Abstract
The aim of this study was to investigate the triethylene glycol (TEGDMA) elution kinetics from light-cured composite with and without chewing simulation over a time period of 86 h. An experimental composite with TEGDMA labeled with a tracer dose of 14C-TEGDMA was used. The material parameters were in the range of commercially available composites. The mastification was simulated with the Fatigue-machine and the MUC-3 chewing simulator. 14C was eluted to 2.55% of the applied 14C-TEGDMA dose within 86 h after chewing simulation with the Fatigue-machine and to 2.60% after chewing simulation with the MUC-3.
Similar 14C-kinetic data were found for 14C-elution with and without chewing simulation with the Fatigue-machine and with MUC-3. During the first 26 h after the beginning of the experiments a linear 14C-elution kinetic was observed, followed by a second linear 14C-elution kinetic with a lower slope up to 86 h in both apparatus. It could be shown that chewing simulation has no significant ( p < 0.05) effect on the release of 14C-TEGDMA (and/or 14C-degradation products) from polymerized composites.
1
Introduction
The increased use of resin-based dental materials has several reasons: on the one hand the high esthetical demands to dental restorative materials and on the other hand the fear of adverse health effects from exposure to mercury released from dental amalgam fillings . Dental composites consist mainly of an organic matrix and dispersed reinforcing inorganic filler particles. Since their introduction to dentistry in the 1960s, dental composites have undergone significant change and improvement, concerning in particular their physical properties like polymerization shrinkage, low resistance to wear, fracture in the body of restoration, voids and insufficient proximal contact .
First studies about elution of uncured monomers, especially triethylene glycol dimethacrylate (TEGDMA) and 2-hydroxyethyl methacrylate (HEMA), degradation products and other additives are reported by Spahl et al. . In the aftermath more in vitro than in vivo examinations tried to estimate the toxicological risk of these eluted substances . The applied substance concentrations used in these test systems are often based on eluting tests with water, artificial saliva and/or methanol. Thereby, the composite filling is immersed into the elution medium over a defined period of time. This does not simulate physiological conditions because chewing and other stress conditions were not considered. No data are available on substance release from dental composites during different chewing situations. From amalgam it is known that the chewing situation leads to an increased release of mercury .
Physiological chewing situations and testing of dental composites can be simulated by two- and three-body abrasive wear . Two body abrasive wear like tooth-grinding can be simulated by Fatigue-machine and chewing simulator MUC-3. One advantage of the chewing simulator MUC-3 compared to the Fatigue-machine is the use of human teeth as antagonists. Three-body abrasive wear, as for example chewing of food, can be simulated with the ACTA-machine.
Up to the present, TEGDMA is one of the mainly eluted substances from composites . The aim of this study was to investigate the TEGDMA elution kinetics from a light-cured composite filling material with and without simulation at two different physiological chewing situations. For these studies an experimental composite was used with TEGDMA, labeled with a tracer dose of 14C-TEGDMA.
2
Methods
2.1
Apparatus and materials
In pre-examinations it could be shown that TEGDMA diffused into plastic tubes. Therefore, all plastic tubes and plastic materials having contact with TEGDMA were replaced by stainless steel or glass products. All experiments were made under photo laboratory conditions to exclude photopolymerisation from eluted substances. For light curing an Elipar Highlight (ESPE, Seefeld, Germany) was used in the 2-step-modus.
The human premolars used as antagonist in the chewing simulator MUC-3 had to be extracted due to orthodontic reasons. After extraction they were stored in Ringer Solution spiked with sodiumazid.
All solvents and reagents used were purchased from Merck, Darmstadt, Germany. All chemicals were of highly pure quality. 14C-TEGDMA was purchased from TNO Prins Maurits Laboratorium (Rijswijk, The Netherlands). The 14C-label was situated on the carbonyl group of the molecule.
2.2
Composite
The experimental composite (S1/32) was produced by ESPE (Seefeld, Germany). S1/32 was produced in a standardized manufacturing process with typical composition for dental composites. Then, 14C-TEGDMA (specific activity: 0.6 Ci/mol) was added to S1/32 and stirred for 1 h in standard mixing unit. The composition is shown in Table 1 . The flexural strength was 386 MPa and the burst strength was 102 MPa. These values were within the range of other commercially available composites. To exclude light curing caused by daylight, the manufacturing process was carried out under photo laboratory conditions. Eighteen milligrams of the 14C-labeled composite had a 14C-activity of 1,000,000 dpm. The 14C-labeled composite was stored in amber glass at 4 °C to reduce autopolymerization caused by radioactive decay.
| Substance | Mass % |
|---|---|
| BisGMA | 18.02 |
| TEGDMA (incl. 14C-TEGDMA) | 7.76 |
| 14C-TEGDMA | 0.6 Ci/mol |
| Campherchinon | 0.79 |
| Amin | 0.19 |
| Quartz | 69.92 |
| Aerosol | 3.19 |
2.3
ACTA-machine
In the ACTA-machine ( Fig. 1 ) (Willytec, Munich, Germany) two electric motors move the sample as well as the antagonist wheel in horizontal direction. A spring force of 15 N induces a constant contact force. The sample wheel with 16 sample chambers made from stainless steel is driven with one rotation per minute, comparable to the physiological mastification. The antagonist wheel (diameter 19 mm) has honeycombed excavation and has a 15% lower rotation speed to simulate the slipping movement during the masticator cycle . The antagonist wheel – with 7 mm in broadness – is smaller than the sample wheel; therefore, a central abrasion groove with untouched edge results for documentation of the starting situation.
