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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

View larger version (40K): [in a new window]   Fig. 1. Finite element model of MC3 (adapted from Gross et al., 1992). (A) The arrow indicates the mid-diaphyseal location of the third metacarpal. (B) Finite element meshes from Gross et al. (1992). Distribution of normal strain (i), shear strain (ii) and strain energy density (iii) acting on the mid-shaft cross-section at the time of peak strain during the gait cycle. Sites of maximum strain are noted by the large arrows, sites of least strain by the small arrows or neutral axis (N-axis). ue=µ. [Reproduced from Journal of Biomechanics vol. 25, `Characterizing bone strain distributions in vivo using three triple rosette strain gages,' pp. 1081-1087, Copyright (1992), with kind permission from Elsevier Science.]

View larger version (16K): [in a new window]   Fig. 2. Compression and tension specimen locations in the MC3. (A) Locations of the six compression `cube' specimens: cranio-lateral (2), lateral (1), caudo-medial (2), and cranio-medial (1). (B) Locations of the four tension `dumb-bell' specimens: cranio-lateral (2), caudo-medial (2). N.A., neutral axis.

View larger version (9K): [in a new window]   Fig. 3. Specimen geometry. (A) Tension (dumb-bell-shaped) and (B) compression. (A reproduced with revision from Equine Veterinary Journal vol. 24, `The response of equine cortical bone to loading at strain rates experienced in vivo by the galloping horse,' pp. 125-128, with kind permission from Equine Veterinary Journal.)

View larger version (14K): [in a new window]   Fig. 4. Regional (intracortical) safety factors in the equine MC3, showing both yield and ultimate safety factors for each cortical region and testing mode. C, compression-tested; T, tension-tested. Values are means ± S.D.

View larger version (11K): [in a new window]   Fig. 5. Comparison of uniform-safety-factor hypothesis versus actual findings in the equine MC3. (A) Cross-section of the MC3 showing equivalent safety factors as proposed by the uniform-safety-factor hypothesis. (B) Cross-section of the MC3 based on present data showing large variations in regional yield safety factors. Values shown are based on `strain-mode-specific' testing [i.e. tension testing (T) of the habitually tensed cranio-lateral cortex and compression testing (C) of the habitually compressed caudo-medial cortex]. N.A., neutral axis.