Quantifying the Shenton line sign in adult hip dysplasia: A reference interval-based radiographic study

Purpose The Shenton line is a qualitative radiographic sign of superior femoral head subluxation in adult hip dysplasia, but binary assessment may miss subtle subluxation. This study aimed to quantify the Shenton line sign using Shenton line distance (SLD) and determine a reference interval-based threshold for SLD positivity. Methods We retrospectively reviewed 211 Crowe type I dysplastic hips and 143 control hips. SLD was defined as the vertical distance between the inferior femoral head-neck junction and the interteardrop line on supine anteroposterior pelvic radiographs. The upper limit of the nonparametric 95% reference interval in controls defined the threshold. Positive classification rates and observer agreement were compared with those of classic Shenton line breakage. Results Median SLD was higher in dysplastic hips than in controls (0.3 mm vs. −6.1 mm, p < 0.001) and increased stepwise from controls to borderline and frank dysplasia. The upper reference limit was 0.0 mm, defining SLD positivity as ≥0 mm. SLD ≥0 mm was present in 61% of dysplastic hips (39% borderline; 73% frank), compared with 25% for classic Shenton line breakage (11% borderline; 33% frank). SLD ≥0 mm had a higher positive classification rate (61% vs. 25%) and greater intra- and interobserver agreement than classic Shenton line breakage (κ = 0.93 and 0.89 vs. 0.71 and 0.66, respectively). Conclusions SLD provides a simple quantitative radiographic measure of superior femoral head subluxation in adult hip dysplasia. The reference interval-based threshold of SLD ≥0 mm may complement acetabular coverage measurements for assessing subtle superior subluxation.