- Email: [email protected]
Improving some selected properties of dental polyacid-modified composite resins
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Improving some selected properties of dental polyacid-modified composite resins Paul J. Milward a , Gabriel O. Adusei b , Christopher D. Lynch a,∗ a b
Tissue Engineering & Reparative Dentistry, School of Dentistry, Cardiff University, Heath Park, Cardiff CF14 4XY, UK Triune Medical Technologies Limited, 22 Pennington Road, Hartford, Huntingdon, Cambridgeshire PE29 1QF, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Polyacid-modified composite resins (compomers) are restorative dental materials
Received 12 November 2009
that exhibit certain features of traditional dental composites and glass-ionomer cements.
Received in revised form
The aim of this paper was to develop experimental compomers with enhanced properties,
16 May 2011
based on adhesive monomers vinyl phosphonic acid and pyromellitic dianhydride glycerol
Accepted 30 June 2011
dimethacrylate, and to compare their properties to those of commercially available products. Methods. Factorial experimental design was employed to optimize both chemical and physical properties. Properties such as biaxial flexural strength (BFS), wear resistance (WR), water
Keywords:
uptake (WU), and adhesion using shear bond strength (SBS) as well as fluoride release (FR)
Mechanical properties
were evaluated and compared with those of commercial products.
Adhesion
Results. Results were subjected to one-way ANOVA (p < 0.05); significant differences were
Composite materials
observed in properties of materials such as WR, BFS and SBS but not in WU and FR com-
Wear
pared to commercial products. Experimental materials exhibited significantly higher WR,
Compomer
BFS and SBS values than commercial materials. Properties of materials were affected by their respective storage media with time. Significance. 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 glassionomers whereas their decrease gives properties similar to conventional dental composite resins with improved properties such as strength and wear resistance. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Polyacid-modified composite resins (compomers) are materials that combine certain features of traditional dental composite resins and glass-ionomer cements [1]. 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 [2]. 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
Corresponding author. Tel.: +44 02920 20744665. E-mail address: [email protected] (C.D. Lynch). 0109-5641/$ – see front matter © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2011.06.006
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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
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 [3,4]. 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 [1,5–7]. 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 acidfunctional monomers with a mixture of reactive glasses to obtain optimized properties found in dental composite resins and glass-ionomers cements [3,4,8–11]. These materials have attracted some attention recently and have been included in commercial and experimental adhesives and resin composites [12,13]. 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 [14]. 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 [9]: • 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.
Table 1 – Materials and their suppliers. Material
Supplier
BisGMA, TEGDMA, PMGDM, and Raysorb Vinyl phosphonic acid Camphorquinone and DMAEM Benzophenone Butylated hydroxytoluene (BHT) Aerosil R972 (silane-treated Silica) G338 glass (silane-treated fluoride-releasing glass) Compoglass® F Dyract® AP F2000 compomer
Esschem, USA Albright & Wilson, UK Aldrich, UK Osi Specialities (UK) Chance and Hunt, UK Degussa AG, Germany 1st Scientific, Germany Ivoclar Vivadent Dentsply 3M ESPETM
2,2-Bis[p-(2 -hydroxy-3 -methacryloxypropoxy) phenyl] propane (BisGMA); triethylene glycol dimethacrylate (TEGDMA); and 2(dimethylamino) ethylmethacrylate (DMAEM).
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.
2.1.
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 cm2 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 [10]. The strength of the specimens was measured using ball on ring (BOR) (12 mm ring) tests and calculated with the equation
BOR =
3(1 + )F 4t2
× 1−
1 + 2 ln
b 2
R 1 − R 2 0 0 b
+
1+
R
R0
Table 2 – Formulation of experimental componers.
2.
Materials and methods
Optimized formulations of experimental componers (Tables 1 and 2) were achieved using experimental factorial design and analysis [6]. 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
Chemical component BisGMA (binder resin) TEGDMA VPA–PMGDM (acid-modified monomer admixture) Camphorquinone DMAEM Benzophenone BHT Aerosil R972 G338 glass Raysorb
Composition (%) 11.55 3.85 Variable 0.20 0.30 0.09 0.01 4.00 Variable Variable
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
where BOR is the strength, is a constant, is the Poisson’s ratio (assumed to be 0.35), R the specimen radius, R0 the ball bearing support, b the contact radius loading area, F is the force, and t the specimen thickness. The b = t/3 (Westergaard approximation) was used. The cross-head speed used was 1 mm/min.
2.2.
Wear resistance measurement
For the WR studies, rectangular strips of specimens of 40 mm, 5 mm wide and 2 mm thickness were manually prepared from both experimental and commercial materials in perspex molds and light-cured for 40 s at both sides through cellulose acetate film. Four specimens of each material were used and stored in water for 24 h. They were then placed in a special perspex molds that held them in position for the toothbrush heads to move freely on the materials during the brushing cycles. For each specimen, a 60 ml of slurry of equal amounts of toothpaste, water and dental pumice powder of grit 60 (Bracon Ltd., East Sussex, UK) was used as a highly abrasive medium to give the abraded sample surfaces high abrasion effect for easy analysis with the profilometer. Four brush heads, each weighing 167 g were used and 30,000 brushing strokes were registered on the counter of the Toothbrushing and Abrasion Test Machine No. 8 (Research Laboratories, The Pepsodent Co., Chicago, IL). A 3D laser measurement system profilometer (UBM/Keyence LC2450 (Germany)), with a point density of 2 m was used to measure the mean vertical loss of material after toothbrush abrasion.
