Anterior cruciate ligament
The anterior cruciate ligament, more commonly known as the ACL, is one of four ligaments in the knee joint. Injury to this ligament is one of the most common injury to the knee. These injuries can be sustained in practically any arena, though most commonly in sporting events. Most patients who injure this ligament need surgery, and rehabilitation after surgery takes anywhere from six to nine months. Although rehabilitation and surgery take place, it is extremely common to re-injury the ligament after the initial injury.
The function of the ACL is to provide rotational stabilization in the knee joint. Other functions include: the prevention of hyperextension of the knee, the secondary stabilization of valgus stress, the reinforcement of the medial collateral; and the controlling rotation of the tibia of the femur.The ACL runs from the back of the femur bone, to the front of the tibia, through the center of the knee joint. It controls of the movement of the tibia in relation to the femur; in other words, it controls how far the tibia can move.  The ACL works with three other ligaments that are connected to the tibia and femur: the medial collateral ligament, lateral collateral ligament, and the posterior cruciate ligament. 
The Anterior Cruciate Ligament is a very easy ligament to tear. Most ACL injuries occur during sport arenas; many times though the tearing of the ACL can occur in non-sports related activities. The hyperextension of the knee 10 degrees beyond the normal straightening is a common reason for the tearing of the ACL. This is often seen when basketball players land straight-legged after jumping, or when baseball players slide into base. Another cause of an ACL injuries, is when a twisting force is exerted on the knee and the foot is still firmly planted on the ground.  Soccer, basketball,and other sports where the athletes pivot and quickly change directions, are some of the most common areas in which injuries like this take place. One other way in which the ACL is most likely to be injured is a hard blow to the side of the knee.  This is seen often in tackle sports such as football and rugby.
Other ways in which the ACL can be damaged or torn include but are not limited to:
- Motor-vehicle accidents
- Repeated trauma
- Rough Pplay in a non-sports environment
- Work-related injuries
Sports in which the tearing of the ACL can occur:
- Martial Arts
Symptoms Extreme pain and extensive swelling within the first six hours is common however, sometimes swelling can be minimal or delayed.  
Other symptoms can include widespread mild tenderness, tenderness at the medial side of the knee joint, and pain when placing weight on the injured leg. Positive signs in special tests such as the anterior drawer test and the Lachman’s test which place stress on the ACL and can detect torn ligaments, are indicators of injury. MRI’s can also be used to determine if the ligament is damaged, or if there are any other injuries within the knee joint. </ref> And finally, although the initial pain and swelling that occurs at the time of the injury goes away after 2-4 weeks, the instability of the joint remains for varying amount of time. 
There are two different types of treatment for an ACL injury: nonsurgical and surgical. Nonsurgical treatment includes the first aid administered at the scene of injury but it mostly focuses on the treatment received instead of or before surgery. When an athlete injures their ACL, the first thing they should do is to immediately stop participating in competition, exercise R.I.C.E. (Rest, Ice, Compression, and Elevation), as well as receiving immediate medical attention.  Taking pain relievers, such as ibuprofen, and wearing a splint to keep the knee straight is also recommended.  A professional can assess the damage done to the joint as well as any other damage done to the surrounding area. If the doctor is unable to determine the extent of the damage they may take an MRI-scan or X-ray. Once the diagnosis has been given, the doctor can then decide to recommend surgery if necessary, as well as provide a rehabilitation program to strengthen the knee and reduce swelling in joint. 
The purpose of nonsurgical treatment is to ease pain, and decrease swelling. Rest and mild pain medication are definitely a part of the treatment, and crutches are typical. Also, the patients may need to undergo aspirating, which is the inserting of a needle into the joint and drawing out as much unnecessary fluid to relieve the swelling. Aspirating gives doctors extremely helpful information because if there is any blood in the fluid, there is a 70% chance that it means a torn ACL as well as sometimes confirming whether or not the cartilage in the knee joint is injured.  Usually patients will go to a Physical Therapist to receive treatment for swelling and pain as well as performing specialized exercises that strengthen the hamstring and quadriceps muscles, and allow for range-of-motion that could be lost. An ACL brace is another type of treatment that is commonly seen, they are custom-made, and recommended when surgery is not planned. 
