Polymerization Contraction Stress Research Articles

The influence of delayed light curing on the degree of conversion and polymerization contraction stress in dual-cured resin luting agents

The aim of the study was to assess the effect of delayed photo-initiation on the polymerization contraction stress (PCS) and degree of conversion (DC) of a dual-cure resin-luting agent. Thirty-five disk (6 mm × 1 mm) samples (n = 10 each group) of dual cure resin luting agent for PCS assessment were fabricated and polymerized using two illuminated quartz rods. Based on the delay in photo-initiation, 30 disks were divided among six groups [group A-0 min (min) delay, group B-2 min, group C-4 min, group D-6 min, group E-8 min and group F-10 min]. A non-photoinitiated group (group G – chemical cure – n = 5) was included as control. The PCS for all specimens was assessed using a Tensometer. For DC evaluation thirty-five specimens were divided into seven groups with delays in photo-initiation (group H-0 min, group I-2 min, group J-4 min, group K-6 min, group L-8 min and group M-10 min, group N-chemical cure). DC was assessed using attenuated total reflectance spectroscopic technique. Statistical comparison among groups was performed using analysis of variance (α = 0.05). The maximum and minimum PCS and DC values with delayed photo-initiation was observed in group-C (3.34 MPa) & group-F (2.44 MPa); and group-M (0.78 MPa) and group-H (0.55 MPa) respectively. Chemically cured samples showed the least PCS (group-G, 1.94) and DC (group-N, 0.53) values in their respective categories. PCS significantly decreased with delayed photo-initiation. A significant increase in DC was noticed when photo-initiation was delayed in the dual cure resin luting agent.

Polymerization stress – Is it clinically meaningful?

ObjectivesThe objective of this article is to discuss the evidence for polymerization shrinkage and shrinkage stress of dental composite restoratives in terms of its potential relevance to the clinical situation MethodsArticles relating to the issue of polymerization contraction stress generation in dental composite materials, and the factors that influence it, were reviewed and included. Particular attention was paid to evidence derived from clinical studies. Articles were identified through PubMed and through the bibliographies of other articles. ResultsThere is extensive evidence for the presence of polymerization contraction stress in dental composites, as well as evidence for its deleterious effects, which include marginal leakage, gap formation, cuspal deflection, tooth cracking, reduced bond strength and lowered mechanical properties of the restorative. There is little, if any, direct evidence for the clinical effect of these contraction stresses. No study has directly established a link between these stresses and enhanced postoperative sensitivity or recurrent caries, for example. However, the concern over these stresses and the manner in which they influence the placement of current composite materials demonstrates that they are considered to be very important. ConclusionThough no direct evidence exists to prove that the generation of contraction stress in dental composite restorations causes reduced clinical longevity, the indirect evidence from numerous in vitro studies and the concern over controlling their effects proves that they are clinically relevant.

Effects of light curing method and resin composite composition on composite adaptation to the cavity wall

This study aimed to evaluate the effects of the light curing method and resin composite composition on marginal sealing and resin composite adaptation to the cavity wall. Cylindrical cavities were prepared on the buccal or lingual cervical regions. The teeth were restored using Clearfil Liner Bond 2V adhesive system and filled with Clearfil Photo Bright or Palfique Estelite resin composite. The resins were cured using the conventional or slow-start light curing method. After thermal cycling, the specimens were subjected to a dye penetration test. The slow-start curing method showed better resin composite adaptation to the cavity wall for both composites. Furthermore, the slow-start curing method resulted in significantly improved dentin marginal sealing compared with the conventional method for Clearfil Photo Bright. The light-cured resin composite, which exhibited increased contrast ratios duringpolymerization, seems to suggest high compensation for polymerization contraction stress when using the slow-start curing method.

