## 4 Discussion
The studies of [PERSON] and [PERSON] (2013) demonstrate that the sealing of denitnal tubules, obtained with the use of a high-power laser, represents an effective treatment strategy in reducing DH [26]. A photo-thermal effect was also reported by [PERSON] et al. for the Nd:YAG laser, which allows a sealing depth of approximately 4 microns due to the melting of dentin and the closure of exposed denitnal tubules [26]. The short-term efficacy of a high-power laser can be precisely correlated to the rapidity with which the physical-thermal effect closes the patent denitnal tubules, but this haste can also cause early relapse and therapy failure if the causal factors persist (i.e., mechanical abrasion or microbial action), which could induce a possible loss of the superficial layer of dentin [20; 27]. In LLLT, the desensitizing action of the low-power laser on DH was compared with the action of desensitizing paste containing 8% arginine-calcium carbonate by [PERSON] et al. (2015) [28]. The efficiency of the low-power laser seems to be greater when lasers with high-tissue penetration are used, such as the 810 nm diode laser and Nd:YAG. [PERSON] et al. (1995) and [PERSON] et al. (2018) verified the analgesic effect of low power using either an Nd:YAG laser (\(1.1\) W) or a diode laser (\(0.1\) W) during routine dental restoration procedures for carious lesions with recovery to baseline pain threshold values after 60 min [29; 30]. This output power showed an analgesic effect on the pulp tissue due to the possible depolarizing effect of C and A\(\delta\) nerve fibers [31] and an anti-inflammatory effect due to the bio-stimulation of the pulp tissue that activated the odontoblasts to counter the production of irregular secondary dentin. The action on the inner layer of dentin with the activation of odontoblasts can explain the slower action of the low-power laser on pain reduction associated with DH and, at the same time, the longer-lasting effect compared with the action of a high-power laser that is restricted to the external surface of dentin [32]. In LHLT, a combined and definite sequence of two different output powers of diode lasers was used on the NCCLs associated with DH. This approach dictated the use of low power first and subsequently high power in order to have a prompt improvement in pain that lasts over time. The rationale for this approach is based on the hypothesis that the mechanical effect, induced by a high-power laser, creates a barrier on the external side of the tubules, which protects the internal environment. In this way, the odontoblasts, already activated by the biological effect and induced by the low-power laser, have a greater chance of organizing the defense on the internal side of the tubules and eliciting the production of secondary dentin. Moreover, the sealing of the external side of dentinal tubules, induced by mechanical action, leads to an immediate improvement of the DH symptomatology. The opposite sequence of the use of output powers should not have the same efficiency in the long term, because, if the external sealing is induced by a high-power laser before bio-stimulation of the internal side of the dentin is achieved, it could create a barrier that prevents the low-power laser from reaching the most inner layer and thus activating the odontoblasts.