Fracture of tooth enamel from incipient microstructural defects.

We present definitive evidence for crack growth from internal defects called 'tufts' in human enamel. Transverse slices (normal to the tooth axis) sawn from extracted human teeth are embedded in a polycarbonate sandwich configuration and tested in simple flexural loading. The evolution of ensuing cracks across the enamel sections is viewed in situ by a video camera. The observations unequivocally identify tufts as sources of internal tooth fracture. In sufficiently thin slices the enamel becomes translucent, allowing for through-thickness observations of the crack topography. Crack segments that appear to be disjointed on a section surface link up into a contiguous primary crack below the surface, suggesting some crack resistance by 'bridging' behind the advancing crack tip. The role of these and other microstructural factors in determining the resilience of tooth structures is considered.