## 1 Introduction  
Dentin hypersensitivity (DH) was described in 2009 by the FDI World Dental Federation as a \"short sharp pain arising from exposed dentin most commonly at the tooth cervical area in response to stimuli (typically thermal, evaporative, tactile, osmotic or chemical), but which cannot be ascribed to any other dental defects, diseases or restorative treatments\" [1]. Increased global average life expectancy and increased number of teeth because the population is now aware of the benefits of prevention and the importance of good oral hygiene [2, 3] led to increased prevalence of DH, which commonly can range up to 74% [4, 5].  
Different theories were proposed throughout the years to explain the pathogenesis of DH, the hydrodynamic theory of Brannstrom being widely accepted nowadays. This theory postulates that external stimuli can rapidly dislocate the contents of dental tubules, distorting the nerve at pulp or predentin level or damaging the odontoblast cells and ultimately causing pain [2, 6, 7, 8, 9]. This is the explanation for two treatment strategies used in practice: denntinal tubule occlusion and nerve excitation prevention [10]. The agents in the first category determine the formation of an intratubular precipitate or the formation of an artificial smear layer over the dentin that blocks the entry into the tubules. Deposition of the thin coating layer determines the occlusion of the tubules, but also remineralization of the exposed dentin surface [11]. However, none of the agents in this category have been considered as a gold standard for denntinal tubule occlusion [12]. Various products are available on the market to be used either by the patients at home or by the dentists in the office, but the most common treatment of DH is the use of dentifrices [13; 14].  
Recommended toothpastes for DH contain different active ingredients such as: fluoride, calcium oxalate, calcium carbonate-phosphate, strontium acetate, arginine, and hydroxyapatite (HAp) [13; 15; 16; 17]. HAp is the major inorganic component of natural teeth and bone and previous studies showed that nano-sized particles have similarities in morphology and structure to the tooth enamel apatite crystals [18; 19]. Some studies and reviews have mentioned the capacity of this biofunctional material to remineralize enamel and dentin [19; 20; 21; 22]. Nano-hydroxyapatite (n-HAp) was considered a promising active ingredient used for the treatment of DH due to high biocompatibility and bioactivity [23; 24; 25]. In toothpastes, HAp was included in the form of nanocrystals because they dissolve easier in this form [19; 20; 26]. Crystals of n-HAp included in dental products have a dimension of 50-1000 nm, which enables them to act like fillers [27]. These products can penetrate and block the exposed denntinal tubules which are responsible for DH. There is no common opinion in the literature regarding the efficiency of n-HAp based products used in DH treatment. Some studies have demonstrated higher desensitizing action and dentin remineralizing potential of n-HAp desensitizing agents [28; 29], while other clinical studies have shown an equivalent result of n-HAp to other desensitizing agents [30].  
This study aims to compare the effectiveness of three different commercial n-HAp toothpastes on denntinal tubule occlusion and on dentin mineral deposition by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. The null hypothesis was that there is no significant difference regarding tubule occlusion between the three n-HAp denitrices.