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The Effect of Obesity on Direct Medical Costs in Total Knee Arthroplasty Hilal Maradit Kremers, MD, MSc, Sue L. Visscher, PhD, Walter K. Kremers, PhD, James M. Naessens, ScD, and David G. Lewallen, MD Investigation performed at the Departments of Health Sciences Research and Orthopedic Surgery, College of Medicine, Mayo Clinic, Rochester, Minnesota
Background: Obesity prevalence continues to rise in the United States. We sought to examine the effect of obesity on length of hospital stay and direct medical costs in a large cohort of patients who underwent total knee arthroplasty. Methods: The study included 8129 patients who had undergone 6475 primary total knee arthroplasties and 1654 revision total knee arthroplasties at a large U.S. medical center from January 1, 2000, to September 30, 2008. Patients with bilateral procedures within ninety days following the index admission were excluded. Data on clinical and surgical characteristics and complications were obtained from the original medical records and the institutional joint registry. Patients were classified into eight groups based on their body mass index at the time of surgery. Direct medical costs were calculated in 2010 U.S. dollars by using standardized, inflation-adjusted costs for services and procedures billed during hospitalization and the ninety-day window. Study end points were hospital length of stay and direct medical costs. End points were compared across the eight body mass index categories in both unadjusted and multivariable risk-adjusted analyses. Linear regression models were used to determine the cost impact associated with increasing body mass index and obesity accounting for comorbidities and complications. Results: Body mass index data were available for 99.5% of patients and ranged from 15 to 73 kg/m2. Length of stay and the direct medical costs were lowest for patients with body mass index values in the normal to overweight range. Increasing body mass index was associated with significantly longer hospital stays and costs. Every 5-unit increase in body mass index beyond 30 kg/m2 was associated with approximately $250 to $300 higher hospitalization costs in primary total knee arthroplasty and $600 to $650 higher hospitalization costs in revision total knee arthroplasty. These estimates persisted after adjusting for comorbidities or complications. Conclusions: Obesity is associated with longer hospital stays and higher costs in total knee arthroplasty. The effect of obesity on costs appears to be independent of obesity-related comorbid conditions and complications. Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.
Peer Review: This article was reviewed by the Editor-in-Chief and one Deputy Editor, and it underwent blinded review by two or more outside experts. It was also reviewed by an expert in methodology and statistics. The Deputy Editor reviewed each revision of the article, and it underwent a final review by the Editor-in-Chief prior to publication. Final corrections and clarifications occurred during one or more exchanges between the author(s) and copyeditors.
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besity prevalence continues to rise in the United States. The most recent national data indicate that more than one-third of adults were obese in 2009 to 20101. The
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.
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negative impact of obesity on the risk and outcomes of various diseases, including knee osteoarthritis, is well established2,3. Obesity is also linked to higher health-care expenditures4-9. For A commentary by Stephen J. Incavo, MD, and Aditya M. Derasari, MD, is linked to the online version of this article at jbjs.org.
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example, lifetime inpatient, outpatient, prescription, and nursing home expenditures for overweight or obese elderly individuals are 6% to 17% higher than normal-weight individuals6. Total knee arthroplasty is increasingly performed in more obese patients. Obese patients represent at least half of the patients undergoing total knee arthroplasty10-12. Although obesity may not seem to interfere with the effectiveness of total knee arthroplasty13-15, evidence is inconclusive whether overweight or obese patients undergoing total knee arthroplasty incur higher costs than their normal-weight counterparts. Obesity may be associated with higher costs in total knee arthroplasty through increased prevalence of obesity-related comorbidities and/or increased risk of short-term total knee arthroplasty complications, such as infections or thrombovascular events16. Whereas some authors11,17-22 demonstrated this relationship in terms of higher complication and readmission rates among obese patients, others failed to observe any pattern23. The discrepancy arises both from methodological differences and from differences in adjustment of comorbidities across studies. Indeed, obesity is a risk factor for several comorbidities, and these comorbidities are in the causal pathway between obesity and costs. Therefore, controlling for comorbidities may result in underestimation of the true incremental cost of obesity, as costs attributable to comorbidities can theoretically be considered attributable to obesity. It is important to examine the effect of obesity on costs in this population and whether any effect is mediated through increased prevalence of comorbidities and complications or if it is independent of them. The primary objective of this study was to examine the effect of obesity on length of hospital stay and direct medical costs in total knee arthroplasty. Materials and Methods
T
he study population included patients who had undergone primary or revision total knee arthroplasty at the Mayo Clinic, Rochester, Minnesota campus from January 1, 2000, to September 30, 2008. Patients who had denied research authorization for use of their medical records in research and patients who had undergone bilateral procedures or revisions during the same hospitalization or up to ninety days following index admission were excluded. Demographic and clinical data were obtained from the institutional Total Joint Registry and the patients’ inpatient and outpatient medical records. The Total Joint Registry contains detailed baseline and follow-up data on all patients 24 who had undergone total knee arthroplasty at our institution . Registry data are extracted through manual review of the patients’ medical records by trained abstractors on an ongoing basis and include clinical and implant details, examination dates and findings, and details of the complete spectrum of total knee arthroplasty complications. All patients are followed by the surgeon at least twice in the first postsurgical year to ascertain subsequent complications and revision arthroplasties. For the purposes of this study, the total knee arthroplasty data from the Total Joint Registry is further supplemented by the electronic medical records to ascertain serial height and weight measures and comorbidities. Body mass index (BMI), which is weight in kilograms divided by height in meters squared, was the primary exposure variable. We recorded all weight and height measurements and calculated BMI for each surgical encounter. Patients were classified into eight groups based on their BMI at the time of the total knee arthroplasty. We used the Elixhauser comorbidity algorithm to capture comorbidities with use of the 25 administrative data . Indications in the setting of primary total knee arthroplasty were grouped into three categories as degenerative arthritis, inflammatory arthritis, and other etiologies. Indications for revision total knee arthroplasty were
THE EFFECT OF OBESITY ON DIRECT MEDICAL C O S T S I N T O TA L K N E E A R T H R O P L A S T Y
grouped into five categories as implant loosening, wear, and/or osteolysis; instability; infection; periprosthetic fractures; and others. We also identified postoperative total knee arthroplasty complications that occurred during hospitalization or the ninety-day window. Complications were captured under five categories as infections (deep and/or superficial), vascular complications (myocardial infarction, stroke, gastrointestinal bleeding, local arterial complications), thrombotic complications (pulmonary embolism, deep vein thrombosis), dislocation and/or instability, and fracture of the bone or prosthesis components. We obtained utilization and cost data from an institutional research database called the Olmsted County Healthcare Expenditure and Utilization 26 Database (OCHEUD). As described previously , this database contains line-item details for every procedure or service billed to the patients. Bottom-up microcosting valuation techniques (all procedures and services are identified, a unit cost is calculated for each, and then it is summed up to estimate total costs for each patient) are employed to generate standardized, inflation-adjusted estimates of the costs in constant dollars. Part A items, which consist primarily of hospitalbilled services and procedures, are valued by multiplying the billed charge for each item by the cost center-specific cost-to-charge ratio for the year in which the service is delivered. Cost-to-charge ratios for each cost center within each hospital are obtained from published Medicare cost reports. Part B services and procedures, which consist primarily of services billed by physicians, are valued with use of national average Medicare reimbursement rates. The costing algorithm applies the gross domestic price implicit price deflator for both Part A and Part B services to express the costs for each year in 2010 constant dollars. The study time window was defined in two ways: index hospitalization period (beginning one day before the index surgery to the day of discharge) and the ninety-day period (ninety days beginning one day before the index surgery). We 27 used Berenson-Eggers Type of Service (BETOS) codes to classify the line-item data, which were then summed by the type of service over both the index hospitalization and the total ninety-day period. We further grouped costs into clinically relevant categories, such as room and board, operating room, and implant costs.
