Hip Osteoarthritis; Is it more than just wear and tear?

Hip Osteoarthritis; Is it more than just wear and tear?

Photo by Sebastian Kaulitzki (iStockphoto)

The hip is one of the most common joints affected by osteoarthritis. It is responsible for hundreds of thousands of hip replacements performed every year in the United States. According to the CDC over 330,000 total hip replacements were performed in the U.S. in 2010. Primary osteoarthritis of the hip is distinguished from arthritis that develops from other causes such as trauma or inflammatory disease such as rheumatoid arthritis.

Osteoarthritis is often described to patients as an unfortunate result of wear and tear on the joints. Recent research, however, suggests that maybe the issue isn't quite so simple.

A Genetic Disease?

A strong argument can be made for osteoarthritis being a genetic disease of Europeans, or people with European genetic heritage. Let's consider a few facts, and see if this theory is reasonable. The prevalence of hip osteoarthritis varies significantly by country: 0.4% in Hong Kong, 0.5% in South Africa, 0.1% in India, compared to a much higher 7% in the United Kingdom and 4% throughout Europe. This is a strong indication that genetics may be involved. The nay-sayers might point to the fact that those countries may have very different environments, and thus the difference in the rates of primary osteoarthritis may be environmental and not genetic.

Two studies looked to further explain this issue.

One study was of Chinese, African Americans, and Hispanics living in San Francisco. This study found that all three groups had relatively low rates of total hip replacements for primary osteoarthritis then white patients in the same city. A similar study was done on Japanese Americans in Hawaii. While both of these studies are interesting they do not provide definitive proof that hip osteoarthritis is purely genetic.

A good place to look at whether a disease is genetic, is at family members of people that have the disease. Studies in Sweden and the UK have shown that first degree relatives of people with hip osteoarthritis have a much higher rate of the disease than the average population. This is even more clear when looking at studies of twins. Studies of hip osteoarthritis using twin registries in the U.K. and the U.S. found that 50% of the heritability of the disease is genetic. Let's take a minute to explain that number. Heritability is a term used to describe what proportion of a disease is genetic. From a genetics standpoint, developing a disease comes from a combination of environmental and genetic factors.

Genes plus environment interact to result in our physical characteristics (whether we are talking about disease, height, weight, etc.). In other words, mother and nurture both play a role. The above twin studies suggest that half of the cause of hip osteoarthritis is genetic and the other half is environmental.

50% is a very high number in terms of heritability, and this can be summed up as saying that hip osteoarthritis has a very strong genetic basis. 

Now that we have established pretty strong evidence for the genetic basis of hip osteoarthritis, the other part of the puzzle is whether it is rooted in European ancestry. This part is the hardest to prove and has the least evidence behind it. From a scientific standpoint, one way to demonstrate this as a likely explanation is to demonstrate that populations with increasing proportion of European ancestry have increasing rates of primary hip osteoarthritis. One study has shown that total hip replacements are lowest in Asians, higher in Blacks, higher still in Hispanics.

This is theoretically proportional to the amount of European genes that those populations would have. However, there is a lot more than just genetics that go into who ends up with a hip replacement, a big part of that is socio-economic in nature. Overall it's fair to say that primary hip osteoarthritis is a genetic disease, and one possible origin of these genes is Northern Europe but more research is necessary before we can confidently make those conclusions.

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