Explore ACL Injuries


The anterior cruciate ligament (ACL) is one of four major ligaments that stabilize the knee joint. A ligament is a tough band of fibrous tissue, similar to a rope, connecting the bones together at a joint. There are two ligaments on the sides of the knee (collateral ligaments) that give stability to sideways motions: the medial collateral ligament (MCL) on the inner side and the lateral collateral ligament (LCL) on the outer side of the knee.

Two ligaments cross over in the center of the knee joint (therefore, called cruciate). The forward ligament is the Anterior Cruciate ligament (ACL) and the ligament behind is the posterior cruciate ligament (PCL). The ACL prevents the shin bone (tibia) from sliding forward relative to the thigh bone (femur) and stabilizes the knee to allow pivoting, twisting, and jumping sports. The PCL prevents the tibia from moving backward relative to the femur.

ACL Injury

How can the ACL tear?

The most common mechanism is a combination of a sudden stopping motion on the leg while quickly twisting at the knee. The planted foot remains stationary while the body rotates toward the opposite side of the body. This can happen in a sport such as basketball, when a player lands on the leg when coming down from a rebound or is running down the court and makes an abrupt stop to pivot.

In football, soccer, or lacrosse, cleats prevent the foot from slipping during this motion. As a result all the stress is transferred to the ACL of the knee. When this stress exceeds the strength of the ACL, the ligament tears.

In skiing, the ACL is commonly injured when the skier sits back while falling. The modern ski boot is stiff, high, and is tilted forward. The boot thus holds the tibia forward. The weight of the body can quickly shift backwards. When the body rotates away from the weight bearing ski, the knee is twisted. Similar toccatas, the ski prevents the foot from moving, causing all the stress to be absorbed by the ACL. When force exceeds the strength of the ligament, it fails (tears).

Signs that an ACL is torn

When the ACL tears, the person feels the knee go out of joint and often hears or feels a "pop". If he or she tries to stand on the leg, the knee may feel unstable and give out. The knee usually swells a great deal immediately (within two hours). Over the next several hours, pain becomes more severe and it becomes difficult to walk.

What other knee structures can be injured when the ACL tears?

There are two types of cartilage in the knee. One type of fibrous cartilage is called the meniscus. Each knee has two crescent shaped menisci that act as a shock absorbers between the femur and tibia. There is a medial (inner) and lateral (outer) meniscus. The menisci are attached to the tibia. The ACL prevents excessive forward tibial motion at the knee joint. When the ACL tears, the knee can partially dislocate (subluxate). Either meniscus can become compressed between the femur and tibia resulting in tears. This abnormal motion of the joint can also bruise the bones.

There is a second type of cartilage in the knee joint called articular cartilage. This joint cartilage is the hard white tissue that is seen at the end of a chicken bone. It is a smooth, white glistening surface that covers the ends of the bones. The surface is slick and much smoother than ice and as a result, there is very little friction when the joint moves. This joint cartilage can get damaged when the ACL tears.

If this articular cartilage is injured, the joint no longer moves smoothly. Stiffness, pain, swelling and grinding can occur. Eventually, arthritis can develop.

With severe contact injuries, such as trauma from motor vehicle accidents, other ligaments can be injured as well. The medial collateral ligament tears when the knee is hit from the outside. The lateral collateral ligament is torn when the knee is hit from the inside. When multiple ligaments are torn, the knee may have injuries to the nerves and blood vessels as well.


What is the initial treatment for a knee that may have a torn ACL?

The initial treatment of the injured joint is to apply ice and gentle compression to control swelling. A knee splint and crutches are used. Medical attention should be sought to confirm the diagnosis and to evaluate the entire extremity. X-rays are taken to rule out a fracture. Sometimes an MRI is needed, but usually the diagnosis can be made by physical examination.

How will the knee function if the ACL is torn?

If no structure other than the ACL is injured, the knee usually regains it range of motion and is painless after six or eight weeks. The knee can then feel completely normal. However, it can be a "trick knee". If a knee does not have an ACL it can give way or be unstable when the person pivots or changes direction. The person can usually run straight ahead and ride a bicycle without a difficulty. However, when the person with a torn ACL makes a quick turning motion away from the planted foot, the knee tends to give way and collapse. This abnormal motion can damage the menisci or articular cartilage and cause further knee problems.

If a person does not participate in pivoting sports and is relatively inactive, the knee can feel quite normal without an ACL. Thus, many patients especially over the age of thirty may not need to have the ACL reconstructed. This is especially true if they do not participate in sports that require quick changes in direction.

In young, athletic patients, however, the knee will tend to give way during activities in which the person quickly changes direction. Each time the knee gives way, the menisci and articular cartilage are exposed to abnormal stresses. Damage to the joint surfaces and menisci can result in early arthritis.

Nonsurgical Treatment: Do all ACL tears need surgery?

No. Some knees function almost normally despite having a torn ACL. Good knee function is more common in patients who are over thirty years old who are relatively inactive in sports. Patients who are less than twenty-five years old, regardless of activity level, tend to have problems with instability and have frequent episodes of giving way. Therefore, surgical reconstruction of a torn ACL is usually recommended for patients who are less than age twenty-five years. However, surgery should be delayed until after the acute injury has subsided (usually at least six weeks following injury).

Surgical Treatment: When should surgery be performed for a torn ACL?

It is best to wait for the pain and swelling to subside and to allow associated injuries to heal before performing surgery for the ACL. If surgery is done too soon after injury, rehabilitation is difficult. The knee may get stiff and have permanent loss of motion. The athlete will usually get back to sports much more quickly if the knee is allowed to recover from initial injury and to regain its full painless range of motion (usually at least six weeks) before performing surgery.

Usually, the best treatment following acute ACL injury is to protect the joint and apply ice and use crutches for several weeks. As the swelling and pain subside, the patient can put weight on the leg. The knee immobilizer and crutches are discontinued. The patient starts therapeutic exercises to regain motion. Resistive exercises to build up strength should not be done during this time to prevent damaging the kneecap (patella).

If the knee also has an injured medial collateral ligament (MCL), it is best to allow the MCL to heal completely (usually six to eight weeks) before reconstructing the ACL. Then an arthroscopic procedure can be performed to reconstruct the ACL. The torn MCL usually does not need to be repaired surgically.

There may be instances when a surgeon wants to perform surgery immediately following an injury. An example is knee dislocation when multiple ligaments are torn. Tears of the outer knee ligament (lateral collateral ligament) should usually be repaired surgically. Individual decisions need to be made on whether or not to reconstruct the ACL soon after injury in these instances where immediate surgery may be required.


There are different surgical options to repair a torn ACL. You and your doctor can discuss which is best for your unique case. The options include one of four ACL grafts - Allograft Graft, Bone-Patellar Tendon-Bone Graft, Hamstring Graft or Quadriceps Tendon Graft - or an Anatomic Footprint ACL Reconstruction.

If you have had surgery to repair a torn ACL, and you think you might need revision ACL surgery, our Sports Medicine surgeons consult on and perform revision ACL surgery frequently.