Statistical Methods Patients were grouped under eight BMI categories based on their BMI at the time of hospitalization. Length of stay, costs, comorbidities, and complications were expressed as means with standard deviations, medians with interquartile ranges, and percentages. The study end points were hospital length of stay and direct medical costs during hospitalization and the ninety-day window. Direct medical costs and categories of costs were calculated separately for the hospitalization period and the ninety-day period (i.e., total costs). End points were compared across the eight BMI categories in both unadjusted and multivariable riskadjusted analyses. Linear regression models were used to determine the cost impact associated with increasing BMI categories and obesity. A number of different BMI cutoff values were examined (>25 kg/m2, >30 kg/m2, and >35 kg/m2) to determine the best fit for BMI data and other adjustment variables. Analyses were conducted in overall study population (all patients who underwent primary and revision total knee arthroplasty during the study period) as well as restricting the analytical sample to subsets of patients. The restricted analytic samples were patients who underwent primary total knee arthroplasty, had degenerative arthritis as the surgical indication, were more than twenty-five years of age, and had no history of cancer; patients who did not have comorbid conditions significantly associated with costs; and patients who did not have complications. Because all of the patients with a BMI of ‡30 kg/m2 had at least one comorbidity as defined by the Elixhauser algorithm (mostly hypertension), it was not possible to restrict the study sample to patients with no comorbidities. Instead, we chose to exclude patients with any one of the fourteen comorbid conditions that had previously 26 been shown to be associated with significantly higher costs. These fourteen comorbid conditions were congestive heart failure, valvular diseases, pulmonary circulation diseases, paralysis, other neurological disorders, chronic pulmonary diseases, diabetes with or without complications, renal failure, metastatic cancer, solid tumors without metastasis, coagulopathy, fluid and electrolyte disorders, and depression. Two other comorbidities (peripheral vascular disease and liver disease) were also excluded. This resulted in the exclusion of 28% of patients undergoing primary total knee arthroplasty and 31% of patients undergoing
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Fig. 1
A line graph showing trends in mean BMI and percentage of obese patients (BMI of ‡30 kg/m2) who underwent total knee arthroplasty (TKA) from 2000 to 2008. revision total knee arthoplasty, and the only remaining comorbidities were hypertension, hypothyroidism, and anemia.
Source of Funding There was no external funding for this study.
Results he study included 8129 patients who had undergone 6475 primary total knee arthroplasties and 1654 revision total knee arthroplasties between January 1, 2000, and September 30, 2008. Overall, the mean patient age was approximately sixtyeight years and 57% of the patients who underwent primary total knee arthroplasty and 53% of patients who underwent revision total knee arthroplasty were female (see Appendix). BMI data were available for 99.5% of patients and ranged from 15 to 73 kg/m2. The mean BMI was similar in both the primary cohort (31.6 kg/m2) and the revision cohort (31.8 kg/m2). An increase in mean BMI was observed in both cohorts over the eight-year period between 2000 and 2008 (Fig. 1). The size of the increase in mean BMI was 1.4 kg/m2 in the primary cohort and 2.1 kg/m2 in the revision cohort. This trend was more prominent in men than in women. The proportion of obese patients (BMI of ‡30 kg/m2) increased from 2000 to 2008 from 49% to 59% in primary total knee arthroplasty and from 51% to 62% in revision total knee arthroplasty. Although the proportion of underweight patients remained 30 kg/m2*
All patients who underwent primary and revision total knee arthroplasty Adjusted for age, sex, surgery type, and surgical indication Adjusted for age, sex, surgery type, surgical indication, and comorbidities Excluded patients with significant comorbidities† and adjusted for age, sex, surgery type, and surgical indication Excluded patients with complications and adjusted for age, sex, surgery type, surgical indication, and comorbidities Patients who underwent primary total knee arthroplasty‡ Adjusted for age and sex Adjusted for age, sex, and comorbidities Excluded patients with significant comorbidities† and adjusted for age and sex Excluded patients with complications and adjusted for age, sex, and comorbidities Patients who underwent revision total knee arthroplasty§ Adjusted for age, sex, and surgical indication Adjusted for age, sex, surgical indication, and comorbidities Excluded patients with significant comorbidities† and adjusted for age, sex, and indication Excluded patients with complications and adjusted for age, sex, indication, and comorbidities
Hospitalization Costs
Total Ninety-Day Costs
421 (282 to 559) 352 (215 to 489) 324 (196 to 452)
524 (301 to 748) 552 (240 to 864) 427 (226 to 627)
340 (228 to 452)
425 (228 to 621)
384 (265 to 504) 285 (168 to 403) 252 (137 to 368) 303 (213 to 341)
404 (167 to 641) 317 (15 to 649) 327 (108 to 545) 324 (143 to 505)
645 (228 to 1063) 592 (185 to 999) 686 (348 to 1023)
979 (479 to 1480) 933 (443 to 1424) 1447 (815 to 2080)
510 (133 to 888)
779 (345 to 1213)
*The values are given as the mean in dollars, with the 95% confidence interval in parentheses. †Patients with selected comorbidities that were significantly associated with costs were excluded. ‡These patients also had degenerative arthritis, were more than twenty-five years of age, and had no recorded malignancies or metastases. §These patients also were more than twenty-five years of age and had no recorded malignancies or metastases.