The 16 chambers aparted by metal bars were filled in multi-layer techniques according to the manufacturer’s instructions. The duration of the load test was 86 h, the rotation frequency 1 Hz, with 309,600 cycles. During the load test, every 1 h two probes of 500 μl were taken.
2.4
Fatique-machine
In the Fatique-machine ( Fig. 2 ) (Willytec, Munich, Germany) a four-chamber aluminum sample wheel with a diameter of 16 mm and a broadness of 7 mm was used. An aluminum collar was used for preparing the composite fillings into the sample wheel. The sample chambers were filled according to the instructions of the manufacturers. The sample wheel was inserted into the Fatique-machine and stressed with 50 N contact force over the antagonist wheel ( Figs. 2 and 3 ). The antagonist wheel was based on stainless steel (diameter: 18 mm, broadness 7 mm). The antagonist wheel was torus formed on contact surface (contact broadness with sample wheel: 1.5 mm).
The test-bed was a funnel-shaped anodized aluminum case filled with bidestilled water. To control the exact content, the dilution factor and the amount of evaporation, the aluminum case was bedded on a precision scale.
The duration of the load test was 86 h, the rotation frequency was 1 Hz, with 309,600 cycles. During the load test, every 1 h three probes of 500 μl were taken (two from the surface, one from the bottom of the funnel-shaped anodized aluminum case). The experiment was repeated four times.
2.5
Chewing simulator MUC-3
The chewing simulator was type MUC-3 ( Fig. 4 ) (Willytec, Munich, Germany). MUC-3 is characterized by the following load cycle details ( Fig. 5 ). The machine uses a sample holder and an antagonist. The antagonist is mounted on a crosshead which performs a vertial motion. The crosshead is lifted with a pneumatic cylinder and slowly sinks back to the surface of the sample. As soon as the antagonist touches the surface it is decoupled from the crosshead and transfers the load of the dead-weights to the sample surface. This mechanism ensures an impact free, equal load to all samples. The samples themselves are mounted on a horizontal carrier which performs a very precise horizontal movement of 0.5 mm which is equivalent to the average “long-centric” movement in the mouth. The load cycle starts with the antagonist touching the surface; next, the horizontal movement starts and at the end of the 0.5 mm movement, the antagonist is lifted from the surface and brought back to the initial position.
MUC-3 has four separated test-beds which are fixed in a steel frame. The antagonist was screwed in vertically adjusted carriers in force-supported (each 50 N) vertical motion elements. The samples were fixed in the test-bed holders in horizontal layer. The MUC-3 was powered by compressed air. All test-beds were free of plastics and had top covers to exclude exposure to light. The surface of the sample was washed continuously with water to prevent accelerated abrasion by particles from the composite filling lying on the surface. In our case the dilution of this thermocycling system was too large, that is why we used a continuously working paddle mixer. The paddle mixer was linked by a rubber belt with a lever system which was powered by a rotation speed modulated electric motor. The test-beds were made from anodized aluminum.
The sample holder was oval (11 mm × 8 mm × 2 mm). The samples were prepared according to the manufacturers instructions. The cusp of a human premolar was used as antagonist. It was fixed in the sample holder by gluing with Pattex Kraft-Mix (Henkel, Düsseldorf, Germany). This glue is based on epoxid-resin, free of methacrylates.
The test duration was 86 h. The frequency of movement was 1 Hz. 309,600 courses of movement were completed. During the load test, every 1 h two probes of 500 μl were taken. The experiment was repeated four times.
2.6
Measurement of 14C-activity
The 14C-radioactivity was measured in aqueous solution with Omni-Szintisols ® . The 14C-radioactivity was determined in a model 2500 TR liquid scintillation analyzer (Canberra-Packard, Dreieich, Germany).
2.7
Calculations and statistics
The data are presented as means ± standard error of mean (SEM). The statistical significance of the differences between the experimental groups was checked using the t -test, corrected according to Bonferroni-Holm .
2.8
Declaration
These experiments comply with the current laws of Germany. The accession number for 14C-compounds is 4/4-8816.352-14783, Bayerisches Landesamt für Umweltschutz, Augsburg, Germany.
2
Methods
2.1
Apparatus and materials
In pre-examinations it could be shown that TEGDMA diffused into plastic tubes. Therefore, all plastic tubes and plastic materials having contact with TEGDMA were replaced by stainless steel or glass products. All experiments were made under photo laboratory conditions to exclude photopolymerisation from eluted substances. For light curing an Elipar Highlight (ESPE, Seefeld, Germany) was used in the 2-step-modus.
The human premolars used as antagonist in the chewing simulator MUC-3 had to be extracted due to orthodontic reasons. After extraction they were stored in Ringer Solution spiked with sodiumazid.
All solvents and reagents used were purchased from Merck, Darmstadt, Germany. All chemicals were of highly pure quality. 14C-TEGDMA was purchased from TNO Prins Maurits Laboratorium (Rijswijk, The Netherlands). The 14C-label was situated on the carbonyl group of the molecule.
2.2
Composite
The experimental composite (S1/32) was produced by ESPE (Seefeld, Germany). S1/32 was produced in a standardized manufacturing process with typical composition for dental composites. Then, 14C-TEGDMA (specific activity: 0.6 Ci/mol) was added to S1/32 and stirred for 1 h in standard mixing unit. The composition is shown in Table 1 . The flexural strength was 386 MPa and the burst strength was 102 MPa. These values were within the range of other commercially available composites. To exclude light curing caused by daylight, the manufacturing process was carried out under photo laboratory conditions. Eighteen milligrams of the 14C-labeled composite had a 14C-activity of 1,000,000 dpm. The 14C-labeled composite was stored in amber glass at 4 °C to reduce autopolymerization caused by radioactive decay.