2.3.
Water uptake
The WU was evaluated using disc-shaped specimens of 13 mm diameter and 1 mm thickness prepared by placing materials in stainless steel molds of appropriate dimensions between two microscope glass slides placed at both sides of the mold. Both faces were irradiated in turn with Cromalux-100 (Mega-Physik Dental Division, Rastatt, Germany) halogen light-curing unit with nominal output of 600 mW cm2 at 470 nm through the glass slides for 20 s. Three specimens of each material were used and they were placed in their respective storage medium with each specimen in a separate stoppered glass vial. They were weighed at regular time intervals at 30 min for 2 h and hourly for 8 h. The weighing continued from the 24th hour at daily intervals, for 1 week and weekly until they had equilibrated at the 12th week. Weighing was performed with blotted dry specimens, using absorbent tissues, determinations being made to the nearest 0.0001 g on a using Mettler AT 250 analytical balance (Mettler-Toledo Ltd., Beaumont Leys Leicester). After weighing, the specimens were returned to the storage medium and stored at 37 ◦ C. The difference between the weight (wt) of the specimen after immersion (Wt(f)) and the initial weight (Wt(i)) was recorded, and the difference in final wt and the initial wt was expressed as percentage hydration at equilibrium using the formula: Wt(f ) − Wt(i) × 100 Wt(i)
999
where Wt(i) and Wt(f) are the initial and is the final weights, respectively.
2.4.
Adhesion analysis
The adhesion analysis employing shear bond strength test used a set of six cylindrical specimens of 4 mm height and 3 mm diameter of each material were bonded to enamel of randomly selected human teeth. The tooth surfaces were acid-etched with 37% phosphoric acid solution for 20 s and oil-free air applied to bonding area to keep the tooth surface dry before materials were placed firmly. A derivative of the active acid-modified monomers, bisGMA, TEGDMA, acetone and polymerization initiators were used for the formulation of experimental bonding agent for materials A–C. For the commercial materials, the recommended bonding agents were applied and the manufacturers’ instructions were followed. All samples were light cured for 40 s and stored in artificial saliva at 37 ◦ C before testing for shear bond strength using Lloyds 10 KN (Lloyd Instruments Ltd., Bognor Regis, West Sussex, UK) at crosshead speed of 0.5 mm/min. A shear blade with blunt edge at point of contact with the material was used to shear the material–tooth bond and the SBS/MPa was calculated by dividing the break force by bonding surface area.
2.5.
Flouride release measurement
Disc shaped specimens of dimensions 13 mm in diameter and 1 mm thickness were used for the fluoride release evaluation. One sample was prepared per material tested. Weight of each specimen equilibrated to 0.35 g and was recorded using a Mettler AT 250 analytical weighing balance. The discs were then placed in glass vials containing 5 ml of equal amounts (2.5 ml) of deionized water and total ionic strength adjustment buffer (TISAB) solution. TISAB is used to maintain constant ionic strength and to remove certain interferences. The ionic fluoride concentration of each solution was measured using a fluoride ion selective electrode under constant temperature at 24 ◦ C and was gently rinsed between with de-ionized water to prevent crossover contamination between samples. Fluoride release measurements were recorded at an hourly interval on the first day, and daily basis up to 12 weeks.
3.