(p = 0.0001) and complications (p = 0.004) as well as in analyses restricted to patients without significant comorbidities or complications. The positive association between BMI and costs per-
sisted among patients who underwent primary total knee arthroplasty, had degenerative arthritis, were more than twentyfive years of age, and had no malignancies. In these patients, every Fig. 4
A line graph showing the mean and median ninetyday costs by BMI categories among patients who underwent total knee arthroplasty (TKA). Blue denotes primary total knee arthroplasty and red denotes revision total knee arthroplasty. Solid lines refer to the mean ninety-day costs. Solid dots refer to the median ninety-day costs. Dotted lines refer to the mean ninety-day costs among patients with none of the fourteen comorbidities (28% of patients who underwent primary total knee arthroplasty and 31% of patients who underwent revision total knee arthroplasty were excluded because of the presence of any one of the fourteen significant comorbidities [noted in the text]). Dashed lines refer to the mean ninetyday costs among patients without complications (7% of patients who underwent primary total knee arthroplasty and 9% of patients who underwent revision total knee arthroplasty were excluded because of the presence of complications).
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5-unit increase in BMI beyond 30 kg/m2 was associated with significantly higher mean costs of $384 for hospitalization (p = 0.0001) and $404 for ninety days (p = 0.005). We then excluded 28% of patients who underwent primary total knee arthroplasty and had at least one of the fourteen significant comorbidities. In these analyses, there were significant associations between BMI (p = 0.002) and costs (p = 0.02), suggesting that obesity is associated with higher costs even among patients with no other significant comorbidities. Similarly, excluding patients who experienced complications had little impact on the cost estimates with a mean cost of $303 for excess hospitalization and $324 for ninety days. Excess costs associated with BMI were higher in revision total knee arthroplasty. Every 5-unit increase in BMI beyond 30 kg/m2 was associated with higher mean costs of $645 for hospitalization and $979 for ninety days. These excess costs correspond to about 4% of the overall costs and were reduced slightly upon further adjustment for comorbidities and complications, but the significant effect (p < 0.05) of increasing BMI persisted. We also adjusted the models for length of stay (an important predictor of costs), and the association between BMI and costs persisted. Discussion n our large contemporary cohort of patients who underwent primary or revision total knee arthroplasty at a large U.S. medical center, we examined the association between BMI and length of stay and costs, taking into account obesity-related comorbidities and postoperative total knee arthroplasty complications. We found that obesity is associated with longer hospital stays and costs. We observed a J-shaped relationship in which the length of stay and costs were lowest for normal BMI or slightly overweight patients and highest for patients at extreme ends of the BMI spectrum. Beyond 30 kg/m2, every 5-unit increase in BMI was associated with higher hospitalization costs (in 2010 U.S. dollars) of approximately $250 to $300 for patients undergoing primary total knee arthroplasty and $600 to $650 for patients undergoing revision total knee arthroplasty. The effect of obesity on costs was partly mediated through obesity-related comorbid conditions, but even among patients without comorbidities, obesity was still associated with higher costs. These findings suggest that an increasing prevalence of obesity in the population undergoing total knee arthroplasty likely contributes to the financial burden of total knee arthroplasty, at least during hospitalization and the early postoperative period. Few studies have examined the potential impact of obesity on length of stay and costs in arthroplasty, and findings are inconclusive at best. In a 1987 study by Epstein and colleagues21, the mean length of stay and hospital charges were 30% to 35% higher in extremely obese patients who underwent total knee arthroplasty or total hip arthroplasty. Extremely underweight patients also had longer stays and higher charges, suggesting a J-shaped relationship between BMI and costs. Epstein et al. found that the length of stay among overweight patients was very similar to patients with normal body weight, which was similar to the results in our study. A more recent study by Batsis et al. from our institution23 addressed the same
I
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question. After adjusting for comorbidities and a number of baseline characteristics, there was no association between BMI and length of stay and hospital costs. In part, the data overlap with the cohorts included in this study, and it is important to discern the differences. In contrast to the Batsis study, we examined BMI more finely, both as a continuous variable and in eight categories, as compared with the four BMI categories in the Batsis study. Also, much of the increase in prevalence of morbid obesity occurred after the Batsis study, and comorbidities were classified and were analyzed differently. The optimal comorbidity measure for analysis depends on the health outcome being studied. The Charlson classification of comorbidities is a very strong predictor of mortality but not necessarily of health-care utilization and costs28. Elixhauser comorbidities (as used in this study) are more strongly associated with health-care utilization and costs. Furthermore, the Batsis analysis may have partially removed the effects of obesity by adjusting for comorbidities highly associated with increased weight. Finally, a more recent study29 relying primarily on Medicare claims data included approximately 1000 elderly patients who underwent total knee arthroplasty and the analyses indicated that obese patients had longer hospital stays, more complications, and thirty-day readmission rates, which persisted after matching on obesity-related comorbidities. These data are consistent with our observations. One study30 suggested that hospitals may shorten the length of stay for obese patients by transferring them to another hospital after their surgery, thus shifting risk and costs of treating medically complex obese surgical patients. This is possibly one of the explanations for the discrepancies across studies. Apart from the arthroplasty literature, the association between obesity and length of stay and costs has been examined in a number of other medical and surgical conditions. A recent study30 relied on administrative data from 122 public hospitals in Australia and suggested that, as compared with non-obese hospitalized patients, hospital stays are longer for medically managed obese patients and shorter for surgically managed obese patients. In the absence of actual height-weight measurements, the study relied on administrative data for defining obesity. Obesity is exceptionally under-reported in administrative data, and therefore, the obesity definition in the Australian study is morbid obesity. Nevertheless, the study included almost 35,000 orthopaedic surgical procedures and the length of stay was 1.1 days longer for morbidly obese patients. Other largescale studies4-9 addressed the financial burden of obesity on the health-care system, but the results are not directly comparable. Our study was limited to a specific time window around total knee arthroplasty, whereas these other studies were primarily focused on lifetime costs. Our report had some limitations. The cost data were limited to direct medical costs and did not include costs incurred by the patients or services outside our delivery system. Although obese patients were more likely than non-obese patients to be discharged to a skilled nursing facility, the costs of skilled care were not captured in our analysis. There is considerable variation31,32 in the practice of total knee arthroplasty,
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and our results may not be generalizable to other settings. However, this would not affect comparisons across BMI categories. We also did not thoroughly examine individual cost components affected by obesity. Although we observed a J-shaped relationship, the number of patients in the lowest BMI category was too small to draw any robust conclusions. However, our ability to combine medical record-derived clinical data with administrative data in a large contemporary total knee arthroplasty cohort provides important insights about the cost of obesity in total knee arthroplasty. Our costing methodology is particularly robust with standardized costs applied to nearly every service, rather than simply relying on charges or average costs of selected services. In conclusion, obesity is associated with longer hospital stays and costs in total knee arthroplasty, even among patients without comorbid conditions or complications. The increasing prevalence of obesity in the population undergoing total knee arthroplasty likely contributes to the financial burden of total knee arthroplasty.
