Are distributions of secondary osteon variants useful for interpreting load history in mammalian bones?

Background/Aims: In cortical bone, basic multicellular units (BMUs) produce secondary osteons that mediate adaptations, including variations in their population densities and cross-sectional areas. Additional important BMU-related adaptations might include atypical secondary osteon morphologies (zoned, connected, drifting, elongated, multiple canal). These variants often reflect osteonal branching that enhances toughness by increasing interfacial (cement line) complexity. If these characteristics correlate with strain mode/magnitude-related parameters of habitual loading, then BMUs might produce adaptive differences in unexpected ways. Methods: We carried out examinations in bones loaded in habitual torsion (horse metacarpals) or bending: sheep, deer, elk, and horse calcanei, and horse radii. Atypical osteons were quantified in backscattered images from anterior, posterior, medial, and lateral cortices. Correlations were determined between atypical osteon densities, densities of all secondary osteons, and associations with habitual strain mode/magnitude or transcortical location. Results: Osteon variants were not consistently associated with 'tension', 'compression', or neutral axis ('shear') regions, even when considering densities or all secondary osteons, or only osteon variants associated with relatively increased interfacial complexity. Similarly, marrow- and strain-magnitude-related associations were not consistent. Conclusion: These data do not support the hypothesis that spatial variations in these osteon variants are useful for inferring a habitual bending or torsional load strain history.