- Email: [email protected]
Experimental dental bio-adhesives for direct restorations: The influence of PMnEDM homologs structure on bond strength
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Experimental dental bio-adhesives for direct restorations: The influence of PMnEDM homologs structure on bond strength c ˛ Tomasz W. Kupka a,b,∗ , Mirosław Gibas c , Agnieszka Dabrowska , a,b d Marta Tanasiewicz , Witold Malec a
Department of Propaedeutics in Dentistry, Medical University of Silesia, Katowice, Poland Experimental Odontology Research Group, Medical University of Silesia, Katowice, Poland c Department of Organic Chemistry, Bioorganic Chemistry and Biotechnology, Silesian University of Technology, Gliwice, Poland d Institute of Non-Ferrous Metals, Gliwice, Poland b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The purpose of this study was to evaluate the effect of PMnEDM dental monomer
Received 5 May 2006
homologs chemical structure on shear bond strength between polymer-based composite
Received in revised form
and alloy.
23 October 2006
Methods. Four light-cured experimental universal dental bio-adhesives (group codes: A
Accepted 16 November 2006
(PMDM), B (PM2EDM), C (PM3EDM), D (PM4EDM)) were preliminarily evaluated with respect to sensitivity to ambient light, curing time, depth of cure, and uncured film thickness according to standardized procedures. Appropriate tests were performed to measure shear bond
Keywords:
strength (SBS) of polymer-based composite to cobalt-based alloy with the use of the adhe-
Restorative dentistry
sives investigated. Variability of results was evaluated by use of the coefficient of variation
Artistic dentistry
(CV). Results were estimated with the aid of one-way analysis of variance (ANOVA), per-
Dental adhesive
formed on the logarithmic values, with ˛ = 0.05 significance level.
Adhesion to metal
Results. All materials passed the requirements according to physicochemical properties.
SBS
Except for formulation D, all results estimating SBS were positive with respect to standardized requirements. The uppermost mean SBS was achieved for the A adhesive (11.45 MPa) and appeared to be significantly different compared to D one (5.07 MPa) (p = 0.0495). Also the B adhesive, having slightly lower mean SBS value (10.50 MPa) exhibited a significant difference in respect to D one (p = 0.0455). The means for other trial pairs did not differ statistically. Significance. The materials here studied might be considered to have a practical use in dental clinics, especially the formulations B and C. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Restorative, and artistic dentistry [1,2] in particular, is closely concerned with the repair and treatment of teeth defects
in order to obtain proper function and adequate esthetics. The application of adhesive techniques to bond dental materials has been long desirable in direct restorative dentistry. True adhesive dentistry began in the 1950s, when it was con-
∗ Corresponding author at: Experimental Odontology Research Group, Department of Propaedeutics in Dentistry, Medical University of Silesia, 17 Akademicki Square, 41-902 Bytom, Poland. Tel.: +48 32 2827993; fax: +48 32 2827993. E-mail address: [email protected] (T.W. Kupka). URL: www.slam.katowice.pl/zpt (T.W. Kupka).
0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.11.019
1270
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
firmed that acrylic resins filled with silica particles would adhere to surface enamel that had been etched with inorganic acid. Immediately after the invention of bis-GMA resin and related composite filling material [3], Bowen introduced the idea of Surface Active Comonomers (SACs), intended to provide chemical bonding between a composite restoration and hard tissues of the tooth. The first SAC-type monomer was NPG-GMA, i.e., the product of addition reaction of Nphenylglycine (NPG) and glycidyl methacrylate (GMA) [4]. Functional groups of SACs were supposed to form chelate-type complexes with dentin hydroxyapatite whereas the methacrylate carbon–carbon double bond copolymerized with those of composite resin constituents [5]. To-date, a number of different functional methacrylates have been synthesized and employed successfully in various dental adhesive systems, including commercial ones. Amongst the best known ones are PMDM [6], HEMA (2hydroxyethyl methacrylate, Sigma-Aldrich) [7], 4-META [8], PENTA-P [9] (Fig. 1). Application of some enamel/dentin bonding agents had also been extended to provide sufficient bond strength to metal/alloy surfaces, e.g. silver amalgams and other metal devices used in restorative direct/indirect dentistry [10–13]. The ability to bond to metals so securely is also due to several proven surface treatment of cast metals, e.g. sandblasting, chemical etching, cast as a mesh, tin plating, silo-coating, or plasma treatment [14,15]. Before the implementation of original treatment methods and new medical devices/dental materials into dental clinical practice, it is recommended to meet some precisely specified standardized requirements at the experimental stage and then in clinical trials. In the effort to maximize adhesives properties and practical qualities of dental restorative materials, it was recognized that interlayer adaptation was an important parameter [16]. Shear strength is particularly important in the study of interfaces between two materi-
als. The influence of monomer structure on dental adhesive properties of related restorative material has not been widely discussed so far. The presence of two methacrylate groups seems to be advantageous, since due to limited conversion during polymerization, at least one of them should be incorporated into a network. Following this line of thinking, one could conclude that the presence of two hydrophilic functional groups, providing adhesion to substrates, should also be advantageous. PMDM, having both two methacrylate and two carboxylic groups, might be considered to be the most efficient dental adhesive monomer, at least from a structural point of view. However, another structural factor never before taken into account was the distance between polymerizable methacrylate groups and hydrophilic carboxylic ones. The structural idea is to introduce longer spacers between these functionalities and to find whether this has any effect on adhesive properties. To do that, synthesis of the PMnEDM series of monomers was performed. These are homologs of PMDM with an increased number of oxyethylene units (n), (for n = 1 the structure corresponds to PMDM). The resultant monomers were added to a typical methacrylate composition with nanoparticle content. To answer the above questions, SBS testing was performed to measure the adhesion of the latter with respect to a metal surface, at the current stage of study.
