The efficacy of desensitizers used in DH management is tested in vitro, in situ or in clinical trials. A well-established laboratory study eliminates clinically uncontrollable factors such as the subjective nature of pain perception, spontaneous remission of DH symptoms, placebo and hawthorne effects [1, 8].
The measurement of hydraulic conductance was reported to be a suitable method for evaluating DP. This method evaluates the dentin tubule occlusion efficacy of desensitizers [6, 17]. It also allows for a comparison between different treatment protocols by producing objective and quantitative values . In our study, we used the CFFM [15, 16] system to test five different dentin desensitizers. It was constituted on the basis of the conventional hydraulic conductance system [17, 19] and the reliability of it was also previously reported .
Dentin discs obtained from human or animal teeth are commonly used for in vitro DP and SEM evaluations of the desensitizers [21-23]. In our study, samples were obtained from bovine dentin around the CEJ which had been reported to be an appropriate substitute for human coronal dentin in terms of permeability characteristics . Standardization and stabilization of the exposed dentin area for DP evaluation were achieved by using O-rings in a split chamber system . Thereby, we evaluated the same area of the same specimen each time to ensure reproducibility of the measurements.
The removal of the smear layer was necessary to simulate the sensitive dentin with dentin tubules patent to the oral cavity and the pulp . EDTA application caused an increase in permeability values of all specimens by removing smear layer and presented the Lpmax. The Lp3 values measured after the desensitizer treatments were significantly lower than Lpmax in all groups. This proved the immediate efficacy of all tested desensitizers in reducing DP. The immediate permeability reduction efficacy of the GSE treatment was significantly superior to the NPP treatment (p<0.05).
Desensitizer treated dentin discs were subjected to CA or AS post-treatments in order to test the resistance of the deposits or layers to the acidic environment or to a wash-out effect that can be encountered in the mouth. Lp4 and Lp5 values measured following post-treatments were significantly greater than Lp3 values in all groups. There were no significant differences (p>0.05) among CA subgroups with regard to their resistance to the CA post-treatment. However, in terms of their durability against AS post-treatment, DSC was superior to BIF (p<0.05) (Table 5).
Gluma Desensitizer Powergel contains 35% HEMA and 5% glutaraldehyde which coagulates the serum albumin in dentinal fluid. The reaction of glutaraldehyde with albumin induces polymerization of HEMA. [26-32] Since dentin desensitization mechanism of glutaraldehyde-based desensitizers depends on protein precipitation, simulating the dentinal fluid in vitro is necessary [28-30]. In order to simulate the protein-containing dentinal fluid, samples in this group were immersed in 2% bovine serum albumin solution (BSA) before GPG treatment. In the current study, BSA acted as an artificial dentin fluid, in accordance with some other studies [28, 29].
SEM images (Fig. 1) proved that post-treatment procedures removed most of the surface layer that was formed as a result of HEMA polymerization, possibly due to the water solubility of HEMA [31, 32]. Transverse septas formed within some tubules as a result of protein precipitation were detected even after post-treatments. This may explain why permeability value did not return to maximum level (100%).
In our in vitro model, we expected the BSA to infiltrate within all tubules completely in order to enable the protein precipitation. However, some empty tubule lumens with no septa formations we observed probaply means that this aim could not be achieved.
Bifluorid 12 is a fluoride varnish containing 6% calcium fluoride (CaF2) and 6% sodium fluoride (NaF). Its mode of action is to create a barrier on the dentin surface by precipitation of CaF2 and NaF [33, 34]. In this study the resinous layer lost its continuity and therefore its quality as a barrier after post-treatments (Fig. 2) and caused a re-increase in permeability.
Gluma Self Etch Bond is a one-step, HEMA containing 7th generation self-etching (SE) dentin adhesive. Since adhesion mechanism of SE adhesives involves integrating the smear into the adhesive interface [35, 36], we created a new smear layer on the surface of each dentin discs in this group, prior to GSE application. Topical application of SE adhesives was reported to be effective in reducing DP by sealing the dentin surface with an acid-resistant hybrid layer and by occluding the tubule orifices with resin tags [34, 37], which was also demonstrated in our study. Porosity of the surface hybrid layer caused by post-treatments explains the re-increase in permeability. However, resin tags that were still present within the tubules even after the post-treatments might explain why permeability value did not return to maximum value (Fig. 3).
D/Sense Crystal is a desensitizer containing 2.5% potassium binoxalate and 2.5% nitric acid in water. DSC was reported to have a dual action in managing DH by reacting with dentin and triggering the precipitation of insoluble calcium oxalate crystals which cover the surface. It has also been reported to reduce nerve excitability via soluble potassium ions penetrating within tubules .
