Geometric properties and the predicted mechanical behavior of adult human clavicles.

An image processing system was used to examine histomorphometric properties of 15 adult male and female human clavicles. Variations in porosity, cross-sectional area, anatomic and principal moments of inertia were assessed at 2.5-5.0% increments along the length of the clavicles. The clavicle's biomechanical behavior (axial, flexural, and torsional rigidities and the critical force for buckling) was modeled from these data using beam theory. Over threefold variations in porosity and moments of inertia were found along the length of the s-shaped clavicle--the greatest porosity and moments of inertia were located in the variably shaped sternal and acromial thirds of the bone in contrast to the denser and smaller, more circulatory shaped central third of the bone. Clavicle orientation, as indicated by the direction of greatest resistance to bending (maximum principal moment of inertia), was found to rotate from a primarily cranio-caudal orientation at the sternum to a primarily anterior-posterior orientation at the acromion. Based on cross-sectional geometry, section moduli, and estimates of flexural and torsional rigidity, the clavicle was found to be weakest in the central third of its length. These data concur with the fracture location most commonly reported clinically. Analysis of Euler buckling predicted a minimum critical force for buckling during axial loading of approximately two to three body weights for an average adult. Thus, buckling, or a combination of axial loading and bending or torsional loading, must be considered as possible failure mechanisms for this commonly injured bone.