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

Abstract

Mechanical strains produced by functional loading influence the cellular activities responsible for normal appendicular bone development and maintenance. Contemporary investigations recognize that modeling and remodeling processes mediate the strain-related structural and material adaptations produced during normal bone development. However, the goals towards which such adaptations are directed remain unclear. hypothesized that a possible objective for regional variations in material organization between cortical locations of a limb bone diaphysis may be the maintenance of uniform stresses throughout a bone's crosssection. Examining mature ovine radii at mid-diaphysis, they found that the narrower caudal 'compression' cortex had a lower elastic modulus than the thicker, less-highly strained cranial (dorsal) 'tension' cortex. Riggs et al. (1993) reported similar elastic modulus differences between the cranial 'tension' and caudal 'compression' cortices of the equine radius at mid-diaphysis, even though these regions have nearly equivalent cortical thickness. These elastic modulus differences were attributed to significant regional variations between the cranial and caudal cortices, including more oblique-to-transverse collagen fiber orientation, lower mineral content, and increased remodeling with secondary osteons in the caudal cortex. These authors suggested that since cranial versus caudal stresses are significantly different in each species, and the associated remodeling responses appeared to amplify this difference, the non-uniform stress distribution represents a goal of developmental adaptation. Because yield and ultimate stress of cortical bone are lower in tension than in compression (Reilly and Burstein, 1975), and the nonuniform stress distributions of ovine and equine radii resulted in roughly equivalent safety factors between the cranial and caudal cortices, the achievement of equivalent or uniform 'regional' safety factors (e.g. cranial cortex = caudal cortex) was offered as an explanation for a major goal of adaptation in these bones Riggs et al., 1993).Safety factors, when considered in skeletal biomechanics, usually refer to an entire bone (Rubin and Lanyon, 1982;Biewener, 1993). In the present study we further examine the idea of 'regional' safety factors; for example, those from a distinct cortical location within the same transverse crosssection Riggs et al., 1993). In either case, 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 ...

The influence of collagen fiber orientation and other histocompositional characteristics on the mechanical properties of equine cortical bone

“…We assumed no histological or material property differences between the MC3s of the left and right limbs. These bones, and the machined specimens obtained from them, were used in our previous study examining 'regional' safety factors (Skedros et al, 2003a). Mechanical test data from our previous study were also used in the present study.…”

“…The cortical locations, the number of animals, and the number of test specimens obtained were as follows: dorsal-lateral (D-L, N=2; 20 total specimens), lateral (L, N=1; 10 total specimens), palmarmedial (P-M, N=2; 20 total specimens) and dorsal-medial (D-M, N=1; 10 total specimens) (Fig.·2A). In the context of the published strain distribution (Gross et al, 1992) and used previously (Skedros et al, 2003a), these locations represent areas that experience in vivo tensile strains, and a range of compression magnitudes [e.g. dorsal-medial (low) to palmarmedial (high)] (Fig.·1, Fig.·2A).…”

“…1081-1087, with permission from Elsevier Science LTD, the Boulevard, Lanford Lane, Kidlington OX5 1GB, UK. This adaptation of the original figure (Gross et al, 1992) has already been used (Skedros et al, 2003a)…”

“…Tension specimens [This adaptation of the original NA (Gross et al, 1992) has already been used (Skedros et al, 2003a) in a study examining 'regional' safety factors.] places more of the lateral cortex in net tension (Nunamaker, 2001).…”

“…In general, we selected the equine MC3 because it has been the focus of studies dealing with its mechanical properties and biomechanical and pathophysiological (e.g. stress fractures) implications of its histocompositional organization (Hiller et al, 2003;Les et al, 1997;Martin and Boardman, 1993;Martin et al, 1997;Nunamaker, 2001;Reilly et al, 1997;Skedros et al, 2003a;Skedros et al, 1996;Stover et al, 1992). More specific reasons for selecting this bone included the fact that independent investigators have described its habitual strain distribution at mid-diaphysis using in vivo strain measurements (Biewener et al, 1983a;Biewener et al, 1983b;Gross et al, 1992).…”