Abstract
Objective
To investigate the surface integrity of solvent-challenged ormocer-matrix composites, photoactivated by different light exposure modes, through surface-hardness measurements at different periods of time; and to compare such behavior with dimethacrylate-based materials.
Methods
One hundred percent ormocer-based matrix (experimental ormocer (ORM)), a commercial mixed dimethacrylate-ormocer-based matrix (Admira (ADR)) and two commercial dimethacrylate-based matrix composites (experimental controls, (Grandio (GRD) and Premise (PRE)) were evaluated. Disk specimens (4 mm × 2 mm) were prepared from each material and light-activated using either a standard (S) or soft-start (SS) light exposure protocol with an LED-curing unit. Top, irradiated surface Knoop hardness (KHN) was measured within the following experimental groups ( n = 5): Group 1: immediately after exposure; Group 2: after dry and dark storage, Group 3: after storage in distilled water, and Group 4: immersion in absolute ethanol. Hardness of Groups 2–4 were measured after 7 days storage. Immediate hardness values were submitted to Student’s t -tests separately for each material. Hardness values after treatments were submitted to two-way ANOVA and Tukey’s post hoc test to compare values among different storage media and light exposure mode protocols. Comparisons among materials were described using percentage of hardness change. Statistical testing was performed at a pre-set alpha of 0.05.
Results
Immediate hardness values were not affected by the light exposure mode, regardless of the material. In general, exposure mode did not significantly affect hardness after 7 days storage, regardless of storage media or material. After 7 days dry storage, hardness values increased for all materials relative to immediate testing, and decreased after water and ethanol storage, with ethanol showing the greatest effect. The experimental ormocer-based material had the lowest percentage hardness change and thus proved more resistant to solvent degradation than the other materials, regardless of the light exposure method.
Significance
Irradiated surface hardness values and surface integrity were unaffected by light exposure mode, regardless of the material tested. The experimental ormocer-based material presented the least change in hardness as a result of solvent challenge than any of the commercial products: ormocer or conventional resin-based, and thus showed better surface integrity.
1
Introduction
An ormocer-resin-based resin composite is a hybrid structure not generated by the classic means of physically mixing ground glass with a polymerizable methacrylate-based resin, but through special processing based on molecular scale technology. This technology combines organic and inorganic components at nanoscopic scale through the sol–gel method . The main characteristic of this type of material is the linking of organic groups to the inorganic backbone, formed by hydrolysis and condensation of alkoxides. The organic component (organic polymer) is responsible for network formation, flexibility, and optical properties. The inorganic unit (glasses, ceramics) lowers thermal expansion as well as provides chemical and thermal stability .
Ormocers are successfully used in high-tech industries, such as optical coatings, electronics, and medical technology. In Dentistry, admixed (a material mixture of dimethacrylate and ormocer-organic matrix) and pure ormocer-matrix have been used to overcome drawbacks of conventional dimethacrylate-matrix composites, such as elution of monomers attributable to toxicity and to allergenic reactions . Special, pure ormocer-matrix composites are believed to offer enhanced clinical performance through high wear resistance, reduced polymerization shrinkage, enhanced biocompatibility, and long-lasting polymer-matrix stability . According to manufacturer information, the new Ormocer formulation has no free monomers, thus eliminating the risk of allergic reaction. However the stability (surface integrity) of this new, experimental material (pure Ormocer-matrix) when challenged by fluids encountered in the oral environment (solvent challenge) is not well known.
Surface integrity influences the long-term clinical performance of resin-composite restorations and can be affected by material formulation and/or degradation due to exposure in the oral environment . Surface characteristics are also influential in the material’s resistance to load application and the generation of scratches . Hardness is a surface characteristic that evaluates material’s surface resistance to localized plastic deformation or scratching by penetration caused by a constant applied load . Surface hardness can also be an indicator of durability, degree of conversion (DC), and crosslink density (CLD) .
The CLD of a polymer network can be indirectly assessed by repeated hardness measurements before and after the immersion in organic solvents . It is generally accepted that highly cross-linked polymers are more resistant to degradation and to solvent uptake, whereas linear polymers present a less dense polymer network allowing solvent molecules to diffuse more readily . By using this methodology, investigations have shown that polymers with similar DC could display distinctly different CLD due to differences in polymer network cross-linking as a result of different rates of polymerization. Thus, elevated light irradiance levels would generate polymers with higher CLD than those using lower levels, which might produce a structure with less extensive CLD . Light exposure protocols based on application of an initial low-level light irradiance, which are applied in hope to reduce polymerization shrinkage, could influence surface network formation, and the resulting surface integrity when challenged by fluids. However, this assumption is a matter of controversy .
Scarce information is available about the surface integrity of materials formulated with a wide range of compositions and submitted to different light exposure protocols. Furthermore, no data can be found related to the surface integrity of pure ormocer-based-matrix composites when challenged by various clinically relevant solvents.
The purpose of this study was to investigate the effect of light exposure protocols (standard or soft-start) on immediate microhardness values (an indirect evaluation of degree of conversion) of different materials, as well as their ability to maintain initial hardness values when treated to various solvents, or remaining dry for a period of 7 days. Thus the surface integrity (maintenance of initial hardness) will be investigated as a function of restorative materials and light exposure methods.
Three research hypotheses are tested. The first assumes that for all restorative materials initial (immediate) hardness would be greater when using continuous exposure than when applying a soft-start method. The second hypothesis was that surface hardness values would be higher for specimens exposed to continuous exposure (compared to soft-start) after 7 days of storage (dry, water and ethanol). The last hypothesis anticipates that the experimental, pure ormocer-based composite would demonstrate the least percentage change in hardness value (immediate to 7 days, within all three storage treatments) among the commercial restorative products tested.
