Our study is first limited by the use of an administrative database, which lacks clinical outcome measures such as the patient's physical function status. We were also unable to investigate procedural or implant information such as the type of surgical approach or use of bone cement as a result of the administrative nature of the data. Second, the identification of PJI for each patient was based on the recording of the 996.66 ICD-9-CM diagnosis code in the claims records. The degree of miscoding is unclear, however, because the majority of these were diagnosed while the patient was hospitalized (66%) or by an orthopaedic surgeon or infection specialist (63%); thus, the incidence is likely reflective of true deep infections. Third, the findings were based on the elderly patient population; it is unclear if our findings extend to patients younger than 65 years old. Fourth, we were unable to identify the specific microorganism(s) implicated in the infection. Despite these limitations, our data suggest that although the incidence of PJI is the greatest within the first 2 years postoperatively, PJI may still present itself at up to 10 years followup.
The risk of PJI was the greatest within the first 2 years after TKA surgery; however, late presentation of PJI accounted for one-fourth of the cases in the study period. The majority of deep infections (approximately two-thirds) were diagnosed while the patient was hospitalized and most were diagnosed by an orthopaedic surgeon or infection specialists. This time-dependent risk of infection may help explain the variability of infection rates reported in the literature. When we limited our followup to 90 days or less, the risk of PJI was comparable to what was previously reported by Katz et al. . In their study of the Medicare population from the first 9 months of 2000, Katz et al.  reported an overall rate of 0.4% with infection in the first 90 days after primary TKA. In a prospective analysis of 4185 patients undergoing primary TKA, Pulido et al.  reported an overall incidence of PJI of 1.1% with an average time to diagnosis of 1.2 years.
This study in a large cohort of patients with up to 10 years followup demonstrates deep prosthetic infection occurs at a relatively high rate in Medicare patients in a large national sample. Patient factors, especially degree of comorbidities and public assistance (eg, receiving public assistance for Medicare premium), were strong risk factors associated with deep infection. Procedure duration, a well-known risk factor of infection, was also confirmed by recent analysis of the Medicare data . Longer operative time was also a predisposing factor for PJI from a cohort of 9245 patients undergoing total joint arthroplasty (4185 knees and 5060 hips) in an unadjusted univariate analysis .
In light of our longitudinal analysis of the incidence of PJI in the Medicare patient population, along with other followup studies  and cross-sectional analysis of national hospitalization records , emphasis should be placed on minimizing the risk of PJI after primary TKA. Because PJI is a challenging complication for patients, hospitals, surgeons, and payers , effective strategies such as stringent patient screening for risk factors or adequate antibiotic prophylaxis  should be developed and implemented to combat this complication.
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