Depth of cure of resin composites: Is the ISO 4049 method suitable for bulk fill materials?

Summary

Objectives

To evaluate if depth of cure D ISO determined by the ISO 4049 method is accurately reflected with bulk fill materials when compared to depth of cure D new determined by Vickers microhardness profiles.

Methods

D ISO was determined according to “ISO 4049; Depth of cure” and resin composite specimens ( n = 6 per group) were prepared of two control materials (Filtek Supreme Plus, Filtek Silorane) and four bulk fill materials (Surefil SDR, Venus Bulk Fill, Quixfil, Tetric EvoCeram Bulk Fill) and light-cured for either 10 s or 20 s. For D new , a mold was filled with one of the six resin composites and light-cured for either 10 s or 20 s ( n = 22 per group). The mold was placed under a microhardness indentation device and hardness measurements (Vickers hardness, VHN) were made at defined distances, beginning at the resin composite that had been closest to the light-curing unit (i.e. at the “top”) and proceeding toward the uncured resin composite (i.e. toward the “bottom”). On the basis of the VHN measurements, Vickers hardness profiles were generated for each group.

Results

D ISO varied between 1.76 and 6.49 mm with the bulk fill materials showing the highest D ISO . D new varied between 0.2 and 4.0 mm. D new was smaller than D ISO for all resin composites except Filtek Silorane.

Conclusions

For bulk fill materials the ISO 4049 method overestimated depth of cure compared to depth of cure determined by Vickers hardness profiles.

Introduction

Energy of the light emitted from a light-curing unit decreases drastically when transmitted through resin composite , leading to a gradual decrease in degree of conversion of the resin composite material at increasing distance from the irradiated surface. Decreases in degree of conversion compromise physical properties and increase elution of monomer and thus may lead to premature failure of a restoration or may negatively affect the pulp tissue. When restoring cavities with light-curing resin composites, it has therefore been regarded as the gold standard to apply and cure the resin composite in increments of limited thickness. The maximal increment thickness has been generally defined as 2 mm . However, restoring cavities, especially deep ones, with resin composite increments of 2 mm thickness is time-consuming and implies a risk of incorporating air bubbles or contaminations between the increments. Thus, various manufacturers have recently introduced new types of resin composites, so-called “bulk fill” materials, which are claimed to be curable to a maximal increment thickness of 4 mm .

A method for defining the maximal increment thickness of resin composites has been introduced by the International Organization for Standardization ISO in the second edition of ISO 4049 in the year 1988 . The method is officially denominated as “ISO 4049; Depth of cure”, and according to the method the resin composite to be tested is filled in a tube-shaped mold, light-cured, pushed out of the mold, and uncured resin composite material is then removed (“ scraped away”) with a spatula leaving a hard cylindrical specimen. Finally, the absolute length of this hard specimen is measured and divided by two. The resulting value is recorded as the depth of cure and defines the maximum increment thickness. The rationale for the division factor two is that not all the hardened specimen is actually optimally cured . The ISO 4049 method was developed using a microfilled resin composite (Durafill, Kulzer & Co GmbH, Bad Homburg, West Germany) , one of the first visible light-curing resin composites. Ever since, the principle of the ISO 4049 method has basically remained the same .

Resin composites, however, have undergone continuous development through the years as regards their various components, e.g. the filler and the initiator. It seems likely that the new bulk fill materials have required certain changes or modifications in the composition, and it is therefore relevant to verify the accuracy of the ISO 4049 method and its division factor.

Since hardness measurement has been shown to be a practical method to indirectly determine degree of conversion for a given resin composite , hardness profiles can be used to alternatively measure depth of cure. Consequently, the aim of this study was to evaluate if depth of cure determined by the ISO 4049 method is accurately reflected with bulk fill materials when compared to depth of cure determined by Vickers hardness profiles. In order to arrive at this aim several subaims were set: (1) to determine the depth of cure by ISO 4049, (2) to measure Vickers hardness at increasing distances from the light-curing source, (3) to determine at which depth 80% of the maximum Vickers hardness was obtained, and (4) to determine which division factor should be used to arrive at this “80% of maximum Vickers hardness” depth. The overall hypothesis to be tested was that the ISO 4049 method accurately reflects the depth of cure determined by Vickers hardness estimations of the degree of conversion.

Materials and methods

Six resin composites ( Table 1 ) were used for investigating the accuracy of the ISO 4049 method: Two control materials (Filtek Supreme Plus and Filtek Silorane), two flowable bulk fill materials (Surefil SDR and Venus Bulk Fill), and two high-viscosity bulk fill materials (Quixfil and Tetric EvoCeram Bulk Fill). All light-curing was performed with an LED light-curing unit (Demi, Kerr Corporation, Middleton, WI, USA) and light power density was verified to be at least 1000 mW/cm 2 at the beginning and end of each day of specimen preparation with a radiometer (Demetron L.E.D. Radiometer, Kerr Corporation).

