It is well established that obesity can have a serious impact on health, including coronary artery disease, hypertension, diabetes, and mortality. The relationship between obesity and outcome following total joint arthroplasty is less clear. Although severe or morbid obesity can result in increased complications and poor functional outcomes, mild to moderate obesity has a relatively small effect that may not be clinically relevant in view of the marked benefits that most patients derive from total joint arthroplasty. This underscores the importance of determining the degree of obesity present and using the most effective methods to make this determination. The current standard is the body mass index (BMI), which is based on the relationship between height and weight and which allows patients to be classified into 5 categories. BMI may be a satisfactory proxy for obesity or body fat percentage, but it has several limitations, including its inability to differentiate among bone, muscle, and fat mass. A more accurate measurement of body fat can be obtained using bioelectric impedance analysis as described by Ledford et al. and other authors. However, an important question is whether this provides a more accurate method of predicting outcome following total joint arthroplasty.
Ledford et al. have evaluated both clinical and patient-reported functional outcomes of 215 patients at a mean time of 24 months after primary total hip arthroplasty or total knee arthroplasty using standard outcome measures. They calculated BMI using height and weight and determined percent body fat using bioelectric impedance analysis. Both BMI and percent body fat were then correlated with several outcome measurements for each patient.
They found some degree of correlation for both BMI and percent body fat, but a greater degree of correlation between percent body fat and functional and clinical outcomes: the greater the obesity, the poorer the outcome. They concluded that percent body fat should be considered when predicting clinical and functional outcomes 2 years following total joint arthroplasty.
Ledford et al. have conducted a careful prospective study and have used a novel approach to determine obesity on the basis of bioelectric impedance analysis to measure body fat. This provides us with information not previously available. Although they found that the percent body fat predicted outcomes following total joint arthroplasty better than BMI, the differences between these 2 techniques were not significant for every parameter of evaluation. A relatively small number of patients were present in each category and the low number of joint-specific complications made analysis difficult. They did not specifically classify patients on the basis of the degree of their obesity, which might have yielded some interesting information. Even if percent body fat determination proves to be a better indicator of obesity than BMI, we have yet to evaluate the clinical importance of this observation. In addition, it does require use of special equipment and techniques. Severe or morbid obesity does affect outcome after total joint arthroplasty, but this can probably be determined effectively using either method of evaluation. Lesser degrees of obesity may not substantially detract from outcome, and more accurate methods of determining obesity in these patients may have little clinical value.
Ledford et al. have presented us with a novel and interesting approach to this problem and have perhaps left us with more questions than answers. How will the information provided affect the care of patients who are candidates for total joint arthroplasty? In addition to considering the role of percent body fat measurements in total joint arthroplasty, this technique might have a much broader application in evaluating a number of obesity-related medical conditions. Further studies along these lines are therefore warranted.