Relationship between body mass index and activity in hip or knee arthroplasty patients

The weight of patients has not been demonstrated to have a consistent effec t on the rate of poly ethylene wear in clinical studies of total joint repl acement. For this reason, we analyzed the relationship between quantitative activity, measured with a pedometer, and body mass index, a measure of obe sity. Data were acquired for 209 individuals, 22-82 years of age; all were independent community walkers. One hundred and fifty-one had a well functio ning total hip or knee replacement. Analysis of variance was used to evalua te the relationship between activity and body mass index, with adjustments for confounding variables. The 58 individuals with no total joint prosthesi s averaged 7,781 steps per day, which was higher (p < 0.01) than those with a total hip (5,869 steps per day) or knee (4,597 steps per day) replacemen t. The subjects with no total joint prosthesis were, however, younger than the patients with a prosthesis (p < 0.01), and the body mass index of the p atients with a total knee replacement was higher than that of the patients with a hip replacement and that of the subjects with no prosthesis (p < 0.0 1). After adjustment for differences in age, gender, and Charnley class, a higher body mass index (greater obesity) was associated with lower activity (p = 0.05). With regard to the rate of polyethylene wear, decreased ambula tory activity may counterbalance increased weight, which could, at least in part, explain why weight has not had a consistent effect on polyethylene w ear in clinical studies. Wear is a function of use, not time. The effect of obesity on activity should be considered in radiographic studies of wear a nd other outcome assessments of total joint replacements.