This study evaluated the effect of composite pre-polymerization temperature and energy density on the marginal adaptation (MA), degree of conversion (DC), flexural strength (FS), and polymer cross-linking (PCL) of a resin composite (Filtek Z350, 3M/ESPE).
For MA, class V cavities (4 mm× 2 mm× 2 mm) were prepared in 40 bovine incisors. The adhesive system Adper Single Bond 2 (3M/ESPE) was applied. Before being placed in the cavities, the resin composite was either kept at room-temperature (25 °C) or previously pre-heated to 68 °C in the Calset™ device (AdDent Inc., Danbury, CT, USA). The composite was then light polymerized for 20 or 40 s at 600 mW/cm 2 (12 or 24 J/cm 2 , respectively). The percentage of gaps was analyzed by scanning electron microscopy, after sectioning the restorations and preparing epoxy resin replicas. DC ( n = 3) was obtained by FT-Raman spectroscopy on irradiated and non-irradiated composite surfaces. FS ( n = 10) was measured by the three-point-bending test. KHN ( n = 6) was measured after 24 h dry storage and again after immersion in 100% ethanol solution for 24 h, to calculate PCL density. Data were analyzed by appropriate statistical analyses.
The pre-heated composite showed better MA than the room-temperature groups. A higher number of gaps were observed in the room-temperature groups, irrespective of the energy density, mainly in the axial wall ( p < 0.05). Composite pre-heating and energy density did not affect the DC, FS and PCL ( p > 0.05).
Pre-heating the composite prior to light polymerization similar in a clinical situation did not alter the mechanical properties and monomer conversion of the composite, but provided enhanced composite adaptation to cavity walls.
The most common reasons for composite restoration replacement are fracture and secondary caries . Therefore, techniques that promote higher conversion and mechanical properties without jeopardizing the marginal sealing are of special interest. The first approach to this aim was made by the use of soft-start light polymerization methods and more recently studies have focused on the influence of composite pre-heating on the composite properties .
It has been reported that increasing composite temperature up to 60 °C might enhance the conversion degree on the top and in 2 mm of the bottom surfaces .
Composite pre-heating (60 °C) under an isothermal condition is capable of increasing monomer conversion, as molecular mobility is enhanced and collision frequency of reactive species is increased . Composites with increased conversion are expected to be highly cross-linked and to have better mechanical properties . As a consequence of this enhanced cross-linking; the free volume within the polymer network is reduced, as well as the solvent uptake and material degradation in the oral environment , although this issue is yet to be investigated.
Despite the aforementioned advantages, Lohbauer et al. showed that composite pre-heating may cause detrimental effects on the restoration margin as it increases the polymerization shrinkage of the resin composite. To the extent of the authors’ knowledge, no other study has so far addressed the effects of composite pre-heating on marginal adaptation of composite restorations, which deserve further investigation.
Furthermore, it is worth mentioning that the studies that have demonstrated optimization in monomer conversion generally maintained the composite temperature constant, a situation that cannot be reproduced in a clinical situation. Once composite is pre-heated, there is a time delay between dispensing it from a syringe or compute, placing it into a preparation, contouring it, and subsequently light polymerizing it. It is estimated that when a composite is heated up to 60 °C and removed from the device, the temperature reduces 50% after 2 min and 90% after 5 min . Therefore, it is clinically important to evaluate the influence of pre-heating under a non-isothermal condition, to simulate the real clinical scenario.
Therefore the aim of this study was to evaluate the effect of pre-heating on marginal adaptation, monomer conversion, flexural strength, microhardness, and polymer cross-linking of a resin composite under a non-isothermal condition for two energy densities (12 and 24 J/cm 2 ). Two hypotheses were tested: (1) pre-heating increases composite monomer conversion, flexural strength, polymer cross-linking and marginal adaptation of composite to the cavity; (2) increased energy density increases composite monomer conversion, flexural strength and polymer cross-linking, and marginal adaptation of the composite to the cavity.
Materials and methods
For all experimental conditions, the nanofiller resin composite Filtek Z350 (3M/ESPE, St. Paul, MN, USA; batch number 6AJ; shade C2) was used and light polymerized at 600 mW/cm 2 for 20 s (energy density = 12 J/cm 2 ) or 40 s (24 J/cm 2 ) ( Table 1 ), with a quartz–tungsten–halogen light polymerization unit (Optilux 501, SDS Kerr, Danbury, CT, USA). Irradiance was checked daily with the built-in radiometer and with a Model 100 Optilux Radiometer (SDS Kerr, Danbury, CT, USA).
|Group||Pre-polymerization composite temperature||Irradiance (mW/cm 2 )||Irradiation time (s)||Energy density (J/cm 2 )|
Composite pre-heating was performed by a Calset™ device (AdDent Inc., Danbury, CT, USA) that elevates composite temperature to 68 °C. The mean time between removing composite from the device and light polymerization was approximately 40 s for all tests. The specimen preparation and testing was performed at controlled room-temperature (25 °C).
