On the fatigue crack growth resistance of human enamel

Although considered hard and wear resistant, human enamel appears to have a relatively low resistance to cracking as small cracks are commonly observed in the enamel of virgin and restored teeth. While small cracks may not result in tooth fracture, their growth over prolonged periods can be detrimental as they may serve as sites for demineralization, and further mechanical degradation. Surprisingly, an investigation on the fatigue crack growth resistance of human enamel has not been reported.

Objectives: To quantify the fatigue crack growth properties of human enamel and compare them with other mineralized tissues such as dentin and bone.

Methods: Special “inset” compact tension (CT) specimens (N=7) embodying a small piece of enamel (2x2x2 mm³) in vit-l-essence resin composite (8x6x2 mm³) were prepared. These inset specimens were loaded under mode I cyclic loads such that crack growth was parallel to the prism orientation. Steady state cyclic crack growth was modeled using the Paris Law. Crack fronts and fracture surfaces were analyzed using scanning probe microscopy (SPM) and scanning electron microscopy (SEM).

Results: The overall steady state crack growth occurred over a stress intensity range from 0.4 to 0.8 MPa•m^0.5. The average Paris Law exponent (m) and coefficient (C) was 7.8 ± 0.42 and 7.9E-04 (mm/cycle)•(MPa•m^0.5)^-m, respectively. The stress intensity threshold (ΔK_th) was approximately 0.33 MPa•m^0.5. SPM and SEM analyses showed that the cracks traversed along and across the prism boundaries with several micro and nano-scale bifurcations along the protein sheath.

Conclusion: Although the most highly mineralized tissue in nature, enamel is less sensitive to fatigue cracks (as identified by m) than young dentin and bone. However, the stress intensity threshold and crack growth resistance of enamel are much lower than either dentin or bone.

(Supported by awards BES 020521467 and BES 0238237 from the National Science Foundation).