Age, comorbidities increase risk of early dislocation after primary THA

During the past decade, orthopaedic surgeons have increased their use of large-diameter femoral heads in primary total hip arthroplasty (THA). But this shift—although it has reduced the incidence of dislocation after primary THA—hasn’t eliminated the problem, reported Arthur L. Malkani, MD, at the 2009 annual meeting of the American Association of Hip and Knee Surgeons.

Dr. Malkani, co-author of the Lawrence D. Dorr Award paper “Short-term dislocation risk following primary hip arthroplasty in the Medicare population,” reviewed the incidence of early and late dislocation and outlined several risk factors.

Bringing the issue to a head
“Despite the use of larger head sizes, dislocation continues to be the most common mode of failure leading to revision arthroplasty,” said Dr. Malkani. He pointed out that dislocation accounts for more than 20 percent of the failures leading to revision arthroplasty, followed by mechanical loosening.

Using data from the National Medicare 5 percent Sample, the authors examined claim records from 1998 through 2007. They were able to identify approximately 39,000 patients who met their inclusion criteria. To see the impact of larger prosthetic femoral head sizes, they divided the sample into two periods (1998–2003 and 2004–2007).

“We looked at procedure and diagnosis codes for primary arthroplasty and dislocation and established three patient categories: those with no dislocation, those with early dislocation (within the first 2 years after surgery), and those with dislocation at 2 years or later,” explained Dr. Malkani.

Of the entire patient cohort, researchers identified 1,500 early dislocations for an incidence of 3.84 percent, and 362 late dislocations for an incidence of less than 1 percent. Throughout the study period, early dislocation rates steadily decreased.

“In 1998,” said Dr. Malkani, “less than 10 percent of primary THAs used large-diameter (32 mm and larger) femoral heads. By 2007, large-diameter femoral heads were used in 77 percent of primary THAs.” The incidence of dislocation during the later period (2004–2007) was 36 percent lower than that during the earlier period (1998–2003).

“Many factors—not just femoral head size—contributed to the decline in dislocation rates,” noted

Dr. Malkani. “The overall decline in dislocation rates has helped decrease the cost of taking care of our patients. Extrapolating from the dislocation cost per patient, we found that declining dislocation rates saved the Centers for Medicare and Medicaid Services about $20 million over a 3-year span.”

Additional factors
Patients with more comorbid conditions were also found to have a higher incidence of dislocation. Patients with 5 or more comorbid conditions had 130 percent higher incidence of dislocation compared to those with 0 comorbidities.

“Decreasing surgeon volume also correlated with an increasing incidence of dislocation,” said Dr. Malkani (Fig. 1). “Surgeons who performed fewer than 5 THAs a year had a 50 percent higher incidence of dislocation compared to surgeons who performed more than 50 THAs a year.”

Researchers also found that patients older than age 85 had the greatest incidence of dislocation by a factor of 44 percent. But they found no difference whatsoever with regard to gender and the risk of dislocation.

“This study helps identify at-risk patients prior to surgery, so that appropriate interventions could be undertaken prior to arthroplasty to minimize the risk of dislocation in this high-risk group,” concluded Dr. Malkani.

Dr. Malkani reports receiving consulting fees and royalties from Stryker, which also funded the study. Dr. Malkani’s co-authors (and their disclosures) include Kevin Ong, PhD (Homer Stryker Center for Orthopaedic Education and Research [HSC]); Edmund Lau, MS (HSC); Steven M. Kurtz, PhD (Stryker, Zimmer, Biomet); and Michael T. Manley, PhD (Stryker).

Mary Ann Porucznik is managing editor of AAOS Now. She can be reached at porucznik@aaos.org

Bottom line

• Dislocation rates after primary THA have declined steadily since 1998.
• Of dislocations that occur, 81 percent occur within the first 2 years after surgery.
• Comorbidities, low surgeon volume, and age are associated with elevated dislocation risks.