Effect of Intermediate Adhesive Resin and Flowable Resin Application on the Interfacial Adhesion of Resin Composite to Pre-impregnated Unidirectional S2-Glass Fiber Bundles Petr Polaceka / Vladimir Pavelkab / Mutlu Özcanc
Purpose: This study evaluated the effect of either an intermediate application of adhesive resin or flowable resin application on the adhesion of particulate filler composite (PFC) to glass fiber-reinforced composite (FRC). Materials and Methods: Unidirectional, pre-impregnated S2-glass fiber bundles (Dentapreg) (length: 40 mm; thickness: 0.5 mm) were obtained (N = 30, n = 10 per group) and secured in translucent silicone material with the adhesion surface exposed and photopolymerized. They were randomly divided into 3 groups for the following adhesion sequence: A) FRC+PFC, B) FRC+intermediate adhesive resin+PFC, C) FRC+flowable resin+PFC. The PFC was applied in a polyethylene mold onto the FRC and photopolymerized. PFCs were debonded from the FRC surface using shear bond test in a universal testing machine (1 mm/min). After debonding, all specimens were analyzed using scanning electron microscopy to categorize the failure modes. The data were statistically analyzed using one-way ANOVA and Tukey’s tests (_ = 0.05). Results: A significant difference was observed between the groups (p < 0.05). The highest mean bond strength value was obtained with the application of an intermediate layer of adhesive resin (group B: 19.4 ± 1.1 MPa) (p < 0.05) followed by group A (14.1 ± 0.6 MPa) and group C (10.4 ± 0.8 MPa), which were also significantly different from one another (p < 0.05). Group A exclusively presented a combination of partial cohesive failure in the PFC and adhesive failure between the FRC and PFC. While group B showed large cohesive defects in the FRC, in group C, only small cohesive failures were observed in the FRC. Conclusion: Based on the highest mean bond strength and the large cohesive failures within the FRC, application of an intermediate layer of adhesive resin on the S2-glass FRC surface prior to incremental build up of the PFC seems to be compulsory. Key words: adhesion, bonding agent, bond strength, dental materials, intermediate adhesive resin, fiber-reinforced composites, flowable resin. J Adhes Dent 2014; 16: 155-159. doi: 10.3290/j.jad.a31812
I
nlay- or surface-retained resin-bonded fixed dental prostheses (RBFDPs) may be an option when implants are not indicated or cannot be afforded by the patients. a
Research Fellow, Faculty of Chemistry, Brno University of Technology, Czech Republic. Performed the experiments, discussed results and commented on manuscript at all stages.
b
Research Fellow, Faculty of Chemistry, Brno University of Technology, Czech Republic. Analyzed the data, discussed results and commented on manuscript at all stages.
c
Professor, University of Zürich, Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zürich, Switzerland. Designed the study, wrote the manuscript, discussed results and commented on manuscript at all stages.
Correspondence: Professor Mutlu Özcan, Dental Materials Unit, Center for Dental and Oral Medicine Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, University of Zürich, Plattenstrasse 11, CH-8032, Zürich, Switzerland. Tel: +41-44-63 45600, Fax: +41-44-63 44305. e-mail: [email protected]
Vol 16, No 2, 2014
Submitted for publication: 24.03.13; accepted for publication: 11.06.13
Furthermore, problems with peri-implantitis or marginal bone loss around the implants have not been thoroughly solved in implant dentistry.12 Especially when abutment teeth contain restorative fillings adjacent to the missing tooth, RBFDPs are minimally invasive alternatives as they require only minimum tooth structure removal. RBFDPs can be made either from all-ceramics or fiberreinforced composites (FRC), constructed indirectly or directly. Wolfart et al reported an 89% survival rate after 4 years for lithium disilicate inlay-retained RBFDPs with failures due to debonding or a combination of debonding and fracture,20 and the survival rate of direct and indirect FRC RBFDPs ranges between 58.8% and 97% from 2 to 5 years.1,5,19,21 Clinical studies have not systematically reported on the location of the fractures, but several in vitro studies indicated that framework design of anterior and posterior FRC RBFDPs can considerably 155
Polacek et al
prevent chipping or complete debonding of veneering resin composite.2-4,14,15 One other way to avoid failures between the FRC framework and the veneering resin11,12 is to improve the interfacial adhesion using application of an intermediate layer of adhesive resins. In the incremental build-up technique, new resin can bond covalently to the already polymerized resin either through free radical polymerization between the old and new monomer of the resin, or through a combination of free radical polymerization and interdiffusion of the monomers of new resin into the substrate resin. Covalent bonding, based on free radicals, takes place when unreacted carbon-carbon double bonds open to form carbon-carbon single bonds. The bonding with interdiffusion happens if the polymerized substrate is partly made of linear polymer and cross-linked polymer and if the monomers of the new polymer can dissolve the substrate.11,18 