Table 1
Resin composites used.
Resin composite Type of resin composite (according to manufacturer) Maximum increment thickness (mm) (according to manufacturer) Shade LOT-number
Filtek Supreme Plus
3 M ESPE, St. Paul, MN, USA
Universal restorative 2 A3 N116619
Filtek Silorane
3 M ESPE, St. Paul, MN, USA
Low shrink posterior restorative 2.5 A3 N138530
Surefil SDR
Dentsply Caulk, Milford, DE, USA
Posterior bulk fill flowable base 4 Universal 100128
Venus Bulk Fill
Heraeus Kulzer, Hanau, Germany
Low stress flowable composite 4 Universal 010030
Quixfil
Dentsply DeTrey, Constance, Germany
Posterior restorative 4 Universal 1007001127
Tetric EvoCeram Bulk Fill
Ivoclar Vivadent, Schaan, Liechtenstein
Moldable posterior composite for bulk-filling technique 4 IVA
(reddish universal shade)
IDS

Depth of cure by ISO 4049

Depth of cure by ISO 4049 was performed with re-usable stainless steel molds according to ISO 4049:2000 . Pretests had found the absolute length of cylindrical specimens of the cured resin composite to vary between 3.5 and 13 mm depending on the resin composite. The ISO 4049 method states that the stainless steel molds shall be at least 2 mm longer than the absolute length of the cylindrical specimens. Thus, stainless steel molds of 6 mm, 9 mm, or 15 mm in length and an internal diameter of 4 mm were custom-made.

Depending on the resin composite, the mold of either 6 mm, 9 mm, or 15 mm in length was placed on a glass slide covered by a Mylar strip (Hawe Stopstrip Straight, KerrHawe, Bioggio, Switzerland). The mold was then filled in bulk with one of the six resin composites. The top side of the mold was covered with a second Mylar strip and the resin material made flush with the mold by use of a second glass slide. The mold was placed on white filter paper (Filter Paper Circles 589/1, Schleicher & Schuell MicroScience GmbH, Dassel, Germany). The second glass slide was removed and the resin composite was light-cured for either 10 s or 20 s keeping the light tip centered and in contact with the second Mylar strip. After light-curing, the cylindrical specimens were pushed out of the mold and the uncured resin composite material was removed with a plastic spatula. The absolute length of the cylindrical specimens of cured resin composite was then measured with a digital caliper of ±0.01 mm accuracy (Mitutoyo IP 65, Kawasaki, Japan). The absolute length ( AL ) was divided by two and the latter value recorded as D ISO . Six specimens were made in each of the 12 groups (i.e. six materials light-cured for either 10 s or 20 s).

Depth of cure by Vickers hardness profiles

Depth of cure by Vickers hardness profiles was performed in a re-usable, block-shaped, and custom-made stainless steel mold with a semicircular notch of 15 mm in length and 4 mm in diameter ( Fig. 1 A ). The semicircular notch was entirely filled with one of the six resin composites. Then, the mold was covered with a Mylar strip (Hawe Stopstrip Straight, KerrHawe) and the resin composite was made flush with the mold by use of a glass slide. Excess resin material was removed and the mold was covered by a stainless steel shell ( Fig. 1 B). A second Mylar strip was placed on the semicircular opening ( Fig. 1 C) and the resin composite was light-cured through the semicircular opening (top surface) for 10 s or 20 s keeping the light tip centered and in contact with the second Mylar strip. After light-curing, the shell and both Mylar strips were removed ( Fig. 1 D) and the mold including the resin composite specimen was placed under a microhardness indentation device (Fischerscope HM2000, Helmut Fischer GmbH, Sindelfingen, Germany). Subsequently, hardness measurements (Vickers hardness, VHN) were made on the resin composite specimen at defined distances, beginning with the resin composite which had been closest to the light tip (i.e. from the “top”) and moving toward the uncured resin composite (i.e. toward the “bottom”) until VHN of the resin composite could not be measured anymore due to its softness. The defined distances ( δ ) were: 0.1 mm, 0.2 mm, 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 5.0 mm, 6.0 mm, 7.0 mm, 8.0 mm, 9.0 mm, 10.0 mm, 11.0 mm, 12.0 mm, and 13.0 mm. Programming of the hardness indentation device for defined distances and reproducible placement of the mold ensured that the VHN measurements were made along the same axis on each specimen. VHN measurements were made at a load of 3 g for 15 s. For each of the 12 groups (i.e. six materials light-cured for either 10 s or 20 s), 22 specimens were prepared and thus 22 VHN measurements were made at each of the defined distances.

