The influence of baseline hardness and chemical composition on enamel demineralization and subsequent remineralization

Abstract

BACKGROUND Several studies have reported that harder enamel with higher contents of calcium (Ca), phosphorus (P) and fluorine (F) coupled with lower contents of carbonate (C), magnesium (Mg) and nitrate (N) was found to be more resistant to demineralization. Additionally, the hardness of dental enamel was found to have a strong correlation with its chemical content. However, yet to be established is the relation between the physical and chemical structure of enamel and its response to de- and remineralizing conditions.

OBJECTIVES The aims of this laboratory study were: 1) To investigate the hardness and chemical content of sound enamel and their influence on demineralization; 2) To investigate these properties in demineralized enamel and their influence on remineralization; and 3) To investigate these properties in sound enamel and their influence on remineralization.

MATERIALS AND METHODS Incipient subsurface caries lesions were created in 94 bovine enamel specimens using Carbopol C907 using three demineralization times. The specimens were then pH-cycled and treated using either 367 ppm F sodium fluoride or a placebo. Knoop surface microhardness (SMH), Energy dispersive X-ray spectroscopy (EDS) and Transverse microradiography (TMR) were performed on the specimens at all stages and compared between them. TMR variables included integrated mineral loss (ΔZ), Lesion depth (L) and maximum mineral density of the surface zone (SZmax). Data were analyzed using three- and four-way ANOVA and Pearson correlation coefficients were calculated.

RESULTS SMH, ΔZ, L and SZmax were significantly different among stages, demineralization times and treatment. The weight% of F at the surface was significantly affected by treatment, irrespective of demineralization time. A statistically significant moderate correlation was found between SZmax and ΔZ and SZmax and L after pH cycling. SMH also correlated weakly to moderately with TMR data.

CONCLUSIONS SMH and SZmax decreased while ΔZ and L increased with increased demineralization time. Both fluoride and non-fluoride specimens were able to remineralize, which emphasizes the role of saliva in mineralization. The Ca:P ratio remained stable at various stages, indicating the stoichiometric dissolution and redeposition of minerals. The greatest deposition of F was at the surface and its increase led to an increase in SMH and SZmax. SMH values showed that harder specimens at baseline and after demineralization remained hard after demineralization and pH-cycling, respectively, although this correlation was weak. Additionally, harder lesions showed less L and ΔZ and greater SZmax.

RELEVANCE This in-vitro study will help better understand the caries process and the impact of physical and chemical characteristics of enamel on de- and remineralization challenges.