The effect of ultrasonic agitation on enamel bond strength of universal adhesives with light-cure versus self-cure activation: an in vitro comparative study

INTRODUCTION

Adhesive dentistry aims to create an effective bonding between resin restorations and teeth’ hard structures, so that they ensure optimal retention, minimize micro-leakage, provide excellent color stability, and warrant life-time clinical durability [1]. The application process has evolved from multi-step systems to simplified methods, with the all-in-one self-etch adhesive being the most simplified system, combining all components in one bottle [2]. In total-etch adhesive systems, the smear layer is dissolved with phosphoric acid and washed away during the rinsing step. While self-etching (SE) adhesive systems use acidic primers to modify or solubilize the smear layer, allowing direct adhesive interaction with the tooth substrate [3], which prevents teeth from becoming hypersensitive after a restoration that occurs in etch-and-rinse approaches. During acid etching, washing, and drying, clinical accidental procedural mistakes are minimized by SE adhesives, ensuring a more comfortable and effective dental procedures [4].
The universal adhesive is the latest dental adhesive generation that can be employed in etch-and-rinse or self-etch modes. The optimal bonding strategy for better clinical outcomes is questionable, and clinicians should use various bonding techniques, according to patient’s preferences and clinical setting [5]. These adhesives contain silane and phosphoric monomers, permitting bonding to tooth structures, metal, and porcelain [6]. Moreover, most adhesive polymerization relies on photoinitiation and free radical production, providing clinician control [7]. However, light-cure adhesive systems can be problematic in areas with compromised light delivery. A small distance between the light source and adhesive resin can significantly reduce the light intensity [8]. Therefore, dual-cured adhesive systems were developed to cure in the absence of light, but the process starts with light activation and should continue with self-cured activators. Light deficiency leads to spontaneous and non-controllable radical generation, resulting in lower bond strength rates [9].
Recently, self-cured adhesives were developed to ensure secure polymerization in areas, where curing light is scarce, reducing the degree of conversion and compromising restoration’s endurance. These adhesive systems are universal two-component and one-stage methods, with their own polymerization initiator [10]. Some manufacturers have introduced self-curing universal adhesives with “no-wait” or “quick bonding” concepts, providing less technical sensitivity and simplified clinical procedures. However, these short application times can have negative consequences for the infiltration of monomials in the tooth structure and solvent evaporation [11]. Notwithstanding, dental adhesives face challenges in bonding to various substrates, and the stability of resin-bonded dentin and marginal sealing effectiveness depend on the bonding to surrounding enamel [12]. Resin-enamel bond occurs through acid etching and resin tags formation. Self-etch adhesives perform etching and priming simultaneously using acidic primers, with etching effect attributed to phosphoric acid or carboxylic acid esters, with higher pH levels. However, self-etch systems are less aggressive than etch-and-rinse systems [13].
Indeed, the SE mode of universal adhesives on enamel reduces bonding effectiveness. Selective phosphoric acid etching is suggested to bypass this restriction and significantly improve one-step SE adhesive performance [14]. On the other hand, unintended dentin etching can occur, especially in the preparation of tiny cavities or with low-viscosity etchants, resulting in poor quality of dentine hybridization [15]. Hence, applying universal adhesives actively in the SE mode can enhance enamel adhesive properties by improving the micro-mechanical interaction with underlying tissue, thus enhancing de-mineralization and bond strength [16].
Nevertheless, the active application of adhesives can be influenced by operator force, which can affect its effectiveness. Standardizing the procedure is crucial, but ultrasonic devices are considered superior due to their ability to reduce finger pressure variations and ensure homogeneous adhesive vibration. In addition, they do not require calibration and are less sensitive to operator experience, making them a more efficient method for adhesive application [17].

OBJECTIVES

The objectives of this study were to conduct a comparative evaluation of the impact of ultrasonic agitation on the micro-shear bond strength (μSBS) of a self-cure universal adhesive system to enamel versus a light-cure adhesive at different storage times (3 and 6 months). The first null hypothesis is that ultrasonic treatment will significantly improve the μSBS, while no significant differences will be observed in the μSBS between the self-cure and light-cure universal adhesive systems. The second null hypothesis is that the storage time will significantly reduce the micro-shear bond strength (μSBS) of both universal adhesive systems.

MATERIAL AND METHODS

Material

As indicated in Table 1, two types of universal adhesives and one nano-filled composite were employed in the present investigation.

