Contributions to Embrittlement of Aging Enamel: Mineral and Organic Content

Objectives: To evaluate the changes in organic content and mineral nanocrystal composition of enamel with age and its effects on hardness and stiffness. The durability of enamel decreases with age. Specifically, the outermost enamel becomes increasingly brittle and prone to fracture with patient age. The mechanisms of embrittlement are not fully understood but are presumed to result from changes in both the organic content and mineral. Here we investigate contributions to decreased enamel durability.
Methods: Enamel was excised from donor teeth collected at dental practices in Seattle, WA with IRB approval and divided into Primary (age≤12 years, n=5), Young Adult (20≤age≤35 years, n=5) and Senior Adult (age ≥ 55, n=5) groups. A methodology was developed to pelletize enamel in KBr to evaluate the organic content via transmission mode FTIR spectroscopy. The organic content was evaluated in the inner and outer enamel regions separately. Raman Spectroscopy was used to assess differences in mineral composition (carbonate substitution, crystallinity, and phosphate peak shift) in the same regions. Complimentary measures of hardness and stiffness were collected via nanoindentation to investigate contributions to embrittlement.
Results: Primary enamel had significantly (P < 0.05) higher organic content than the young and senior adult groups. The organic matter decreased non-linearly with age. The inner enamel for all groups had significantly (P < 0.05) higher organic content than the outer enamel. Carbonate content in the mineral was significantly (P < 0.05) higher in the inner enamel for all three age groups. The opposite trend was seen for crystallinity and phosphate peak shift. The nanomechanical properties were inversely related to organic and carbonate content and directly related to crystallinity and peak position.
Conclusions: The reduction in enamel durability with age is attributed to a decrease in organic content and simultaneous changes in mineral structure-composition, which increases the hardness and stiffness.