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Release: Nov. 15, 2001

UI researcher studies causes of post-traumatic arthritis

IOWA CITY, Iowa -- Intraarticular injury (injuries occurring within a joint) with cartilage damage can lead to post-traumatic arthritis, a condition characterized by pain and stiffness in the affected joint. Intraarticular injuries are often the result of high-energy injuries such as traffic accidents or bad falls.

A recent study by researchers at the University of Iowa and Oregon State University may shed some light on the causes of post-traumatic arthritis and explain why this arthritic condition is more severe and develops faster in ankles than in knees.

"Of the three major joints in the leg, the hip, the knee and the ankle, the knee is the most tolerant of injury and the ankle is the least tolerant," said Todd O. McKinley, M.D., UI assistant professor of orthopaedic surgery and lead author of the study. "Interestingly, although knees are very forgiving of post-traumatic arthritis, they have a fairly substantial incidence of idiopathic arthritis, or arthritis for no reason. However, ankles, which almost never get wear-and-tear arthritis, have a substantial incidence of post-traumatic arthritis. So the two joints behave very differently."

Using high-resolution digital images McKinley and his colleague, Brain K. Bay, Ph.D., associate professor of mechanical engineering at Oregon State University, measured bone strain in knee and ankle joints with cartilage defects. The researchers found that cartilage defects in the ankle joint increased strain on bone near the joint, but the opposite was true for the knee joint, where cartilage defects actually led to a decrease in bone strain next to the joint.

"The increase in strain in the ankle was expected," McKinley said. "The decrease in strain in the knee was completely unexpected but the results consistently indicated that the strain on the bone was going away."

The causes of post-traumatic arthritis are not well understood. However, it is likely that joint injuries cause changes to load-transmission through weight-bearing bones and this may play a role in development of the arthritic condition.

The researchers took slices through ankle and knee joints from cadavers and made defects of various sizes in the cartilage of those joints. The cartilage defects were similar to the kind that might occur after an intraarticular injury. The researchers then measured the bone strain in these samples using a technique called digital image correlation whereby a computer program compares high-resolution digital images of the bone under loaded and unloaded conditions and analyzes how much strain the trabecular bone experiences. Trabecular bone is the inner lattice of bone that sits underneath the cartilage near joints.

The study clearly shows that knees and ankles respond quite differently to cartilage injury in the way the joint transmits load from the joint surfaces through the bone. This finding may help explain why the knee and ankle behave differently when injured.

McKinley, who also is a consultant physician at the Veterans Affairs Medical Center in Iowa City, suggests that several features of the knee joint might explain the decrease in bone strain and the greater ability to tolerate intraarticular injuries.

"The knee is not a very close-fitting joint and much of the knee's stability is provided by the ligaments. Also, the cartilage is very thick," McKinley said. "The knee is more tolerant of a defect in the cartilage because there is enough remaining cartilage to pick up the slack."

In contrast, the ankle is a very constrained joint with a fairly precisely mated surface between the tibia (shinbone) and the talus (anklebone). In addition, the cartilage at the ankle joint is about one third as thick as in the knee.

"The ankle has a lot less reserve to accommodate loss of cartilage," McKinley said.

The study appeared in the September issue of the Journal of Orthopaedic Research and was funded by grants from the Orthopaedic Research and Education Fund, Orthopaedic Trauma Association and the Giannini Medical Scholars Foundation.

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