Micro-shear bond strength of different composites and glass-ionomers used to reinforce root dentin

PurposeComparison of microshear bonding strength of reinforcing root dentin materials (microhybrid composite, nano-composite, conventional glass ionomer, nano-glass ionomer). Materials & methodsA total of 20 roots of similar-sized undamaged one rooted lower premolars were used, 40 root halves were embedded in the acrylic block and were randomly divided into four equal groups (10 each) according to the reinforcing materials. Group I: microhybrid composite, group II: nano-composite, group III: conventional glass ionomer was used and group IV: light cured nano-ionomer was used. Teeth were sliced and loaded to a testing machine. A shearing load with tensile mode of force was applied via materials. The mode of failure was observed under stereomicroscope. The results were statistically analyzed using one way ANOVA and t-test. ResultsNano-composite group recorded the highest μ-shear bond strength mean value (23.52 ± 3.997 MPa) followed by μ-Hybrid composite group (16.88 ± 3.356 MPa) then nano glass-ionomer group (8.77 ± 1.341 MPa) while conventional glass-ionomer group showed the lowest μ-shear bond strength mean value (4.062 ± 0.9623 MPa). The difference was statistically significant between all groups (P < 0.05). The failure modes indicated that high bond strength showed cohesive or mixed modes, while low bond strength groups tended to exhibit adhesive, cohesive or mixed modes. ConclusionNano-composite recorded the highest microshear bond strength close to that required to resist the polymerization contraction stress.

Effect of Preheat Temperatures on Mechanical Properties and Polymerization Contraction Stress of Dental Composites

Objectives: The purpose of this study was to investigate the effect of preheat temperatures on polymerization contraction stress and mechanical properties of three resin composites. Methods: Three resin composites (Filtek Supreme XT, GC Kalore, and Gradia Direct X) at room temperature, 37°C, and 60°C were investigated. Stress development and maximum contraction stress of the composites were evaluated. Directly after preheating, samples were light-cured for 40 seconds and the force recorded for 15 minutes. Subsequent calculations were done to account for the system’s compliance and to obtain the shrinkage stress of the composites. In addition, composite discs (5 mm? and1 mmthick) were light-cured for 40 seconds at the preheat temperature. Hardness, elastic modulus, and creep of composites were investigated using a nano-indentation system (UMIS 2000). The results were analyzed using Two-way Analysis of Variance (2-way ANOVA) and Tukey’s Post-Hoc test (α = 0.05). Results: The results indicated that preheating composites to 37°C and 60°C increased the polymerization contraction forces, but did not significantly affect hardness, elastic modulus, and creep behaviour of the materials. Analysis of the contraction force upon allowing for thermal contraction indicated only a minor influence of preheat temperature. Significance: Preheating composites, upon allowing for system thermal contraction, showed a slight increase of the polymerization contraction stress but did not significantly affect the composites’ mechanical properties.

Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

The loss of interfacial integrity was identified as one of the major causes for replacement of resin composite restorations. Preheating procedure has been proven to enhance flowability and adaptation of resin composites and increase their degree of conversion. The purpose of this study was to investigate polymerization contraction stress produced in resin composites after preheating to 37℃ and 60℃, and measure microleakage of Class V restorations restored with preheated composites. Three resin composites (GC Kalore, Gradia Direct X, Filtek Supreme XT) at room temperature, 37℃, and 60℃ were investigated. Maximum contraction stress of the composites (n = 5) was evaluated in a modified low-compliance device. Samples were light-cured for 40 seconds and the maximum force was recorded during 15 minutes. Calculations were done to adjust for the system’s compliance and obtain linear shrinkage values of composites. Data were analyzed by Multivariated Analysis of Variance (MANOVA) and Tukey’s test for multiple comparisons (α = 0.05). Seventy-two Class V cavities were prepared on the buccal surfaces of extracted premolars and divided into 9 groups. The teeth were restored with composites at 3 temperatures and were thermo-cycled between 5℃ and 55℃ with a one-minute dwell-time for 1000 cycles. The teeth were sealed with wax and nail vanish before placed in 0.5% toluidine blue dye for 24 hours. The teeth were embedded in self-curing resin and sectioned bucco-lingually with a slow-speed diamond saw, providing 3 sections per restoration. Microleakage was rated by two evaluators using a 0 - 4 ordinal scale at the occlusal and cervical margins under light microscope. Microleakage data were analyzed with Kruskal-Wallis ANOVA and Mann-Whitney U test (α = 0.05). Results indicate that preheating composites to 37℃ and 60℃ significantly increased polymerization contraction stress of composites (p 0.05). A significantly greater amount of leakage was found at the cervical margins (p 0.05). For all tested materials, preheating composites to 60℃ resulted in significantly less microleakage at the cervical margin.

