Evidence of strain-mode-related cortical adaptation in the diaphysis of the horse radius

The relative importance that certain strain features, including mode (e.g., tension vs. compression) and magnitude, have in affecting adaptive bone remodeling seen in normal skeletally mature bones remains controversial. The equine radius is used as a model because in vivo strain data show that the mid-to-proximal diaphysis receives a consistent history of predominantly cranial-caudal bending loads, in contrast to the distal diaphysis which receives relatively more torsional loading superimposed on cranial-caudal bending. Medial and lateral cortices serve as control regions because they correspond to a neutral axis of bending. Equine radii were sectioned transversely at 65% (proximal), 50%, and 35% (distal) of length and cortical bone from the cranial (“tension”), caudal (“compression”), medial, and lateral regions was examined to determine if one, of many, structural and material features could be distinguished as being consistently related to the distribution of the prevailing strain modes. Mineral content (percent ash) differences, though statistically significant (p< 0.01), vary less than 1% between regions of the cortex at all sections. Porosity is not significantly different between any of the regions (p = 0.13). In the 65% and 50% sections, secondary osteon population density (OPD, osteons per square millimeter) and fractional area of secondary bone (FASB) are each nearly two times as great in the caudal regions than in the other three regions (p< 0.01). The 35% section shows a pattern opposite of that in the other sections—there are more than two times as many osteons in the cranial cortex than in the caudal cortex. However, at all section locations, collagen fibers have more oblique-to-transverse orientation in the caudal cortex and have relatively more longitudinal orientation in the cranial cortex (p< 0.01). Collagen fiber orientation is not significantly different between medial and lateral cortices at any section location (p> 0.3). Because the caudal cortex of the 35% section is significantly less remodeled (lower OPD and FASB) than the opposite cranial cortex, the oblique-to-transverse collagen seen in the caudal cortex of this section cannot be attributed to prior remodeling activity. These data show that the greatest amount of remodeled bone does not consistently occur in the caudal cortex, where strain magnitudes are invariably greatest. However, collagen fiber orientation is markedly different between the opposite cortices in a pattern that consistently coincides with the cranial-caudal distribution of bending strains. These data support the hypothesis that the primary bone and/or bone within secondary osteons of the cranial and caudal cortices is organized with some recognition of prevailing strain mode.