In cortical bone, regional variations in predominant collagen fiber orientation (CFO) and other histocompositional (material) characteristics may represent biomechanically important adaptations related to specific strain modes of tension, compression, or shear. For example, regions habitually loaded in compression have relatively more oblique-to-transverse collagen compared to regions loaded in tension, which have relatively more longitudinal collagen (Carando et al., 1991;Kalmey and Lovejoy, 2002;Mason et al., 1995;Riggs et al., 1993a;Skedros, 2001;Skedros and Kuo, 1999;Skedros et al., 1996). Standard material tests (conducted This study examined relative influences of predominant collagen fiber orientation (CFO), mineralization (% ash), and other microstructural characteristics on the mechanical properties of equine cortical bone. Using strain-mode-specific (S-M-S) testing (compression testing of bone habitually loaded in compression; tension testing of bone habitually loaded in tension), the relative mechanical importance of CFO and other material characteristics were examined in equine third metacarpals (MC3s). This model was chosen since it had a consistent non-uniform strain distribution estimated by finite element analysis (FEA) near mid-diaphysis of a thoroughbred horse, net tension in the dorsal/lateral cortices and net compression in the palmar/medial cortices. Bone specimens from regions habitually loaded in tension or compression were: (1) tested to failure in both axial compression and tension in order to contrast S-M-S vs non-S-M-S behavior, and (2) analyzed for CFO, % ash, porosity, fractional area of secondary osteonal bone, osteon cross-sectional area, and population densities of secondary osteons and osteocyte lacunae. Multivariate multiple regression analyses revealed that in S-M-S compression testing, CFO most strongly influenced total energy (pre-yield elastic energy plus post-yield plastic energy); in S-M-S tension testing CFO most strongly influenced post-yield energy and total energy. CFO was less important in explaining S-M-S elastic modulus, and yield and ultimate stress. Therefore, in S-M-S loading CFO appears to be important in influencing energy absorption, whereas the other characteristics have a more dominant influence in elastic modulus, pre-yield behavior and strength. These data generally support the hypothesis that differentially affecting S-M-S energy absorption may be an important consequence of regional histocompositional heterogeneity in the equine MC3. Data inconsistent with the hypothesis, including the lack of highly longitudinal collagen in the dorsal-lateral 'tension' region, paradoxical histologic organization in some locations, and lack of significantly improved S-M-S properties in some locations, might reflect the absence of a similar habitual strain distribution in all bones. An alternative strain distribution based on in vivo strain measurements, without FEA, on non-Thoroughbreds showing net compression along the dorsal-palmar axis might be more characteristic of ...

The limb bones of cursorial mammals may exhibit regional structural/material variations for local mechanical requirements. For example, it has been hypothesized that mineral content (%ash) and secondary osteon population density (OPD) progressively change from proximal (e.g., humerus) to distal (e.g., phalanx), in accordance with corresponding progressive changes in stress and mechanical/metabolic cost of functional use (both greatest in the distal limb). We tested this hypothesis in wild-shot Rocky Mountain mule deer by examining transverse segments from mid-diaphyses of medial proximal phalanges, principal metacarpals, radii, and humeri, as well as the lateral aspects of sixth ribs from each of 11 mature males. Quantified structural parameters included the section modulus (Z), polar moment of inertia (J), cortical area/total area ratio (CA/TA), bone girth, and cortical thickness. In addition, %ash and the prevalence of in vivo microcracks were measured in each bone. Thin sections from seven animals were further examined for OPD and population densities of new remodeling events (NREs). Results showed a significant progressive decrease in %ash from the humerus (75.4% Ϯ 0.9%) to the phalanx (69.4% Ϯ 1.1%) (P Ͻ 0.0001), with general proximal-to-distal increases in OPD and general decreases in J and Z. Thirteen microcracks were identified in the rib sections, and only two were observed in the limb bones. Although the ribs had considerably greater NREs, no significant differences in NREs were found between the limb bones, indicating that they had similar remodeling rates. Equivalent microcrack prevalence, but nonequivalent structural/material organization, suggests that there are regional adaptations that minimize microcrack production in locations with differences in loading conditions. The progressive proximal-to-distal decrease in %ash (up to 6%); moderate-to-high correlations between OPD, %ash, J, and CA/TA; and additional moderateto-high correlations of these parameters with each bone's radius of gyration support the possibility that these variations are adaptations for regional loading conditions. Anat Rec Part A 274A: 837-850, 2003.

