EVIDENCE OF STRUCTURAL AND MATERIAL ADAPTATION TO SPECIFIC STRAIN FEATURES IN CORTICAL BONE

Background: Functionally induced strains provide epigenetic signaling for bone modeling and remodeling activities. Strain gauge documentatio n of the equine third metacarpal reveals a neutral axis passing throug h the craniolateral cortex, resulting in a narrow band of cortex loade d predominantly in tension, with the remainder of the cortex experienc ing a wide range of compression strain magnitudes that are maximal in the caudomedial cortex. This predictable strain pattern provides a mod el for examining the hypothesis that strain mode, magnitude, and strai n energy density are potential correlates of compact bone structural a nd material organization. Methods: Structural and material variables w ere quantified in nine equine (standard breeds) third metacarpals for comparison with the in vivo strain milieu that was evaluated in thorou ghbred horses. The variables quantified included secondary osteon popu lation density (OPD), fractional area of secondary bone (FASB), fracti onal area of porous spaces, collagen fiber orientation, mineral conten t (% ash), and cortical thickness. Each bone was sectioned transversel y at 50% of length, with subsequent quantification of eight radial sec tors and three intracortical regions (periosteal, middle, endosteal). Linear regression analysis compared these variables to magnitudes of c orresponding regional in vivo longitudinal strain, shear strain, and s train energy density values reported in the literature. Results: The c raniolateral (''tension'') cortex of this bone is distinguished by its 30% lower FASB and with the lateral cortex exhibits 20% darker gray l evel (more longitudinal collagen) compared with the average of all oth er locations. Conversely, the remaining (''compression'') cortices as a group have a high OPD, are more extensively remodeled, and contain m ore oblique-to-transverse collagen. The caudal cortices (caudomedial, caudal, caudolateral) are significantly thinner (P < 0.01) and have 4% lower mineral content (P < 0.05) than all other locations. Moderately strong correlations exist between collagen fiber orientation and norm al strain (r = 0.752) and shear strain (r = 0.555). When normal and sh ear strains were transformed to their respective absolute values, thus eliminating the effects of strain mode (tension vs. compression), the se correlation coefficients decreased markedly. Conclusions: Collagen fiber orientation is related to strain mode and may function to accent uate rather than attenuate bending. These differences may represent ad aptations that function synergistically with bone geometry to promote a beneficial strain distribution and loading predictability during fun ctional loading. (C) 1996 Wiley-Liss, Inc.