Effects of Artificial Aging of Direct Resin Nano-Hybrid Composite on Mean Bond Strength Values for Veneer Ceramic Samples

Introduction

The conceptualization of dental caries disease has evolved to be seen more as a noncommunicable disease that is linked to lifestyle and behavior [1]. The clinical presentation of the disease has changed dramatically from earlier higher prevalence rates in pits, fissures, and posterior teeth to smooth surfaces and anterior teeth [2], thereby leading to the development of interventional concepts, such as minimally invasive dentistry, in which adhesive-bonded restorations form a core treatment option [3]. With advantages such as marginal sealing, decreased cement solubility, aesthetic compatibility, and economic feasibility [4], such restorations can range from simple inlay, onlay, laminates, and crowns to complex occlusal veneers of the posterior teeth, whose retentive ability is not based mainly on mechanical principles, making their cementation (adhesive) vital for extended restoration durability [5]. Long-term successful cementation depends on many external factors (occlusion, anterior guidance, and parafunction) that are not related to material composition and properties [6]. A few adhesive treatment options, including laminates, veneers, and crowns, are specifically indicated in clinical scenarios in which existing composite resin restorations are present and have aged (exposed to oral conditions, discolored, biofilm deposited, abraded, eroded, or worn) [7]. Veneers can be either of resin or ceramic, are usually very thin, and may or may not cover the incisal areas. In many instances, a definitive restoration cannot be provided unless the foundation is appropriately built [8]. Existing foundation resin restorations are also difficult to remove due to the difficulty in discerning color between composite and natural tooth structure [9], increasing the clinical chances of depleting the natural tooth. Other factors, such as replacement costs, compromising or weakening existing natural teeth, or causing pulpal threats, preclude the choice of replacing an existing restoration with a new one. Simultaneously, clinical failures of laminates and crowns have been attributed to defects in underlying/existing previous restorations [7,10]. Despite the known disadvantages of old composite resin restorations, studies show a considerably higher percentage of aged resin restorations being encountered while a definitive restoration is being cemented [11].

Adhesive restoration can be made from restorative materials, such as composite resin (direct and indirect), alloys (base metal or noble metal), or porcelain (ceramics) [12]. Nano-filled resin composites contain a blend of small (nanometer)-sized particles dispersed into larger secondary resin particles, while nanohybrid resins use a different approach of combining micrometer-sized and nanometer-sized fillers [3,11]. Adhesive ceramics originated in 1959, when lithium disilicate (Li2Si2) glass ceramics were discovered in the form of a binary glass ceramic system after precipitation in glass (silver acting as a nucleating agent for crystallization) [13]. Ivoclar Vivadent has to date introduced 2 lithium disilicate-based glass ceramics, IPS Empress II (pressable) in 1998, and IPS e.max Press (castable) in 2001, with improved mechanical and optical properties [14]. High flexural strength and lifelike translucency make them better treatment options than earlier castable (Dicor and Mirage) and present day leucite ceramics in treatment options such as veneers and laminates, in which a thin surface of the natural tooth is restored [15,16]. IPS e.max Press is a crystalline-dominated, pressable lithium disilicate-based ceramic produced using bulk casting methods [17]. Sluggish controlled cooling after melting minimizes internal defects, thereby improving optical and mechanical properties. Restorations made from IPS require cementation by resin cements, which, when bonded to the underlying substrate (natural tooth or foundation restorations), increase restoration fracture resistance, surface and marginal adaptation, and retention [18]. However, the interface between the ceramic (glass) and the resin (plastic) cement requires mechanical (sand blasting or chemical etching [8%–10% hydrofluoric acid]) and chemical (silane coupling agent application) surface treatment [4,11,19]. Together, they thus provide a combination of micromechanical (honeycomb-like) and chemical bonds at the interface between the ceramic and the resin cement. The bond strength and adhesive failures of resin cements to pressable ceramics have been widely studied [20] using different bond strength tests, including push-out tests, tensile and shear bond strengths, and their respective microforms (micro tensile and micro shear) [21].