In this study, the dentin surface appeared to be covered with a crystalline layer, even after both post-treatments (Fig. 4). Considering this, lower permeability values might be expected. However, it is important to note that the hydrostatic pressure we utilised (1835.49 cm H2O) was much higher than the physiological pulpal pressure (15 cm H2O) . This might have facilitated the removal of crystalline deposits from the surface during permeability measurements, thereby leading to a greater Lp value than we would have obtained if we had chosen a hydrostatic pressure value close to the physiological pulp pressure. On the contrary to the permeability evaluation, SEM analyses were conducted without simulating pulpal pressure and no hydrostatic pressure was applied on samples.
Our study is consistent with several in vitro studies [10, 14] in terms of reporting the immediate efficacy of DSC treatment in reducing the permeability. Various studies [10, 14, 39-41] with different experimental designs also reported the solubility of the calcium oxalate crystals, as well as the re-increase in permeability after AS post-treatment over time.
Nupro Sensodyne Prophylaxis Paste with novamin contains 15% novamin as an active ingredient. Novamin is a trade name for calcium sodium phosphosilicate bioactive glass. It is incorporated into dentifrices and in-office products as a remineralizing agent . Novamin-based remineralizing products are reported to reduce DH by physically occluding dentin tubules with hydroxycarbonate apatite (HCA) and with residual novamin particles .
In our study, following the post-treatments, some of the apatite-like surface deposits were removed, thereby an increase in Lp occurred. However, some reduction in the diameter of tubule lumens was maintained even after the post-treatments (Fig. 5) and this may explain why the permeability value did not reach to maximum.
The results of this study are compatible with several studies [11, 44, 45] reporting the decrease in permeability following novamin-based treatment and some other studies [11, 44] demonstrating the re-increase in permeability following CA post-treatment.
Novamin-based bioactive glasses are assumed to react in an aqueous environment (ie. saliva), resulting in HCA deposition [12, 42, 43]. In the current study, we did not detect a remineralization effect of the NPP treatment following AS post-treatment for 24 hours. In addition, the wash-away effect of AS was superior to a possible remineralization mechanism. This finding of our study was comparable with the results of several investigations that studied novamin-based products and did not detect dentin remineralization after 24 or 48 hours of storage in RSS solution  or immersion in AS  for 24 hours. It has been suggested that 24 hours of storage in AS might not be sufficient to test for possible remineralization .
Recently, several investigators indicated that bioglass in dentrifices causes desensitization by physically occluding the tubules rather than by enhancing remineralization.  It was also noted that conventional bioglasses (e.g. novamin) incorporated into dentifrices cause HCA deposition. However, fluoride containing novel bioglass components are able to form a more stable and acid-resistant fluoroapatite layer on tooth surfaces and may give better results in DH management .
Dentin permeability evaluation allows for a comparison between different treatment protocols by producing objective, quantitative values. However, a direct comparison of the quantitative data from different studies might not be possible because of the differences in methodologies e.g. differences in samples, treatment and post-treatment protocols, hydrostatic pressure values and artificial dentinal fluids.
All the tested desensitizers were effective in reducing DP by covering or occluding dentin tubules. From a clinical standpoint, it is important that a dental clinician should be aware of the post-treatment durability of the desensitizers to be used and consider the need to recall the patient and repeat the treatment. In the current study, CA and AS post-treatments caused some reduction in the efficacy of all desensitizers by partly removing the deposits or layers formed on/within the tubules and caused a re-increase in permeability.
In our study, we evaluated the efficacy and durability of five different desensitizers for their tubule occluding and permeability reducing abilities. In cases where they possessed another mode of action in DH management (e.g. changing nerve excitability), these properties were not evaluated.
Research into the development of desensitizer treatment modalities more resistant to the oral environment may eventually contribute to more prolonged patient relief from discomfort associated with DH.
CONFLICT OF INTEREST
The authors confirm that this article content has no conflict of interest.
This study was supported by The Scientific and Technological Research
Council of Turkey (project number 114S019). The authors also thank Heraus Kulzer
TURKEY for donating the Gluma Desensitizer Power Gel, Dentsply UK for donating
the Nupro Sensodyne Prophylaxis Paste with novamin, Dr. Sevgi CANBAZ
(Ondokuz Mayis University) for the statistical analyses and Gregory T. Sullivan
(GPEM, University of Queensland, Brisbane, Australia) for editing the English in an
earlier version of this manuscript.