Use of cementless, monoblock dual mobility implants in primary total hip arthroplasty (THA) failed to reduce the risk of dislocation, with a dislocation rate of 3.2 percent and survivorship of 93 percent, according to a study that will be presented today at 11:20 a.m. in Ballroom 20A.
Together, these outcomes imply that “dual mobility constructs may not represent the panacea to instability that previous reports may suggest,” noted the authors, led by Benjamin Kelley, MD, an orthopaedic surgery resident at UCLA Health in Los Angeles.
Dr. Kelley and coauthors reviewed data from 168 consecutive primary THAs in 152 patients performed by a single surgeon between 2011 and 2017. THAs were performed via a posterior approach with a cementless, monoblock, dual mobility implant construct.
The analysis involved a total of 156 hips in patients who had been followed for at least two years. The researchers looked at outcomes including implant failures, complications, and Harris Hip Scores (HHS).
Approximately one-half of THAs were performed in men (46.7 percent; n = 73), and patients had a mean age of 67 years. The mean duration of follow-up was 5.3 years. The authors also reported that 30.8 percent of THAs were performed in patients with BMI >30. Common etiologies preceding THA were primary osteoarthritis (81.4 percent) and hip fracture (7.7 percent).
Regarding implant characteristics, the mean cup size (diameter) was 50.32 mm and the mean head size (outer diameter) was 44.32 mm.
The researchers identified 10 cases of implant failure requiring revision, for a failure rate of 6.4 percent. Those cases included three postoperative periprosthetic femur fractures (2 percent), three recurrent dislocations (2 percent), two loose femoral components (1.3 percent), one stem failure at a modular junction (0.6 percent), and one painful anterior impingement (0.6 percent). Five dislocations occurred (3.2 percent) at an average of 28.4 days postoperatively. There were no intraprosthetic dislocations.
Two of the dislocations were managed with closed reduction and bracing, the authors noted. In addition, one was treated with a modular dual mobility head exchange. Two patients with recurrent instability were revised to modular acetabular revision shells with constrained liners.
Of patients who experienced complications postoperatively, mean HHS improved by 44 points (range = 11–84) in the 146 hips with original components.
At final follow-up, the rate of survivorship was 93 percent. This outcome included a revision-free survival rate of 93.6 percent and a dislocation-free survival rate of 96.8 percent.
Multivariable logistic regression analysis to determine factors that increased the risk for implant failure or HHS improvement revealed that age, sex, obesity, and etiology were not significantly associated with failure.
Only cup size significantly increased the risk of failure (odds ratio = 2.25; P = 0.049). Obesity and stem type were the only factors significantly related to improvement in HHS score (P = 0.018 for both).
The researchers noted that the findings of the analysis are limited by the use of data from a single surgeon. Further long-term data are needed to support routine use of these implants in the primary setting.
Dr. Kelley’s coauthors of “No Dislocation Panacea: Mid-Term Outcomes of Dual Mobility Bearing Primary Total Hip Arthroplasty” are Michael Orden, MD; Matthew Dipane, BA; Adam Anton Sassoon, MD, MS; and Edward J. McPherson, MD.
Ariel DeMaio is the managing editor of AAOS Now. She can be reached at email@example.com.