Results
FED yielded formulations A–C as the optimized experimental materials with three different percent compositions of acid-modified monomers and reactive glass fillers in their respective formulations. It was also observed that the effects of only one of the active acid-modified monomers in other formulations did not confer desired properties and were inferior to the ones containing both PMGDM and VPA, which showed synergistic effect with better properties. The results of the evaluated properties of the experimental and commercial materials are given in Tables 3–7 with standard deviations (sd) in parentheses. The results were subjected to one-way ANOVA and Tukey test (p < 0.05); significant differences were observed in the properties of materials such as
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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
Table 3 – Biaxial flexure strength (BFS) of experimental and commercial compomers stored in distilled water. Material
A B C Dyract AP Compoglass F F 2000
BFS/MPa at various storage times (sd) 24 h
1 week
4 weeks
188.6 (±2.2) 186.4 (±4.6) 176.8 (±4.2) 177.6 (±2.5) 182.4 (±2.4) 186.8 (±2.0)
182.4 (±2.6) 186.2 (±2.8) 178.2 (±1.2) 168.5 (±4.2) 174.4 (±2.0) 176.0 (±2.2)
171.8 (±4.2) 172.6 (±5.6) 188.4 (±2.4) 154.8 (±2.2) 165.6 (±2.2) 160.2 (±2.2)
Table 4 – Wear analysis of experimental and commercial compomers stored in distilled water using toothbrushing technique. Material
Wear of materials/micron at various storage times (sd) 24 h
A B C Dyract AP Compoglass F F 2000
36.4 (±6.2) 40.2 (±2.2) 44.8 (±5.3) 56.6 (±3.2) 48.0 (±6.2) 46.4 (±2.2)
1 week
4 weeks
42.4 (±2.4) 48.2(±4.2) 58.2 (±6.2) 62.8 (±2.2) 56.4 (±1.2) 50.8 (±3.2)
44.6 (±2.8) 55.4 (±2.2) 60.4 (±5.2) 64.6 (±4.6) 62.4 (±4.2) 58.4 (±2.6)
WR, BFS and SBS but not in WU and FR. Experimental materials exhibited somewhat greater BFS (Table 3), WR (Table 4) and SBS (Table 6) values than those of commercial materials. The results of the BFS (Table 3) show that all the materials decreased in strength with time under wet conditions except experimental material C, which increased steadily to significantly higher strength than the rest at the 4th week. The decrease in wear resistance was observed in all materials under the different storage regime. Within the experimental materials, C showed the lowest wear resistance whereas A showed the highest and the difference was significant. Experimental materials exhibited similar SBS (Table 6) as compared to the commercial products, which increase with time; however, the increase in C at the 4th week was significantly higher than all the materials tested. The cumulative amounts of FR from all materials in 10 ml of water over a period of 12 weeks were similar (Table 7), however, material C released the highest amount of fluoride ions whereas material A released the lowest.
4.
Discussion
Wear analysis showed that, among the experimental materials, C had lower wear resistance although the wear characteristics were similar to those of the commercial products, however, material A showed the highest wear resistance. The difference between the wear in materials A and C up to the
Table 5 – Percentage water uptake of experimental and commercial compomers at various times. Material
% water uptake at various storage times (sd) 24 h
A B C Dyract AP Compoglass F F 2000
0.4 (±0.2) 0.6 (±0.2) 0.8 (±0.3) 0.6 (±0.2) 0.6 (±.0.1) 0.4 (±0.2
1 week
4 weeks
12 weeks
0.6 (±0.1) 0.8 (±0.2) 1.0 (±0.1) 0.8 (±0.2) 1.0 (±0.2 0.8 (±0.2)
0.7 (±0.2) 1.0 (±0.1) 1.4 (±0.2) 1.2 (±0.2) 1.4 (±0.2) 1.0 (±0.1)
1.1 (±0.2) 1.2 (±0.2) 1.6 (±0.3) 1.4 (±0.1) 1.6 (±0.2) 1.2 (±0.6)
Table 6 – Shear bond strength (SBS) of experimental and commercial compomers stored in distilled water. Material
SBS/MPa at various storage times (sd) 24 h
A B C Dyract AP Compoglass F F 2000
13.5 (±2.2) 12.6 (±2.0) 14.8 (±5.0) 12.6 (±2.2) 12.4 (±6.2) 16.4 (±4.2)
1 week
4 weeks
12.4 (±1.4) 14.2 (±1.2) 18.4 (±4.2) 12.8 (±2.0) 16.4 (±1.2) 16.8 (±3.6)
14.6 (±2.8) 18.4 (±1.4) 24.4 (±1.2) 16.6 (±4.4) 16.8 (±4.2) 18.4 (±1.6)
Table 7 – Cumulative fluoride release from experimental and commercial compomers stored in distilled water. Material
A B C Dyract AP Compoglass F F 2000
Cumulative fluoride release/ppm in 10 ml of water 24 h
1 week
4 weeks
225.2 256.4 386.8 309.8 388.0 315.6
316.8 360.2 466.8 384.4 460.0 402.6
408.2 460.8 526.4 456.9 522.6 485.0
12 weeks 422.9 492.6 592.4 524.5 586.8 512.2
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
4th week is due to their dissimilarity. The high wear resistance of materials A can be attributed to the relatively small amount of acid-modified monomers present, since that is the only difference between the experimental materials. This pattern of wear suggests that material A exhibits wear characteristics similar to composite resin whereas that of C is similar to glassionomer cement [15] considering the latter wore less as it aged in water during maturation period. While there is some debate, it would appear that the post cure acid–base neutralization reaction that takes place in compomers is similar to that of glass-ionomer cements [16,17]. Water has a number of essential roles in the glass-ionomers cement chemistry [18,19]. It acts as the medium for the setting reaction, without which the polymeric acid would be unable to act as an acid. Water is needed so that the acids may dissociate, in principle, yield protons, thereby