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Health Library

Anterior Cruciate Ligament (ACL) Injuries


The ACL (anterior cruciate ligament) is one of the four major ligaments that help to stabilize and support the knee. The ACL works along with the PCL (posterior cruciate ligament) and the medial and lateral collateral ligaments (on the sides of your knee) to stabilize the knee joint. The ACL prevents the tibia (shin bone) from moving too far forward on the femur (thigh bone) and limits rotational movement of the knee.


An ACL injury results from overstretching or tearing this ligament. Immediately after an ACL injury, your knee may swell, feel unstable, and become too painful to bear weight. Many people report that they feel a “popping” sensation in their knee when this type of injury occurs.


The ACL can be injured in several different ways. Causes of injuries can include changing direction quickly, landing incorrectly from a jump, direct contact from a collision, or stopping suddenly. Movements of the knee that result in a tear are often described as a hyperextension injury (the knee straightens beyond its normal fully straightened position) or a pivoting injury (excessive inward turning of the lower leg).


Diagnosis begins with a physical examination and a review of patient history with a qualified musculoskeletal expert. A physician will compare a patient’s non-injured knee with the injured knee. Most injured ligaments can be diagnosed after an examination using special manual tests. Imaging tests such as MRI or x-rays may be needed to see if there are additional injuries such as broken bones or additional tissue injuries (to muscle, tendons, ligaments, or cartilage).


There are both non-surgical and surgical options available. Non-surgical treatment could be effective for patients that have a low activity level or who are elderly.


Bracing and physical therapy are two non-surgical treatment options. A brace may be recommended to protect the knee from instability or buckling. To keep from putting weight through the knee joint, you may be given crutches. Specific exercises will help restore function and strengthen the leg muscles as directed by a physical therapist.


The ligament must be reconstructed to restore knee stability. Usually surgery is not performed right away to give inflammation a chance to resolve. Prior to surgery, your doctor may also suggest that you see a physical therapist to try and regain as much normal range of motion as possible at the knee joint. Arthroscopic surgery (a small incision) is performed to rebuild the ACL using a new ligament for the damaged one (allograft) or using tendons from other parts of the body to substitute as the ACL (usually a part of the patella tendon or hamstring muscle).


After surgery, you will use crutches or a walker at first. You will gradually increase the amount of weight you can put on your repaired leg. You will likely be wearing a brace to give you support while the knee is healing and regaining strength. You may initially have a machine that moves your leg to help you progress with range of motion. Physical therapy will be a very important part of your recovery. A physical therapist will guide you in many stretches and exercises to help you restore your range of motion, strength, and stability.


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