Marginal adaptation (MA)
Forty sound bovine incisors were stored in 0.5% chloramine solution for no longer than three months after extraction. Teeth were randomly assigned to one of four experimental groups ( Table 1 , n = 10). Cylindrical diamond burs (ref #2094, KG Sorensen, Barueri, SP, Brazil) were used to prepare class V cavities (4 mm wide × 2 mm long × 2 mm deep) with enamel margins and axial wall in dentin. The internal walls of each cavity ( C -factor = 3) were perpendicular to the top and bottom surfaces, with round angles as defined by the bur shape.
Preparations were acid etched (35% phosphoric acid, 3M/ESPE, St. Paul, MN, USA) for 15 s, washed and gently dried. The adhesive system Adper Single Bond 2 (3M/ESPE, St. Paul, MN, USA; batch number 6FT) was applied in two layers according to the manufacturer’s directions and light polymerized for 10 s at 600 mW/cm 2 . Resin composite either at room-temperature or pre-heated to 68 °C was placed in bulk and light polymerized for 20 or 40 s.
Restorations were then stored in distilled water at 37 °C for 7 days. Each specimen was sectioned in half through the center of the restoration with a diamond saw (Extec-Blade XL 12235, Extec Corp., Enfield, CT, USA) in a cutting machine (Labcut 1010 Extec Corp., Enfield, CT, USA), resulting in two fragments with the adhesive interface exposed. Both fragments were embedded in epoxy resin and polished down using decreasing grit abrasive papers (600, 1200, 2400 and 4000, Buehler Ltd., Lake Bluff, IL, USA) and 0.5 μm diamond paste (Buehler Ltd., Lake Bluff, IL, USA) with a polishing cloth. Impressions of polished surfaces were taken with low viscosity vinyl polysiloxane material (Express, 3M/ESPE, St. Paul, MN, USA), which served as molds to fabricate epoxy resin replicas (Buhler Epoxicure Resin, Lake Bluff, IL, USA).
Replicas were platinum coated (MED 020, Bal-Tec, Liechtenstein) and the gaps in the interface were analyzed at 200× magnification in a scanning electron microscope (SEM) (Stereo Scam/LEO, Cambridge, United Kingdom). Adhesive interfaces were divided into seven sections ( Fig. 1 ). Each section received a score according to the presence of gap: 0 = no gaps observed; 1 = at least 1 gap observed. Higher SEM magnifications (300–1600×) were used to allow visualization of the gaps at the adhesive interface.
Two statistical analyses were performed. First, the scores given for the two tooth halves were averaged and a single score was then used per tooth. Data from the four experimental groups were analyzed by Kruskall–Wallis and Mann–Whitney’s test ( α = 0.05). Second, a mean of the scores attributed to the axial wall for the two tooth halves (sections 3–5) was averaged, as well as the proximal walls (sections 1, 2, 6 and 7), in order to compare the frequency of gaps in the proximal and axial walls. Data were analyzed by Wilcoxon’s test for segment mean score comparisons and Friedman’s test ( α = 0.05).
Degree of conversion (DC)
Cylindrical specimens ( n = 3), 2 mm high and 5 mm in diameter were built-in a split Teflon mold positioned between mylar strips. Specimens were stored dry for 24 h at 37 °C and then submitted to degree of conversion (DC) analysis using FT-Raman spectroscopy (RFS 100/S, Bruker Analytische Meßtechnik, Karlsruhe, Germany), with an Nd-Yag laser. Both irradiated and non-irradiated surfaces of the composite were analyzed. Spectra were obtained by co-addition of 128 scans, at a resolution of 4 cm −1 . DC was obtained by standard baseline techniques. The percentage of unreacted carbon–carbon double bonds (% C C) was determined from the ratio of absorbance intensities of aliphatic C C (peak height at 1640 cm −1 ) against internal standard before and after specimen polymerization. The aromatic carbon–carbon bond (peak height at 1610 cm −1 ) absorbance was used as an internal standard. The degree of conversion (DC) was determined by subtracting the % C C from 100%.
Flexural strength (FS)
For FS measurements, 10 specimens of each experimental condition were prepared using a stainless steel mold (10 mm× 2 mm× 1 mm), positioned over a polyester strip. After filling the mold to excess, the material surface was covered with a polyester strip and a glass slide and compressed under a 500 g load to extrude excess material. The specimens were light activated according to the protocols already described in Table 1 . Specimen dimensions were measured using a digital caliper (Digimatic Caliper CD-6” OS, Mitutoyo, Japan). Samples were then stored in distilled water at 37 °C for 24 h.
The three-point bending test was carried out in a universal testing machine (Kratos Dinamômetro, São Paulo, Brazil), at a cross-head speed of 0.5 mm/min and 8 mm span between supports. FS was calculated with the following formula:
F S = 3 × L × D 2 × W × H 2