The anterior cruciate ligament (ACL) is a band of tough, fibrous connective tissue in the middle of the human knee joint. It is one of four main ligaments in the knee. The anterior cruciate ligament is connected to both the tibia, or shin bone, and the femur, or thigh bone.
Three other ligaments also connect to the tibia and femur. The medial collateral ligament runs along the inside of the knee joint and prevents it from bending inward. The lateral collateral ligament runs along the outside of the knee joint and stops the knee joint from bending outward. The posterior cruciate ligament works in conjunction with the ACL and prevents the tibia from moving out of alignment and sliding backward under the femur.
The anterior and posterior cruciate ligaments cross over one another inside the knee. The anterior and posterior ligaments are responsible for maintaining the correct anatomical position of the femur and tibia. Another function of the anterior cruciate ligament is to allow stable rotation of the knee joint. If this ligament sustains an injury such as a tear, the knee joint becomes far less stable.
A tear of the anterior cruciate ligament is one of the most common sports-related knee injuries. This injury can occur when the knee suffers a sudden direct blow, when landing after a jump or when the joint twists while the foot in firmly placed on the ground. The ligament then becomes overextended and cannot contend with the force being exerted on it. The end result of this is a tear in the ligament. ACL injuries are more likely to occur in females than males, which is believed to be because of the anatomical, muscular and hormonal differences found between males and females.
There is often an audible popping sound when the anterior cruciate ligamentis torn. Severe pain and a feeling of instability usually accompany this injury. Extensive swelling usually occurs along with widespread tenderness. Movement of the joint will be restricted, including the inability to completely straighten the knee.
An extensive or complete tear of the anterior cruciate ligament usually requires surgery to replace the ligament. Surgery is followed by a period of immobilization to allow healing. This is followed by an intense rehabilitation regime designed to regain maximum flexibility, mobility and rotation of the joint. Rehabilitation can take between six and nine months before normal activity can be resumed.