Functional and anatomic orientation of the femoral head

Background Femoral neck geometry directly affects load transmission through the hip. Orientations may be described anatomically or using functional definitions that consider load transmission. Questions/purposes This study introduces and applies a new method for characterizing functional femoral orientation based on the distribution of subchondral bone density in the femoral head and compares it with orientation measures generated via established anatomic landmark-based methods. Both orientation methods then are used to characterize side-to-side symmetry of orientation and differences between men and women within the population. Patients and Methods A retrospective review of CT imaging data from 28 patients was performed. Anatomic orientation was determined using established two-dimensional and three-dimensional landmarking methods. Subchondral bone density maps were generated and used to define a density-weighted surface normal vector. Orientation angles generated by the three methods were compared, with side-to-side symmetry and differences between genders also investigated. Results The three methods measured substantially different angles for anteversion and neck-shaft angle. Weak correlations were found between anatomic and functional orientation measures for neck-shaft angle only. Conclusions Neck-shaft angles calculated using the functional orientation method corresponded well with previous in vivo loading data. An absence of strong correlation between functional and anatomic measures reinforces the concept that bone geometry is not solely responsible for determining loading of the femoral head. Level of Evidence Level II, Diagnostic Studies-Investigating a Diagnostic Test. See the Guidelines for Authors for a complete description of levels of evidence.