2.
Materials and methods
The syntheses of PMnEDM monomers were carried out on ca. 0.005 mole of PMDA (pyromellitic anhydride; 1,2,4,5-benzenetetracarboxylic anhydride, Sigma-Aldrich) and 0.02 mole of appropriate monomethacrylates of oligoethylene glycols (HEMA, DEGMMA, TEGMMA and TTEGMMA; either commercial or synthesized as previously reported [17]). The mixture of the reagents was refluxed under nitrogen in acetone (POCh)
Fig. 1 – Some of the most important dental adhesive monomers.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
solution (13 wt%) for 8 h in the presence of 400 ppm of HQME (hydroquinone monomethyl ether, Sigma-Aldrich) (inhibitor) and 2% of DMAEMA (N,N-dimethylaminoethyl methacrylate, Merck) (catalyst). The products were examined by NMR spectroscopy with the aid of a UNITY/INOVA spectrometer (Varian) working at 300 MHz. The samples were prepared as 5–10% solutions in deuteroacetone with tetramethylsilane (TMS) used as an internal reference. The products – the reaction mixtures containing particular PMnEDM monomers and an excess of respective monomethacrylate – were mixed at a 1:9 weight ratio with a methacrylate resin composed of common dental monomers, i.e. bis-GMA, UDMA and TEGDMA (triethylene glycol dimethacrylate, Fluka) in a 1:1:1 weight ratio. To form a one-component light-cured formulation, CQ (camphorquinone, Aldrich)/DMAEMA photoinitiating system (0.6 and 2.0 wt%, respectively, in respect to the total resin) was dissolved therein and finally the mixture was homogenized with silanized amorphous silica at a10:1 weight ratio. Four light-cured experimental dental bio-adhesives (group codes: A (PMDM), B (PM2EDM), C (PM3EDM), D (PM4EDM)), series I/2005, prepared as above, were evaluated with respect to sensitivity to ambient light, curing time, depth of cure, and uncured film thickness according to standardized procedures. The Shear Bond Strength (SBS) test of a light-cured polymer-based dental composite material to an alloy substrate employing four bio-adhesives was measured according to the standardized procedure. Thus, 20 burnished metal plates made of cobalt-based alloy were randomly assigned to four equal groups according to 4 coded experimental bio-adhesives used. A polymer-based composite material was bonded onto cobalt-based alloy surfaces using the materials investigated. Shear strengths (MPa) were tested on an Instron machine at (1 ± 0.3) mm/min cross-head speed. The initial post-fracture
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surface analysis on the CoCr samples was performed using a microscope (Stereo-Light Microscopy), at 25× magnification in order to assign failure modes categorized as adhesive, cohesive, or mixed fractures in the bio-adhesive or composite. Results concerning A, B, C and D adhesives, representing their continuous quality factor, were assembled into four independent trials. Consistence of the distributions with a normal distribution was confirmed with Shapiro–Wilk’s test. Normality of trials’ distributions allowed for the presentation of their descriptive characteristics in the form of an arithmetic mean and a standard deviation. Variability of results was evaluated by use of coefficient of variation (CV). The influence of adhesive formulation on the average SBS value was estimated by comparing arithmetic means for A, B, C and D trials with the aid of one-way analysis of variance (ANOVA). Application of this test requires verifying an assumption of the homogeneity of variance, which was not satisfied for the trials compared (Levene’s test). As a consequence, ANOVA test was performed on the logarithmic values. Rejection of the hypothesis on equality of means allowed multiple comparisons by HSD Tukey’s test. Significance level ˛ = 0.05 was assumed for all the tests. The calculations were conducted by use of Statistica 6.0 software.
3.
Results
Examination of the final reaction mixtures by 1 H NMR spectroscopy proved the formation of the assumed products, i.e., PMnEDM series of monomers, according to the reaction scheme given in Fig. 2. A typical, representative fragment of the spectrum recorded for PM3EDM is given in Graph 1. Thus, two equivalent ring protons in the para isomer exhibit a singlet in the middle position whereas unequivalent ones in the meta
Fig. 2 – Reaction scheme for syntheses of PMnEDM monomers.