Fibers are usually impregnated using monomers, polymers, or a combination of the two in order to achieve good attachment of the veneering resin.6 Effective pre-impregnation also allows the matrix to increase the surface wetting properties of the fibers and help to keep the fibers in close contact in the fiber bundle.18 Good impregnation of fibers with the surrounding monomer matrix is important, since fiber reinforcement is only successful when the loading force can be transferred from the resin matrix to the fiber.3 Pre-impregnated systems usually involve monomers like urethane dimethacrylate (UDMA), urethane tetramethacrylate (UTMA), bisphenol glycidylmethacrylate (bis-GMA), or polymethylmethacrylate (PMMA) either already impregnated by the manufacturer or performed by the clinician. The rigidity and strength of such appliances made from FRCs are dependent on the polymer matrix of the FRC and the type of fiber.4 Criticism has been focused on the inadequate interfacial adhesion between ultrahigh molecular weight polyethylene fibers and dental polymers even after electrochemical “plasma” treatments.15 On the other hand, more reliable impregnation can be achieved to silanized glass FRC materials.15 As opposed to PMMA/ bis-GMA multiphase impregnated E-glass FRCs, S2-glass fibers are directly impregnated with dimethacrylate resins during plasma-enhanced chemical vapour deposition, which eliminates hydrolysis of silane.16 Although the number of FRC materials on the market is increasing, there are still no established guidelines for the clinical application of FRC RBFDPs, especially regarding layering procedures. Since the manufacturers of the available FRC materials advise either the application of adhesive resin (Ribbond, StickTech products) or flowable resin only (ADM products) on the FRC surfaces during buildup, some confusion about clinical procedures exists. In fact, application of an intermediate adhesive resin layer on the FRC could be anticipated to increase the wettability of the PFC.7,8,11 Linear polymers allow diffusion of monomers resulting in an interpenetrating polymer network (IPN), but diffusion of monomers into the cross-linked polymer is difficult to obtain.8 The objectives of this study were a) to evaluate the effect of intermediate adhesive resin or flowable resin ap156
plication on the adhesion of PFC to S2-glass FRC bundle surfaces and b) to analyze the failure types after debonding. The hypothesis tested was that application of an intermediate adhesive resin on the FRC bundle would deliver better adhesion of the PFC on the FRC as opposed to flowable resin or PFC application only.
MATERIALS AND METHODS Unidirectional, silanized, pre-impregnated S2-glass FRC bundles, batch No. PFU10-102009 (50 to 60 wt%; ADM, Dentapreg PFU; Brno, Czech Republic), were obtained (N = 30, n = 10 per group) and secured in translucent silicone with the adhesion surface exposed. The specimens were randomly divided into 3 groups for the following adhesion sequences: A. FRC+PFC: After FRC was photopolymerized for 2 min (Targis Power, Ivoclar Vivadent; Schaan, Liechtenstein) with a light output of 1000 mW/cm2, PFC (Boston C&B, Arkona; Lublin, Poland; batch No. 160209) was applied and photopolymerized for 5 min using the same polymerization device. B. FRC+intermediate adhesive resin+PFC: After FRC was photopolymerized as in group A, an intermediate application of adhesive resin (Adper Single Bond Plus, 3M ESPE, Seefeld, Germany; Batch no: 20070509) was performed in one coat and gently air thinned. Then, PFC (Boston C&B, Arkona, Lublin, Poland; Batch no: 160209) was applied and photopolymerized for 5 min as in group A. C. FRC+flowable resin+PFC: After FRC was photopolymerized as in group A, flowable resin (Boston Flow, Arkona; batch No. 110615) was applied with a brush. Then, PFC was applied and photopolymerized as in group A. The chemical composition of the intermediate adhesive resin was based on methacrylate and dimethacrylate mixtures (bis-GMA and 2-HEMA) with a small amount of treated silica nanofillers (10 to 20 wt%). Flowable resin and PFC were also based on methacrylate and dimethacrylate mixtures, but with different volume fraction of silica (50% and 75 to 80 wt%, respectively) as a filler. The PFC was applied in a polyethylene mold onto the FRC and photopolymerized for 5 min. PFCs were debonded from the FRC surface using the shear bond test in a universal testing machine (1 mm/min) (Zwick Z010; Ulm, Germany).16 The specimen dimensions and test set-up are presented in Fig 1. After debonding, substrate and adherend of all specimens were analyzed using a scanning electron microscope (SEM) (Philips 30; Brno, Czech Republic) to categorize the failure modes (Fig 2). The failure types were classified as follows: combination of partial cohesive failure within the PFC and adhesive failure between the FRC and PFC (score b); a large cohesive defect in the FRC accompanied by a small adhesive defect between the FRC and PFC (score c); a small cohesive defect in the FRC accompanied by a small adhesive defect between the FRC and PFC (score d). The Journal of Adhesive Dentistry
Polacek et al
0.5
FRC
40
1.3
PFC
4
3
3
Fig 1a Schematic drawing of the PFC bonded on the FRC surface and the dimensions of the specimens in mm.
