to correlate polymerization stress data obtained under two compliance conditions with those from different interfacial quality tests.
Six commercial composites were tested (Filtek Z250/3M ESPE, Heliomolar/Ivoclar Vivadent, Aelite LS Posterior/Bisco, Filtek Supreme/3M ESPE, ELS/Saremco and Venus Diamond/Heraeus Kulzer). Bond strength (BS) was evaluated by push-out test on slices of bovine dentin (2-mm thick) with tapered cavities. For microleakage (ML) and gap analysis, cylindrical cavities in bovine incisors (4-mm diameter and 1.5-mm height) were restored and epoxy replicas of the cavo-surface margins were prepared for analysis under scanning electron microscopy (200×). The same specimens were submitted to a microleakage protocol using AgNO 3 as tracer. After sectioned twice perpendicularly, ML was determined under a stereomicroscope (60×). Polymerization stress (PS, n = 5) was determined by the insertion of the composite ( h = 1.5 mm) between poly(methyl methacrylate), PMMA, or glass rods (Ø = 4 mm) attached to a universal testing machine. Data were analyzed using Kruskal–Wallis (ML and gaps), and ANOVA/Tukey (BS and PS, α = 5%). Pearson’s correlation test was used to verify correlations between stress and interfacial quality.
BS varied from 4.7 to 7.9 MPa. Average ML data ranged from 0.34 to 0.89 mm. Maximum ML varied from 0.61 to 1.34 mm. Gap incidence varied from 13 to 47%. PS ranged from 2.5 to 4.4 MPa in PMMA, and between 2.1 and 8.2 in glass. Statistically significant correlations were observed between stress and interfacial quality, except between BS and PS in glass. These correlations were stronger when PMMA was used as bonding substrate.
PS data obtained using a high compliance testing system showed a stronger correlation with “in vitro” interfacial integrity results, compared to data from a low compliance system.
According to clinical studies, drawbacks such as postoperative sensitivity, marginal discoloration and possibly secondary caries are often associated with loss of marginal integrity in composite restorations . One of the possible causes for interfacial debonding is polymerization stress. When composites polymerize confined in a cavity preparation, shrinkage associated with the development of modulus of elasticity generate stresses in the tooth/restoration interface, which may lead to debonding .
Several research groups have focused on developing mechanical tests to quantify polymerization stress . In the most commonly used test, the composite is inserted and polymerized between two flat surfaces of glass, metal or poly(methyl methacrylate), PMMA, rods attached to an universal testing machine . The load cell records the contraction force exerted by the composite on the substrate during polymerization and the nominal stress is calculated, in MPa, by dividing this value by the cross sectional area of the rod. This method has been widely used to compare commercial and experimental composites , photoactivation methods and to evaluate several factors associated with stress development . Some studies have correlated the stress values from mechanical tests with interfacial integrity, noting that microleakage and cuspal deflection increase proportionally with increasing stress . However, no relationship was found between stress and gap formation in porcelain inlays . A study evaluating polymerization stress as a function of photoactivation methods observed that modulated photoactivation results in lower stress values, leading to higher bond strengths .
The studies mentioned above have in common the fact that stress was determined in low compliance systems, using glass as bonding substrate for the composite. However, the system’s compliance has great influence on ranking materials regarding stress magnitude . The lower the compliance of the testing system, the lower is its ability to elongate and relief the stress. Consequently, the recorded value is higher. In the past few years, bonding substrates with lower modulus of elasticity have been used polymerization stress testing .
Even assuming that data from mechanical tests cannot be extrapolated to the clinic, a question arises regarding which system would be more closely related to the interfacial quality of composite restorations. It is possible that the use of low compliance testing systems could overestimate the stress values, in comparison with those found in high compliance conditions, more akin to the behavior of a prepared tooth. Estimating the compliance of the tooth in a clinical situation is nearly impossible. The stiffness of the dental tissues varies among teeth and even in the same tooth there is a substantial difference in stiffness between enamel and dentin. But even being that complex, the tooth cannot be considered as a rigid system. In fact, several studies have shown that polymerization shrinkage could lead to tooth deformation . In a previous study, several commercial composites ranked similarly for microleakage and stress values obtained in a high compliance system, but the same did not apply to stress data obtained in a low compliance system .
Considering the above, it is important to verify if data from polymerization stress tests can be correlated with results from interfacial quality tests, namely, bond strength, microleakage and gap formation. The null hypothesis was that the polymerization stress values shows no correlation to interfacial integrity, regardless of the system compliance. Additionally, a second null hypothesis was tested, stating the compliance of the testing system did not influence polymerization stress values.