Methods

Calculating sample size: Based on Muñoz et al. [17], the sample size for the current research was determined using G*Power test analysis program, resulting in a total of 24 samples. These samples were divided into 2 primary groups, with 12 samples in every group, based on the application method (manual or ultrasonic). Every group was further sub-divided into 2 sub-groups (n = 6), each based on the type of universal adhesive (self-cure or light-cure). Sample size was deemed adequate to identify a significant impact size (f = 0.8) using a two-sided hypothesis test’s intended power (1-β error) of 0.8 (80%) and significance level (α error) of 0.05 (5%). Since the research involved two storage periods (3 months and 6 months), specimens required to be duplicated, resulting in a total of 48 samples, with 6 specimens in each of the 8 sub-groups.

Teeth selection

A total of 16 recently extracted wisdom teeth were utilized in the current research. These molars were extracted from patients aged between 20 and 40 years at the Oral Surgery Clinic of Faculty of Dentistry, Al-Azhar University Cairo (Boys). Following extraction, any remaining tissue was removed by scraping off the teeth, molars were cleaned with tap water, and a 10× magnifying glass was used to check for cracks. The chosen teeth were placed in a glass jar filled with distilled water until further use.

Samples grouping

A total of 16 wisdom teeth were utilized in this study, resulting in 48 specimens (with 3 samples from each tooth). The samples were divided into 2 primary groups (n = 24) according to the type of application, i.e., group A with sonic application, and group B without sonic application. Furthermore, each primary group was sub-divided into 2 sub-groups (n = 12) according to adhesive’s curing mode. Sub-group S1 involved samples treated with a self-cure universal adhesive system, while sub-group S2 included samples treated with a light-cure system. Finally, each sub-group was split into two divisions (n = 6) according to the storage time. Division T1 represented the three-month storage time, while division T2 was the six-month storage time.

Enamel surface preparation

A specially fabricated Teflon mold (15 × 20 mm) was filled with self-curing acrylic resin. Each wisdom tooth was positioned upright in the mold and the highest point at the cemento-enamel junction, resulting in an occlusal surface extending over the mold’s surface. Following acrylic curing, wisdom molars were extracted from the mold. Occlusal surfaces of every wisdom tooth were grounded flat using a low-speed diamond disk, with sufficient water cooling mounted into IsoMet 4000 microsaw device (IsoMet 4000 Linear Precision Saw, Buhler, Germany) to obtain a flat enamel surface.

Mold fabrication for preparation of specimens

A specially fabricated aluminum mold was created using Autodesk Inventor Professional 2020 software in conjunction with a CNC Haas machine. The mold was tailored specifically for the research purposes. A combination of digital modeling software and precise machining allowed for an accurate fabrication of the mold to meet the desired specifications. This metal mold contained three main parts, including frame, cover, and condenser (Figure 1). The 3 mm-thick frame had 4 horizontal protrusions in its half, each measuring 1 mm in length. These protrusions enabled full sitting across the occlusal surface. The cover had five protrusions, which were 1 mm in length and 1.2 mm in width, spaced 2.5 mm apart. The resin composite was packed into the holes made in the silicon mold, using a specially fabricated condenser with a dimension of 1.2 mm. To account for sample deformation throughout examination or storage, four samples from every wisdom molar were acquired.

Fabrication of customized silicon sheets for specimens’ preparation

The 4 projections of frame were used to position the frame on the occlusal surfaces. Next, clear silicon was introduced into the frame, and the cover was applied under pressure to remove any spare silicon (Figure 2). Following material’s setting, the cover and frame were taken off to reveal a specially produced silicon sheet, with holes of 1.2 mm in width and 1 mm in thickness (Figure 3).

Adhesive systems applications

Manual application
The enamel occlusal surface was covered with a specially designed silicon sheet. The universal adhesive systems were applied in accordance with the manufacturer’s instructions. The self-cure universal adhesive (PALFIQUE Universal Bond) was packaged in a pair of containers. One droplet from every container was put into the rubber mixture dish, and combined with the company’s micro-brush. After that, the bond was placed in the silicon sheet holes and allowed to dry gently for 10 sec without light curing. Regarding the light-cure universal adhesive (single-bond universal adhesive), the container was completely shacked. The bond was placed into the silicon sheet holes and scraped for 20 sec using a micro-brush, dried for 5 sec, and followed by 10 sec of light curing with a LED curing device (Elipar S10, 3M ESPE, USA).
Ultrasonic application
Following the application of both universal adhesive systems into the enamel occlusal surface via silicon sheet holes, the micro-brush was sliced and connected to ultra­sonic equipment (Spark Innovators Corp., New York, USA) (Figure 4) with a frequency of 170 Hz, and used to spread both bonds into the holes to generate ultrasonic movement within bonds for 20 sec [17].