enabling the property of acidity to be manifested as stated in Bronsted–Lowry’s definition. The polarity of water allows the various metal ions to enter the liquid phase and thus react. It is one of the components of the set cement and actually becomes incorporated into the cement as it hardens. As a reaction product of the acid–base reaction that takes place during setting of the cement. Water uptake of dental biomaterials with hydrophilic moieties is characterized by very complex mechanisms. For the water uptake studies, the presence of phosphonic and carboxylic acids in the experimental materials was observed to influence the extent of hydration, thus the higher the content of such hydrophilic components the higher the water uptake. The results of the water uptake studies are supported by the findings of another published research with a clean bisGMA and its acid-modified version, the later which absorbs more water than the former [20]. Reactive glass has also been identified as hydrophilic component in compomers and therefore the higher the amount, the more higher the hydration [21]. This observation reflects on the nature of water uptake of material C, which absorbed highest percentage of water than its counterpart A and B. Phosphate ions constitute a greater part of the mineral phase of the tooth and therefore any interaction with a molecule that carries a large number of –PO– groups, e.g. organic polyphosphates or polyphosphate ions (polyfunctional complexing agent) will exhibit a stronger affinity for chemisorption, resulting in stronger adhesion to hydroxyapatite crystals of the tooth [22]. Hydroxyapate crystals carry three PO4 3− ions per unit cell that are readily exchangeable by other phosphate ions and this is easily shown by isotopic exchange [23]. The COOH groups from the PMGDM also promote adhesion through ionic interactions with the mineral content of the tooth. Material C which had the highest acidmodified monomers and reactive glasses showed enhanced interactions with the tooth. It must be emphasized that, the experimental bonding agent appears to act in multifunctional ways thus both the phosphonic and carboxylic acid groups of the monomers chelate with the calcium ions of hydroxyapatite of the tooth [24], and in addition react with the glass surface to promote the release of fluoride ions. The FR studies showed that the higher percentage content of the both the acid-modified monomers and fluoridecontaining leachable glass in the experimental materials, the
1001
higher the amount of fluoride ions released. This may be due to the fact that the post-cure acid–base reaction that occurs in compomers when water is absorbed enhances the FR [17] and this occurs to a greater extent in materials C than all the other experimental and commercial materials. Fluoride release is of clinical importance in limiting the occurrence of recurrent caries and restoration failure [25]. Published release profiles indicate that the loss of fluoride from these materials is not linear. While not measured here, the potential also exists to “recharge” the fluoride content of these materials [26]. The amounts of fluoride released from compomers are relatively lower than those released from glass-ionomer cements [2,19] and therefore in products such as Compoglass F, the use of fluoride-containing leachable glass filler systems with slightly soluble fluoride salt such as ytterbium trifluoride has been employed [20] as shown in the following equation: YbF3 + 3OH → Yb(OH)3 + 3F−
5.
Conclusions
The use of FED in the development of new dental materials is very important in the optimization of properties. The experimental materials evaluated showed improved properties and in most cases better than commercially existing compomers under the tests chosen, however, long-term and further studies at both laboratory and clinical levels and at other different conditions are required to ascertain their stability. Higher amounts of VPA, PMGDM and reactive glass render the material with properties similar to glass-ionomers whereas their decrease gives properties similar to composite resins. VPA and PMGDM can be copolymerized and used as active monomers in compomer restoratives with improved properties.
references
[1] Nicholson JW. Poly-acid modified composite resins (“compomers”) and their use in clinical dentistry. Dent Mater 2007;23:615–22. [2] Meyer JM, Cattani-Lorente MA, Dupuis V. Compomers: between glass-ionomer cements and composites. Biomaterials 1998;19:529–39. [3] Nicholson JW, Singh G. The use of organic compounds of phosphorus in clinical dentistry. Biomaterials 1996;17:2023–30. [4] Anusavice KJ. Phillip’s science of dental materials. 10th ed. Philadelphia, PA, USA: W.B. Saunders Co.; 1996. [5] Nicholson JW, Alsarheed M. Changes on storage of polyacid-modified composite resins. J Oral Rehabil 1998;25:616–20. [6] Dahl JE, Li J, Ruyter IE. Long-term water uptake of compomers and its effect on mechanical properties. J Dent Res 1998;77(Special issue B):657 [Abstract 207]. [7] Adusei GO, Deb S, Nicholson JW. A comparison of some properties of contemporary polyacid-modified composite resins (“compomers”) for use in clinical dentistry. Dent Forum 2004;31:19–24. [8] Venz S, Dickens B. Modified surface-active monomers for adhesive bonding to dentin. J Dent Res 1993;72:582–6. [9] Fisher RA. The design of experiment. Edinburgh: Oliver and Boyd Ltd.; 1951.
