A New Computed Tomography-based Measure that Provides Insight into Hip Stability in Patients with Posterior Wall Acetabular Fractures

Objective: Describe the technique and results of a new sagittal plane computed tomography (CT) based angular measure for predicting stability following posterior wall acetabular fractures (PWF). Design: Retrospective review Setting: Academic Level II Trauma center Patients: Fifty-eight consecutive patients with PWF (AO/OTA class 62A.1), 98% were high energy injuries. Intervention: A new sagittal CT (sCT) measure of PWF based upon the angle subtending the joint center, cranial and caudal fracture exits. Outcome measures and Comparisons: Hip incongruity or dislocation demonstrated using gold standard test, exam under anesthesia (EUA), or instability on static images. Prediction of hip instability utilizing a sCT angular measure based upon cranial and caudal fracture exits was compared to previous axial CT (aCT) measures suggestive of increased risk for instability including PW size >50%, and those with cranial exit within 5.0 mm of the acetabular dome. Results: There were 32 operative and 26 nonoperatively treated fractures. Thirty were determined to be unstable, and 28 stable following EUA. Measurements of >70° using the sCT angular measure predicted instability in 28/28 patients, and ≤70° predicted stability in 30/30 patients (sensitivity 100% specificity 100%). Prevalence of EUA confirmed instability for subgroups with PWF based on prior aCT measures were as follows: ≥50% wall involvement 11/16 (sensitivity 67% specificity 60%; 95%CI 45-89%/45-75%), fracture within 5.0 mm of dome 5/18 (sensitivity 86%, specificity 73%; 95%CI 71-100%/59-87%), fracture within 5.0 mm of dome and ≥50% involvement 1/9 (sensitivity 89%, specificity 56%, 95%CI 69-100%/24-88%). Conclusion: In a sample of 58 mostly high energy posterior wall fractures all having had an EUA, a new sagittal angular CT measurement of ≤70° predicted hip stability and >70° predicted instability with 100% sensitivity and specificity. Level of evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.