Resin composite application

Afterwards, the specially constructed silicon sheet was removed, allowed to dry fully, and re-applied. Using a specially designed condenser, the nano-filled resin composite (PALFIQUE, LX5) was applied, packed inside the holes, and light-cured for 40 sec. Subsequently, the silicon sheet was immediately withdrawn to create 4 composite projections of 1 mm in length and 1.2 mm in width at the occlusal enamel surface (Figure 5). After finishing of μSBS assessment, the samples were stored in water for three and six months at 37°C in a chamber with a 100% humidity level.

Micro-shear bond strength assessment

The specimen within the acrylic block was fastened to the bottom fixed head of the universal assessment apparatus after every storage period. A 0.14-inch-diameter stainless steel wire, fastened to the assessment apparatus’s top movable head, was applied to evaluate every composite projection for μSBS. It was placed as near as possible to the enamel-composite contact, and the shear stress was applied at a crosshead speed of 1.0 mm per min, till the sample’s separation. The μSBS value was determined in MPa using the apparatus’s software (BlueHill 3; Instorn, England) by dividing the force required for breakdown (Newton) by the surface area (mm²).

Statistical analysis

Data’s normality was assessed using Shapiro-Wilk test. IBM SPSS Statistics, version 25.0, was employed for statistical evaluation. One-way ANOVA was used in statistical analysis to contrast data, and Tukey post-hoc test was applied for multiple comparisons of μSBS mean scores. Statistical significance was predetermined with p-value ≤ 0.05.

RESULTS

The impact of various application methods on the micro-shear bond strength

Based on the Tukey post-hoc test, the findings showed that the difference among the various application modalities under similar storage time and the type of universal adhesive system was statistically significant (p ≤ 0.001). For each adhesive type under evaluation, the ultrasonic application method yielded a statistically significant larger μSBS value (p ≤ 0.05), as compared with the manual application (Table 2).

The impact of the type of universal adhesive system on the micro-shear bond strength

Based on the Tukey post-hoc assessment, comparing both universal adhesive systems within similar storage time and the type of application, the findings were not statistically significant (p > 0.05). On the other hand, the self-cure universal adhesive system demonstrated not significantly lower μSBS comparing with the light-cure adhesive (Table 3).

The impact of storage period on the micro-shear bond strength

Based on the Tukey post-hoc analysis of data, the comparison of storage times using similar universal adhesive systems and application methods, revealed not statistically significant values (p > 0.05). However, for the tested universal adhesive systems, the median μSBS decreased non-significantly; the greatest median μSBS value was noted at 3 months, whereas the lowest median μSBS was observed at six months (Table 4).

