My sister just had her first hip replacement. I always knew she had arthritis but didn’t know how bad it was. She is about 10 years older than me. I’ve heard that arthritis runs in families. Does this mean I’ll probably start getting it too as I get closer to her age?

It turns out that primary osteoarthritis is considered a genetic disease — but only among people of European descent. People who are of African or Asian lineage are much less likely to develop this condition. In fact, according to recent research findings, primary osteoarthritis is completely absent in true Asian and African people. Only those adults who are a mixture of African (or Asian) and European develop primary OA.

Primary osteoarthritis of the hip refers to loss of joint space from a degenerative process affecting the hip joint cartilage. It only occurs in adults 55 year old or older. It is not caused by previous hip problems in childhood like Perthes disease, trauma, developmental dysplasia, or slipped capital femoral epiphysis (SCFE). Other potential causes for the arthritic changes are also ruled out (e.g., rheumatoid arthritis, Paget disease, sepsis).

Naturally, after making this discovery that there is a difference in rates of hip replacement between Europeans and non-Europeans, the scientists started looking for an environmental or genetic explanation. They used data from family and twin studies to look for factors that might explain the European versus non-European differences in rates of hip osteoarthritis. They did not find any environmental risk factor that could account for these differences. But there were some genetic links.

Putting this finding into statistical terms, here’s what they found. Primary hip osteoarthritis is the reason for 65 to 70 per cent of all hip replacements (around the world). And 100 per cent of those hip replacements are in people with European ancestry. Intermarriage among Europeans and Asians or Africans eventually (over 20 generations) results in the same risk for osteoarthritis as among those who are 100% European.

Curiously, having osteoarthritis (OA) in any part of the body (e.g., hip, knee, hand) does NOT increase the risk of developing arthritis in any other part of the body. So having hip OA does not mean you will be getting knee arthritis later (or vice versa). But if you are of European ancestry, having a family member develop osteoarthritis does increase the likelihood that you might develop it, too.