Acetabulotrochanteric Distance in Slipped Capital Femoral Epiphysis

Abstract

A pathognomonic finding of slipped capital femoral epiphysis (SCFE) is a combination of the decrease in the height of the slipped epiphysis and the position of external rotation and flexion of hip. We believe that decrease in the acetabulotrochanteric distance (ATD) on an anteroposterior (AP) radiograph represents this finding. In a retrospective study of 25 consecutive cases of unilateral SCFE, we defined ATD as the distance between a line connecting the superolateral margins of the acetabulae with a second line, parallel to the first line, which goes through the tip of the greater trochanter on each hip and acetabulotrochanteric angle (ATA) as the angle between a line connecting the superolateral margins of the acetabulae with a second line connecting the tip of greater trochanter on each side. The difference in ATD and ATA between both hips in each case was evaluated and their correlation was determined using the Pearson correlation coefficient. Nineteen cases (76%) showed difference in ATD of > 2 mm and positive ATA divergence of >1 degree. The average difference in ATD was 6.6 mm (range, 0 to 25 mm); the average ATA divergence was 2.4 degree (range, 0 to 5.3 degree). Our findings show that the difference in acetabulotrochanteric distance (ATD) between hips is an easy, reliable, and sensitive finding present on an AP radiograph of patients with unilateral SCFE. ATD should be taken into consideration while evaluating AP radiographs of a patient suspected to have SCFE, and further evaluation with lateral hip radiographs should be carried out to confirm the diagnosis. Level IV.