Shear bond strength of three dual-cured resin cements to dentin analyzed by finite element analysis

ObjectivesTo determine the shear bond strength to bovine dentin of dual-cured resin cements cured in different circumstances, the contraction stress and volumetric shrinkage in both polymerization modes, and to review the failure stress distribution at the cement–tooth interface with finite element analysis. MethodsThe volumetric shrinkage of RelyX Unicem, Panavia F 2.0 and DC Core Automix was determined by mercury dilatometry. Polymerization contraction stress was determined using a constraint tensilometer set-up. For the shear bond strength test, cement discs on bovine root dentin (self-cured and dual-cured), composite discs cemented to dentin (self-cured and dual-cured), and dentin cemented to dentin (self-cured) specimens were fabricated. Specimens were stored in water for 24h (37°C, 100% humidity) and tested (crosshead speed 1mmmin−1). FE modeling of the specimens was carried out in order to calculate the maximum shear stresses in the cement–dentin interface. Differences between groups were determined using two-way ANOVA with Tukey post hoc tests, and paired samples t-tests (α<0.05). ResultsPanavia F2.0 showed significantly lower volumetric shrinkage than the other cements. Dual-curing lead to higher contraction stresses for all tested cements compared to self-curing. RelyX Unicem showed higher volumetric shrinkage when dual-cured. Shear bond strength and maximum shear stress was positively influenced by dual-curing. DC Core Automix performed best and Panavia F2.0 worst in terms of shear bond strength and maximum shear stress. SignificanceCuring mode may play an important role in the final bond strength to dentin of indirect restorations, depending on the material used.

Polymerization profile analysis of resin composite dental restorative materials in real time

ObjectivesIn this study, the hypothesis that the polymerization shrinkage profile of “low shrinkage” non-methacrylate based composite; Silorane and “low shrinkage” high molecular mass methacrylate based composite; Kalore is not different from that of three conventional methacrylate based composites (Gradia Direct X, Filtek Supreme XT and Beautifil II) was tested. MethodsFive commercially available composites were analysed: one “low shrinkage” non-methacrylate based composite (Silorane); one “low shrinkage” high molecular mass methacrylate based composite (Kalore) and three conventional methacrylate based composites (Gradia Direct X, Filtek Supreme XT and Beautifil II). Polymerization shrinkage was measured using an electromagnetic balance which recorded changes in composite buoyancy occurring due to volumetric changes during polymerization. This instrument allowed real time volumetric shrinkage measurements to be made at 40ms intervals. ResultsAll five resin composites demonstrated a similar volumetric shrinkage profile during polymerization. The rate of shrinkage of all five composites decreased from t=0 at a rate approximating x=t. After 170s the rate of shrinkage of all five composites was at or below 0.01%/s. During the initial 5s of light exposure Silorane and Kalore exhibited a significantly lower (p<0.05) rate of contraction relative to the three conventional methacrylate composites. After 640s of analysis, Silorane exhibited a significantly lower (p<0.05) percentage volumetric contraction compared to the other four analysed materials. ConclusionsThe newly developed “low shrinkage” composites (Silorane, Kalore) in the present study demonstrated significantly lower (p<0.05) shrinkage rates and shrinkage volumes compared to the three conventional methacrylate composites. Investigation to identify whether polymerization shrinkage profile analysis is a good predictor of relative polymerization contraction stress levels generated by different composites, is warranted. Clinical significanceClinicians making a resin composite selection with the view to minimizing the clinical effects of polymerization shrinkage must consider the rate of polymerization as well as the total volumetric shrinkage of a composite. Silorane (non methacrylate composite) and Kalore (high molecular mass methacrylate composite) have the ability to exhibit lower shrinkage rates and lower shrinkage volumes compared to conventional methacrylate composites.

