Femoral head decentration on hip MRI: comparison between imaging planes, methods of contrast administration, and hip deformities.

OBJECTIVES To compare the prevalence of femoral head decentration (FHD) on different MR imaging planes in patients undergoing direct/indirect hip MR arthrography (MRA) with asymptomatic controls and to evaluate its association with osseous deformities. METHODS IRB-approved retrospective single-center study of symptomatic hips undergoing direct or indirect hip MRA at 3 T. Asymptomatic participants underwent non-contrast hip MRI at 3 T. FHD was defined as a continuous fluid layer between the acetabulum and femoral head and assessed on axial, sagittal and radial images. The association of intra-articular/intra-venous contrast agents and the prevalence of FHD was evaluated. The association of FHD with osseous deformities and joint damage was assessed using multiple logistic regression analysis. RESULTS Three-hundred ninety-four patients (447 hips, mean age 31 ± 9 years, 247 females) were included and compared to 43 asymptomatic controls (43 hips, mean age 31 ± 6 years, 26 females). FHD was most prevalent on radial images and more frequent in symptomatic hips (30% versus 2%, p < 0.001). FHD prevalence was not associated with the presence/absence of intra-articular contrast agents (30% versus 22%, OR = 1.5 (95% CI 0.9-2.5), p = 0.125). FHD was associated with hip dysplasia (OR = 6.1 (3.3-11.1), p < 0.001), excessive femoral torsion (OR = 3.0 (1.3-6.8), p = 0.010), and severe cartilage damage (OR = 3.6 (2.0-6.7), p < 0.001). CONCLUSION While rare in asymptomatic patients, femoral head decentration in symptomatic patients is associated with osseous deformities predisposing to hip instability, as well as with extensive cartilage damage. CRITICAL RELEVANCE STATEMENT Decentration of the femoral head on radial MRA may be interpreted as a sign of hip instability in symptomatic hips without extensive cartilage defects. Its presence could unmask hip instability and yield promise in surgical decision-making. KEY POINTS The best method of identifying femoral head decentration is radial MRI. The presence/absence of intra-articular contrast is not associated with femoral head decentration. Femoral head decentration is associated with hip deformities predisposing to hip instability