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Appendix A table showing the characteristics of patients who underwent primary or revision total knee arthroplasty from 2000 to 2008 is available with the online version of this article as a data supplement at jbjs.org. n
Hilal Maradit Kremers, MD, MSc Sue L. Visscher, PhD Walter K. Kremers, PhD James M. Naessens, ScD David G. Lewallen, MD Departments of Health Sciences Research (H.M.K., S.L.V., W.K.K., and J.M.N.) and Orthopedic Surgery (D.G.L.), College of Medicine, Mayo Clinic, 200 First Street S.W., Rochester, MN 55905. E-mail address for H.M. Kremers: [email protected]
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17. Dowsey MM, Liew D, Choong PFM. Economic burden of obesity in primary total knee arthroplasty. Arthritis Care Res (Hoboken). 2011 Oct;63(10):1375-81. 18. J¨amsen E, Nevalainen P, Eskelinen A, Huotari K, Kalliovalkama J, Moilanen T. Obesity, diabetes, and preoperative hyperglycemia as predictors of periprosthetic joint infection: a single-center analysis of 7181 primary hip and knee replacements for osteoarthritis. J Bone Joint Surg Am. 2012 Jul 18;94(14):e101. 19. Namba RS, Paxton L, Fithian DC, Stone ML. Obesity and perioperative morbidity in total hip and total knee arthroplasty patients. J Arthroplasty. 2005 Oct;20(7) (Suppl 3):46-50. 20. Dowsey MM, Choong PFM. Obese diabetic patients are at substantial risk for deep infection after primary TKA. Clin Orthop Relat Res. 2009 Jun;467(6):1577-81. Epub 2008 Oct 8. 21. Epstein AM, Read JL, Hoefer M. The relation of body weight to length of stay and charges for hospital services for patients undergoing elective surgery: a study of two procedures. Am J Public Health. 1987 Aug;77(8):993-7. 22. Kim SH. Morbid obesity and excessive hospital resource consumption for unilateral primary hip and knee arthroplasty. J Arthroplasty. 2010 Dec;25(8):1258-66. Epub 2009 Oct 30. 23. Batsis JA, Naessens JM, Keegan MT, Huddleston PM, Wagie AE, Huddleston JM. Body mass index and the impact on hospital resource use in patients undergoing total knee arthroplasty. J Arthroplasty. 2010 Dec;25(8):1250-7: e1. Epub 2010 Feb 19. 24. Berry DJ, Kessler M, Morrey BF. Maintaining a hip registry for 25 years. Mayo Clinic experience. Clin Orthop Relat Res. 1997 Nov;(344):61-8. 25. Elixhauser A, Steiner C, Harris DR, Coffey RM. Comorbidity measures for use with administrative data. Med Care. 1998 Jan;36(1):8-27. 26. Maradit Kremers H, Visscher SL, Moriarty JP, Reinalda MS, Kremers WK, Naessens JM, Lewallen DG. Determinants of direct medical costs in primary and revision total knee arthroplasty. Clin Orthop Relat Res. 2013 Jan;471(1):206-14. 27. Centers for Medicare & Medicaid Services. Berenson-Eggers type of service (BETOS) codes. 2011. http://www.cms.gov/Medicare/Coding/HCPCSReleaseCodeSets/ BETOS.html.Accessed 2014 Jan 1. 28. Dominick KL, Dudley TK, Coffman CJ, Bosworth HB. Comparison of three comorbidity measures for predicting health service use in patients with osteoarthritis. Arthritis Rheum. 2005 Oct 15;53(5):666-72. 29. Silber JH, Rosenbaum PR, Kelz RR, Reinke CE, Neuman MD, Ross RN, EvenShoshan O, David G, Saynisch PA, Kyle FA, Bratzler DW, Fleisher LA. Medical and financial risks associated with surgery in the elderly obese. Ann Surg. 2012 Jul;256(1):79-86. 30. Hauck K, Hollingsworth B. The impact of severe obesity on hospital length of stay. Med Care. 2010 Apr;48(4):335-40. 31. Stargardt T. Health service costs in Europe: cost and reimbursement of primary hip replacement in nine countries. Health Econ. 2008 Jan;17(1)(Suppl):S9-20. 32. Tomek IM, Sabel AL, Froimson MI, Muschler G, Jevsevar DS, Koenig KM, Lewallen DG, Naessens JM, Savitz LA, Westrich JL, Weeks WB, Weinstein JN. A collaborative of leading health systems finds wide variations in total knee replacement delivery and takes steps to improve value. Health Aff (Millwood). 2012 Jun;31(6):1329-38. Epub 2012 May 9.
