The recent emergence of specialty hospitals focusing on procedural aspects of medicine has generated wide-spread controversy1,2. Supporters have suggested that a focused approach improves outcomes and reduces cost3,4, whereas opponents have countered that specialty hospitals select patients who are the most profitable and with the lowest risk of complications without improving outcomes5,6. Moreover, critics have claimed that specialty hospitals focus on efficiency and profitability while forsaking important societal obligations including the delivery of emergency services and charity care7,8. This controversy has evolved into a polarizing debate involving hospital trade groups, the American Medical Association, and the lay press3,9-12.
Despite the controversy, rigorous data addressing the quality of care delivered by specialty hospitals remain limited and largely derived from cardiac hospitals13,14. Only a single preliminary investigation has assessed outcomes in specialty orthopaedic hospitals, the single largest category of specialty hospital6,15. That analysis, performed by consultants for the Centers for Medicare and Medicaid Services (CMS), found evidence of improved outcomes in specialty orthopaedic hospitals16, but interpretation of these results is complicated by important methodological limitations17.
Thus, we conducted a retrospective cohort study of all Medicare beneficiaries who underwent total hip replacement or total knee replacement in specialty orthopaedic and general hospitals in the same geographic regions between 1999 and 2003 to compare the demographic data, rates of comorbid illnesses, socioeconomic status, and risk-adjusted outcomes. We hypothesized that specialty hospitals would care for patients with a lower prevalence of comorbidities and a higher socioeconomic status than those in the comparison group of general hospitals. We also hypothesized that after adjusting for patient characteristics and hospital procedural volume, outcomes would be similar in specialty and general hospitals.
Materials and Methods
Aconsecutive series of Medicare beneficiaries who were sixty-five years of age or older and underwent a major joint replacement (432,579 who had total hip replacement and 719,482 who had total knee replacement) between 1999 and 2003 were identified from the Medicare Provider Analysis and Review (MedPAR) Part-A data files with use of the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) procedure codes (81.51 for primary total hip replacement, 81.53 for revision total hip replacement, 81.54 for primary total knee replacement, and 81.55 for revision total knee replacement)18. Data elements included demographic data, zip code and state of residence, primary and secondary diagnoses and procedures, admission source (e.g., outside hospital or emergency department), admission and discharge dates, discharge disposition (e.g., home or another acute care hospital), date of death up to three years after discharge, the Medicare identifier of each hospital. Socioeconomic measures, including median home values and per capita income, were determined at the zip code level for each patient by linking zip code of residence with zip code-specific United States Census data19.
Each hospital performing total hip replacement or total knee replacement was assigned to one of 307 unique hospital referral regions with use of hospital zip code-based algorithms available from the Dartmouth Atlas of Health Care20. Hospital referral regions represent distinct regional markets for medical care. Additional hospital-level information was obtained from the American Hospital Association Annual Survey Database for fiscal year 2000, including hospital teaching status and ownership (i.e., for profit or not for profit)21,22.
Definition of Specialty Orthopaedic Hospitals and General Hospitals
There is no established definition of a specialty orthopaedic hospital, but previous studies have identified specialty hospitals with use of the ratio of orthopaedic admissions to total admissions6. We built on this approach by first developing an index of orthopaedic specialization, defined for each hospital as the proportion of all 2003 Medicare admissions categorized as Major Diagnostic Category 8 (diseases of the musculoskeletal system). Thus, each hospital's specialization index was between 0 (no admissions within Major Diagnostic Category 8) and 1.0 (all admissions within Major Diagnostic Category 8), with a higher index correlating with a greater degree of orthopaedic specialization. The index was used to identify the 100 hospitals with the greatest orthopaedic specialization (i.e., highest specialization index). From this list of 100 hospitals, we then excluded all fifty-five general hospitals, defined as those hospitals that provided obstetric and/or general pediatric services and all seven hospitals with formal medical school affiliations as defined in the American Hospital Association survey, because we assumed that the new specialty hospitals generally do not play a major role in the training of medical students or residents5,6. This resulted in the identification of thirty-eight specialty orthopaedic hospitals whose specialization was verified by a review of hospital web sites and/or confirmatory telephone calls.
