There are many ways the ACL can be torn; the ligament tears because it is overstretched. The movements of the knee that can result in a tear is when the knee is straightened more than 10 degrees beyond its normal maximal position (hyperextension) - the lower leg is forced forward in relation to the upper leg.^[2] Tears in the anterior cruciate ligament often take place when the knee receives a direct impact from the front of the thigh while the leg is in a stable position, for example a standing football player is tackled sideways when his feet are firmly planted.

Torn ACLs are most often related to high impact sports or when the knee is forced to make sharp changes in movement and during abrupt stops from high speed. These types of injuries are prevalent in alpine skiing,^[3]Soccer, American football, Australian rules football, basketball, rugby, professional wrestling, martial arts, and artistic gymnastics. Research has shown that women involved in sports are more likely to have ACL injuries than men. ACL tears can also happen in older individuals through slips and falls and are seen mostly in people over 40 due to wear and tear of the ligaments. An ACL tear can be diagnosed by a popping sound heard after impact, swelling after a couple of hours, severe pain when bending the knee, and when the knee buckles or locks during movement or gives way while standing still with weight on the affected knee.

The ACL primarily serves to stabilize the knee in an extended position and when surrounding muscles are relaxed; so if the muscles are strong, many people can function without it. Fluids will also build the muscle.

The term for non-surgical treatment for ACL rupture is "conservative management", and it often includes physical therapy and using a knee brace. Lack of an ACL increases the risk of other knee injuries such as a torn meniscus, so sports with cutting and twisting motions are strongly discouraged. For patients who frequently participate in such sports, surgery is often indicated.

Patients who have suffered an ACL injury should always be evaluated for other knee injuries that often occur in combination with an ACL tear. These include cartilage/meniscus injuries, bone bruises, PCL tears, posterolateral injuries and collateral ligament injuries.

Conservative[edit]

A torn ACL is less likely to restrict the movement of the knee. When tears to the ACL are not repaired it can sometimes cause damage to the cartilage inside the knee because with the torn ACL the tibia and femur bone are more likely to rub against each other. Immediately after the tear of the ACL, the person should rest the knee, ice it every 15 to 20 minutes, provide compression on the knee, and then elevate above the heart; this process helps decrease the swelling and reduce the pain. The form of treatment is determined based on the severity of the tear on the ligament. Small tears in the ACL may just require several months of rehab in order to strengthen the surrounding muscles, the hamstring and the quadriceps, so that these muscles can compensate for the torn ligament. Falls associated with knee instability may require the use of a specific brace to stabilize the knee. Women are more likely to experience falls associated with the knee giving way. Sudden falls can be associated with further complications such as fractures and head injury.

Surgery[edit]

If the tear is severe, surgery may be necessary because the ACL cannot heal independently, as there is no blood supply to this ligament. Surgery is usually required among athletes because the ACL is needed in order to perform sharp movements safely and with stability. The reconstructive surgery is usually done several weeks after the injury in order to allow the swelling and inflammation to go down. During surgery the ACL is not repaired, instead, it is reconstructed using other tendons in the body. There are three different types of ACL surgery. Patella tendon-bone auto graft and hamstring auto graft are the most common and preferred and tend to produce the best results. For the Patella tendon-bone auto graft, the central 1/3 of the patella tendon is removed along with a piece of bone at the attachment sites on the kneecap and tibia. The advantages of using this method is that the patella tendon and ACL are relatively the same length and it uses a bone to bone attachment which most surgeons agree is much stronger than other healing methods. Disadvantages of this method is common anterior knee pain due to the removal of bone from the kneecap. For the hamstring auto graft, two tendons are taken from the hamstring muscles and wrapped together to form the new ACL. Advantages of this method are less pain associated with post surgery healing than that of the patella tendon-bone graft due to the fact no bone is removed, and the small incision. The disadvantage of this method is that the new ligament takes longer to heal since there is no bone to bone healing and the tendon to bone connection takes relatively long to become rigid.^[16] After the surgery, rehabilitation is required in order to strengthen the surrounding muscles and stabilize the joint.

There are two main options for ACL graft selection, allograft and autograft. Autografts are the persons own tissues, and options include the hamstringtendons or middle third of the patella tendon but it is not known which is best.^[17] Allografts are cadaveric tissue sourced from a tissue bank. Each method has its own advantages and disadvantages; hamstring and middle third of patella tendon having similar outcomes. Patellar grafts are often incorrectly cited as being stronger, but the site of the harvest is often extremely painful for weeks after surgery and some patients develop chronic patellartendinitis. Replacement via a posthumous donor involves a slightly higher risk of infection. Additionally, donor grafts eliminate tendon harvesting which, due to improved arthroscopic methods, is responsible for most post-operative pain.

The surgery is typically undertaken arthroscopically, with tunnels drilled into the femur and tibia at approximately the original ACL attachments. The graft is then placed into position and held in place. There are a variety of fixation devices available, particularly for hamstring tendon fixation. These include screws, buttons and post fixation devices. The graft typically attaches to the bone within six to eight weeks^{[citation needed]}. The original collagen tissue in the graft acts as a scaffold and new collagen tissue is laid down in the graft with time. Hence the graft takes over six months to reach maximal strength.^{[citation needed]}

After surgery, the knee joint loses flexibility, and the muscles around the knee and in the thigh tend to atrophy. All treatment options require extensivephysical therapy to regain muscle strength around the knee and restore range of motion (ROM). For some patients, the lengthy rehabilitation period may be more difficult to deal with than the actual surgery. In general, a rehabilitation period of six months to a year is required to regain pre-surgery strength and use.^{[citation needed]} This is very dependent on the rehabilitation assignment provided by the surgeon as well as the person who is receiving the surgery. External bracing is recommended for athletes in contact and collision sports for a period of time after reconstruction. It is important however to realize that this type of prevention is given by a 'surgeon to surgeon' basis; all surgeons will prescribe a brace and crutches for post surgery recovery. Total usage time is one month. After surgery, no sports are allowed for 6 to 7 months. Whether the ACL deficient knee is reconstructed or not, the patient is susceptible to early onset of chronic degenerative joint disease.