Ceramic restorations, when luted to composite resin restorations using resin cement, have comparatively greater fracture resistance than other restorative materials [22]. Chen et al investigated the bond strength of feldspathic porcelain (VMK 68) to direct composite restorative resin (Clearfil APX) using different hydrofluoric acid etchant concentrations and different etching times and found that lower etchant concentrations (2.5%) produced greater bond strengths than higher concentrations (5%) [23]. The use of silane coupling agents has also been found to improve the bond strength of resin composites to various types of porcelain [24,25]. Kilnic et al [26] assessed the SBS of resin composites (Filtek Z550) for different types of aged and non-aged ceramics (nanoceramic, resin ceramic, feldspathic, and lithium disilicate). Results showed material and surface treatment types significantly changed SBS [26]. Makishi et al investigated the SBS of 2 multimode adhesives (Scotchbond Universal and All-Bond Universal) on 1-year-aged indirect resin composites and IPS e.max Press and reported higher SBS after 24 h for indirect resin composites than for 1 year and did not find any significant difference between the 2 adhesives used after 1 year [27]. One of the reasons for decreased SBS in indirect resin composites is the use of air abrasion [28], which promotes water absorption into the primed layer. Studies investigating the adhesion of 2 composite resin layers for repair have found that the adhesion that is generally achieved in the presence of an oxygen-inhibited layer of unpolymerized resin is absent in aged composite resin [11,29], which can be improved by various surface treatments, such as roughening, etching, airborne particle abrasion, or using silanes/intermediate resins [29,30]. Recent studies on the surface conditioning of polymeric materials have favored airborne particle abrasion (silica-coated alumina particles) with silanization, to produce a more effective bond than acid etching and silanization [31]. The monomeric ends within silane molecules interact with the methacrylate of adhesive resins by free radical polymerization [32]. The protocol of aging for composite resin research has been generally performed through thermocycling for 5000 to 10000 cycles, which equals 6 to 12 months of clinical usage [33]. These time periods do not actually reflect the clinical scenario. In many cases, including complete occlusal rehabilitations and implant supported restorations, there are short aging periods, within 1 to 3 months, between placement of foundation restoration and cementation of ceramic restoration. In other cases, the composite restoration may be further exposed to oral aging, because the definitive restoration needed to be repeated or did not fit.

To the best of our knowledge, there are studies that focus on the influence of resin adhesion to new or fresh polymerized composite resin [26,28,34]. However, studies investigating adhesion to aged composite resin are limited to composite repair [10,30,35] and feldspathic porcelain [36]. Also, no studies have investigated the short-term aging cycles (1 and 3 months), which broaden the clinical spectrum encountered by practitioners. Therefore, in this study, we aimed to evaluate the effects of various aging (thermocycling) cycles of resin nanohybrid composites on mean bond strength values for ceramic (lithium disilicate pressed) samples. We hypothesize that since aged composites will not present a full array of free surface radicals, the adhesion of a pressable ceramic through resin cement will yield inferior bond strengths to that of aged composites. Alternately, the null hypothesis states that there is no difference in bond strength after composite resin undergoes various aging cycles.

Material and Methods

ETHICS:

This study received its ethics clearance from the concerned Ethics Committee of the College of Dentistry, Jazan University, via approval number CODJU-21151. This in vitro experimental research study was part of the requirement that was conducted by a group of intern students under the direct supervision of staff of the Department of Prosthetic Dental Sciences during the academic year 2022-2023.

STUDY DESIGN:

This study followed a comparative approach between control and experimental groups, with the control group serving as the baseline and experimental groups serving as the test groups. The independent variables for the study were the materials (pressable ceramic, composite resin, and resin cement) and thermocycling (aging cycles of 1, 3, 6, and 12 months), while the dependent variables were the SBS and adhesive failure analysis. Figure 1 represents the study flowchart, highlighting the sequence and the concerned study variables. Operators who performed testing were blinded to specimen identification and research outcomes.

