Bond Strength and Monomer Conversion of Indirect Composite Resin Restorations, Part 1: Light vs Heat Polymerization Daniel Alexandre Menezes Pedrosa Maltaa / Pascal Magneb / Sylvio Monteiro-Juniorc Purpose: To assess the resin microtensile bond strength (MTBS) and the monomer conversion (MC) of indirect composite resin restorations made of three different materials. Materials and Methods: Two light-polymerized direct materials (Filtek Z100 and Premise) and one light- and heat-polymerized indirect material (Premise Indirect) were used. For MTBS testing, 42 cylindrical samples were fabricated (7 pairs per material). Surface conditioning included airborne-particle abrasion, cleaning, and application of a silane. Cylinders were bonded to each other using adhesive resin (Optibond FL). Specimens were stored in water for 24 h. Another 15 cylinders (5 per material) were fabricated for MC measurements (FT-IR) immediately and at 24 h. The MTBS data were submitted to one-way ANOVA and the MC to two-way ANOVA (material and storage time) (α = 0.05), followed by post-hoc comparisons with the Tukey test. Results: The MTBS to Z100 was 72.2 MPa, significantly higher than that to Premise (48.4 MPa) and Premise Indirect (52.7 MPa). The immediate MC was similar for all materials (range 51% to 56%) and significantly increased at 24 h (range 57% to 66%), except for Z100. Premise Indirect showed the highest MC (66% at 24 h). Conclusion: Z100 showed better “bondability” than Premise and Premise Indirect. Premise Indirect, with its heat initiator, did not present a higher MC. Keywords: adhesion, bond strength, direct composite resin, dental materials, indirect composite resin, monomer conversion. J Adhes Dent 2014; 16: 517–522. doi: 10.3290/j.jad.a33199
sthetic and biomimetic adhesive dentistry has been made possible by the numerous advances in dentin-enamel bonding and the development of composite resin materials with improved mechanical properties.4,15,20,21,24,40 Nowadays, survival rates of direct composite resin exceed those of amalgam and present less risk of tooth/restoration fracture and cracking.31 One of the drawbacks of direct composite resin restorations is polymerization shrinkage. The ultimate solution to this problem is the use of indirect restorations. When used in the laboratory, composite resin inlays/
Interim Professor, School of Dentistry, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil. Wrote the manuscript, performed the experiments.
Professor, Division of Primary Oral Health Care, Herman Ostrow School of Dentistry of USC, University of Southern California, Los Angeles, CA, USA. Cowrote and proofread the manuscript, performed some of the experiments.
Professor, School of Dentistry, Federal University of Santa Catarina, Florianópolis, Santa Catarina, Brazil. Co-wrote and proofread the manuscript.
Correspondence: Dr. Daniel Malta, Universidade Federal de Santa Catarina, Centro de Ciências da Saúde, Departamento de Odontologia, Disciplina de Dentística, Campus Universitário Trindade, Florianópolis, Santa Catarina, Brazil 88040-970. Tel/Fax: +55-48-3721-9880. e-mail: [email protected]
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Submitted for publication: 28.03.14; accepted for publication: 14.08.14
onlays may be a valuable socioeconomic alternative to costly indirect ceramic restorations.13,25,26,38 Indirect techniques facilitate the generation of ideal anatomic form, marginal adaptation, and appropriate proximal contact, contour, and occlusion. 22,34,44 The survival rates of indirect composite resin restorations, however, may have suffered from the early marketing of materials with particularly low elastic modulus (less than 6 GPa) and filler content (less than 60% in volume). Those early indirect systems were designed to satisfy the dental technician’s requirements to simulate handling properties similar to those of porcelain. This flowable behavior allowed them to be placed with a brush (like ceramics) but it affected their physical performance.6,27 Currently, indirect composite restorations can be fabricated with materials originally intended for direct use by the dentist or with specific indirect materials for laboratory use only (light and heat polymerized). In spite of the similar composition, indirect composite restorations are thought to have superior mechanical properties due to higher levels of monomer conversion usually obtained with the use of different polymerization procedures that involve photo-activation, heat, or both.7,11,17,35,41,44 517
