According to hydrodynamic theory, blocking the opened dentinal tubules could reduce discomfort from dentin hypersensitivity by reducing fluid movement through dentinal tubules3. In this study, the intratubular crystals were formed in PBS after the experimental material was applied to the exposed dentin surface and filled the dentinal tubules more densely as the period of storage in PBS increased.
In the case of intratubular crystal formation in previous study18, when using a calcium silicate based sealer for root canal treatment, calcium silicate was continuously present inside the root canal and in contact with the dentinal tubules. For dentin hypersensitivity treatment, a desensitizer should be applied to the dentin surface exposed to the outside, which is not in contact with the surface of dentin continuously exposed due to physical and chemical factors in the oral cavity. Therefore, in order to improve intratubular occlusion, desensitizer must efficiently penetrate the dentinal tubules.
In this study, two efforts were made to achieve efficient penetration of experimental particles as greater penetration of particles into the dentinal tubules would have occurred. First, the experimental used in this study consisted of particles with a smaller diameter than in other studies using calcium silicate (lower than 85 µm or 0.5 mm)19,20 or calcium silicate in commercially available mineral trioxide aggregate (MTA) products 21. The diameter of dentinal tubules of the sensitive dentin is larger than that of the non-sensitive dentin but is only 0.83 µm on average2. The small dimension of dentinal tubules makes it difficult for desensitizers to be efficiently penetrated12. According to a study by Kim et al.22, comparing the degree of absorption according to the particle size of the desensitizer, the desensitizer with small-sized particles had more opportunities to penetrate the dental tubules. Therefore, the experimental materials consisting of specially designed nano-scale particles used in this study would effectively penetrate the dentinal tubules and form occluding plugs, as in other studies23. Second, C2S/C3S were applied as a brushing motion to the dentin surface. This simulated that calcium silicate was contained in toothpaste and applied to the surface of the dentin when brushing. A total of 10,000 repeated strokes (1 strokes/second) of brushing simulated about 18.5 days assuming brushing three minutes/time and three times a day24. It can have a chance to push desensitizing materials into dentinal tubules compared with just dropping or rubbing with a micro-brush on the specimen surface. As a result, the occluding plug was formed below the dentinal tubule orifice in applying experimental material to the brushing motion (Fig. 1).
The occluding plugs can act as a reservoir of calcium ions, continuously dissolving calcium ions and making the inside of the dentinal tubules into a supersaturation state25. The supersaturation condition was expected to cause local aggregation of calcium ions and phosphate ions due to interaction with ions present on the inner surface of the dentinal tubules, which caused the growth of the intratubular crystals23. This reaction can be inferred that the lower part of the occluding plug was rough, and plate-shaped crystals were formed below (Fig. 1b white arrowheads).
The intratubular crystal formation reaction by diffusion according to the concentration gradient of ions can occur at a deep point in the dentinal tubules. In this study, the intratubular crystals were formed at a depth of more than 50 µm from the exposed dentin surface (Fig. 1a white arrows). In a clinical situation, superficial occlusion of dentinal tubules has a short-term effect as the precipitates can be easily removed due to daily tooth brushing, dissolution by saliva, and acidic beverages26. For the long-term effect of desensitizer, the material blocking the dentinal tubules should be deep enough27.
Crystal formation reaction continuously takes place in dentinal tubules forming denser crystal complex as the period of storage in PBS increased (Fig. 2). The crystal complex is expected to contribute to preventing the movement of the pulpal fluid through the dentinal tubules and reduce discomfort from dentin hypersensitivity more effectively7,28. Further research will be needed on the clinical effectiveness of the experimental material.
When C2S and C3S are in contact with water, they are hydroxylated, and the surface dissolves according to below29;
$$2(2CaO\bullet Si{O}_{2})+4{H}_{2}O\to 3CaO\bullet 2Si{O}_{2}\bullet 3{H}_{2}O+Ca{\left(OH\right)}_{2}$$
1
$$2(3CaO\bullet Si{O}_{2})+6{H}_{2}O\to 3CaO\bullet 2Si{O}_{2}\bullet 3{H}_{2}O+3Ca{\left(OH\right)}_{2}$$
2
As a result of these reactions, calcium and hydroxyl ions are released, resulting in a highly alkaline environment30. After the hydration reaction of calcium silicate, the hydroxyapatite-like crystals were formed under contact with PBS31,32. A previous study showed calcium-deficient and B-type carbonated apatite with a 1.4–1.5 Ca/P ratio formed from Portland cement in PBS32. Other studies showed calcium silicate in PBS could make hydroxyapatites after hydration33–35. In this study, the Ca/P ratio of formed intratubular crystals was 1.68 after three months. Considering that the Ca/P ratio of hydroxyapatite is 1.6736, it can be expected that the intratubular crystals made in this experiment would be hydroxyapatite-like crystals.
However, this experimental design could not simulate an oral environment, such as an acid challenge from the diet. Therefore, further studies need to be performed to evaluate the effect of the acid-neutral cycle on intratubular crystal formation.
This study demonstrated that the experimental material could form intratubular crystals in PBS after being applied to the exposed dentin surface. The crystal formation occurred at more than 50 µm from the dentin surface, and the crystals more densely filled the dentinal tubules over time. The experimental material consisting of C2S and C3S with nano-scaled particle size used in this study allowed for effective penetration to dentinal tubules and the formation of intratubular crystals, which makes the material a promising alternative for clinical use to reduce discomfort from dentin hypersensitivity.