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  logo elsevier Dental Materials 19 (2003) 517-522
 

A comparative study of the properties of dental resin composites polymerized with plasma and halogen light

Sanjukta Deb*, and Harminder Sehmi,

Department of Biomaterials, Guy's, King's and St Thomas' Dental Institute, King's College London, Floor 17, Guy's Tower, London Bridge, London SE1 9RT, UK Received 22 February 2002; revised 19 June 2002; accepted 23 July 2002. ; Available online 11 March 2003.
 
*Corresponding author. Tel.: +44-207-955-2969; fax: +44-207-955 2963
E-mail address: sanjukta.deb@kcl.ac.uk(S.Deb).

Abstract

Objectives.
Newly developed curing units utilizing plasma arc methodology have been advocated for rapid curing of dental composites. This study was conducted to investigate the effect of plasma light using a 3 s and a step cure regime on the properties of four dental restorative materials and compare it with properties resulting from halogen light curing of the same materials.

Methods.
Composites Quadrant, Filtek and two polyacid modified composites (compomers) Dyract AP and Compoglass F were cured, using a conventional halogen light, a plasma light for 3 s (Apollo95E) and a plasma step cure (Apollo 95E) method. The parameters studied for characterization of the restorative materials were polymerization exotherm, surface hardness and their interactions with saline.

Results.
Irradiation with plasma light for 3 s or step cure produced an order of hardness: Filtek>Compoglass F>Dyract AP>Quadrant (p<0.001), however, halogen cure yielded an order of hardness: Filtek>Quadrant>Dyract AP>Compoglass F. No significant differences in hardness were observed on the exposed and non-exposed surfaces of the materials cured by plasma step cure whereas a 3 s cure yielded a significant difference in the cases of Quadrant, Compoglass F and Dyract AP (p<0.001). Mass losses were also found to be greater in the specimens cured by plasma light for 3 s in comparison with plasma step cure and halogen cure.

Significance.
Plasma step and halogen curing were found to yield composites with superior properties in comparison to a 3 s plasma cure, suggesting, that a step cure regime is a preferred method, when a plasma light unit is used. A 3 s curing with a plasma light may lead to less than optimum properties of the composite cements.

© 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Resin composites; Plasma light; Halogen light
1. Introduction
Dimethacrylate monomers are widely used in dentistry and form an essential ingredient in dentin bonding agents, restorative dental composites, luting agents and fissure sealants. Light cured dental restorative materials set via an addition polymerization and exposure to light of a requisite wavelength and intensity, initiates the generation of free radicals that propagate the polymerization, leading to the set material. The lights available on the market today include, halogen and plasma arc light curing units. The former is a well-established technology in dentistry whilst the latter is a relatively new tool in dentistry. The main difference between the two light units is that the plasma arc lights have higher intensities of light emitted over a narrow range of wavelengths and due to the high irradiance, it offers a much shorter curing time in comparison to conventional halogen light units. The halogen light consists of a halogen bulb with a filament and as current passes through the filament, the wire heats up and as a result electromagnetic radiation is emitted from the filament [1]. Using filters the emitted radiation at the light curing tip is mainly visible light and infrared energy.

