For obese patients in the range 30–40 kg/m 2 versus those with a normal BMI, the estimated sub-HR for revision was weakly significant for THR (15.7% increase, 95% CI 0.2% to 33.7%, p=0.048) but not for TKR (17.9% increase, 95% CI −1.9% to 41.6%, p=0.079).
19 However, a recent Canadian study of 3290 THRs did categorise BMI to include a morbidly obese group (BMI>40 kg/m 2 ) and although the authors found no difference in time to revision between BMI categories in an unadjusted analysis, there was a marginally significant difference for septic revisions. 25 Our results suggest that there may be a 1.5–2% rise in the risk of knee and hip revision, respectively, for each extra unit of BMI.