Fig 1b PFC-FRC assembly subjected to shear forces in the universal testing machine.
Bond Strength (MPa)
25 20 15 10 5 a
b
c
d
0
A
B
C
Fig 2 Schematic illustration of the failure profiles. a) Original test specimen prior to debonding. b) Combination of partial cohesive failure within the PFC and adhesive failure between the FRC and PFC. c) Large cohesive defect in the FRC accompanied with small adhesive failure between the FRC and PFC. d) Small cohesive defect in the FRC accompanied with small adhesive failure between the FRC and PFC.
Fig 3 Mean bond strength values (MPa) and standard deviations for groups A (FRC+PFC), B (FRC+intermediate adhesive resin+PFC), and C (FRC+flowable resin+PFC).
Statistical analysis was performed using SPSS 12.00 (SPSS; Chicago, IL, USA). The means of each group were analyzed with one-way ANOVA. Because of the significant group factor (p < 0.05), multiple comparisons were made using the Tukey-Kramer adjustment test. p-values < 0.05 were considered statistically significant in all tests.
According to the SEM analysis, group A exclusively presented a combination of partial cohesive failure in the PFC and adhesive failure between the FRC and PFC. While group B showed large cohesive defects in the FRC, in group C, only small cohesive failures were observed in the FRC (Fig 4).
RESULTS
DISCUSSION
A significant difference was observed between the groups (p < 0.05). The highest mean bond strength value was obtained in group B with the intermediate application of adhesive resin (19.4 ± 1.1 MPa; p < 0.05), followed by group A (14.1 ± 0.6 MPa) and group C (10.4±0.8 MPa), which also differed significantly from one another (p < 0.05) (Fig 3).
Since the group in which FRC surfaces were activated with an intermediate adhesive resin demonstrated the significantly highest mean bond strength, the hypothesis could be accepted. In contrast, application of the flowable resin on the surface of the FRC bundle resulted in significantly lower bond strengths compared to that of direct PFC application on the FRC. In this study, PFC
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FRC Surface
FRC Surface
( a
b
c
FRC Surface
FRC Surface
( d
e
g
FRC Surface
FRC Surface
( g
i
h
Figs 4a to 4i SEM images of the FRC surfaces after debonding (50X, 150X, 300X) from three different groups. a to c: Small cohesive defects in the FRC with some remnants of PFC (score b). d to f: Extensive cohesive defects in the FRC with less PFC remnants on the FRC (score c). g to i: Small cohesive defects in the FRC limited to the bonding area with some PFC remnants (score d). See Fig 2 for failure type descriptions. * indicates the FRC rupture and PFC remnants. The arrows indicate the direction of loading (a,d,g). The dotted lines signify the boundary between the FRC and PFC.
