According to a study published in the Annals of the Rheumatic Diseases, women who have many children, used hormone replacement therapy, and had early puberty are more likely to have surgery performed on their joints - especially on their knees.

Researchers from the United Kingdom studied 1.3 million middle-aged UK women who took part in the Million Women Study. The health of the participants was tracked beginning around age 50, and they were monitored for about six years. Baseline information provided data on the age of the participants when they had their first and last menstrual events, how many children to whom they had given birth, and their use of oral contraceptives and hormone replacement therapy (HRT). The researchers used the follow-up data to see if the women were admitted to hospital for a knee or hip replacement for the inflammatory joint disease, osteoarthritis.

Dr Bette Liu (Cancer Epidemiology Unit, University of Oxford, Oxford, UK) and colleagues report that over 12,000 of the women needed a hip replacement and a little less than 10,000 required a knee replacement during the follow-up time period.

What factors most predicted these surgeries? Serial motherhood - or having additional children - increased the probability of a hip replacement by 2% and of a knee replacement by 8%. Women who began menstruation before the age of 11 increased their likelihoods of both types of surgery by 9 to 15%. The researchers found that previous use of oral contraceptives did not seem to have a significant effect on the risk of joint surgery. However, use of HRT raised the probability of a hip replacement by 38% and of a knee replacement by 58%.

The researchers point out that women on HRT are probably making better use of health services, which may partly explain their highly likelihood of joint surgery. They also suggest that female sex hormones, such as estrogen, may help explain the difference in osteoarthritis, a condition that is more common among women than men.

"Given the large burden of osteoarthritis and the associated burden of joint replacement surgery in women worldwide, it is important to understand the role of potentially modifiable factors for these conditions. We found that parity, age at menarche and HRT use are all associated with the risk of hip and knee replacement and that the knee joint is affected more by these factors than the hip joint. However the underlying reasons for these findings remain unclear," conclude the authors.

Reproductive history, hormonal factors and the incidence of hip and knee replacement for osteoarthritis in middle-aged women
B Liu, A Balkwill, C Cooper, A Roddam, A Brown, V Beral, on behalf of the Million Women Study Collaborators
Annals of the Rheumatic Diseases (2008).
doi:10.1136/ard.2008.095653
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Written by: Peter M Crosta