Factors associated with dislocation after total hip arthroplasties performed for nontraumatic osteonecrosis of the femoral head: a multicenter cohort study of 5,983 hips.

Background and purpose:  Nontraumatic osteonecrosis of the femoral head (ONFH) patients are at a higher dislocation risk after primary total hip arthroplasties (THAs) than osteoarthrosis patients. It has not been clear how large prosthetic heads should be to reduce dislocation. A nationwide multicenter follow-up cohort study of THAs performed for ONFH aimed to evaluate risk factors associated with dislocation and whether larger head size could reduce the dislocation risk.

Methods:  A multivariable logistic regression model analyzed factors associated with dislocation in 5,983 THAs performed for ONFH between 1996 and 2022 with a median of 7.1 (0.5-27)-year follow-up. Patient age at surgery was 52 years and BMI was 22.9, as medians. A posterior approach was employed in 59%. The head diameter was 22 mm in 4%, 26 mm in 15%, 28 mm in 24%, 32 mm in 36%, and ≥ 36 mm in 21%.

Results:  288 THAs (4.8%) dislocated. Younger (1st quartile, ≤ 41 years) patient age (odds ratio [OR] 1.45 CI [95% confidence interval] 1.02-2.07 vs. 2nd quartile), higher BMI (OR 1.05, CI 1.02-1.08 per 1), posterior approach (OR 3.33, CI 1.96-5.56 vs. anterior or anterolateral approach, OR 2.27 CI 1.59-3.23 vs. lateral approach), and smaller heads were identified as risk factors. However, ≥ 36-mm heads were not different from 32-mm heads (OR 1.06 CI 0.69-1.63).

Conclusion:  Risk factors associated with dislocation were younger patient age, higher BMI, posterior approach, and smaller heads; however, 32-mm heads were large enough to reduce dislocation.