Curing potential of experimental resin composites filled with bioactive glass: A comparison between Bis-EMA and UDMA based resin systems

Highlights

• Curing potential of composites with 0–40 wt% of bioactive glass was evaluated.

• Experimental composite series were prepared based on Bis-EMA and UDMA resins.

• Bis-EMA series showed direct polymerization inhibition by bioactive glass.

• No direct polymerization inhibition was identified in UDMA series.

• UDMA series demonstrated markedly better curing potential than Bis-EMA series.

Abstract

Objectives

To evaluate the degree of conversion, light transmittance, and depth of cure of two experimental light-curable bioactive glass (BG)-containing composite series based on different resin systems.

Methods

Experimental composite series based on either Bis-EMA or UDMA resin were prepared. Each series contained 0, 5, 10, 20, and 40 wt% of BG 45S5. Reinforcing fillers were added up to a total filler load of 70 wt%. The degree of conversion was evaluated using Raman spectroscopy, while light transmittance was measured using visible light spectroscopy. The depth of cure was estimated from the degree of conversion data and using the ISO 4049 scraping test.

Results

Replacement of reinforcing fillers with BG can diminish the degree of conversion, light transmittance, and depth of cure. The effect of BG on the aforementioned properties was highly variable between the experimental series. While in the Bis-EMA series, the degree of conversion was significantly impaired by BG, all of the composites in the UDMA series attained clinically acceptable degree of conversion values. The reduction of the degree of conversion in the Bis-EMA series occurred independently of the changes in light transmittance. The UDMA series showed better light transmittance and consequently higher depth of cure than the Bis-EMA series. The depth of cure for all composites in the UDMA series was above 2 mm.

Significance

While the Bis-EMA series demonstrated clinically acceptable curing potential only for 0–10 wt% of BG loading, an excellent curing potential in the UDMA series was observed for a wide range (0–40 wt%) of BG loadings.

Introduction

The addition of various functional fillers into resin composites has been investigated as a means to prevent secondary caries and increase restoration longevity [1]. Secondary caries has often been mentioned as the most common reason for composite restoration replacement, while about 60% of all restorative procedures are related to the replacement of failed restorations [2]. Among the various compounds which are being used for functionalizing experimental resin composites, bioactive glasses (BGs) are interesting candidates due to their potential to exert various beneficial effects when embedded into light-curable methacrylate resins, including remineralization of dental hard tissues [3], deposition of hydroxyapatite precipitates [4], inhibition of bacterial penetration through the microgap [5], antibacterial activity [6], and favorable biocompatibility [7]. Unlike enamel, in which an increased mineral content generally translates into improved mechanical properties, dentin is more challenging to remineralize because its mechanical properties are improved only if intrafibrillar mineralization is attained [8].

BGs encompass a heterogeneous group of substances of varying compositions, which determine their properties. The basic BG 45S5 composition (45 wt% SiO₂, 24.5 wt% CaO, 24.5 wt% Na₂O, and 6 wt% P₂O₅) is generally more reactive than compositions with lower sodium content [9]. Composites admixed with unsilanized BG 45S5 fillers have shown high solubility, which implies high bioactive potential [10]. This comes at the cost of mechanical properties [11], which tend to be better in composites containing more stable BG compositions [12].

An experimental light-curable composite series with a total filler load of 70 wt% and variable amounts of unsilanized BG 45S5 (0–40 wt%) in a Bis-GMA/TEGDMA resin matrix has recently been investigated on their curing potential [13] and polymerization kinetics [14]. These studies showed that the replacement of reinforcing fillers with BG can diminish curing efficiency, which was manifested as a decrease in the degree of conversion (DC) and depth of cure (DoC) [13], as well as reduced polymerization rate [14]. The negative effect of BG 45S5 on resin polymerization was observed independently of the changes in light transmittance and was ascribed to a premature termination of free-radical-mediated polymerization by the oxides on the surface of unsilanized BG fillers. The BG-induced polymerization inhibition can be partly responsible for impaired performance that was reported for the aforementioned experimental series, including reduced flexural strength/modulus [11] and dentin bond strength [15], as well as increased water sorption and solubility [10].

Due to these drawbacks of Bis-GMA/TEGDMA resin [10], [11], [13], [14], [15], alternative resin systems were sought in order to achieve better polymerization of experimental BG-containing composites. This study aimed to investigate the curing potential of Bis-EMA/TEGDMA and UDMA/TEGDMA resin systems by evaluating the DC, light transmittance, and DoC in composite series containing 0–40 wt% of BG, using the same study design as that performed previously for the Bis-GMA/TEGDMA resin system [13]. The DC of experimental composites was evaluated in relation to the resin system, BG amount, layer thickness, and curing time. Light transmittance for 2-mm layers was assessed as a function of the resin system and BG amount. The DoC was estimated as the layer thickness at which the DC amounts to 80% of the maximum attainable DC, and according to the ISO 4049 scraping test. The null hypotheses were that (I) the DC, (II) the light transmittance, and (III) the DoC of the experimental composite series would not differ between the two resin systems investigated.