The knee is much more than just a hinge between the thigh bone and the shin bone. Seriously, the knee joint allows hinge like motion primarily, but also allows us to pivot with some degree of rotational movement, providing stability to quick turns and change from forward to lateral movement. The the strong ligament that runs from the outside back part of the femur (thigh bone) to the inside front part of the tibia (shin bone) is called the anterior cruciate ligament (ACL). The posterior cruciate ligament (PCL) runs from the inside of the back of the knee to the outside of the front of the knee, and forms an X shape along with the ACL, giving them the name cruciate, from the Latin from crux or crusis meaning “cross”. When a person plants the foot and quickly turns toward the other direction, e.g. plants the right foot to cut left, the femur rotates on the tibia, and this rotation is limited by the ACL. The ACL is a very strong ligament, ranging in diameter from 7-12 mm, larger in men than women. Still a violent stress on this ligament, either from the individual’s own weight with the toe and usually the heel fixed in place, or when additional stress is added from contact with another person while doing this maneuver, can rupture this ligament. This usually leads to acute pain and a large amount of swelling in the knee quickly from bleeding and fluid accumulation. Any knee injury that quickly leads to a large knee effusion (sometimes called water on the knee) is suspicious for an ACL injury. Often the athlete will describe a “popping” sound or feeling.
Without an intact ACL an athlete lacks stability of the knee with any pivoting movement, and is not likely to be effective in competitive sports. Fortunately now we have great surgeries involving arthroscopic repairs that often allow athletes to fully recover. The graft materials used need several months to mature to a strength that will allow stress to be placed across them without re-injury, so even with today’s advanced surgery, recovery takes a minimum of 7-8 months, often a year or more. Aggressive therapy to regain strength in the supporting muscles of the knee is key to a good recovery of function and to prevent repeat injury.
For various reasons women are more prone to ACL tear than men. The reasons for this may include a different angle from the pelvis to the femur, hormone differences, strength differences, and size of the ACL itself. For a discussion of proven ways to reduce ACL injury in women see: ACL injury prevention in female athletes.