On the Reduction in Toughness of Dentin with Patient Age

Advancements in medicine and an overall increase in the quality of life have resulted in a growing number of partially and/or fully dentate seniors. Objectives: To quantify the influence of patient age on the crack growth resistance of human dentin. Methods: Compact Tension (CT) specimens were sectioned from the coronal dentin of molars classified as young (18≤age≤35), middle (40≤age≤45) and old (55≤age≤83). All specimens were subjected to incremental mode I quasi-static loads. Digital images of the specimens were obtained during loading using a stereomicroscope and Digital Image Correlation (DIC) was performed with the images to identify the crack length and to evaluate the near-tip displacement field. Facture surfaces were examined using scanning electron microscopy (SEM) under secondary and backscatter emission modes. Results: Crack growth resistance curves showed that dentin exhibits an increase in toughness with crack extension (i.e. rising “R-curve”) and that there is a significant (p≤0.05) reduction in toughness with increasing patient age (i.e. at initiation, growth and plateau toughness). In the young dentin there was a 25% increase in the crack growth resistance from the onset of extension (Ko =1.34 MPa•m^0.5) to the “plateau” toughness (Kp = 1.65 MPa•m^0.5). In old dentin the crack growth resistance increased with extension by less than 10 % from Ko = 1.08 MPa•m^0.5 to Kp = 1.17 MPa•m^0.5. In the young dentin crack growth was accompanied by microcracking of the peritubular dentin and bridging by both unbroken ligaments of tissue and collagen fibrils. These mechanisms were suppressed in the old dentin due to changes in the microstructure, thereby resulting in significantly lower toughness. Conclusion: Results showed that aging results in a significant reduction in the toughness of dentin and a reduction in the “damage tolerance” of the tissue. (Supported by the National Science Foundation BES 0238237 and the NIDCR R01 DE016904).