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Modified CT Sagittal Angle Measure: Accuracy in Determining Hip Stability for Posterior Wall Acetabulum Fractures and
Todd Jaeblon1,2, Colin Harrington2,3, Umar Khan2,3
1Department of Orthopaedic Traumatology, University of Maryland Capital Region Medical Center, Largo, MD.
Objectives:
The aim of this study was to describe modifications to the original CT sagittal angle measure (SAM) for predicting hip stability after posterior wall acetabular fractures (PWF), and to assess modified SAM (mSAM) accuracy and resident agreement and accuracy with the mSAM relative to examination under anesthesia (EUA).
Design:
Retrospective review of prospectively collected data.
Setting:
Academic Level 2 Trauma Center.
Intervention:
Results of a modification of the sagittal CT based PWF measurement (mSAM) and gold standard EUA were collected and evaluated.
Patient Selection Criteria:
Patients with PWFs (AO/OTA 62A.1) requiring an EUA from June 2017 through July 2025 were included.
Outcome Measures And Comparisons:
The accuracy of mSAMs by the lead author and residents in predicting hip stability was quantified by sensitivity, specificity, positive predictive value, and negative predictive value using EUA, performed by fellowship trained orthopaedic surgeons, as the reference standard. A receiver operating characteristic curve analysis was used to determine whether mSAM reproduced the published SAM threshold angles most closely associated with hip stability, using EUA as the reference standard. Modified SAM reliability and agreement was assessed with intraclass correlation coefficients (ICC) after measurements in 2 rounds, separated by 1 month. Resident agreement for mSAM in predicting hip stability after PWF was compared with their agreement for the axial CT-based percentage of posterior wall fracture (PPWF) measurement.
Results:
The mSAM sensitivity and specificity for predicting hip stability from 84 patients [male: 65 (77%); median age: 35.0 years (range: 17-75) years] was 97.7% and 100.0%, respectively [95% confidence interval (CI) 87.7%-99.9%, 91.4%-100%]. Receiver operating characteristic analysis identified the optimal mSAM thresholds of ≥71 degrees for instability and ≤70 degrees for stability. Resident measurements of mSAM resulted in 100.0% sensitivity and 94.4% specificity for predicting stability in round 1 (95% CI 91.6%-100.0%, 72.7%-99.9%) and 100.0% sensitivity and specificity in round 2 (95% CI 91.6%-100%, 81.5%-100.0%). Interrater reliability for mSAM was good in round 1 (ICC = 0.83, 95% CI 0.70-0.93) and round 2 (ICC = 0.86, 95% CI 0.74-0.95). Intrarater reliability was good for two residents and excellent for 4 (range 0.79-0.99). Interrater reliability for PPWF was good (ICC = 0.78, 95% CI 0.62-0.91).
Conclusions:
Optimal mSAM measures predictive of hip instability were ≥71 and ≤70 degrees for stability after PWF. The mSAM demonstrated good-to-excellent intrarater reliability among residents, and the intraclass correlation coefficient was higher than the PPWF measure.
Level Of Evidence:
Therapeutic Level III, Diagnostic. See Instructions for Authors for a complete description of levels of evidence.
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