The effect of low pH on surface hardness, volume, surface area and morphology of direct dental restorative materials

Background: Dental erosions as a disease is an increasing problem today, in particularly among adolescents. They may be caused by intrinsic factors such as gastric juice, frequently present in the oral cavity of bulimic patients. Direct dental restorative materials may be used to treat erosions, but the data regarding the effects of low pH on volume changes of these materials are limited. Objective: To investigate the effect of a hydrochloric acidic solution with a pH of 1.2 on surface hardness, volume, surface area and morphology of three direct dental restorative materials, using microhardness test, micro computed tomography (micro-CT) and scanning electron microscopy (SEM). Material and methods: Using Tetric EvoCeram, Tetric EvoFlow and GC Fuji II LC, 13 specimens of each material were made. The prepared specimens were thereafter randomly allocated within their respective material group and exposed to a hydrochloric acidic solution (pH 1.2) for 6 and 48 hours (equivalent to 2 and 16 years of vomiting, respectively), before they underwent microhardness test, micro-CT and SEM. The tests were performed both before and after acidic exposure. Statistical analyses were performed using a significance level set to 5%. Results: A reduction in microhardness was seen in all the materials, but only Tetric EvoCeram had a significant reduction in Knoop hardness number (KHN) (P = 0.039) after 48 hours of acidic exposure (mean values from 29.73 to 27.13 KHN). Significant volume reductions were seen after 48 hours for Tetric EvoFlow (P = 0.039) (mean values from 56.04 to 54.17 mm3) and GC Fuji II LC (P = 0.023) (mean values from 63.86 to 55.11 mm3). No significance was found regarding surface area. SEM showed varying results regarding surface morphology; several images exhibit an apparent increase in roughness with acidic exposure, while others did the opposite. Conclusion: In general – in the absence of mechanical stimulus – the microhardness of the materials may be reduced, there may be a loss of volume, and differences in surface area and morphology may appear after acidic exposure. The observed changes were small, and few were significant. Consequently, this indicates that dental restorative materials may have a protective effect against erosions on non-loaded areas of the dentition. However, there are several limitations of our study; a low sample size, multiple clinical factors we did not take into account, and uncertainty about the validity of the exposure times used. This complicates the drawing of any certain conclusions, making further investigations required to obtain complementary results. Keywords: Erosion, bulimia, direct dental restorative materials, resin-based composites, resin-modified GIC, acidic exposure, microhardness, micro-CT, SEM