Instability of the Hip Joint After Posterior Acetabular Wall Fracture

Background: Exact determinants of hip instability have not been established for fractures of the posterior wall of the acetabulum involving ≤50% of the wall. Therefore, examination of the hip under anesthesia (EUA) is routinely performed. Recently, the superior exit point of the fracture has been reported to be an important identifiable risk factor. Pre-existing adult hip dysplasia (developmental dysplasia of the hip [DDH]) is thought to have a similar role. The purpose of this study was to determine if any known radiographic measurements and signs associated with DDH, or any fracture characteristics, are independent risk factors for hip instability after fracture of the posterior wall of the acetabulum. Methods: All patients with a posterior acetabular wall fracture (OTA 62.A1) treated at our institution between 2004 and 2015 were considered for the study. Inclusion criteria were an age of ≥18 years, an isolated posterior acetabular wall fracture involving ≤50% of the acetabular wall, adequate imaging, and documented EUA results. Evaluated variables included fracture fragment size, superior exit point of the fracture, center-edge angle, acetabular index, Tönnis angle, lateralized head sign, crossover sign, posterior wall sign, ischial spine sign, and hip version. Data were examined using univariate testing, followed by a multivariate logistic regression analysis. Results: Sixty-eight patients met all of the inclusion criteria. Univariate analyses identified the posterior wall sign (p = 0.033), ischial spine sign (p = 0.030), and proximity of the superior exit point of the fracture to the acetabular dome (p = 0.044) as having a significant association with hip instability. However, multivariate logistic regression modeling revealed that none of these factors were significant independent risk factors. Conclusions: Consistent with previous studies, univariate analyses identified certain radiographic findings as significant risk factors for hip instability in the setting of a fracture of the posterior wall of the acetabulum. However, subsequent multivariate logistic regression modeling showed that no studied variable was an independent risk factor. Our results indicate that important factors leading to hip instability are yet to be identified or the contributions of the measured variables are relatively small. Therefore, EUA should remain the main clinical determinant of hip stability status. Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.