Traumatic arthritis
Symptoms include swelling, pain, tenderness, joint instability, and internal bleeding. So how does traumatic arthritis develop? Injury to a joint, such as a bad sprain or fracture, can cause damage to the articular cartilage. The cartilage can be "bruised" when too much pressure is exerted on it. This damages the cartilage, although if you look at the surface it may not appear to be any different. The injury to the material doesn't show up until months later. Sometimes the cartilage surface is damaged even more severely and pieces of the cartilage are ripped from the bone. These pieces do not heal back and usually must be removed from the joint surgically. If not, they may float around in the joint causing the joint to catch and be painful. These fragments of cartilage may also do more damage to the joint surface. Once this cartilage is ripped away, it does not normally grow back. Unlike bone, holes in the surface are not simply replaced by the cartilage tissue around the hole. Instead the defects are filled with scar tissue. The scar tissue that forms is not nearly as good a material for covering joint surfaces as the cartilage it replaces. It just can't support weight and isn't smooth like true articular cartilage. An injury to a joint--even if it does not injure the articular cartilage directly--can alter how the joint works. This is true for a fracture where the bone fragments heal differently from the way they were before the break occurred. It is also true when ligaments are damaged that lead to instability in the joint. When an injury results in a change in the way the joint moves, the injury may increase the forces on the articular cartilage. This is similar to any mechanical device or machinery. If the mechanism is out of balance, it wears out faster. Over many years this imbalance in the joint mechanics can lead to damage to the articular surface. Since articular cartilage cannot heal itself very well, the damage adds up. Finally, the joint is no longer able to compensate for the increasing damage, and it begins to hurt. The damage occurs well before the pain begins. In summary, arthritis may come from differences in how each of us is put together based on our genes--a condition best described as OA. Or arthritis may develop years after an injury that leads to slow damage to the joint surfaces--a condition probably best described as post-traumatic arthritis. Either way the joint is worn out, and it hurts. For the purposes of this document, we will refer to both types as OA.
Intra-articular 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. Intra-articular 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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