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BY
T HE J OURNAL
OF
B ONE
AND J OINT
S URGERY, I NCORPORATED
The Effect of Obesity on Direct Medical Costs in Total Knee Arthroplasty Hilal Maradit Kremers, MD, MSc, Sue L. Visscher, PhD, Walter K. Kremers, PhD, James M. Naessens, ScD, and David G. Lewallen, MD Investigation performed at the Departments of Health Sciences Research and Orthopedic Surgery, College of Medicine, Mayo Clinic, Rochester, Minnesota
Background: Obesity prevalence continues to rise in the United States. We sought to examine the effect of obesity on length of hospital stay and direct medical costs in a large cohort of patients who underwent total knee arthroplasty. Methods: The study included 8129 patients who had undergone 6475 primary total knee arthroplasties and 1654 revision total knee arthroplasties at a large U.S. medical center from January 1, 2000, to September 30, 2008. Patients with bilateral procedures within ninety days following the index admission were excluded. Data on clinical and surgical characteristics and complications were obtained from the original medical records and the institutional joint registry. Patients were classified into eight groups based on their body mass index at the time of surgery. Direct medical costs were calculated in 2010 U.S. dollars by using standardized, inflation-adjusted costs for services and procedures billed during hospitalization and the ninety-day window. Study end points were hospital length of stay and direct medical costs. End points were compared across the eight body mass index categories in both unadjusted and multivariable risk-adjusted analyses. Linear regression models were used to determine the cost impact associated with increasing body mass index and obesity accounting for comorbidities and complications. Results: Body mass index data were available for 99.5% of patients and ranged from 15 to 73 kg/m2. Length of stay and the direct medical costs were lowest for patients with body mass index values in the normal to overweight range. Increasing body mass index was associated with significantly longer hospital stays and costs. Every 5-unit increase in body mass index beyond 30 kg/m2 was associated with approximately $250 to $300 higher hospitalization costs in primary total knee arthroplasty and $600 to $650 higher hospitalization costs in revision total knee arthroplasty. These estimates persisted after adjusting for comorbidities or complications. Conclusions: Obesity is associated with longer hospital stays and higher costs in total knee arthroplasty. The effect of obesity on costs appears to be independent of obesity-related comorbid conditions and complications. Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.
Peer Review: This article was reviewed by the Editor-in-Chief and one Deputy Editor, and it underwent blinded review by two or more outside experts. It was also reviewed by an expert in methodology and statistics. The Deputy Editor reviewed each revision of the article, and it underwent a final review by the Editor-in-Chief prior to publication. Final corrections and clarifications occurred during one or more exchanges between the author(s) and copyeditors.
O
besity prevalence continues to rise in the United States. The most recent national data indicate that more than one-third of adults were obese in 2009 to 20101. The
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.
J Bone Joint Surg Am. 2014;96:718-24
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negative impact of obesity on the risk and outcomes of various diseases, including knee osteoarthritis, is well established2,3. Obesity is also linked to higher health-care expenditures4-9. For A commentary by Stephen J. Incavo, MD, and Aditya M. Derasari, MD, is linked to the online version of this article at jbjs.org.
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example, lifetime inpatient, outpatient, prescription, and nursing home expenditures for overweight or obese elderly individuals are 6% to 17% higher than normal-weight individuals6. Total knee arthroplasty is increasingly performed in more obese patients. Obese patients represent at least half of the patients undergoing total knee arthroplasty10-12. Although obesity may not seem to interfere with the effectiveness of total knee arthroplasty13-15, evidence is inconclusive whether overweight or obese patients undergoing total knee arthroplasty incur higher costs than their normal-weight counterparts. Obesity may be associated with higher costs in total knee arthroplasty through increased prevalence of obesity-related comorbidities and/or increased risk of short-term total knee arthroplasty complications, such as infections or thrombovascular events16. Whereas some authors11,17-22 demonstrated this relationship in terms of higher complication and readmission rates among obese patients, others failed to observe any pattern23. The discrepancy arises both from methodological differences and from differences in adjustment of comorbidities across studies. Indeed, obesity is a risk factor for several comorbidities, and these comorbidities are in the causal pathway between obesity and costs. Therefore, controlling for comorbidities may result in underestimation of the true incremental cost of obesity, as costs attributable to comorbidities can theoretically be considered attributable to obesity. It is important to examine the effect of obesity on costs in this population and whether any effect is mediated through increased prevalence of comorbidities and complications or if it is independent of them. The primary objective of this study was to examine the effect of obesity on length of hospital stay and direct medical costs in total knee arthroplasty. Materials and Methods
T
he study population included patients who had undergone primary or revision total knee arthroplasty at the Mayo Clinic, Rochester, Minnesota campus from January 1, 2000, to September 30, 2008. Patients who had denied research authorization for use of their medical records in research and patients who had undergone bilateral procedures or revisions during the same hospitalization or up to ninety days following index admission were excluded. Demographic and clinical data were obtained from the institutional Total Joint Registry and the patients’ inpatient and outpatient medical records. The Total Joint Registry contains detailed baseline and follow-up data on all patients 24 who had undergone total knee arthroplasty at our institution . Registry data are extracted through manual review of the patients’ medical records by trained abstractors on an ongoing basis and include clinical and implant details, examination dates and findings, and details of the complete spectrum of total knee arthroplasty complications. All patients are followed by the surgeon at least twice in the first postsurgical year to ascertain subsequent complications and revision arthroplasties. For the purposes of this study, the total knee arthroplasty data from the Total Joint Registry is further supplemented by the electronic medical records to ascertain serial height and weight measures and comorbidities. Body mass index (BMI), which is weight in kilograms divided by height in meters squared, was the primary exposure variable. We recorded all weight and height measurements and calculated BMI for each surgical encounter. Patients were classified into eight groups based on their BMI at the time of the total knee arthroplasty. We used the Elixhauser comorbidity algorithm to capture comorbidities with use of the 25 administrative data . Indications in the setting of primary total knee arthroplasty were grouped into three categories as degenerative arthritis, inflammatory arthritis, and other etiologies. Indications for revision total knee arthroplasty were
THE EFFECT OF OBESITY ON DIRECT MEDICAL C O S T S I N T O TA L K N E E A R T H R O P L A S T Y
grouped into five categories as implant loosening, wear, and/or osteolysis; instability; infection; periprosthetic fractures; and others. We also identified postoperative total knee arthroplasty complications that occurred during hospitalization or the ninety-day window. Complications were captured under five categories as infections (deep and/or superficial), vascular complications (myocardial infarction, stroke, gastrointestinal bleeding, local arterial complications), thrombotic complications (pulmonary embolism, deep vein thrombosis), dislocation and/or instability, and fracture of the bone or prosthesis components. We obtained utilization and cost data from an institutional research database called the Olmsted County Healthcare Expenditure and Utilization 26 Database (OCHEUD). As described previously , this database contains line-item details for every procedure or service billed to the patients. Bottom-up microcosting valuation techniques (all procedures and services are identified, a unit cost is calculated for each, and then it is summed up to estimate total costs for each patient) are employed to generate standardized, inflation-adjusted estimates of the costs in constant dollars. Part A items, which consist primarily of hospitalbilled services and procedures, are valued by multiplying the billed charge for each item by the cost center-specific cost-to-charge ratio for the year in which the service is delivered. Cost-to-charge ratios for each cost center within each hospital are obtained from published Medicare cost reports. Part B services and procedures, which consist primarily of services billed by physicians, are valued with use of national average Medicare reimbursement rates. The costing algorithm applies the gross domestic price implicit price deflator for both Part A and Part B services to express the costs for each year in 2010 constant dollars. The study time window was defined in two ways: index hospitalization period (beginning one day before the index surgery to the day of discharge) and the ninety-day period (ninety days beginning one day before the index surgery). We 27 used Berenson-Eggers Type of Service (BETOS) codes to classify the line-item data, which were then summed by the type of service over both the index hospitalization and the total ninety-day period. We further grouped costs into clinically relevant categories, such as room and board, operating room, and implant costs.
