Cause
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 happen in sports that put high demand on the ACL such as basketball, football, soccer, hockey, and gymnastics).
- Landing 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 meniscus.
An ACL injury may develop into long-lasting and recurrent (chronic) 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.
