Micromechanical Properties of the Rat Frontoparietal Suture Upon Nanoindentation

The mechanical properties of cranial sutures have rarely been investigated. Objectives: The objective of the present study was to characterize the mechanical properties of the rat frontoparietal suture and its adjacent sutural mineralization front (SMF). Methods: Nanoindentation with atomic force microscopy (AFM) was used to characterize the mechanical properties of both the frontoparietal suture and its corresponding SMF in five, 8-week-old Sprague-Dawley rats.  Upon tissue harvest, the middle 1/3 of the suture was dissected in a resulting tissue block of approximately 5×3×1 mm³ (length×width×thickness) in size, including at least 2 mm of adjacent bone on each side of the suture.  The cerebral surface of the entire specimen was glued to a metallic disk and mounted to the AFM piezoscanner.  The subcutaneous sutural surface, irrigated with PBS, was subjected to nanoindentation to obtain topographic and force-spectroscopic images of both the sutural connective tissue and the SMF that were approximately 30 microns apart. Results: The sutural connective tissue and the SMF demonstrated significant differences in surface topography: 484.74±147.09 nm for the suture and 1169.24±301.05 nm for SMF (p < 0.01), suggesting that the sutural connective tissue is less variable in surface contour than its adjacent mineralizing bone that was only 30 microns away. The average Young's modulus of the SMF (2.34±0.05 MPa) was significantly higher than that of the sutural connective tissue (0.69±0.11 MPa) (p < 0.01).  Conclusion: In addition to providing the dynamic micromechanical properties of the rat frontoparietal suture and sutural mineralization front upon nanoindentation, the present data suggest that sutural mineralization is regulated in small fields such as 30 microns. These findings lead to a new hypothesis that micromechanical properties of the suture and sutural mineralization front are general for all cranial sutures. Supported by USPHS research grants DE13088 and DE13964 from NIH/NIDCR, Bethesda, MD 20892.