Rectus Femoris Indirect Head Enthesis: A Reliable Adjunct Landmark to Detect Hip Dysplasia With Ultrasound in Infants

Abstract

Objectives

To evaluate the indirect head of rectus femoris tendon (IHRFT) enthesis as a stable adjunct anatomical landmark for standardizing coronal plane acquisition in ultrasound detection of developmental dysplasia of the hip (DDH) and characterize its diagnostic utility.

Methods

In this prospective study, 200 infants (400 hips, median age 89 days) underwent hip ultrasound. Spatial relationships between the Graf standard coronal plane and IHRFT enthesis were quantified. A modified coronal plane was acquired using the posterior IHRFT margin as an adjunct landmark with flexed hips. Alpha angles from both planes were compared. IHRFT enthesis thickness was measured in an axial oblique plane; its diagnostic accuracy for DDH (Graf type ≥IIa) was assessed.

Results

The posterior IHRFT margin consistently aligned with the Graf standard plane (mean offset: −1.0 ± 1.1 mm). Alpha angles derived using the IHRFT as an adjunct landmark (62.2° ± 3.9°) showed no significant difference from Graf plane angles (62.9° ± 4.4°, p = 0.08), with good inter-operator reliability (ICC = 0.77). IHRFT enthesis thickness was significantly greater in dysplastic hips (n = 36) vs. normal hips (n = 364) (4.1 ± 1.1 mm vs. 2.4 ± 0.3 mm, p < 0.0001). Enthesis thickness ≥3.2 mm predicted DDH with high accuracy (AUC = 0.91, sensitivity 91%, specificity 88%).

Conclusions

Incorporating the posterior margin of the IHRFT enthesis as an adjunct landmark for defining the standard coronal plane enhances inter-operator consistency in alpha angle measurements, reducing dependence on strict neutral positioning. IHRFT enthesis thickening (≥3.2 mm) on axial oblique view is a novel sonographic index for diagnosis of DDH.