top of page

Knee Pain

ACL Tear

An anterior cruciate ligament (ACL) injury is the over-stretching or tearing of the ACL in the knee. A tear may be partial or complete. The ACL is probably the most commonly injured ligament of the knee. It is usually injured during sports.

The ACL is located in the center of the knee joint where it runs from the backside of the femur (thighbone) to connect to the front of the tibia (shinbone). The ACL is the main controller of how far forward the tibia moves under the femur. If the tibia moves too far, the ACL can rupture. The ACL is also the first ligament that becomes tight when the knee is straightened. If the knee is forced past this point, or hyperextended, the ACL can also be torn.

ACL injuries often occur with other injuries. For example, an ACL tear often occurs along with tears to the medial collateral ligament (MCL) and the shock-absorbing cartilage in the knee (meniscus).


The mechanism of injury for many ACL ruptures is a sudden deceleration, hyperextension, or pivoting in place. Sports-related injuries are the most common.


Basketball, football, soccer, and skiing are common sports linked to ACL tears. Common causes include:

  • Getting hit very hard on the side of your knee (e.g. during a football tackle)

  • Overextending your knee joint

  • Quickly stopping movement and changing direction while running, landing from a jump, or turning

  • A “popping” sound at the time of injury

  • Obvious knee swelling within 6 hours of injury

  • Pain, especially when you put weight on the injured leg

  • Difficulty continuing with sport

  • Instability in the knee, feeling like it is “giving way”


The history and physical examination are probably the most important ways to diagnose a ruptured or deficient ACL. In the acute (sudden) injury, the swelling is a good indicator. A good rule of thumb that orthopedic surgeons use is that any tense swelling that occurs within two hours of a knee injury usually represents blood in the joint, or a hemarthrosis. If the swelling occurs the next day, the fluid is probably from the inflammatory response.


Placing a needle in the swollen joint and aspirating (or draining as much fluid as possible) gives relief from the swelling and provides useful information to your doctor. If blood is found when draining the knee, there is about a 70 percent chance it represents a torn ACL. This fluid can also show if the cartilage on the surface of the knee joint was injured.


During the physical examination, special stress tests are performed on the knee. Three of the most commonly used tests are the Lachman test, the pivot-shift test, and the anterior drawer test. The doctor will place your knee and leg in various positions and then apply a load or force to the joint. Any excess motion or unexpected movement of the tibia relative to the femur may be a sign of ligament damage and insufficiency.


Another way to check for anterior tibial translation is with the KT-1000 and KT-2000 arthrometers. The patient’s leg is bent and supported on a wedge with the knee in 30 degrees of flexion. The arthrometer is placed against the knee to be tested and strapped to the lower leg. Usually, the normal knee is tested first. The arthrometer applies an anterior force of 15 pounds against the tibia. The amount of anterior tibial translation is measured. The test is repeated with a force of 20 pounds. A third test applies a manual maximal force to the posterior (back) of the tibia. This is similar to the Lachman test.


The results of these tests will help your doctor determine how badly the ACL was injured. Other tests may be combined with tests of ACL integrity to determine whether other knee ligaments, joint capsule, or joint cartilage have also been injured.


Your doctor may order X-rays of the knee to rule out a fracture. Ligaments and tendons do not show up on X-rays, but bleeding into the joint can result from a fracture of the knee joint, or when portions of the joint surface are chipped off.


Magnetic resonance imaging (MRI) is probably the most accurate test for diagnosing a torn ACL without actually looking into the knee. The MRI machine uses magnetic waves rather than X-rays to show the soft tissues of the body. 


In some cases, arthroscopy may be used to make the definitive diagnosis if there is a question about what is causing your knee problem.

Non-surgical treatments
  • Using the RICE (rest, ice, compression, elevation) method: The RICE method involves resting your knee, icing your knee, wearing an elastic bandage around your knee (compression) and elevating your knee while you’re resting. This helps to reduce pain and swelling.

  • Taking pain relievers: Your healthcare provider may recommend taking pain relievers (non-steroidal anti-inflammatory medications, or NSAIDs) to help reduce pain and swelling in your knee. Acetaminophen can be used as an alternative if you cannot tolerate NSAIDs or are allergic.

  • Wearing a knee brace: Your provider may have you wear a knee brace that prevents your knee from moving side to side so that your ACL can heal.

  • Using crutches: Your provider may have you use crutches to walk so that you can limit the amount of weight you put on your affected knee.

  • Doing physical therapy: Your provider will most likely have you do physical therapy exercises to improve your strength and range of motion in your knee. Exercises may involve strengthening your thigh muscles, cycling and doing resistance exercises. If you have an ACL tear and play a sport, your healthcare team will tailor your physical therapy to the types of movements you do for the sport that you play.


You can return to your sporting activities when your quadriceps and hamstring muscles are back to nearly their full strength and control, you are not having swelling that comes and goes, and you aren't having problems with the knee giving way.