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Table 1 Groups, materials, and polymerization protocol Group
40 s light polymerization
40 s light polymerization
40 s light polymerization + 10 min oven
Composition, batch numbers, and manufacturers of the resins used in this study Shade/batch
Average particle size
A3,5 / 234295 B2 / 240201
Bis-GMA, TEG-DMA, zirconia, silica, camphorquinone
85% in weight 66% in volume
3M ESPE; St Paul, MN, USA
Opaque A3 / 3435851 Opaque B1 / 346919
Bis-GMA, TEG-DMA, barium, silica, camphorquinone
84% in weight 70% in volume
Kerr; Orange, CA, USA
Primary Dentin A3,5 / 3461351 Primary Dentin B1 / 3364736
Bis-GMA, TEG-DMA, barium, silica, camphorquinone, proprietary heat initiator
87% in weight 72% in volume
There is a general concern, however, that increased levels of monomer conversion might be associated with decreased resin “bondability” due to a reduction in amount of reactive double bonds.3,23,32 At present, there is a paucity of literature on the relationship between polymerization rates and bond strength. Thus, the aim of this study was to assess the monomer conversion and bond strength of indirect composite resin restorations made of three different materials with different polymerization processes. The null hypothesis is that there is no difference in bondability and monomer conversion between materials intended for direct use and those intended for laboratory use.
MATERIALS AND METHODS The materials and the polymerization protocols are listed in Table 1. Two direct composite resin materials – Filtek Z100 (3M ESPE; St Paul, MN, USA) and Premise (Kerr; Orange, CA, USA) – and one indirect, Premise Indirect (Kerr), were used in this study (Table 2). The composite resins were light polymerized with an LED unit (Valo Curing Light, Ultradent; South Jordan, UT, USA) in standard power mode with 1000 mW/cm2 of intensity for 40 s per increment, and an additional heat treatment (Premise Indirect Curing Oven, Kerr) at 138°C for 10 min for Premise Indirect composite resin. Microtensile bond strength (MTBS) test The MTBS samples were made from 42 cylindrical composite resin specimens using a plastic mold (8 mm depth x 6 mm diameter) on a glass slide. The samples were fabricated in pairs using two distinct shades of material (Table 2). Seven pairs (n = 7) were made for each group (14 cylinders) and bonded to each other. The dif518
ferent shades within each pair were necessary to identify the adhesive interface and analyze the failure mode (Fig 1). For Filtek Z100 and Premise, cylinders were produced by incremental layering (2-mm-thick layers, light polymerized for 40 s each). For Premise Indirect, a bulk placement technique was used because pilot tests revealed lack of interlayer cohesion when using incremental placement. All cylinders were light polymerized for 240 s through the plastic mold (40 s, 6 times). The cylinders were then removed from the plastic mold. For Premise Indirect only, all specimens were placed into the proprietary oven for 10 min at 138°C without nitrogen pressure. All specimens were then reduced to a 6-mm height with a diamond saw (Diamond Wheel, M4D220-N75M99-1/8, Saint Gobain North America; Valley Forge, PA, USA) under water refrigeration on a cutting machine (IsoMet low-speed Saw, Buehler; Lake Bluff, IL, USA). The fitting surface of each cylinder was treated by wet-grinding flat with a 400-grit silicon carbide paper (Wetordry, 3M ESPE; St Paul, MN, USA) for 5 s, followed by airborne-particle abrasion (RONDOFlex Plus 360, KaVo; Biberach, Germany) for 15 s at 30 psi with 50-μm aluminum oxide (Danville; San Ramon, CA, USA) at a distance of approximately 10 mm and at a 90-degree angle. The surfaces were then cleaned by etching with 35% phosphoric acid (UltraEtch, Ultradent; South Jordan, UT, USA) and gently scrubbing with a microbrush for 1 min followed by rinsing with water. Finally, the samples were placed in an ultrasonic bath with distilled water for 2 min and air dried. A silane (Silane, Ultradent) was applied for 5 s, air dried and heat dried for 1 min (D.I.-500, Coltène Whaledent; Altstätten, Switzerland). Seven pairs of cylinders of the same type of composite resin but dissimilar shades were bonded together using a coat of adhesive resin (Optibond FL Adhesive; Kerr) on each fitting surface. A constant force of 1 kg was apThe Journal of Adhesive Dentistry
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Fig 1 Composite resin sticks obtained by the non-trimming method.
