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).

References

Bertram JEA, Biewener AA. 1988. Bone curvature: Sacrificing strength for load predictability? J Theor Biol 131:75-92.

Burger E, Klein-Nulend J, Smit T. 2003. Strain-derived canalicular fluid flow regulates osteoclast activity in a remodelling osteon–a proposal. J Biomech 36:1453-1459.

Carter DR, Caler WE, Spengler DM, Frankel VH. 1981. Fatigue behavior of adult cortical bone: The influence of mean strain and strain range. Acta Orthop Scand 52:481-490.

Ehrlich PJ, Lanyon LE. 2002. Mechanical strain and bone cell function: a review. Osteoporos Int 13:688-700.

Francillon-Vieillot H, de Buffrénil V, Castanet J, Géraudie J, Meunier F, Sire J, Zylberberg L, de Ricqlès A. 1990. Microstructure and mineralization of vertebrate skeletal tissues. In: Carter J, editor. Skeletal Biomineralization: Patterns, Processes and Evolutionary Trends. New York: Van Nostrand Reinhold. p 471-530.

Ganey TM, Odgen JA. 1998. Pre- and post-natal development of the hip. In: Callaghan JJ, Rosenbery AG, Rubash HE, editors. The adult hip. Philadelphia: Lippincott-Raven Publishers. p 39-55.

Judex S, Gross TS, Bray RC, Zernicke RF. 1997a. Adaptation of bone to physiological stimuli. J Biomech 30:421-429.

Judex S, Gross TS, Zernicke RF. 1997b. Strain gradients correlate with sites of exercise-induced bone-forming surfaces in adult skeleton. J Bone Miner Res 12:1737-1745.

Lee TC, Staines A, Taylor D. 2002. Bone adaptation to load: microdamage as a stimulus for bone remodelling. J Anat 201:437-446.

Reilly GC, Currey JD. 1999. The development of microcracking and failure in bone depends on the loading mode to which it is adapted. J Exp Biol 202:543-552.

Rubin CT, Fritton S, Sun YQ, McLeod KJ. 1996. Biomechanical parameters which stimulate bone formation: The ugly duckling of the skeletal growth factors. In: Davidovitch Z, Norton LA, editors. Biological mechanisms of tooth movement and craniofacial adaptation. Boston: Harvard Society for the Advancement of Orthodontics. p 51-59.

Skedros JG, Mason MW, Nelson MC, Bloebaum RD. 1996. Evidence of structural and material adaptation to specific strain features in cortical bone. Anat Rec 246:47-63.

Skedros JG, Sorenson SM, Hunt KJ, Holyoak JD. 2007. Ontogenetic structural and material variations in ovine calcanei: a model for interpreting bone adaptation. Anat Rec 290:284-300.