Material BehaviourMonomer conversion at different dental composite depths using six light-curing methods
Introduction
Visible light-activated resin composite was introduced as a direct restorative material over 25 years ago. However, several factors limit the performance of this material, especially depth of cure and degree of conversion (DC). Variations in these parameters are related to material composition and characteristics of the light source. With respect to the composite, factors such as resin formulation, type and concentration of catalyst system, shade and translucency of the organic matrix, filler type and loading exert important influences on depth of cure and DC [1], [2]. Conversely, characteristics related to the light source, such as energy density and spectral flux, also alter final material properties [1], [2], [3], [4], [5], [6]. Energy density is the product of power density and the duration of exposure [4], whilst spectral flux represents the optical power output from the light-curing unit, in milliwatts, at each given wavelength [5], [6].
Light of an appropriate wavelength initiates photo-polymerization of methacrylate groups producing a highly cross-linked polymer matrix. Light from the curing source must be able to adequately polymerize deeper composite regions than just the top, irradiated surface. However, as light passes through the composite, it is absorbed and scattered, reducing its effectiveness to initiate polymerization, and consequently resulting in variation of cure with depth [4], [7], [8]. Regulations [9] advocate a scraping test to determinate the depth of cure, where the uncured material is removed and the height of the remaining polymerized composite cylinder is measured. This value is divided by two and recorded as the depth of cure. However, this test may not to be correlated with the final material properties, as is DC.
Conversion is an important parameter influencing the final physical, mechanical, and biological properties of composite [10]. Higher DC values may be obtained by applying high power density [1], [3], [11]. However, higher DC also correlates with increased polymerization shrinkage, which may result in stress formation at bonded interfaces, and lead to disruption of the bond [12], [13], [14]. Thus, efforts to maximize one aspect of light-curing usually result in a compromise in another.
In an attempt to maximize DC and reduce polymerization shrinkage, several photo-polymerization techniques have been suggested. Traditionally, quartz–tungsten–halogen (QTH) lights have been used in a continuous output mode while emitting a fairly high power density [1], [3], [14], [15]. Radiation from this type source can, however, also is applied in different ways. A classification of exposure techniques, all labeled as ‘soft-start’ methods, employ an initial low intensity for a specific duration followed by a higher one equivalent in value to that of the later continuous phase [12], [13], [14], [15], [16]. These methods have been shown to reduce polymerization shrinkage and stress, as well as result in enhanced marginal integrity [12], [13], [14], [15], [16]. Ramped exposure begins at a low intensity and increases power density in a linear or exponential manner over a specific duration to finally reach maximal level where it is maintained for the exposure duration [15], [17], [18]. Intermittent light exposure alternates periods of light and darkness for specified time intervals [15], [16].
Another light source commonly used is classified as the plasma arc light (PAC), and generates very high power density levels during a short exposure [4], [15]. The blue light emitting diode (LED) is the latest light source marketed and supplies a lower power density, but with a narrower spectral range than the QTH or PAC lights, and is more specific toward the absorption needs of the common photo-initiator, camphorquinone [5], [6], [19], [20].
Whatever the type of light source or exposure method, it is essential that adequate conversion result from any curing method used, especially in deeper regions of the material bulk. Having uniform, high DC values throughout the bulk would ensure homogeneous mechanical and physical properties of the restoration, and should lead to enhanced clinical performance.
The aim of this study, was to measure and compare monomer conversion at different depths from the top irradiated surface in a commercial composite using a variety of light sources and exposure methods. The hypotheses tested were: (a) that neither light-curing source nor exposure method has a significant influence on conversion at depths less than 2 mm, and (b) at depths greater than 2 mm, conversion values will be significantly lower than those at shallower depths when using any curing source or exposure method.
Section snippets
Materials and methods
A submicron, hybrid composite was used (Filtek Z250, shade A3, batch 1NL 2004-08, 3M, St Paul, MN, USA). This material is a bis-GMA/UDMA-based hybrid composite. The photo-initiator is camphorquinone, but the system incorporates a proprietary compound greatly enhancing the reactivity of this initiator.
Uncured composite was placed in a cylindrical silicon mould made of heavy body vinyl polysiloxane impression material (Express, 3M, St Paul, MN, USA): 7 mm in diameter, 5 mm in height. The material
Results and discussion
Results are presented in Table 2 and Fig. 1, Fig. 2. As regards the data in Table 2, in the rows, the lower case letters indicate comparison of the DC of the resin composite at different depths for each light-curing method. In columns, the capital letters indicate comparison of light-curing methods at each polymerization depth. The data in graphical form (Fig. 1, Fig. 2) helps to comprehend the totality of the results. The general trend is best seen in Fig. 1, but the standard deviations in
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