Factors influencing marginal cavity adaptation of nanofiller containing resin composite restorations

Objectives The aim of this study was to investigate the effects of polymerization contraction, shrinkage stress and Young's modulus of nanofiller containing resin composites on early marginal adaptation of restorations in cavities. Methods Six nanofiller containing and two reference resin composites were studied. Marginal gap widths of restorations in cylindrical 4.2 mm wide and 1.5 mm deep dentin cavities, non-bonded or bonded with a self-etch adhesive, and in Teflon cavities of same dimensions were determined 15 min after irradiation ( n = 8). Polymerization shrinkage strains were measured using the bonded-disk ( n = 8) and a strain gage method ( n = 8). For determination of contraction stress the composites ( n = 10) were bonded to and cured in Araldit molds using a photoelastic method. Flexural moduli of the restoratives were studied according to ISO specification 4049 ( n = 5). Statistical analysis was performed with one- and two-way ANOVA, Kruskal–Wallis ANOVA test and post hoc tests ( p < 0.05). Results Only two nanofiller composites (Kalore, GC, Japan) and Venus Diamond (Heraeus Kulzer, Germany) showed consistently gap-free margins in bonded dentin cavities. The mean gap widths in non-bonded and in Teflon cavities were 6.1–12.8 and 14.1–25.5 μm, and linearly correlated ( r 2 < 0.85). Significant linear relationships were observed between strain, stress and marginal gap widths in non-bonded and Teflon cavities ( p < 0.01). Flexural moduli (15 min) were between 1.66 and 8.63 GPa. Significance Marginal cavity adaptation of restorations in bonded dentin cavities reflects complex interactions between adhesive bonding on the one hand, and polymerization contraction strain, stress and elastic modulus, on the other.

Formation of gaps at the filler–resin interface induced by polymerization contraction stress: Gaps at the interface

Objectives To investigate the movement of resin matrix with respect to the filler particles of filled composites during their photo cure without or with polymerization contraction stress (PCS). Methods Two types of composites were prepared. Glass beads as macroscopic fillers were placed into the center of a bis-GMA/TEGDMA resin to make single bead-embedding “composites” and a variety of fillers of different compositions, sizes, and shapes were mixed with another bis-GMA/TEGDMA resin to make lightly filled composites. They were photo cured in a cavity constructed with an acrylic or aluminum ring sitting on a polyester strip. Bonding to the ring constrained the polymerization shrinkage and thus produced a PCS. The formation of gaps between the filler and the resin was detected by optical microscopy for the glass bead–resin systems, and by light attenuation and scanning electron microscopy (SEM) for the filler–resin composites. Results In general, for composites with untreated fillers, the optical microscopy and SEM revealed gaps at the filler–resin interface only when they were cured under constrained shrinkage conditions. These composites attenuated more light when cured under constrained shrinkage conditions than when under non-constrained conditions. For the composites with silane-treated fillers, no gaps were observed. Some did not show any significant difference in light attenuation when cured under either constrained or non-constrained conditions. Conclusions The resin tends to move away from the filler particles under the influence of PCS. Strengthening the filler–resin interaction, such as by the use of silane-treated filler, may help prevent the resin departure and thus the formation of gaps.

Polymerization Contraction Stress of Resin Composite

Polymerization Contraction Stress of Resin Composite

Polymerization contraction stress in resin–tooth bonds under hydrated and dehydrated conditions

Objective This study hypothesizes that, with enamel or dentin as a bonding substrate, intrinsic water affects the development of polymerization contraction stress in the bonds of self-etching adhesives during bonding. Materials and methods The influence of the water content in dentin and enamel (wetness with water as control and acetone-dried specimens) on the stress development in self-etching adhesives was determined with a tensilometer. Thin layers of self-etching primer and/or adhesive resins were created between a glass plate and a flat enamel or dentin surface. Results After an initial maximum shortly after light curing for 30 min, the contraction stress was decreased in the dentin (30–70%) and enamel (approximately 20%). In the acetone-dried specimens, the stress was continuously increased for 20–50%. Significance The intrinsic water content of tooth tissue influences the initial polymerization of polymers. This effect is favorable for stress relief in resin restoration but causes unwanted nanoleakage channel formation in resin–tooth bonds.