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
Table 1 – Some spectroscopic data on PMnEDM syntheses Chemical shift of ring protons ı (ppm)
Compound
ı (ppm)
Molar (%)
7.98; 8.32 8.0; 8.3*
–
Not seen
7.94; 8.28 7.99; 8.19 8.00; 8.20
8.52; 8.70 8.55; 8.68 8.53; 8.64
4 5 6
para isomer
meta isomer
PMDM
8.14 8.15a
PM2EDM PM3EDM PM4EDM
8.11 8.09 8.10
a
Intermediate product
According to [6].
Table 2 – Standardized properties of experimental bio-adhesives containing PMnEDM monomers Property evaluated
Sensitivity to ambient light (s) Curing time (s) Depth of cure (mm) Uncured film thickness (m)
Standardized requirement Not less than 25 s Not more than 60 s Not less than 1.5 mm Not more than 0.1 mm (100 m)
isomer give two satellite-like singlets around the former. The values of the intensities of the signals indicates the formation of an equimolar ratio of the isomers. Two minor signals at the lower field side might be assigned to the intermediate product having a structure as shown in Fig. 2. From the signal intensities ratio in respect of the main product, the content of intermediate is ca. 5% (molar). Table 1 summarizes the values of chemical shifts for aromatic protons of PMnEDM monomers as well as of respective intermediates. The results of physicochemical examination of the experimental bio-adhesive mixtures, carried out according to standardized procedures, are given in Table 2. The results of SBS test are given in Table 3, Figs. 3 and 4 and Graphs 2 and 3. The uppermost mean SBS was achieved for the A adhesive (11.45 MPa) and appeared to be significantly different with respect to D one only (5.07 MPa) (p = 0.0495). Also the B adhesive, having slightly lower mean SBS value (10.50 MPa), exhibited a significant difference with respect to D one (p = 0.0455). Means for other trail pairs analyzed did not differ statistically (Table 4). Based on the coefficient of variation, B adhesive was found to be characterized by the highest homo-
Experimental bio-adhesives containing PMnEDM homologs A (PMDM) 117 (6) 11.6 (0.2) 4.18 (0.05) 3.13 (1.40)
B (PM2EDM)
C (PM3EDM)
203 (4) 11.3 (0.4) 4.11 (0.10) 2.84 (1.05)
154 (7) 10.9 (0.2) 4.11 (0.07) 4.18 (2.42)
D (PM4EDM) 116 (2) 10.5 (0.1) 4.24 (0.11) 1.21 (0.33)
Fig. 3 – A representative SLM picture of composite-alloy interface after fracture; bio-adhesive B; residues of a separator around the bonding area.
geneity of results (CV = 16.02%). For D material V amounted to 33.73% whereas the highest variability of results had A and C ones: 48.30% and 49.85%, respectively.
4.
Graph 1 – Aromatic region fragment of 1 H NMR spectrum of PM3EDM.
Discussion
Durable bonding to a metal framework is required for conservative preparation designs [18]. Present developments in dental polymeric materials are mainly focused on the reduction of polymerization shrinkage and improvement of biocompatibility, wear resistance, and processing properties. This can be achieved by using new tailor-made monomers or optimized filler particles and adequate adhesives [19]. On the other hand, esthetic properties are of great importance for clinicians, in terms of performing highly sophisticated restorations.
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
Table 3 – Results of shear bond strength measurements Bio-adhesives containing PMnEDM homologs
Content of the carboxyl groups (mmol/g)
Individual SBS values (MPa)
SBS mean (MPa)
A (PMDM)
0.271
10.0 4.59 14.38 19.14 9.08
11.45 (5.53)
B (PM2EDM)
0.218
8.67 9.47 10.85 12.90 10.63
10.50 (1.61)
C (PM3EDM)
0.183
3.07 8.85 5.23 11.45 5.11
6.74 (3.36)
D (PM4EDM)
0.158
7.82 4.89 4.46 3.13 5.05
5.07 (1.71)
Graph 3 – A representative shear curve graph for the experimental dental bio-adhesive B.
Fig. 4 – A representative SLM picture of composite-alloy interface after fracture; bio-adhesive C; residues of a separator around the bonding area.
Graph 2 – SBS results for experimental dental bio-adhesives tested.
