Evolution of head-shaft angle and neck-shaft angle in childhood

The current retrospective study investigates the natural evolution of head-shaft angle (HSA) and neck-shaft angle (NSA) in childhood. It is not known if a high HSA in early childhood leads to a high HSA in adulthood. This study aims to characterize the evolution of HSA and compares it with the more commonly known NSA in healthy children. We measured radiographs of 84 different healthy hips of children between 3 and 14.5 years old who underwent at least 2 radiographs of the pelvis, corresponding to 286 measurements. We used a linear mixed model to determine the covariance between random intercept and slope while allowing each individual hip to change over time. The covariance for HSA between random intercept and random slope was -4.262 (p < 0.001), corresponding to a high negative correlation of -0.717, for NSA -2.754 (p = 0.031) or a high negative correlation of -0.779. HSA and NSA were strongly correlated, a value of 0.736 (p < 0.001) was measured. The high negative correlation for random intercept and random slope means that the higher the initial value (intercept), the steeper the decline (slope). Therefore HSA decreases faster in hips with high HSA at an early age. Hips with high HSA in early childhood do not necessarily lead to hips with high HSA in adulthood. Our results may aid in future clinical decision making in patients with developmental dysplasia of the hip (DDH) with high HSA in particular.