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The negative effects of obesity following total joint arthroplasty, such as increased morbidity and mortality, have been well documented in literature. However, little is known about whether specific body mass indices can be used as cutoffs to determine which patients are most at risk for having a poor postoperative outcome. We evaluated the effects of differing levels of obesity as measured by body mass index (BMI) on implant survivorship, Knee Society scores, complications, and radiographic outcomes. A systematic review of the literature was performed to identify all studies reporting outcomes of total knee arthroplasty in obese (30 ≤ BMI < 40 kg/m2) and morbidly obese patients (40 ≤ BMI < 50 kg/m2). Twenty-four studies were identified in our literature search. At a mean 5-year follow-up, morbidly obese patients (88%) had significantly lower implant survivorship than obese patients (95%) and nonobese patients (97%). Significantly, lower postoperative mean Knee Society objective and function scores (71 and 60 points) were observed for morbidly obese patients than for nonobese patients (75 and 90 points), but obese patients did not have significantly lower Knee Society objective and function scores than nonobese patients (78 and 84 points). Complication rates for nonobese, obese, and morbidly obese patients were 9, 15, and 22%, respectively, all of which were significantly different. However, no significant difference was observed in the incidence of radiolucent lines that were 12, 19, and 14%, respectively. Thus, we conclude that a BMI greater than 40 kg/m2 may be used as a cutoff to help guide patient education and treatment options for primary total knee arthroplasty.
McElroy et al. (Mon,) studied this question.