The value of bending: Predictability, fluid flow, nutrient delivery, and beneficial signals.
If bending creates the dilemma of regionally prevalent, but potentially deleterious, shear and/or tension, then why is it such a common load history? In turn, does this load regime have some less obvious or fundamental biological value even though strain-mode-specific nanostructural/microstructural toughening accommodates the tension/shear/compression strain distribution of bending? There appears to be a link between “load predictability” and the answers to these questions as discussed below.
Investigators who have been written about the “value” of bending point out examples of how a bone’s asymmetric cross-sectional shape and/or longitudinal curvature might be designed to allow bending to occur while controlling its magnitude (Bertram and Biewener, 1988). This “load predictability” is structurally beneficial because the bone can differentially adapt its histology for spatially and temporally predictable non-uniform strain mode/magnitude distributions (Reilly and Currey, 1999). Load predictability is also beneficial because it is linked to a predictably non-uniform strain distribution that in turn ensures predictability in the fluid-flow dynamics that are essential for nutrient delivery to bone cells (Skedros et al., 1996; Judex et al., 1997b; Ehrlich and Lanyon, 2002). Predictable bending might also produce spatial-temporal epigenetic and “extra-genetic” strain-related signals (e.g., electrical potentials) that are vital in the development and maintenance of some bones (Carter et al., 1981; Bertram and Biewener, 1988; Francillon-Vieillot et al., 1990; Rubin et al., 1996; Skedros et al., 1996; Judex et al., 1997a; Ganey and Odgen, 1998, pg. 46-47; Lee et al., 2002; Burger et al., 2003; Skedros et al., 2007).
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