BIOMECHANICAL IMPLICATIONS OF MINERAL-CONTENT AND MICROSTRUCTURAL VARIATIONS IN CORTICAL BONE OF HORSE, ELK, AND SHEEP CALCANEI

Background: Artiodactyl and perissodactyl calcanei have been recently introduced as models for examining bone for mechanically mediated adap tation. We have reported substantial regional variations in cortical b one microstructure and mineral content within the same cross-section o f mule deer calcanei. In part, these variations may be adaptations acc ommodating the customary presence of predominantly tension, compressio n, and shear strain modes in mutually exclusive cortical locations. Ca lcanei from skeletally mature horses, elk, and sheep were examined in order to corroborate these previous findings. Methods: From each speci es, one calcaneus was obtained from each of 13 animals. Each bone was cut transversely near mid-shaft into two segments and examined for min eral lash) content. From each species, an additional segment obtained from each of 7 of the original 13 bones was examined for microstructur e using 50x backscattered electron images. Regions examined included t he compression (cranial), tension (caudal), and medial and lateral (sh ear) cortices. Periosteal (P), middle (M), and endosteal (E) regions w ere also examined separately within the compression and tension cortic es. Quantified microstructural parameters included: (1) secondary oste on population density (OPD), (2) fractional area of secondary bone (FA SB), (3) porosity, (4) population density of new remodeling events (NR E = resorption spaces and newly forming secondary osteons), and (5) se condary osteon diameter and minimum-to-maximum chord ratio. Results: R esults in each species showed variations that are considered to be mec hanically important and are similar to those reported in mule deer cal canei. Mineral content data suggest that remodeling activity in the co mpression, medial, and lateral cortices was occurring at a slower rate than remodeling in the tension cortex. In comparison to the tension c ortices, the compression cortices have approximately 6.0% higher miner al content (P < 0.007) and 35% higher OPD (P < 0.01). Additionally, th e compression cortices have more nearly perfectly round osteons and lo wer FASB, porosity, NRE, and osteon diameter (P < 0.05; except for FAS B in horse where P = 0.087 and NRE in sheep where P = 0.520). However, patterns of microstructural variations between intracortical regions (P, M, E) are inconsistent when compared to data reported in mule deer calcanei. Microstructural characteristics between the medial and late ral cortices were similar although some significant differences were i dentified. In general, the microstructure of the medial and lateral co rtices differ from the neighboring compression and tension cortices. C onclusions: Differences in mineral content and microstructure between opposing compression and tension cortices of these three species resem ble differences previously reported in mule deer calcanei. The majorit y of the microstructural variations can be explained in the context of strain-magnitude-based rules of Frost's Mechanostat Theory of mechani cally induced bone adaptation. These variations may also be strongly i nfluenced by the strain mode predominating in each cortical location. The hypothesis that intracortical material adaptations are correlated with progressive transcortical strain magnitude variations is not supp orted by the inconsistent transcortical variations in material organiz ation. These interpretations do not preclude the possibility that othe r specific strain features may contribute to a complex adaptive signal . (C) 1997 Wiley-Liss, Inc.