Patient factors, medical comorbidities drive infection rates

Most serious complications of total knee arthroplasty (TKA) have decreased in the past 10 years, but not infections, according to Daniel J. Berry, MD. Despite the use of perioperative antibiotics and the introduction of antibiotic-loaded cement, the incidence of infection after TKA remains fairly stable.

To understand why there has not been a reduction in the rate of infection, Dr. Berry and his colleagues examined the incidence of infection in a large national database.

Dr. Berry presented “Incidence and risk factors for prosthetic joint infection following TKA,” which details the results of a large, longitudinal study of Medicare patients, at the 2009 Knee Society/American Association of Hip and Knee Surgeons Specialty Day.

Large numbers reveal major risk factors
Most infection studies have been done by a single institution or with a moderate number of patients.

Dr. Berry and his colleagues, however, wanted to know what the “incidence of infection was in the real world, not just in a single academic institution.” They also wanted to identify the risk factors.

Through the use of large national databases, they were able to identify more than 82,000 TKAs done between 1997 and 2006. “The patients all had encrypted Medicare beneficiary numbers that enabled us to track them longitudinally so we could determine what happened to them over time,” Dr. Berry said.

They then stratified patients according to comorbidities.

“The prosthetic infection rate for the first 2 years was 1.8 per­cent. From 2 to 10 years, infections developed or were recognized in an additional 0.6 percent of patients. Over the first 10 years, this brought the cumulative risk of infection to 2.47 percent in this Medicare population,” he explained, which might be a modest overestimation, based on the study’s methodology.

Patient factors and medical comorbidities were the greatest risk factors for infection. “An extremely strong correlation exists between how sick the patient was and the likelihood of both early and late infection developing in that patient,” he said (Fig. 1).

“When patients with almost no major medical problems were compared with those who had the most problems, the risk of infection was more than two-fold higher in the sickest patients—very important data that we can used in our clinical practices,” Dr. Berry said.

Patients who received public assistance for Medicare premiums also had a higher risk of infection. Women were found to have “slight­ly fewer infections than men with an odds ratio of 0.81,” he said. Age did not prove to be a risk factor.

What the databases didn’t include
Dr. Berry acknowledged that the study didn’t analyze the “subpopulations of patients with comorbidities who are at exceptionally high risk for infection.”

These included patients who are immunocompromised—those with diabetes, rheumatoid arthritis, human immunodeficiency virus (HIV) infection or transplantations, for example.

“Patients who have renal or liver failure, were previously institutionalized, or had a prolonged hospitalization have been shown to be at higher risk of infection in other studies. Patient risk factors related to the surgical site—previous knee surgery, the super-obese, previous infection, psoriasis, and lymphedema—are also important,” he said.

Knowing these risk factors gives the surgeon an opportunity to have a “careful discussion with the pa­tient about whether the risks versus the benefits of the operation are worth it to that patient,” he added.

Dr. Berry also noted that patients who have “malnutrition, psoriasis, skin flora from institutionalization, or HIV may be able to be treated for some of those factors before the surgery to reduce the risk of infection.”

He encouraged the use of rigorous perioperative intravenous antibiotics and noted that for high risk patients some surgeons consider the use of antibiotic-loaded cement.

“Infection is our biggest un­solved complication in the first 10 years after TKA. That tells us that we need to focus more efforts on new methods of prevention,” he said.

Dr. Berry would like young investigators to see this as a “fertile area of research both as orthopaedic surgeons and as orthopaedic researchers.” He also hopes that the orthopaedic industry will turn their attention to this serious problem.

Co-authors for “Incidence and risk for prosthetic joint infection following TKA in the Medicare population” include Kevin J. Bozic, MD, MBA; Steven M. Kurtz, PhD; Kevin Ong, PhD; Edmund Lau, MS, and Javad Parvizi, MD.

Dr. Berry reported the following conflicts--Royalties: DePuy, A Johnson & Johnson Company; Speakers bureau/paid presentations: DePuy, A Johnson & Johnson Company; Paid consultant or employee: DePuy, A Johnson & Johnson Company; Research or institutional support from companies or suppliers: DePuy, A Johnson & Johnson Company; Smith & Nephew; Stryker; Zimmer

Annie Hayashi is the senior science writer for AAOS Now. She can be reached at hayashi@aaos.org