Influences of desensitizing agents on bond strength of etch-and-rinse and self-etch adhesive system to dentin.

BACKGROUND: Cervical lesions requiring adhesive restoration may present with dentin hypersensitivity, but the effect of desensitizer treatment on microtensile bond strength (MBS) is inconclusive. AIM: The aim of the present study was to assess the influence of two types of dentin desensitizing treatment on bond strength to dentin.
MATERIALS AND METHODS: Thirty-six extracted human mandibular first premolars were used to obtain superficial buccal cervical dentin. The samples were assigned to two groups (18/group): etch-and-rinse (ERS) (Optibond® Solo) and self-etch (Palfique) adhesive systems. Eighteen teeth in each group were randomly assigned to three desensitizer treatments (6/treatment); control (no desensitization), Gluma™ desensitizerand Novamin-containing toothpaste (Sensodyne repair and protect™). Bonded dentin-composite specimens were stored in artificial saliva for 2 weeks prior to scanning electron microscope (SEM) evaluation and MBS testing. Univariate analysis of variance and Tukey tests were performed (α = 0.05) for the data analysis.
RESULTS: The mean MBS with self-etch system (SES) was significantly higher than ERS system, irrespective of the dentin desensitizer regimen. SEM showed no hybrid layer or resin tag formation in the SES group. Tubular occlusion was observed in all specimens treated with desensitizer.
CONCLUSION: Mild self-etch adhesive can be used on dentin treated with Gluma or bioglass-containing desensitizer without compromising the bond strength. CLINICAL RELEVANCE: Dentin desensitizer such as in-office Gluma or at-home Novamin-containing toothpaste could be used without compromising the bond strength of mild self-etch adhesive systems. Copyright:

INTRODUCTION

Dentin hypersensitivity (DH) is characterized by short, sharp, pain, arising from exposed tooth dentin in response to external stimuli (thermal, tactile, osmotic, evaporative, or chemical) that cannot be attributed to any other dental defect or pathology.[1] Exposure of dentin is closely related with DH.[2] Management of DH can be invasive (gingival surgery, pulpectomy, application of resin, and laser) or noninvasive (topical agent application and dentifrices containing desensitizing ingredients).[3] Noninvasive management options are preferred as the first line of treatment for DH for majority of patients because it is simple and cost-effective.[45] Patient-applied topical agents used for DH contain fluoride, oxalate, potassium nitrate, or nanohydroxyapatite to occlude the dentinal tubules to relief DH.[4] A commonly used professionally applied desensitizer is the Gluma™ (Kulzer GmbH, Germany), which contains 5% glutaraldehyde and 35% hydroxyethylmethacrylate (HEMA). On the application of Gluma to exposed dentin surface, the two aldehyde groups of glutaraldehyde interlace themselves with amino groups of dentin collagen, leading to fixing of proteins and occlusion of the tubules.[67]

Adhesive restorations may be performed following dentin desensitizing treatment, but the effect of the desensitizer treatment on dentin bond strength is inconclusive.[3] A systematic review concluded that the important factor influencing retention of noncarious cervical lesion (NCCL) restorations is the type of adhesive used (etch- and-rinse [ERS] or self-etch), despite the fact that the observation leading to this conclusion is elusive.[8] Earlier investigations have reported that dentin desensitizing treatment adversely affects self-etch primer bond strength.[39] Contrary to these conclusions, another investigation reported no adverse effect on bond strength after dentin desensitizing agent application.[10] ERS type of adhesive system has been observed to show reduced bond strength after desensitizer application.[11]

With these diverse reports in the literature, the present study was proposed with the primary aim of investigating the influence of two types of dentin desensitizing treatment, (1) an in-office method, Gluma™ (Kulzer GmbH, Germany) desensitizer application and (2) at-home NovaMin-containing toothpaste (Sensodyne repair and protect®; GlaxoSmithKline, India), on microtensile dentin bond strength of ERS and self-etch system (SES) adhesive systems. Our null hypothesis was that there was no significant difference in bond strength between the two adhesive systems and among the desensitizing treatments.

