The anterior cruciate ligament is a thick band of tissue which has two major strands that extend from the lower leg bone (tibia) to the thigh bone (femur). This ligament is very important for maintaining stability of the knee. When it is injured or torn the patient feels the instability of the knee when they turn or pivot. This instability is particularly problematic when participating in pivoting sports such as soccer and football. The ligament sits just in front of its counterpart, the posterior cruciate ligament, directly in the middle of the knee joint.
Mechanism of Injury
Most anterior cruciate ligament tears occur during a sporting activity and usually in younger patients. When you consider the number of sport hours played, they are more common in women. There have been a variety of reasons proposed for this, such as muscle imbalance and slight variations in the anatomy of the knee joint in women compared to men. The most common sports are football and basketball in younger patients; skiing injuries predominate in older patients. It is, however, possible to injure the anterior cruciate doing a variety of activities. We’ve seen bilateral ACL tears in a weight lifter who was doing an incline bench and popped both his knees at the same time when bench-pressing 350 pounds. It can also be a work-related injury. Interestingly, most people would expect that it is due to contact, but this is not true. Mostly it is a non-contact deceleration where the athlete suddenly turns to the opposite side of the planted and injured knee. As the patient turns and pivots the ligament tears. In basketball it is usually a result of a hyperextension and internal rotation of the tibia on the femur, associated with deceleration.
Usually the patient will feel a sudden pop in their knee immediately in injury to the knee. Surprisingly, sometimes the knee will not get very swollen, although it certainly can. The injury is often missed because the physical examination requires some experience and training. It might actually be easily missed in the initial stages.