Basically, the synthesis of PMnEDM monomers has been carried out in a similar way to the procedure given by Bowen [5], except that acetone was used instead of tetrahydrofurane, an amine catalyst was used in a much lower concentration, and an excess of hydroxymethacrylate was 100% instead of 10% (molar). The reaction conditions required much longer reaction time to yield a complete conversion of PMDA. However, the resulting mixtures of products (PMnEDM and respective intermediates; the latter compounds are structurally similar to 4-META, a monomer well known in adhesive dentistry [8]) and unreacted species (excessive hydroxymethacrylate and DMAEMA) did not require purification and could be introduced directly into the methacrylate resin with nanoparticle content. All four bio-adhesive formulations containing particular PMnEDM monomers comply with the standardized requirement concerning light-cured polymer-based dental materials, as shown in Table 2. Thus, these materials might be considered to have a practical use in dental practice. Except for the formulation D containing PM4EDM, all the results estimating shear bond strength of the composite/adhesive/metal systems,
1274
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
Table 4 – Results of multiple comparisons; p (HSD Tukey’s test) Pairs compared A and B A and C A and D B and C B and D C and D ∗ ∗∗
Acknowledgement p
0.99997* 0.2261* 0.0494** 0.21111* 0.0455** 0.8252*
No significant differences. Significant differences.
where the adhesive resin contained PMnEDM monomers, are positive with respect to the standardized requirements, i.e. at least 4 the SBS values are not less than 5 MPa (in the case of PM4EDM just 2). However, although the formulations contained almost equal, i.e. ca. 6 wt%, content of particular PMnEDM monomers, concentrations of carboxyl groups, probably mostly responsible for adhesive properties, differ considerably due to differences in molecular weight of the former. As illustrated in Table 3 the concentration of carboxyls in the resin containing PMDM is almost twice as large as in that containing PM4EDM. This could be a reason for the poor mechanical performance of the latter. Along with the increase in bond strength values, fundamentally above 10 MPa, cohesive failures were observed, especially in the bio-adhesive, but also in the surrounding area of polymer-based composite. A variety of tests have been developed to measure bond strength between material and tooth and between two materials. Some of them are designed to place the bond in tension or in shear. To simulate oral conditions many test specimens are subjected to cycles of varying temperature before measuring bond strength. Thermo-cycling influences many more specimens than traditional water immersion [20]. There is no close analogy between laboratory simulations and clinical conditions [21]. The relative contribution of widely used artificial aging methodology, like thermo-cycling, is strongly dependent on the specific test set-up and the adhesive used [22]. Although laboratory results cannot be directly extrapolated to the clinical situation, they constitute a proper prognosis and deliver some appropriate tips with respect to the probable in vivo behavior. The authors anticipate performing thermo-cycling simulations and SBS on hard tissues of tooth in subsequent stages of the present study.
5.
Conclusions
1. All the 4 dental bio-adhesive formulations particularly those containing PMnEDM monomers comply with the standardized requirements concerning physicochemical properties of light-cured dental materials. 2. Experimental dental bio-adhesives code A, B and C, comply with standardized requirements concerning SBS. 3. The results of non-clinical tests suggest that experimental dental bio-adhesive, especially code B and C, might be useful in restorative dentistry.
This research was supported by research grant no. 3T09B 06227.
references
[1] Kupka T, Tanasiewicz M. Artistic dentistry—rhetorical fiction or key to therapeutic success? Magazyn Stomatol 2004;10:76–9. [2] Tanasiewicz M, Gibas M, Kupka TW. 1H and 13C NMR spectroscopy of PM2EDM novel bioadhesive monomer for artistic dentistry. 2nd International Congress on adhesive dentistry, new challenge and derivation, Tokyo 22–24.04.2005, vol. 4. Adhes Dent 2005 [abstr. no. P-10]. [3] Bowen RL. Dental filling material comprising vinyl silane treated fused silica and a binder consisting of the reaction product of bis-phenol and glicydyl acrylate. US patent 3,066,112. November, 1962. [4] Bowen RL. Adhesive bonding of various materials to hard tooth tissues. II. Bonding to dentin promoted by a surface-active comonomer. J Dent Res 1965;44: 895–902. [5] Bowen RL. Adhesive bonding of various materials to hard tooth tissues. IX. The concept of polyfunctional surface-active comonomers. J Biomed Mater Res 1975; 9:501. [6] Farahani M, Johnston AD, Bowen RL. The effect of catalyst structure on the synthesis of a dental restorative monomer. J Dent Res 1991;70:67–71. [7] Nicholson JW. Adhesive dental materials—a review. Int J Adhes Adhes 1998;18:229–36. [8] Atsuta M, Abell AK, Turner DT, Nakabayashi N, Takeyama M. A new coupling agent for composite materials: 4-Methacryloxyethyl trimellitic anhydride. J Biomed Mater Res 1982;16:619–28. [9] Gibas M. Application of radically cured methacrylate systems in conservative dentistry. Wiadomo´sci