Effects of Silver Diamine Fluoride and Nano-Hydroxyapatite Toothpaste on Surface Morphology, Elemental Composition, and Microhardness of Mechanically Exposed Dentin—An In Vitro Study

Objectives: This study aimed to evaluate and compare the effects of 38% silver diamine fluoride (SDF) and nano-hydroxyapatite (nHAp) toothpaste on dentinal tubule surface area, elemental composition, and microhardness of mechanically exposed dentin. Methods: Thirty extracted human molars with standardized exposed occlusal dentin were randomly assigned to control, SDF, or nHAp groups (n = 10 each). Surface morphology and dentinal tubule area were assessed using scanning electron microscopy, elemental composition by energy-dispersive X-ray spectroscopy, and microhardness by Vickers testing before and after application of SDF and nHAp. Data were analyzed using one-way ANOVA with Bonferroni post hoc tests and paired t-tests (α = 0.05). Results: Baseline dentinal tubule area and microhardness were comparable among groups. SDF showed the strongest effects, reducing dentinal tubule area by 84.5% and producing the greatest microhardness increase (p < 0.001), while nHAp reduced dentinal tubule area by 45.2% and increased microhardness within the group (p < 0.001). Calcium content was highest in the SDF group (p < 0.001), whereas phosphorus showed no significant difference (p = 0.195). The Ca/P ratio differed overall (p = 0.046), mainly between Control and SDF (p = 0.024). Scanning electron microscopy confirmed dense tubule occlusion with SDF, partial occlusion with nHAp, and open tubules in controls. Conclusions: SDF produced greater dentinal tubule occlusion and microhardness improvement than nHAp toothpaste in this short-term in vitro model using mechanically exposed dentin. nHAp toothpaste produced moderate tubule occlusion and a smaller improvement in microhardness under the tested conditions.