An ACL tear is a partial or complete rip of the anterior cruciate ligament, one of the four knee ligaments that stabilize the joint, according to MedicineNet.com. An ACL tear is a frequent knee injury, occurring when a patient quickly changes direction or pivots when the knee is locked.
Athletes suffering from an ACL tear frequently feel the knee pop, followed by swelling and pain in the joint. Because of differences in muscle function and anatomy, women tear their ACLs more frequently than men. The treatment protocol is designed to bring the knee back to its quality of function before the injury, although this may take as long as nine months, notes MedicineNet.com.