A virtual geometric morphometric approach to the quantification of long bone bilateral asymmetry and cross-sectional shape.

Abstract

The widespread use of three-dimensional digitization means that models of whole bone geometry are routinely captured for many applications in virtual anthropology. In this article, we test whether a geometric morphometric (GMM) approach can be used to accurately quantify directional bilateral asymmetry in cross-sectional shape using virtual bone models. We introduce a method to (1) orient virtual long bone models and extract cross sections and (2) collect GMM landmarks to analyze cross-sectional shape. Landmark data, captured using polar radii, were analyzed for an example study set of paired humeri from Andaman Islanders sampled at 35%, 50%, and 65% locations. The GMM method can (1) detect significant differences in left and right cross-sectional shape and (2) allow the directionality of shape change (distribution of bone from centroid) to be evaluated in the context of whole outline shape and in relation to the axis of maximum bending rigidity. The protocol may be used with models created from laser (surface) scan or computed tomography (CT) scan data, and applied to cross-sectional images that were collected using periosteal molding techniques. The results demonstrate directional bilateral asymmetry in shape, but do not recover the same signal for size measurements. Our method offers a pathway to quantify both the pattern of variation in shape and the relationship between size and shape variation, opening new questions about how those patterns manifest over ontogeny, change temporally or differ in relation to the nature and intensity of the activity, and bone loading conditions.