Chemiczne 1994;48:593–631. [10] Lambert RL, Scrabeck JG, Robinson FB. Esthetic composite resin facings for amalgam restorations. Gen Dent 1983;31:222–4. [11] Flores AR, Saez G, Barcelo F. Metalic bracket to enamel bonding with a photopolymerizable resin-reinforced glass ionomer. Am J Orthod Dentofacial Orthop 1999;116:514–7. [12] Roda RS, Zwicker PF. The combined composite resin and amalgam restoration for posterior teeth: a clinical report. Quintessence Int 1992;23:9–13. ¨ [13] Ozcan M. Shear bond strength of composite resin to differently conditioned amalgam. J Dent Res 2003;70(Special issue B) [abstr. no. 1266]. [14] Quaas Ac, Heide S, Freitag S, Kern M. Influence of metal cleaning methods on the resin bond strength to NiCr alloy. Dent Mater 2005;21:192–200. [15] Maruo Y, Oka M, Nishigawa G, Takahashi S, Hamanaka M, Nagao N, et al. Shear bond strength of cobalt-chromium alloy to self-curing acrylic resin with plasma treatment and adhesive primer. J Adhes Dent 2002 [abstr. no. 303]. [16] Blixt M, Coli P. The influence of lining techniques on the material seal of class II composite resin restorations. Quintessence Int 1995;7:471–8. [17] Korytkowska A, Barszczewska-Rybarek I, Gibas M. Side-reactions in the transesterification of oligoethylene glycols by methacrylates. Des Mon Pol 2001;4:27–37.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
[18] Taira YH, Atsuta M. Effects of a fluoride etchant on resin bonding to titanium–aluminium–niobium alloy. Eur J Oral Sci 2004;112:384–7. [19] Moszner N, Salz U. New developments of polymeric dental composites. Prog Polym Sci 2001;26:535–76. [20] Tanaka T, Kamada I, Matsumura H, Atsuta M. A comparison of water temperatures for thermocycling of metal-bonded resin specimens. J Prosth Dent 1995;74:345–9.
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[21] Swift EJ, Perdigao J, Heymann HO. Bonding composite materials to enamel and dentine. Short history and current study results. Quintessence Int 1996;5:307–10. [22] De Munc J, Van Landuyt K, Coutinho E, Poitevin A, Peumans M, Lambrechts P, Van Meerbeek B. Micro-tensile bond strength of adhesives bonded to class-I cavity-bottom dentin after thermo-cycling. Dent Mater 2005;21: 999–1007.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Experimental dental bio-adhesives for direct restorations: The influence of PMnEDM homologs structure on bond strength c ˛ Tomasz W. Kupka a,b,∗ , Mirosław Gibas c , Agnieszka Dabrowska , a,b d Marta Tanasiewicz , Witold Malec a
Department of Propaedeutics in Dentistry, Medical University of Silesia, Katowice, Poland Experimental Odontology Research Group, Medical University of Silesia, Katowice, Poland c Department of Organic Chemistry, Bioorganic Chemistry and Biotechnology, Silesian University of Technology, Gliwice, Poland d Institute of Non-Ferrous Metals, Gliwice, Poland b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The purpose of this study was to evaluate the effect of PMnEDM dental monomer
Received 5 May 2006
homologs chemical structure on shear bond strength between polymer-based composite
Received in revised form
and alloy.
23 October 2006
Methods. Four light-cured experimental universal dental bio-adhesives (group codes: A
Accepted 16 November 2006
(PMDM), B (PM2EDM), C (PM3EDM), D (PM4EDM)) were preliminarily evaluated with respect to sensitivity to ambient light, curing time, depth of cure, and uncured film thickness according to standardized procedures. Appropriate tests were performed to measure shear bond
Keywords:
strength (SBS) of polymer-based composite to cobalt-based alloy with the use of the adhe-
Restorative dentistry
sives investigated. Variability of results was evaluated by use of the coefficient of variation
Artistic dentistry
(CV). Results were estimated with the aid of one-way analysis of variance (ANOVA), per-
Dental adhesive
formed on the logarithmic values, with ˛ = 0.05 significance level.
Adhesion to metal
Results. All materials passed the requirements according to physicochemical properties.
SBS
Except for formulation D, all results estimating SBS were positive with respect to standardized requirements. The uppermost mean SBS was achieved for the A adhesive (11.45 MPa) and appeared to be significantly different compared to D one (5.07 MPa) (p = 0.0495). Also the B adhesive, having slightly lower mean SBS value (10.50 MPa) exhibited a significant difference in respect to D one (p = 0.0455). The means for other trial pairs did not differ statistically. Significance. The materials here studied might be considered to have a practical use in dental clinics, especially the formulations B and C. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Restorative, and artistic dentistry [1,2] in particular, is closely concerned with the repair and treatment of teeth defects
in order to obtain proper function and adequate esthetics. The application of adhesive techniques to bond dental materials has been long desirable in direct restorative dentistry. True adhesive dentistry began in the 1950s, when it was con-
∗ Corresponding author at: Experimental Odontology Research Group, Department of Propaedeutics in Dentistry, Medical University of Silesia, 17 Akademicki Square, 41-902 Bytom, Poland. Tel.: +48 32 2827993; fax: +48 32 2827993. E-mail address: [email protected] (T.W. Kupka). URL: www.slam.katowice.pl/zpt (T.W. Kupka).