:

The term cementation has been operationally defined as the process of attaching parts by means of a dental cement, in this case, a resin cement. Cohesive failure is a type of bond failure within a dental material as a result of tension or shearing forces, while adhesive failure is a type of bond failure that takes place at the interface between 2 materials due to shearing or tensile forces. The adhesive can be applied partially or completely to one or both of the substrates, depending on the type of bond failure. Castable ceramic for dental applications refers to a form of glass ceramic that has restorative characteristics and can be cast using the lost wax technique.

SAMPLE SIZE: The study conducted by comparing 5 groups (1 control and 4 experimental). The total number of specimens for the study and the number of these specimens in each group were statistically estimated using software (Nquery, Version 7, Informer Technologies, USA) using the formula N=2σ2×(Zα+Zβ) 2/2 [38]. The calculated samples for the total study came out to be 90 specimens, with each group having a minimum of 18 samples (derivation standards of type 1 error rate a=0.05, effect size D2=0.28, and study power assumption 80%), which were guided by earlier similar studies [19,24]. Compensation for faulty sample loss was compensated by keeping 2 additional samples for each subgroup that would replace the defective ones.

SPECIMEN PREPARATION: Materials used with their respective brands, manufacturers, batch numbers, chemical composition, and working characteristics are listed in Table 1. The study was sequenced as the preparation of resin composite specimens followed by their respective aging cycles, and in the concluding stage, the ceramic specimens were prepared and adhesively bonded to aged composite specimens after respective surface modifications.

For the composite resin specimen (n=90), the nanohybrid composite resin (Tetric N-Ceram, Ivoclar Vivodent, Switzerland) that would represent the existing aged restorations under the ceramic restoration was prepared by packing the specimens into a cylinder-shaped polyethylene mold with a height of 3 mm and a diameter of 5 mm. Incremental photopolymerization was conducted by a halogen unit (Demetron LC, Kerr; Orange, CA, USA; intensity 1200 MW/cm2; duration, 40 s; and distance, 2 mm), as per manufacturer recommendations. To standardize the light intensity, verification using a radiometer (Demetron LC, Kerr) was performed after preparing every 10 specimens. To protect the formation of an oxygen-inhibited surface layer, a clean, dry glass slab was used to create a smooth surface on each specimen. Once polymerization was accomplished, a total of 90 test specimens were removed and distributed into 5 different groups (1 control and 4 experimental) that were based on the time duration (T) of aging (thermocycling). The specimens in the control group (TC) were placed in distilled water for 24 h at a controlled body temperature (37°C) before subjecting them to testing. The remaining experimental groups T1, T3, T6, and T12 represented aging of 1 month (850 cycles), 3 months (2500 cycles), 6 months (5000 cycles), and 12 months (10000 cycles), respectively. Aging was conducted in a thermocycle bath (Mechatronik, Bayern, Germany), which circulated the samples with alternate immersions in warm and cold bath temperatures (5 to 55 °C), with a dwell time of 5 s. The thermocycling was representative of clinical usage, as indicated in previous studies [11,32].