(
alone or the FR/PFC combination was debonded from the FRC surface using the shear test, where the force was applied parallel to the fiber orientation. The tensile test was not considered in order to avoid pulling out and separating the fibers during debonding. A previous study, in which a methacrylate-based PFC was debonded from a PPMA/bisGMA-impregnated E-glass FRC using the shear test, noted that FRC oriented perpendicular to the PFC showed the highest bond strength (44.8 MPa) compared to transverse positioning (24.8 MPa).9 In that study, no flowable resin 158
but only an intermediate adhesive resin containing mainly 10-methacryloyloxydecyl dihydrogen phosphate (MDP) was used. In another study, the use of an intermediate adhesive resin based on bis-GMA/HEMA significantly improved the adhesion between the PFC and E-glass FRC with PPMA/bis-GMA multiphase impregnation (16 MPa) after aging compared to the groups where no adhesive was used (6.5 MPa).8 Controversial reports exist as to whether aging decreases the adhesion of PFC to FRC or not.7,8 However, it was noted that OH groups of the The Journal of Adhesive Dentistry
Polacek et al
HEMA monomer make it hydrophilic and cause relatively high water uptake. Similarly, in another study, bis-GMA/ TEG-DMA-based adhesive use increased the shear bond strength of PFC to multiphase FRC (21 MPa) compared to the control group, but thermocycling decreased the bond strength (14.3 MPa). The results of this study with the use of self-adhesive resin (19.4 ± 1.1 MPa) based on bisGMA and 2-HEMA were comparable to those of previous studies, even though polymerization duration (5 min) was less than those of previous studies (30 min).8 Chemically, in fact, flowable resin and PFC are similar; both are based on the mixtures of methacrylates and dimethacrylates. The difference between them is in the amount of filler content. While PFC contains 75 to 80 wt% microsilica, flowable composite contains about 50 wt% microsilica. The reason for the lower shear bond strength of the FRC+flowable+PFC group is possibly a consequence of the higher viscosity of flowable composite. Moreover, the self-adhesive bonding system partially etches the inhibited layer of photopolymerized FRC, which increases micromechanical adhesion between PFC and FRC. Together with lower viscosity of the adhesive resin, better wettability of the FRC surface was achieved more effectively than with flowable composite. The large cohesive failures in the FRC observed with the application of an intermediate adhesive resin (group B) indicates higher adhesive strength between PFC and FRC than the cohesive strength of PFC. Possibly, self-adhesive resin etched the inhibited surface layer of the photopolymerized FRC and enabled better micromechanical retention between FRC and PFC. On the other hand, the combination of cohesive failure in FRC and adhesive failure between FRC and PFC in group A indicates less surface wettability of FRC with the highly filled PFC. Finally, only small cohesive failures were observed in the FRC in group C, implying apparently low adhesive strength of the composite compared to the cohesive strength of PFC. In this group, debonding occured in a thin layer of flowable composite and partially on the surface of FRC. Fiber pull-out was more common when the multiphase FRCs were impregnated with a bisGMA/HEMA8 or bis-GMA-, UDMA-, and bis-EMA-based adhesive resin,9 whereas in this study, fiber pull-out was accompanied by cohesive fractures of fibers. The forces applied during chewing are complex in nature. FRC-supported PFCs are usually prone to a combination of shear and tensile forces. In that respect, forces applied parallel to the FRC/PFC interface may be considered unfavorable and perhaps the worst-case scenario. The bond strength results and failure types may change with other material combinations.
2. Application of flowable resin on the FRC surface may be excluded prior to PFC buildup.
REFERENCES 1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12. 13. 14.
15.
16.
17.
18. 19.
20.
21.
CONCLUSIONS 1. Considering both the high bond strength and the cohesive FRC failure types experienced after debonding, the adhesion of the veneering PFC-FRC combination with the tested materials was better when the FRC surface was conditioned with an intermediate adhesive resin. Vol 16, No 2, 2014
Bohlsen F, Kern M. Clinical outcome of glass-fiber-reinforced crowns and fixed partial dentures: a three-year retrospective study. Quintessence Int 2003;34:493-496. Dyer SR, Lassila LV, Jokinen M, Vallittu PK. Effect of fiber position and orientation on fracture load of fiber-reinforced composite. Dent Mater 2004;20:947-955. Ellakwa AE, Shortall AC, Shehata MK, Marquis PM. The influence of fibre placement and position on the efficiency of reinforcement of fibre reinforced composite bridgework. J Oral Rehabil 2001;28:785-791. Garoushi S, Lassila LV, Tezvergil A, Vallittu PK. Static and fatigue compression test for particulate filler composite resin with fiber-reinforced composite substructure. Dent Mater 2007;23:17-23. Göhring TN, Roos M. Inlay-fixed partial dentures adhesively retained and reinforced by glass fibers: clinical and scanning electron microscopy analysis after five years. Eur J Oral Sci 2005;113:60-69. Isaac DH. Engineering aspects of the structure and properties of polymer fiber composites. In: Vallittu PK (ed). The First International Symposium on Fibre-Reinforced Plastics in Dentistry, Institute of Dentistry and Biomaterials Project, University of Turku, Finland, 1999:27-29. Kallio