Anterior cruciate ligament (ACL) injuries are caused when the knee is straightened beyond
its normal limits (hyperextended), twisted, or bent side to side.
Typical situations that can lead to ACL injuries include:
- Changing direction quickly or cutting around an
obstacle or another player with one foot solidly planted on the ground (as can
sports that put high demand on the ACL such as
basketball, football, soccer, hockey, and gymnastics).
after a jump with a sudden slowing down, especially if the leg is straight
(such as in basketball).
- Falling off a ladder, stepping off a curb,
jumping from a moderate or extreme height, stepping into a hole, or missing a
step when walking down a staircase. Injuries from these situations tend to be
caused by stopping suddenly, with the leg straight.
Inactive people and some older adults who have weak leg
muscles may injure their knees during normal daily activities. But they usually
injure bones, not ligaments.
When contact causes an ACL injury, it
can be from playing a sport, from a sudden and severe accident, or from less
obvious contact injuries. In football, receiving a clipping contact injury-in
which the bent knee is struck from the outside-can cause an ACL injury.
Clipping often damages several knee structures at the same time, including the
ACL, the medial collateral ligament (MCL), and the pads in the
knee (menisci) that protect and cushion the joint surface
and bone ends. Clipping injures the medial meniscus more often than the lateral
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 your ACL has not been
treated or has been treated unsuccessfully, or if you had an ACL injury in the
past and did not know it.
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