## 4 Discussion
In our study, SEM investigation of dentin samples treated by all three n-HAp toothpastes had shown mineral deposition that occluded the dentinal tubules and covered the intertubular dentin. Similar precipitate layer formation on the dentin surface was reported in previous studies that tested 10% and 15% n-HAp solutions [37]. Efficient dentinal tubule occlusion and mineral crystal growth on the dentin surface and into the dentinal tubules due to application of an n-HAp desensitizer was highlighted in a recent study [38]. Our study also reported increased calcium, phosphorous, carbon, and oxygen ion concentrations when n-HAp toothpastes had been applied on the dentin surface. In the remineralization process it was shown that n-HAp may act as a template in mineral crystal nucleation and growth to form a structure similar to dentin. Calcium and phosphate ions from the environment may be attracted on the dentin surface during the precipitation process and will fill the vacant places in the crystal structure [39]. Some previous studies have shown that toothpastes containing ZnCO\({}_{3}\)/n-HAp or n-HAp have similar enamel and dentine remineralization capacity, and the remineralization potential was higher when compared to fluoride toothpastes [20]. On the contrary, another study that compared the remineralization capacity of HAp toothpaste to fluoride and bioactive glass toothpastes concluded that the effectiveness of the remineralization process was lower for HAp products than for fluoride toothpaste [33]. To our knowledge, there is no study in the scientific literature to evaluate Karex toothpaste efficacy on dentinal tubule occlusion and very few articles analyzed the efficacy of Biorepair Plus and [PERSON]'s Biorepair toothpastes only in comparison with other desensitizing dentifrices. In our study there was no statistically significant difference in tubular occlusion between the study groups, so our null hypothesis was not rejected. This is in accordance with the studies of [PERSON] et al. [19] and [PERSON] et al. [13] which have demonstrated that different n-HAp toothpastes have similar capacities in the remineralization process [13; 19]. However, [PERSON] et al. did not use the toothpastes tested in the present study and performed the experiment on bovine dentin specimens. It is difficult to compare the results recorded in different studies due to different methodology of sample preparation and application technique. The present study simulates an acid attack using 40% citric acid in order to obtain the sensitive tooth dentin pattern. In our product application protocol, the dentin samples were brushed using a tooth brushing machine and this might preclude the probability of applying different forces on the manual toothbrush handle. One limitation of our study was that the dentine sample was obtained by cutting the tooth crown in the middle third. DH usually involves exposure of the cervical dentinal tubules, which have a number, diameter, and orientation that are different from those of the middle part of the crown. Therefore, the deposition pattern of the crystals might be different in vivo compared to our experiment. Another limitation was the evaluation of the toothpaste's action in the absence of any acidic attack between tooth brushing, which normally occurs in the oral cavity. Furthermore, the samples were stored in artificial saliva between tooth brushing sessions, but neither buffer effect of saliva nor enzyme activity were simulated. Further clinical studies are needed to evaluate the action of commercial n-HAp toothpaste on DH treatment.