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. 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 three times as much as males. However, ACL tears happen during American football more than any other sport. One reason females typically experience more ACL injuries is an increased angle between the 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 is 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. These positions place the ACL at risk for injury due to abnormal forces placed across the knee.

There are measures that can be taken to help minimize the risk of an ACL tear. Many ACL prevention programs recommend hamstring strengthening, core strengthening, hip strengthening, and balance and coordination exercises. The FIFA 11 ACL prevention program, originally designed for soccer players, can be used for any sport involving cutting or pivoting movements.

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 centimeters. Deep in the joint, it is attached to the thigh bone and the shinbone. The image on the left shows the ACL during an arthroscopic surgery.

ACL tears are classified into three grades. In grade one, the ACL is stretched with little to no pain or swelling, and the ligament remains functionally intact. Grade two injuries are partial tears with bleeding in the joint, swelling, tenderness, and some instability, often limiting daily activities.

Grade three injuries are complete tears. Significant pain and swelling are present, and the knee demonstrates increased movement when examined by a health care provider. Patients typically experience immediate pain, swelling, and a sensation of instability or giving way, especially during planting or pivoting.

During examination, patients may feel their knee is loose or unstable when special tests are performed to evaluate for an ACL tear. X-rays are obtained to rule out fractures, and if an ACL tear is suspected, an MRI is ordered to confirm the diagnosis and evaluate cartilage and other soft tissues.

Once a diagnosis is made, the doctor will discuss treatment options with the patient, and a decision will be made to proceed with surgery or attempt nonoperative management.