Reference values for proximal femoral anatomy in adolescents based on sex, physis, and imaging plane

Background: Morphological alterations of the hip joint are important contributors to the development of osteoarthritis. While plane-specific variations in the shape of the proximal femur have been described, there are no defined reference standards for measurements in adolescent patients. Purpose: To evaluate hips in asymptomatic adolescent patients using radially reformatted multidetector computed tomography (MDCT) to define the morphological characteristics of the femoral head-neck (FHN) junction with respect to patient sex and physeal status, and to establish reference values for α angle, FHN offset, and epiphyseal extension (EE). Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 132 pelvic MDCT scans in adolescent patients with abdominal pain were reformatted into radially oriented planes along the femoral necks. The diameter of the femoral head, α angle, EE, and FHN offset were measured. Reference values for a angle, EE, and FHN offset were determined, and the α angle, EE, and FHN offset were compared between open versus closed physeal status for each sex. Results: The α angle measurements in male patients were higher than in female patients in anterior (A), anterosuperior (AS), and superior (S) planes (P < .001). The median α angle was highest for all patients in the AS plane (male, 52°; female, 49°). Open physes correlated with higher α angles compared with closed physes in all imaging planes except the AS plane. The FHN offset was lowest in the AS plane for all patients and was increased in female patients with closed physes compared with open physes in the A plane and in the A, S, posterosuperior, and posterior planes in male patients (P<.05). There were subtle plane-specific variations in EE. Conclusion: The a angle is higher and FHN offset is lower in the AS plane in patients with closed versus open physes, whereas the opposite is true in all other planes. Theα angles in male patients were higher than in female patients, although there were no significant sex-based differences in the FHN offset. The α angles were highest and FHN offset was lowest in the AS plane. There were subtle variations in EE across all planes, and the EE was higher in patients with closed versus open physes. Clinical Relevance: Plane- and sex-specific reference values for α angle, FHN offset, and EE in asymptomatic adolescent patients will assist orthopaedic surgeons and radiologists in appropriately suggesting femoroacetabular impingement based on the imaging evaluation of patients with hip pain, whose measurements lie at the periphery of or outside the reference intervals in the appropriate clinical context.