TT, Lastumäki TM, Vallittu PK. Bonding of restorative and veneering composite resin to some polymeric composites. Dent Mater 2001;17:80-86. Keski-Nikkola MS, Alander PM, Lassila LV, Vallittu PK. Bond strength of Gradia veneering composite to fibre-reinforced composite. J Oral Rehabil 2004;31:1178-1183. Lassila LV, Tezvergil A, Dyer SR, Vallittu PK. The bond strength of particulate-filler composite to differently oriented fiber-reinforced composite substrate. J Prosthodont 2007;16:10-17. Lastumäki TM, Kallio TT, Vallittu PK. The bond strength of light-curing composite resin to finally polymerized and aged glass fiber-reinforced composite substrate. Biomaterials 2002;23:4533-4539. Lastumäki TM, Lassila LV, Vallittu PK. The semi-interpenetrating polymer network matrix of fiber-reinforced composite and its effect on the surface adhesive properties. J Mater Sci Mater Med 2003;14:803-809. Meffert RM. Periodontitis and periimplantitis: one and the same? Pract Periodontics Aesthet Dent 1993;5:79-80, 82. Nielsen LE. Mechanical properties of polymer and composites, vol 2. New York: Marcel Dekker, 1974:468-488. Özcan M, Breuklander MH, Vallittu PK. The effect of box preparation on the strength of glass fiber-reinforced composite inlay-retained fixed partial dentures. J Prosthet Dent 2005;93:337-345. Özcan M, Koekoek W, Pekkan G. Load-bearing capacity of indirect inlayretained fixed dental prostheses made of particulate filler composite alone or reinforced with E-glass fibers impregnated with various monomers. J Mech Behav Biomed Mater 2012;12:160-167. Polacek P, Jancar J. Effect of filler content on the adhesion strength between UD fiber reinforced and particulate filled composites. Composites Sci Technol 2008;68:251-259. Vallittu PK. The effect of void space and polymerization time on transverse strength of acrylic-glass fibre composite. J Oral Rehabil 1995;22:257-261. Vallittu PK. Oxygen inhibition of autopolymerization of polymethyl methacrylate-glass fibre composite. J Mater Sci Mater Med 1997;8:489-492. van Heumen CC, Kreulen CM, Creugers NH. Clinical studies of fiberreinforced resin-bonded fixed partial dentures: a systematic review. Eur J Oral Sci 2009;117:1-6. Wolfart S, Bohlsen F, Wegner SM, Kern M. A preliminary prospective evaluation of all-ceramic crown-retained and inlay-retained fixed partial dentures. Int J Prosthodont 2005;18:497-505. Wolff D, Schach C, Kraus T, Ding P, Pritsch M, Mente J, Joerss D, Staehle HJ. Fiber-reinforced composite fixed dental prostheses: a retrospective clinical examination. J Adhes Dent 2011;13:187-194.
Clinical relevance: When PFC is built up on the S2glass FRC bundle, the FRC surface needs to be activated with an intermediate adhesive resin prior to application of the PFC.
159
Purpose: This study evaluated the effect of either an intermediate application of adhesive resin or flowable resin application on the adhesion of particulate filler composite (PFC) to glass fiber-reinforced composite (FRC). Materials and Methods: Unidirectional, pre-impregnated S2-glass fiber bundles (Dentapreg) (length: 40 mm; thickness: 0.5 mm) were obtained (N = 30, n = 10 per group) and secured in translucent silicone material with the adhesion surface exposed and photopolymerized. They were randomly divided into 3 groups for the following adhesion sequence: A) FRC+PFC, B) FRC+intermediate adhesive resin+PFC, C) FRC+flowable resin+PFC. The PFC was applied in a polyethylene mold onto the FRC and photopolymerized. PFCs were debonded from the FRC surface using shear bond test in a universal testing machine (1 mm/min). After debonding, all specimens were analyzed using scanning electron microscopy to categorize the failure modes. The data were statistically analyzed using one-way ANOVA and Tukey’s tests (_ = 0.05). Results: A significant difference was observed between the groups (p < 0.05). The highest mean bond strength value was obtained with the application of an intermediate layer of adhesive resin (group B: 19.4 ± 1.1 MPa) (p < 0.05) followed by group A (14.1 ± 0.6 MPa) and group C (10.4 ± 0.8 MPa), which were also significantly different from one another (p < 0.05). Group A exclusively presented a combination of partial cohesive failure in the PFC and adhesive failure between the FRC and PFC. While group B showed large cohesive defects in the FRC, in group C, only small cohesive failures were observed in the FRC. Conclusion: Based on the highest mean bond strength and the large cohesive failures within the FRC, application of an intermediate layer of adhesive resin on the S2-glass FRC surface prior to incremental build up of the PFC seems to be compulsory. Key words: adhesion, bonding agent, bond strength, dental materials, intermediate adhesive resin, fiber-reinforced composites, flowable resin. J Adhes Dent 2014; 16: 155-159. doi: 10.3290/j.jad.a31812
I
nlay- or surface-retained resin-bonded fixed dental prostheses (RBFDPs) may be an option when implants are not indicated or cannot be afforded by the patients. a
Research Fellow, Faculty of Chemistry, Brno University of Technology, Czech Republic. Performed the experiments, discussed results and commented on manuscript at all stages.
b
Research Fellow, Faculty of Chemistry, Brno University of Technology, Czech Republic. Analyzed the data, discussed results and commented on manuscript at all stages.
c
Professor, University of Zürich, Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zürich, Switzerland. Designed the study, wrote the manuscript, discussed results and commented on manuscript at all stages.
