An anterior cruciate ligament (ACL) injury is a tear in one of the knee ligaments that join the upper leg bone to the lower leg bone. The ACL helps keep your knee stable.
Your ACL can be injured
if your knee joint is bent backward, twisted, or bent side to side. The chance
of injury is higher if more than one of these movements occurs at the same
time. Contact (being hit by another person or object) also can cause an ACL
An ACL injury can cause knee pain, swelling, and weakness. Rest and physical rehabilitation (rehab)—and sometimes surgery—are important to prevent a long-lasting knee problem.
ACL injuries range from mild, such as a small tear, to severe, such as when the ligament tears completely or when the ligament and part of the bone separate from the rest of the bone.
William H. Blahd, Jr., MD, FACEP - Emergency Medicine & E. Gregory Thompson, MD - Internal Medicine & Freddie H. Fu, MD - Orthopedic Surgery
The Health Encyclopedia contains general health information. Not all treatments or services described are covered benefits for Kaiser Permanente members or offered as services by Kaiser Permanente. For a list of covered benefits, please refer to your Evidence of Coverage or Summary Plan Description. For recommended treatments, please consult with your health care provider.