The knee is a hinge-type joint. The thigh bone (femur) attaches to the shin bone (tibia) with the oval kneecap in the front. The joint may be thought of has having three compartments; the inner, outer and front compartments. Deep inside the knee are two ligaments, the ACL and PCL ligaments. They help to stabilize the knee against forward and backward pressures. On each side of the joint (medial = inside and lateral = outside) are the collateral ligaments and provide side-to-side stability. The kneecap protects the front of the joint, and also acts as a pulley mechanism for the large thigh muscles (quadriceps) to straighten the leg at the knee joint.
One of the most common knee injuries is a sprain or tear of the ACL (anterior cruciate ligament). If the knee experiences a sudden or abnormal (especially sudden) strain or force on it, the ACL may stretch (sprain) or tear. The PCL (posterior cruciate ligament) may also tear, but this is less common.
There is usually a very loud pop at the time of the injury. A torn ACL usually causes sudden pain. The knee usually swells up due to bleeding in the joint immediately after the injury. A complete ACL tear may then make the knee feel unstable or loose with walking, or give out without warning. Over time, abnormal wear and tear on the knee cartilage may result in osteoarthritis.
Along with a history about the injury and knee symptoms, a knee examination often suggests an ACL tear is present when the knee is moved and rotated. X-rays may be normal, and an MRI often is necessary.
Treatment of ACL injuries depends on whether the ligament is completely or partially torn. Partially torn ACL ligaments may be treated with physical therapy. Sometimes braces are helpful. If the ACL is completely torn, full recovery usually requires surgery.This is normally done using a device called an arthroscope (a flexible surgical instrument with a fiberoptic camera) that is inserted through small incisions into the knee joint by an orthopedic surgeon, and the ACL is repaired.