SAN ANTONIO, Texas -- If your patient has an identical twin and he or she undergoes knee or hip replacement surgery for osteoarthritis, the patient's risk of having the same procedure is 5 times that of the general population.

Researchers in Sweden, scrutinizing the largest twin registry in the world as well as a hospital discharge database, discovered the relationship between identical twins and their tendency to have similar diseases. However, Stefan Lohmander, MD, Professor of Orthopedics at Lund University, Sweden, said the high risk does not hold up for fraternal twins: "Their risk is about the same as that for other people who have osteoarthritis that ‘runs in the family,'" an hereditary risk about twice that found in the general population.

"This demonstrates that a very large part of the heritability is due to shared genes and not family-shared environment," Dr. Lohmander said at a press briefing on Monday, October 18, during the 2004 annual scientific meeting of the American College of Rheumatology. "Finding those genes may help us understand osteoarthritis and possibly treat it better."

Dr. Lohmander added that family history has already proven to be a significant factor in predisposing patients to severe osteoarthritis. Prior studies have not discriminated between genetic profiles and the environmental factors that close family members also share.

From the twin registry compiled at the Karolinska Institute in Stockholm, Dr. Lohmander and colleagues identified 1000 individuals, half of whom had undergone knee and the other half hip replacement surgery. The doctors scrutinized records of the Swedish Discharge Register between 1987 and 2000. The twin registry contains medical information on 85,000 sets of siblings.

"We have just scratched the surface of this database," Dr. Lohmander said, explaining that numerous other studies are underway in attempts to further define how genetics plays a role in osteoarthritis, especially among identical twins. For example, investigators are looking into the differences between the time when one twin undergoes surgery and the time when his or her sibling needs a joint replacement.

"At this point in time, the suggestion is that the genetic contribution to osteoarthritis is small," Joseph Flood, MD, Clinical Assistant Professor of Medicine at Ohio State University, Columbus, Ohio, pointed out. "Remember that osteoarthritis is really joint failure; failure of the cartilage to perform its function, and there are probably lots of causes of that. One of those is probably genetic."

Holding his midsection, Dr. Flood added, "Another cause is what I characterize as my Milwaukee goiter; ie, increased load on the knees. For every pound of weight one loses, about 5 pounds of stress are taken off the knee joints. If one loses 10 pounds, it is similar to carrying 50 fewer pounds with every step one takes. Injury to the knee causes osteoarthritis, but we know that some families have a tendency to develop osteoarthritis earlier than others, and we have isolated the gene for that."

References:

Lohmander S, Johnell O, Pedersen NL. Genetic Contribution to Severe Osteoarthritis of the Hip and Knee Leading to Arthroplasty. A Twin Study. Poster presented at: Annual Meeting of the American College of Rheumatology; October 18, 2004; San Antonio, Texas. Abstract 228.