If you have a sudden (acute) anterior cruciate ligament (ACL) injury, you typically know when it happens. You may feel or hear a pop, and the knee may give out, causing you to fall. The knee swells and often is too painful or unstable for you to continue any activity.
An ACL injury can cause small or medium tears of the ligament, a complete tear of the ligament (rupture), a separation of the ligament from the upper or lower leg bone (avulsion), or a separation of the ligament and part of the bone from the rest of the bone (avulsion fracture). When any of these occur, the lower leg bone moves abnormally forward on the upper bone, with a sense of the knee giving out or buckling.
How an anterior cruciate ligament (ACL) injury
is treated and how it heals depends on:
- The condition of the ACL before the injury. This includes prior injuries, partial tears, ACL deficiency, and changes due to age.
- The general condition and health of the rest of your
knee before this injury.
- The amount of damage or
injury to the ACL. Injuries are usually grouped into
grade I, II, or III sprains (tears) according to the
amount of damage.
- Other injuries to the knee joint, such as to
menisci, or to bones in the knee.
age, how active you are, and how committed you are to treatment and
- The time of diagnosis. If the ACL diagnosis is not
made soon after the injury, the knee may be further damaged with use.
An ACL injury may develop into long-lasting and
ACL deficiency that leads to an unstable knee-the knee buckles or gives out,
sometimes with pain and swelling. This can occur if you had an ACL injury in the
past and didn't know it or if your ACL has not been
treated or has been treated unsuccessfully. ACL deficiency can cause damage to the joint,
osteoarthritis. But not everyone with an ACL injury
gets ACL deficiency.
People with minor ACL injuries usually begin treatment with
a physical rehabilitation (rehab) program. Rehab exercises build strength and
flexibility in the muscles on the front of the thigh (quadriceps) and
strengthen and tighten the muscles in the back of the thigh (hamstrings). Most people return to their normal activities after a few weeks of
More serious ACL injuries may need several
months of rehab or surgery followed by several months of
rehab to regain your knee strength, knee stability, and range of
Not all ACL injuries require surgery. But whether you have
surgery or not, you need to start strengthening your knee and regaining motion
soon after you injure it. This prepares you for your rehab program if
you choose not to have surgery. It also helps prepare the knee for surgery if
you choose to have it.