Is the association between obesity and hip osteoarthritis surgery explained by familial confounding?

Several studies found the associations between obesity and severe hip osteoarthritis (OA). Recent twin studies have estimated that genetic factors account for as much as 47%-73% of the variation in disease risk for hip OA. Twin studies are valuable for their ability to adjust for unmeasured confounding by familial factors. The result of the current research showed familial confounding might explain the association between body mass index (BMI) and hip OA for men.

Familial confounding is confounding due to environmental or genetics exposures experienced by family members. The study analysed whether familial confounding illustrates the relationship between severe hip osteoarthritis (OA) and body mass index (BMI).

A prospective population-based cohort analysis of same-sex twins born 1915-60 was conducted by joined information from the Norwegian Arthroplasty Registry and Norwegian Twin Registry. BMI was determined using the self-reported height/weight. Incident hip arthroplasty due to OA was taken as the primary outcome. Cox regression models were applied to perform sex-specific co-twin control analyses of monozygotic and dizygotic twin pairs and bivariate twin models to explore reasons for any familial confounding.

A total of 614 patients had hip surgery due to OA with 22.6 mean (SD) BMI and 25.6 peak lifetime BMI. In cohort analyses, the men and women of hip OA were known to association with BMI. The relationship got stronger for women after adjusting for familial confounding within MZ twins. No overlapping was found between BMI and hip OA, yet suggestive evidence regarding the relationship could be illustrated by shared environmental factors provided by the familial confounding in men.

The familial confounding for men explains the relationship between BMI and hip OA. No evidence of familial confounding was seen among the women.