0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.11.019
1270
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
firmed that acrylic resins filled with silica particles would adhere to surface enamel that had been etched with inorganic acid. Immediately after the invention of bis-GMA resin and related composite filling material [3], Bowen introduced the idea of Surface Active Comonomers (SACs), intended to provide chemical bonding between a composite restoration and hard tissues of the tooth. The first SAC-type monomer was NPG-GMA, i.e., the product of addition reaction of Nphenylglycine (NPG) and glycidyl methacrylate (GMA) [4]. Functional groups of SACs were supposed to form chelate-type complexes with dentin hydroxyapatite whereas the methacrylate carbon–carbon double bond copolymerized with those of composite resin constituents [5]. To-date, a number of different functional methacrylates have been synthesized and employed successfully in various dental adhesive systems, including commercial ones. Amongst the best known ones are PMDM [6], HEMA (2hydroxyethyl methacrylate, Sigma-Aldrich) [7], 4-META [8], PENTA-P [9] (Fig. 1). Application of some enamel/dentin bonding agents had also been extended to provide sufficient bond strength to metal/alloy surfaces, e.g. silver amalgams and other metal devices used in restorative direct/indirect dentistry [10–13]. The ability to bond to metals so securely is also due to several proven surface treatment of cast metals, e.g. sandblasting, chemical etching, cast as a mesh, tin plating, silo-coating, or plasma treatment [14,15]. Before the implementation of original treatment methods and new medical devices/dental materials into dental clinical practice, it is recommended to meet some precisely specified standardized requirements at the experimental stage and then in clinical trials. In the effort to maximize adhesives properties and practical qualities of dental restorative materials, it was recognized that interlayer adaptation was an important parameter [16]. Shear strength is particularly important in the study of interfaces between two materi-
als. The influence of monomer structure on dental adhesive properties of related restorative material has not been widely discussed so far. The presence of two methacrylate groups seems to be advantageous, since due to limited conversion during polymerization, at least one of them should be incorporated into a network. Following this line of thinking, one could conclude that the presence of two hydrophilic functional groups, providing adhesion to substrates, should also be advantageous. PMDM, having both two methacrylate and two carboxylic groups, might be considered to be the most efficient dental adhesive monomer, at least from a structural point of view. However, another structural factor never before taken into account was the distance between polymerizable methacrylate groups and hydrophilic carboxylic ones. The structural idea is to introduce longer spacers between these functionalities and to find whether this has any effect on adhesive properties. To do that, synthesis of the PMnEDM series of monomers was performed. These are homologs of PMDM with an increased number of oxyethylene units (n), (for n = 1 the structure corresponds to PMDM). The resultant monomers were added to a typical methacrylate composition with nanoparticle content. To answer the above questions, SBS testing was performed to measure the adhesion of the latter with respect to a metal surface, at the current stage of study.
2.
Materials and methods
The syntheses of PMnEDM monomers were carried out on ca. 0.005 mole of PMDA (pyromellitic anhydride; 1,2,4,5-benzenetetracarboxylic anhydride, Sigma-Aldrich) and 0.02 mole of appropriate monomethacrylates of oligoethylene glycols (HEMA, DEGMMA, TEGMMA and TTEGMMA; either commercial or synthesized as previously reported [17]). The mixture of the reagents was refluxed under nitrogen in acetone (POCh)
Fig. 1 – Some of the most important dental adhesive monomers.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1269–1275
solution (13 wt%) for 8 h in the presence of 400 ppm of HQME (hydroquinone monomethyl ether, Sigma-Aldrich) (inhibitor) and 2% of DMAEMA (N,N-dimethylaminoethyl methacrylate, Merck) (catalyst). The products were examined by NMR spectroscopy with the aid of a UNITY/INOVA spectrometer (Varian) working at 300 MHz. The samples were prepared as 5–10% solutions in deuteroacetone with tetramethylsilane (TMS) used as an internal reference. The products – the reaction mixtures containing particular PMnEDM monomers and an excess of respective monomethacrylate – were mixed at a 1:9 weight ratio with a methacrylate resin composed of common dental monomers, i.e. bis-GMA, UDMA and TEGDMA (triethylene glycol dimethacrylate, Fluka) in a 1:1:1 weight ratio. To form a one-component light-cured formulation, CQ (camphorquinone, Aldrich)/DMAEMA photoinitiating system (0.6 and 2.0 wt%, respectively, in respect to the total resin) was dissolved therein and finally the mixture was homogenized with silanized amorphous silica at a10:1 weight ratio. Four light-cured experimental dental bio-adhesives (group codes: A (PMDM), B (PM2EDM), C (PM3EDM), D (PM4EDM)), series I/2005, prepared as above, were evaluated with respect to sensitivity to ambient light, curing time, depth of cure, and uncured film thickness according to standardized procedures. The Shear Bond Strength (SBS) test of a light-cured polymer-based dental composite material to an alloy substrate employing four bio-adhesives was measured according to the standardized procedure. Thus, 20 burnished metal plates made of cobalt-based alloy were randomly assigned to four equal groups according to 4 coded experimental bio-adhesives used. A polymer-based composite material was bonded onto cobalt-based alloy surfaces using the materials investigated. Shear strengths (MPa) were tested on an Instron machine at (1 ± 0.3) mm/min cross-head speed. The initial post-fracture
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surface analysis on the CoCr samples was performed using a microscope (Stereo-Light Microscopy), at 25× magnification in order to assign failure modes categorized as adhesive, cohesive, or mixed fractures in the bio-adhesive or composite. Results concerning A, B, C and D adhesives, representing their continuous quality factor, were assembled into four independent trials. Consistence of the distributions with a normal distribution was confirmed with Shapiro–Wilk’s test. Normality of trials’ distributions allowed for the presentation of their descriptive characteristics in the form of an arithmetic mean and a standard deviation. Variability of results was evaluated by use of coefficient of variation (CV). The influence of adhesive formulation on the average SBS value was estimated by comparing arithmetic means for A, B, C and D trials with the aid of one-way analysis of variance (ANOVA). Application of this test requires verifying an assumption of the homogeneity of variance, which was not satisfied for the trials compared (Levene’s test). As a consequence, ANOVA test was performed on the logarithmic values. Rejection of the hypothesis on equality of means allowed multiple comparisons by HSD Tukey’s test. Significance level ˛ = 0.05 was assumed for all the tests. The calculations were conducted by use of Statistica 6.0 software.
3.
Results
Examination of the final reaction mixtures by 1 H NMR spectroscopy proved the formation of the assumed products, i.e., PMnEDM series of monomers, according to the reaction scheme given in Fig. 2. A typical, representative fragment of the spectrum recorded for PM3EDM is given in Graph 1. Thus, two equivalent ring protons in the para isomer exhibit a singlet in the middle position whereas unequivalent ones in the meta
Fig. 2 – Reaction scheme for syntheses of PMnEDM monomers.
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Table 1 – Some spectroscopic data on PMnEDM syntheses Chemical shift of ring protons ı (ppm)
Compound
ı (ppm)
Molar (%)
7.98; 8.32 8.0; 8.3*
–
Not seen
7.94; 8.28 7.99; 8.19 8.00; 8.20
8.52; 8.70 8.55; 8.68 8.53; 8.64
4 5 6
para isomer
meta isomer
PMDM
8.14 8.15a
PM2EDM PM3EDM PM4EDM
8.11 8.09 8.10
a
Intermediate product
According to [6].
Table 2 – Standardized properties of experimental bio-adhesives containing PMnEDM monomers Property evaluated
Sensitivity to ambient light (s) Curing time (s) Depth of cure (mm) Uncured film thickness (m)
Standardized requirement Not less than 25 s Not more than 60 s Not less than 1.5 mm Not more than 0.1 mm (100 m)
isomer give two satellite-like singlets around the former. The values of the intensities of the signals indicates the formation of an equimolar ratio of the isomers. Two minor signals at the lower field side might be assigned to the intermediate product having a structure as shown in Fig. 2. From the signal intensities ratio in respect of the main product, the content of intermediate is ca. 5% (molar). Table 1 summarizes the values of chemical shifts for aromatic protons of PMnEDM monomers as well as of respective intermediates. The results of physicochemical examination of the experimental bio-adhesive mixtures, carried out according to standardized procedures, are given in Table 2. The results of SBS test are given in Table 3, Figs. 3 and 4 and Graphs 2 and 3. The uppermost mean SBS was achieved for the A adhesive (11.45 MPa) and appeared to be significantly different with respect to D one only (5.07 MPa) (p = 0.0495). Also the B adhesive, having slightly lower mean SBS value (10.50 MPa), exhibited a significant difference with respect to D one (p = 0.0455). Means for other trail pairs analyzed did not differ statistically (Table 4). Based on the coefficient of variation, B adhesive was found to be characterized by the highest homo-
Experimental bio-adhesives containing PMnEDM homologs A (PMDM) 117 (6) 11.6 (0.2) 4.18 (0.05) 3.13 (1.40)
B (PM2EDM)
C (PM3EDM)
203 (4) 11.3 (0.4) 4.11 (0.10) 2.84 (1.05)
154 (7) 10.9 (0.2) 4.11 (0.07) 4.18 (2.42)
D (PM4EDM) 116 (2) 10.5 (0.1) 4.24 (0.11) 1.21 (0.33)
Fig. 3 – A representative SLM picture of composite-alloy interface after fracture; bio-adhesive B; residues of a separator around the bonding area.
geneity of results (CV = 16.02%). For D material V amounted to 33.73% whereas the highest variability of results had A and C ones: 48.30% and 49.85%, respectively.
