Effect of exposure time and pre-heating on the conversion degree of conventional, bulk-fill, fiber reinforced and polyacid-modified resin composites
Introduction
The marginal seal is an essential factor in the longevity of a dental restoration. Leakage at the deep gingival margin can lead to secondary caries development resulting in the failure of the restoration and compromising the health of the vital pulp tissue [1]. In root canal treated teeth the penetration of microorganisms through the coronal orifice of the root canal may also cause recontamination and subsequent failure of the endodontic treatment [2].
Resin-based composite restorative materials (RBC) are widely used among dentists as the most common restorative material. Evolution in both filler and polymer technology led to a wide selection of materials that provide the adequate characteristics required for each clinical situation [3]. Besides the conventional RBCs, bulk-fill and fiber reinforced RBCs are also available in the market as improved materials. Low and high viscosity bulk filling composites usually have higher translucency, and sometimes a modified initiator system to ensure better curing in depth, as compared to conventional composites. These materials are recommended to use in 4 mm or even 5 mm in thickness without stratification [4], and promise adequate curing depth, physical and mechanical properties. Many bulk-fill composite resins have been investigated regarding different parameters like mechanical features, degree of conversion, polymerization stress or microleakage. On the one hand, such studies have shown that bulk-fill composite resins have similar physical and chemical properties as conventional RBCs [5], [6], [7], [8], on the other hand, bulk filling RBCs were found to have lower mechanical properties, higher shrinkage rate and lower degree of conversion in the recommended 4 mm thickness compared to 2 mm of the conventional RBC’s [9], [10], [11].
Fiber reinforcement of conventional dental composites were also introduced with the aim of enhancing their physical properties [12]. The enhancement was due to the stress transfer from the matrix to the fibers depending on the fibers length and diameter, leading to high resistance to fracture [13], [14]. Besides the above mentioned developments, manufacturers are looking for continuous improvements to eliminate disadvantageous properties, like the polymerization shrinkage and the inadequate rate of polymerization. The presence of the aforementioned drawbacks adversely affects the marginal or coronal leakage. To avoid it, flowable RBCs could be used at the gingival margins of a proximal cavity or as a barrier to seal the orifices of the root canals [15]. Flowable RBCs have better adaptation to the cavity walls owing to their high tooth surface wetting ability, ensuring penetration into all irregularities [16].
Pre-heating of RBC can also decrease microleakage. Increasing the polymerization temperature leads to lower viscosity thus increasing the fluidity and improving the adaptation of the RBC material to the cavity [17], [18]. Pre-heating in turn results in greater mobility of monomer molecules within the resin matrix, enhances free radical formation, which results in a higher value of the DC and shorten curing time [19], [20]. The increased mobility of monomers at elevated temperature can lead to delayed autodeceleration stage of the polymerization reaction thus contribute to increased monomer conversion [21]. In addition, pre-heating significantly reduces the generation of shrinkage forces in both high-viscosity bulk-fill and conventional resin composites [22].
Clinical restoring procedures meet complex cavity shapes which could be challenging. Occasionally, cavity preparations that are 7–10 mm deep with a narrow orifice, as well as the angulation of the light curing tip may influence the polymerization rate of RBCs. Incomplete curing can lead to the early degradation, wear of the RBC restoration and also affect the functional durability, eventually leading to failure [23]. Light-curing an RBC is a complex process, as the depth of cure is affected by material composition, layer thickness, irradiance, curing time and variety of other factors [24]. For adequate polymerization the conventional RBC should receive a radiant exposure within the 16–24 J/cm2 range [25]. This radiant exposure or energy density is calculated by multiplying the irradiance level coming from the light curing unit (LCU) by its duration [25]. Curing time is set depending of the irradiance level of the LCU. The “exposure reciprocity low” proposes reciprocity between the irradiance level and exposure duration to achieve equivalent DC of RBCs. This low has been evaluated in the literature and found not to apply, as it depends on the photoinitiator- and monomer-system of the RBC, the spectral radiant power of the LCU and is even time-dependent [26], [27], [28]. Selig et al. showed that an exposure time of only 10 s and above gave a sufficient DC [29], thus increasing the light exposure time results in higher radiant exposure reaching the RBC increment, especially with conservative cavity preparation (small orifice) and increased distance between the LCU tip and the RBC surface [30].
Selecting the proper material from the wide range available in the market is also a hard decision. In deep, occasionally irregular cavities the RBC should be easy to handle – if it is possible without the conventional layering – well adaptable and must be converted at an acceptable degree to provide good sealing and mechanical properties with low solubility. When sealing the orifices in root canal treated teeth, the use of a well distinguished material could be also advisable supposing a possible future re-treatment.
The purpose of this study was to measure the conversion degree with micro-Raman spectroscopy at the top and bottom of the first layer of a conventional sculptable and flowable, two flowable bulk-fill, a fibre-reinforced high-viscosity bulk-fill and a low-viscosity, coloured polyacid-modified RBC applied in a clinically relevant in vitro model, where an 8 mm distance from the light guide tip to the bottom side of the cavity was compiled. Further aim was to determine the effect of the recommended and the doubled curing time, as well as the RBC’s pre-heating to 35 °C or 55 °C on the polymerization rate of the investigated materials.
