Anterior Cruciate Ligament (ACL) Tears: Overview

Anterior Cruciate Ligament (ACL) Tears: Overview

Though it looks like a simple hinge joint, the knee is very complex and must withstand significant force with many activities in the knee. The two main functions of the anterior cruciate ligament, or ACL, are to help keep the shin bone from shifting forward, as well as preventing the shin bone from rotating too far during twisting or turning activities.

The ACL has a partner ligament right next to it called the posterior cruciate ligament or PCL. Together, these two main stabilizing ligaments of the knee help limit excessive forward and backward movement and twisting movement while still allowing the knee to bend and straighten.

ACL tears can happen to anyone, not just athletes. This injury can happen at work or from a fall while at home, but most ACL injuries do occur in the athletic population. Some studies show that they can account for over 60% of all athletic knee injuries during sports that require planting or pivoting movements.

Historically, ACL tears have been reported more often in females, even up to 3× as much as males. However, ACL tears happened during American football more than any other sport. One of the reasons females typically get more ACL injuries is an increased angle between their thigh bone and shinbone placing more stress on the ACL.

Noncontact ACL tears typically happen in one of three ways. The first is during high-energy, high-speed, noncontact athletic activity that involves a plant and pivot motion to quickly change direction. Second, a jumping and landing activity that causes the knee to fall inward. Lastly, a low-energy, low-speed fall during which the knee twists and the patient falls backward.

Contact ACL injuries are less common and happen when the foot is planted on the ground and there is direct contact by an outside force, either bending the knee inward or bending it backward. When the knee bends inward, the thigh rotates in, the shin rotates out, and since the foot is planted, it rotates out as well. The knee can also bend backward. If someone is standing or running with their foot planted, it is hit from the front and the knee, then hyperextends. Both of these positions put the ACL at risk for injury.

There are certain measures that can be taken to help minimize the risk of an ACL tear. Many ACL prevention programs that exist typically recommend the following exercises: hamstring strengthening, core strengthening, hip strengthening, and balance and coordination. For example, the FIFA 11 ACL prevention program was originally designed for soccer players but can be used for any sport involving cutting or pivoting. It uses 11 different exercises that can help minimize the risk of getting an ACL tear.

The three main bones in the knee are the distal femur or thigh bone, the proximal tibia or shinbone, and the patella or kneecap. The ACL itself is a little over an inch long or about 3 cm. Deep in the joint, it is attached to the thigh bone here and to the shinbone here. The image on the left is a picture of the ACL during an arthroscopic surgery.

ACL tears can be classified into 3 different grades. In grade 1, the ACL is only stretched. There is little to no pain or swelling, and the patient likely will not feel any kind of instability. The ligament is considered functionally intact. Grade 2 injuries are labeled partial or incomplete tears. There will be hemarthrosis or bleeding in the joint, swelling, and some tenderness. The patient will feel some instability as well and will likely have some difficulty performing daily activities.

Grade 3 injuries are complete tears, significant pain and swelling will be present and the knee will have increased movement and shifting when examined by a health care provider. A patient with an ACL tear will typically have symptoms of immediate pain and swelling. The knee also will feel unstable or have a giving way sensation. This feeling is worse when the patient plants or pivots on their leg.

During the knee examination, the patient will feel like their knee is loose or unstable when the health care provider performs the special tests that check for an ACL tear. X-rays are typically taken to make sure there are no fractures that occurred from the injury. If an ACL tear is suspected, an MRI is ordered to confirm the diagnosis. Since soft tissue is better seen with this imaging, cartilage and other tissues can be checked as well.

Once a diagnosis is made, the doctor will discuss treatment options with the patient and a decision will be made to do surgery or try nonoperative measures.