4.
Graph 1 – Aromatic region fragment of 1 H NMR spectrum of PM3EDM.
Discussion
Durable bonding to a metal framework is required for conservative preparation designs [18]. Present developments in dental polymeric materials are mainly focused on the reduction of polymerization shrinkage and improvement of biocompatibility, wear resistance, and processing properties. This can be achieved by using new tailor-made monomers or optimized filler particles and adequate adhesives [19]. On the other hand, esthetic properties are of great importance for clinicians, in terms of performing highly sophisticated restorations.
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Table 3 – Results of shear bond strength measurements Bio-adhesives containing PMnEDM homologs
Content of the carboxyl groups (mmol/g)
Individual SBS values (MPa)
SBS mean (MPa)
A (PMDM)
0.271
10.0 4.59 14.38 19.14 9.08
11.45 (5.53)
B (PM2EDM)
0.218
8.67 9.47 10.85 12.90 10.63
10.50 (1.61)
C (PM3EDM)
0.183
3.07 8.85 5.23 11.45 5.11
6.74 (3.36)
D (PM4EDM)
0.158
7.82 4.89 4.46 3.13 5.05
5.07 (1.71)
Graph 3 – A representative shear curve graph for the experimental dental bio-adhesive B.
Fig. 4 – A representative SLM picture of composite-alloy interface after fracture; bio-adhesive C; residues of a separator around the bonding area.
Graph 2 – SBS results for experimental dental bio-adhesives tested.
Basically, the synthesis of PMnEDM monomers has been carried out in a similar way to the procedure given by Bowen [5], except that acetone was used instead of tetrahydrofurane, an amine catalyst was used in a much lower concentration, and an excess of hydroxymethacrylate was 100% instead of 10% (molar). The reaction conditions required much longer reaction time to yield a complete conversion of PMDA. However, the resulting mixtures of products (PMnEDM and respective intermediates; the latter compounds are structurally similar to 4-META, a monomer well known in adhesive dentistry [8]) and unreacted species (excessive hydroxymethacrylate and DMAEMA) did not require purification and could be introduced directly into the methacrylate resin with nanoparticle content. All four bio-adhesive formulations containing particular PMnEDM monomers comply with the standardized requirement concerning light-cured polymer-based dental materials, as shown in Table 2. Thus, these materials might be considered to have a practical use in dental practice. Except for the formulation D containing PM4EDM, all the results estimating shear bond strength of the composite/adhesive/metal systems,
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Table 4 – Results of multiple comparisons; p (HSD Tukey’s test) Pairs compared A and B A and C A and D B and C B and D C and D ∗ ∗∗
Acknowledgement p
0.99997* 0.2261* 0.0494** 0.21111* 0.0455** 0.8252*
No significant differences. Significant differences.
where the adhesive resin contained PMnEDM monomers, are positive with respect to the standardized requirements, i.e. at least 4 the SBS values are not less than 5 MPa (in the case of PM4EDM just 2). However, although the formulations contained almost equal, i.e. ca. 6 wt%, content of particular PMnEDM monomers, concentrations of carboxyl groups, probably mostly responsible for adhesive properties, differ considerably due to differences in molecular weight of the former. As illustrated in Table 3 the concentration of carboxyls in the resin containing PMDM is almost twice as large as in that containing PM4EDM. This could be a reason for the poor mechanical performance of the latter. Along with the increase in bond strength values, fundamentally above 10 MPa, cohesive failures were observed, especially in the bio-adhesive, but also in the surrounding area of polymer-based composite. A variety of tests have been developed to measure bond strength between material and tooth and between two materials. Some of them are designed to place the bond in tension or in shear. To simulate oral conditions many test specimens are subjected to cycles of varying temperature before measuring bond strength. Thermo-cycling influences many more specimens than traditional water immersion [20]. There is no close analogy between laboratory simulations and clinical conditions [21]. The relative contribution of widely used artificial aging methodology, like thermo-cycling, is strongly dependent on the specific test set-up and the adhesive used [22]. Although laboratory results cannot be directly extrapolated to the clinical situation, they constitute a proper prognosis and deliver some appropriate tips with respect to the probable in vivo behavior. The authors anticipate performing thermo-cycling simulations and SBS on hard tissues of tooth in subsequent stages of the present study.
5.
Conclusions
1. All the 4 dental bio-adhesive formulations particularly those containing PMnEDM monomers comply with the standardized requirements concerning physicochemical properties of light-cured dental materials. 2. Experimental dental bio-adhesives code A, B and C, comply with standardized requirements concerning SBS. 3. The results of non-clinical tests suggest that experimental dental bio-adhesive, especially code B and C, might be useful in restorative dentistry.
This research was supported by research grant no. 3T09B 06227.
references
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