Mechanical implications of collagen fibre orientation in cortical bone of the equine radius

Abstract

Abstract. Mechanical test specimens were prepared from the cranial and caudal cortices of radii from eight horses. These were subjected to destructive tests in either tension or compression. The ultimate stress, elastic modulus and energy absorbed to failure were calculated in either mode of loading. Analysis was performed on the specimens following mechanical testing to determine their density, mineral content, mineral density distribution and histological type. A novel technique was applied to sections from each specimen to quantify the predominant collagen fibre orientation of the bone near the plane of fracture. The collagen map for each bone studied was in agreement with the previously observed pattern of longitudinal orientation in the cranial cortex and more oblique to transverse collagen in the caudal cortex. Bone from the cranial cortex had a significantly higher ultimate tensile stress (UTS) than that from the caudal cortex (160 MPa vs 104 MPa; P<0.001) though this trend was reversed in compression, the Caudal cortex becoming relatively stronger (185MPa vs 217MPa; P<0.01). Bone from the cranial cortex was significantly stiffer than that from the caudal cortex both in tension (22 GPa vs 15 GPa; P<0.001) and compression (19 GPa vs 15 GPa; P<0.01). Of all the histo-compositional variables studied, collagen fibre orientation was most closely correlated with mechanical properties, accounting for 71% of variation in ultimate tensile stress and 58% of variation in the elastic modulus. Mineral density and porosity were the only other variables to show any significant correlation with either UTS or elastic modulus. The variations in mechanical properties around the equine radius, which occur in close association with the different collagen fibre orientations, provide maximal safety factors in terms of ultimate stress, yet contribute to greater bending of the bone as it is loaded during locomotion, and thus lower safety factors through the higher strains this engenders.Correspondence to .' A. Boyde

Variability and anisotropy of mechanical behavior of cortical bone in tension and compression

“…However, it is too early to draw the conclusion from these results. Riggs et al (1993b) analyzed the collagen fiber orientation of the equine radius and revealed the contrasting relation between four quadrants. Potentially, this implies that the variation of material properties or microstructure may differ between species or bone types.…”

Circularly polarized light standards for investigations of collagen fiber orientation in bone

“…Studies of the horse radius (Boyde and Riggs, 1990;Mason et al, 1995;Riggs et al, 1993a,b); the macaque circumorbital region (Bromage, 1992); and the calcanei of horse, sheep, and elk (Skedros et al, 1997) helped to support existing hypotheses of the relationship between preferred collagen fiber orientation and mechanical loading. Martin and Ishida (1989), Martin and Boardman (1993), and Riggs et al (1993b) examined several material properties of bone and determined that collagen fiber orientation made the most significant contribution to the overall strength of bone tissue. McMahon et al (1995) presented data from the sheep calcaneus, suggesting this relationship might be more complex than previously thought: the authors propose that low levels of localized tension may have a more significant impact on collagen fiber distribution than higher levels of peak compression in regions of some bone elements.…”

Techniques to assess bone ultrastructure organization: orientation and arrangement of mineralized collagen fibrils

“…Among them, the significant contribution of the orientation and arrangement of bone's ultrastructure to its mechanical properties has long been suggested [30 -33] and experimentally investigated [34][35][36][37][38][39][40][41]. In addition, several studies conducted at different structural scales have shown that ultrastructure orientation and arrangement are among the best predictors of mechanical properties such as bone strength or elastic modulus [42][43][44][45]. Many approaches have been proposed in previous years to investigate the threedimensional (3D) orientation of at least one of the four structural elements of bone ultrastructure mentioned above, including methods based on visible light, X-rays, electrons or magnetic fields, with some of them providing very promising results.…”