Correspondence: Professor Mutlu Özcan, Dental Materials Unit, Center for Dental and Oral Medicine Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, University of Zürich, Plattenstrasse 11, CH-8032, Zürich, Switzerland. Tel: +41-44-63 45600, Fax: +41-44-63 44305. e-mail: [email protected]
Vol 16, No 2, 2014
Submitted for publication: 24.03.13; accepted for publication: 11.06.13
Furthermore, problems with peri-implantitis or marginal bone loss around the implants have not been thoroughly solved in implant dentistry.12 Especially when abutment teeth contain restorative fillings adjacent to the missing tooth, RBFDPs are minimally invasive alternatives as they require only minimum tooth structure removal. RBFDPs can be made either from all-ceramics or fiberreinforced composites (FRC), constructed indirectly or directly. Wolfart et al reported an 89% survival rate after 4 years for lithium disilicate inlay-retained RBFDPs with failures due to debonding or a combination of debonding and fracture,20 and the survival rate of direct and indirect FRC RBFDPs ranges between 58.8% and 97% from 2 to 5 years.1,5,19,21 Clinical studies have not systematically reported on the location of the fractures, but several in vitro studies indicated that framework design of anterior and posterior FRC RBFDPs can considerably 155
Polacek et al
prevent chipping or complete debonding of veneering resin composite.2-4,14,15 One other way to avoid failures between the FRC framework and the veneering resin11,12 is to improve the interfacial adhesion using application of an intermediate layer of adhesive resins. In the incremental build-up technique, new resin can bond covalently to the already polymerized resin either through free radical polymerization between the old and new monomer of the resin, or through a combination of free radical polymerization and interdiffusion of the monomers of new resin into the substrate resin. Covalent bonding, based on free radicals, takes place when unreacted carbon-carbon double bonds open to form carbon-carbon single bonds. The bonding with interdiffusion happens if the polymerized substrate is partly made of linear polymer and cross-linked polymer and if the monomers of the new polymer can dissolve the substrate.11,18 Fibers are usually impregnated using monomers, polymers, or a combination of the two in order to achieve good attachment of the veneering resin.6 Effective pre-impregnation also allows the matrix to increase the surface wetting properties of the fibers and help to keep the fibers in close contact in the fiber bundle.18 Good impregnation of fibers with the surrounding monomer matrix is important, since fiber reinforcement is only successful when the loading force can be transferred from the resin matrix to the fiber.3 Pre-impregnated systems usually involve monomers like urethane dimethacrylate (UDMA), urethane tetramethacrylate (UTMA), bisphenol glycidylmethacrylate (bis-GMA), or polymethylmethacrylate (PMMA) either already impregnated by the manufacturer or performed by the clinician. The rigidity and strength of such appliances made from FRCs are dependent on the polymer matrix of the FRC and the type of fiber.4 Criticism has been focused on the inadequate interfacial adhesion between ultrahigh molecular weight polyethylene fibers and dental polymers even after electrochemical “plasma” treatments.15 On the other hand, more reliable impregnation can be achieved to silanized glass FRC materials.15 As opposed to PMMA/ bis-GMA multiphase impregnated E-glass FRCs, S2-glass fibers are directly impregnated with dimethacrylate resins during plasma-enhanced chemical vapour deposition, which eliminates hydrolysis of silane.16 Although the number of FRC materials on the market is increasing, there are still no established guidelines for the clinical application of FRC RBFDPs, especially regarding layering procedures. Since the manufacturers of the available FRC materials advise either the application of adhesive resin (Ribbond, StickTech products) or flowable resin only (ADM products) on the FRC surfaces during buildup, some confusion about clinical procedures exists. In fact, application of an intermediate adhesive resin layer on the FRC could be anticipated to increase the wettability of the PFC.7,8,11 Linear polymers allow diffusion of monomers resulting in an interpenetrating polymer network (IPN), but diffusion of monomers into the cross-linked polymer is difficult to obtain.8 The objectives of this study were a) to evaluate the effect of intermediate adhesive resin or flowable resin ap156
plication on the adhesion of PFC to S2-glass FRC bundle surfaces and b) to analyze the failure types after debonding. The hypothesis tested was that application of an intermediate adhesive resin on the FRC bundle would deliver better adhesion of the PFC on the FRC as opposed to flowable resin or PFC application only.
