Causes

Ligaments are tough, non-stretchable fibers that hold bones that span the knee joint together. The anterior and posterior cruciate ligaments in your knee give you stability on your feet. ACL tears most often occur by changing direction rapidly, slowing down from running or landing from a jump. You might hear a popping noise when your ACL tears. This is followed by your knee giving out, pain and swelling.

Excessive twisting or hyperextension of the knee is most likely to happen in football, basketball, skiing and soccer. For example, a football player might twist his knee running during an aggressive cut or hyperextend it while being tackled. Injuries during basketball typically occur during a foot plant followed by a quick change in direction.

Women suffer ACL tears more often than men, depending on the sport as much as two to four times. Although not proven, it has been postulated that the female anatomy may leave women more vulnerable to ACL tears and that the female hormone estrogen may weaken the ligaments.