A comparison group of hospitals was identified by selecting all general hospitals performing major joint replacement in the same hospital referral region as the specialty hospitals. The comparison group was limited to hospitals within the same hospital referral regions to minimize bias introduced due to regional variations in care23,24.
Outcomes of Interest
We developed a composite end point of death and preventable surgical complications by combining outcomes assessed in previous studies with use of administrative data to assess orthopaedic outcomes25-28 with selected Patient Safety Indicators developed by the Agency for Healthcare Research and Quality (AHRQ) that are most relevant to surgical quality29. The resulting composite end point consisted of six separate outcomes occurring within ninety days of surgery: sepsis, hemorrhage, pulmonary embolism, deep vein thrombosis, wound infection requiring readmission, or death. Secondary outcomes included postoperative hospital length of stay and the proportion of patients requiring transfer from the hospital where they underwent the initial arthroplasty to another acute care hospital (a potential measure of complications that could not be managed in the primary hospital).
Differences between the patients in the specialty hospitals and those in the general hospitals with regard to demographic data, the mean number of comorbid conditions as measured by Elixhauser et al.30, predicted risk of the composite outcome, unadjusted outcomes, and hospital length of stay were compared with use of the chi-square statistic or Wilcoxon signed-rank test. The characteristics of the specialty and general hospitals (e.g., procedural volume) were assessed with use of similar tests on the basis of 2003 Medicare data to reflect the fact that procedural volume benchmarks set by external agencies (e.g., the Leapfrog Group) are usually based on annual volume.
Multivariable models were developed from patient characteristics available in the MedPAR database. Candidate variables included demographic data, admission source, surgical priority (i.e., emergent, urgent, or elective), and comorbid conditions. Comorbid conditions were defined with use of algorithms developed by Elixhauser et al.30. The algorithms were developed to identify common comorbid medical conditions that were associated with adverse outcomes for a wide array of medical and surgical diagnoses and were expressly designed for use with administrative data. A list of these conditions is provided in the Appendix. Additional high-risk conditions specific to joint replacement surgery (prior joint replacement, active joint infection, acute fracture, and cancer with bone involvement) were identified with use of published algorithms28,31,32.
Multivariable models were initially developed independently for primary and revision total hip replacement and total knee replacement cohorts. However, because we found these models to be similar, patients who had total hip replacement and those who had total knee replacement were combined with use of separate indicator variables to represent the surgical procedure each patient underwent (e.g., primary total hip replacement or revision total hip replacement). Fifty-seven candidate variables associated with mortality in bivariate analyses (with use of a statistical criterion of alpha = 0.05) were entered into stepwise logistic regression analyses; variables were retained in the model if they demonstrated independent association with mortality (with use of alpha = 0.01)33,34. The final risk adjustment models contained thirty-three variables that are listed in the Appendix. These models were used to provide estimates of the predicted risk of an adverse outcome for each patient, a composite measure of case complexity.
Generalized estimating equations were used to compare the occurrence of the composite end point in specialty hospitals relative to general hospitals after accounting for clustering of patients within hospitals35,36. A series of three sequential analyses were conducted. First, the unadjusted odds of the composite outcome was calculated. Second, the odds ratio was adjusted for the patient characteristics identified above and, third, it was adjusted for patient characteristics and hospital procedural volume.