Statistical Methods Patients were grouped under eight BMI categories based on their BMI at the time of hospitalization. Length of stay, costs, comorbidities, and complications were expressed as means with standard deviations, medians with interquartile ranges, and percentages. The study end points were hospital length of stay and direct medical costs during hospitalization and the ninety-day window. Direct medical costs and categories of costs were calculated separately for the hospitalization period and the ninety-day period (i.e., total costs). End points were compared across the eight BMI categories in both unadjusted and multivariable riskadjusted analyses. Linear regression models were used to determine the cost impact associated with increasing BMI categories and obesity. A number of different BMI cutoff values were examined (>25 kg/m2, >30 kg/m2, and >35 kg/m2) to determine the best fit for BMI data and other adjustment variables. Analyses were conducted in overall study population (all patients who underwent primary and revision total knee arthroplasty during the study period) as well as restricting the analytical sample to subsets of patients. The restricted analytic samples were patients who underwent primary total knee arthroplasty, had degenerative arthritis as the surgical indication, were more than twenty-five years of age, and had no history of cancer; patients who did not have comorbid conditions significantly associated with costs; and patients who did not have complications. Because all of the patients with a BMI of ‡30 kg/m2 had at least one comorbidity as defined by the Elixhauser algorithm (mostly hypertension), it was not possible to restrict the study sample to patients with no comorbidities. Instead, we chose to exclude patients with any one of the fourteen comorbid conditions that had previously 26 been shown to be associated with significantly higher costs. These fourteen comorbid conditions were congestive heart failure, valvular diseases, pulmonary circulation diseases, paralysis, other neurological disorders, chronic pulmonary diseases, diabetes with or without complications, renal failure, metastatic cancer, solid tumors without metastasis, coagulopathy, fluid and electrolyte disorders, and depression. Two other comorbidities (peripheral vascular disease and liver disease) were also excluded. This resulted in the exclusion of 28% of patients undergoing primary total knee arthroplasty and 31% of patients undergoing
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Fig. 1
A line graph showing trends in mean BMI and percentage of obese patients (BMI of ‡30 kg/m2) who underwent total knee arthroplasty (TKA) from 2000 to 2008. revision total knee arthoplasty, and the only remaining comorbidities were hypertension, hypothyroidism, and anemia.
Source of Funding There was no external funding for this study.
Results he study included 8129 patients who had undergone 6475 primary total knee arthroplasties and 1654 revision total knee arthroplasties between January 1, 2000, and September 30, 2008. Overall, the mean patient age was approximately sixtyeight years and 57% of the patients who underwent primary total knee arthroplasty and 53% of patients who underwent revision total knee arthroplasty were female (see Appendix). BMI data were available for 99.5% of patients and ranged from 15 to 73 kg/m2. The mean BMI was similar in both the primary cohort (31.6 kg/m2) and the revision cohort (31.8 kg/m2). An increase in mean BMI was observed in both cohorts over the eight-year period between 2000 and 2008 (Fig. 1). The size of the increase in mean BMI was 1.4 kg/m2 in the primary cohort and 2.1 kg/m2 in the revision cohort. This trend was more prominent in men than in women. The proportion of obese patients (BMI of ‡30 kg/m2) increased from 2000 to 2008 from 49% to 59% in primary total knee arthroplasty and from 51% to 62% in revision total knee arthroplasty. Although the proportion of underweight patients remained 30 kg/m2*
All patients who underwent primary and revision total knee arthroplasty Adjusted for age, sex, surgery type, and surgical indication Adjusted for age, sex, surgery type, surgical indication, and comorbidities Excluded patients with significant comorbidities† and adjusted for age, sex, surgery type, and surgical indication Excluded patients with complications and adjusted for age, sex, surgery type, surgical indication, and comorbidities Patients who underwent primary total knee arthroplasty‡ Adjusted for age and sex Adjusted for age, sex, and comorbidities Excluded patients with significant comorbidities† and adjusted for age and sex Excluded patients with complications and adjusted for age, sex, and comorbidities Patients who underwent revision total knee arthroplasty§ Adjusted for age, sex, and surgical indication Adjusted for age, sex, surgical indication, and comorbidities Excluded patients with significant comorbidities† and adjusted for age, sex, and indication Excluded patients with complications and adjusted for age, sex, indication, and comorbidities
Hospitalization Costs
Total Ninety-Day Costs
421 (282 to 559) 352 (215 to 489) 324 (196 to 452)
524 (301 to 748) 552 (240 to 864) 427 (226 to 627)
340 (228 to 452)
425 (228 to 621)
384 (265 to 504) 285 (168 to 403) 252 (137 to 368) 303 (213 to 341)
404 (167 to 641) 317 (15 to 649) 327 (108 to 545) 324 (143 to 505)
645 (228 to 1063) 592 (185 to 999) 686 (348 to 1023)
979 (479 to 1480) 933 (443 to 1424) 1447 (815 to 2080)
510 (133 to 888)
779 (345 to 1213)
*The values are given as the mean in dollars, with the 95% confidence interval in parentheses. †Patients with selected comorbidities that were significantly associated with costs were excluded. ‡These patients also had degenerative arthritis, were more than twenty-five years of age, and had no recorded malignancies or metastases. §These patients also were more than twenty-five years of age and had no recorded malignancies or metastases.
