Longitudinal evaluation of demineralized enamel at the critical pH

Objectives: To evaluate longitudinal change of mineral density in demineralized bovine enamel at the critical pH. Methods: Six bovine enamel specimens were coated with nail varnish. On each specimen, 4 square windows (1 mm2) were created. The specimens were incubated in lactic acid solution (pH=5.5) at 38 ºC. During incubation, the windows were covered by nail varnish one by one at 15, 30, and 45 minutes, respectively. At 60 minutes, the specimens were removed from the solution. After the nail varnish was removed, X-ray microradiography was performed by SKYSCAN1172 at settings of 100 kVp and 100 µA. The mean Hounsfield unit values (HUV) of enamel in a non-window area and those in the window areas were calculated every 180 µm over a depth (D) of 0-900 µm and analyzed by three-way ANOVA, selecting fixed factors as enamel specimen, incubation time, and the depth from the enamel surface, and HUV as an objective variable. Scheffe method was performed for multiple comparisons among three or more independent groups. A p-value <0.05 was considered significant. Results: Three-way ANOVA showed that enamel specimen, incubation time, and the depth from the enamel surface were significant (p<0.01, respectively). The HUV seemed to began to decrease at the depth of 720-900 µm, and the decrease of the HUV seemed to spread through increasing and decreasing according to the demineralized time. Conclusions: Mineral density decreased through increasing and decreasing at the critical pH. It is suggested that mineral loss began near the depth of 720-900 µm.