Effects of restoration and substrate on polymerization contraction stress of dental composites

The aim of this study was to treat both restoration and substrate as a combined factor (RS-factor) to complement the popular C-factor in prediction of polymerization contraction stress (PCS). A simple model consists of a uniaxial restoration with a curing composite sandwiched between two solid mountings (substrates). By using the equal stress principle and taking into account substrate deformation, a set of equations were developed and solved, resulting in a mathematical relationship between PCS and the size and stiffness of the substrate and the restoration. The strain gage method was used to experimentally assess the PCS of a light-cured composite encircled in an aluminum ring. Differently sized inserts made of cured composites and glass were placed in the center of the ring to control the thickness of the composite to be cured and created different RS factors as well as C-factors. According to the model, a restoration with a small RS-factor will produce a high PCS. The model also predicts that a restoration with a small C-factor will have a high PCS because of the compliance of substrates. These predictions were tentatively confirmed by the strain gage measurements. A higher PCS was detected when a smaller insert was used, which created a smaller RS-factor or C-factor, or when a glass insert was used instead of a less stiff composite insert, in which the former created a smaller RS-factor. The RS-factor may be a dominant factor in determining the PCS outcome in some special cases.

Polymerization contraction stress in dentin adhesives bonded to dentin and enamel

Objective In a previous study on of polymerization contraction stress determinations of adhesives bonded to dentin a continuous decline of stress was observed after the adhesives had been light-cured. The decline was ascribed to stress relief caused by diffusion into the adhesive layer of water and/or solvents, left in the impregnated dentin surface after drying and/or evaporation in the application procedure. The purpose of the present study was to test the hypothesis that the contraction stress of adhesives bonded to enamel will not decline after light-curing, based on the assumption that water and/or solvents are more efficiently removed from impregnated enamel surfaces in the drying and/or evaporation step. Materials and methods Contraction stress was determined in a tensilometer for three total-etching adhesives Scotchbond multi-purpose, Single bond and One-step plus and four self-etching adhesives Clearfil SE Bond, Clearfil Protect Bond, AdheSE, and Xeno III. The adhesives were placed in a thin layer between a glass plate and a flat dentin or enamel surface pre-treated with phosphoric acid or self-etching primer and light-cured under constrained conditions. Results All adhesives bonded to enamel showed a stress decline, but significantly less than for dentin with the exception of two self-etching adhesives. The greatest decline was found for the total-etching adhesive systems bonded to dentin. The presence of hydrophobic monomers in the adhesives had a significant influence on the decline. Significance The experiments indicate that fluids are withdrawn from the resin impregnated tooth structures, which may result in small defects in the tooth–resin interfaces.

Interactions of self-etch adhesives with resin composites

Objective Aim of this in vitro study was to investigate the correlation of shear bond strength and marginal cavity adaptation, together with polymerization shrinkage and contraction stress, using the combination of four self-etch adhesives and three resin composites. Materials and methods Interactions were studied between one two-step and three one-step adhesives, and the hybrid-type resin composites, Beautifil (BEU, Shofu) and Venus (VEN, Heraeus), and an experimental nano-hybrid resin composite NEUN (NEU, Heraeus). For all 12 combinations shear bond strengths (SBS) were determined on human dentin. Marginal adaptation (MGW) was assessed in cylindrical butt-joint dentin cavities. Further, polymerization contraction and contraction stress of the resin restoratives were measured. Results Significant determinants of SBSs on dentin were time of testing (10 min or 24 h) and adhesives ( p < 0.001). Marginal adaptation was best for NEU, followed by VEN and BEU. Only the resin composite used was a highly significant determinant of cavity adaptation. Polymerization shrinkage after 5 min was 2.58, 2.74, and 1.53% for BEU, VEN, and NEU, respectively. Polymerization contraction stresses were largest for BEU, less for VEN, and smallest for NEU ( p < 0.05). Conclusions No correlation was found between bond strength and marginal cavity adaptation. In contrast, reduced shrinkage and low polymerization contraction stress of resin composites were identified as important determinants of marginal cavity adaptation.

