Acetabular sector angles in asymptomatic and dysplastic hips

Abstract: Background: Radiographic evaluation plays an important role in detecting and grading hip dysplasia. Acetabular sector angles (ASAs) measure the degree of femoral head coverage provided by the acetabulum on computed tomographic (CT) scans. In this study, we aimed to determine ASA values at different axial levels in a control cohort with asymptomatic, high functioning hips without underlying hip pathology and a study group with symptomatic, dysplastic hips that underwent periacetabular osteotomy (PAO), thereby defining the ASA thresholds for hip dysplasia.Methods: This was a cross-sectional study evaluating a control group of 51 patients (102 hips) and a study group of 66 patients (72 hips). The control group was high-functioning and asymptomatic, with an Oxford Hip Score of >43, did not have osteoarthritis (Tonnis grade <= 1), underwent a pelvic CT scan, had a mean age (and standard deviation) of 52.1 +/- 5.5 years, and was 52.9% female. The study group had symptomatic hip dysplasia treated with PAO, had a mean age of 29.5 +/- 7.3 years, and 83.3% was female. Anterior ASA (AASA) and posterior ASA (PASA) were measured at 3 axial CT levels to determine equatorial, intermediate, and proximal ASA. The thresholds for dysplasia were determined using receiver operating characteristic (ROC) curve analysis, including the area under the curve (AUC).Results: Patients with dysplasia had significantly smaller ASAs compared with the control group; the differences were most pronounced for proximal AASAs and proximal and intermediate PASAs. The control group had a mean proximal PASA of 162 degrees +/- 17 degrees, yielding a threshold for dysplasia of 137 degrees (AUC, 0.908). The mean intermediate PASA for the control group was 117 degrees +/- 11 degrees, yielding a threshold of 107 degrees (AUC, 0.904). The threshold for anterior dysplasia was 133 degrees for proximal AASA (AUC, 0.859) and 57 degrees for equatorial AASA (AUC, 0.868). The threshold for posterior dysplasia was 102 degrees for intermediate PASA (AUC, 0.933). Conclusions: Measurement of ASA is a reliable tool to identify focal acetabular deficiency with high accuracy, aiding diagnosis and management. A proximal PASA of <137 degrees or an intermediate PASA of