Impact of citric acid exposures on the erosion susceptibility and microhardness of anatomically different enamel surfaces

Objective: To determine the indentation-load dependency of the observed changes in surface microhardness of occlusal and buccal enamel polished surfaces before and after a dietary acid exposure. Methods: Enamel specimens (n = 20) were obtained from previously extracted human molars. The samples were divided into two groups, buccal and occlusal (n = 10). Both groups were exposed to 0.3% citric acid at pH 2.7 over different time periods. The mean 3D step-heights defined by ISO 5436–1 standard of the erosion scar was calculated using a white light profilometer. Vickers microhardness testing was carried out to discriminate the surface mechanical changes and different loading regimens used to provide an estimate of depth related changes to the enamel sub-surface. Differences between groups were assessed using two-way ANOVA and post-hoc Sidak's multiple comparisons tests at α = 0.05. Results: A two-way ANOVA identified that erosion duration significantly influenced step-height (p < 0.01). The factorial analysis also identified a significant increase in step-height on occlusal surfaces compared to buccal surfaces (p < 0.01). At 5 min the mean 3D step-heights µm ± (SD) on the occlusal surface was 3.6 ± (0.65) and on the buccal surface was 3.1 ± (0.60) which increased after 60 mins to 32.9 ± (3.43) and 30.1 ± (3.34), respectively. In contrast to the non-eroded surface, increasing the indentation load on eroded enamel resulted in a significantly increased HV (p < 0.01) suggesting a significant modification to the enamel's response to an increasing indentation load. Significance: Occlusal surfaces following acid exposure were more prone to erosion scar relative to buccal surfaces in vitro. Different loading regimens estimated the hardness of the erosive lesion from the outermost surface through the unaffected enamel.