Placing dental restoratives such as composites and polyacid modified composites (compomers) in deep cavities, requires an incremental placement technique to help minimize the effects of polymerization shrinkage and address the problems of a limited depth of cure due to light attenuation [2]. Thus for medium to large size cavities more than one incremental placement of material is necessary followed by several curing cycles which results in an increase in the time spent by the clinician. It is known that dental composites cured by halogen light require a minimum energy density, which in turn is related to the intensity of light and time of exposure, to allow an optimum curing [3]. The plasma curing light in contrast, provides rapid setting, however, it has been associated with intra-pulpal temperature rise [4] and this limits the range of intensities that may be applied. Rapid curing may also lead to the formation of short polymer chains and the faster rate of cure may not allow sufficient time for the pre-gel phase [5] of the material to absorb the polymerization/contraction stresses. The degree of polymerization of a photoinitiated material is dependent on the wavelength and intensity of light output from the curing unit, curing time, the size, location and orientation of tip of the source, shade, thickness and composition of the material [6, 7 and 8]. Partial polymerization can increase water absorption [9] and solubility of unreacted monomers [10] which can affect the longevity and aesthetics of the restoration. Several studies have shown that the high irradiance delivered by the plasma arc lamp over a few seconds is not adequate to bring about optimum properties in resin composites and it also has a marked influence on the degree of polymerization [11]. The concern about the possible detrimental effects of post rapid curing on restorative materials has led to this study, wherein the effects of halogen and plasma light curing have been compared by evaluating surface hardness, interaction with saline and polymerization exotherm of the materials. For any given material involving polymerization, the degree of conversion among others, is one factor that influences the properties. In general, a high degree of conversion yields greater hardness and strength [12 and 13] thus surface hardness of the cured materials was determined both on the incident light surface and the under surface to determine the degree of cure. In the present study, two types of restorative materials namely, composites and polyacid modified resin composites (compomers) were selected, and curing was conducted using a conventional halogen light and a plasma arc light using two different curing regimes, namely a 3 s cure and a plasma step cure. The polymerization exotherm was measured for each type of material at the core of the material to ascertain the temperature rise within the matrix as a result of the exothermic polymerization reaction. Also as a consequence of increasing the rate of polymerization it is expected to result in the average reduction of the length of the polymer chains, hence the interaction with physiological saline is presented to examine the mass loss, water solubility and equilibrium uptake of these materials since these properties are likely to reflect the effects of the presence of lower molecular weight species and unreacted monomers present in the matrices of the cured composites.

2. Materials and methods
The materials used in the study were Quadrant, Filtek, Dyract AP and Compoglass F (Table 1). Two curing lights, one a conventional halogen light curing unit (Prismetics Lite II, Dentsply) and the other a plasma arc curing unit (Apollo 95E, DMDS, UK) were used. An output of 460 mW/cm2 was measured for the Prismetics halogen light using a curing radiometer (Model 110, Demetron curing Radiometer), however, a new plasma light unit with an irradiance value (1370 mW/cm2) provided by the manufacturer was used. The curing regimes for the composites with halogen light (H) were, 20 s for the composites, 40 s for the compomers (H40), 3 s with plasma arc (P3) and a step curing with plasma arc (Psc) for each material. The step curing setting Psc allows for polymerization in two steps, i.e. 1.5 s at half-power followed by 3.8 s at full-power. The 20 and 40 s cure for the composites and compomers, respectively, were used as per manufacturers recommendation.
tabel 1
Polymerization exotherm. The exothermic polymerization temperature was recorded using a thermocouple connected to a digital thermometer (Comark). The restorative materials were packed into a PTFE mold (8 mm diameter×1.5 mm height) with a thermocouple positioned within the center of the mold, that was then placed in a circular Teflon mold and curing was carried out by placing a glass cover slip over the material.

Vickers hardness. The surface hardness was measured using a micro-Vickers hardness testing machine (Leitz, Germany). The composites were packed into a circular ring mold and placed on a brass plate (1.5 mm thickness). The other end of the ring was covered with a clear glass slip and cured through the cover slip using the different curing methods. The three curing methods were applied to each material and three specimens per material were prepared. Two indentations were made on each specimen, giving a total of six hardness values for each material and curing method. Hardness was measured on the surface on which the light was incident, referred to as the `exposed surface' and on the lower surface away from the light (non-exposed). The loads applied were either 0.981 or 0.961 N for 15 s and the indents measured using a microscope. A lower load had to be applied to some of the composite materials due to too large an indent being produced for accurate reading, because of the softness of the material. Thereafter, the specimens were stored individually in 5 ml of 0.9% saline at 37 °C and hardness was determined after 100 days.

Interaction with saline. Disc shaped specimens (8 mm×1.5 mm thickness) were prepared by placing them in brass ring molds and cured using the different curing methods. Specimens were stored at 37 °C in a 0.9% aqueous sodium chloride and weighed at regular intervals until they equilibrated. They were then subjected to desorption (removal of unbound water at 37 °C) until they reached constant weight. Finally, a second cycle of absorption and desorption was carried out to calculate equilibrium uptake.