(p = 0.0001) and complications (p = 0.004) as well as in analyses restricted to patients without significant comorbidities or complications. The positive association between BMI and costs per-
sisted among patients who underwent primary total knee arthroplasty, had degenerative arthritis, were more than twentyfive years of age, and had no malignancies. In these patients, every Fig. 4
A line graph showing the mean and median ninetyday costs by BMI categories among patients who underwent total knee arthroplasty (TKA). Blue denotes primary total knee arthroplasty and red denotes revision total knee arthroplasty. Solid lines refer to the mean ninety-day costs. Solid dots refer to the median ninety-day costs. Dotted lines refer to the mean ninety-day costs among patients with none of the fourteen comorbidities (28% of patients who underwent primary total knee arthroplasty and 31% of patients who underwent revision total knee arthroplasty were excluded because of the presence of any one of the fourteen significant comorbidities [noted in the text]). Dashed lines refer to the mean ninetyday costs among patients without complications (7% of patients who underwent primary total knee arthroplasty and 9% of patients who underwent revision total knee arthroplasty were excluded because of the presence of complications).
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5-unit increase in BMI beyond 30 kg/m2 was associated with significantly higher mean costs of $384 for hospitalization (p = 0.0001) and $404 for ninety days (p = 0.005). We then excluded 28% of patients who underwent primary total knee arthroplasty and had at least one of the fourteen significant comorbidities. In these analyses, there were significant associations between BMI (p = 0.002) and costs (p = 0.02), suggesting that obesity is associated with higher costs even among patients with no other significant comorbidities. Similarly, excluding patients who experienced complications had little impact on the cost estimates with a mean cost of $303 for excess hospitalization and $324 for ninety days. Excess costs associated with BMI were higher in revision total knee arthroplasty. Every 5-unit increase in BMI beyond 30 kg/m2 was associated with higher mean costs of $645 for hospitalization and $979 for ninety days. These excess costs correspond to about 4% of the overall costs and were reduced slightly upon further adjustment for comorbidities and complications, but the significant effect (p < 0.05) of increasing BMI persisted. We also adjusted the models for length of stay (an important predictor of costs), and the association between BMI and costs persisted. Discussion n our large contemporary cohort of patients who underwent primary or revision total knee arthroplasty at a large U.S. medical center, we examined the association between BMI and length of stay and costs, taking into account obesity-related comorbidities and postoperative total knee arthroplasty complications. We found that obesity is associated with longer hospital stays and costs. We observed a J-shaped relationship in which the length of stay and costs were lowest for normal BMI or slightly overweight patients and highest for patients at extreme ends of the BMI spectrum. Beyond 30 kg/m2, every 5-unit increase in BMI was associated with higher hospitalization costs (in 2010 U.S. dollars) of approximately $250 to $300 for patients undergoing primary total knee arthroplasty and $600 to $650 for patients undergoing revision total knee arthroplasty. The effect of obesity on costs was partly mediated through obesity-related comorbid conditions, but even among patients without comorbidities, obesity was still associated with higher costs. These findings suggest that an increasing prevalence of obesity in the population undergoing total knee arthroplasty likely contributes to the financial burden of total knee arthroplasty, at least during hospitalization and the early postoperative period. Few studies have examined the potential impact of obesity on length of stay and costs in arthroplasty, and findings are inconclusive at best. In a 1987 study by Epstein and colleagues21, the mean length of stay and hospital charges were 30% to 35% higher in extremely obese patients who underwent total knee arthroplasty or total hip arthroplasty. Extremely underweight patients also had longer stays and higher charges, suggesting a J-shaped relationship between BMI and costs. Epstein et al. found that the length of stay among overweight patients was very similar to patients with normal body weight, which was similar to the results in our study. A more recent study by Batsis et al. from our institution23 addressed the same
I
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question. After adjusting for comorbidities and a number of baseline characteristics, there was no association between BMI and length of stay and hospital costs. In part, the data overlap with the cohorts included in this study, and it is important to discern the differences. In contrast to the Batsis study, we examined BMI more finely, both as a continuous variable and in eight categories, as compared with the four BMI categories in the Batsis study. Also, much of the increase in prevalence of morbid obesity occurred after the Batsis study, and comorbidities were classified and were analyzed differently. The optimal comorbidity measure for analysis depends on the health outcome being studied. The Charlson classification of comorbidities is a very strong predictor of mortality but not necessarily of health-care utilization and costs28. Elixhauser comorbidities (as used in this study) are more strongly associated with health-care utilization and costs. Furthermore, the Batsis analysis may have partially removed the effects of obesity by adjusting for comorbidities highly associated with increased weight. Finally, a more recent study29 relying primarily on Medicare claims data included approximately 1000 elderly patients who underwent total knee arthroplasty and the analyses indicated that obese patients had longer hospital stays, more complications, and thirty-day readmission rates, which persisted after matching on obesity-related comorbidities. These data are consistent with our observations. One study30 suggested that hospitals may shorten the length of stay for obese patients by transferring them to another hospital after their surgery, thus shifting risk and costs of treating medically complex obese surgical patients. This is possibly one of the explanations for the discrepancies across studies. Apart from the arthroplasty literature, the association between obesity and length of stay and costs has been examined in a number of other medical and surgical conditions. A recent study30 relied on administrative data from 122 public hospitals in Australia and suggested that, as compared with non-obese hospitalized patients, hospital stays are longer for medically managed obese patients and shorter for surgically managed obese patients. In the absence of actual height-weight measurements, the study relied on administrative data for defining obesity. Obesity is exceptionally under-reported in administrative data, and therefore, the obesity definition in the Australian study is morbid obesity. Nevertheless, the study included almost 35,000 orthopaedic surgical procedures and the length of stay was 1.1 days longer for morbidly obese patients. Other largescale studies4-9 addressed the financial burden of obesity on the health-care system, but the results are not directly comparable. Our study was limited to a specific time window around total knee arthroplasty, whereas these other studies were primarily focused on lifetime costs. Our report had some limitations. The cost data were limited to direct medical costs and did not include costs incurred by the patients or services outside our delivery system. Although obese patients were more likely than non-obese patients to be discharged to a skilled nursing facility, the costs of skilled care were not captured in our analysis. There is considerable variation31,32 in the practice of total knee arthroplasty,
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and our results may not be generalizable to other settings. However, this would not affect comparisons across BMI categories. We also did not thoroughly examine individual cost components affected by obesity. Although we observed a J-shaped relationship, the number of patients in the lowest BMI category was too small to draw any robust conclusions. However, our ability to combine medical record-derived clinical data with administrative data in a large contemporary total knee arthroplasty cohort provides important insights about the cost of obesity in total knee arthroplasty. Our costing methodology is particularly robust with standardized costs applied to nearly every service, rather than simply relying on charges or average costs of selected services. In conclusion, obesity is associated with longer hospital stays and costs in total knee arthroplasty, even among patients without comorbid conditions or complications. The increasing prevalence of obesity in the population undergoing total knee arthroplasty likely contributes to the financial burden of total knee arthroplasty.
