Elsevier

Dental Materials

Volume 35, Issue 2, February 2019, Pages 217-228
Dental Materials

Effect of exposure time and pre-heating on the conversion degree of conventional, bulk-fill, fiber reinforced and polyacid-modified resin composites

https://doi.org/10.1016/j.dental.2018.11.017Get rights and content

Abstract

Objective

To determine the degree of conversion (DC) of different type of resin-based composites (RBC) in eight-millimeter-deep clinically relevant molds, and investigate the influence of exposure time and pre-heating on DC.

Methods

Two-millimeter-thick samples of conventional sculptable [FiltekZ250 (FZ)], flowable [Filtek Ultimate Flow (FUF)] and polyacid-modified [Twinky Star Flow (TS)] RBCs, and four-millimeter-thick samples of flowable bulk-fill [Filtek Bulk Fill Flow (FBF), Surefil SDR (SDR)] and sculptable fibre-reinforced [EverX Posterior (EX)] RBCs were prepared in an eight-millimeter-deep mold. The RBCs temperature was pre-set to 25, 35 and 55 °C. The RBCs were photopolymerized with the recommended and its double exposure time. The DC at the top and bottom was measured with micro-Raman spectroscopy. Data were analyzed with ANOVA and Scheffe post-hoc test (p < 0.05).

Results

The differences in DC% between the top/bottom and the recommended/extended exposure time were significant for the materials, except SDR (64.5/63.0% and 67.4/63.0%). FUF (69.0% and 53.4%) and TS (64.9% and 60.9%) in 2 mm provided higher DC% at the top and bottom with the recommended curing time, compared to the other materials, except SDR. Pre-heating had negative effect on DC at the bottom in flowable RBCs (FUF: 48.9%, FBF: 36.7%, SDR: 43%, TS: 54.7%). Pre-heating to 55 °C significantly increased the DC% in fibre-reinforced RBC (75.0% at the top, 64.7% at the bottom).

Significance

Increased exposure time improves the DC for each material. Among bulk-fills, only SDR performed similarly, compared to the two-millimeter-thick flowable RBCs. Pre-heating of low-viscosity RBCs decreased the DC% at the bottom. Pre-heating of fibre-reinforced RBC to 55 °C increased the DC% at a higher rate than the extended curing time.

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)

  • T.T. Tauböck et al.

    Pre-heating of high-viscosity bulk-fill resin composites: effects on shrinkage force and monomer conversion

    J Dent

    (2015)
  • J.L. Ferracane

    Resin composite-state of the art

    Dent Mater

    (2011)
  • M.M. AlShaafi

    Factors affecting polymerization of resin-based composites: a literature review

    Saudi Dent J

    (2017)
  • R.H. Halvorson et al.

    Energy dependent polymerization of resin-based composite

    Dent Mater

    (2002)
  • J.W. Wydra et al.

    The reciprocity low concerning light dose relationships applied to BisGMA/TEGDMA photopolymers: theoretical analysis and experimental characterization

    Dent Mater

    (2014)
  • D. Selig et al.

    Examining exposure reciprocity in a resin based composite using high irradiance levels and real-time degree of conversion values

    Dent Mater

    (2015)
  • R.L. Erickson et al.

    Curing characteristics of a composite. Part 2: the effect of curing configuration on depth and distribution of cure

    Dent Mater

    (2014)
  • J.G. Leprince et al.

    Progress in dimethacrylate-based dental composite technology and curing efficiency

    Dent Mater

    (2013)
  • R.H. Halvorson et al.

    The effect of filler and silane content on conversion of resin-based composite

    Dent Mater

    (2003)
  • N. Ilie et al.

    Investigations on a methacrylate-based flowable composite based on the SDR™ technology

    Dent Mater

    (2011)
  • K. Baroudi et al.

    Shrinkage behaviour of flowable resin-composites related to conversion and filler-fraction

    J Dent

    (2007)
  • K. Al-Ahdal et al.

    Polymerization kinetics and impact of post polymerization on the degree of conversion of bulk-fill resin-composite at clinically relevant depth

    Dent Mater

    (2015)
  • I.D. Sideridou et al.

    Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites

    Dent Mater

    (2009)
  • J. Zorzin et al.

    Bulk-fill resin composites: polymerization properties and extended light curing

    Dent Mater

    (2015)
  • C. Goracci et al.

    Polymerization efficiency and flexural strength of low-stress restorative composites

    Dent Mater

    (2014)
  • N. Ilie

    Impact of light transmittance mode on polymerisation kinetics in bulk-fill resin-based composites

    J Dent

    (2017)
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