Section snippets
Preparation of the composite resin specimens
During this in vitro study six brands of resin composite material – a conventional sculptable microhybrid, a flowable nanofill, two flowable bulk-fill RBC, a fibre-reinforced bulk-fill material and a polyacid-modified RBC – were analyzed. The brand, the chemical composition and the manufacturer are presented in Table 1. According to the sample preparation and polymerization method, four experimental groups of specimens were divided. In each group, from each material, 5 specimens were prepared.
Results
Fig. 2 shows the degree of conversion at the top of the investigated materials according to the method of polymerization. Regarding the top of the samples, conversion degree of the different materials ranged between 38.9% and 75.6%. The lowest value was measured in case of conventional sculptable microhybrid RBC (FZ_20) irradiated with the recommended exposure time at room temperature, meanwhile the highest DC% was detected in the case of the polyacid-modified resin composite (TS_80) with
Discussion
In this study a clinically relevant 8 mm deep, 5 mm wide mold was filled with six different RBCs in the recommended layer thicknesses, irradiated with the recommended and its doubled exposure time as well as pre-heated to 35 and 55 °C. The degree of conversion at the top and bottom surfaces was assessed using micro-Raman spectroscopy. Different type of RBC materials were included in this investigation: a commercial sculptable microhybrid RBC (FZ), a conventional flowable nanofill RBC (FUF), a
Conclusion
Within the limitations of this in vitro study – simulating an eight mm deep clinically relevant simulated cavity – the following conclusions can be stated:
- 1)
Significantly higher DC levels were measured at the top of the samples compared to the bottom in each investigated material, in each experimental group, except SDR in Group 1 and 4.
- 2)
Doubling the exposure time had a significant effect on DC% except for SDR. It provided the highest DC% at the bottom of the samples in Group 1 and 4, regardless
Acknowledgments
This work was supported by PTE-ÁOK-KA-2016/1 and GINOP-2.3.2.-15-2016-00022 Research Grant.
References (53)
- et al.
Longevity of posterior composite restorations: not only a matter of materials
Dent Mater
(2012) - et al.
Curing behavior of high-viscosity bulk-fill composites
J Dent
(2014) - et al.
Long-term sorption and solubility of bulk-fill and conventional resin-composites in water and artificial saliva
J Dent
(2015) - et al.
Degree of conversion of bulk-fill compared to conventional resin-composites at two time intervals
Dent Mater
(2013) - et al.
Bulk-filling of high C-factor posterior cavities: effect on adhesion to cavity-bottom dentin
Dent Mater
(2013) - et al.
Marginal quality of flowable 4-mm base vs. conventionally layered resin composite
J Dent
(2011) - et al.
Physical properties and depth of cure of a new short fiber reinforced composite
Dent Mater
(2013) - et al.
In vitro fracture resistance of molar teeth restored with a short fibre-reinforced composite material
J Dent
(2014) - et al.
Composite pre-heating: effects on marginal adaptation, degree of conversion and mechanical properties
Dent Mater
(2010) - et al.
The effect of resin composite pre-heating on monomer conversion and polymerization shrinkage
Dent Mater
(2009)
Pre-heating of high-viscosity bulk-fill resin composites: effects on shrinkage force and monomer conversion
J Dent
Resin composite-state of the art
Dent Mater
Factors affecting polymerization of resin-based composites: a literature review
Saudi Dent J
Energy dependent polymerization of resin-based composite
Dent Mater
The reciprocity low concerning light dose relationships applied to BisGMA/TEGDMA photopolymers: theoretical analysis and experimental characterization
Dent Mater
Examining exposure reciprocity in a resin based composite using high irradiance levels and real-time degree of conversion values
Dent Mater
Curing characteristics of a composite. Part 2: the effect of curing configuration on depth and distribution of cure
Dent Mater
Progress in dimethacrylate-based dental composite technology and curing efficiency
Dent Mater
The effect of filler and silane content on conversion of resin-based composite
Dent Mater
Investigations on a methacrylate-based flowable composite based on the SDR™ technology
Dent Mater
Shrinkage behaviour of flowable resin-composites related to conversion and filler-fraction
J Dent
Polymerization kinetics and impact of post polymerization on the degree of conversion of bulk-fill resin-composite at clinically relevant depth
Dent Mater
Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites
Dent Mater
Bulk-fill resin composites: polymerization properties and extended light curing
Dent Mater
Polymerization efficiency and flexural strength of low-stress restorative composites
Dent Mater
Impact of light transmittance mode on polymerisation kinetics in bulk-fill resin-based composites
J Dent
Cited by (28)
Degree of conversion and in vitro temperature rise of pulp chamber during polymerization of flowable and sculptable conventional, bulk-fill and short-fibre reinforced resin composites
2021, Dental MaterialsCitation Excerpt :As a limitation it should be mentioned that the present study investigated the DCs and PTs changes of conventional RBCs with an extended exposure time, while bulk-fill and SFRC RBCs were tested only with an exposure duration of 20 s. Several investigations demonstrated a higher DC of bulk-fills and SFRCs with an extended irradiation time [82,83]. Its effect pertaining to bulk-fills however is strongly material dependent [10,58]. Even though increased irradiation time is proven to be beneficial for mechanical properties [10], it may also increase the pulpal temperature thereby compromising the health of the pulp tissue.