The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Drifting osteons seem unlikely to be a mechanical adaptation in view of inconsistent regional distributions and overall low prevalence in adult chimpanzee and human femora


Dept. of Orthopaedics, University of Utah School of Medicine

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Drifting osteons (DOs) are common osteon variants that might have important mechanical or metabolic functions. As shown previously, DOs are more prevalent in "tension" cortices of chimpanzee proximal femora, but did not exceed 11% of the total osteon population density (OPD) (Keenan et al., 2010 AAPA). Based on this previous study, we expected an increased prevalence of DOs in "tension" cortical regions as an adaptation for the inferior mechanical properties in tension compared to compression. Twenty-four sections from 12 modern adult human proximal femoral shafts were embedded in methacrylate, ultramilled, and imaged in circularly polarized light. DOs were identified/counted using published methods, and percentages of DOs (%DOs = #DOs/OPD) were obtained for each image. Unexpectedly, more DOs were found in the medial "compression" cortex of the proximal shaft (p<0.05). The relatively small %DOs (4.04% + 4.39%; range: 0%-27.1%) is similar to our findings in chimpanzee femora. But the regional differences in human femora are opposite to our previous findings in chimpanzee femora (human femora = more DOs in medial "compression" cortex; chimpanzee femora = more DOs in lateral "tension" cortex). These results reduce the likelihood that drifting osteons are an important toughening mechanism, at least not in the context of regionally prevalent/predominant tension vs. compression. If there is a preferential mechanical or metabolic 'purpose' for DOs, then it has not been discovered. It may be that DOs are sporadic manifestations of the plasticity of a remodeling process and as such have little metabolic or mechanical functions in adult osteonal bone.

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