MATERIALS AND METHODS Unidirectional, silanized, pre-impregnated S2-glass FRC bundles, batch No. PFU10-102009 (50 to 60 wt%; ADM, Dentapreg PFU; Brno, Czech Republic), were obtained (N = 30, n = 10 per group) and secured in translucent silicone with the adhesion surface exposed. The specimens were randomly divided into 3 groups for the following adhesion sequences: A. FRC+PFC: After FRC was photopolymerized for 2 min (Targis Power, Ivoclar Vivadent; Schaan, Liechtenstein) with a light output of 1000 mW/cm2, PFC (Boston C&B, Arkona; Lublin, Poland; batch No. 160209) was applied and photopolymerized for 5 min using the same polymerization device. B. FRC+intermediate adhesive resin+PFC: After FRC was photopolymerized as in group A, an intermediate application of adhesive resin (Adper Single Bond Plus, 3M ESPE, Seefeld, Germany; Batch no: 20070509) was performed in one coat and gently air thinned. Then, PFC (Boston C&B, Arkona, Lublin, Poland; Batch no: 160209) was applied and photopolymerized for 5 min as in group A. C. FRC+flowable resin+PFC: After FRC was photopolymerized as in group A, flowable resin (Boston Flow, Arkona; batch No. 110615) was applied with a brush. Then, PFC was applied and photopolymerized as in group A. The chemical composition of the intermediate adhesive resin was based on methacrylate and dimethacrylate mixtures (bis-GMA and 2-HEMA) with a small amount of treated silica nanofillers (10 to 20 wt%). Flowable resin and PFC were also based on methacrylate and dimethacrylate mixtures, but with different volume fraction of silica (50% and 75 to 80 wt%, respectively) as a filler. The PFC was applied in a polyethylene mold onto the FRC and photopolymerized for 5 min. PFCs were debonded from the FRC surface using the shear bond test in a universal testing machine (1 mm/min) (Zwick Z010; Ulm, Germany).16 The specimen dimensions and test set-up are presented in Fig 1. After debonding, substrate and adherend of all specimens were analyzed using a scanning electron microscope (SEM) (Philips 30; Brno, Czech Republic) to categorize the failure modes (Fig 2). The failure types were classified as follows: combination of partial cohesive failure within the PFC and adhesive failure between the FRC and PFC (score b); a large cohesive defect in the FRC accompanied by a small adhesive defect between the FRC and PFC (score c); a small cohesive defect in the FRC accompanied by a small adhesive defect between the FRC and PFC (score d). The Journal of Adhesive Dentistry
Polacek et al
0.5
FRC
40
1.3
PFC
4
3
3
Fig 1a Schematic drawing of the PFC bonded on the FRC surface and the dimensions of the specimens in mm.
Fig 1b PFC-FRC assembly subjected to shear forces in the universal testing machine.
Bond Strength (MPa)
25 20 15 10 5 a
b
c
d
0
A
B
C
Fig 2 Schematic illustration of the failure profiles. a) Original test specimen prior to debonding. b) Combination of partial cohesive failure within the PFC and adhesive failure between the FRC and PFC. c) Large cohesive defect in the FRC accompanied with small adhesive failure between the FRC and PFC. d) Small cohesive defect in the FRC accompanied with small adhesive failure between the FRC and PFC.
Fig 3 Mean bond strength values (MPa) and standard deviations for groups A (FRC+PFC), B (FRC+intermediate adhesive resin+PFC), and C (FRC+flowable resin+PFC).
Statistical analysis was performed using SPSS 12.00 (SPSS; Chicago, IL, USA). The means of each group were analyzed with one-way ANOVA. Because of the significant group factor (p < 0.05), multiple comparisons were made using the Tukey-Kramer adjustment test. p-values < 0.05 were considered statistically significant in all tests.
According to the SEM analysis, group A exclusively presented a combination of partial cohesive failure in the PFC and adhesive failure between the FRC and PFC. While group B showed large cohesive defects in the FRC, in group C, only small cohesive failures were observed in the FRC (Fig 4).
RESULTS
DISCUSSION
A significant difference was observed between the groups (p < 0.05). The highest mean bond strength value was obtained in group B with the intermediate application of adhesive resin (19.4 ± 1.1 MPa; p < 0.05), followed by group A (14.1 ± 0.6 MPa) and group C (10.4±0.8 MPa), which also differed significantly from one another (p < 0.05) (Fig 3).