For the IPS e.max Press (LT) specimens (n=90), a total of 90 ceramic specimens with diameter of 10 mm and thickness at 3 mm were manufactured to facilitate an accurate point of testing apparatus. Specific wax pattern discs were obtained using modeling base plate wax. Each wax disc was then sprued with 3 mm sprue wax (Bego, Germany), and 3 such sprued wax patterns were placed on the IPS muffle (Ivoclar Vivadent, Liechtenstein), followed by investing in manufacturer-recommended investment material (IPS PressVest Premium, Ivoclar Vivadent). After setting up the investment, the ring base and gauge were removed, and burnout was conducted in an automatic furnace (Ney, US Dental) to eliminate wax at the recommended temperature of 900°C (1650°F). The obtained mold was then placed in the porcelain furnace (EP 3000, Ivoclar Vivadent), where lithium disilicate glass ceramic ingots (IPS e.max Press LT) of one particular shade (A1) were heat-pressed. The pressed mold was then cooled slowly at room temperature for 1 h, and the mold was divested (4 bar pressure, 110 μm alumina particles). Each specimen, after divesting, was placed in hydrofluoric acid (1% Invex liquid, Ivoclar Vivadent) for 10 min, followed by water wash, air drying, and air abrasion (110 μm alumina, 2 bar pressure) to remove the reaction layer, as per the manufacturer’s recommendations. The discs were then separated from the sprues, and each specimen was examined for surface defects.

SURFACE MODIFICATIONS AND CEMENTATION: For the resin composite specimens, the surface to be bonded was prepared using an abrasive paste (Qartz Prophylaxis Paste) that simulates the clinical application of oral prophylaxis. The samples were then air abraded with an intraoral air abrasion device (Dento-Prep, Daugaard, Denmark), which uses 30 μm particles of alumina that are coated with silica. The device was kept at a distance of 10 mm for 4 s at a pressure setting of 2.5 bar [39]. Surfaces were cleaned with air, followed by the application of an oxysilane coupling agent (3-methacryloxypropyl trimethoxysilane, ESPE-Sil, 3M ESPE) for 5 min, before the application of the priming agent (Monobond N), as recommended for the resin cement (Variolink N). The primer was left to react for a period of 60 s. For ceramic discs, Monobond Etch & Prime (an alcoholic-aqueous solution of ammonium polyfluoride) was applied for a period of 60 s (applied for 20 s, followed by shaking, and then left for another 40 s), followed by drying with oil- or water-free dry air (10 s) using low pressure. Before cementation, the ceramic specimens were applied with the silane coupling agent and allowed to react for 60 s, which is essentially a step of the final cementation with the resin cement.

For cementation, the 2 conditioned substrates of aged resin composite and ceramic were bonded to the resin cement (Variolink N, Ivoclar Vivadent), as per the manufacturer’s instructions and recommendations. A customized aligning device allowed both specimens to be aligned so that surfaces contacted evenly under the constant load (750 g). This ensured that the thickness of resin cement would be uniform for all specimens in each group. Excess resin cement was removed with a microbrush, followed by photopolymerization (40 s) in each direction from a distance of 2 mm. After cementation, an oxygen-inhibiting gel was applied to the free surface, which was kept on the specimens for 5 min. The specimens were then washed, rinsed, and dried. The specimens from all groups were then stored in distilled water before undergoing shear bond testing. The cemented specimens before testing for bond strength were embedded in a hard polyethylene ring (diameter of 2 cm and height of 1 cm) using auto polymerizing polymethylmethacrylate (quick repair). One surface of the specimen was thus embedded within the acrylic, while the other side was uncovered for testing.

MEASURES, DATA COLLECTION, AND INTERPRETATION:

All embedded specimens were tested for SBS by mounting them in a jig of a universal testing machine (Instron Corp, Canton, MA, USA), using a shear force on the interface between the 2 cemented specimens until failure occurred, which falls as per standard ISO regulations (PN-EN ISO 29022: 2013-10). The force was applied at a crosshead speed of 1 mm/min, while the analysis of stress and strain and the failure load were recorded automatically within the machine software. The load required to debond each specimen was measured in newtons, and the bond strength was then represented in megapascals (by dividing the load by the brackets’ mean surface area).

