Hidden contributions of the enamel rods on the fracture resistance of human teeth

Abstract

The enamel of human teeth is generally regarded as a brittle material with low fracture toughness. Consequently, the contributions of this tissue in resisting tooth fracture and the importance of its complex microstructure have been largely overlooked. In this study an experimental evaluation of the crack growth resistance of human enamel was conducted to characterize the role of rod (i.e. prism) orientation and degree of decussation on the fracture behavior of this tissue. Incremental crack growth was achieved in-plane, with the rods in directions longitudinal or transverse to their axes. Results showed that the fracture resistance of enamel is both inhomogeneous and spatially anisotropic. Cracks extending transverse to the rods in the outer enamel undergo a lower rise in toughness with extension, and achieve significantly lower fracture resistance than in the longitudinal direction. Though cracks initiating at the surface of teeth may begin extension towards the dentin–enamel junction, they are deflected by the decussated rods and continue growth about the tooth’s periphery, transverse to the rods in the outer enamel. This process facilitates dissipation of fracture energy and averts cracks from extending towards the dentin and vital pulp.

Influence of structural hierarchy on the fracture behaviour of tooth enamel

“…These crystallite interconnections between the prisms and ipm are responsible for crack bridges (also evident in [7,25]) when they run along the prism-ipm boundaries as shown in figure 8e. Unbroken prism ligaments (a few prisms), as demonstrated in figure 8f , are also reported in various studies of human and bovine enamel and have a significant contribution to the fracture toughness by shielding the crack tip and further reducing the local stress intensity available to drive fracture [7,8,11,12,25]. Observations of the crack paths in transversal oriented samples revealed crack branching (figure 8b) as a major visible toughening mechanism [8,11,12], whereas the term 'meandering cracks' was introduced [11] to describe branches that rejoin the main crack again after some micrometres of propagation.…”

Overcoming the brittleness of glass through bio-inspiration and micro-architecture

“…In some extreme cases, minerals form more than 95% vol. of the material, as in tooth enamel 1 or mollusk shells 2 . With such high concentrations of minerals, one would expect these materials to be fragile, yet these materials are tough, durable, damage tolerant and can even produce 'quasiductile' behaviours.…”

“…Tooth enamel (Fig. 1a) is made of long rods perpendicular to the surface of the tooth and 4-8 mm in diameter 1 and held together by a small fraction of proteins (1% vol.). Similarly, nacre (Fig.…”

“…Deeper within enamel the rods bend and criss-cross, forming a complex decussation, which impedes further crack growth (Fig. 1a) 1 . More severe stresses may propagate the crack through the enamel layer where they meet another line of defence: the dentino-enamel junction 22 and underlying dentin 15,[23][24][25] .…”

Universal composition–structure–property maps for natural and biomimetic platelet–matrix composites and stacked heterostructures

“…From this perspective, the condensation of our results in the form of 'Universal Composition-StructureProperty Design maps', containing only four characteristic parameters, is a significant first step in the field, which places the platelet-matrix composites in equal footing with nanocrystalline metallic systems and alloys in the current applications of structure-property relationships to significantly alter the material design landscape. Broadly, the innovative concepts, strategies and techniques of this work (specially the notion of universal maps), although casted in the context of platelet-matrix architectures, have important implications for understanding and mimicking several other fascinating damage-tolerant natural composites, which are similarly made of hard and soft building blocks but arranged in different architectures (for example, criss-cross decussation in tooth enamel 34 , Bouligand arrangement in ARTICLE exoskeleton of anthropod 2 , helicoidal matrix of hammer-like dactyl clubs of the stomatopods 35 and so on). Furthermore, one can employ the novel concept and strategies of this work to create universal design maps for other synthetic material classes such as cermets, metallic glasses, combinatorial and particulate oxides, boron nitride layered composites and emerging high entropy alloys [36][37][38][39][40][41][42] .…”