Patient, surgeon, health system, and device factors are all known to influence THA outcomes . Prior studies have identified risk factors associated with reduced implant survivorship, such as younger age, male sex, higher activity level, and metal-on-metal bearings [10, 21, 22]. However, early failures that occur within the first 12 months after primary THA often result in devastating outcomes, and patient-related factors associated with an increased risk of early revision are poorly understood, particularly in elderly patients.
Elderly patients who undergo THA often have comorbid illnesses and are known to be at increased risk for perioperative morbidity and mortality [13-15]. Previous investigators have identified independent risk factors for periprosthetic joint infection (rheumatologic disease, obesity, coagulopathy, preoperative anemia) and mortality (congestive heart failure, metastatic cancer, psychosis, renal disease, dementia, cerebrovascular disease, chronic pulmonary disease) after primary THA .
Using the Medicare 5% national sample administrative database, we identified specific demographic and clinical factors associated with an increased risk of revision during the first 12 months after primary THA in Medicare patients.
Patients and Methods
We used the 5% sample of the Centers for Medicare & Medicaid Services (CMS) Medicare claims database, which includes Part A (inpatient), Part B (physician), and outpatient claims, to identify patients who underwent primary THA (ICD-9-CM Procedure Code 81.53) between 1998 and 2010. Patients younger than 65 years old were excluded from the study because they had major disabilities and therefore represented a population that differed from the general Medicare population. Patients enrolled in a health maintenance organization (HMO) were excluded because HMO claims are not available in the Medicare 5% sample database and therefore these patients would have incomplete claims history. Patients not enrolled in both Part A and Part B of Medicare were also excluded because of their incomplete claim history. Nonresidents were also excluded for the same reason.
Unique encrypted Medicare beneficiary identifiers were used to track patients longitudinally during the 13-year study period. A linked denominator file that accompanies the analytic data sets was used to track each patient's Medicare enrollment status. This annual Medicare denominator file contains information regarding the date of death of the enrollees and was used to identify patients who died during the 1-year followup period. Revision surgery during the study period (ie, through December 31, 2010) was identified by the occurrence of the ICD-9-CM Procedure Code 81.53 or 00.70-00.73 from the Part A files of the Medicare data set.
The preoperative comorbid conditions analyzed in our study were compiled from diagnoses included in Part A, Part B, or outpatient claims submitted during the 12-month period before the primary THA. Only patients who had been enrolled in Medicare during the entire 12-month period before their THA were included in the study; this ensured that a full year of baseline comorbidity information would be available for each patient. To avoid misclassification of postoperative complications as preexisting comorbid conditions, only comorbid conditions that were identified in administrative claims dated at least 30 days before the date of the index THA procedure were classified as preexisting conditions.
The study cohort consisted of 56,030 Medicare patients who underwent primary THA between 1998 and 2010. The mean age was 75.2 years (SD = 6.2 years), 65% were women, and 94.2% were white. Patients were geographically dispersed throughout the different US Census regions, 7.4% had public assistance for their Medicare premiums, and 76.8% had a Charlson Comorbidity Index  of less than 3 (Table 1). The prevalence of each comorbid condition ranged from a high of 75% for hypertension to a low of 0.4% for drug abuse (Table 2). The cumulative incidence of revision during the first 12 months after primary THA for the entire cohort was 2.03%.
We examined the impact of 29 comorbid conditions on the risk of revision THA within 12 months following primary THA using multivariate Cox regression, controlling for age, sex, race, US Census region, public assistance (indicated by Medicare premiums and deductibles subsidized by the state because of the patient's financial status), and all other baseline comorbidities. The 29 comorbid conditions included the specific diseases used to determine the Charlson Comorbidity Index  and other diseases used as comorbidity indexes in previous analyses of administrative databases because of their association with increases in the length of hospital stay, hospital charges, and mortality . In addition, other conditions identified in previous clinical studies as risk factors for early revision THA were included .
We calculated both the crude relative risk (RR) and the adjusted hazard ratio (HR) for each comorbid condition and used the Wald chi-square statistic to evaluate the significance of the HR. The resulting p value for the HR was used to rank the degree of association of each comorbid condition with risk of revision surgery within 1 year after primary THA. Thus, the ranking of a specific condition indicated the relative degree of association or significance of its presence with risk of revision surgery within 1 year after primary THA.
