Intra-articular air predicts hip joint instability following posterior wall fracture, irrespective of fracture size or history of dislocation.

Abstract

To determine if the presence of intra-articular air on computed tomography (CT) correlates with hip joint instability following posterior wall acetabular fractures. Design: Retrospective chart and radiographic review. Two academic level 1 trauma centers. Skeletally mature patients with posterior wall acetabulum fractures (AO/OTA 62A1) that underwent stress examination under anesthesia (EUA) from 2014 to 2024. Patients with positive and negative EUAs were compared with regards to history of dislocation, presence of intra-articular air on CT scan, amount of fracture displacement, sagittal angle measurement, and fracture size. There were 38 positive (unstable) and 33 negative (stable) EUAs for posterior wall acetabulum fractures. Demographics were similar between positive (mean age 28 years (range (18-58 years), 34% female) and negative (mean age 36 years (range (18-61 years), 30% female) groups. Patients with a positive EUA had a significantly higher incidence of intra-articular air (82% vs 9%, P < 0.001), fracture displacement greater than 5 millimeters (76% vs 33%, P < 0.001), sagittal angle measurement greater than 70° (74 vs 21%, P < 0.001), and hip dislocation on injury radiograph (61 vs 18%, P < 0.001). In 43 patients who presented without a history of hip dislocation, intra-articular air was present significantly more often in the positive EUA group (n = 16) compared to the negative EUA group (n = 27) (75 vs 7%, P < 0.001). Hip joint instability following posterior wall acetabulum fracture was significantly higher in patients with intra-articular air on CT scan - regardless of a history for hip dislocation. Fracture displacement greater than 5 millimeters and sagittal angle measurement greater than 70° were also found to be predictors of positive EUA. When these characteristics are found on injury imaging, an EUA should be considered to rule out hip joint instability. Diagnostic Level III.