DISCUSSION

Currently, the simplicity of usage, fewer sensitive methods, and streamlined application processes of universal adhesive systems, have made them popular in clinical utilization. Dentists can apply them in selective enamel etch, etch-and-rinse, and self-etch approaches. One-step self-etch adhesive systems, which merge etching, priming, and bonding steps, are less technique-sensitive [18].
The findings of the present study showed that both adhesives’ µSBS significantly increased during ultrasonic administration. This could be explained by the chemical adhesion of functional monomers into the enamel mine­rals, which may have been strengthened by this application approach, with more extensive and retentive etch design generated by the ultrasonic vibration modes [17]. Moreover, this might be linked to the ultrasonic vibration of micro-brush, which delivers energy to the adhesive liquid, agitating monomers to access regions outside of the bristles’ contact. This leads to deeper de-minerali­zation of the sub-superficial enamel. Because the solution molecules are stimulated by high-speed vibration, waves of pressure and shear stresses are produced, and microscopic bubbles are aggressively pushed on the applied surfaces [19]. Our outcomes coincide with Muñoz et al. [17], who reported that the universal adhesives applied with ultrasonic technology improved the bond strength to the enamel.
Using the ultrasonic technology in SE approach placement may outweigh the disadvantage of this mode, resulting in the enamel’s lesser bonding effectiveness [20]. In previous studies, it was observed that universal adhesives’ etching capability did not leave a retentive enamel surface pattern when applied in the SE approach [21, 22]. The acidic monomers were probably prevented from de-mineralizing the enamel surface to the point of producing effective etching design for micro-mechanical inter-locking by the surface reaction of using SE solutions with the enamel [23]. Therefore, the μSBS of the manual method was significantly lower than the μSBS of the ultrasonic application.
Regarding the type of universal adhesive system, the present findings showed that the light-cure adhesive’s µSBS was greater than the self-cure adhesive’s, in a non-significant way. This might be because every universal adhesive system has a distinct chemical composition. For example, the self-cure adhesive comprises HEMA only, whereas the light-cure contains HEMA and 10-MDP monomer; although HEMA weakens the bond strength to the tooth structure and 10-MDP strengthens the bond [24]. Furthermore, Tekce et al. [25] investigated a combination of 10-MDP and HEMA, and reported improvement of calcium ion chelation and surface wetness of tooth hard structure. Such findings are in line with those of Melkumyan et al. [26], who demonstrated that the light-cure universal adhesive system outperformed the self-cure in terms of enamel bond strength, with non-statistically significant difference. Additio­nally, 10-MDP is a hydrophobic monomer that has mild etching capabilities, which makes it appropriate for use in universal adhesives. When ionic bonds with calcium ions, an insoluble MDP-Ca salt is produced, chemically bound to hydroxyapatite crystals [27].
According to the current findings, the self-curing adhesive’s µSBS was comparable with the light-cure adhesive. This might be due to the self-curing adhesive containing borate catalyst as an initiator, which increases the adhesive layer’s conversion degree by encouraging polymerization and producing free radicals when interacting with acidic phosphoric monomer [28]. Similarly, by producing a large number of calcium-dependent ionic bonding sites, self-cure adhesive based on 3D self-reinforcing (SR) technology, may chemically connect to the hard tooth structure [29]. Considering the current findings, the first null hypothesis is accepted, since the micro-shear bond strength was considerably raised by ultrasonic usage, and there was no significant difference between the tested universal adhesive systems.
However, the present results differ from those of Sai et al. [30], who discovered that the self-curing adhesive has a stronger enamel bond than the light-cure adhesive. They claimed that the strong borate catalyst could impact the self-curing adhesive’s enhanced bond efficiency.
Regarding storage time, the current results showed no evident difference in the µSBS between the storage times. It is explained by the dual bonding processes employed by self-etch adhesive systems (chemical bonding), which decelerate hydrolysis breakdown as well as prolongs the marginal sealing of restorations and micro-mechanical bonding, which offers strength contrary to mechanical forces [31]. These outcomes are in line with those obtained by Ageel et al. [32], who discovered that the bonding strength of light-cure universal adhesives was decreased to enamel following thermocycling aging at 5,000 cycles, equivalent to six months of water storage as well as after 14 days of storage, although lacking a significant variation. Concerning the present results, the second null hypothesis is rejected as there was no statistically significant difference in the µSBS between the two storage times.
Zhang et al. [33] demonstrated that the existence of 10-MDP as well as the subsequent strong and stable adherence to calcium in hydroxyapatite, increase the bond endurance in light-cure universal adhesives, improving the adhesive efficacy of self-etching systems, because it creates insoluble nano-layers and inhibits the hybrid layer’s hydrolytic breakdown. Moreover, the solubility and stability of 3D-SR in self-cure adhesive are probably similar to that of 10-MDP, since the phosphate monomer in 3D-SR may create multifunctional monomer constructs with different phosphate groups by partly self-organizing within an adhesive. These phosphate groups have the ability to engage and create ionic connections with calcium at various locations [34]. On the other hand, the present findings showed that the µSBS decreased non-significantly during water storage within both adhesives, which might result from simpler resin bonding systems’ capacity to absorb water. This causes the hybrid layer to gradually degrade hydrolytically, which worsen by water seeping through nano-leakage pathways. Therefore, the bonding strength weakens with aging [35].
Limitations of this research include laboratory conditions, due to which clinical performance of the tested universal adhesives could not be accurately assessed. Additionally, the evaluation period was short (at six months) and only one approach to the adhesive system was investigated. Thus, further studies employing different adhesive systems with extended storage times and clinical evaluations are essential.

CONCLUSIONS

According to the current results, it may be concluded that ultrasonic agitation can be recognized as an efficient method to enhance the enamel μSBS of both universal adhesive systems. The enamel’s μSBS of the self-cure universal adhesive was similar to that of the light-cure one. Additionally, over six months of water storage, the enamel μSBS was comparatively constant in both investigated universal adhesives.

DISCLOSURES

1. Institutional review board statement: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References