MATERIALS AND METHODS

After obtaining Institutional Ethical Committee approval (CSICDSR/IEC/0035/2017), 36 freshly extracted intact mandibular first premolar teeth were used in this study. The teeth were examined under an operating microscope (Labomed Prima, USA) for any defects. Teeth with cracks, previous restorations, and caries were excluded from the investigation. Selected teeth were cleaned and stored in 0.5% chloramine T trihydrate (Sigma Aldrich, Bengaluru, India) prior to the experimental procedure. The teeth were randomly assigned to two experimental groups of 18 teeth each: ERS or self-etch adhesive systems.

Roots of the teeth were cut off using a slow-speed diamond disc at the cement–enamel junction. Coronal portion of the teeth were sectioned into buccal and lingual halves along the central groove using a water-cooled diamond disc. Buccal half of all the teeth were utilized for the experimental procedure. Using a standard-grit diamond rotary cutting instrument (102–125 μm, TF-11, Mani Dia-Burs, Mani Co., Tochigi, Japan), the cervical region of the buccal surface of each sample was ground flat to expose a dentin surface of approximately 4 mm × 4 mm. The exposed dentin surface was finished with a fine-grit diamond rotary cutting instrument (20–30 μm, TF-11, Mani-Dia-Burs, Mani Co., Tochigi, Japan).

The 18 samples assigned to each adhesive system were further randomly assigned to three desensitizing treatment subgroups (A, B, and C) with six samples per subgroup [Figure 1]. The exposed cervical dentin surfaces on samples in both ERS (Kerr Optibond™ Solo, Kerr Corp., CA, USA) and self-etch (Palfique, Tokuyama Dental Corp., Tokyo, Japan) adhesive systems were treated with desensitizing agents as follows:

Figure 1

Flowchart showing the group allotment

Subgroup A: No dentin treatment was done; this subgroup served as the control

Subgroup B: Gluma® desensitizer (Kulzer GmbH, Hanau, Germany) was applied to the dentin surface using micro-applicator tips with a gentle rubbing motion. The dentin surface was allowed to dry for 30 s

Subgroup C: NovaMin-containing (Sensodyne repair and protect™, GlaxoSmithKline, Mumbai, India) desensitizing toothpaste (approximately 2 cm) was applied to the dentin surface with a soft toothbrush with rubbing motion for 1 min.

Following the above desensitizing treatment, the sample was rinsed carefully with distilled water. The desensitizing treatment was performed two times each day with a 12-h interval between treatments for 2 weeks, with a total of 28 applications per sample. The samples were kept in artificial saliva at room temperature during nontreatment periods over this 2 week period. Subsequently, all specimens from the three subgroups were stored in artificial saliva (Wet Mouth, ICPA Health Products Ltd, Mumbai, India) at room temperature for an additional 2 weeks before the bonding procedure.

Bonding procedure

ERS system: Optibond™ Solo (Kerr, CA, USA) bonding agent was used according to the manufacturers' recommendation. Briefly, the exposed cervical dentin was etched with 37.5% phosphoric acid gel (Kerr, CA, USA) for 15 s followed by rinsing water with the aid of the dental air–water syringe. Following slight drying of the dentin, the bonding agent was applied with microbrush for 15 s using light brushing motion, air thin for 3 s, and then, light cured (Woodpecker LED D unit, Guilin Woodpecker, Guangzhou, China) for 20 s. Following this, the exposed dentin (4 mm × 4 mm) was restored with 4-mm thickness of bulk-fill composite (Tetric N Ceram, Ivoclar Vivadent, Liechtenstein).

Self-etch adhesive system: Palfique™ (Tokuyama Dental Corp., Tokyo, Japan) bonding agent application was performed as per the manufacturers' recommendation. Briefly, the bonding agent was applied over the dentin surface with a disposable microbrush applicator and waited for 10 s before it was mildly air-dried for 5 s and then light-cured for 10 s. Following this procedure, the exposed dentin was restored with a 4-mm thickness of bulk-fill composite as with the ERS system.

Scanning electron microscope examination

Following restoration, all samples from both the groups were stored in artificial saliva for 2 weeks at room temperature before bond strength testing. Then, one sample was randomly selected from each of the six subgroups for scanning electron microscope (SEM) analysis. Each of the selected samples was sectioned into two by a cut through the middle of and perpendicular to the surface of the 4-mm thick composite restoration. The sectioned surface of each sample was hand polished. Following polishing, the samples were decalcified by immersion in 6 M hydrochloric acid to demineralize the dentin tissue that was not infiltrated by a resin material. This procedure was performed to increase the contrast between resin infiltrated and noninfiltrated regions. Subsequently, the samples were immersed in 10% sodium hypochlorite to remove the exposed collagen in the demineralized region. The sectioned surface of each sample was air dried, gold sputtered, and examined with SEM (TESCAN VEGA3, Brno-Kohoutovice, Czech Republic) at 8.0 kV with ×2000 magnification for changes in the dentin surface consequent to desensitizer application and bonding agent interaction.