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[10] de With G, Wagemans HHM. Ball on ring test revisited. J Am Ceram Soc 1989;72:1538. [11] De Moor RJ, Verbeeck RM. Changes in surface hardness of conventional restorative glass ionomer cements. Biomaterials 1998;19:2269–75. [12] Dickens SH, Cho BH. Interpretation of bond failure through conversion and residual solvent measurements and Weibull analyses of flexural and microtensile bond strengths of bonding agents. Dent Mater 2005;21:354–64. [13] Lopez-Suevos F, Dickens SH. Degree of cure and fracture properties of experimental acid–resin modified composites under wet and dry conditions. Dent Mater 2008;24:778–85. [14] Adusei GO, Deb S, Nicholson JW. A preliminary study of experimental polyacid-modified composite resins (‘compomers’) containing vinyl phosphonic acid. Dent Mater 2005;21:491–7. [15] Peutzfeldt AF, Garcia-Godoy F, Asmussen E. Surface hardness and wear of glass ionomers and compomers. Am J Dent 1997;10:15–7. [16] Arrondo JL, Collado MI, Triana IS, Ellacuria RJ. Setting reaction of polyacid modified composite resins or compomers. Open Dent J 2009;3:197–201. [17] Eliades G, Kakaboura A, Palaghias G. Acid–base reaction and fluoride release profiles in visible light-cured polyacid-modified composite restoratives (compomers). Dent Mater 1998;14:57–63.
[18] Nicholson JW. Glass-ionomers in medicine and dentistry. Proc Inst Mech Eng [H] 1998;212:121–6. [19] Wilson AD, Nicholson JW. Acid–base cements: their biomedical and industrial applications. Cambridge: The University Press; 1993. [20] Adusei G. Polymerization behavior of an organophosphorus monomer for use in dental restorative materials. J Appl Polym Sci 2003;88:565–9. [21] Adusei G, Deb S, Nicholson JW. The role of the ionomer glass component in polyacid modified composite resin dental restorative materials. J Mater Sci Mater Med 2004;15:751–4. [22] Anbar M, St John GA. Adsorption of polyphosphonated polyethylene on enamel of teeth. J Dent Res 1971;50:778. [23] Sognnaes RF, Shaw JH, Bogoroch R. Radiotracer studies on bone, cementum, dentin and enamel of rhesus monkeys. Am J Physiol 1955;180:408. [24] Cabasso I, Sahni S. Acrylated phosphonate esters containing 1,3-dioxane and 1,3-dioxolane moieties as adhesion-promoting agents for dentin and hard tissues, I. J Biomed Mater Res 1990;24:705–20. [25] Mjor IA, Smith DC. Frequency of secondary caries at various anatomical locations. Oper Dent 1985;10:88–92. [26] Xu X, Burgess JO. Compressive strength, fluoride release and recharge of fluoride-releasing materials. Biomaterials 2003;24:1687–96.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Improving some selected properties of dental polyacid-modified composite resins Paul J. Milward a , Gabriel O. Adusei b , Christopher D. Lynch a,∗ a b
Tissue Engineering & Reparative Dentistry, School of Dentistry, Cardiff University, Heath Park, Cardiff CF14 4XY, UK Triune Medical Technologies Limited, 22 Pennington Road, Hartford, Huntingdon, Cambridgeshire PE29 1QF, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Polyacid-modified composite resins (compomers) are restorative dental materials
Received 12 November 2009
that exhibit certain features of traditional dental composites and glass-ionomer cements.
Received in revised form
The aim of this paper was to develop experimental compomers with enhanced properties,
16 May 2011
based on adhesive monomers vinyl phosphonic acid and pyromellitic dianhydride glycerol
Accepted 30 June 2011
dimethacrylate, and to compare their properties to those of commercially available products. Methods. Factorial experimental design was employed to optimize both chemical and physical properties. Properties such as biaxial flexural strength (BFS), wear resistance (WR), water
Keywords:
uptake (WU), and adhesion using shear bond strength (SBS) as well as fluoride release (FR)
Mechanical properties
were evaluated and compared with those of commercial products.
Adhesion
Results. Results were subjected to one-way ANOVA (p < 0.05); significant differences were
Composite materials
observed in properties of materials such as WR, BFS and SBS but not in WU and FR com-
Wear
pared to commercial products. Experimental materials exhibited significantly higher WR,
Compomer
BFS and SBS values than commercial materials. Properties of materials were affected by their respective storage media with time. Significance. 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 glassionomers whereas their decrease gives properties similar to conventional dental composite resins with improved properties such as strength and wear resistance. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Polyacid-modified composite resins (compomers) are materials that combine certain features of traditional dental composite resins and glass-ionomer cements [1]. 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 [2]. 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
Corresponding author. Tel.: +44 02920 20744665. E-mail address: [email protected] (C.D. Lynch). 0109-5641/$ – see front matter © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2011.06.006
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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
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 [3,4]. 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 [1,5–7]. 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 acidfunctional monomers with a mixture of reactive glasses to obtain optimized properties found in dental composite resins and glass-ionomers cements [3,4,8–11]. These materials have attracted some attention recently and have been included in commercial and experimental adhesives and resin composites [12,13]. 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 [14]. 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 [9]: • 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.
Table 1 – Materials and their suppliers. Material
Supplier
BisGMA, TEGDMA, PMGDM, and Raysorb Vinyl phosphonic acid Camphorquinone and DMAEM Benzophenone Butylated hydroxytoluene (BHT) Aerosil R972 (silane-treated Silica) G338 glass (silane-treated fluoride-releasing glass) Compoglass® F Dyract® AP F2000 compomer
Esschem, USA Albright & Wilson, UK Aldrich, UK Osi Specialities (UK) Chance and Hunt, UK Degussa AG, Germany 1st Scientific, Germany Ivoclar Vivadent Dentsply 3M ESPETM
2,2-Bis[p-(2 -hydroxy-3 -methacryloxypropoxy) phenyl] propane (BisGMA); triethylene glycol dimethacrylate (TEGDMA); and 2(dimethylamino) ethylmethacrylate (DMAEM).