Materials and methods
Six dimethacrylate-based commercial composites shade A3 were tested ( Table 1 ). Three of them (Heliomolar, Filtek Supreme and Filtek Z250) were chosen based on their filler content (by volume). The other three (Venus Diamond, ELS and Aelite LS) are considered as “low shrinkage” or “low stress” materials by the respective manufacturers. Venus Diamond has TCD-urethane in its composition, in addition to conventional dimethacrylates, while ELS has no diluent monomer (TEGDMA) and Aelite LS has a very high filler content. Elastic moduli (determined by three point bending test) and post-gel shrinkage (determined by the strain-gage method) were obtained in a previous study, and correspond to the values recorded 10 min after phtoactivation using the same irradiance and radiant exposure adopted in the present study .
|Material (abbreviation)||Filler content (vol.)||Average size of filler particles||Manufacturer||Organic matrix||Post-gel Shrinkage (%)||Elastic Modulus (GPa)|
|Heliomolar (HM)||46%||0.04–0.2 μm||Ivoclar Vivadent, Schaan, Liechtenstein||BisGMA, UDMA, D 3 MA||0.43 (0.02)||3.1 (0.3)|
|ELS (EL) a||50%||0.07–2.6 μm||Saremco, Rohnacker, Switzerland||BisGMA, BisEMA||0.35 (0.02)||2.0 (0.2)|
|Filtek Supreme (SU)||57%||75 nm–1.4 μm||3M ESPE||BisGMA, BisEMA, UDMA, TEGDMA||0.64 (0.07)||6.0 (0.7)|
|Filtek Z250 (FZ)||60%||0.19–3.3 μm||3M ESPE||BisGMA, BisEMA, UDMA, TEGDMA||0.52 (0.04)||5.6 (0.6)|
|Venus Diamond (VD) a||64%||5 nm–20 μm||Heraus Kulzer GmbH, Hanau, Alemanha||TCD-uretano||0.39 (0.03)||4.5 (0.3)|
|Aelite LS Posterior (AE) a||74%||0.06 μm||Bisco, Schamburg, IL, EUA||BisGMA, BisEMA, TEGDMA||0.51 (0.04)||9.3 (0.7)|
Push-out bond strength
Bovine incisors ( n = 15) had their crowns removed at the cement-enamel junction with a diamond disc under refrigeration. The buccal surface was flattened with wet sandpaper until the enamel was completely removed. The lingual surface was sectioned using a diamond disc (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) to obtain a slice with 2 mm thickness. Tapered cavities with 2.9-mm diameter on the buccal surface and 3.5 mm diameter on the lingual surface were prepared using cylindrical and truncated cone diamond burs.
The cavity walls were etched with 37% phosphoric acid for 15 s and then rinsed in running water for 15 s. Excess water was removed with short air blasts, leaving the surface visibly moist. Two layers of an one-bottle adhesive system (Single Bond 2, 3M ESPE) were applied and photoactivated with a radiant exposure of 12 J/cm 2 (400 mW/cm 2 × 30 s – VIP Jr, Bisco, Schaumburg, IL, USA). The tooth was placed on a mylar tape over a glass slab, with the buccal surface facing up. After inserting the composite in bulk, a second mylar strip was placed on the buccal surface of the restoration and the curing tip was placed in contact therewith, so the light was directed from buccal to lingual surfaces. The composite was light cured with a radiant exposure of 18 J/cm 2 (570 mW/cm 2 × 32 s). Specimens were stored for 24 h in distilled water at 37 °C. Both buccal and lingual surfaces were slightly ground with finishing discs (Soft-Lex, 3M ESPE).
For the push-out test, the specimen was placed on a stainless steel base under the actuator of a universal testing machine (Instron 5565, Canton, MA EUA). The smallest radius (buccal surface) was placed in contact with a 2.5-mm diameter stainless steel tip, connected to the load cell. This tip applied a compressive force (cross-head speed: 0.5 mm/min) on the composite surface until the rupture of the bonded interface. Values in MPa were obtained by dividing the maximum force (N) by the bonded area of the specimen (in mm 2 ). The bonded area was calculated by the formula of the lateral area of the truncated cone:
Bonded area = [ π ⋅ ( R + r ) ] h 2 + ( R − r ) 2
where π = 3.1416; R larger cavity radius, r smaller cavity radius; h cavity high.
Microleakage and marginal gap analysis
Bovine incisors ( n = 15) had their buccal surfaces flattened with #400 grit sandpaper to provide an enamel surface large enough to make sure the cavity margins were entirely surrounded by enamel, and then received cylindrical cavities with 4-mm diameter and 1.5-mm depth (C-factor: 2.5, volume: 19 mm 3 ), all of them with enamel margins. The restorative procedure was the same as described for the push-out test. Immediately after polymerization, the restorations were ground and polished with silicon carbide sandpaper (grits 600–4000) to remove composite excess and expose the restoration margins. After 24 h storage in distilled water at 37 °C, the specimens were sonicated for cleaning the surface. Then, the restored surfaces were molded using an addition silicone (Express XT, light consistency, 3M ESPE) and the impressions were poured with epoxy resin (Buhler Epothin, Epoxicure Resin, Lake Bluff, IL, USA). After 9 h at 37 °C, the replicas were separated from the molds, fixed in metal stubs and coated with gold for analysis in a scanning electron microscope (LEO, AEG-Zeiss, Germany) under 200× magnification.
