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Year : 2020  |  Volume : 3  |  Issue : 4  |  Page : 116-120

The effect of body mass index on the functional outcome of patients with hip replacement

1 Department of Rehabilitation Medicine, Emory University School of Medicine, Atlanta, Georgia, USA
2 Department of Behavioral Medicine, Sultan Qaboos University, Muscat, Oman
3 Medical Simulation and Skills Development Centre, Oman Medical Specialty Board, Muscat, Oman

Date of Submission16-Mar-2020
Date of Decision03-Jun-2020
Date of Acceptance04-Jun-2020
Date of Web Publication19-Oct-2020

Correspondence Address:
Dr. David T Burke
Department of Rehabilitation Medicine, Emory University School of Medicine, Atlanta, Georgia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jisprm.jisprm_18_20

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Objective: The objective of this study is to investigate the association between body mass index (BMI) and the functional progress among patients with a recent hip replacement. Data suggest that a number of medical societies and organizations are restricting patient access to joint replacement surgeries based on the patient's BMI. This restriction seems to be related to the increased risk of complications, primarily wound infections, at the time of surgery. Few studies have been conducted on the relationship between BMI and outcomes during rehabilitation after hip replacement surgery. Design: Retrospective cohort study. Setting: All patients admitted to a rehabilitation hospital, with a recent diagnosis of hip replacement and available hospital admission height and weight data during the years 2014-2017. Main Outcome Measure: Functional independence measure (FIM) gain per day of patients by BMI category. Results: The chart review identified 479 patients had undergone hip replacement surgery. Over half of the patients were overweight or obese. FIM efficiency (FIM change/day) was the highest in the obese class II category (3.65), followed by obese class III (3.60). The differences in FIM efficiency between the normal and obese I groups reached statistical significance in favor of obese I (P = 0.024). Conclusion: This study did not demonstrate that obesity significantly impairs functional outcomes during the rehabilitation process after hip replacement surgery.

Keywords: Body mass index, functional independence measure, hip replacement, total hip arthroplasty

How to cite this article:
Burke DT, Burke D, Al-Adawi S, Bell RB, McCargo T, Panchatcharam SM. The effect of body mass index on the functional outcome of patients with hip replacement. J Int Soc Phys Rehabil Med 2020;3:116-20

How to cite this URL:
Burke DT, Burke D, Al-Adawi S, Bell RB, McCargo T, Panchatcharam SM. The effect of body mass index on the functional outcome of patients with hip replacement. J Int Soc Phys Rehabil Med [serial online] 2020 [cited 2023 Mar 27];3:116-20. Available from: https://www.jisprm.org/text.asp?2020/3/4/116/298608

  Introduction Top

Globally, 39% of adults currently fall into the overweight or obese category.[1] In the United States (US), the Centers for Disease Control and Prevention (CDC) estimates that two-thirds of the population are now overweight and/or obese.[2] The World Health Organization (WHO) has linked obesity with an annual mortality of 2.8 million adults.[3] Associated with the rise in body mass index (BMI) is an increased risk of noncommunicable diseases, which include, cardiovascular, cerebrovascular, cancer, and among others, degenerative joint disease (DJD).[4] As with elevated BMI, the prevalence of DJD has increased significantly in recent years, with the prevalence of DJD in the US now estimated to be over 30 million.[5] The WHO now lists DJD as one of the most disabling diseases in developed countries with 80% of those affected experiencing limitations in movement and 25% unable to perform daily tasks.[6]

Primary management of DJD symptoms includes weight loss, oral anti-inflammatory drugs, topical agents, bracing, and physical therapy. When these conservative treatments are found to be insufficient, patients seek more invasive treatment, including joint replacement. The rise in numbers of these surgeries in past decades has reflected an increased need, combined with improved technique and hardware. Overall, the results of these surgeries have been one of decreased pain, increased mobility, and improved patient satisfaction.[7] The number of hip replacements in the US is expected to increase by 174% by the year 2030.[8]

Despite the efficacy of these procedures, some patients experience reluctance on the part of their physicians to perform these surgeries.[9],[10],[11] Examples of this reluctance include United Kingdom National Health Service policy, and an American Academy of Orthopedic Surgeons' position statement, either suggesting or mandating restricted access to joint replacement surgery based on BMI.[9],[12] Some have suggested however, that the deleterious effect of obesity on surgical outcomes may have been overestimated.[9],[13] This study was designed to better understand the effect of BMI on the outcome of total hip arthroplasty (THA).