3. Results
The peak temperatures are shown in Fig. 1. The composite Quadrant cured with halogen light exhibited an average peak temperature of 43.6 °C [0.43], whereas, the plasma light produced an average peak temperature of 34.8 °C [0.32]. Filtek exhibited peak polymerization temperatures at 35.8 [0.1] and 39.1 °C [0.1] for halogen and plasma light, respectively. The peak temperatures for Dyract AP and Compoglass F were 46.0 [0.2] and 34.8 °C [0.2], respectively, for halogen light cure and 35.8 [0.23] and 36.2 °C [0.64], respectively, for plasma light.
grafiek 1
Surface hardness. The results of the micro-Vickers hardness measurements are shown in Fig. 2 and Fig. 3 for the dry and wet samples, respectively. The hardness values on the exposed surfaces showed that halogen curing provided significantly higher values in comparison to plasma 3 s curing or plasma step cure (p<0.001), for Quadrant, Dyract AP and Compoglass F, however, no differences were observed for Filtek. A comparison of the hardness of the exposed and non-exposed surfaces of the specimens cured with 3 s of plasma light showed significant differences for Filtek (p<0.006), Dyract AP (p<0.001) and Compoglass F (p=0.013). Halogen curing showed that other than Quadrant, the differences in hardness were not significant between exposed and the non-exposed surfaces. Plasma step curing of the materials yielded greater values of surface hardness in comparison to plasma 3 s cure except in the case of Filtek where no significant differences were observed. Furthermore, no significant differences were found in the surface hardness of the exposed and non-exposed surface post plasma step curing, although the values were lower in comparison to halogen cure.
grafiek2
Fig. 2. VHN of Quadrant (Q), Filtek (F), Dyract AP (D) and Compoglass F (C) for the exposed (E) and non-exposed (NE) surface.
grafief3
Fig. 3. VHN of Quadrant (Q), Filtek (F), Dyract AP (D) and Compoglass F (C) for the exposed (E) and non-exposed (NE) surface at 100 days conditioning in saline at 37 °C.

3.1. Effect of storage in saline on hardness
The dental composites and compomers were conditioned in saline at 37 °C for 100 days and surface hardness for each material recorded (Fig. 3). No significant differences were observed in the values of hardness except a decrease in hardness of halogen cured Quadrant (p=0.018), plasma 3 s for Filtek (p=0.001) and Compoglass F (p=0.012) cured by plasma light for 3 s.

3.2. Interaction with saline
All the materials were found to have differences in solubility in saline solutions, depending on the type of cure (Table 2). The greater solubility of the materials cured by plasma light was statistically significant. A comparison between plasma step curing and a 3 s plasma cure also showed that the net amount of solubles (total amount of leachable components) was greater for the latter treatment. The composite Filtek had fewer soluble components in comparison to Quadrant, Dyract AP and Compoglass F. From the loss values and change in mass, Quadrant showed a greater mass loss (5.8%) when cured with a 3 s plasma light. There was significantly more mass loss for specimens cured with 3 s plasma light when compared to Psc and halogen curing. Equilibrium uptake values calculated from the second cycles of absorption-desorption are shown in Table 3.

tabel 2

tabel 3
4. Discussion
When dental composites and compomers undergo polymerization, various factors govern the amount of heat that the pulp may experience. The two main sources of heat are, the energy from the light curing unit and the polymerization exotherm temperature. The former is related to the intensity and duration of exposure [11] while the latter is related to the thickness, thermal conductivity and factors such as the composition of the resin component. The effect of light curing has been suggested by some workers as to play the more influential role on the effect of heat experienced by the pulp [4], however, factors such as dentin thickness and the proximity of the material to the pulp contribute. In the present study polymerization exotherms were measured within the composite material of a fixed dimension in an insulated double walled Teflon chamber and cured by plasma and halogen light. The thermocouple was placed in the center of the sample and the temperature rise was expected to be a reflection of the temperature rise within the matrix, as a result of the exothermic polymerization reaction. Maximum polymerization exotherms were recorded for Quadrant and Dyract AP when cured with the halogen light, the temperatures being lower with plasma light. The higher exotherm is also indicative of a higher degree of conversion of double bonds to single bonds [14] and these results are in accordance with the surface hardness measured in this study.

