Aim: Inability to lose weight is often attributed to restriction of mobility due to painful arthritic joints. We hypothesise that removing the pain by total hip replacement or knee replacement will improve patients exercise tolerance thereby helping them lose weight.

Method: A prospective study of obese patients (BMI > 30) undergoing consecutive primary total hip or knee replacement. BMI was measured pre operatively and 1 year post operatively. Patients with medical conditions or complications of surgery that would limit mobility were excluded. A questionnaire detailing pre and postoperative activity levels, walking distance/aids and diet was completed.

Results: 37 patients were included in the study. All patients had symptomatic osteoarthritis. There were 21 female and 16 male patients with an average age of 70 (range 51 – 85). 23 patients had total knee replacements and 14 had total hip replacement. The average follow up was 11.4 months (range 8 – 12 months).

The International classification of adult weight according to BMI was used to categorise patients. Obesity is defined as a BMI greater than 30 and is graded in severity. Class I is a BMI of 30.0 – 34.9, Class II is 35.0 – 39.9 and Class III is greater than 40.0

There were 14 patients in obese class I. 5 patients increased their BMI, 4 patients remained the same and 5 patients decreased their BMI. There was an average increase in BMI of 0.36. 13 patients had an improvement in walking distance and in 1 patient it remained unchanged.

There were 16 patients in obese class II. 6 patients increased their BMI, 5 patients remained the same and 5 patients decreased their BMI. There was an average increase in BMI of 0.62. 14 patients had an improvement in walking distance and in 2 patients it remained unchanged.

There were 7 patients in obese class III. 1 patient increased their BMI, 1 patient remained the same and 5 patients decreased their BMI. There was an average decrease in BMI of 1.3. 5 patients had an improvement in walking distance and in 2 patients it remained unchanged.

There was an average increase of BMI of 0.19 in all patients. All patients reported an improvement in activity levels and a reduction in the use of walking aids.

Conclusion: Overall there was an increase in the BMI of obese patients undergoing total knee or hip replacement. This study suggests that total knee or hip replacement in obese patients for osteoarthritis despite improved activity levels does not result in a reduction in BMI. This is either as the improvement in mobility is not sufficient to confer a significant loss of weight or that pre-operative obesity is unrelated to inactivity due to osteoarthritis. Interestingly, the weight loss in obese class III patients (BMI > 40) who undergo total hip or knee replacement suggests that they benefit more from surgery than other classes of obesity.