Influence of soft-start irradiation on polymerization shrinkage stress of universal hybrid composite resins

To compare the difference between soft-start curing mode and standard curing mode in polymerization shrinkage stress of universal hybrid composite resins and to study effect of the soft-start curing mode on the decrease of shrinkage stress. Three universal hybrid resins (A: Charisma, B: TPH Spectrum, and C: Esthet-X) were respectively filled in cavities (4 mm in diameter) of epoxide resin disks, 16 specimens of each. Off them, eight of the specimens for each composite resin were exposed using soft-start mode and the other eight using standard mode. Polymerization contraction stress was calculated during 48 h after curing with photo-elastic stress analysis. Three composite resins cured with soft-start mode showed the same trend of shrinkage stress changing as that with standard curing mode and values of polymerization shrinkage stress at 24 h after curing were (3.80 +/- 0.31) MPa, (3.21 +/- 0.40) MPa, and (2.84 +/- 0.22) MPa respectively for A, B and C composite resins. The corresponding figures for the composites with standard curing mode were (4.19 +/- 0.24) MPa, (3.69 +/- 0.33) MPa, and (3.14 +/- 0.28) MPa. Three composite resins cured with soft-start mode had significantly lower polymerization shrinkage stress compared with standard curing mode at 24 h after curing (P < 0.05). Using soft-start curing mold can reduce, to some extent, the polymerization shrinkage stress of universal hybrid composite resins.

Assessment of polymerization contraction stress of three composite resins

Objectives The purpose of this study was to measure the development of contraction stress of three composite resin restorative materials during photo-polymerization: a micro-hybrid composite (Filtek Z250, 3M ESPE, St. Paul, MN, USA); a nano-filled composite (Filtek Supreme, 3M ESPE, St. Paul, MN, USA); and a low-shrinkage composite (Ælite LS, Bisco Inc., Schaumburg, IL, USA). Methods Curing shrinkage stress was measured using a stress-analyzer. Composites were polymerized with a halogen-curing unit (VIP, Bisco Inc., Schaumburg, IL, USA) for 40 s. The contraction force (N) generated during polymerization was continuously recorded for 150 s after photo-initiation. Contraction stress (MPa) was calculated at 20, 40, 60 and 150 s. Data were statistically analyzed. Results The low-shrinkage composite Ælite LS exhibited the lowest stress values compared to other materials ( p < 0.05). Statistical analysis did not show significant differences between Filtek Z250 and Filtek Supreme. Significance The low-shrinkage composite showed lower contraction stress than micro-hybrid and nano-filled composite. Ideally, non-shrinking resins would represent the ultimate solution to overcome polymerization contraction and stress-related problems.

Shrinkage stress in light-cured composite resins: Influence of material and photoactivation mode

Objectives The aim of this study was to record the effect of composite type and photoactivation mode on the stresses resulting from polymerization of five established composite resins: packable (Solitaire, Solitaire 2), micro-hybrid (Aelitefil, Z100) and hybrid (Clearfil AP-X). Methods A mechanical testing machine was used to record the polymerization contraction stress (MPa) of cylindrical composite specimens ( d = 5 mm; h = 2 mm; C-factor = 1.25) at 0.1 s intervals over a period of 400 s. The samples were photopolymerized using a halogen light curing device under two types of light exposure: group 1, Standard (800 mW·cm −2 × 60 s); group 2, Exponential (logarithmic increase from 150 to 800 mW·cm −2 over 15 s + 800 mW·cm −2 × 45 s). The stress rate (SR: slope MPa>0–60 s ) and the maximum shrinkage stress (MSS: MPa 400 s ) of each material (five replications) were statistically analysed by one-way ANOVA/Scheffe's test and Pearson's correlation procedure ( α = 0.05). Finally, Student's t-test (two matched series) enabled the assessment of the effect of the irradiation method on the results. Results For group 1, in decreasing order, the MSS was 1.51 ± 0.07 MPa (Solitaire) statistically equivalent to 1.45 ± 0.06 MPa (Aelitefil), 1.29 ± 0.08 MPa (Solitaire 2), and 1.04 ± 0.03 MPa (Z100) statistically equivalent to 0.92 ± 0.05 MPa (Clearfil AP-X). Z100 showed the highest SR (0.045 ± 6 × 10 −3) and Solitaire, the lowest (0.017 ± 2 × 10 −3). There was no correlation between SR and MSS ( r < −0.33, p < 0.05). For group 2, the MSS and SR values were distributed in a similar way to those from group 1. There was a negative correlation between SR and MSS ( r < −0.43 and p < 0.01). The exponential ramp successfully reduced the MSS (−3.9%) and SR (−11%) values. Significance There is no relationship between composite resin type, stress rate and shrinkage stress levels. The slower stress rate development, resulting from ramped light intensity, helped slightly to reduce the maximum polymerization stress.

