The most widely accepted theory for DH is the hydrodynamic theory proposed by Brännström. This theory postulates that temperature, physical or osmotic changes can disturb the fluids within the dentinal tubules stimulating the baroreceptor on the plasmalemma of the odontoblasts which leads to neural discharge [3].
Different systems have been proposed in order to block or reduce the DH symptomatology. Some of these act by producing occlusion of the dentinal tubule either by precipitation of calcium salts [11, 12] or by coagulation of proteins contained in dentine fluid[13], others act by causing depolarization of the membrane of the nerve endings reducing the nerve transmission of pain[14].
Finally, the ability of different types of lasers in sealing patent dentinal tubules have been evaluated, but the studies carried out on which laser achieves the best outcome for the treatment of DH showed discordant evidences which could be explained considering several factors such as: types of lasers, fibers, power outcome, exposure time and radiation frequency[14].
Several studies have showed the efficiency of diode laser [8, 15–18] and of its different wavelengths (780, 900 and 810 nm) in the treatment of DH [19].
In the present study we have appraised the effectiveness of three different treatments based on the use of diode laser (LLLT, HPLT and combined LHLT) focusing on the different percentage of pain reduction and on the persistence of the effect over time.
Our results underline that more immediate improvements, obtained with a single output power, are not long-lasting while others that can be achieved in a longer lass of time are more persistent [9, 20].
The effectiveness of each laser therapy was gauged by estimating the improvement of DH symptomatology on 3 different NCCLs comparing the mean VAS score obtained at the end of treatment (t1), after 15 days (t2) and after 3 months (t3) with the mean VAS values at baseline (t0).
A clear improvement, both in the short and long term from therapy, has been documented for all the NCCls in all study groups with the best results obtained in SG-3 (LHLT). In the short term, immediately after the end of treatment (t1), the HPLT and the LHLT showed the same improvement of DH symptomatology compared to the LLLT (59.29% and 60,12
% vs 49.38%). In fact, no significant difference was estimated between the mean VAS score of SG2 (HPLT) and SG3 (LHLT) at time t1, while both averages resulted statistically different from the mean VAS score of SG1 (LLLT) for the same time point. In the long term, after 15 days or after 3 months from the end of treatment, the effectiveness of the LHLT, applied in the SG3, was still greater (56.44% and 50.62% respectively) than the laser treatments based on the use of a single output power (LLLT and HLT), applied in the SG1 and SG2 respectively. This result was supported by the evidence that the averages of VAS scores both for comparison SG1 vs SG3 and SG2 vs SG3 were statistically different.
The rationale behind the use of two different outcome powers of the diode laser is based on the different interaction that each intensity of laser has with the tissues: photo-thermo effect with thermodynamic behavior of tissue water content and bio-stimulant effects on dental tissues.
In the HPLT (SG2 group) the lasers with high power can improve the symptomatology of DH mainly with the photo- thermal effect that melts the hard tissue on the surface with the smear layer occluding the tubules. In this way the laser blocks the movement of the fluids in the dentinal tubules, which is the fundament of the hydrodynamic theory of DH pain. The studies of Lopes and Aranha demonstrate that the sealing of dentinal tubules, obtained with the use of high-power laser, represents an effective treatment strategy in reducing dentine hypersensitivity [21]. The photo-thermal effect is reported by Liu et al. also for the Nd:YAG laser which allows to have a sealing depth of approximately 4 microns due to the melting of dentin and the closure of exposed dentinal tubules[22]. The short-term efficacy of high-power laser can be precisely correlated to the rapidity with which the physical-thermal effect closes the patent dentinal tubules, but this haste can also expose to an early relapse and a failure of the therapy if the causal factors persist (i.e.: mechanical abrasion or microbial action) which could induce a possible loss of superficial layer of dentine[22].
In the LLLT (SG1 group) the desensitizing action of the low-power laser on DH compared to action of desensitizing paste containing 8% arginine-calcium carbonate has been demonstrated by Bal et al [23].
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. Whitters et al. and Femiano et al. (2018) verified the analgesic effect of low-power using either a Nd:YAG laser (1.1 Watt) or a diode laser (0.1 Watt) during routine dental restoration procedures for carious lesions with the recovery to baseline values of pain threshold after 60 min[24, 25].
This output power could show an analgesic effect on the pulp tissue due to the possible depolarizing effect of C- and Aδ- nervous fibers [14] and an anti-inflammatory effect due to the bio-stimulation of the pulp tissue that could activate the odontoblasts to counter with the production of an irregular secondary dentine. The action on the inner layer of dentine with the activation of odontoblasts could explain the slower action of low-power laser on pain reduction associated with DH and, at the same time, the longer-lasting effect compared with the action of high power laser that is restricted to the external surface of dentine[26].
In the LHLT (SG3 group), a combined and definite sequence of two different output powers of diode laser was used on the NCCLs associated with DH. This approach dictated the use a low power first and subsequently a high-power set in order to have a prompt improvement of pain that also lasts over time. The rationale of this clinical evidence is based on the hypothesis that the mechanical effect, induced by 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 in the internal side of the tubules and
elicit the production of secondary dentine. On the other hand, the sealing of the external side of dentinal tubules, induced by mechanical action, consents to have 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 high-power laser before the bio-stimulation of the internal side of the dentine is achieved, it could create a barrier that prevents the low power laser from reaching the most inner layer and thus activating the odontoblasts.