  Materials and Methods Top

This study received approval from the Institutional Review Board. The data reviewed for this study were obtained from the American Medical Rehabilitation Providers Association (AMRPA), and gathered by eRehab during the years 2014-2017. AMRPA collects the data from a nationwide group of inpatient rehabilitation facilities (IRFs) with standardized data collection practices. We chose data which included patients that received a THA. All patients included in this analysis had their height and weight measured on admission. To measure the patients' functional ability, the functional independence measure (FIM) was scored for each patient upon admission and at discharge. The FIM is a broadly recognized, 18-item assessment of physical, psychological and social function, developed in 1983.[14] Of the 18 items, 13 constitute the motor subscale and the remaining five items from the cognitive subscale. During the study period, 479 patients met the requirement of THA. The BMI for each patient was calculated by taking the body weight (in kilograms) and dividing by height (in meters squared). These patients were then separated into six groups according to BMI as defined by the WHO.[3] The groups were separated as follows: <18.5 kg/m 2 (underweight), 18.5–24.9 kg/m 2 (normal weight), 25.0–29.9 kg/m 2 (overweight), 30–34.9 kg/m 2 (obese class I), 35–39.9 kg/m 2 (obese class II), and 40 kg/m 2 or greater (obese class III). FIM motor gains per day (FIM motor efficiency) and FIM cognitive gains per day (FIM cognitive efficiency) were calculated for each individual, then averaged for each BMI category and compared, looking for statistically significant differences between the six BMI groups. The overall FIM gains per day (FIM efficiency) were calculated by summing FIM motor efficiency and FIM cognitive efficiency for each individual, then averaging the results for each BMI category. The BMI group results were then compared.

  Results Top

During the study period, data were obtained for 479 patients who underwent THA and for whom complete records were available. Of the patients, 161 were male and 318 female, with an overall average age of 72 years. The BMI calculations for these patients demonstrate that 3% were underweight, 29% were normal weight, 30% were overweight, 19% were obese I, 10% were obese II, and 8% were obese III [Table 1]. The overall length of stay (LOS) for all patients was 11 days. At discharge, the average FIM score by BMI category was 88.20 for the underweight, 90.99 for the normal weight, 98.42 for the overweight, 99.01 for the obese class I, 103.69 for the obese class II, and 103.03 for the obese class III.
Table 1: Baseline characteristics and functional independence measure score efficiency

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During hospitalization, functional motor capacity improvement was measured as FIM motor change in all the weight categories [Table 2]. The mean FIM motor change was determined to be 25.67 for underweight, normal range as 26.86, 27.41 for overweight, 28.66 for obese class I, 29.19 for obese class II, and 32.26 in obese class III. The mean cognitive improvement, as measured by FIM cognitive change was 2.93 points in the underweight, 3.19 points in the normal weight, 3.31 points in the overweight, 2.92 points in obese class I, 2.13 points in obese class II, and 2.77 points in obese class III.
Table 2: Functional independence measure score change by body mass index

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Normalizing these changes for LOS, the overall FIM efficiency for each was calculated. The mean FIM efficiency for underweight, normal, overweight, obese I, obese II, and obese III was 2.31, 2.92, 3.30, 3.63, 3.49, and 3.65, respectively. Using a multinomial regression analysis, the differences in FIM efficiency between the normal (2.77) and obese I (3.50) groups reached statistical significance, P = 0.024. Compared with the normal weight group, other comparisons failed to reach statistical significance (underweight P = 0.095, overweight P = 0.257, obese II P = 0.274, and obese III P = 0.120). As the FIM score reflects both motor and cognitive changes, we looked at subtest scores separately. The mean FIM motor efficiency by BMI category were 2.13 in the underweight, 2.65 in the normal weight, 2.96 in the overweight, 3.34 in the obese class I, 3.26 in obese class II, and 3.38 in obese class III. Compared with the normal weight group, none of the weight classes reached statistical significance (underweight, P = 0.334, overweight, P = 0.906, obese I, P = 0.529, obese II, P = 0.068, and obese III, P = 0.408). For the cognitive subtest, the mean FIM cognitive efficiency was 0.18 for the underweight, 0.28 for the normal weight, 0.34 for the overweight, 0.29 for the obese class I, 0.23 for obese class II, and 0.27 for obese class III. Compared to the normal weight group, none of these comparisons reached statistical significance (underweight, P = 0.373, overweight, P = 0.393, obese I, P = 0.959, obese II, P = 0.248, and obese III, P = 0.648).

  Discussion Top

The results from this study demonstrate that patients who present for hip replacement surgery with a BMI above the normal range, do not have a significantly reduced rate of recovery as compared to those whose weight is normal. We found this to be true even among those at the highest levels of BMI. The data, in fact, show quite the contrary. Patients in our study demonstrated an overall rate of FIM recovery that was better in the overweight and lower obese weight groups as compared to those in the normal weight and underweight groups.

Given long-standing medical beliefs about the fatiferous effects of obesity on health, one might have expected quite different results. For instance, among studies of surgical outcome, researchers have shown that obesity is associated with longer time during surgery, more intraoperative blood loss, elevated rates of superficial wound infection and deep infection, an increased risk of return to operating room, and increased total LOS.[15],[16] Due to these risks, surgical intervention is often denied to obese patients, or delayed until a sufficient weight loss is achieved.

