Polyacid-modified composite resins (compomers) are restorative dental materials that exhibit certain features of traditional dental composites and glass-ionomer cements. The aim of this paper was to develop experimental compomers with enhanced properties, based on adhesive monomers vinyl phosphonic acid and pyromellitic dianhydride glycerol dimethacrylate, and to compare their properties to those of commercially available products.
Factorial experimental design was employed to optimize both chemical and physical properties. Properties such as biaxial flexural strength (BFS), wear resistance (WR), water uptake (WU), and adhesion using shear bond strength (SBS) as well as fluoride release (FR) were evaluated and compared with those of commercial products.
Results were subjected to one-way ANOVA ( p < 0.05); significant differences were observed in properties of materials such as WR, BFS and SBS but not in WU and FR compared to commercial products. Experimental materials exhibited significantly higher WR, BFS and SBS values than commercial materials. Properties of materials were affected by their respective storage media with time.
Based on the results of this study, higher amounts of vinyl phosphonic acid (VPA), pyromellitic dianhydride glycerol dimethacrylate (PMGDM) and reactive glasses render the material with enhanced fluoride release and adhesion with properties similar to glass-ionomers whereas their decrease gives properties similar to conventional dental composite resins with improved properties such as strength and wear resistance.
Polyacid-modified composite resins (compomers) are materials that combine certain features of traditional dental composite resins and glass-ionomer cements . They are based on bulky monomers that undergo addition polymerization, generally as a result of irradiation of light at 470 nm in the presence of light-activated initiators. At least one of the monomer components contains a small proportion of carboxylic acid functional groups, insufficient to confer water solubility on either the monomer or its polymer, but capable of promoting acid–base neutralization reaction following sorption of water after polymerization to release fluoride ions. Compomers are predominantly composite in structure and have many properties found in conventional composite resins as well as glass-ionomer cements .
A major disadvantage with conventional composite resins is their lack of adhesion to dentin and therefore application of appropriate bonding agents are necessary to achieve adhesive strength of clinical significance. In restorative dentistry, the use of organophosphorus-based and acid-modified monomers in dental materials, e.g. bonding agents has shown some advantages such as improving adhesion to the tooth . Furthermore there is evidence to demonstrate that the absorption of water, as part of the setting reaction of these materials, may have an adverse effect on the mechanical properties of the material. A number of studies have demonstrated that properties such as compressive strength, flexural strength, biaxial and diametral tensile strengths of compomers decease when exposed to water . Clearly there is scope for modification of compomer materials to optimize its clinical application.
The aim of this study was to develop experimental compomers with enhanced properties using a combination of pyromellitic dianhydride glycerol dimethacrylate (PMGDM) based on glycerol dimethacrylate and pyromellitic dianhydride offering a tetramethacrylate and two acid (COOH) groups and vinyl phosphonic acid (VPA) as active acid-functional monomers with a mixture of reactive glasses to obtain optimized properties found in dental composite resins and glass-ionomers cements . These materials have attracted some attention recently and have been included in commercial and experimental adhesives and resin composites . Incorporation of VPA into compomers has advantages in comparisons to other materials; however, the performance of restorations completed from compomers containing VPA is quite sensitive to the concentration included . The properties of the experimental compomers produced were then compared with those found in commercial compomers.
Factorial experimental design (FED) will be used in this study as it provides an opportunity to identify the key factors that affect product and optimize the required properties. There are three specific advantages to use a factorial design in experimentation rather than classical methods :
Greater efficiency – a number of factors (or variables) are evaluated with equal precision with a fraction of the number of observations that would otherwise be necessary.
Greater comprehensiveness – in addition to the determination of the effects of single factors, interactions of the factors are also evaluated.
Wider inductive basis (Fisher’s term) – conclusions based on an experiment in which many factors have been varied have been tested under a broader range of conditions than if only one variable had been changed at a time.
Materials and methods
Optimized formulations of experimental componers ( Tables 1 and 2 ) were achieved using experimental factorial design and analysis . Three different optimized experimental compomers A–C were formulated with varying amounts of VPA and PMGDM. The ratio of VPA to PMGDM in each formulation was 1:1. From the optimized formulations, the total amounts of acid-modified adhesive-enhancing monomers in A, B and C were 0.1%, 0.5% and 1.0% (w/w), respectively. Properties of experimental materials such as biaxial flexural strength (BFS), wear resistance (WR) using abrasion test, water uptake (WU), and adhesion using shear bond strength (SBS) as well as fluoride release (FR) were evaluated and compared with those of commercial products tested under the same conditions.
|BisGMA, TEGDMA, PMGDM, and Raysorb||Esschem, USA|
|Vinyl phosphonic acid||Albright & Wilson, UK|
|Camphorquinone and DMAEM||Aldrich, UK|
|Benzophenone||Osi Specialities (UK)|
|Butylated hydroxytoluene (BHT)||Chance and Hunt, UK|
|Aerosil R972 (silane-treated Silica)||Degussa AG, Germany|
|G338 glass (silane-treated fluoride-releasing glass)||1st Scientific, Germany|
|Compoglass ® F||Ivoclar Vivadent|
|Dyract ® AP||Dentsply|
|F2000 compomer||3M ESPE™|
|Chemical component||Composition (%)|
|BisGMA (binder resin)||11.55|
|VPA–PMGDM (acid-modified monomer admixture)||Variable|
Biaxial flexural strength (BFS) measurement
Three sets of six disc-shaped specimens of 12 mm diameter and 1 mm thickness were light cured for 20 s from each side and were stored in distilled water (Cromalux-100 (Mega-Physik Dental Division, Rastatt, Germany) halogen light-curing unit with nominal output of 600 mW cm 2 at 470 nm through the glass slides for 20 s). The stored specimens were tested for BFS at 24 h, 1 week and 4 weeks. During the test, the specimens were supported by a cylindrical ring of 12 mm diameter. The formula used to evaluate the BFS was developed by de With and Wagemans .
The strength of the specimens was measured using ball on ring (BOR) (12 mm ring) tests and calculated with the equation
σ BOR = 3 ( 1 + ν ) F 4 π t 2 1 + 2 ln R 0 b + 1 − ν 1 + ν R 0 R 2 1 − b R 0 2