The hip joint as an egg shape: a comprehensive study of femoral and acetabular morphologies

Abstract

ABSTRACT Understanding the morphological features characterising a normal femoral head and acetabular cavity is critical for a more comprehensive and updated definition of hip anatomy. Based on anatomical observations, MacConaill introduced the notion that spheroidal articular surfaces are better represented by ovoidal shapes, in comparison with the still well-established spherical shape. This work tests MacConaill’s classification by using a surface-fitting framework to assess the goodness-of-fit regarding the largest assortment of sphere-like shapes presented in a single study (i.e. a total of 10 different shapes: sphere, rotational conchoids, rotational ellipsoid, ellipsoid, superellipsoid, Barr’s superellipsoid, tapered ellipsoid, Barr’s tapered superellipsoid, ovoid, superovoid). Anatomical data of the femoral head and acetabular cavity were obtained from computed tomography scans of a gender-balanced, asymptomatic 15 population of 30 adult subjects. The framework involved image segmentation with active contour methods, mesh smoothing and decimation, and surface fitting to point clouds was performed with genetic algorithms. The statistical analysis of the surface-fitting errors revealed the superior approximation of non-spherical shapes: superovoids provided the best fit for each femoral head and acetabular cavity, whereas spheres presented the worst fitting values. We also addressed gender variability in bony 20 hip geometry as sphericity, ellipticity, conicity and squareness were measured.