Remarkable resilience of teeth

Abstract

Tooth enamel is inherently weak, with fracture toughness comparable with glass, yet it is remarkably resilient, surviving millions of functional contacts over a lifetime. We propose a microstructural mechanism of damage resistance, based on observations from ex situ loading of human and sea otter molars (teeth with strikingly similar structural features). Section views of the enamel implicate tufts, hypomineralized crack-like defects at the enamel-dentin junction, as primary fracture sources. We report a stabilization in the evolution of these defects, by "stress shielding" from neighbors, by inhibition of ensuing crack extension from prism interweaving (decussation), and by self-healing. These factors, coupled with the capacity of the tooth configuration to limit the generation of tensile stresses in largely compressive biting, explain how teeth may absorb considerable damage over time without catastrophic failure, an outcome with strong implications concerning the adaptation of animal species to diet.

Fig. 1.

Fracture of teeth indented along a vertical axis with the flat end of a WC rod. (A) Human molar, maximum load 390 N. (B) Sea otter molar, maximum load 450 N. Radial–median (R) cracks have propagated part way downward from the contact zone, margin (M) cracks all of the way upward from the cervical base. Some plastic flattening of the indented cusp is evident immediately beneath the indenter.

Fig. 2.

Optical micrographs showing how cracks grow from tufts at the EDJ in the vicinity of the vertical load axis, in human molar. Specimen is a 1.8-mm-thick longitudinal slice before (A) and after (B) indentation with WC rod (upper cusp not shown). Cracks in B have extended upward from tufts toward the cuspal surface. The faintly visible fringes (Hunter–Schreger bands) mark changes in prism orientation.

Fig. 3.

Optical micrographs from longitudinal sections of human (A) and sea otter (B) molars, showing disruption of cracks at Hunter–Schreger bands. (A) Location of field of view is near EDJ, immediately adjacent to the tooth axis. (B) Field of view is ≈2 mm below the cusp surface, midway between EDJ and outer enamel surface.

Fig. 4.

FEA calculations of stress intensity factor K as function of reduced crack size c/w, for extension of periodic array of cracks in curved bilayer slice. Solid curves are calculations for crack arrays in inhomogeneous contact stress field, with w/d = 0.1 mm/1.8 mm = 0.055. Heavy dashed curve is for single crack in same stress field. Light dashed curve is for same single crack under uniform stress σ.

Fig. 5.

Segment of transverse section view through molars. (A) Human, loaded to 450 N and sectioned to depth 4.4 mm below the cuspal surface. (B) Sea otter, loaded to 550 N and sectioned to depth 2.2 mm. Dentin is exposed at top of field of view. Cracks (arrows) appear to initiate from tufts.

Fig. 6.

Scanning electron micrographs from transverse section of human molar, showing intersection of Vickers corner crack CC′ with (A) tuft TT′, showing delamination at lower tuft interface along C′T and C′T′, and (B) margin crack MM′, showing penetration through interface to adjacent enamel.