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.
When you twist, bend, or over-straighten your knee, you can injure your ACL. This 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.