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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
JAMA. Author manuscript; available in PMC 2011 October 20.
Published in final edited form as:
PMCID: PMC3108186

Clinical characteristics and outcomes of Medicare patients undergoing total hip arthroplasty, 1991–2008

Peter Cram, MD MBA,1,2 Xin Lu, MS,1 Peter J. Kaboli, MD, MS,1,2,3 Mary S. Vaughan-Sarrazin, PhD,1,2 Xueya Cai, PhD,1 Brian Wolf, MD MS,4 and Yue Li, PhD1



Total hip arthroplasty (THA) is a common surgical procedure but little is known about longitudinal trends.


To examine demographics and outcomes of patients undergoing primary and revision THA between 1991 and 2008.

Design, Setting, and Participants

Observational cohort of 1,453,493 Medicare beneficiaries who underwent primary THA and 348,596 who underwent revision THA.


Changes in patient demographics and comorbidity; hospital length of stay (LOS); mortality; discharge disposition; and all-cause readmission rates.


Between 1991 and 2008 the mean age for primary THA increased from 74.1 (95% CI, 74.0-74.1) years to 75.1 (95% CI, 75.1-75.2)(P=0.01) and 75.8 (95% CI, 75.7-75.9) to 77.3 (95% CI, 77.2-77.4) for revision THA (P<.001). The mean number of comorbid illnesses per patient increased from 1.0 (95% CI, 1.0-1.0) to 2.0 (95% CI, 2.0-2.0) for primary and 1.1 (95% CI, 1.1-1.1) to 2.3 (95% CI, 2.3-2.3) for revision THA (P<.001 for both). For primary THA, LOS decreased from 9.1 days (95% CI, 9.1-9.2) in 1991–1992 to 3.7 days (95% CI, 3.7-3.7) in 2007–2008 (P=0.002); unadjusted in-hospital and 30-day mortality decreased from 0.5% (95% CI, 0.5%-0.5%) to 0.2% (95% CI, 0.2%-0.2%)(P< 0.001) and 0.7% (95% CI, 0.7%-0.7%) to 0.4% respectively (95% CI, 0.4%-0.4%)(P< 0.001). The proportion of primary THA patients discharged home declined from 68.0% (95% CI, 67.8%-68.3%) to 48.2% (95% CI, 48.0%-48.4%)(P<.001); the proportion discharged to skilled care increased from 17.8% (95% CI, 17.6%-18.1%) to 34.3% (95% CI, 34.1%-34.5%) (P<.001); 30-day all-cause readmission increased from 5.9% (95% CI, 5.8%-6.1%) to 8.5% (95% CI, 8.4%-8.6%) (P<.001). For revision THA similar trends were observed in hospital LOS, in-hospital mortality, discharge disposition, and hospital readmission rates.


Among Medicare beneficiaries who underwent primary and revision hip arthroplasty between 1991 and 2008, there was a decrease in hospital LOS, but an increase in the rates of post-acute care and readmission.


Total hip arthroplasty (THA) is a safe and effective therapy for patients with advanced degenerative joint disease.1,2 In recent years there has been a dramatic increase in utilization of this procedure both in the United States (U.S) and abroad.36 There is a general assumption that increasing experience with THA has resulted in improvements in patient outcomes as has been observed in other procedures,79 but rigorous empirical data documenting such improvement are limited.1014 This lack of data is striking given that an estimated 280,000 THA procedures are performed annually at a cost of more than $12 billion.5

The lack of rigorous evaluation of THA outcomes is somewhat surprising given ongoing efforts to regionalize surgical procedures to higher volume and higher quality hospitals.1517 As a generally elective surgery, THA is precisely the type of procedure that should be amenable to regionalization. Moreover, the introduction of the Medicare prospective payment system (PPS) in 1983 provided hospitals with incentive to control costs.18,19 A particularly important cost-control mechanism has been through reducing hospital length-of-stay (LOS). However, there is growing concern that hospitals may be reducing LOS by discharging patients prematurely resulting in increased utilization of skilled-care facilities20,21 and increased patient readmissions thus eliminating much of the cost savings originally envisioned by the PPS.22,23

With this background, the objective of this study was to evaluate long term trends in the outcomes of Medicare beneficiaries undergoing primary and revision THA and explore whether reductions in hospital LOS might be associated with increases in discharge of patients to post-acute care settings and/or increases in readmission rates.



