Resin viscosity determines the condition for a valid exposure reciprocity law in dental composites

Abstract

Objective

To provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites.

Methods

Composites made from different mass ratios of resin blends (Bis-GMA/TEGDMA and UDMA/TEGDMA) and silanized micro-sized glass fillers were used. All the composites used camphorquinone and ethyl 4-dimethylaminobenzoate as the photo initiator system. A cantilever beam-based instrument (NIST SRI 6005) coupled with NIR spectroscopy and a microprobe thermocouple was used to simultaneously measure the degree of conversion (DC), the polymerization stress (PS) due to the shrinkage, and the temperature change (TC) in real time during the photocuring process. The instrument has an integrated LED light curing unit providing irradiances ranging from 0.01 W/cm² to 4 W/cm² at a peak wavelength of 460 nm (blue light). Vickers hardness of the composites was also measured.

Results

For every dental composite there exists a minimum radiant exposure required for an adequate polymerization (i.e., insignificant increase in polymerization with any further increase in the radiant exposure). This minimum predominantly depends on the resin viscosity of composite and can be predicted using an empirical equation established based on the test results. If the radiant exposure is above this minimum, the exposure reciprocity law is valid with respect to DC for high-fill composites (filler contents >50% by mass) while invalid for low-fill composites (that are clinically irrelevant).

Significance

The study promotes better understanding on the applicability of the exposure reciprocity law for dental composites. It also provides a guidance for altering the radiant exposure, with the clinically available curing light unit, needed to adequately cure the dental composite in question.

Introduction

In photochemistry, using different combinations of irradiance (radiant flux per unit area) and exposure time is justified by the Bunsen–Roscoe reciprocity law [1] (also referred to as the exposure reciprocity law, ERL). For example, in restorative dentistry the ever-increasing demand from dentists for reducing the exposure time (i.e., reducing chairside time) in photocuring composites has led to high irradiance LED curing units (e.g., >2 W/cm²) [2]. The ERL states that for a given radiant exposure (defined as the total radiant energy received per unit area = irradiance × exposure time), the polymerization (the degree of conversion from monomers to polymers, DC) of the resin does not change with any combination of irradiance and exposure time. However, studies in the literature have debated rather inconclusively about the validity of the ERL in dentistry [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. The results of these studies depended on the combination of the photocuring conditions (irradiances and radiant exposures) and materials (model dental resins/composites and commercial composites) used. The objective of this work is to address the discrepancies regarding the validity debate and, hence, promote a better understanding of the applicability of the ERL as it pertains to resin-based dental composites.

Although a few studies showed that the ERL for dental materials was valid [14,15], a majority of the studies showed that the ERL was invalid. These studies can be divided broadly into two categories. In the first category, the ERL has been demonstrated not to be valid since a higher DC was obtained using a higher irradiance in comparison to lower irradiances [5,13,17]. For example, Wydra et al. [13] showed that using a high irradiance (0.024 W/cm²) UV-cure on a dimethacrylate resin mixture resulted in a higher DC when compared to using a low irradiance (0.003 W/cm²). Leprince et al. [5] showed the same trend when a dimethacrylate resin and composite were cured with Lucirin-TPO as the initiator. Some theoretical arguments made in these studies, in support of their results, state that the ERL cannot be valid since the DC, derived from the classical equation for the rate of photopolymerization [18,19], is not proportional to the first order of the radiant exposure. Conversely, in the second category, a large number of studies have indicated the ERL not to be valid as a lower DC was obtained using a higher irradiance in comparison to lower irradiances [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12],16]. For example, Hadis et al. [10] showed that DC achieved using a higher irradiance of 3 W/cm² was significantly lower than that using an irradiance of 0.4 W/cm² for a range of commercial dental composites. Similar trends were seen by Feng et al. [3,4] when they examined the validity of the ERL on a range of multifunctional acrylate resins, methacrylate resins, and commercial composites. Due to the clinical resemblance of the materials used in these studies, this study will primarily address the ERL validity in this category. Also, subsequently, results and discussion are provided regarding the first category as mentioned earlier.

In addition to exploring the validity of the ERL, some studies also discussed the effect of irradiance, at a constant radiant exposure, on other key properties of the composites. These properties involve, but are not limited to, polymerization stress (PS) due to shrinkage, the temperature change (TC) due to the reaction exotherm and absorbance associated with the photocuring process, and the hardness. For example, it has been discussed that photocuring with high irradiance may increase PS and effectively decrease the bond strength to the tooth [[20], [21], [22]]. Although an increase in irradiance will increase the rate of PS, it is speculative as to whether this will lead to a higher PS [13,[23], [24], [25], [26], [27]]. The exothermic temperature increases due to the increase of irradiance, along with the temperature rise due to the absorbance. This can potentially lead to pulpal and dental tissue damage [[28], [29], [30]]; however, only a few studies on the validity of the ERL [17,31] have measured TC. The hardness of the composites was also evaluated, sometimes as a substitute to measuring the DC, to test the validity of the ERL [16]. As these properties are clinically relevant, in the current study they will also be measured and discussed in conjunction with the discussion of the ERL.

A systematic study involving the variation of the composition of model dental composites and the photocuring conditions has been carried out in this work to address the validity of the exposure reciprocity law (ERL) with respect to the DC. Model dimethacrylate dental resins blended of Bis-GMA (bisphenol A glycidyl methacrylate)/TEGDMA (triethyleneglycol dimethacrylate) or UDMA (urethane dimethacrylate)/TEGDMA at different ratios were used. The resin blends were mixed with a fixed mass of visible-light initiator system (camphorquinone and ethyl 4-dimethylaminobenzoate) and with varied content of a silanized micro-sized glass filler. A NIST-developed standard reference instrument (NIST SRI 6005) [32,33] coupled with NIR spectroscopy and a microprobe thermocouple, which simultaneously measures DC, PS, and TC in real time during the photocuring process, was used in this study. After the measurement of the polymerization properties, the Vickers hardness test was performed for the composites. The result from this study indicates that for every dental composite there exists a minimum radiant exposure required for adequate polymerization (i.e., insignificant increase in polymerization will be produced with any further increase in the radiant exposure). This minimum depends primarily on the resin viscosity of composite; an empirical model as a function of the resin viscosity is established and verified based on experimental results to predict it. The validity of the ERL should be discussed only if the radiant exposure used in the photocuring process is above this minimum. The ERL is valid with respect to DC for high-fill composites (filler contents >50% by mass, which are clinically relevant to dental composites) while invalid for low-fill composites. The difference between the high-fill and low-fill composites on the ERL validity is largely due to the significance of polymerization temperature effect on the resin viscosity during the photocuring process. The result of the study clarifies discrepancies reported in the literature and gives guidance on the validity of the exposure reciprocity law for dental composites. Also, the empirical model can enable one to determine the exposure time required to adequately cure a given dental composite with the available curing light unit (irradiance). More importantly, when employing a high irradiance and the corresponding exposure time, based on the law, care should be taken such that the associated temperature change in the underlying tissue is clinically acceptable. Finally, although the study is based on dimethacrylate-based resin composites, the conclusions made in this study are applicable to other resin composite systems where the nature of the photopolymerization reaction is primarily influenced by the diffusion-controlled polymerization.