Friday, May 30, 2008

Gender Gap for Obesity and Osteoarthritis-Caused Hip Replacement

By Judith Groch
LUND, Sweden, 30 may 2008-- A high body mass index increases the risk of osteoarthritis-caused hip arthroplasty for men but not for women, according to a case-control study here.
However, both men and women who were overweight were likelier to need knee-replacement surgery, and the heavier they were, the greater the risk, Jonas Franklin, M.D., of Lund University and colleagues reported on line in the Annals of Rheumatic Diseases.
Studies have shown that a high BMI precedes knee osteoarthritis, but the relationship between high BMI and hip osteoarthritis has been less well defined, the researchers said.
The investigators compared 1,473 Icelandic men and women who'd had hip or knee replacement before the end of 2002 with 1,103 individuals who did not have joint replacement.
First-degree relatives of participating patients served as controls, while a randomly selected sample, representative of the Icelandic population, was used as a secondary control group.
Among the 1,473 patients, 872 were women, while among 1,103 controls, 599 were women, all born from 1910 through 1939. Participants had answered a questionnaire that included height and weight.
Adjusted for age, occupation, and presence of hand osteoarthritis, the odds ratio (OR) for having a total hip replacement was 1.1 (95% CI 0.9 to 1.5) for overweight men (BMI 25-30) and 70% greater for obese men (BMI>30), (OR 1.7 CI 1.0 to 2.9).
However, women who were overweight (BMI 25-30) or obese (BMI>30) were no more likely to need a new hip than normal-weight women. The OR for having a total hip was 1.0 (CI 0.8 to 1.3) for overweight women and 1.0 (CI 0.6 to 1.5) for obese women.
Being overweight or obese were factors in requiring knee arthroplasty for both men and women, the researchers reported.
Overweight men were 70% more likely to need total knee replacement (OR 1.7, CI 1.1 to 2.6), while the risk for obese men was more than five times higher (OR 5.3, CI 2.8 to 10.1).
Overweight women were 60% more likely to need knee replacement (OR 1.6, 95% CI 1.1 to 2.2), while obese women were four times more likely to need a new knee joint (OR 4.0, 95% CI 2.6 to 6.1).
For the knee, this study corroborates conclusions from earlier studies, the investigators said. For the hip, they added, some studies have claimed a positive relationship; others say not so. This study suggests that both may be right with a risk for men but not for women.
There may be anatomical, mechanical, and biochemical reasons for the different effects of high BMI on hip and knee osteoarthritis, the investigators said.
For example, it has been proposed that the gender differences for the effect of weight on the knee are caused by a difference in anatomical structure. Joint alignment and BMI may interact as risk factors for the knee, whereas such interactions may not be present for the hip joint.
Study limitations included voluntary participation so that non-participants may have differed from the study group. Because high BMI correlates with higher mortality, the results may have been influenced toward the null. However, this was not so in women's hip osteoarthritis where BMI was similar to that of the controls.
In addition, height and weight were self-estimated and many high-risk obese patients may never have been offered joint replacement.
Although the BMI of the controls was more like that of the cases than would be expected, such a bias would have a similar effect on men and women, which does not explain the weaker association for women and hip replacement, the researchers said. The fact that primary controls were relatives of the patients and secondary controls were from the general population probably contributed to the correlation of BMI among patients and controls.
This study supports a positive association between high BMI and total knee replacement in both sexes, but the association for hip replacement and BMI seems to be weaker, and possibly negligible for women, the investigators concluded.
No competing interests were reported. This study was supported by the Scientific Foundation of Akureyri Central Hospital, The Swedish Research Council (medicine), Lund Medical Faculty and University Hospital, the King Gustaf V 80-year Fund, The Swedish Rheumatism Association, and the Kock Foundations.
Primary source: Annals of Rheumatic DiseasesSource reference:Franklin J, et al "Sex differences in the association between body mass index and total hip or knee joint replacement resulting from osteoarthritis" Ann Rheum Dis 2008; DOI: 10.1136/ard.2007.086868.

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