Fig. 1
Specimen preparation for depth of cure determination by Vickers hardness measurements. A = stainless steel mold with semicircular notch, B = stainless steel shell, C = semicircular opening for light-curing, and D = mold including the resin composite specimen.

Statistical analysis

In each of the 12 groups, the maximum VHN max of the VHN values obtained at the defined distances δ = {0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0} was identified for each of the 22 specimens. For each group, the median <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='VHN˜max’>VHN˜maxVHN˜max
V H N ˜ max
of the 22 VHN max values as well as the median <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='D˜ISO’>D˜ISOD˜ISO
D ˜ ISO
of the six D ISO values was derived. Then the VHN value VHN ISO at the biggest depth that was equal to or smaller than <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='D˜ISO’>D˜ISOD˜ISO
D ˜ ISO
was determined ( Fig. 2 ). To assess which percentage p ISO of <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='VHN˜max’>VHN˜maxVHN˜max
V H N ˜ max
was attained at <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='D˜ISO’>D˜ISOD˜ISO
D ˜ ISO
, VHN ISO was divided by <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='VHN˜max’>VHN˜maxVHN˜max
V H N ˜ max
. The biggest depth D new , above which at least 80% of <SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='VHN˜max’>VHN˜maxVHN˜max
V H N ˜ max
was attained, and the factor f new , by which the absolute length should be divided in order to arrive at D new , was calculated for each specimen. For certain specimens VHN was never above 80% of <SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax style="POSITION: relative" data-mathml='VHN˜max’>VHN˜maxVHN˜max
V H N ˜ max
. For these specimens D new and f new could not be calculated and the number of such incalculable cases was called N miss . For D ISO , VHN max , p ISO , D new and f new medians and 95% bootstrap confidence intervals for the medians were determined for each of the 12 groups.

Fig. 2
Schematic depiction of the identification of VHN max and VHN ISO (the VHN value at the biggest depth that was equal to or smaller than <SPAN role=presentation tabIndex=0 id=MathJax-Element-9-Frame class=MathJax style="POSITION: relative" data-mathml='D˜ISO’>D˜ISOD˜ISO
D ˜ ISO
) as well as the calculation of D new (the depth above which at least 80% of <SPAN role=presentation tabIndex=0 id=MathJax-Element-10-Frame class=MathJax style="POSITION: relative" data-mathml='VHN˜max’>VHN˜maxVHN˜max
V H N ˜ max
was attained).

All calculations were performed with R version 2.13.0 (The R Foundation for Statistical Computing, Vienna, Austria; www.R-project.org ), using descriptive methods.

Materials and methods

Six resin composites ( Table 1 ) were used for investigating the accuracy of the ISO 4049 method: Two control materials (Filtek Supreme Plus and Filtek Silorane), two flowable bulk fill materials (Surefil SDR and Venus Bulk Fill), and two high-viscosity bulk fill materials (Quixfil and Tetric EvoCeram Bulk Fill). All light-curing was performed with an LED light-curing unit (Demi, Kerr Corporation, Middleton, WI, USA) and light power density was verified to be at least 1000 mW/cm 2 at the beginning and end of each day of specimen preparation with a radiometer (Demetron L.E.D. Radiometer, Kerr Corporation).

Table 1
Resin composites used.
Resin composite Type of resin composite (according to manufacturer) Maximum increment thickness (mm) (according to manufacturer) Shade LOT-number
Filtek Supreme Plus
3 M ESPE, St. Paul, MN, USA
Universal restorative 2 A3 N116619
Filtek Silorane
3 M ESPE, St. Paul, MN, USA
Low shrink posterior restorative 2.5 A3 N138530
Surefil SDR
Dentsply Caulk, Milford, DE, USA
Posterior bulk fill flowable base 4 Universal 100128
Venus Bulk Fill
Heraeus Kulzer, Hanau, Germany
Low stress flowable composite 4 Universal 010030
Quixfil
Dentsply DeTrey, Constance, Germany
Posterior restorative 4 Universal 1007001127
Tetric EvoCeram Bulk Fill
Ivoclar Vivadent, Schaan, Liechtenstein
Moldable posterior composite for bulk-filling technique 4 IVA
(reddish universal shade)
IDS

Depth of cure by ISO 4049

Depth of cure by ISO 4049 was performed with re-usable stainless steel molds according to ISO 4049:2000 . Pretests had found the absolute length of cylindrical specimens of the cured resin composite to vary between 3.5 and 13 mm depending on the resin composite. The ISO 4049 method states that the stainless steel molds shall be at least 2 mm longer than the absolute length of the cylindrical specimens. Thus, stainless steel molds of 6 mm, 9 mm, or 15 mm in length and an internal diameter of 4 mm were custom-made.