To investigate the possibility that specialty hospital performance differed for specific patient populations, a number of analyses were conducted with use of the models described above for defined patient subgroups. First, we conducted analyses for all patients in aggregate, patients who had primary joint replacement alone, and patients who had revision joint replacement alone. Second, we stratified all patients into three subgroups on the basis of the number of comorbid conditions (zero or one comorbid condition, two or three comorbid conditions, and four or more comorbid conditions), as measured according to Elixhauser et al.30. In these analyses, we controlled for the type of procedure (primary total hip replacement, primary total knee replacement, revision total hip replacement, or revision total knee replacement) using indicator variables. Then, we assessed the performance of specialty hospitals and general hospitals separately for each subgroup to further investigate whether specialty hospitals performed better (or worse) for specific patient groups. Third, we compared outcomes after grouping hospitals into low-volume (fifty or fewer major joint replacement procedures in 2003), intermediate-volume (fifty-one to 250 procedures), and high-volume (>250 procedures) strata. Finally, linear regression models were used to compare unadjusted and adjusted log-transformed hospital length of stay in specialty and general hospitals.
The analyses described above were also conducted separately for each component of the composite outcome, and the results were similar to the findings for the composite outcome. Similar analyses were conducted after excluding patients admitted as transfers from outside hospitals and patients admitted through the emergency department, and outcomes were again similar. To confirm that the effects of specialty hospitals were similar for patients who had total hip replacement and those who had total knee replacement, additional models that included interactions between specialty hospital status and type of procedure were developed; none of the interactions were significant (p > 0.05). All analyses were performed with use of Stata SE 8.2 (Stata, College Station, Texas).
For total hip replacement, the thirty-eight specialty orthopaedic hospitals performed 4683 procedures, while the comparison group of 517 general hospitals performed 47,105 procedures on Medicare beneficiaries between 1999 and 2003. The percentage of total hip replacements that were classified as revisions was smaller for specialty hospitals (mean, 8.2%; range, 1.8% to 20.0%) than for general hospitals (mean, 12.8%; range, 1.3% to 33.3%), but this difference was not significant (p = 0.19)
The patients who underwent total hip replacement in specialty hospitals were less likely than the patients in general hospitals to have diabetes (10.5% and 11.7%, respectively; p = 0.02), congestive heart failure (2.9% and 6.4%; p < 0.001), and renal failure (0.3% and 0.9%; p < 0.001). The patients in specialty hospitals also had a smaller number of comorbid conditions, as measured by Elixhauser et al.30, than the patients in general hospitals (mean, 2.1 [range, zero to nine conditions] and 2.3 [range, zero to nine conditions], respectively) (p < 0.001), were less likely to have a number of important high-risk orthopaedic conditions such as an acute fracture (2.7% and 11.1%; p < 0.001), and were less likely to be admitted from the emergency department (p < 0.001) (Table I). In addition, the patients treated in specialty hospitals resided in zip codes with higher home values and per capita incomes (p < 0.001 for both). The mean predicted risk of the composite outcome, a summary estimate of case complexity, was lower for the patients who underwent total hip replacement in specialty hospitals (4.2%) than for those treated in general hospitals (6.7%) (p < 0.001).
For total knee replacement, the thirty-eight specialty hospitals performed 10,234 procedures while the 517 general hospitals performed 89,531 procedures. The percentage of total knee replacements that were classified as revisions was larger for the specialty hospitals (mean, 6.3%; range, 0.7% to 50.0%) than for the general hospitals (mean, 5.4%; range, 0.8% to 50.0%), but this difference was not significant (p = 0.63). The patients in specialty hospitals had lower rates of most comorbid conditions than the patients in general hospitals; these included diabetes (13.0% and 15.8%, respectively; p < 0.001), heart failure (2.1% and 4.4%; p < 0.001), and renal failure (0.3% and 0.5%; p = 0.03). The patients in the specialty hospitals had a smaller mean number of comorbid conditions than the patients in the general hospitals (2.1 conditions [range, zero to nine conditions] and 2.3 conditions [range, zero to ten conditions], respectively) (p < 0.001), but they had statistically similar rates for all high-risk orthopaedic conditions (p > 0.05 for all). In addition, the patients who underwent total knee replacement in the specialty hospitals were less likely (p < 0.001) to be admitted from the emergency department and were more likely to live in areas with higher home values and per capita incomes (p < 0.001). The mean predicted risk of the composite outcome was also lower for the patients who had total knee replacement in specialty hospitals (3.2%) than for those who had the procedure in general hospitals (3.7%) (p < 0.001).
