Percent Body Fat Is More Predictive of Function After Total Joint Arthroplasty Than Body Mass Index

Background: Obesity has an important impact on the future of total joint arthroplasty; however, the definition and influence of obesity on surgical risks and outcomes remain controversial. Our hypothesis was that percent body fat was better than body mass index (BMI) at identifying clinical risks and patient-reported functional outcomes following arthroplasty.

Methods: Clinical and functional outcomes were collected prospectively in 215 patients undergoing primary total knee arthroplasty (115 patients) or total hip arthroplasty (100 patients) at a mean time of twenty-four months (range, twelve to forty months). Clinical data included patient demographic characteristics, preoperative evaluation including measurements of BMI and percent body fat, intraoperative records, hospital course or events, and postoperative outpatient follow-up. Patient-reported outcomes were obtained through a series of questionnaires: a surgical satisfaction survey; the University of California, Los Angeles (UCLA) activity scale; the Knee Injury and Osteoarthritis Outcome Score (KOOS) for total knee arthroplasty; and the Hip Disability and Osteoarthritis Outcome Score (HOOS) for total hip arthroplasty. Multivariable regression models were used to identify significant body mass predictors of outcomes (p < 0.05).

Results: Higher percent body fat predicted occurrence of any medical or surgical complication (odds ratio per one standard deviation increase, 1.58 [95% confidence interval, 1.04 to 2.40]; p = 0.033). Percent body fat was also a predictor of the UCLA activity score (risk ratio, 0.92 [95% confidence interval, 0.85 to 0.98]; p = 0.013) and pain scores (risk ratio, 1.18 [95% confidence interval, 1.03 to 1.36]; p = 0.017), and it trended toward significance for the surgical satisfaction score (odds ratio, 1.96 [95% confidence interval, 0.93 to 4.15]; p = 0.078), whereas BMI was not predictive of these functional outcomes. Additionally, with regard to surgical procedure-specific outcome scores, percent body fat was predictive of outcomes after total knee arthroplasty (KOOS pain, p = 0.015, and KOOS activities of daily living, p = 0.002), but not for those after total hip arthroplasty.

Conclusions: Percent body fat should be considered when predicting clinical and functional outcomes at two years following total joint arthroplasty. Percent body fat may help surgeons to improve risk stratifications, to project patient-reported functional outcomes, and to better educate obese patients with regard to postoperative expectations prior to undergoing elective total joint arthroplasty.

Level of evidence: Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.