To determine the effects of various monomers on conversion and cytocompatibility of dental composites and to improve these properties without detrimentally affecting mechanical properties, depth of cure and shrinkage.
Composites containing urethane dimethacrylate (UDMA) or bisphenol A glycidyl methacrylate (Bis-GMA) with poly(propylene glycol) dimethacrylate (PPGDMA) or triethylene glycol dimethacrylate (TEGDMA) were characterized using the following techniques: conversion (FTIR at 1 and 4 mm depths), depth of cure (BS EN ISO 4049:2009 and FTIR), shrinkage (BS EN ISO 17304:2013 and FTIR), strength and modulus (biaxial flexural test) and water sorption. Cytocompatibility of composites and their liquid phase components was assessed using three assays (resazurin, WST-8 and MTS).
UDMA significantly improved conversion, BFS and depth of cure compared to Bis-GMA, without increasing shrinkage. UDMA was cytotoxic at lower concentrations than Bis-GMA, but extracts of Bis-GMA-containing composites were less cytocompatible than of those containing UDMA. PPGDMA improved conversion and depth of cure compared to TEGDMA, without detrimentally affecting shrinkage. TEGDMA was shown by all assays to be highly toxic. Resazurin, but not WST-8 and MTS, suggested that PPGDMA exhibited improved cytocompatibility compared to TEGDMA.
The use of UDMA and PPGDMA results in composites with excellent conversion, depth of cure and mechanical properties, without increasing shrinkage. Composites containing UDMA appear to be slightly more cytocompatible than those containing Bis-GMA. These monomers may therefore improve the material properties of dental restorations, particularly bulk fill materials. The effect of diluent monomer on cytocompatibility requires further investigation.
Dental composites are widely used as dental restorative materials for their high strength and excellent aesthetics. These consist primarily of: a liquid phase containing monomers and an initiator system (typically photo-activated); a filler phase (typically boro-aluminosilicate glass particles), which provides optimal mechanical and aesthetic properties; and silane, a coupling agent which enables bonding of the polymer to the filler. Bisphenol A glycidyl methacrylate (Bis-GMA), urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) are some of the most commonly used monomers in dental composites. Upon photoinitiation, they form a cross-linked polymer network which hardens and entraps the fillers .
Composites are typically layered in increments and this is time-consuming for clinicians, since it requires curing each increment before proceeding with the next. This is particularly an issue in deeper posterior cavities, in which many increments may be required. As a result, bulk fill materials aim to overcome this issue by utilizing photoinitiators which are effective at depths of 4–5 mm, as well as monomers with low double bond concentration and, in some cases, monomers which are cleaved during polymerization.
One of the major limiting factors of composites is the close interplay between degree of conversion and other characteristics, including mechanical properties, polymerization shrinkage, water sorption and elution of toxic components. Since the level of residual monomer in a composite affects its biocompatibility, mechanical properties and aesthetics , high conversion is ideal for optimization of these properties. Conversely, however, high conversion is typically associated with high volumetric shrinkage. In the patient, this can result in microbial microleakage (penetration of pathogens between the composite and tooth), recurrent caries and, ultimately, failure of the restoration. Monomers with low double bond concentration and subsequent low shrinkage are therefore optimal, particularly in the case of bulk fill materials, due to the larger volume of each increment.
The aim of the present research was to improve the conversion, strength and cytocompatibility of dental composites without detrimentally affecting polymerization shrinkage or depth of cure by fully replacing Bis-GMA with UDMA and TEGDMA with PPGDMA. This is due to the greater flexibility and cross-linking density of UDMA than Bis-GMA and the greater flexibility and significantly lower double bond concentration of PPGDMA than TEGDMA. In order to investigate the effect of each monomer on cytocompatibility, human gingival fibroblasts (HGF) were cultured in solutions of each individual liquid phase component at varying concentrations, as well as in extracts of each composite formulation. Due to the variability between different cell viability assays, which arises from the targeting of different enzymes within the cell, and the ambiguity of the widely used term ‘biocompatibility’ , three assays were compared. The null hypothesis was that replacement of Bis-GMA with UDMA and TEGDMA with PPGDMA would have no effect on these properties.