Ten specimens of each experimental group were randomly selected for gap analysis. Each specimen required between 38 and 48 images using 200× magnification to scan the entire perimeter of the restoration. Examples of debonded and gap-free margins are shown in Fig. 1 . ImageJ software (National Institute of Health, Bethesda, USA) was used to measure the length of the debonded segments at the enamel-composite margins, as well as the entire perimeter of the restoration. The scale bar of the SEM images was used for calibration. The value obtained in millimeters was converted to percentage based on the total perimeter of the interface for each specimen.
After the silicone impression was obtained, each specimen was coated with nail polish, except in an area of 1 mm around the restoration. They were immersed in 50% AgNO 3 for 2 h in the dark, followed by a period of 6 h immersion in developing solution (Kodak, São José dos Campos, SP, Brazil) under fluorescent light. After that, the specimens were sectioned with 0.3-mm diamond discs under water cooling (1000 Isomet, Buehler Ltd., Lake Bluff, IL, USA) twice perpendicularly through the center of the restoration. Since some of the tooth substance was lost during sectioning (due to the thickness of the diamond disc) eight surfaces (rather than four pairs of adjacent surfaces) were considered for microleakage evaluation. Images of each surface were digitized using a 60× magnifying stereomicroscope (model SZ61, Olympus Inc., Tokyo, Japan) equipped with a CCD camera (Q-Color 3, Olympus). The depth of penetration of the tracer was measured (in mm) using the ImageJ software, and both the average penetration of the eight surfaces and the maximum dye penetration were recorded. Additionally, the thickness of the enamel layer was also recorded.
Polymerization stress was measured under two compliance conditions, defined by the material used as bonding substrate for the composite: PMMA (“high compliance”) and glass (“low compliance”). Rods with 4 mm in diameter were sectioned in segments with 13 or 28 mm in length. For the 13 mm rods, one of the surfaces was polished with silicon carbide sandpaper (600–2000 grit) and felt disks with alumina paste (Alumina 3, ATM, Altenkirchen, Germany) in order to allow the highest light transmission possible during photoactivation. The opposite surface and both surfaces of the 28 mm rods were sandblasted with aluminum oxide (250 μm).
For the PMMA rods, the sandblasted surfaces received a layer of methyl methacrylate monomer (JET Acrílico Auto Polimerizante, Artigos Odontológicos Clássico, São Paulo, Brasil), while the glass rods received a layer of silane (Ceramic Primer, 3M ESPE), followed by two layers of unfilled resin (Scotchbond Multipurpose Plus, bottle 3, 3M ESPE), light-cured with a radiant exposure of 12 J/cm 2 (400 mW/cm 2 × 30 s). The rods were attached to a universal testing machine (Instron – Fig. 2 ). Those with 13 mm were attached to the lower clamp and those with 28 mm to the upper clamp. The space between them was fixed at 1.5 mm (C-Factor: 1.3, volume: 19 mm 3 ). The composites were inserted into this space and shaped as a cylinder following the perimeter of the rods. An extensometer was attached to the rods (model 2630-101, Instron) to monitor the specimen height and provide a feedback to the testing machine to move the actuator in order to keep the specimen height into a minimum range.
The value registered by the load cell corresponded to the force necessary to counteract the polymerization shrinkage force and maintain the specimen’s initial height. The tip of the light guide (VIP Jr, Bisco) was positioned in contact with the polished surface of the 13 mm rod. The irradiance effectively reaching the composite was determined using a radiometer (model 100, Demetron Res. Corp., Orange, California, EUA) and duration of the exposure was adjusted to obtain a radiant exposure of 18 J/cm 2 . Force development was monitored for 10 min from the beginning of photoactivation and the maximum nominal stress was calculated by dividing the maximum force value recorded by the cross-section of the rods ( n = 5).
Polymerization stress data were analyzed using two-way ANOVA (composite and compliance) and Tukey test. Microleakage and marginal gap were analyzed using Kruskal–Wallis due to the lack of homocedasticity. In both tests, the pre-set global significance level was 5%.
Pearson’s tests were used to verify the presence of statistically significant correlations between polymerization stress (in both substrates, glass and PMMA) and bond strength, microleakage or marginal gaps. In order to be considered statistically significant, the critical “ r ” value (Pearson’s correlation coefficient) was 0.811, according to the number of data pairs (six) and global significance level of 5% . Regression analyses involving the same variables were also performed to determine the equations for the regression curves.