Relative Effects of Collagen Fiber Orientation, Mineralization, Porosity, and Percent and Population Density of Osteonal Bone on Equine Cortical Bone Mechanical Properties in Mode-specific Loading

INTRODUCTION Predominant collagen fiber orientation (CFO) can significantly influence the mechanical properties of cortical bone (e.g., Martin and Ishida, 1989; Martin and Broadman, 1993; Riggs et al., 1993). Previous studies have demonstrated that CFO is a strong predictor of tensile, compressive, or bending strength. Few of these studies, however, have examined the relative effects of CFO, mineralization, and other microstructural variables in the context of the in vivo habitual loading mode (“mode-specific loading” e.g., compression testing specimens from a habitually compressed cortical region, or tension testing specimens from a habitually tensed cortical region) (Riggs et al., 1993). Evaluating mechanical influences of such histologic variables in the context of a habitual loading mode is important since: 1) in vivo studies have shown long bones of all animals studied receive directionally consistent bending during typical daily gait-related activities (Biewener, 1993), and 2) cortical bone is substantially stronger and stiffer, has different fatigue behavior, and likely has greater toughness in compression than in tension. Consequently, a long limb bone must accommodate regional strain-mode-related disparities in habitual loading. Analyses of bone in specific loading modes may demonstrate a rather different role for CFO than suggested by studies that have not evaluated mechanical properties in modespecific loading. This study tested the hypothesis that CFO will more strongly correlate with energy absorption, a measure of toughness, than with elastic modulus, yield stress, or ultimate stress in compression-tested specimens from “compression” regions.