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.
2.1.
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 cm2 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 [10]. The strength of the specimens was measured using ball on ring (BOR) (12 mm ring) tests and calculated with the equation
BOR =
3(1 + )F 4t2
× 1−
1 + 2 ln
b 2
R 1 − R 2 0 0 b
+
1+
R
R0
Table 2 – Formulation of experimental componers.
2.
Materials and methods
Optimized formulations of experimental componers (Tables 1 and 2) were achieved using experimental factorial design and analysis [6]. 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
Chemical component BisGMA (binder resin) TEGDMA VPA–PMGDM (acid-modified monomer admixture) Camphorquinone DMAEM Benzophenone BHT Aerosil R972 G338 glass Raysorb
Composition (%) 11.55 3.85 Variable 0.20 0.30 0.09 0.01 4.00 Variable Variable
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
where BOR is the strength, is a constant, is the Poisson’s ratio (assumed to be 0.35), R the specimen radius, R0 the ball bearing support, b the contact radius loading area, F is the force, and t the specimen thickness. The b = t/3 (Westergaard approximation) was used. The cross-head speed used was 1 mm/min.
2.2.
Wear resistance measurement
For the WR studies, rectangular strips of specimens of 40 mm, 5 mm wide and 2 mm thickness were manually prepared from both experimental and commercial materials in perspex molds and light-cured for 40 s at both sides through cellulose acetate film. Four specimens of each material were used and stored in water for 24 h. They were then placed in a special perspex molds that held them in position for the toothbrush heads to move freely on the materials during the brushing cycles. For each specimen, a 60 ml of slurry of equal amounts of toothpaste, water and dental pumice powder of grit 60 (Bracon Ltd., East Sussex, UK) was used as a highly abrasive medium to give the abraded sample surfaces high abrasion effect for easy analysis with the profilometer. Four brush heads, each weighing 167 g were used and 30,000 brushing strokes were registered on the counter of the Toothbrushing and Abrasion Test Machine No. 8 (Research Laboratories, The Pepsodent Co., Chicago, IL). A 3D laser measurement system profilometer (UBM/Keyence LC2450 (Germany)), with a point density of 2 m was used to measure the mean vertical loss of material after toothbrush abrasion.
2.3.
Water uptake
The WU was evaluated using disc-shaped specimens of 13 mm diameter and 1 mm thickness prepared by placing materials in stainless steel molds of appropriate dimensions between two microscope glass slides placed at both sides of the mold. Both faces were irradiated in turn with Cromalux-100 (Mega-Physik Dental Division, Rastatt, Germany) halogen light-curing unit with nominal output of 600 mW cm2 at 470 nm through the glass slides for 20 s. Three specimens of each material were used and they were placed in their respective storage medium with each specimen in a separate stoppered glass vial. They were weighed at regular time intervals at 30 min for 2 h and hourly for 8 h. The weighing continued from the 24th hour at daily intervals, for 1 week and weekly until they had equilibrated at the 12th week. Weighing was performed with blotted dry specimens, using absorbent tissues, determinations being made to the nearest 0.0001 g on a using Mettler AT 250 analytical balance (Mettler-Toledo Ltd., Beaumont Leys Leicester). After weighing, the specimens were returned to the storage medium and stored at 37 ◦ C. The difference between the weight (wt) of the specimen after immersion (Wt(f)) and the initial weight (Wt(i)) was recorded, and the difference in final wt and the initial wt was expressed as percentage hydration at equilibrium using the formula: Wt(f ) − Wt(i) × 100 Wt(i)
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where Wt(i) and Wt(f) are the initial and is the final weights, respectively.
2.4.
Adhesion analysis
The adhesion analysis employing shear bond strength test used a set of six cylindrical specimens of 4 mm height and 3 mm diameter of each material were bonded to enamel of randomly selected human teeth. The tooth surfaces were acid-etched with 37% phosphoric acid solution for 20 s and oil-free air applied to bonding area to keep the tooth surface dry before materials were placed firmly. A derivative of the active acid-modified monomers, bisGMA, TEGDMA, acetone and polymerization initiators were used for the formulation of experimental bonding agent for materials A–C. For the commercial materials, the recommended bonding agents were applied and the manufacturers’ instructions were followed. All samples were light cured for 40 s and stored in artificial saliva at 37 ◦ C before testing for shear bond strength using Lloyds 10 KN (Lloyd Instruments Ltd., Bognor Regis, West Sussex, UK) at crosshead speed of 0.5 mm/min. A shear blade with blunt edge at point of contact with the material was used to shear the material–tooth bond and the SBS/MPa was calculated by dividing the break force by bonding surface area.
2.5.
