What is the anterior cruciate ligament (ACL)?
The anterior cruciate ligament (ACL) is one of four knee ligaments that connect the upper leg bone (femur) with the lower leg bone (tibia). The ACL stabilizes knee movement by:

1.Preventing the lower leg bone from sliding forward or turning inward when the leg is straight.
2.Preventing the knee from being stretched or straightened beyond its normal limits (hyperextended).
3.Supporting the knee ligaments that keep the knee from bending sideways.?
What is an ACL injury?
An ACL injury can involve a small or medium tear of the ligament, a complete tear of the ligament (rupture), a separation of the ligament from the upper or lower leg bone (avulsion), or a separation of the ligament and part of the bone from the rest of the bone (avulsion fracture). When any of these occur, the lower leg bone may move abnormally forward on the upper bone, with a sense of the knee giving out.

Without treatment, an ACL injury may develop into long-lasting and recurrent (chronic) ACL deficiency. The ability of the ACL to control knee movement decreases, and the abnormal bone movement also can trap and damage the tissue that covers the ends of bones (cartilage) and the pads that cushion the knee joints (menisci). This can lead to premature osteoarthritis.

When you injure your ACL, other parts of the knee can be injured at the same time, including the meniscus, other knee ligaments, cartilage, or bones in the knee joint, which may be broken.

What causes an ACL injury?
Your ACL can be injured if your knee is hyperextended, twisted, or bent side to side. The risk of injury increases if a combination of these movements occurs at one time. Contact (being hit by another person or object) also can cause an ACL injury.

An ACL injury often occurs in sports when the foot is firmly planted on the ground and a sudden force is applied to the knee while the leg is straight. Examples of this include changing direction rapidly, slowing down when running, and landing from a jump. This type of injury is common in athletes who play sports that require stop-and-go movements, jumping, or weaving through other players or obstacles. These sports include football, soccer, basketball, and skiing.

ACL injuries outside of sports may be caused by accidents, such as falling off a ladder or missing a step on a staircase. The ACL, like any other body structure, becomes weaker with age, so a tear is more likely in those 40 years of age or older who are active or still involved in sports. When contact causes an ACL injury, it may be from playing a sport (such as being tackled in football), from a sudden and severe accident, or from less obvious injuries that were caused by contact.

What are the symptoms?
The symptoms of an ACL injury depend on whether the injury is acute (sudden) or you have chronic ACL deficiency.

An acute ACL injury happens suddenly, often when playing sports. After an acute injury, you will almost always have to stop the activity you are engaged in, but you may be able to walk. Symptoms of an acute ACL injury include:

1.Feeling or hearing a pop in the knee at the time of injury.
2.Sudden instability in the knee (the knee buckles or gives out) after a jump or change in direction or after a direct blow to the side of the knee.
3.Pain on the outside and back of the knee.
4.Knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the knee joint (hemarthrosis). Swelling that occurs suddenly is usually a sign of a serious knee injury.
5.Limited knee movement because of pain and/or swelling.
The main symptom of chronic ACL deficiency is the knee buckling or giving out, sometimes with pain and swelling. This will happen more and more often if the ACL is not treated.
How is an ACL injury diagnosed?
An ACL injury is diagnosed through a medical history and physical examination. Your health professional will ask you how you injured your knee and whether you have had any other knee injuries. He or she will check for stability, movement, and tenderness in both the injured and uninjured knee.

You may need X-rays, which can show damage to the knee bones, or other imaging tests, such as a MRI, which can show damage to ligaments, tendons, muscle, or knee cartilage. Arthroscopy, in which your health professional inserts instruments through one or more small incisions in the knee to examine the inside of the knee, may also be done.

How is it treated?
Initial treatment of an acute ACL injury consists of taking measures to reduce inflammation and pain. These include limiting movement of the knee (immobilization), resting the knee, applying ice, using gentle compression with an elastic bandage, elevating the leg, and taking nonsteroidal anti-inflammatory drugs (NSAIDs). You may need crutches the first few days after the injury.

Later treatment may include several months of rehabilitation or surgery with rehabilitation. Your treatment will depend on how much of the ACL is torn, whether other parts of the knee are injured, how active you are, your age, your overall health status, and when the injury occurred.

The goal of treatment is to stabilize the knee if it is unstable at least stabilize it enough to suit your lifestyle to reduce the likelihood of further damage to the knee.