Body Mass Index Affects Proximal Femoral but Not Acetabular Morphology in Adolescents Without Hip Pathology

Increased mechanical load secondary to a large body mass index (BMI) may influence bone remodeling. The purpose of this study was to investigate whether BMI is associated with the morphology of the proximal part of the femur and the acetabulum in a cohort of adolescents without a history of hip disorders.

We evaluated pelvic computed tomographic (CT) images in 128 adolescents with abdominal pain without a history of hip pathology. There were 44 male patients (34%) and the mean patient age (and standard deviation) was 15 ± 1.95 years. The alpha angle, head-neck offset, epiphysis tilt, epiphyseal angle, and epiphyseal extension were measured to assess femoral morphology. Measurements of acetabular morphology included lateral center-edge angle, acetabular Tönnis angle, and acetabular depth. BMI percentile, specific to age and sex according to Centers for Disease Control and Prevention growth charts, was recorded.

BMI percentile was associated with all measurements of femoral morphology. Each 1-unit increase in BMI percentile was associated with a mean 0.15° increase in alpha angle (p < 0.001) and with a mean 0.03-mm decrease in femoral head-neck offset (p < 0.001). On average, a 1-unit increase in BMI percentile was associated with a 0.0006-unit decrease in epiphyseal extension (p = 0.03), a 0.10° increase in epiphyseal angle (p < 0.001), and a 0.06° decrease in tilt angle (p = 0.02; more posteriorly tilted epiphysis). There was no detected effect of BMI percentile on acetabular morphology including lateral center-edge angle (p = 0.33), Tönnis angle (p = 0.35), and acetabular depth (p = 0.88).

Higher BMI percentile was associated with increased alpha angle, reduced head-neck offset and epiphyseal extension, and a more posteriorly tilted epiphysis with decreased tilt angle and increased epiphyseal angle. This morphology resembles a mild slipped capital femoral epiphysis deformity and may increase the shear stress across the growth plate, increasing the risk of slipped capital femoral epiphysis development in obese adolescents. BMI percentiles had no association with measurements of acetabular morphology. Further studies will help to clarify whether obese asymptomatic adolescents have higher prevalence of a subclinical slip deformity and whether this morphology increases the risk of slipped capital femoral epiphysis and femoroacetabular impingement development.