We used Medicare Provider Analysis and Review (MedPAR) Part A data files to identify fee-for-service beneficiaries who underwent primary or revision THA between 1991 and 2008. Patients were identified using International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9CM) procedure codes (81.51 for primary and 8005, 8153, 0070, 0071, 0072, 0073for revision THA).2427 The Part A files contain a range of data collected from discharge abstracts for all hospitalized fee-for-service Medicare enrollees including: patient demographics; ICD-9CM codes for primary and secondary diagnoses and procedures; admission source (e.g., emergency department or transfer from outside hospital); admission and discharge dates; discharge disposition (coded as 25 separate categories); death occurring up to three years after discharge; a unique patient identifier that allows for identification of patient readmissions; and each hospital’s unique six digit identification number. For purposes of this analysis patient race was categorized as white, black, other and missing. Comorbid illnesses present on the index admission were identified using algorithms described by Elixhauser et al.28,29 which consider 30 specific conditions and exclude comorbid conditions that may represent complications of care or that are related to the primary reason for hospitalization.

To perform our 18-year longitudinal study, we used two separate Medicare Part A data files obtained from the Centers for Medicare and Medicaid Services (CMS): one file extended from 1991 through 2005; and a second from 2006 through 2008. Each of these files contains an internally consistent unique patient identifier that allows for the tracking of individual patients across time within the file. However, because the unique patient identifiers differ across the two different data files, it was impossible to link individual patients across the two different files. Thus, for example, if a single patient were hospitalized in December, 2005 and readmitted again in January, 2006 the lack of a consistent unique identifier across the two files precluded us from determining whether a single patient were admitted and readmitted or whether two different patients were admitted (one in December, 2005 and another in January, 2006). From a practical standpoint, this had minimal impact on our results but did influence our methodological approach, as described below.

Our intention was to examine changes in outcomes of patients undergoing primary and revision THA procedures. As primary THA is most often an elective procedure while revision THA can be either an elective or more urgent procedure, we applied separate exclusion criteria to the primary and revision THA populations in accordance with prior studies.27,30,31 For primary THA we sequentially excluded patients with acute fractures (N=136,887), patients admitted through the emergency department (N=19,721), and patients admitted after transfer from another acute-care hospital (N=2,856); these exclusion criteria were developed to select a population of primary elective THA patients. Our revision THA population did not exclude these types of patients because revision THA is often an emergent or unscheduled procedure and thus exclusion of these populations would not make sense.

Outcomes of Interest

Our outcomes of interest included hospital length of stay (LOS), mortality (in-hospital, within 30-days and 90-days of admission), discharge disposition, and all-cause readmission within 30-days and 90-days of admission. Discharge disposition was stratified into six mutually exclusive categories: home; skilled care/intermediate care; transfer to another acute-care hospital; inpatient rehabilitation; dead; and other. We limited our 30-day readmission analysis to patients whose index hospitalization occurred prior to November 30, 2005 for procedures performed between 1991 and 2005, and prior to November 30, 2008 for procedures performed between 2006 and 2008 to allow for a full 30-day follow-up period. Similarly, our 90-day readmission analysis was limited to patients whose index hospitalization occurred prior to September 30, 2005 for procedures performed between 1991 and 2005 and prior to September 30, 2008 for procedures performed between 2006 and 2008. Patients with hospital LOS greater than one-year were excluded from the LOS analysis in accordance with other recent studies.22

Statistical Analysis

We examined the demographic characteristics and prevalence of key comorbid illnesses for patients who underwent THA during the study period; for simplicity, data are presented separately for each two-year period (e.g., 1991–92, 1993–94, etc). We used analysis of variance for comparisons of continuous variables and the Mantel-Hanzel chi-square test for categorical variables. All analyses were performed separately for primary and revision THA patients.

We compared rates of each of the outcomes described above for each of the two-year periods using similar statistical methods. We used graphical techniques to examine changes in LOS, mortality, discharge disposition, and readmission rates over the course of the study period. To account for the changing demographics of the primary and revision THA populations over time, we calculated risk-standardized mortality ratios (RSMR) that adjusted for age, sex (male, female), race (white, black, other, and unknown).32 Inclusion of patient demographics in calculating these rates is important given that prior studies have demonstrated differential complication and utilization rates for joint arthroplasty among different demographic groups.3335 Again, all analyses were conducted separately for the primary and revision THA cohorts.

All p-values are 2-tailed, with p-values less than .05 deemed statistically significant. All statistical analyses were performed using SAS 9.2 (SAS Institute Inc., Cary, NC). This project was approved by the University of Iowa Institutional Review Board.

Sensitivity Analysis

To assess the robustness of our results, we performed a number of supplemental analyses. First, we repeated our analyses while restricting our analysis to the first primary or first revision hip arthroplasty procedure performed on each patient during a 12-month period; this eliminates patients who undergo “staged” bilateral arthroplasty procedures or complicated revisions. Second, we repeated our analyses after adding back the excluded populations described previously (e.g., fracture patients). Results of these analyses are available from the authors by request.


