The importance of microstructural variations on the fracture toughness of human dentin

Abstract

The crack growth resistance of human dentin was characterized as a function of relative distance from the DEJ and the corresponding microstructure. Compact tension specimens were prepared from the coronal dentin of caries-free 3rd molars. The specimens were sectioned from either the outer, middle or inner dentin. Stable crack extension was achieved under Mode I quasi-static loading, with the crack oriented in-plane with the tubules, and the crack growth resistance was characterized in terms of the initiation (K(o)), growth (K(g)) and plateau (K(p)) toughness. A hybrid approach was also used to quantify the contribution of dominant mechanisms to the overall toughness. Results showed that human dentin exhibits increasing crack growth resistance with crack extension in all regions, and that the fracture toughness of inner dentin (2.2 ± 0.5 MPa·m(0.5)) was significantly lower than that of middle (2.7 ± 0.2 MPa·m(0.5)) and outer regions (3.4 ± 0.3 MPa·m(0.5)). Extrinsic toughening, composed mostly of crack bridging, was estimated to cause an average increase in the fracture energy of 26% in all three regions. Based on these findings, dental restorations extended into deep dentin are much more likely to cause tooth fracture due to the greater potential for introduction of flaws and decrease in fracture toughness with depth.

Figure 1

Preparation of a compact tension (CT) specimen machined from a human 3rd molar. (a) section of a human 3rd molar indicating the three coronal regions where the specimens were obtained. I, M and O represent inner, middle and outer, respectively; (b) view of a sectioned tooth and potential specimen. The dentin (D) and enamel (E) are evident from the difference in gray scale. Note that the crack front is in-plane with the tubules, but perpendicular to their axes.

Figure 2

Finite element model details. (a) geometry, boundary conditions and loading, and (b) crack tip mesh (element type CPE8R).

Figure 3

Constitutive behavior of the springs used to model extrinsic toughening. Notice that the maximum elongation of the spring is 3.5 µm.

Figure 4

Quasi-static loading response and crack extension in the dentin. (a) a load-line displacement distribution for stable crack extension. Region I entails crack opening and the initiation of growth. Region II corresponds to incremental stable crack extension; (b) a digital image of a crack in a dentin specimen and the gray scale distribution used for the DIC analysis. Note the crack path highlighted by the white arrows.

Figure 5

Crack growth resistance (R-curve) responses for the dentin specimens. (a) a typical Rcurve for young dentin obtained from a 23 year old female. The initiation (Ko), growth (Kg) and plateau (Kp) toughness defined in this response. (b) results obtained from all of the specimens an in all three coronal regions evaluated.

Figure 6

Relationship between the region of evaluation, and resistance to crack growth. (a) initiation toughness, b) growth toughness, and (c) plateau toughness. In both (a) and (c) the columns with different letters are significantly different (p≤ 0.05).

Figure 7

Relationship between the microstructure and resistance to crack growth.(a) the initiation toughness as a function of the percentage area of the fracture surface occupied by lumens. (b) the growth toughness presented in terms of the percentage fracture area occupied by the peritubular cuff. (c) the plateau toughness as a function of the lumen area.

Figure 8

Scanning electron micrographs illustrating the toughening mechanisms during crack growth in human coronal dentin. (a) crack extension in inner dentin. The crack primarily extends from lumen to lumen as shown with the white arrows. Some degree of peritubular microcraking is also observed at the crack tip as outlined (white encircled area). (b) Micrograph showing the presence of ligaments bridging the crack in inner dentin (arrows). Notice that the majority of bridging involves bundles of tubules as evident at this lower magnification. (c) Microcracking of 2 the peritubular cuffs is prevalent in central dentin as shown in the highlighted region of the crack wake; (d) example of ligaments bridging the crack and microcracks observed during crack growth in outer dentin.

Figure 9

Comparison of the crack opening displacement from the symmetry plane (COD/2) as a function of distance from the crack tip for the experiments and the numerical model. These are representative distributions for (a) inner, (b) middle and, (c) outer dentin. Table 1 Finite element results for the components of fracture energy and toughness for inner, middle and outer dentin.