Fig 2 Composite resin cylinder placed in measuring device for FT-IR.
plied to the blocks for 10 s before light polymerizing for 40 s (20 s, twice). The samples were stored in distilled water for 24 h at room temperature before testing. After the storage, each specimen was individually secured with sticky wax (Sticky Wax, Kerr) to a transparent plastic sectioning block. Using the non-trimming technique developed by Shono et al,39 multiple beams were prepared, with the resin-to-resin adhesive interface in the center of the beam. To do so, specimens were vertically sectioned into 1-mm-thick slabs using a low-speed diamond saw. The slabs were sectioned again into beams with approximately 1-mm2 cross-sectional areas. The specimens were attached to a table-top material tester (Micro Tensile Tester, Bisco; Schaumburg, IL, USA) using cyanoacrylate glue (Professional Super Glue, Loctite Henkel; Avon, OH, USA). A chemical accelerator (Zapit Accelerator, DVA; Corona, CA, USA) was applied with a microbrush on the cyanoacrylate. The specimens were subjected to microtensile testing at a crosshead speed of 5.4 kg/min. After testing, the failure mode of each beam was determined under a stereomicroscope (MZ125, Leica Microsystems; Heerbrugg, Switzerland) under 40X magnification. Failures were considered adhesive when the fracture site was located entirely between the adhesive and the composite resin, cohesive when the fracture occurred exclusively within the composite resin, or mixed when fragments of composite resin were detected along the adhesive fracture.
resin was placed in the matrix with a clean spatula in one bulk layer and covered with a sheet of clear plastic matrix. Another glass slide was placed over the plastic and pressed to force the composite to conform to the metal matrix dimensions to acquire a flat, smooth surface. The percent monomer conversion at the surface of the samples (top surface) was collected between 1680 and 1550 cm-1 with the use of a Fourier transform infrared spectrometer (Spectrum 100 FT-IR Spectrometer, PerkinElmer; Beaconsfield, Buckinghamshire, UK) using 4 scans at 2 cm-1 of resolution. The baseline measurement was obtained from unpolymerized material by carefully removing the glass slide and the plastic matrix and placing the sample on the spectrometer. The same procedure was repeated with polymerized samples (1000 mW/cm2 intensity for 40 s applied to top surface) (Fig 2). For Premise Indirect, light polymerization was followed by heat treatment by placing the assembly (metal matrix and composite) into the proprietary oven without nitrogen pressure. The same samples were then stored in water at room temperature until the 24-h FT-IR measurements. The percent monomer conversion was calculated by standard methods from the ratio of absorbance intensities of aliphatic-to-aromatic bonds (C=C) between the nonpolymerized and polymerized composite resin spectra:
Fourier Transform Infrared Spectrometry The shade of each material that was closest to that used for MTBS testing was selected (B2 for Z100; Opaque B1 for Premise; and Primary Dentin B1 for Premise Indirect) for this part of the experiment. Cylindrical composites (5 per material) were made in a metal matrix (3 mm depth x 6 mm diameter) on a glass slide. The composite Vol 16, No 6, 2014
[Abs (aliphatic) / Abs (aromatic)]polymer × 100 [Abs (aliphatic) / Abs (aromatic)]monomer
where Abs was the area of absorption band. Monomer conversion was determined by subtracting the residual percentage of aliphatic C=C from 100% (DC%=100[%C=C]).1,12,30,36 The bond strength data were submitted to the Lilliefors normality test. The monomer conversion data were sub519
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Analysis of variance for bond strength and monomer conversion
Sum of squares
One-way ANOVA of bond strength data Composite
Two-way ANOVA (composite and storage time) of monomer conversion data Composite
Composite x storage time
Table 4 Mean (standard deviation) bond strength in MPa and monomer conversion Filtek Z100 Bond strength (SD)
