Limited reciprocity in curing efficiency of bulk-fill resin-composites

Abstract

Objective

The aim of the study was to identify experimental limits of the general reciprocity hypothesis that the same photo-cure outcomes will result from applying essentially constant energy densities, despite reciprocal variations in the irradiance and irradiation time-period, for a representative set of bulk fill (BF) and non-BF resin composites.

Methods

Six BF and two non-BF resin-composites were selected. The unset pastes were inserted into white acetal molds (5 mm id) with ( n = 6) depths (1, 2, 3, 4, 5 and 6 mm). Three light curing units (LCUs) of increasing radiant emittance capability: 1200, 2000 and 3200 mW/cm 2 were used. Composite specimen groups ( n = 3, per depth, per LCU) were irradiated on the upper surface only. For each specific composite, the irradiation times for each LCU were reduced reciprocally, as the LCU irradiance increased, to deliver a constant energy density (J/cm 2 ) to that composite. However, the required energy density for a given composite differed in accordance with each composite manufacturer recommendations. After storing for 24 h at 37 °C, light transmission measurements were made through each specimen and re-expressed as Apparent Absorbance ( A ′). Vickers hardness ( H V ) measurements ( n = 10) were made on both top and bottom surfaces, for each specimen, and H V versus “depth” profile plots created. From the top-surface data, a Depth-of-Cure parameter could be derived. Data were statistically evaluated for differences between top and bottom H V values and for other predefined variables of interest.

Results

Irradiation with the LCU of 1200 mW/cm 2 generally gave the highest H V /depth for most materials tested compared to the other curing lights with higher power output, regardless of top and bottom measurements ( p < 0.001). However, this difference was material-dependent. With one BF composite, 1200 and 2000 mW/cm 2 irradiance did not show a significant difference between top and bottom H V . Composites with higher translucency showed reduced differences in top/bottom H V than more opaque composites.

Significance

Reciprocity was found to be limited with most materials examined, such that irradiance periods of 10 s, gave generally better H V outcomes than by using LCUs of superior radiant emittance while reciprocally reducing irradiance time to maintain constant dose of energy density.

Introduction

Over recent decades, dental amalgam has been increasingly replaced as a restorative material by alternative restorative materials, such as glass ionomer cements, compomers, resin composites and derivate products. Although amalgam is still available for direct posterior fillings, resin composite is increasingly used due to its superior esthetic properties . Longevity and annual failure rates were shown for both materials . Relatively more secondary caries was related to composite and more fracture failures to amalgam restorations by another retrospective study, comparing posterior composite and amalgam restorations without differences in longevity . It was found that larger-sized restorations showed reduced longevity .

Light cured resin composites are usually placed in increment thicknesses of 2 mm or less into cavity preparations through incremental layering . Exposure to blue light from an occlusal direction follows, before repeating increments until the cavity preparation is filled . Improved physical properties, enhanced marginal adaptation and reduced cytotoxicity of the resin composite is achieved by adequate polymerization . A second rationale for the incremental technique is to reduce shrinkage occurring during polymerization. If polymerization shrinkage is not reduced, the associated shrinkage-stress may cause cuspal deformation with resulting sensitivity and micro-cracks in the resin structure , which can result in marginal gaps, micro-leakage and secondary caries.

There are several disadvantages of the incremental layering technique: the possibility of contamination between the layers, bonding failures and increased processing time. Bulk fill (BF) resin composites allow the placement of increments up to 4 mm in thickness . As opposed to placement and curing of multiple increments, BF resin composites require less chairside time .

Investigation of the depth of cure of resin-composites has been made by various methods. Apart from the simple “scrape test” used in International Standard: ISO 4049, one of the most widely adopted procedures is to consider the profile of hardness versus depth and to note the depth for which hardness just equals or exceeds 80% of the maximum hardness – at or close to the surface .

To achieve adequate physical and mechanical properties, sufficient curing of BF resin composites is essential . Improving the absorption spectrum and the photo initiator’s reactivity may increase the depth of cure of light curing resin composites, as well as optimizing the LCU and increasing the translucency of the materials . One of the most commonly used photoinitiators is a combination of camphorquinone (CQ) and different types of tertiary amine . New photoinitiators such as dibenzoyl germanium derivates (Ivocerin), which are more light reactive than CQ, are incorporated in TEC-BF as an additional photoinitiator, to increase the depth of cure . Another important aspect for sufficient curing is light energy: Insufficient light energy and duration of exposure may result in a low degree of conversion , reduced microhardness and inferior mechanical properties . The depth of cure is obviously translucency dependent and some BF resin composites are more translucent to blue light than some conventional resin composites . For conventional composites an incremental filling technique is used, as it has been shown that light energy transmitted through resin composite decreases exponentially with resin composite thickness. More precisely, this is the case after a correction has been made for surface reflection and the Apparent Absorption ( A ′) then increases linearly with depth .