Microtensile bond strength evaluation

Using a slow-speed diamond disc under water coolant, each of the remaining 30 restored teeth (5 samples/subgroup) was sectioned so as to produce composite-dentin block approximately 1 mm width × 1 mm thickness × 6 mm length in dimension (i.e., with an adhesive surface area of 1 mm2) for microtensile bond strength (MBS) testing [Figure 2a]. Dimensions of the specimens were checked using digital Vernier caliper. Using cyanoacrylate (Henkel Adhesives Technologies Pvt Ltd, Mumbai, India), the rectangular blocks were glued to Gripit cable tie (white zip self-locking nylon wire tag) [Figure 2b]. After half an hour, the samples were tested in universal testing machine of 100 N at crosshead speed of 0.5 mm/min [Figure 2c]. MBS was expressed in MPa.

Figure 2

(a) Dentin-composite sticks prepared for microtensile bond strength testing. (b) Dentin-composite sticks glued to Gripit cable tie for loading in universal testing machine. (c) Dentin-composite sticks mounted in universal testing machine (Tinius Olsen India Pvt Ltd, Noida, India)

Statistical analysis

The results were analyzed using univariate analysis of variance (ANOVA) and Tukey tests at a preset α of 0.05. Statistical analysis was performed using SPSS version 23.0 (IBM Corp., Washington, USA).

RESULTS

Comparing the subgroups treated with similar dentin desensitizer from the two adhesive groups, univariate ANOVA analysis revealed a statistically significant difference (P < 0.001) in MBS between the two adhesive systems [Table 1]. There was no statistically significant difference in MBS among the three dentin desensitizer treatment methods (subgroups) within each adhesive group (Tukey test). Pulling together the data from the three subgroups in one adhesive group (ERS or self-etch adhesive), the mean MBS with ERS (10.40 ± 2.01) adhesive system was significantly (P < 0.001) lower than that observed with the self-etch (13.70 ± 2.85) adhesive system.

Table 1

Mean microtensile bond strength for two different adhesive system groups in three different dentin desensitizer treatment regimen

Etch type	Group mean MBS (MPa)*	Dentin desensitizer treatment	Subgroups mean MBS (MPa)
			Mean*	SD	Minimum	Maximum
Etch-and-rinse	10.40±2.01c	Control	9.52a	0.99	8.32	10.72
		Gluma	9.13a	1.47	7.00	10.52
		Desensitizing toothpaste	12.56a	1.52	10.58	14.40
Self-etch	13.70±2.85d	Control	13.27b	3.39	7.28	15.40
		Gluma	14.24b	3.44	9.80	18.42
		Desensitizing toothpaste	13.59b	2.15	11.40	16.26

*Different superscript alphabets indicate statistically significant difference (Univariate ANOVA analysis), *Same superscript alphabets denote no statistical difference. MSB: Microtensile bond strength, SD: Standard deviation

SEM images showed contrasting details for the two adhesive groups and the three desensitizer subgroups Figures 3 and 4. In all microstructure images (from both adhesive groups), it could be clearly seen that dentin treated with desensitizer application has occluded tubules, unlike the untreated (control) samples. In those samples treated with Gluma desensitizer, dentin tubules were occluded with amorphous-like material, while in those treated with NovaMin™ toothpaste, tubular occlusion was by mineral deposits. In ERS adhesive group, a clear hybrid layer formation is visible only in the control subgroup, while resin tags were visible without the presence of a hybrid layer in the desensitizer-treated subgroups [Figure 3]. Images from the desensitizer and control subgroups of the self-etch adhesive groups did not display any resin tag formation [Figure 4]

Figure 3

Scanning electron microscope images of etch-and-rinse adhesive (optibond solo) agent applied dentin. (a) Control dentin. (b) Gluma applied dentin. (c) NovaMin-containing toothpaste applied dentin. Arrows point to hybrid layer mark

