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The Journal of Experimental Biology 206, 2431-2439 (2003)
doi: 10.1242/jeb.00466

Are uniform regional safety factors an objective of adaptive modeling/remodeling in cortical bone?

John G. Skedros^1,2,*, Michael R. Dayton², Christian L. Sybrowsky^1,2, Roy D. Bloebaum² and Kent N. Bachus³

¹ Utah Bone and Joint Center, Salt Lake City, UT 84115, USA
² Bone and Joint Research Laboratories, Department of Veterans Affairs Medical Center, Salt Lake City, UT 84148, USA
³ Orthopedic Bioengineering Research Laboratory, University of Utah, Salt Lake City, UT 84112, USA

^* Author for correspondence (e-mail: jskedros{at}utahboneandjoint.com)

Accepted 22 February 2003

It has been hypothesized that a major objective of morphological adaptation in limb-bone diaphyses is the achievement of uniform regional safety factors between discrete cortical locations (e.g. between cranial and caudal cortices at mid-diaphysis). This hypothesis has been tested, and appears to be supported in the diaphyses of ovine and equine radii. The present study more rigorously examined this question using the equine third metacarpal (MC3), which has had functionally generated intracortical strains estimated by a sophisticated finite element model. Mechanical properties of multiple mid-diaphyseal specimens were evaluated in both tension and compression, allowing for testing of habitually tensed or compressed regions in their respective habitual loading mode (`strain-mode-specific' loading). Elastic modulus, and yield and ultimate strength and strain, were correlated with in vivo strain data from a previously published finite element model. Mechanical tests revealed minor variations in elastic modulus, and yield and ultimate strength in both tension and compression loading, while physiological strains varied significantly between the cortices. Contrary to the hypothesis of uniform safety factors, the MC3 has a broad range of tension (caudo-medial, 4.0; cranio-lateral, 37.7) and compression (caudo-medial, 5.7; cranio-lateral, 68.9) safety factors.

Key words: horse, safety factor, bone adaptation, cortical bone, equine third metacarpal, bone remodeling

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