This resistance has been demonstrated by hesitation and rejection due to institutional guidelines and health policy.[9],[12] This opposition would seem a reasonable considering “elective” surgeries may place the patient at risk, and the term obesity has long been synonymous with poor health outcomes. The concern about the risk of surgery seems a logical extension from these data. However, recent clinical data have given us reason to challenge this hesitation and rethink our position.[7]

Seicean et al. found that preobese, obese class I, and obese class II patients were not at an increased risk of adverse outcomes, with the exception of a longer LOS.[17] The association between postoperative complications and obesity may be dependent on confounding variables that are found within some of the obese categories including a higher incidence of diabetes, smoking, and hypertension.[17] Further and more recent examinations of data have even suggested that patients in the overweight and obese categories may recover faster and better than those in normal weight categories.[18] In a recent study of patients undergoing primary total hip replacement, overweight, obese class I and obese class II were found to have a significantly lower 90-day mortality risk when compared to normal weight patients.[18]

The phrase “obesity paradox” describes overweight and obese cardiovascular patients having better outcomes compared to normal weight patients.[19] Others have since discovered that this “paradox” was not isolated to cardiology and in fact might not be isolated only to hospitalized patients. Flegal et al. found that while obesity was related with increased all-cause mortality, those in the overweight category of BMI 25–30 kg/m 2 experienced a decreased all-cause mortality as compared to those in the normal weight category.[20] More than 70% of American adults reside in the overweight classification.[21] It was also found that individuals with class 1 obesity (BMI of 30–35 kg/m 2) were not at an increased risk for mortality.[20] Importantly, however, researchers have extended the focus to include other hospitalized cohorts. One such analysis included data that demonstrated that poststroke mortality is found to be lower among stroke survivors who are obese.[22]

As health is not measured merely by survival, surveillance should extend beyond mortality as an outcome measure when reviewing the effect of BMI on health. The data for recovery during hospitalization seem to resonate with the mortality data. In a study of hospitalized patients, it was found that among patients with acute stroke, the overweight patients demonstrated better recovery per hospital day than did all other weight categories.[23] The findings of slightly better though not statistically significant improvement during hospitalization have also been found in patients hospitalized for pulmonary disease, cardiovascular disease, traumatic brain injury, and those hospitalized for amputation.[24],[25],[26],[27] In these and a growing number of other studies, the patients in the normal weight categories fail to demonstrate significantly better gains during hospitalization as compared to those with greater BMI. Similar findings for hospital LOS were also noted.


This study is limited by the retrospective nature of the analysis. The sample was unequal across the weight categories and reflected progress among patients who were referred for care in a postacute rehabilitation hospital after hip joint replacement surgery. It is not possible from our data to determine whether the results in patients followed in our study are representative of the entire population of joint replacement patients. These patients are referred by medical teams at the surgical site and accepted for admission after review by an acute care team at the acute rehabilitation hospital. The decision process for this transfer is unlikely to result in a completely representative sample of this surgical cohort. For this reason, it is difficult to generalize from these data. It is important to review the selection process at acute care facilities to understand whether the BMI of the patient is inappropriately influences nonmedical factors in the referral or the acceptance process. Thus, these processes may be skewed by the very factor that we study. Data do suggest a bias among the medical community, providing a face validity to this concern.[7]

It would be important to review all patients hospitalized for hip replacement and to follow them independently of the discharge destination. This has major drawbacks. First, the destinations to which patients are discharged have far different abilities to influence outcome, ranging from facilities that offer daily inpatient care, to home health care, to home without care. Furthermore, each has different standard measurement tools, ranging from the FIM in IRFs to subjective reports by patients in their homes.

In addition, we studied individuals admitted to acute rehabilitation hospitals. These may not be indicative of the entire THA patient population. Recent data have shown that over 90% of patients undergoing a hip or knee replacement will be discharged to home.[28] Additional data collection for a comprehensive population should be considered to clarify our findings, though patients admitted to postoperative care facilities other than rehabilitation hospitals often do not take the same measurements used in our study (FIM efficiency). Therefore, comparable findings may be difficult to obtain.

  Conclusions Top

As the population ages, and as obesity rises, it seems inevitable that the number of individuals denied joint replacement surgery due to BMI is certain to grow in number. According to the US Census Bureau, the US had a growth in median age largely attributed to the aging “baby boomer” population who began turning 65 years old in 2011.[29] As individuals age, their joints often deteriorate with a growing number failing conservative treatment. Many of them seek surgical intervention. Therefore, with an aging population, the prevalence of joint replacements is expected to increase. This is true not only in the US, but reflects a global trend.

Although the prevalence of joint replacements, including THA, is expected to increase along with the prevalence of obesity, many patients will be denied access to surgical intervention solely based on BMI.

As these numbers are based on current eligibility restrictions, this curve may artificially underrepresent the true need. Our study, along with others, shows that for the vast majority BMI alone is not a proper restrictive factor for limiting a candidate for THA. By determining that BMI alone should not disqualify patients who seek total joint replacement, we can focus on creating more medically appropriate restrictions that will allow more patients to receive proper treatment.

In conclusion, this nationwide sample of patients hospitalized for THA demonstrates that there is no significant adverse effect of BMI on the rate of recovery during hospitalization. This study resonates with previous data suggesting that patients with elevated BMIs should be expected to do well after surgery, and should not be denied this option based on BMI alone.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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