MTA cement exposed to acidic environment: Surface and internal microhardness

Purpose: The surface deterioration that mineral trioxide aggregate (MTA) cements undergo during setting in acidic envi- ronment has already been demonstrated. Our purpose was to investigate the effect of acidic environment on the internal microhardness of MTA. Methods and materials: White ProRoot MTA powder (Dentsply) was mixed with sterile distilled water and compacted into 30 cylindrical molds (diameter 3 mm, depth 6 mm) making use of ultrasonic instruments. In order to allow proper setting time, samples were stored at 100% rh in direct contact to gauzes soaked with pH 7.4 (G1, n = 15) or pH 4.4 (G2, n = 15) buffer solutions (butyric acid and sodium butyrate). A Vickers indenter was used to assess the microhardness on 10 randomly selected areas of the top surface of each sample. The samples were cross-sectioned at 1 mm from the surface with a microtome and microhardness analysis was carried out again. The arithmetic mean of the readings was regarded as statistical unit. Statistical analysis was performed by means of t-tests for paired and independent data (p < 0.05). Results: Microhardness mean values ± standard deviations (expressed in HV) were: G1/top surface, 58.3 ± 6.4; G1/1 mm depth, 58.8 ± 6.7; G2/top surface, 26.3 ± 5.1; G2/1 mm depth, 60.5 ± 8.7. The statistical analysis revealed that the surface microhardness of MTA hardened at acidic pH was significantly lower than the internal microhardness (p < 0.001); at pH 7.4, surface and internal microhardness were similar and significantly greater than surface microhardness at pH 4.4 (p < 0.001). Conclusion: The already known effect that the acidic environment exerts on the setting of MTA cement occurred also in the present study. Nevertheless, the microhardness of the cement 1mm below the exposed surface was comparable in all experimental conditions, regardless of exposure to different pH solutions.