Flouride release measurement
Disc shaped specimens of dimensions 13 mm in diameter and 1 mm thickness were used for the fluoride release evaluation. One sample was prepared per material tested. Weight of each specimen equilibrated to 0.35 g and was recorded using a Mettler AT 250 analytical weighing balance. The discs were then placed in glass vials containing 5 ml of equal amounts (2.5 ml) of deionized water and total ionic strength adjustment buffer (TISAB) solution. TISAB is used to maintain constant ionic strength and to remove certain interferences. The ionic fluoride concentration of each solution was measured using a fluoride ion selective electrode under constant temperature at 24 ◦ C and was gently rinsed between with de-ionized water to prevent crossover contamination between samples. Fluoride release measurements were recorded at an hourly interval on the first day, and daily basis up to 12 weeks.
3.
Results
FED yielded formulations A–C as the optimized experimental materials with three different percent compositions of acid-modified monomers and reactive glass fillers in their respective formulations. It was also observed that the effects of only one of the active acid-modified monomers in other formulations did not confer desired properties and were inferior to the ones containing both PMGDM and VPA, which showed synergistic effect with better properties. The results of the evaluated properties of the experimental and commercial materials are given in Tables 3–7 with standard deviations (sd) in parentheses. The results were subjected to one-way ANOVA and Tukey test (p < 0.05); significant differences were observed in the properties of materials such as
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Table 3 – Biaxial flexure strength (BFS) of experimental and commercial compomers stored in distilled water. Material
A B C Dyract AP Compoglass F F 2000
BFS/MPa at various storage times (sd) 24 h
1 week
4 weeks
188.6 (±2.2) 186.4 (±4.6) 176.8 (±4.2) 177.6 (±2.5) 182.4 (±2.4) 186.8 (±2.0)
182.4 (±2.6) 186.2 (±2.8) 178.2 (±1.2) 168.5 (±4.2) 174.4 (±2.0) 176.0 (±2.2)
171.8 (±4.2) 172.6 (±5.6) 188.4 (±2.4) 154.8 (±2.2) 165.6 (±2.2) 160.2 (±2.2)
Table 4 – Wear analysis of experimental and commercial compomers stored in distilled water using toothbrushing technique. Material
Wear of materials/micron at various storage times (sd) 24 h
A B C Dyract AP Compoglass F F 2000
36.4 (±6.2) 40.2 (±2.2) 44.8 (±5.3) 56.6 (±3.2) 48.0 (±6.2) 46.4 (±2.2)
1 week
4 weeks
42.4 (±2.4) 48.2(±4.2) 58.2 (±6.2) 62.8 (±2.2) 56.4 (±1.2) 50.8 (±3.2)
44.6 (±2.8) 55.4 (±2.2) 60.4 (±5.2) 64.6 (±4.6) 62.4 (±4.2) 58.4 (±2.6)
WR, BFS and SBS but not in WU and FR. Experimental materials exhibited somewhat greater BFS (Table 3), WR (Table 4) and SBS (Table 6) values than those of commercial materials. The results of the BFS (Table 3) show that all the materials decreased in strength with time under wet conditions except experimental material C, which increased steadily to significantly higher strength than the rest at the 4th week. The decrease in wear resistance was observed in all materials under the different storage regime. Within the experimental materials, C showed the lowest wear resistance whereas A showed the highest and the difference was significant. Experimental materials exhibited similar SBS (Table 6) as compared to the commercial products, which increase with time; however, the increase in C at the 4th week was significantly higher than all the materials tested. The cumulative amounts of FR from all materials in 10 ml of water over a period of 12 weeks were similar (Table 7), however, material C released the highest amount of fluoride ions whereas material A released the lowest.
4.