SurgeryThe surgical reconstruction of the anterior cruciate ligament is carried out more often than not after injury. Surgery is determined by the doctor who looks at a number of factors: patient’s age; lifestyle; involvement in sporting arenas; job; knee joint instability and insecurities; as well as other injuries to surrounding areas. 
Other factors that affect whether or not surgery would be performed include the degree of participation of activities that require a efficient ACL (sporting or not), as well as whether or not the tear is partial or complete. If the ACL is completely torn, it will require surgery, if it isn’t torn, it may not need surgery.The main goal of surgery in the case of the ACL is to keep the tibia from sliding too far forward under the femur, as well as allowing the knee joint to begin functioning properly again. There are two types of surgery, repair and reconstruction. Surgical repair is where, if the damage is in the middle of the ligament, it is stitched back up; if the ligament has been torn form the bone, then that piece of bone is reattached.  In surgical reconstruction there are two different techniques used: extraarticular technique and intraarticular technique.
The extraarticular technique is where a suture, such as a part of the hamstring tendon, is inserted in place of the ACL.  One example of this is the Allograft Reconstruction.  The intraarticular technique is where the doctor uses a graft or structure that is already located within the knee to replace the ACL.  An example of this is the Patellar Tendon Graft.
- Arthroscopic Method: The reconstruction of the ACL that uses a graft from another ligament or tendon to replace the damaged ACL. This procedure is done with the help of the arthroscope which allows the surgeon to leave the joint closed. Usually, a patient is able to go home the same day however, some stay at the hospital for one or two nights to recuperate. 
- Patellar Tendon Graft: The surgeon takes a strip of the patellar tendon, which connects the kneecap to the tibia and replaces the damaged ACL with it. 
- Hamstring Tendon Graft: Similar to the Patellar Tendon Graft in that the surgeon takes a graft from one of the hamstring tendons to reconstruct the ACL. When arranged in three or four strips, the graft has almost the same amount of strength as the Patellar Tendon Graft. 
- Allograft Reconstruction: The surgeon uses a tissue from a deceased organ donor whose organs have been checked for diseases, sterilized, and frozen. These grafts can be from the tibialis tendon, the patellar tendon, the hamstring tendon, or the Achilles tendon. However, surgeons usually use the patellar tendon because it comes with the bone attached to the ligament. Allografts are popular because it does not disturb any normal tissue, or take away strength from the joint. 
The rehabilitation process after an ACL surgery is the most important part of surgery. After surgery the doctor typically recommend a formal progressive therapy for four to six months. At the beginning it is probable that the patient will see the therapist 2-3 times per week. Once the patient has shown remarkable improvement, visits to the therapist will decrease and home therapy exercises will take place. Eventually, the patient will see the therapist once every few weeks until completely recovered.  Their are four reasons for rehabilitation: joint repair; stabilization; ability to continue in physical activities; as well as a quicker return to those activities 
There is also nonsurgical rehab, which lasts for typically six to eight weeks. Physical Therapists use electrical stimulation and ice to reduce pain and swelling. Range-of-motion exercises will also help to gradually strengthen muscles. Another service that the physical therapists provide is to help the patient find and use a brace that is custom-made for them. 
- ANTERIOR CRUCIATE LIGAMENT INJURIES: Treatment and Rehabilitation by Mervyn J. Cross, North Sydney Orthopaedic and Sports Medicine Centre.
- Orthopedic Information: Ankle eOrthopod Web. Accessed December 11, 2014.
- Anterior Cruciate Ligament Tears: What Is The Anterior Cruciate Ligament? by ehealthMD
- Torn Anterior Cruciate Ligament / ACL Knee Injury by Sports Injury Clinic
- What You Need To Know About ACL Tears By Jonathan Cluett, M.D., About.com Guide
- National Library of Medicine
- Sports Injury Clinic
- Encyclopedia of Sports Medicine and Science
- e-medicine from WebMD