Discussion

       The gross shape and heterogeneous mechanical properties of the MC3 reflect its function in response to evolutionary adaptation for efficient and safe high speed ambulation [3 1]. Its length and large cortical cross sectional area give it an inherent resistance to both buckling and static compression failure: safety factors (SFs) exceed 5 on buckling and approach 2 on compressive strength for horses in a trot [5,16], but only slightly exceed 1 on fatigue strength [16]. The low fatigue SF reflects the high incidence of stress fractures in racing horses, up to 7000 in young thoroughbreds [37]. These fractures occur on the dorsal aspect of the MC3, an area of predominant tension [37], diametrically opposite the location of the foramen. Thus, the foramen exists in a region of predominate longitudinal compression.

       It is well known that a transverse tension stress field exists in the "up-" and

"downstream" regions near an elliptical hole in a plate subjected to far field longitudinal compression. Such regions exist near the proximodistal apexes of the MC3 foramen. It is in these regions that we observed endoperiosteal osteon trajectories, especially evident nearest the periosteal surface of the MC3. The periosteal surface represents the surface of greatest compressive bending normal stress, and, thus, the greatest transverse tension in the foramen apexes. Osteons perpendicular to this tension present their cement lines as possible crack arrestors and, along with diverging osteons which may deflect cracks, may increase toughness in these regions.