Infection risk may be time-dependent
Patients who receive an intra-articular steroid or hyaluronic acid injection in the knee prior to undergoing total knee arthroplasty (TKA) may be at an elevated risk of postsurgical infection, according to Nicholas A. Bedard, MD. Furthermore, infection risk may be time-dependent, increasing when less time has elapsed between injection and undergoing TKA, noted Dr. Bedard during the 2015 annual meeting of the American Association of Hip and Knee Surgeons.

"Five studies in the literature have evaluated this question," said Dr. Bedard. One large study of 35,890 Medicare patients found significantly increased odds of infection when TKA patients received an injection within 3 months prior to undergoing the surgery. The other four studies were retrospective and involved much smaller patient cohorts (64 to 250 patients). Although one of these studies found increased risk of deep infection with preoperative injection, the other three found no significant differences in infection rate tied to injections.

Dr. Bedard, who cited the "disastrous consequences" that can occur in patients who sustain infection after TKA, noted that he and his colleagues sought to shed light on injections as a possible infection risk factor by analyzing data from a multicenter database on a large patient cohort.

Conducting the study
The investigators queried the administrative claims database of a large private insurer for data from 2007 to 2014 on both private and Medicaid/Medicare Advantage-insured patients. Using Current Procedural Terminology (CPT) codes and laterality modifiers, they identified 83,684 patients—29,603 patients (35.4 percent) who had an injection in the ipsilateral knee at least one year prior to primary TKA, and 54,081 patients (64.6 percent) who did not receive a previous injection within 1 year of undergoing TKA.The researchers stratified the injection cohort—ie, those who had undergone injection followed by ipsilateral TKA within 1 year of injection—into 12 subgroups by monthly intervals out to 12 months, as designated by the number of months that had elapsed between injection and TKA. The investigators then broke down each cohort by age and sex and determined the Charlson Comorbidity Index (CCI) for both cohorts. Finally, they identified postoperative rates of surgical site infection (SSI) and TKA infection that required surgical intervention within 6 months of primary TKA.

Females outnumbered males in both cohorts; the injection cohort was 62.5 percent female and the no injection cohort was 66.7 percent female. No significant differences in CCI scores were found between the two cohorts; the average CCI score was 2.9 in both groups.

Using the chi-square test, the researchers determined significant differences and also used an independent two-sample t-test for continuous variables.

Assessing results
The researchers found a significantly higher rate of postoperative infection in patients who had received an injection compared to those who had not (4.4 percent versus 3.6 percent, P < 0.001). Furthermore, the investigators found that time between injection and TKA played a role in infection rates. The odds of patients sustaining a postoperative infection were significantly higher for the injection cohort when the injection was administered within 6 months of undergoing TKA. Odds of surgical intervention for TKA infection were higher when the injection was received within 7 months of undergoing TKA. But when the time between the injection and the surgical procedure exceeded 7 months, odds ratios were no longer significantly increased for either endpoint.

Thus, concluded the researchers, "injection prior to TKA increases the odds of postoperative infection, with increasing odds found when less time has elapsed between injection and TKA.

"Further research—in the form of a prospective, multicenter study with large patient cohorts—is needed to better clarify this relationship," said Dr. Bedard.

Dr. Bedard's coauthors for "Do Injections Increase the Risk of Infection Following TKA?" included Andrew J. Pugely, MD; Jacob M. Elkins, MD, PhD; Kyle R. Duchman, MD; Robert W. Westermann, MD; Steve S. Liu, MD; Yubo Gao, PhD; and John J. Callaghan, MD. The authors' disclosure information can be accessed at www.aaos.org/disclosure

Jennie McKee is a senior science writer for AAOS Now. She can be reached at mckee@aaos.org

Bottom Line

• This study sought to determine whether injection prior to TKA elevates the risk of postoperative infection and to identify whether the amount of time between injection and TKA affects postoperative infection rates.
• The researchers obtained data from 2007 to 2014 on 83,684 patients—29,603 patients (35.4 percent) who had an injection in the ipsilateral knee prior to primary TKA, and 54,081 patients (64.6 percent) who did not—from a large, multicenter database.
• Patients in the injection cohort had significantly higher odds of postoperative infection out to a duration of 6 months between injection and TKA. Odds of operative intervention for TKA infection were also higher when the injection was received within 7 months of undergoing TKA.
• When time between the injection and the surgical procedure exceeded 7 months, odds ratios were no longer significantly increased at either endpoint.
• The researchers concluded that undergoing injection prior to TKA increases the odds of postoperative infection, with odds increasing when less time has elapsed between injection and undergoing the surgery.