December 2009

Finding—and breaking—the ACL-OA link

By Barbara Boughton

In recent years, scientific studies have shown that anterior cruciate ligament (ACL) injury in athletes can lead to later osteoarthritis in a large proportion of cases. In fact, some studies have documented osteoarthritis rates of 50% to 100% in former athletes who have experienced ACL injury, when followed for 15 to 20 years.

Newer studies have also documented a link between ACL injury and osteoarthritis in non-athletes. In one study published in Arthritis and Rheumatism in 2005, for instance, researchers found that 22.8% of older patients with painful knee osteoarthritis had complete ACL rupture. But only 47.9% of these patients reported a previous knee injury.

Several mechanisms for the increased risk have been proposed. And this line of research is now beginning to suggest that changing the management of ACL injuries could decrease the risk of osteoarthritis later on.

One theory is that ACL injury triggers the same kind of cartilage degeneration process that occurs with age-related osteoarthritis. Chrondrocyte senescence—the gradual loss of function in chondrocytes—leads to osteoarthritis in the aging patient. But this process may be accelerated following a traumatic injury such as one to the ACL. Researchers have also found a protein called HMGB2 associated with age-related articular cartilage loss, which may play a similar role following joint trauma.

Researchers also have reason to believe that biomechanics may play a role. In a study published in the British Journal of Sports Medicine in May, Robert J Butler, PhD, PT, and colleagues showed that altered gait mechanics were significantly more common in 17 people who had previously undergone ACL reconstruction compared to a matched control group without such injury. The study showed that peak knee-abduction movement was increased by 21% in the ACL group compared with the control group.

“Those with an ACL reconstruction had elevated loads in the medial compartment of the knee that would be associated with a great risk of wear and tear on the joint, as well as a greater risk of progression to knee osteoarthritis,” said Butler, an assistant professor of physical therapy at the University of Evansville (IN).

Researchers are only now beginning to piece together what kind of clinical interventions might decrease the risk of osteoarthritis, if altered gait mechanics are indeed part of the mechanism. Foot orthoses may be one possibility.

“If we alter the alignment of the foot, we can alter the load of the knee,” Butler said. “The risk of osteoarthritis after ACL injury is a mechanical issue as well as a wear and tear issue on the articular cartilage level.”

Another option might be knee bracing, although getting patients to wear braces long-term might be a compliance issue, he said.

Another theory is that proper rehabilitation after ACL injury, particularly exercises that strengthen the musculature around the knee joint, may help it absorb force and insults and thus reduce the risk of joint degeneration. Proprioceptive rehabilitation may be particularly helpful.

“The ACL is a sensor—it senses where the center of the knee joint is in three dimensional space, senses the forces and torques on that joint, and reacts to them. But after you tear your ACL, that sensor may never be the same,” said Tim Hewett, PhD, professor and director of the Sports Medicine Biodynamics Center at Cincinnati Children’s Hospital and the University of Cincinnati.

Hewett and colleagues are following athletes who have had ACL injury or reconstruction to determine if proprioceptive rehabilitation affects long-term outcomes. The exercises include  dynamic single leg hopping and holding techniques, single leg balancing and balancing on uneven surfaces.

“At the same time that the patient is doing the exercise, you give him or her a lot of feedback about joint position and proper joint alignment,” Hewett said. “When you put someone on an unbalanced surface, for instance, you can teach them to move the joint with an optimal amount of control.”

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