Contraction stress and bond strength to dentinfor compatible and incompatible combinations of bonding systems and chemical and light-cured core build-up resin composites

Recent studies have shown that adhesives containing acidic monomers combined with composites can adversely effect the polymerization reaction producing low bond strengths. This phenomenon may also occur in making composite build-ups, jeopardizing one of the key factors for a successful core build-up restoration. The aim of this study was to investigate the contraction stress development and bond strength to dentin of core build-up resin composites combined with adhesives of various acidities. In addition the hypothesis tested was that light irradiation through chemical-cured composites during curing does not influence contraction stress or bond strength to dentin. The chemical-cured (Clearfil Core) and light-cured (Clearfil Photo Core) core build-up resin composites were combined with two light-cured adhesives, Clearfil SE Bond (pH=1.8) and One-Step Bond (pH=4.3) and two dual-cured adhesives, Clearfil Photo Bond (pH=2.5) and All-Bond 2 (pH=6.1). Contraction stress development (at C=3) was determined for a period of 30 min in a universal testing machine where the opposing bonding surfaces were glass and dentin. After the 30 min period, the specimens were loaded in tension to determine the bond strength to dentin. To test the hypothesis, the combinations of the chemical-cured composites with the four bonding systems were also light irradiated for 40s right at the start of curing. For all composite-adhesive combinations tested, the adhesion to dentin resisted the developing polymerization contraction stresses. Both, dentin as a substrate to bond at and the use of adhesives, were showed to play an important role in keeping the contraction stresses low. The chemical-cured composite (Clearfil Core) combined with the light-cured adhesive SE Bond (pH=1.8) showed for both contraction stress and bond strength significant lower values than the other combinations. The hypothesis was accepted for combinations of the chemical-cured composite with All-Bond 2 and One-Step Bond, but was not supported by combinations with Clearfil SE Bond or Clearfil Photo Bond, as a significant increase in contraction stress was found. The higher values found for bond strength were not significant. Besides combinations of chemical-cured core build-up composites with light or dual-cured adhesives as recommended by the manufacturer, also combinations with adhesives of other manufacturers are compatible, provided that the pH is higher than approximately 4.3. Chemical-cured core build-up composites combined with light-cured adhesives with a pH as low as 1.8 lead to a significantly lower stress and bond strength compared to other combinations. Light irradiation during curing through a combination of a chemical-cured composite and a low pH adhesive reactivates polymerization.

A mechanism on why slower polymerization of a dental composite produces lower contraction stress

It has been well documented that the rate of polymerization of a dental composite often affects its polymerization contraction stress. In most cases, a slower cure produces a lower stress. To investigate the mechanism behind this, we prepared an unfilled dimethacrylate resin sample and photocured it using two light irradiances, both with the same total irradiation energy. We measured the polymerization-induced shrinkage from the unbonded surface of a class I restoration, contraction stress, extent of polymerization, and flexural modulus. The resin specimens cured under the two irradiances achieved the same extent of polymerization and developed an identical amount of shrinkage from the unbonded surface. But those cured under the lower irradiance possessed a lower contraction stress and a lower flexural modulus than those cured under the higher irradiance. We demonstrated that the stress level did not respond to the extent of viscous flow of the curing resin because the slower polymerization did not produce more shrinkage from the unbonded surface. Instead, the lower stress is likely due to a lower modulus of the cured resin. To explain why the cured resin with an identical extent of polymerization can have different moduli, we proposed that slower polymerization produces a higher level of structural inhomogeneity, which reduces the rigidity of the cured resin.