL is the most commonly injured knee ligament. Damage to this ligament frequently results in knee instability. The ACL is often stretched and/or torn as a result of a sudden twisting motion that takes place when the feet stay planted one way, but the knees turn the other way. Injuries can range from mild to severe with the majority a result of participation in sports like football, basketball, lacrosse, soccer, and skiing. Women for a variety of reasons are at a higher risk of sustaining an ACL injury than men.

Typical symptoms of an ACL tear include pain, immediate difficulty with walking and a sensation that the knee is unstable, buckling or giving way. Additionally, a ‘pop’ is often heard and swelling often develops quickly. Fluid within the joint may make it difficult to extend or straighten the knee.

Diagnosis of an ACL injury includes a complete history and a physical exam during which a range of movement will be evaluated. Visual imaging tests may include x-rays to reveal any associated fractures or dislocations and an MRI scan to check for evidence of ACL damage as well as injuries to other knee ligaments, meniscus cartilage, or articular cartilage. Diagnostic knee arthroscopy when necessary allows for a direct visualization of the ACL and associated damage.

ACL injuries can range from a partial ACL tear to a complete ACL tear to bone avulsions. Treatment depends on the severity of the tear. Approximately 50% of ACL injuries occur with injuries to other structures in the knee. While some cases can be treated with non-surgical methods including bracing and knee rehabilitation, almost all active patients and athletes will require surgery to reconstruct the torn ACL. A full recovery can take several months and will depend in large part on a strong commitment to a physical rehabilitation regimen.