Figure 4

Scanning electron microscope images of self-etch adhesive (Palfique bond) agent applied dentin. (a) Control dentin. (b) Gluma-applied dentin. (c) NovaMin toothpaste-applied dentin. Arrows point to hybrid layer mark

DISCUSSION

NCCLs requiring adhesive restoration may present with DH that needs treating before restoration; however, the effect of desensitizer treatment on dentin bond strength is inconclusive. Thus, the present study investigated the influence of two types of dentin desensitizing treatment on MBS to dentin. The current observation of the ERS adhesive system significantly reducing the MBS when compared to the SES rejects the null hypothesis that there was no statistically significant difference in bond strength between the two adhesive systems. On the contrary, the dentin desensitizing treatment regimen not influencing the MBS accepts the null hypothesis that there was no statistically significant difference in bond strength among the desensitizing treatments. The result of the present study is in agreement with previous investigation by Sabatini and Wu.[10] Palfique bonding agent has a pH of about 2.3 categorized under “mild” self-etch adhesive. SEM images of self-etch adhesive system in the present investigation agreed with earlier observations of minimal resin tag and hybrid layer formation with self-etch adhesive system.[12] The exhibition of higher bond strength by self-etch adhesive system than ERS system, as observed in the present study, is in contrast to earlier studies.[12] van Meerbeek et al.[12] reported that if self-etch adhesive systems were developed with functional monomers tailored to have good chemical bonding, this will guarantee almost durable dentin bonding. However, a review of adhesive strategies for NCCL restoration concluded that no one adhesive system is superior to another.[13] To the best of the authors' knowledge, no previous MBS evaluation with Palfique bond is available in the literature. The results of the present study could not be compared with any previous study due to a lack of studies comparing different types of adhesive systems based on their bond strength after treating the dentin surface with a desensitizer.[8] Manufacturer of Palfique bond claimed that the improved three-dimensional (3D)-SR functional monomer has more number of groups interacting with calcium groups and enhanced 3D cross-linking, thus this may be the reason for the higher bond strength.

The observation that Gluma desensitizer did not affect the dentin bond strength is in agreement with the report by Sabatini and Wu.[10] It is believed that this phenomenon is due to the water and hydroxyethyl methacrylate (HEMA) content in Gluma, both of which have been shown to aid the resin diffusion into the partially demineralized dentin and increase the resin–dentin bond strength.[8141516] Furthermore, infiltration of Gluma into dentin has been reported to improve mechanical properties which can minimize the resin–dentin bond degradation.[10] SEM images in the present study showing amorphous tubule occlusion in Gluma treated dentin may be attributed to the interlacing of the two aldehyde groups of glutaraldehyde with the amino groups of dentin collagen, leading to fixing of proteins and occlusion of the tubules.[67]

NovaMin®-containing toothpaste has been previously shown not to hinder but rather improve dentin bonding with either ERS or the self-etch adhesive systems.[17181920] The bioglass particles in NovaMin-containing toothpaste have been demonstrated to get deposited on dentinal surfaces, forming hydroxyapatite crystals that are chemically similar to mineral structure found in dentin.[21] These calcium-containing mineral deposits may have aided in bonding mechanism for both the adhesive systems tested in the present study. SEM images of samples treated with NovaMin™ toothpaste in both the adhesive groups display dentinal tubules occluded with mineral deposits. These mineral deposits may have been the NovaMin particles. Potassium nitrate-containing paste when applied to the dentin surface prior to bonding had shown inferior bond strength.[22] The demonstration of significantly higher bond strength by the self-etch adhesive system than the ERS is in agreement with a previous report on self-etch adhesive bonding agent used for dual-cure resin-based cement.[17] Palfique bonding agent evaluated in our study with a pH ≈ 2.3 can be considered mild and its improved performance is in agreement with earlier observations with mild self-etch adhesives in desensitizer-treated dentin.[101923]

SEM images displayed intact intra-tubular occlusion in desensitizer-treated dentin in both the adhesive groups. This reinforces the concept that postoperative sensitivity encountered following adhesive restoration could be controlled with desensitizing agent application without adversely affecting the bond strength.[1823] Future research on the aging effects and bond degradation of desensitizer-treated self-etch adhesive-bonded restorations needs to be evaluated.

CONCLUSION

Within the limit of the present study, it can be concluded that “mild” self-etch adhesive can be used on dentin surface treated with Gluma or bioglass-containing desensitizer without compromising the bond strength.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.