Depending on the resin composite, the mold of either 6 mm, 9 mm, or 15 mm in length was placed on a glass slide covered by a Mylar strip (Hawe Stopstrip Straight, KerrHawe, Bioggio, Switzerland). The mold was then filled in bulk with one of the six resin composites. The top side of the mold was covered with a second Mylar strip and the resin material made flush with the mold by use of a second glass slide. The mold was placed on white filter paper (Filter Paper Circles 589/1, Schleicher & Schuell MicroScience GmbH, Dassel, Germany). The second glass slide was removed and the resin composite was light-cured for either 10 s or 20 s keeping the light tip centered and in contact with the second Mylar strip. After light-curing, the cylindrical specimens were pushed out of the mold and the uncured resin composite material was removed with a plastic spatula. The absolute length of the cylindrical specimens of cured resin composite was then measured with a digital caliper of ±0.01 mm accuracy (Mitutoyo IP 65, Kawasaki, Japan). The absolute length ( AL ) was divided by two and the latter value recorded as D ISO . Six specimens were made in each of the 12 groups (i.e. six materials light-cured for either 10 s or 20 s).

Depth of cure by Vickers hardness profiles

Depth of cure by Vickers hardness profiles was performed in a re-usable, block-shaped, and custom-made stainless steel mold with a semicircular notch of 15 mm in length and 4 mm in diameter ( Fig. 1 A ). The semicircular notch was entirely filled with one of the six resin composites. Then, the mold was covered with a Mylar strip (Hawe Stopstrip Straight, KerrHawe) and the resin composite was made flush with the mold by use of a glass slide. Excess resin material was removed and the mold was covered by a stainless steel shell ( Fig. 1 B). A second Mylar strip was placed on the semicircular opening ( Fig. 1 C) and the resin composite was light-cured through the semicircular opening (top surface) for 10 s or 20 s keeping the light tip centered and in contact with the second Mylar strip. After light-curing, the shell and both Mylar strips were removed ( Fig. 1 D) and the mold including the resin composite specimen was placed under a microhardness indentation device (Fischerscope HM2000, Helmut Fischer GmbH, Sindelfingen, Germany). Subsequently, hardness measurements (Vickers hardness, VHN) were made on the resin composite specimen at defined distances, beginning with the resin composite which had been closest to the light tip (i.e. from the “top”) and moving toward the uncured resin composite (i.e. toward the “bottom”) until VHN of the resin composite could not be measured anymore due to its softness. The defined distances ( δ ) were: 0.1 mm, 0.2 mm, 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 5.0 mm, 6.0 mm, 7.0 mm, 8.0 mm, 9.0 mm, 10.0 mm, 11.0 mm, 12.0 mm, and 13.0 mm. Programming of the hardness indentation device for defined distances and reproducible placement of the mold ensured that the VHN measurements were made along the same axis on each specimen. VHN measurements were made at a load of 3 g for 15 s. For each of the 12 groups (i.e. six materials light-cured for either 10 s or 20 s), 22 specimens were prepared and thus 22 VHN measurements were made at each of the defined distances.

Fig. 1
Specimen preparation for depth of cure determination by Vickers hardness measurements. A = stainless steel mold with semicircular notch, B = stainless steel shell, C = semicircular opening for light-curing, and D = mold including the resin composite specimen.

Statistical analysis

In each of the 12 groups, the maximum VHN max of the VHN values obtained at the defined distances δ = {0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0} was identified for each of the 22 specimens. For each group, the median ˜VHNmax
V H N ˜ max
of the 22 VHN max values as well as the median ˜DISO
D ˜ ISO
of the six D ISO values was derived. Then the VHN value VHN ISO at the biggest depth that was equal to or smaller than ˜DISO
D ˜ ISO
was determined ( Fig. 2 ). To assess which percentage p ISO of ˜VHNmax
V H N ˜ max
was attained at ˜DISO
D ˜ ISO
, VHN ISO was divided by ˜VHNmax
V H N ˜ max
. The biggest depth D new , above which at least 80% of ˜VHNmax
V H N ˜ max
was attained, and the factor f new , by which the absolute length should be divided in order to arrive at D new , was calculated for each specimen. For certain specimens VHN was never above 80% of ˜VHNmax
V H N ˜ max
. For these specimens D new and f new could not be calculated and the number of such incalculable cases was called N miss . For D ISO , VHN max , p ISO , D new and f new medians and 95% bootstrap confidence intervals for the medians were determined for each of the 12 groups.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Depth of cure of resin composites: Is the ISO 4049 method suitable for bulk fill materials?
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