Structure and mechanical properties of the soft zone separating bulk dentin and enamel in crowns of human teeth: insight into tooth function

The 200-300 microm soft zone of dentin, found beneath enamel in crowns of human teeth, is thought to fulfill important roles in tooth function, but little is known about its structure-mechanical relations. Scanning electron microscopy images of fracture surfaces showed that near the dentino-enamel junction (DEJ), a porous reticulate matrix of intertubular-dentin contains tubules with no peritubular lining. Peritubular-dentin however is found at some distance from the DEJ, and it gradually thickens with increasing depth into the bulk dentin. Concurrently, tighter packing of the collagen fibers is observed with a gradual increase in mineral deposits on and between the fibers. This structurally graded zone is known to be softer when tested for micro-hardness. It undergoes greater strain compared to bulk dentin, when measured using Moiré interferometry. We investigated the deformation and stiffness of this zone by means of non-contact laser-speckle interferometry (ESPI), and nanometer-scale deformations were tracked during compression-testing performed in water. We report a significantly reduced stiffness of this zone compared to bulk dentin, with mid-buccal regions of teeth averaging 3.5 GPa compared with 9.7 GPa in mid-lingual regions. Our results support and expand upon the hypothesis that the durability of the whole tooth relies upon a bucco-lingual asymmetric matching of stiffness by means of an interphase: a cushioning soft layer between enamel and bulk dentin.