Since the group in which FRC surfaces were activated with an intermediate adhesive resin demonstrated the significantly highest mean bond strength, the hypothesis could be accepted. In contrast, application of the flowable resin on the surface of the FRC bundle resulted in significantly lower bond strengths compared to that of direct PFC application on the FRC. In this study, PFC
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FRC Surface
FRC Surface
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FRC Surface
FRC Surface
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Figs 4a to 4i SEM images of the FRC surfaces after debonding (50X, 150X, 300X) from three different groups. a to c: Small cohesive defects in the FRC with some remnants of PFC (score b). d to f: Extensive cohesive defects in the FRC with less PFC remnants on the FRC (score c). g to i: Small cohesive defects in the FRC limited to the bonding area with some PFC remnants (score d). See Fig 2 for failure type descriptions. * indicates the FRC rupture and PFC remnants. The arrows indicate the direction of loading (a,d,g). The dotted lines signify the boundary between the FRC and PFC.
(
alone or the FR/PFC combination was debonded from the FRC surface using the shear test, where the force was applied parallel to the fiber orientation. The tensile test was not considered in order to avoid pulling out and separating the fibers during debonding. A previous study, in which a methacrylate-based PFC was debonded from a PPMA/bisGMA-impregnated E-glass FRC using the shear test, noted that FRC oriented perpendicular to the PFC showed the highest bond strength (44.8 MPa) compared to transverse positioning (24.8 MPa).9 In that study, no flowable resin 158
but only an intermediate adhesive resin containing mainly 10-methacryloyloxydecyl dihydrogen phosphate (MDP) was used. In another study, the use of an intermediate adhesive resin based on bis-GMA/HEMA significantly improved the adhesion between the PFC and E-glass FRC with PPMA/bis-GMA multiphase impregnation (16 MPa) after aging compared to the groups where no adhesive was used (6.5 MPa).8 Controversial reports exist as to whether aging decreases the adhesion of PFC to FRC or not.7,8 However, it was noted that OH groups of the The Journal of Adhesive Dentistry
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HEMA monomer make it hydrophilic and cause relatively high water uptake. Similarly, in another study, bis-GMA/ TEG-DMA-based adhesive use increased the shear bond strength of PFC to multiphase FRC (21 MPa) compared to the control group, but thermocycling decreased the bond strength (14.3 MPa). The results of this study with the use of self-adhesive resin (19.4 ± 1.1 MPa) based on bisGMA and 2-HEMA were comparable to those of previous studies, even though polymerization duration (5 min) was less than those of previous studies (30 min).8 Chemically, in fact, flowable resin and PFC are similar; both are based on the mixtures of methacrylates and dimethacrylates. The difference between them is in the amount of filler content. While PFC contains 75 to 80 wt% microsilica, flowable composite contains about 50 wt% microsilica. The reason for the lower shear bond strength of the FRC+flowable+PFC group is possibly a consequence of the higher viscosity of flowable composite. Moreover, the self-adhesive bonding system partially etches the inhibited layer of photopolymerized FRC, which increases micromechanical adhesion between PFC and FRC. Together with lower viscosity of the adhesive resin, better wettability of the FRC surface was achieved more effectively than with flowable composite. The large cohesive failures in the FRC observed with the application of an intermediate adhesive resin (group B) indicates higher adhesive strength between PFC and FRC than the cohesive strength of PFC. Possibly, self-adhesive resin etched the inhibited surface layer of the photopolymerized FRC and enabled better micromechanical retention between FRC and PFC. On the other hand, the combination of cohesive failure in FRC and adhesive failure between FRC and PFC in group A indicates less surface wettability of FRC with the highly filled PFC. Finally, only small cohesive failures were observed in the FRC in group C, implying apparently low adhesive strength of the composite compared to the cohesive strength of PFC. In this group, debonding occured in a thin layer of flowable composite and partially on the surface of FRC. Fiber pull-out was more common when the multiphase FRCs were impregnated with a bisGMA/HEMA8 or bis-GMA-, UDMA-, and bis-EMA-based adhesive resin,9 whereas in this study, fiber pull-out was accompanied by cohesive fractures of fibers. The forces applied during chewing are complex in nature. FRC-supported PFCs are usually prone to a combination of shear and tensile forces. In that respect, forces applied parallel to the FRC/PFC interface may be considered unfavorable and perhaps the worst-case scenario. The bond strength results and failure types may change with other material combinations.
2. Application of flowable resin on the FRC surface may be excluded prior to PFC buildup.
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CONCLUSIONS 1. Considering both the high bond strength and the cohesive FRC failure types experienced after debonding, the adhesion of the veneering PFC-FRC combination with the tested materials was better when the FRC surface was conditioned with an intermediate adhesive resin. Vol 16, No 2, 2014
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Clinical relevance: When PFC is built up on the S2glass FRC bundle, the FRC surface needs to be activated with an intermediate adhesive resin prior to application of the PFC.
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