Material property analysis in craniofacial cortical bone of small mammals

Objectives: Material properties are an essential feature for functional analysis of cortical bone. One difficulty in assessing regional and phenotypic variation in craniofacial cortical bone is the anisotropy coupled with variable orientation of the axis of maximum stiffness. This difficulty is often compounded by small skull size in experimental animals. This study compares ultrasonic and indentation techniques for assessing directional differences in material properties in the skulls of rats and bats. We hypothesize that variations by direction will be similar.

Methods: Ultrasonic velocities generated with a longitudinal Panametrics transducer (10MHz) were measured around the perimeters of cylinders of cortical bone (2 or 3mm in diameter) from 10 rat and 11 bat specimens at 10„a intervals. Maximum and minimum velocities determined the orientation of maximum and minimum material stiffness. These values were compared with Knoop microhardness (HK) measurements made in the same orientations.

Results: Ultrasonic measurements showed variable amounts of anisotropy in both rat and bat cranial bone ranging from a minimum/maximum velocity of 0.69-0.96 (X=0.90;SD=0.08). These results suggest that some of the cortical bone, despite a lack of cortical remodeling, is highly oriented, and similar to that in humans. Results from microindenture studies did not show the degree of anisotropy suggested by the ultrasound. Min/max ratios ranged from 0.80-1.22 (X=0.98, SD=0.11. A correlation between the two ratios was weak (R=0.50).

Conclusion: Rat and bat cranial bone is significantly anisotropic suggesting that variations in elastic moduli by orientation should be considered when characterizing the mechanical structure of the crania in small mammals. Although initial microindenter results did not match the ultrasonic results, further work may yet validate the usefulness of this technique for studying directional differences in small cortical bone specimens. (Supported by the Baylor Oral Health Foundation and NSF HOMINID grant 0523159).