The mean number of major joint replacements performed in 2003 was higher in the specialty hospitals (108 replacements; range, one to 637 replacements) than in the general hospitals (sixty replacements; range, one to 1045 replacements) (p = 0.01), but the percentage of major joint replacements classified as revision procedures was similar in specialty and general hospitals (5.9% and 6.0%, respectively; p = 0.92) (Table II). As expected, the mean specialization index (i.e., the proportion of Medicare admissions categorized as Major Diagnostic Code 8) was higher for the specialty orthopaedic hospitals than for the general hospitals (0.82 and 0.22; p < 0.001). In addition, specialty hospitals were more likely than general hospitals to be for profit (36% and 23%, respectively), although this difference was not significant (p = 0.06).
For both total hip and total knee replacement, the patients treated in specialty hospitals were less likely than those treated in general hospitals to experience adverse outcomes such as postoperative hemorrhage, wound infection, and death (Table III). Thus, the composite outcome (defined as the proportion of patients experiencing one or more of the individual outcomes) was less common in the specialty hospitals than in the general hospitals with regard to primary total hip replacement (2.8% and 6.2%, respectively; p < 0.001), revision total hip replacement (3.5% and 11.0%; p < 0.001), primary total knee replacement (2.1% and 3.8%; p < 0.001), and revision total knee replacement (2.4% and 6.0%; p < 0.001).
In the regression analyses, the unadjusted odds of the composite outcome among all patients in the specialty hospitals compared with the general hospitals was 0.47 (95% confidence interval, 0.41 to 0.54; p < 0.001) (Table IV). After adjustment for patient characteristics, the odds of the composite adverse outcome were 0.57 (95% confidence interval, 0.48 to 0.68; p < 0.001), and they remained significant after further adjustment for patient characteristics and hospital procedural volume (odds ratio, 0.62; 95% confidence interval, 0.54 to 0.72; p < 0.001). In stratified analyses, specialty hospitals demonstrated improved outcomes for both primary joint replacement (odds ratio, 0.64; 95% confidence interval, 0.56 to 0.75; p < 0.001) and revision joint replacement (odds ratio, 0.49; 95% confidence interval, 0.36 to 0.66; p < 0.001) after adjusting for patient characteristics and hospital procedural volume (Table IV). When patients were stratified according to the number of comorbid conditions, as measured by Elixhauser et al.30, the risk of adverse outcomes was again lower for patients treated in specialty hospitals for each of the patient subgroups (Table V).
Moreover, adverse outcomes were less common for patients in both high-volume and intermediate-volume specialty hospitals after adjustment for patient characteristics and hospital procedural volume but were statistically similar in low-volume hospitals (Table VI). Additional analyses comparing the occurrence of individual outcomes (e.g., death and joint infection) yielded similar results.
In secondary analyses, patients who were treated in specialty orthopaedic hospitals were more likely than those in general hospitals to require transfer to another acute-care hospital for primary total hip replacement (3.1% and 1.6%, respectively; p < 0.001), revision total hip replacement (4.0% and 1.7%; p < 0.001), primary total knee replacement (3.0% and 1.6%; p < 0.001), and revision total knee replacement (3.6% and 1.0%; p < 0.001). The mean length of hospitalization was statistically similar for the patients in specialty and general hospitals for both total hip replacement (4.53 and 4.54 days, respectively; p = 0.80) and total knee replacement (4.54 and 4.52 days; p = 0.64). After adjustment for patient characteristics and hospital procedural volume, the patients in the specialty hospitals had a 0.6% shorter length of stay for total hip replacement and a 0.5% shorter length of stay for total knee replacement, although neither of these differences was significant.