Our final study population included 1,453,493 elective primary total hip arthroplasty procedures and 348,596 revision hip arthroplasty procedures performed on Medicare beneficiaries between 1991 and 2008. For primary THA (Table 1) mean age increased from 74.1 (95% CI, 74.0-74.1) in 1991–1992 to 75.1 (95% CI, 75.1-75.2) in 2007–2008. The prevalence diabetes increased from 7.1% (95% CI, 7.0%-7.3%) to 15.5% (95% CI, 15.4%-15.7) and the prevalence of obesity increased from 2.2% (95% CI, 2.2%-2.3%) to 7.6% (95% CI, 7.5%-7.7%) (P< 0.001 for each).

Table 1
Characteristics of medicare beneficiaries receiving primary THA between 1991 and 2008

Trends were similar for revision THA (Table 2). In particular, mean age increased from 75.8 (95% CI, 75.7-75.9) in years 1991–1992 to 77.3 (95% CI, 77.2-77.4) in 2007–2008. The prevalence of diabetes increased from 7.2% (95% CI, 6.9%-7.4%) to 15.7% (95% CI, 15.3%-16.0%) and the prevalence of obesity increased from 1.4% (95% CI, 1.3%-1.5%) to 4.7% (95% CI, 4.5%-4.9%) (P< 0.001 for each).

Table 2
Characteristics of medicare beneficiaries receiving revision THA between 1991 and 2008

For primary THA mean hospital LOS declined from 9.1 days (95% CI, 9.1-9.2) in 1991–1992 to 3.7 days (95% CI, 3.7-3.7) in 2007–2008 (Table 3 and Figure 1), a 59.3% relative decrease (P< 0.001). In-hospital mortality after primary THA dropped from 0.5% (95% CI, 0.5%-0.5%) in 1991–1992 to 0.2% (95% CI, 0.2%-0.2%) in 2007–2008, a 60.0% relative reduction (P< 0.001). Likewise, 30-day mortality decreased from 0.7% (95% CI, 0.7%-0.7%) in 1991–1992 to 0.4% (95% CI, 0.4%-0.4%) in 2007–2008 (a relative reduction of 42.9%, P=0.004) and 90-day mortality declined from 1.2% (95% CI, 1.2%-1.3%) to 0.8%(95% CI, 0.7%-0.8%) (P<0.001). After adjustment for patient characteristics, risk-adjusted 30-day mortality over the study period declined from 0.7% (95% CI, 0.7%-0.8%) to 0.3% (95% CI, 0.3%-0.4%) and 90-day mortality dropped from 1.3% (95% CI, 1.2%-1.3%) to 0.7% (95% CI, 0.7%-0.7%) (P<0.001 for each).

Table 3
Hospital LOS, discharge disposition, mortality, and readmission rates for primary THA

The proportion of primary THA patients discharged home declined from 68.0% (95% CI, 67.8%-68.3%) in 1991–1992 to 48.2% (95% CI, 48.0%-48.5%) in 2007–2008 while the proportion of patients discharged to skilled or intermediate care increased from 17.8%% (95% CI, 17.6%-18.1%) to 34.3% (95% CI, 34.1%-34.5%) (P<.001 for both) (Table 3 and Figure 1). The 30-day all-cause readmission rate declined from 5.9% (95% CI, 5.8%-6.1%) in 1991–1992 to 4.6% (95% CI, 4.5%-4.7%) in 2001–2002 (P<.001) before increasing to 8.5% (95% CI, 8.4%-8.6%) in 2007–2008 (P<.001); results were similar for 90-day readmission rates.

For revision THA mean hospital LOS declined from 12.3 days (95% CI, 12.2-12.4) in 1991–1992 to 6.0 days (95% CI, 6.0-6.1)in 2007–2008 (Table 4 and Figure 2), a 51.2% relative decrease (P< 0.001). Unadjusted in-hospital mortality after revision THA dropped from 1.8% (95% CI, 1.6%-1.9%) in 1991–1992 to 1.2% (95% CI, 1.1%-1.3%) in 2007–2008, a 33.3% relative reduction (P< 0.001). Alternatively during the study period unadjusted 30-day mortality increased from 2.0% (95% CI, 1.8%-2.1%) in 1991–1992 to 2.4% (95% CI, 2.2%-2.5%) in 2007–2008 (an 20.0% relative increase) (P=0.004) and 90-day mortality increased from 4.0% (95% CI, 3.8%-4.2%) to 5.2% (95% CI, 5.0%-5.4%) (a 30.0% relative increase) (P<0.001). However, after adjustment for patient characteristics, risk-adjusted 30-day mortality remained stable within a narrow range between 1.9% and 2.3% (P = 0.22) while adjusted 90-day mortality remained near 4.5% throughout the study period (P = 0.16).