The overall aim of this study was to obtain further experimental information about a possible reciprocity between light irradiance (mW/cm 2 ) and irradiation time (s) for resin-composites. The product of these quantities has the units of energy density (J/cm 2 ). So the general reciprocity hypothesis is that: the same photo-cure outcomes will result from applying essentially constant energy densities despite reciprocal variations in the irradiance and time-period. The specific clinical motivation is to achieve equivalent curing by using high irradiance coupled with shorter irradiation times. Previous studies suggest that such reciprocity may only hold to a very limited extent. Hence the overall aim can be re-expressed as the identification of limits to reciprocity for representative bulk-filled (BF) and conventional (non-BF) resin-composites when subjected to different irradiation regimes : i.e. polymerized by a series of light-curing units (LCUs) of increasing radiant emittance (mW/cm 2 ), for systematically varied irradiation times.

The specific objectives were to investigate:

  • (i)

    Hardness versus Depth profiles of representative BF and non-BF resin composites, when each material is subjected to different irradiation regimes.

  • (ii)

    Whether or not curing with high energy density at short irradiation time generates acceptable hardness profiles.

  • (iii)

    Whether translucent materials are cured more efficiently than high-viscosity resin composites in 4 mm depth molds.

  • (iv)

    Whether maximum hardness correlates with composite filler loading.

Based on these objectives the following null hypotheses were formulated:

  • (I)

    Shorter curing times with high energy density do not result in similar depth of cure to prolonged curing times with moderate energy density.

  • (II)

    Translucent materials do not show more efficient curing than more opaque resin composites in 4 mm cavity preparations.

  • (III)

    Maximum hardness does not correlate with composite filler loading.

Materials and methods

Sets of flat-cylindrical solid acetal test molds with a circular cavity were prepared with increasing thickness (1, 2 3, 4, 5 and, 6 mm), and with an internal diameter of 5.0 mm. To reduce light reflection back up the materials filling the cavities, dark covered microscope slides were used as the base. Transparent plastic film was positioned between the dark base and the acetal mold to achieve a plane surface. Six BF composite materials (four low viscosity [X-BF, V-BF, F-BF, SDR], two high viscosity materials [TEC-BF, SF-BF]) and two conventional composite materials (TEC, CXM) ( Table 1 ) were filled into the mold cavities. After filling the cavities, the upper surface of each specimen was marked and covered with another piece of transparent film. Specimens were produced in triplicate. The resin composites were cured from the marked top side with specific irradiation times ( Table 2 ), which were calculated for each composite separately as specified by the manufacturers for three different curing lights. The energy densities (J/cm 2 ) received by the resin composites, are the mathematical product of irradiance (mW/cm 2 ) and the exposure time (s) . E D = I × t.

Table 1
Material product codes and their filler size and loading (manufacturer data).
Product Filler size (μm) Filler content (wt.%) Manufacturer
TEC Tetric EvoCeram 0.6 79–81 Ivoclar Vivadent AG, Liechtenstein
CXM Ceram.X mono+ 1.2–1.6 77 Dentsply Sirona, USA
X-BF x-tra base Bulkf Fill 3.5 75 VOCO GmbH, Germany
V-BF Venus Bulk Fill 0.02–5 65 Kulzer GmbH, Germany
F-BF Filtek Bulk Fill 0.01–5 64 3M Germany, Germany
SDR SDR 4.2 68 Dentsply Sirona, USA
TEC-BF Tetric EvoCeram Bulk Fill 0.55 77 Ivoclar Vivadent AG, Liechtenstein
SF-BF Sonic Fill 0.02–0.08
0.04–4.0
83 KaVo Kerr, USA

Table 2
Range of irradiant energy densities delivered to each material and durations of exposure for each LED-LCU.
Composite Energy density, range (J/cm 2 ) Elipar S10
1200 mW/cm 2
Bluephase 20i Turbo
2000 mW/cm 2
Valo Xtra Power
3200 mW/cm 2
Irradiation times (s)
TEC 10.0–12.8 9 5 4
CXM 10.0–12.8 9 5 4
X-BF 6.0–6.4 5 3 2
V-BF 12.0–12.8 10 6 4
F-BF 12.0–12.8 10 6 4
SDR 12.0–12.8 10 6 4
TEC-BF 10.0–12.8 9 5 4
SF-BF 22.0–22.4 19 11 7

After curing the resin composites, removing the transparent film and inserting the specimens into glass bottles, they were stored dry, in the dark for 24 h at 37 °C.

Light transmission measurements

The light irradiance transmitted through the composite depends on the amounts of light reflected, scattered and absorbed . The transmittance ( τ ) of light is defined as the ratio of the light quantity transmitted through the material to the total amount of incident light :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='τ=It/I0′>𝜏=𝐼𝑡/𝐼0τ=It/I0
τ=It/I0

To apply the Beer–Lambert relationship, it is necessary to apply a correction for the reflected light. The apparent absorbance ( A ′) of a material is defined as the logarithmic ratio of the light quantity falling upon the material to the light quantity transmitted through the material .

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='A′=log10{1/τ}’>𝐴=log10{1/𝜏}A′=log10{1/τ}
A′=log10{1/τ}
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Aug 5, 2020 | Posted by in Dental Materials | Comments Off on Limited reciprocity in curing efficiency of bulk-fill resin-composites
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