Mechanical Properties of Incisors in Rodents

Rodents sharpen their incisors by grinding the lingual and buccal surfaces of opposing teeth. Normally the buccal surface is covered with enamel, whereas the lingual surface is not. While grinding enamel against dentin, the dentin wears and a sharp tip is created. We studied lower incisors in mice carrying mutations in sprouty genes, the protein products of which are antagonists of receptor‐tyrosine kinase signaling. Sprouty loss‐of‐function mutants (Spry2 +/− ;Spry4 −/− ) have phenotypic features that mimic the ancestral condition in rodents, which include second incisors and enamel on the lingual surfaces of the incisors. We found that in some individuals the phenotype was asymmetrical, with enamel on the lingual surface of one lower incisor but not on the other. We compared the properties of mutated and regular lower incisors from sprouty mutated mice. Teeth were axially loaded and scanned within a micro‐CT (Xradia). Normal incisors, without lingual enamel, bent lingually primarily at the worn and sharp part near the tip, whereas the teeth with enamel on both sides bent at a more apical location, with the result that the entire tooth bent. Much smaller excursions of the loading device were required to fracture the incisors with enamel on both sides. Our results demonstrate that enamel deposited on the lingual surface lowers the extent to which teeth bend and elevates the brittleness of the teeth.