Microstructural correlates of anisotropy in human zygomatic cortical bone

Objectives: Remodeling of cortical bone has been suggested to result in a change in 3D elastic properties from orthotropy to transverse isotropy. Such relationships are more apparent in postcranial long bones because of their consistent material orientation. In the craniofacial skeleton, material orientation is often pronounced but less consistent between regions and individuals. The objective of this study was to examine osteonal structure in human zygomatic bone and compare it to 3D elastic properties as determined ultrasonically. We hypothesize that material orientation increases in bone in which there is greater amounts of remodeling.

Methods: Ultrasonic methods were used to determine 3D elastic properties in 100 cylindrical cortical specimens from 10 frozen human crania. MicroCT was used to quantify variations in Haversian canal orientations in 23 specimens, selected from the 100 specimens, so that a range of anatomical locations and anisotropies were represented. Mineralized histological preparations of serial sections of the 23 specimens were used to quantify bone matrix ultrastructural composition.

Results: The ratio of the elastic modulus in the direction of minimum stiffness to that in the direction of maximum stiffness ranged from 0.56 to 0.92. Average osteon orientation weighted by osteon length was within 15„a of the direction of maximum stiffness in all but 2 specimens. The anisotropy ratio was moderately correlated with the standard deviation of osteon orientation at R=0.48 (P<0.05). The cortical bone was only partially remodeled in all sections.

Conclusion: Results suggest that osteon orientation is a structural correlate of cortical anisotropy. The moderate correlation suggests that other factors, such as collagen orientation, or the quantity and structure of primary bone, are also important in determining bulk cortical bone anisotropy. We suggest that the incomplete remodeling of most sections reduced anisotropies. (Supported by the Baylor Oral Health Foundation and NSF HOMINID grant 0523159).