A strong correlation has been reported between micro-hardness and the degree of cure within one type of dental composite [15]. The measurement of the hardness of the surface exposed to the source of light when compared to the hardness of the surface below gives an indication of the degree of cure through the thickness of the material. The difference between the hardness values of restorative grade materials is dependent on many factors such as shade, amount of filler, type of filler, and the energy and wavelength of the light emitted by the curing light [16]. Plasma cure or Psc yielded significantly lower values of surface hardness for Quadrant, however, the differences between the hardness of the exposed and non-exposed surfaces were not significant which suggests that although the entire depth is cured by using the two plasma curing regimes, the increase in speed of polymerization leads to the reduction in the average lengths of the polymer chains [16]. As expected, of all the materials tested, Filtek produced the highest values of VHN for all curing methods and on storage in saline. It is designed as a posterior composite, which has high translucent filler content that allows superior transmission of light through the matrix. Since the filler phase is almost always harder than the polymer phase, increasing filler eventually leads to an overall increase in the surface hardness of the composite. It is also possible that the amine-peroxide combination and their concentration resulted not only in a higher degree of conversion of the double bonds, but also resulted in greater molecular weights. However, as information regarding the details of amine-peroxide systems used in these materials are difficult to obtain, it is difficult to draw conclusions. Since the compomers have much lower filler content and are of a different type, they therefore yielded a lower surfaces hardness. However, a comparison within each group of material suggested that the type of light curing was important and not all materials were suited to plasma curing, since the composite Quadrant, compomers Dyract AP and Compoglass F showed a markedly lower surface hardness. The fact that the surface hardness of samples cured with the halogen light did not correlate with the surface hardness of the materials cured with the plasma light is indicative of the influence of the incident light. The order of material hardness cured by the halogen was Filtek>Quadrant>Dyract AP>Compoglass F and curing with either plasma 3 s or step curing produced an order of hardness that was Filtek>Compoglass F>Dyract AP>Quadrant. The study also illustrates that Quadrant may not be suitably cured with plasma light as compared to halogen light. This may be due to the initiator system in Quadrant being activated at a wavelength that is not ideal for the wavelengths and peak intensities emitted by the plasma arc. Peutzfeldt et al. [11] recently tested a set of restorative materials and reported that the net amount of conversion of double bonds was lower on curing with Apollo95E light for certain materials. Although storage in solution is known to influence surface hardness [17], the values remained unchanged for the compomers Dyract AP and Compoglass F, post saline conditioning for 100 days irrespective of the curing regime which may be attributed to post hardening due to the acid-base reaction.

In almost all cases the loss in mass of the specimens cured by plasma 3 s light was significantly greater than those cured with halogen light. Interestingly, plasma step curing gave reduced weight loss in the materials as compared to plasma 3 s cure. The two types of materials studied were found to have differences in their solubility in saline depending on the type of cure. The higher intensity of the plasma light is likely to generate more free radical centers per unit volume leading to low molecular weight species and also greater amounts of residual monomer due to the rapid reaction [18]. This is also confirmed by the fact that net loss of mass was lower for the plasma step cure method for all the materials tested. Both Dyract AP and Compoglass F when cured with the halogen light showed higher values of percentage equilibrium uptake compared to Quadrant and Filtek, regardless of curing method, this can be accounted for by the presence of hydrophilic moieties present within the matrix. Since compomers have the potential to undergo an acid-base reaction, it can to an extent behave as conventional glass-ionomer cements, wherein the loosely bound water becomes tightly bound during maturation. Thus it can be suggested that plasma light led to an inefficient curing causing higher than the usual levels of residual free monomer to remain, which led to a decrease in mass. The mass loss can also be explained by the loss of free monomer, filler components and other small components being replaced by loosely bound water. This is evident from the increase in mass loss of Dyract AP and Compoglass F cured with the plasma light over the test period as compared to curing with the halogen light.

5. Conclusion For the materials and the light sources studied, the results show that polymerizable dental restoratives cured with a plasma light cure may not be adequate for curing different types of restorative materials, thus leading to less than optimum properties of the cements. Plasma step curing was found to yield cured composites with superior properties in comparison to a 3 s plasma cure, suggesting that rapid polymerization may hinder the development of optimum properties in some materials.

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