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If you feel a sudden pop in your knee, accompanied by pain and instability, you may have suffered an ACL tear. See your doctor immediately.

The anterior cruciate ligament (ACL) is one of the four major ligaments of the knee. The ACL is located in the center of the knee joint and is one of the structures that connect the femur (thigh bone) to the tibia (shin bone). Its primary function is to keep the tibia from moving forward abnormally in relation to the femur. When the ACL is torn, active patients usually develop a feeling of knee instability. This may limit an active lifestyle and can cause injury to other structures in the knee, such as tears in the shock absorbing medial and lateral meniscus cartilages. This can lead to the long-term development of arthritis.

Causes
Tears of the ACL are common in patients involved in sports that require cutting or jumping, and are usually non-contact injuries. In basketball, the ACL may be torn by a player landing from a rebound. In football, a common mechanism causing an ACL tear is a sudden change in direction on a cleated shoe that is fixed in the turf. In skiing, a classic ACL injury involves falling forward and outward over a ski tip that has become caught in the snow.

The ligament may tear only partially, but is more frequently completely ruptured. ACL sprains are graded as Grade I (minimal damage), Grade II (moderate ligament damage and partial tearing), and Grade III (complete ligament tear). Most anterior cruciate ligament injuries are Grade III.

Symptoms
When an ACL is torn, the patient often reports a 'pop' inside the knee. There is typically transient pain, often on the lateral side (outside) of the knee, and there may be a sensation of instability. The patient is typically unable to continue participating in the activity. The knee usually swells within a few hours and a sense of tightness develops within the knee. Depending on associated injuries, the patient may develop a significant limp and may have difficulty bearing weight on the injured knee.

Diagnosis
Your doctor can detect an ACL tear by examining the knee and learning more about the history of the injury. X-rays cannot be used to detect a tear in the ACL, but are often used to identify other problems and injuries to the bones about the knee. A magnetic resonance imaging (MRI) examination may be used to view the ACL as well as other knee structures.

Treatment
Initial treatment of an acute ACL injury consists of rest, ice, elevation, anti-inflammatory medications, protected weight-bearing and range of motion exercises.

  • Rest . All activities that require active use of the knee should be restricted. Talk with your doctor about a plan for gradually returning to full activity.
  • Ice . Ice packs may be used for about 20 minutes at a time, each hour to help alleviate pain and decrease swelling.
  • Bandage/Compression Wraps . Your doctor may recommend that you use a soft elastic wrap or bandage at the knee to help reduce swelling.
  • Elevation . Elevating the knee should help ease pain and swelling. Ideally, the foot should be elevated higher than the knee, and the knee should be elevated higher than the hip.
  • Medication . Your doctor may prescribe an anti-inflammatory or pain medication.
  • Knee Brace . Your doctor may prescribe a knee brace to help stabilize the joint.
  • Crutches . Your doctor may advise you to use crutches to take pressure off the knee, reduce pain and increase mobility. You should be protected with crutches if weight bearing is painful or if you are limping.
  • Physical Therapy . Your doctor or physical therapist may recommend a range of motion and muscle stretching and strengthening program for your knee.

The primary reason for surgical reconstruction of an acute ACL tear is the patient's desire to return to a high level of activity. Age is of minor importance; older patients who wish to return to a lifestyle that includes cutting or jumping activities should be considered for ACL reconstruction. Patients without other knee joint injuries, who are willing to modify their lifestyles or do not develop knee instability, may be treated with bracing and physical therapy.

If they subsequently develop 'giving way,' especially associated with pain or swelling, this approach should be abandoned in favor of an ACL reconstruction. It may be recommended that children with a torn ACL delay surgery until after they have stopped growing. Talk with your doctor about whether surgery is the best approach for your injury.

Recovery Outlook
After surgery to repair an ACL injury, a minimum of six months of recovery is usually required before you can return to full contact sports. Your doctor may ask you to wear a brace for at risk activities when initially returning to sports after ACL surgery. Full recovery can easily take 12 months. Recovery time depends in large part on the extent of associated injuries and on the care of your knee during the rehabilitation process.