Materials and methods
Composite paste preparation
Microhybrid dental composites were prepared using 10 wt% 40 nm fumed silica (Aerosil OX-50, Evonik Industries AG, Essen, Germany) and 90 wt% silane-treated barium boro-aluminosilicate glass particles of various sizes (DMG Chemisch-Pharmazeutische Fabrik GmbH, Hamburg, Germany). These were combined with four dimethacrylate-based liquid phases. The liquid phases consisted of bulk monomer UDMA (DMG) or Bis-GMA (Polysciences Inc., Eppelheim, Germany) combined with diluent monomer PPGDMA (Polysciences) or TEGDMA (DMG). The bulk to diluent molar ratio was 3.5:1. The liquid phases also contained 40 mM (0.58–0.61 wt%) photoinitiator camphorquinone (CQ, DMG), 60 mM (0.82–0.86 wt%) co-initiator N , N -dimethylaminoethyl methacrylate (DMAEMA, Sigma–Aldrich, Gillingham, UK) and 100 ppm inhibitor butylated hydroxytoluene (Sigma–Aldrich). The two phases were combined to form composite pastes using a centrifugal planetary mixer (SpeedMixer, Hauschild Engineering, Hamm, Germany), in order to minimize air incorporation and ensure complete wetting of filler particles. The powder to liquid ratio (PLR) was kept constant at 40 vol% liquid (19.3–20.3 wt%, depending on liquid phase density). Dental composites were designated abbreviations based on their bulk and diluent monomer content: UP, UT, BP and BT, where U, B, P and T represent UDMA, Bis-GMA, PPGDMA and TEGDMA, respectively. Commercial composite Filtek Z250 (3M ESPE, St. Paul, MN, USA) was used for comparison.
Disc specimen production
Except where otherwise stated, disc-shaped specimens were moulded by applying composite pastes to metal circlips (internal diameter 10.2 mm, thickness 1 mm) and pressing them between two sheets of acetate. This prevents oxygen inhibition during polymerization and expels excess paste, ensuring similar specimen thickness. Specimens were photo-polymerized using a blue light emitting diode curing unit with a wavelength of 450–470 nm and power output with periodic level shifting of 1100–1330 mW/cm 2 (Demi Plus, Kerr Dental, Orange, CA, USA), in direct contact with the acetate. The curing duration for each testing method varied and is detailed in each corresponding section.
Handling properties and wet-point determination
The wet-point of each liquid was determined by gradually adding a small quantity of liquid phase to a known mass of filler phase and mixing, until the filler was sufficiently wetted and a cohesive paste had been formed. The quantity of liquid phase added was recorded and notes were made regarding the handling properties of each formulation. The wet-point (vol%) was then calculated from the total mass of liquid and the density of each component.
Degree of conversion
The conversion of each composite was determined using Fourier transform infrared spectroscopy (FTIR, System 2000, PerkinElmer, Seer Green, UK). Composite paste was applied to either a single or four stacked circlips of the same dimensions. These were placed on the diamond of an attenuated total reflectance accessory (Golden Gate ATR, Specac Ltd., Orpington, UK) and covered with a sheet of acetate. After an initial spectrum of the uncured composite had been obtained, spectra were recorded continuously for 1000 s ( n = 3). The specimens were photo-polymerized from the top for the first 20 s. Spectra were recorded at a wavelength range of 800–1800 cm −1 and resolution of 8 cm −1 . Absorbance profiles were obtained at 1319 ± 1 cm −1 (C–O stretch bond) and 1334 ± 2 cm −1 (baseline) and used to calculate conversion at 1 mm and 4 mm depths using Eq. (1) :