Relationship of height, weight and body mass index to the risk of hip and knee replacements in middle-aged women

Objectives. To examine the effect of height, weight and body mass index (BMI) on the risk of hip and knee replacement in middle-aged women. Methods. In a prospective cohort study 490 532 women aged 50-69 yrs who were recruited in the UK in 1996-2001 were followed over 2.9 yrs for incident primary hip and knee replacements. Results. Height, weight and BMI were all associated with the risk of hip and knee replacement. Comparing the tallest group (>= 170 cm) with the shortest (< 155 cm) the relative risks were 1.90 (95%CI 1.55-2.32) for hip replacement and 1.55 (95%CI 1.19-2.00) for knee replacement. Comparing the heaviest group (>= 75 kg) with the lightest (< 60 kg) the relative risks of hip and knee replacement were 2.37 (95%CI 2.04-2.75) and 9.71 (95%CI 7.39-12.77), respectively. Comparing obese women (BMI >= 30 kg/m(2)) to women with a BMI < 22.5 kg/m(2), the relative risks for hip and knee replacement were 2.47 (95%CI 2.11-2.89) and 10.51 (95%CI 7.85-14.08), respectively. These effects did not vary according to age, education, alcohol and tobacco consumption, or with use of hormonal therapies. Currently, an estimated 27% of hip replacements and 69% of knee replacements in middle-aged women in the UK are attributable to obesity. Conclusion. In middle-aged women, the risk of having a hip or knee replacement increases with both increasing height and increasing BMI. From a clinical perspective, relatively small increases in average BMI among middle-aged women are likely to have a substantial impact on the already increasing rates of joint replacement in the UK.