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Appendix A table showing the characteristics of patients who underwent primary or revision total knee arthroplasty from 2000 to 2008 is available with the online version of this article as a data supplement at jbjs.org. n
Hilal Maradit Kremers, MD, MSc Sue L. Visscher, PhD Walter K. Kremers, PhD James M. Naessens, ScD David G. Lewallen, MD Departments of Health Sciences Research (H.M.K., S.L.V., W.K.K., and J.M.N.) and Orthopedic Surgery (D.G.L.), College of Medicine, Mayo Clinic, 200 First Street S.W., Rochester, MN 55905. E-mail address for H.M. Kremers: [email protected]
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17. Dowsey MM, Liew D, Choong PFM. Economic burden of obesity in primary total knee arthroplasty. Arthritis Care Res (Hoboken). 2011 Oct;63(10):1375-81. 18. J¨amsen E, Nevalainen P, Eskelinen A, Huotari K, Kalliovalkama J, Moilanen T. Obesity, diabetes, and preoperative hyperglycemia as predictors of periprosthetic joint infection: a single-center analysis of 7181 primary hip and knee replacements for osteoarthritis. J Bone Joint Surg Am. 2012 Jul 18;94(14):e101. 19. Namba RS, Paxton L, Fithian DC, Stone ML. Obesity and perioperative morbidity in total hip and total knee arthroplasty patients. J Arthroplasty. 2005 Oct;20(7) (Suppl 3):46-50. 20. Dowsey MM, Choong PFM. Obese diabetic patients are at substantial risk for deep infection after primary TKA. Clin Orthop Relat Res. 2009 Jun;467(6):1577-81. Epub 2008 Oct 8. 21. Epstein AM, Read JL, Hoefer M. The relation of body weight to length of stay and charges for hospital services for patients undergoing elective surgery: a study of two procedures. Am J Public Health. 1987 Aug;77(8):993-7. 22. Kim SH. Morbid obesity and excessive hospital resource consumption for unilateral primary hip and knee arthroplasty. J Arthroplasty. 2010 Dec;25(8):1258-66. Epub 2009 Oct 30. 23. Batsis JA, Naessens JM, Keegan MT, Huddleston PM, Wagie AE, Huddleston JM. Body mass index and the impact on hospital resource use in patients undergoing total knee arthroplasty. J Arthroplasty. 2010 Dec;25(8):1250-7: e1. Epub 2010 Feb 19. 24. Berry DJ, Kessler M, Morrey BF. Maintaining a hip registry for 25 years. Mayo Clinic experience. Clin Orthop Relat Res. 1997 Nov;(344):61-8. 25. Elixhauser A, Steiner C, Harris DR, Coffey RM. Comorbidity measures for use with administrative data. Med Care. 1998 Jan;36(1):8-27. 26. Maradit Kremers H, Visscher SL, Moriarty JP, Reinalda MS, Kremers WK, Naessens JM, Lewallen DG. Determinants of direct medical costs in primary and revision total knee arthroplasty. Clin Orthop Relat Res. 2013 Jan;471(1):206-14. 27. Centers for Medicare & Medicaid Services. Berenson-Eggers type of service (BETOS) codes. 2011. http://www.cms.gov/Medicare/Coding/HCPCSReleaseCodeSets/ BETOS.html.Accessed 2014 Jan 1. 28. Dominick KL, Dudley TK, Coffman CJ, Bosworth HB. Comparison of three comorbidity measures for predicting health service use in patients with osteoarthritis. Arthritis Rheum. 2005 Oct 15;53(5):666-72. 29. Silber JH, Rosenbaum PR, Kelz RR, Reinke CE, Neuman MD, Ross RN, EvenShoshan O, David G, Saynisch PA, Kyle FA, Bratzler DW, Fleisher LA. Medical and financial risks associated with surgery in the elderly obese. Ann Surg. 2012 Jul;256(1):79-86. 30. Hauck K, Hollingsworth B. The impact of severe obesity on hospital length of stay. Med Care. 2010 Apr;48(4):335-40. 31. Stargardt T. Health service costs in Europe: cost and reimbursement of primary hip replacement in nine countries. Health Econ. 2008 Jan;17(1)(Suppl):S9-20. 32. Tomek IM, Sabel AL, Froimson MI, Muschler G, Jevsevar DS, Koenig KM, Lewallen DG, Naessens JM, Savitz LA, Westrich JL, Weeks WB, Weinstein JN. A collaborative of leading health systems finds wide variations in total knee replacement delivery and takes steps to improve value. Health Aff (Millwood). 2012 Jun;31(6):1329-38. Epub 2012 May 9.
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