Discussion
Wear analysis showed that, among the experimental materials, C had lower wear resistance although the wear characteristics were similar to those of the commercial products, however, material A showed the highest wear resistance. The difference between the wear in materials A and C up to the
Table 5 – Percentage water uptake of experimental and commercial compomers at various times. Material
% water uptake at various storage times (sd) 24 h
A B C Dyract AP Compoglass F F 2000
0.4 (±0.2) 0.6 (±0.2) 0.8 (±0.3) 0.6 (±0.2) 0.6 (±.0.1) 0.4 (±0.2
1 week
4 weeks
12 weeks
0.6 (±0.1) 0.8 (±0.2) 1.0 (±0.1) 0.8 (±0.2) 1.0 (±0.2 0.8 (±0.2)
0.7 (±0.2) 1.0 (±0.1) 1.4 (±0.2) 1.2 (±0.2) 1.4 (±0.2) 1.0 (±0.1)
1.1 (±0.2) 1.2 (±0.2) 1.6 (±0.3) 1.4 (±0.1) 1.6 (±0.2) 1.2 (±0.6)
Table 6 – Shear bond strength (SBS) of experimental and commercial compomers stored in distilled water. Material
SBS/MPa at various storage times (sd) 24 h
A B C Dyract AP Compoglass F F 2000
13.5 (±2.2) 12.6 (±2.0) 14.8 (±5.0) 12.6 (±2.2) 12.4 (±6.2) 16.4 (±4.2)
1 week
4 weeks
12.4 (±1.4) 14.2 (±1.2) 18.4 (±4.2) 12.8 (±2.0) 16.4 (±1.2) 16.8 (±3.6)
14.6 (±2.8) 18.4 (±1.4) 24.4 (±1.2) 16.6 (±4.4) 16.8 (±4.2) 18.4 (±1.6)
Table 7 – Cumulative fluoride release from experimental and commercial compomers stored in distilled water. Material
A B C Dyract AP Compoglass F F 2000
Cumulative fluoride release/ppm in 10 ml of water 24 h
1 week
4 weeks
225.2 256.4 386.8 309.8 388.0 315.6
316.8 360.2 466.8 384.4 460.0 402.6
408.2 460.8 526.4 456.9 522.6 485.0
12 weeks 422.9 492.6 592.4 524.5 586.8 512.2
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 997–1002
4th week is due to their dissimilarity. The high wear resistance of materials A can be attributed to the relatively small amount of acid-modified monomers present, since that is the only difference between the experimental materials. This pattern of wear suggests that material A exhibits wear characteristics similar to composite resin whereas that of C is similar to glassionomer cement [15] considering the latter wore less as it aged in water during maturation period. While there is some debate, it would appear that the post cure acid–base neutralization reaction that takes place in compomers is similar to that of glass-ionomer cements [16,17]. Water has a number of essential roles in the glass-ionomers cement chemistry [18,19]. It acts as the medium for the setting reaction, without which the polymeric acid would be unable to act as an acid. Water is needed so that the acids may dissociate, in principle, yield protons, thereby enabling the property of acidity to be manifested as stated in Bronsted–Lowry’s definition. The polarity of water allows the various metal ions to enter the liquid phase and thus react. It is one of the components of the set cement and actually becomes incorporated into the cement as it hardens. As a reaction product of the acid–base reaction that takes place during setting of the cement. Water uptake of dental biomaterials with hydrophilic moieties is characterized by very complex mechanisms. For the water uptake studies, the presence of phosphonic and carboxylic acids in the experimental materials was observed to influence the extent of hydration, thus the higher the content of such hydrophilic components the higher the water uptake. The results of the water uptake studies are supported by the findings of another published research with a clean bisGMA and its acid-modified version, the later which absorbs more water than the former [20]. Reactive glass has also been identified as hydrophilic component in compomers and therefore the higher the amount, the more higher the hydration [21]. This observation reflects on the nature of water uptake of material C, which absorbed highest percentage of water than its counterpart A and B. Phosphate ions constitute a greater part of the mineral phase of the tooth and therefore any interaction with a molecule that carries a large number of –PO– groups, e.g. organic polyphosphates or polyphosphate ions (polyfunctional complexing agent) will exhibit a stronger affinity for chemisorption, resulting in stronger adhesion to hydroxyapatite crystals of the tooth [22]. Hydroxyapate crystals carry three PO4 3− ions per unit cell that are readily exchangeable by other phosphate ions and this is easily shown by isotopic exchange [23]. The COOH groups from the PMGDM also promote adhesion through ionic interactions with the mineral content of the tooth. Material C which had the highest acidmodified monomers and reactive glasses showed enhanced interactions with the tooth. It must be emphasized that, the experimental bonding agent appears to act in multifunctional ways thus both the phosphonic and carboxylic acid groups of the monomers chelate with the calcium ions of hydroxyapatite of the tooth [24], and in addition react with the glass surface to promote the release of fluoride ions. The FR studies showed that the higher percentage content of the both the acid-modified monomers and fluoridecontaining leachable glass in the experimental materials, the
1001
higher the amount of fluoride ions released. This may be due to the fact that the post-cure acid–base reaction that occurs in compomers when water is absorbed enhances the FR [17] and this occurs to a greater extent in materials C than all the other experimental and commercial materials. Fluoride release is of clinical importance in limiting the occurrence of recurrent caries and restoration failure [25]. Published release profiles indicate that the loss of fluoride from these materials is not linear. While not measured here, the potential also exists to “recharge” the fluoride content of these materials [26]. The amounts of fluoride released from compomers are relatively lower than those released from glass-ionomer cements [2,19] and therefore in products such as Compoglass F, the use of fluoride-containing leachable glass filler systems with slightly soluble fluoride salt such as ytterbium trifluoride has been employed [20] as shown in the following equation: YbF3 + 3OH → Yb(OH)3 + 3F−
5.
Conclusions
The use of FED in the development of new dental materials is very important in the optimization of properties. The experimental materials evaluated showed improved properties and in most cases better than commercially existing compomers under the tests chosen, however, long-term and further studies at both laboratory and clinical levels and at other different conditions are required to ascertain their stability. Higher amounts of VPA, PMGDM and reactive glass render the material with properties similar to glass-ionomers whereas their decrease gives properties similar to composite resins. VPA and PMGDM can be copolymerized and used as active monomers in compomer restoratives with improved properties.
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