Table 4
Hospital LOS, discharge disposition, mortality, and readmission rates for revision THA

The proportion of revision THA patients discharged home declined from 57.4% (95% CI, 56.8%-57.9%)in 1991–1992 to 35.4% (95% CI, 34.9%-35.8%) in 2007–2008 while the proportion of patients discharged to skilled or intermediate care increased from 26.7% (95% CI, 26.2%-27.1%) to 42.4% (95% CI, 42.0%-42.9%) (P<.001 for both) (Table 4 and Figure 2). The 30-day all-cause readmission rate decreased from 8.7% (95% CI, 8.3%-9.0%) in 1991–1992 to 8.2% (95% CI, 7.9%-8.5%) in 1999–2000 (P<.001) before increasing to 14.1% (95% CI, 13.8%-14.5%) in 2007–2008 (P<.001) with similar results for 90-day readmission rates.


In an analysis of Medicare administrative data from 1991–2008, we identified a number of interesting trends. We found that despite increasing patient complexity, both unadjusted and adjusted mortality for primary THA showed substantial improvement over time. Alternatively, for revision THA unadjusted mortality appeared to increase modestly but this increase was largely explained by increasing patient complexity. Most importantly, marked declines in hospital LOS for both primary and revision THA seemed to correspond with an increase in the proportion of patients who were discharged to post-acute care and an increase in patient readmissions.

A number of our findings warrant further discussion. First, our finding of increasing complexity of both primary and revision THA patients is important. While a number of studies have documented increasing complexity of patients in the cardiovascular disease literature,22,23,36 few studies have evaluated trends in patient complexity in orthopaedics and most have focused on knee arthroplasty.1,3739 Our finding of an increase in the average age of both primary and revision THA patients suggests that at least some of the increase in patient complexity is real and is not simply an artifact of more aggressive coding practices (aka, upcoding).40

Our finding of increased patient complexity is particularly important when considering the changes in patient mortality that we observed. We found clinically and statistically significant reductions in primary THA mortality, albeit from very low baseline levels. Alternatively, the finding of an increase in revision THA mortality in unadjusted analyses was somewhat unexpected; the finding that this increase in mortality was no longer significant in adjusted analyses is reassuring. The absence of similar large-scale studies of the U.S. hip arthroplasty population with which to compare our results highlights the need for more rigorous study of THA outcomes.

Second, the marked decrease in hospital LOS for both primary and revision THA is interesting and has significant policy implications. The motivation for hospitals to reduce LOS under the Medicare prospective payment system has been well described previously,41,42 but the impact of declining LOS is much less clear. While Bueno et al. noted that reductions in LOS for Medicare beneficiaries hospitalized with congestive heart failure were accompanied by an increase in readmission rates,22 a prior study by Baker et al. found that reductions in LOS were not associated with an increase in readmissions.43 To the best of our knowledge no prior studies have evaluated the impact of reductions in LOS in regard to joint arthroplasty.

We found that the reduction in hospital LOS in both primary and revision THA patients was accompanied by a significant increase in the proportion of patients discharged to post-acute care facilities (i.e., nursing homes and rehabilitation centers) and a significant reduction in the proportion of patients discharged directly home. Moreover, we found that while the reduction in hospital LOS was not associated with an increase in readmission rates between 1992 and 1999, in more recent years readmission rates have risen markedly. Together these findings suggest that the old adage that there is no such thing as a free lunch may hold when it comes to hospital LOS; reductions in LOS accompanied by an increase in patient discharges to skilled care facilities and an increase in readmission rates. These findings reinforce the potential wisdom of moving to bundled payments and/or reimbursement for episodes of care as a way for incentivizing the “correct” LOS rather than perpetual reductions in LOS that seem to be occurring.

Our study has a number of limitations that warrant brief mention. First, our study was limited to fee-for-service Medicare beneficiaries and thus extrapolation to other populations should be done with caution. That said, more than 60% of all THA procedures are performed on Medicare enrollees, making this an appropriate data-set for studying THA. Second, our study relied upon administrative data and thus we were unable to evaluate a number of important arthroplasty outcomes including functional status and patient satisfaction.

In conclusion, mortality after primary THA has declined moderately over time and mortality after revision THA has remained stable despite substantial increases in patient complexity. There have also been marked reductions in hospital LOS and rising readmission rates.


Dr. Cram was supported by a K23 career development award (RR01997201) from the NCRR at the NIH and the Robert Wood Johnson Physician Faculty Scholars Program. This work is also funded by R01 HL085347-01A1 from NHLBI at the NIH (Cram) and R01 AG033202 (Li). The views expressed in this article are those of the authors and do not necessarily represent the views of the Department of Veterans Affairs. The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation, review, or approval of the manuscript. Dr. Cram has received consulting fees from The Consumers Union (publisher of Consumer Reports Magazine) and Vanguard Health Inc. for advice he provided on quality improvement initiatives. Dr. Cram and Ms. Lu had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.


None of the authors have any conflicts of interest.


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