Introduction: The Femoro-Epiphyseal Acetabular Roof (FEAR) index has recently been proposed as a useful tool in identifying hips with instability in the setting of borderline acetabular dysplasia. Beck et al. were the first to describe this parameter and demonstrated a FEAR index as the angle between a line connecting the most medial and lateral part of the sourcil and a line connecting the most medial and lateral part of the straight central third of the physeal scar. Purpose: The purposes of the current study were 1) external validation of intra-observer and inter-observer reliability and 2) to determine the correlation/association of FEAR with a clinical diagnosis of instability. Methods: The current study was a retrospective review of patients diagnosed with borderline acetabular dysplasia by a single surgeon. The study period included January 2008-April 2017 and identified patients with LCEA 20°-25°, via prospectively collected radiographic measurements in a hip preservation database. Inclusion criteria were treatment with either hip arthroscopy or PAO, LCEA of 20°-25°, and 14-40 years of age. Patients were excluded if they had a Tӧnnis grade ≥2, prior ipsilateral hip surgery, residual deformities from SCFE or Perthes. Demographics and radiographic measurements were recorded. Two individuals read all radiographs after obtaining an intra observer reliability of 97% and inter observer reliability of 88%. Sensitivity and specificity were calculated for FEAR predicting instability. A t-test was used to assess correlation of LCEA, acetabular inclination (AI), and ACEA with FEAR. Results: A total of 186 patients were included, FEAR was unable to be assessed in 5% of hips because of inability to visualize the proximal femoral physeal scar. Of the remaining 176 hips, 18% of hips had a FEAR index >5°. FEAR positive mean was 7.6°±2.8° (range=5.1°-17.5°) and FEAR negative mean was –6.9°±6.4° (range=-29.4°-3.7°). Results of FEAR predicting instability was sensitivity=33% (23/70), specificity=92% (98/106), PPV=74% (23/31), and NPV=67% (98/145). The mean AI was significantly high in FEAR(+) hips (AI mean=11.2° and FEAR(-) AI mean=9.5°; p=0.005). No difference in LCEA or ACEA was seen relative to the presence of FEAR. Discussion: In the current study, a positive FEAR index was generally indicative of the presence of clinical instability. However, the FEAR index was poorly sensitive to hip instability with 67% hips diagnosed with instability having a negative FEAR index.
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