Among the clinical and demographic factors analyzed (Table 3), the most significant independent risk factors for revision THA within 12 months (in order of significance; p ≤ 0.038 for all comparisons) were depression (HR = 1.64; 95% CI, 1.39-1.93), rheumatologic disease (HR = 1.32; 95% CI, 1.11-1.57), psychoses (HR = 1.34; 95% CI, 1.08-1.68), renal disease (HR = 1.29; 95% CI, 1.06-1.58), chronic urinary tract infection (HR = 1.15; 95% CI, 1.01-1.32), and congestive heart failure (HR = 1.20; 95% CI, 1.01-1.43). The most common overall reasons for revision were mechanical complication, dislocation, infection, and periprosthetic fracture (Table 4).
THA outcomes have previously been characterized at the population level for the Medicare population [3, 16, 17]. Most studies have shown that THA is associated with high rates of success as defined by decreased pain and improved function in patients who suffer from disabling arthritis of the hip . However, complications requiring revision surgery within the first 12 months after primary THA are associated with increased costs and poor clinical outcomes. Identifying risk factors for early failure is important for informing shared medical decision making in elderly patients who are considering elective THA for management of their hip pain and for risk stratification of publicly reported outcomes. We identified specific demographic and clinical factors associated with an increased risk of revision during the first 12 months after primary THA in Medicare patients.
Our study has several important limitations. First, we relied on administrative claims data (which did not include laterality) to identify revisions and risk factors for early failure. However, previous investigators have reported reasonable correlation between administrative claims and the clinical record when evaluating causes and types of revision TJA procedures . Second, our study was limited to Medicare patients, and therefore our findings may not be generalizable to the broader population of younger patients who undergo primary THA. However, given that older patients have more comorbid conditions, we believe that it is important to identify risk factors for early revision THA in the elderly population. Finally, our findings are limited to risk factors for failures that occur during the first 12 months after primary THA, and therefore it is unclear whether the same or other risk factors are associated with a higher long-term risk of revision THA. However, the impact of patient comorbidities on the risk of early failure after THA has important clinical and policy implications for surgeons, hospitals, and patients.
Previous investigators have evaluated risk factors for complications and revision after THA. Conroy et al.  investigated risk factors for revision for dislocation after primary THA as reported in the Australian Orthopaedic Association National Joint Replacement Registry and found patients with fractured neck of the femur (RR = 2.0), rheumatoid arthritis (RR = 2.0), or avascular necrosis (RR = 1.6) had a significantly increased risk of revision for dislocation. Pederson et al.  used the Danish Hip Arthroplasty Registry to evaluate the extent to which diabetes affects the risk of revision THA. They found that diabetes was associated with an increased risk of revision due to deep infection (RR = 1.45; 95% CI, 1.00-2.09), particularly in patients with Type 2 diabetes (RR = 1.49; 95% CI, 1.02-2.18), those with diabetes for less than 5 years before THA (RR = 1.69; 95% CI, 1.24-2.32), those with complications due to diabetes (RR = 2.11; 95% CI, 1.41-3.17), and those with cardiovascular comorbidities before surgery (RR = 2.35; 95% CI, 1.39-3.98). Graw et al.  compared time to revision between primary minimally invasive (MI) THA and traditional THA among patients who were referred to their practice for revision THA. They found that MI THA was associated with a significantly increased risk of early revision compared with conventional THA (1.4 years versus 14.7 years, odds ratio = 26.5). Finally, Schrama et al.  evaluated differences in the risk of revision THA for infection among patients with rheumatoid arthritis and osteoarthritis. Using data from the Norwegian Arthroplasty Register, they reported a higher risk of late (> 6 years postoperatively) infection leading to revision THA in patients with rheumatoid arthritis compared to patients with osteoarthritis .
As the United States shifts from volume- to value-based payment and delivery systems [5, 9, 23], increased emphasis has been placed on public reporting of physician performance and patient outcomes, including complication and revision rates for elective surgical procedures such as THA. Furthermore, the United States has initiated a THA outcomes registry, the American Joint Replacement Registry, which will be used to track revision rates among patients with THA in the United States. It is important that publicly reported THA outcomes and THA registry results be appropriately risk adjusted for factors outside the control of the surgeon, such as patient comorbidities, which are known to influence patient outcomes. The patient comorbidities identified in our study could be used in risk adjustment models for public reporting of THA outcomes and THA registries to appropriately account for patient characteristics associated with an increased risk of early revision THA.
In conclusion, depression, rheumatologic disease, psychoses, renal disease, chronic urinary tract infection, and congestive heart failure are associated with an increased risk of early revision after primary THA in Medicare patients. This information is important when counseling elderly patients with regarding the risk of early failure and for risk stratifying publicly reported outcomes in Medicare patients with THA.
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