What are the anterior cruciate ligament (ACL) and an ACL injury?
The anterior cruciate ligament (ACL) is one of four knee
ligaments that connect the upper leg bone (femur) with the large lower leg bone
(tibia). The ACL stabilizes knee movement by:
- Preventing the lower leg bone from sliding
forward or turning when the leg is straight.
- Preventing the knee
from being stretched or straightened beyond its normal limits (hyperextended).
- Supporting the knee ligaments that keep the knee from bending
See a picture of the
knee and the ACL .
An ACL injury can involve a small or medium tear
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 may move abnormally forward on the
upper bone, with a sense of the knee giving out, and possibly further injuring
the inside structures of the knee.
What are the risks of an ACL injury?
The risks of
an ACL injury depend on whether you stop or modify activities that require a
stable knee, how severe the injury was, whether other parts of the knee also
were injured, and whether you take part in and complete a rehabilitation
If nothing is done, the ACL injury may develop into
chronic ACL deficiency. The knee becomes more and more unstable and may give
out more often. The abnormal sliding within the knee also can damage
cartilage and trap and damage the
menisci in the knee and can lead to premature
- If you can live within the limits that a
somewhat loose knee requires and avoid repeated episodes of instability, your
knee will not necessarily develop osteoarthritis.
- If you
repeatedly do things that cause your knee to give way and become painful and
swollen, the joint will develop degenerative changes that can become disabling.
If you wait too long to reconstruct a torn ACL and you
develop chronic ACL deficiency, the surgeon may not be able to repair all the
joint damage, and you may still have pain and swelling even though the surgery
makes the knee stable again. It is important to have surgery before knee
instability leads to degenerative changes.
What are the possible complications of surgery and rehabilitation?
ACL reconstruction surgery is generally safe.
Complications from surgery or problems that may arise
during rehabilitation and recovery include loss of motion in the knee joint,
grating of the knee cap, and pain or swelling during activities ranging from
daily activities to strenuous sports.
How successful is ACL surgery?
About 60% of people
who have ACL surgery return to the full level of activity they had before their
injury.1 But between 80% and 90% of people who have
ACL surgery have favorable results, with reduced pain, good knee function and
stability, and a return to normal levels of activity.2
Between 3% and 10% of people who have ACL surgery still have knee pain and