2
Materials and methods
2.1
Restorative materials
Compositions and manufacturer information for each restorative material used are presented in Table 1 . An experimental ormocer consisting of one hundred percent ormocer-matrix (ORMOCER, Voco (ORM)) as well as a commercial, mixed ormocer-dimethacrylate-matrix (Admira, Voco (ADR)) were used. Experimental controls consisted of two commercial, dimethacrylate-matrix composites (Grandio, Voco (GRD) and Premise, Kerr (PRE)).
| Material | Type | Matrix | Fillers | Manufacturer |
|---|---|---|---|---|
| (ADR) | ORMOCER | ORMOCER a matrix with substantial addition of conventional dimethacrylate monomers Bis-GMA, UDMA, TEGDMA | 78 wt% SiO 2 and glass-ceramic | Voco, Cuxhaven, Germany |
| (ORM) | ORMOCER | ORMOCER a matrix with no additional dimethacrylate monomers | 87 wt% SiO 2 and glass-ceramic | Voco, Cuxhaven, Germany |
| (GRD) | Nanohybrid | Bis-GMA, UDMA, TEGDMA | 87 wt% SiO 2 and glass-ceramic | Voco, Cuxhaven, Germany |
| (PRE) | Nanohybrid | Bis-EMA, TEGDMA | 84 wt% SiO 2 and barium glass | Kerr Corporation, USA |
a ORMOCERS are inorganic/organic hybrid structures incorporating covalently-bonded methacrylate end-groups for free-radical cross-linking developed by the Fraunhofer-Institute, Germany.
2.2
Light exposure protocols and samples’ preparation
A mono-wave (blue) light emitting diode (LED) unit was used (Elipar Freelight 2, 3M ESPE Dental Products, St Paul, MN, USA). Uncured paste of each material was placed into a cylindrical brass mould (2 mm height × 4 mm inner diameter) and, covered with a Mylar strip and pressed with a microscope slide to form a flat surface. The light unit was held rigidly in place and specimens were light-activated with the light guide positioned directly on the Mylar strip. Two different light exposure protocols were used: continuous (C) consisting of a single 40-s exposure at 1100 mW/cm 2 , (total of 44 J/cm 2 ) or an internal, selectable soft-start exposure was applied, providing an initial low, increasing irradiance value for 10 s, and then an irradiance similar to that of the continuous exposure for the remaining 30 s (total of 35.5 J/cm 2 ). Irradiance was measured using a hand-held dental curing radiometer (Model 200, Demetron Research Corporation, Danbury, CT, USA). Twenty specimens of each material were made for each light curing mode. After polymerization, the specimens were mounted in 1-in. diameter phenolic rings (Buehler, Lake Bluff, USA) and embedded in self-curing polystyrene resin (Castoglas Resin, Buehler, Lake Bluff, USA) before polishing. The embedded specimens were submitted to mechanical polishing in a grinding/polishing machine (Buehler, Lake Bluff, USA) with a sequence of 800- and 1200-grit SiC papers under continuous water cooling.
Samples were then randomly assigned to one of four groups ( n = 5): Group 1: immediate hardness testing after exposure; Group 2: hardness determined 7 days after dry, dark storage at 37 °C (control group); Group 3: hardness obtained 7 days following immersion in distilled water at 37 °C; and Group 4: 7 days soaking in absolute ethanol (100%) at 37 °C.
2.2.1
Surface hardness
Knoop hardness values was measured on the irradiated surfaces using an indenter (Future Tech FM-700, Tokyo, Japan) under a load of 25 g applied for 20 s. Measurements were performed at five locations on each specimen, and the average value was recorded as the hardness for a given specimen.
2.2.2
Statistical analyses
It is not appropriate to directly compare absolute hardness values among products, only with a product between light exposure modes. Thus, for each material, Student’s t -test was used to compare immediate hardness values between continuous or soft-start modes. A two-way ANOVA was applied to examine the effect of storage condition and exposure mode within a given restorative material, followed by Tukey’s post hoc test for pair-wise comparisons. All statistical testing was performed using a pre-set alpha of 0.05. Hardness comparison among the different materials was performed by determining the percentage change in hardness for a given material between the immediate reading and the 7 days storage value. Statistics were not performed on this data.
2
Materials and methods
2.1
Restorative materials
Compositions and manufacturer information for each restorative material used are presented in Table 1 . An experimental ormocer consisting of one hundred percent ormocer-matrix (ORMOCER, Voco (ORM)) as well as a commercial, mixed ormocer-dimethacrylate-matrix (Admira, Voco (ADR)) were used. Experimental controls consisted of two commercial, dimethacrylate-matrix composites (Grandio, Voco (GRD) and Premise, Kerr (PRE)).
| Material | Type | Matrix | Fillers | Manufacturer |
|---|---|---|---|---|
| (ADR) | ORMOCER | ORMOCER a matrix with substantial addition of conventional dimethacrylate monomers Bis-GMA, UDMA, TEGDMA | 78 wt% SiO 2 and glass-ceramic | Voco, Cuxhaven, Germany |
| (ORM) | ORMOCER | ORMOCER a matrix with no additional dimethacrylate monomers | 87 wt% SiO 2 and glass-ceramic | Voco, Cuxhaven, Germany |
| (GRD) | Nanohybrid | Bis-GMA, UDMA, TEGDMA | 87 wt% SiO 2 and glass-ceramic | Voco, Cuxhaven, Germany |
| (PRE) | Nanohybrid | Bis-EMA, TEGDMA | 84 wt% SiO 2 and barium glass | Kerr Corporation, USA |
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