The Unstable Knee
By Steven A. Marciniak, MD

During physical activity the knee joint is subjected to enormous forces. With the increasing popularity of sports and fitness knee ligament injuries are becoming more common. Fortunately, treatment for these injuries is very successful. In the past a “blown out” knee was often a career-ending injury. Improved understanding of anatomy, joint mechanics and surgical techniques have allowed orthopaedic surgeons to get athletes back out on the field. This year there will be over 100,000 anterior cruciate ligament (ACL) injuries. Recent studies have shown that this injury is up to 20 times more common in females than males. The exact reason for this large discrepancy is not completely understood.

Anatomy

The knee joint is composed of three bones; the femur (thigh bone), tibia (shin bone) and the patella (knee cap). The knee is stabilized by a series of ligaments. The medical collateral ligament complex is located on the inside of the knee and stabilizes the knee from bending inward. The lateral collateral ligament complex is located on the outside of the knee and protects the knee from bending outward. The anterior cruciate (ACL) and posterior cruciate ligament (PCL) are located inside the knee joint. They prevent the tibia from moving forward and backward with respect to the femur and also control rotational forces. All of these ligaments act in concert to counteract forces placed across the knee joint.

Mechanism of Injury

The most common mechanism of injury is a non-contact deceleration force accompanied by rotation or hyperextension. Tears also result from direct trauma. The ACL has two main bundles, which in turn are composed of multiple strands of collagen. When the ACL is torn it is analogous to a nylon rope breaking. At the time of injury about 50% of patients feel or hear a loud pop. A feeling of the knee momentarily coming apart is often experienced. The knee wills well within the first three hours in the majority of patients. The knee is usually painful enough to prevent further participation, and many people are unable to walk without assistance.

Treatment

Early treatment should consist of ice, analgesics, protected weight bearing and bracing as needed for comfort. Prompt medical attention by a physician familiar with knee injuries is paramount. Initial evaluation involves a good history and physical examination. Radiographs are obtained to look for fractures, and an MRI scan may be ordered to confirm the diagnosis and look for other ligament or meniscal injuries. Treatment during the first several weeks focuses on range-of-motion exercises, muscle strengthening, edema control and establishing a normal gait pattern. Patients who wish to resume an active lifestyle or patients who experience recurrent instability are candidates for ACL reconstruction. Surgical treatment of ACL injuries requires the ligament to be reconstructed with a graft, since the ligament can’t be sewn back together. It will take about six months of rehabilitation before full activities are allowed.

Prevention

There is really no effective prevention for ACL or other injuries. However, a good strengthening, stretching and conditioning program, along with good warm-up practices, should be part of any active lifestyle.

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