Medicare patients undergoing joint replacement in specialty orthopaedic hospitals had a 50% lower risk of adverse outcomes than patients in a comparison group of general hospitals as measured by administrative data. The differences in outcomes remained after adjustment for patient characteristics and were consistent among both the patients who had a primary total joint replacement and those who had a revision. Specialty hospitals also had higher procedural volumes, but after accounting for this difference as well, the risk of adverse outcomes in specialty hospitals remained 40% lower. The consistency of these findings suggests that specialty orthopaedic hospitals may deliver care that leads to improved outcomes, although interpretation of these findings is limited by our lack of access to important clinical outcomes.
The finding that better outcomes in specialty hospitals persist after adjusting for procedural volume suggests that there may be benefits to the orthopaedic specialization of a hospital above and beyond the benefits of increased procedural volume alone. While the association between increased procedural volume and improved clinical outcomes has been well established28,37-39, the relationship between hospital specialization and clinical outcomes is unclear40-42. The current findings suggest that hospital specialization may lead to improved patient outcomes, although the mechanism through which this might occur is currently unknown. Possible mechanisms may include the procedural volumes performed by individual orthopaedic surgeons, the expertise and experience of ancillary staff, or organizational factors including the use of clinical pathways or communication among the clinical teams43. Future studies are required to clarify which of these factors are most important.
The finding that patients admitted to specialty hospitals for either total hip replacement or total knee replacement were significantly more likely than patients admitted to general hospitals to be transferred to another acute-care hospital following their procedures is also important. The fact that specialty hospitals had higher transfer rates for both primary and revision procedures suggests that this finding should not be attributed solely to the differences in the patient populations served by specialty and general hospitals. There are a number of potential explanations for this finding. Specialty hospitals claim that they are expressly designed to provide care to low-risk patients and that they are not designed to manage all complications that arise; thus, from this standpoint, the higher transfer rate from specialty hospitals may be justified for optimal patient outcomes. Alternatively, prior studies have demonstrated that hospitals may have financial incentives to transfer patients with more complicated (and more expensive) cases to other hospitals under the current Medicare prospective payment system44,45. From this perspective, it is possible that specialty hospitals might have other reasons to transfer patients with complications to other hospitals rather than manage such complications on-site.
Our finding that hospital length of stay was similar in specialty and general hospitals is somewhat unexpected, given the fact that specialty hospitals admit less complicated cases and these patients have lower complication rates, both of which would be expected to result in a longer length of stay at general hospitals. Moreover, our recent analysis of specialty cardiac hospitals demonstrated a similar finding14. One possible explanation is that general hospitals, under intense financial pressure, have already reduced hospital length of stay as much as possible, resulting in this small difference. Alternatively, it is possible that many new specialty hospitals, having opened relatively recently, have yet to optimize their hospital length of stay, resulting in a higher than expected length of stay.
The current findings add to our understanding of the relative quality of care provided by specialty hospitals in a number of ways. Most prior studies have focused on specialty cardiac hospitals. To our knowledge, the sole previous investigation of quality of care in specialty orthopaedic hospitals was performed by consultants under contract for CMS; the analysis found that after stratifying patients according to severity of illness (with use of the All-Patient Refined Diagnosis-Related Group methodology)46, specialty orthopaedic hospitals had lower mortality rates than did general hospitals16. However, that prior analysis had a number of methodological limitations, which were addressed by the current study, including our use of a more refined risk-adjustment model, examination of a more robust set of adverse outcomes, and our assessment of outcomes in important subpopulations (e.g., patients undergoing primary and revision joint replacement). Moreover, our results suggest that the improved outcomes in specialty orthopaedic hospitals persist after adjustment for differences in hospital volume.
Our finding of significantly improved outcomes in specialty orthopaedic hospitals contrasts with the available data assessing outcomes in specialty cardiac hospitals. While some studies have described mortality rates that were 20% to 50% lower in specialty cardiac hospitals16, we found that mortality rates in specialty cardiac and general hospitals were similar for percutaneous coronary interventions and coronary artery bypass graft surgery after adjustment for patient characteristics and hospital volume14. This led us to conclude that there was no inherent advantage to specialty cardiac hospitals.
