Is BMI an Independent Risk Factor for Unfavorable Outcomes Following Total Hip Arthroplasty

“These two commentaries are editorial balance to an article ‘ Effect of Body Mass Index on Complications and Reoperations After Total Hip Arthroplasty’. I did not post the original article as the commentaries explain the research and the healthcare problem that is developing. This problem is that CMS has changed payment for surgery in total hip replacement to a bundle, a single payment to the hospital, who then pays the surgeon. This starts in April, 2016. The amount of payment is affected by the complication and readmission rate. Both hospitals and surgeons will be pressured, probably forced into selecting healthy patients who will have lower complications and faster recovery. Both of those factors change what the hospital is paid and the profile of the surgeon. There is not an answer I can see inside this new payment construct, which begins April 1 for most hospitals and the surgeons who operate there. This is my best understanding of the real conflict that will effect healthcare decisions. I am not a part of the discussions occurring as I retired July, 2015.” Bill Chesnut, MD


Is BMI an Independent Risk Factor for Unfavorable Outcomes Following Total Hip Arthroplasty?

Commentary on an article by Eric R. Wagner, MD, et al.: “Effect of Body Mass Index on Complications and Reoperations After Total Hip Arthroplasty”

Nader Toossi, MD; Norman A. Johanson, MD

J Bone Joint Surg Am, 2016 Feb 03; 98 (3): e11 .


High body mass index (BMI) has been shown to have a strong association with unfavorable outcomes following total hip arthroplasty1. An increasing proportion of patients undergoing total hip arthroplasty have a high BMI value2. The article by Wagner et al. is a single institution’s retrospective review of prospectively collected data regarding 21,361 primary total hip arthroplasties performed between 1985 and 2012. The relationship between BMI, as an independent variable, and outcomes were reported as hazard ratios, and survival estimates were made by the Kaplan-Meier method. The findings are largely consistent with those of the current literature; however, the treatment of BMI as a continuous variable is unique and represents an advance in understanding obesity in more of a “dose-response” relationship to surgical complications and points to a more refined risk-adjustment strategy. This is the greatest strength of the study.

A limitation of the study is its failure to address the combined effect of diabetes and high BMI on the surgical outcomes. Diabetes mellitus is strongly associated with both high BMI values and certain surgical complications, such as periprosthetic joint infection and wound dehiscence3. Although the authors offered a credible explanation for the increased risk of infection, they did not present statistical evidence of controlling for the confounding effect of diabetes in their analysis. There are different ways to control for the confounding effect of diabetes on the outcome, such as case-matching, stratification, or adjusting for diabetic status in a multivariable analysis. The investigators could have stratified the patients on the basis of their diabetic status, or they could have adjusted for diabetes in the Cox model in addition to the other variables, such as age, sex, and preoperative diagnosis.

Although the large number of cases in this study is an advantage, and the focus on long-term follow-up (an average of ten years for the total cohort) is usually considered to be a point of strength in studies of arthroplasty outcomes, the long follow-up duration in this specific study is fraught with a limitation. Independent variables, such as sex and race, are constant throughout the lifetime of a patient. Some independent variables, such as age, follow a predictable trend over time. However, BMI neither remains constant nor follows a predictable trend over time4,5. This variability may confound the interpretation of outcomes, especially those that have a significant dose-response relationship. Additionally, the normal-BMI cohort may change its weight and, accordingly, BMI over the long term. The BMI value at the time of a complication or revision years later might vary substantially from the value at the time of total hip arthroplasty. Thus, one of the weaknesses of the present study is due to the inability to follow trends of BMI for individual patients during a long duration of follow-up postoperatively. The longer the follow-up duration, the higher the chances that the BMI value would change over the period of study.

Despite these limitations, the article paves the way for future and more comprehensive studies. This study points toward the importance of examining larger and more contemporary databases to elucidate the impact of higher levels of BMI, such as morbid obesity (a BMI of ≥40 kg/m2) and super obesity (a BMI of ≥50 kg/m2), on the array of commonly encountered complications.


What Will We Do Now with the Super-Obese Patient Undergoing Total Hip Arthroplasty?

Commentary on an article by Kimona Issa, MD, et al.: “Bariatric Orthopaedics: Total Hip Arthroplasty in Super-Obese Patients (Those with a BMI of ≥50 kg/m2)”

Raymond Paul Robinson, MD

J Bone Joint Surg Am, 2016 Feb 03; 98 (3): e12 .


The increasing body weights of our patients and the growing responsibility to avoid postoperative adverse events have put a new urgency into expanding our knowledge of the impact of comorbidities such obesity on our patients and health-care system. Thirty years ago in my own practice, performing a total hip replacement on a patient with a body mass index (BMI) of >40 kg/m2 was unusual. Higher complication rates were expected and were addressed as necessary. Now even heavier patients are appearing in our clinics, putting more pressure on surgeons and hospitals. The implementation of bundled payment requiring that hospitals cover any postoperative complication or need for readmission within ninety days will further increase these pressures. Not surprisingly, there is concern that patients with comorbidities such as extreme obesity may not be able to obtain care.

Issa et al. present new data about outcomes and complications of super-obese patients undergoing total hip arthroplasty. These data will be helpful in understanding the cost of care and expectations of the patient and health-care professionals. The authors present an excellent retrospective review of forty-five patients (forty-eight hips) with a minimum BMI of 50 kg/m2 who underwent total hip arthroplasty at one of four high-volume joint centers. They compare implant survival, complication rates, Harris hip scores, Short Form-36 (SF-36) scores, and University of California Los Angeles (UCLA) activity scores with a matched control group of patients with a BMI of <30 kg/m2. Patients were followed for four to twelve years, which further contributes to the previous study by Issa et al. on super-obese patients undergoing total hip arthroplasty followed for three years1.

In the current study, Issa et al. report an 8.2% lower implant survival rate in the super-obese patient at a mean time of six years and a 4.5% rate of septic revision. Compared with the non-obese patients, super-obese patients had inferior clinical results, as demonstrated by significantly lower postoperative scores. It is important to note that these results were submitted from experienced surgeons practicing in high-volume joint centers.

Equally important is the documentation that, even from 2001 to 2010, super-obese patients had difficulty finding surgeons who would perform the surgical procedure on them. Although the authors do not tell us why other surgeons would not operate on these patients, bundled payment will likely make access to care even harder for these patients. This article provides needed data that will help us to understand the increased burden of the super-obese patient on our health-care system.

Issa et al. point out that the study was retrospective, although many of the data were accumulated prospectively. The authors also do not discuss the option of delaying the surgical procedure on these patients until and unless they lose weight or undergo a bariatric surgical procedure, which are relevant alternative approaches.