Femoro-Epiphyseal Acetabular Roof Index Values on Anteroposterior Pelvic Radiographs Significantly Increase With Hip Adduction and Decrease With Abduction

Purpose

To investigate the impact of hip adduction and abduction on the femoro-epiphyseal acetabular roof (FEAR) index using conventional anteroposterior pelvic radiographs of patients with borderline hip dysplasia (BHD).

Methods

Patients with BHD (lateral center edge angle [LCEA] with values of 18° ≤ LCEA <25°) and who were subject to periacetabular osteotoym were selected from a consecutive retrospective cohort from January 2009 to January 2016. The hip ad-/abduction (adduction handled as negative values) and the FEAR index (lateral opening angles handled as positive values) were measured in the initially standardized acquired pelvic radiographs. Thereafter, using surgical planning software, different degrees of hip adduction and abduction were simulated from 20° of adduction to 30° of abduction, and the FEAR index was measured at each position. Pearson correlation was used to identify a potential correlation between the hip ad-/abduction and the FEAR index.

Results

Eighty-nine hips were included. Initial radiographs showed unintentional mean ad-/abduction of –5.3° ± 3.1° (range, –14° to 2°). The mean FEAR index at the initial ad-/abduction angle was –3.6° ± 7.2° (range, –18.0° to 16.0°) and was significantly different from neutral position (0° of ad-/abduction) –9.2° ± 7.0° (range, –27.0° to 10.0°) (P < .001). The FEAR index was negatively correlated to the hip ad-/abduction angle (r = 1, P < .001), with lower values for the FEAR index with the hip in abduction.

Conclusions

Significant differences in FEAR index measurements were observed depending on hip ad-/abduction on pelvic radiographs. A linear change of the FEAR index of 5° per 5° of hip adduction or abduction could be demonstrated, with adduction resulting in increasing and abduction in decreasing values for the FEAR index.

Level of Evidence

Level III, diagnostic study.