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 ...

When the skeleton is catabolically challenged, there is great variability in the timing and extent of bone resorption observed at cancellous and cortical bone sites. It remains unclear whether this resorptive heterogeneity, which is often evident within a single bone, arises from increased permissiveness of specific sites to bone resorption or localized resorptive events of varied robustness. To explore this question, we used the mouse model of calf paralysis induced bone loss, which results in metaphyseal and diaphyseal bone resorption of different timing and magnitude. Given this phenotypic pattern of resorption, we hypothesized that bone loss in the proximal tibia metaphysis and diaphysis occurs through resorption events that are spatially and temporally distinct. To test this hypothesis, we undertook three complimentary in vivo/μCT imaging studies. Specifically, we defined spatiotemporal variations in endocortical bone resorption during the 3 weeks following calf paralysis, applied a novel image registration approach to determine the location where bone resorption initiates within the proximal tibia metaphysis, and explored the role of varied basal osteoclast activity on the magnitude of bone loss initiation in the metaphysis using μCT based bone resorption parameters. A differential response of metaphyseal and diaphyseal bone resorption was observed throughout each study. Acute endocortical bone loss following muscle paralysis occurred almost exclusively within the metaphyseal compartment (96.5% of total endocortical bone loss within 6 days). Using our trabecular image registration approach, we further resolved the initiation of metaphyseal bone loss to a focused region of significant basal osteoclast function (0.03 mm3) adjacent to the growth plate. This correlative observation of paralysis induced bone loss mediated by basal growth plate cell dynamics was supported by the acute metaphyseal osteoclastic response of 5-wk vs. 13- month-old mice. Specifically, μCT based bone resorption rates normalized to initial trabecular surface (BRRBS) was 3.7-fold greater in Young vs. Aged mice (2.27 ± 0.27 μm3/μm2/day vs. 0.60 ± 0.44 μm3/μm2/day). In contrast with the focused bone loss initiation in the metaphysis, diaphyseal bone loss initiated homogeneously throughout the long axis of the tibia predominantly in the second week following paralysis (81.3% of diaphyseal endocortical expansion between days 6 and 13). The timing and homogenous nature is consistent with de novo osteoclastogenesis mediating the diaphyseal resorption. Taken together, our data suggests that tibial metaphyseal and diaphyseal bone loss induced by transient calf paralysis are spatially and temporally discrete events. In a broader context, these findings are an essential first step toward clarifying the timing and origins of multiple resorptive events that would require targeting to fully inhibit bone loss following neuromuscular trauma.

Natural loading of the calcanei of deer, elk, sheep and horses produces marked regional differences in prevalent ⁄ predominant strain modes: compression in the dorsal cortex, shear in medial-lateral cortices, and tension ⁄ shear in the plantar cortex. This consistent non-uniform strain distribution is useful for investigating mechanisms that mediate the development of the remarkable regional material variations of these bones (e.g. collagen orientation, mineralization, remodeling rates and secondary osteon morphotypes, size and population density). Regional differences in strain-mode-specific microdamage prevalence and ⁄ or morphology might evoke and sustain the remodeling that produces this material heterogeneity in accordance with local strain characteristics. Adult calcanei from 11 animals of each species (deer, elk, sheep and horses) were transversely sectioned and examined using light and confocal microscopy. With light microscopy, 20 linear microcracks were identified (deer: 10; elk: six; horse: four; sheep: none), and with confocal microscopy substantially more microdamage with typically non-linear morphology was identified (deer: 45; elk: 24; horse: 15; sheep: none). No clear regional patterns of strain-mode-specific microdamage were found in the three species with microdamage. In these species, the highest overall concentrations occurred in the plantar cortex. This might reflect increased susceptibility of microdamage in habitual tension ⁄ shear. Absence of detectable microdamage in sheep calcanei may represent the (presumably) relatively greater physical activity of deer, elk and horses. Absence of differences in microdamage prevalence ⁄ morphology between dorsal, medial and lateral cortices of these bones, and the general absence of spatial patterns of strain-mode-specific microdamage, might reflect the prior emergence of non-uniform osteon-mediated adaptations that reduce deleterious concentrations of microdamage by the adult stage of bone development.