Premise 48.43 (1.64)b
Table 5 Distribution of MTBS failure modes as observed by optical microscopy
Premise Indirect 52.67 (2.98)b
Monomer Imme53.17 (5.18)Aa 56.13 (4.47)Aa 50.81 (4.68)Aa diate conversion in % 24 h 56.77 (6.90)Aa 66.28 (5.46)Bbc 58.25 (6.08)Bac (SD)
Similar superscript letters indicate no significant difference at p < 0.05. Uppercase letters refer to columns; lowercase letters refer to rows.
mitted to the Shapiro-Wilk normality test. Both datasets were normally distributed (p ≥ 0.05) and a parametric test was chosen. The bond strength data were analyzed with a one-way ANOVA and the monomer conversion with a twoway ANOVA (material and storage time) (α = 0.05). When interactions were significant in the full factorial model, Tukey’s HSD post-hoc test was used to detect pairwise differences (α = 0.05) (Table 3).
RESULTS The results are presented in Table 4. The MTBS to Filtek Z100 was 72.2 MPa, significantly higher than that to Premise (48.4 MPa) and Premise Indirect (52.7 MPa). The immediate monomer conversion was similar for all materials (51% to 56%) and significantly increased at 24 h (57% to 66%), except for Z100. The highest monomer conversions, 58% and 66%, were obtained at 24 h with Premise Indirect and Premise, respectively. Table 5 shows that adhesive failure was the most common failure type found in all groups.
DISCUSSION The null hypothesis can be rejected, in part because Filtek Z100 (intended for direct use) showed significantly higher MTBS and slightly lower monomer conversion (only at 24 h) than Premise and Premise Indirect. 520
The bondability of a composite resin is expected to be correlated to the amount of C=C double bonds available at the surface of the material, which, in turn, correlates with monomer conversion rate and to the amount of resin matrix in the material.3,23,32,47 Composite resins generally contain three main components: filler, monomer, and initiator. Those components, along with the monomer conversion, play an important role in the physical behavior of composite resins.2,8,9,34 In turn, monomer conversion and related physical properties can be influenced by the intensity of the light polymerization unit, the temperature of the polymerization oven, and the variability of the resin formulation among products.7,11,17,35,41,44,47,48 The physicochemical composition of the materials used in the present study (Table 2) reveals similar monomers (bis-GMA and TEGMA) and almost identical amounts of filler by weight (84%–87%). The zirconia present in Filtek Z100 in combination with the amount of bis-GMA and TEG-DMA might be responsible for the high mechanical properties found in this restorative material. The fillers play a prominent role in determining the properties of the composite resins,6,8 and the use of a resin matrix with 50% bis-GMA and 50% TEG-DMA was found to present the highest strength and elastic modulus.2,42 Resin matrix strength is dependent upon monomer composition, being greatest when stiff bis-GMA molecules are used and the monomer conversion of the methacrylate groups is maximized.16 It appears that the stiffness and hardness of solely light-polymerized Filtek Z100 exceeds that of indirect light- and heat-polymerized composite resins.6 The Journal of Adhesive Dentistry
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The flexural strength of Filtek Z100 was also reported to be higher, approximately 180 MPa,6 vs 128 and 158 MPa for Premise and Premise Indirect, respectively (manufacturer’s information). It can be hypothesized that the proprietary spheroidal filler promoted improved rheological properties and optimized bis-GMA/TEGMA ratios in Filtek Z100. Its slightly lower volume percentile of filler might also explain the improved bondability (more resin matrix exposed). The immediate monomer conversion of the three composites tested was similar. Premise Indirect was expected to demonstrate better monomer conversion. As an indirect composite, Premise Indirect was activated by using a combination of light- and heat-curing