For adhesive failure analysis, the failure sites were examined by 2 independent and calibrated reviewers who were blinded to the study outcome and the specimen samples. All observations were made under an optical microscope (magnification ×20; Amscope, USA). The review consisted of a visual microscopic inspection as well as a digital image of the failed surface, using ImageJ software. Five different types of failures were identified (prefailure, substrate failure, mixed failure, cohesive failure, and adhesive failure). Depending upon the amount of adhesive left over the surface of the specimen, the failure was categorized as no adhesive left (adhesive failure), adhesive left partially (mixed), and complete adhesive left (cohesive).

STATISTICAL ANALYSIS:

After entering the obtained data into Microsoft Excel sheets, correction, refinement, and coding were performed before analysis was conducted in SPSS version 22.0 software (IBM Corp, Armonk, NY, USA) using a desktop computer (Lenovo, CT55AG7) through Windows 10 Pro. The mean shear bond strength values and their standard deviations were derived. Differences in means between the experimental and control groups were subjected to one-way analysis of variance test (ANOVA), with bond strength being dependent and aging being independent variables. For differences between pairs (multiple pairwise) of group means, a post hoc Tukey honestly significant difference test was used. All differences were considered significant statistically if the P value was either equal to or less than 0.05 (P≤0.05). Types of failures were evaluated in terms of frequency distribution (percentage).

Results

SBS BETWEEN AGED RESIN AND PRESSABLE CERAMIC:

Table 2 presents the mean SBS values of the groups investigated in this study. The highest SBS was observed in specimens that belonged to the control group (m=20.97), while the lowest SBS was observed in the T12 group (m=18.27), indicating that the SBS showed a time-dependent decline in the aged composite resin. Among the 4 experimental groups, the highest SBS was observed in specimens that were aged for the least amount of time (1 month), while the greatest decrease in SBS was observed in specimens aged 12 months. One-way ANOVA showed that the differences between the groups from the control were statistically significant (P≤0.05). The post hoc pairwise group tests are presented in Table 3. All subgroups except T1 were found to differ significantly from the control group, with differences between 24 h (control) and 1 month (T1) being not significant (P≤0.05). Although there was a continuous reduction in SBS at succeeding time intervals (T3, T6, and T12), the differences between T3 and T6, T3 and T12, and T6 and T12 were not found to be significant, indicating that most of the reduction in SBS occurred during the first 3 months of aging. The clinical application of these findings is that a composite resin restoration that is older than 3 months will have a significant reduction in SBS and should be either replaced or other means of surface modification must be investigated that will enhance the SBS.

ADHESIVE FAILURE ANALYSIS:

The frequency distribution of different failures observed in the specimens of each subgroup is presented in Figure 2. More cohesive failures were observed in specimens aged between 1 and 3 months, while more adhesive failures were observed in specimens that were aged between 6 and 12 months. Other types of failures (substrate, mixed, and prefailure) occurred with less frequency, with all of them occurring in the samples that were aged up to 3 months.

Discussion

STRENGTHS AND LIMITATIONS OF THE STUDY:

This study is perhaps the first to investigate the effect of precementation aging of resin composites on the SBS of a pressable ceramic using resin cement. The study highlights the significance of aging restoration earlier than the routinely investigated 6 months in most research on SBS composites. The study has, however, limitations in that we investigated only 1 composite type, 1 ceramic type, and 1 resin cement. We also did not investigate different surface treatments that have been shown to improve bond strength, as mentioned in the literature.

Conclusions

Within the scope and limitations of the present study, it can be concluded that the aging of composite resin decreases its SBS to pressed lithium disilicate glass ceramic (IPS e.max Press). The decrease in SBS is greater as the resin composite ages. Composites that have aged for 3 months do not show any significant changes in bond strength, compared with those that have aged for 6 or 12 months. Clinically, the results of the study indicate that resin restorations older than 1 month should be removed before being cemented to a pressed ceramic with resin cement. Clinicians should also be aware that all laboratory procedures should be done within the stipulated 1-month timeframe so that the ceramic restoration is cemented to the composite that has not become old. Further studies are recommended to investigate the role of various surface treatments on the SBS of aged composite to ceramic restoration.