Methods: Lower left incisors of seven 12-week old wild-type mice were surveyed for enamel and dentin hardness, a surrogate for degree of mineralization. Freshly extracted incisors were embedded in epoxy resin, ground and sequentially polished longitudinally in the sagittal plane. Vickers microhardness indentions were made at the center of the enamel and dentin thickness along the length of the incisor, from incisal to radicular. Hardness was plotted against the distance of the indentation from the incisal tip in mm. Data was segregated into biomechanical sections by line-fitting, y=mx+c.
Results: For enamel, the overall mean hardness and standard deviation (SD) was 2.5 (0.6) GPa (n=63); but 3 distinct zones were discerned. The mid-zone of enamel, 0.5 to 8mm from the incisal tip, had the least slope, m=-0.01; a mean hardness of 2.6 GPa; and the lowest SD, 0.4. For dentin, the mean hardness was 0.7 (0.2) GPa (n=91); 3 distinct dentin zones were discerned. The mid-zone of dentin, 1 to 6mm from the incisal tip, had the least slope, m=-0.001; a mean hardness of 0.8 GPa; and the lowest SD, 0.2. The most incisal zones of enamel and dentin, had a decrease in hardness and positive slopes; possibly attributable to oral dissolution.
Conclusion: Distinct zones of biomechanical properties exist in murine incisors. Comparative biomechanical testing might best be limited to the mid zones described above.