modes. A higher temperature may enhance radical mobility and polymerization rate, resulting in a superior cross-linking density and final degree of conversion.12,19,45 Filtek Z100 monomer conversion ranged from ca 53% to 57%. This data is in agreement with other studies.14,33,34 Premise Indirect ranged approximately from 51% to 58% in the present study (10 min in oven). In a subsequent pilot experiment, Premise Indirect ranged from about 62% to 73% after 20 min in the oven, confirming the positive effect of heat polymerization on monomer conversion. There is no data in the literature about the monomer conversion of Premise and Premise Indirect. Neither of the composite resins, direct or indirect, were able to reach a high monomer conversion rate (>80%) within the limitations for the present study. Bis-GMA is an extremely reactive molecule and has the ability to overcome diffusion limitations to react. However, the creation of a networked polymer limits the mobility of the reacting system. For this reason, the final monomer conversion in dental resins (ca 50% to 75% bis-GMA by weight) ranges between 55% to 75%.1,12,18,23,34,45,47 Dark polymerization was confirmed in the present study with a significant increase of the monomer conversion after 24 h, although there was no significant difference in the case of Filtek Z100. Hence, a 24-h delay before delivery is suggested for Premise and Premise Indirect. Monomer conversion rates of direct composite resins have been reported to vary with the composition of the resin matrix and polymerization-light intensity and duration.10,14 Manufacturers usually recommend light polymerizing for 20 s given a lighter shade and 40 s for darker shades.48 In the present experiment, the composite resins were light polymerized for 40 s because both light and dark shades were used. Pilot tests failed to reveal differences between light and dark shades, probably because the monomer conversion was measured only at the specimen’s surface. The light source must also be considered, because newer LED units can easily reach 4000 mW/ cm2 of intensity, leading to higher monomer conversion.29 Another parameter is the narrow wavelength distribution of LED units, which coincides with the maximum peak of absorption of camphorquinone.14,46 Consequently, it can be hypothesized that modern LED units (with the effective combination of intensity and narrow wavelength) have reduced the differences in monomer conversion between light-polymerized and heat-polymerized materials. Vol 16, No 6, 2014
However, it must be emphasized that indirect composite resins must be placed in the oven to completely eliminate the toxic benzoyl peroxide heat initiator.15 The high mechanical properties and slightly lower vol% of filler of Filtek Z100, originally used as a direct material, seems to explain the MTBS results achieved in this research. The present work, along with the existing literature, confirms that strong and highly bondable indirect restorations can be fabricated with composite resins originally intended for direct use.5,6,22,28,37,43
CONCLUSION Within the limitations of the present study, it can be concluded that Filtek Z100 showed better “bondability” than Premise and Premise Indirect, probably due to its physicochemical composition and mechanical properties. Premise Indirect, with its heat-initiator, did not present higher monomer conversion.
ACKNOWLEDGEMENTS The authors wish to thank the Kerr Sybron Corporation and 3M ESPE for supplying the composites and adhesive used in this study. Moreover, a special thanks goes to Hector Garza and Al Kobashigawa from Kerr/Sybron for the assistance in FT-IR analysis. This project was supported by the CAPES Foundation Brazil (BEX 3853/10-1).
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Clinical relevance: Highly “bondable” indirect restorations can also be fabricated with composite resins originally intended for direct use. A 24-h delay before delivery is suggested for Premise and Premise Indirect.
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