There are several possible explanations for the differences between the results of the current study and our prior analysis of cardiac hospitals. One possibility is that the benefits of specialization are greatest in surgical settings. Thus, specialty orthopaedic hospitals receive substantial benefit from their specialization, while specialty cardiac hospitals that typically perform a combination of surgical procedures (e.g., bypass surgery), medical procedures (e.g., angioplasty), and general cardiac care may receive less benefit from specialization. Alternatively, it is possible that the stringent regulation of cardiac care in many states through certificate-of-need programs has resulted in regionalization of cardiac care to a relatively smaller number of hospitals, thereby minimizing the differences in outcomes between specialty and general cardiac hospitals; since major joint replacement has generally not faced such regulatory oversight, differences in outcomes between specialty and general hospitals may be larger47.
The policy implications of the current study are important, particularly given the recent decision by the United States Congress to allow a temporary moratorium on specialty hospital growth to expire, thereby setting the stage for further specialty hospital expansion1,2. Our results support the contention that specialty hospitals focus primarily on low-risk patients, thus taking advantage of well-documented inconsistencies in Medicare's diagnosis-related-group-based reimbursement system that effectively overreimburses hospitals for providing care to low-risk patients while underreim-bursing them for high-risk patients48. CMS is aware of this potential problem and is taking steps to update the prospective payment system49. By demonstrating improved outcomes in orthopaedic specialty hospitals, the current study is likely to add to the momentum favoring the growth of specialty orthopaedic hospitals. It will be important for general hospitals to investigate the clinical factors (e.g., the use of clinical pathways) and organizational factors (e.g., the composition of patient-care teams) that specialty orthopaedic hospitals are using and consider adapting them to their own institutions.
It is important to acknowledge the potential limitations to our work. First, our analysis was limited to Medicare beneficiaries so extrapolation of the results to other patients must be done with caution; future analyses should include a more heterogeneous mix of patients such as those with Medicaid or private insurance. Second, our analysis relied on claims data, which may underestimate differences in patients treated in specialty and general hospitals. Even though our findings were consistent among patients having primary and revision joint replacement, it is possible that our models were unable to adequately adjust for differences between the patients in specialty hospitals and those in general hospitals. In addition, our reliance on administrative data may have resulted in the underestimation of the rates of certain adverse outcomes (e.g., deep vein thrombosis)27. Third, our results may be subject to systematic differences between specialty and general hospitals in the capture of ICD-9-CM diagnosis and procedure codes, resulting in biased estimates of risk-adjusted differences in outcomes; however, it is unlikely that unadjusted risk would fully account for the 40% reduction in adverse outcomes that was observed in specialty orthopaedic hospitals. Fourth, the study examined a limited number of outcomes and did not consider important clinical outcomes such as functional status or patient satisfaction. Finally, while our definition of specialty orthopaedic hospitals is consistent with definitions used by both policy makers and other investigators, there remains debate over precisely how to define specialty hospitals. In particular, it is unclear to what extent specialty hospitals should be defined by the presence of physician owner-investors and for-profit or not-for-profit status. Future research is needed to clarify these issues.
In summary, the current findings demonstrate lower rates of adverse outcomes in specialty orthopaedic hospitals relative to general hospitals after adjustment for patient characteristics and hospital procedural volume. This suggests that orthopaedic specialization may lead to better patient outcomes. Further analyses with use of clinical registries are warranted to verify our findings.
Tables showing comorbid conditions, as measured according to Elixhauser et al.30, and the independent variables included in the final risk adjustment model are available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on “Supplementary Material”) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ▪
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the Department of Veterans Affairs, Health Services Research and Development Program to the Center for Research in the Implementation of Innovative Strategies for Practice (CRIISP), and a K23 career development award (RR01997201) from the National Center for Research Resources, National Institutes of Health. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM).
The views expressed in this article are those of the authors and do not necessarily represent the views of the Department of Veterans Affairs.
Investigation performed at the Division of General Internal Medicine, the University of Iowa Carver College of Medicine, Iowa City, Iowa
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