Shear bond strength ofchemical and light-cured glass ionomer cements bonded to resin composites

Australian Dental Journal 1998;43 :(2):81-6 Shear bond strength ofchemical and light-cured glass ionomer cements bonded to resin composites Camile S. Farah, BDSc* Vergil G. Orton, BSct Stephen M. Collard, DDS, PhDt A bstract A bond between glass ionomer cements (GIC) and resin composites is desirable for the success of the 'sandwich' restoration. Chemically cured glass ionomer cements have been the traditional materials used in this technique since its development, but etching the GIC was necessary to obtain a bond to the composite facing. Producing a very smooth GIC surface has aided in better determining the magnitude of the chemical bond between glass ionomers and resin composites. Shear testing of bonded specimens has revealed that chemical bonding is minimal (0.21 MPa) in conventional glass ionomers, but does exist (4.92 MPa) between GIC and resin composite regardless of the filler content (microfilled vs hybrid) of the composite. Thermal stressing affects the bond to resin-modified glass ionomers, but has no significant effect on selfcured cements. Of all combinations tested, VitremerlScotchbond/Silux Plus showed the highest mean shear bond strength. Based on the clinical need for an adhesive bond between GIC linerlbase and resin composite, the resin-modified glass ionomer would appear to be the material of choice. Key words: Resin-mod ified GIC, resin composite, 'sandwich' technique, adhesive bond. (Received for publication.september 1997. Revised November 1997. Accepted November 1997.) Introduction A satisfactory bond between glass ionomer linerslbases and resin composites is critical to the success of restorations involving cervical defects for which the gingival margin must rest on cementum rather than enamel. The ' sandwich' or laminate *Recipient of Australian Dental Research Fund (now Foundation) Scholarship 1992-93. Currently, National Health and Medical Resear ch Council Postgraduate Scholar, Department of Pathology, Faculty of Medicine and Dentistry, The University of Western Australia. tschool of Oral Health Sciences, The University of Western Australia. Australian Dental Journal 1998;43:2. technique, as developed by McLean et al.,i employs the dentine adhesive properties ofglass ionomer to seal the cavity and reduce microleakage, and the aesthetic and enamel bonding properties of resin composite to enhance clinical serviceability. Bond strength between conventional glass ionomer cements and composites is limited by the low cohesive strength of the glass ionomers, and by minimal chemical bonding due to the different chemical reactions of these materials.' As applied clinically, these glass ionomer lininglbase materials contain micro-porosities and have a surface roughness that will provide micro-mechanical bonding to the resin composite. Cohesive failure through the glass ionomer cement has been reported to be the predominant failure mode in shear testing of both self-cure and resin-modified glass ionorner cements. 3 With the introduction of photocurable resin polyalkenoate mixes, the cohesive strength of glass ionomers has been significantly improved.' Bond strengths as measured by previous investigators were the combination of chemical and mechanical bonding with the true contribution of chemical bonding not clearly determined. The aim of the present study was to determine whether chemical bonding is enhanced by the use of resin-modified glass ionomer cements bonded to microfilled and hybrid resin composites, and to study the effect of thermal stresses on the bond strength. A comparison is made between one selfcured glass ionomer cement, Ketac-Bond,:j: and two resin-modified glass ionomer cements, Photac-Fil:j: and Vitremer. Materials and methods Two resin-modified glass ionorner cements, Photac-Fil and Vitremer and a single self-cure glass ionomer cement Ketac-Bond were examined in this ; ESPE GmbH, Germ any. 3M Dental Products Division, USA. 81

Table 1. Combinations, means and standard deviations (MPa) of glass ionomer cements and resin composites (N=10) Glass Ionomer Bonding Resin Bond strength cement agent composite Mean ± (SD) MPa Non-thermocycled Thermocycled Ketac-Bond Visiobond Pertac 0.04 (0.06) 0.02 (0.10) Ketac-Bond Visiobond Visiodispers 0.04 (0.02) 0.07 (0.10) Ketac-Bond Visiobond Z-IOO 0.11 (0.16) 0.20 (0.22) Ketac-Bond Visiobond Silux Plus 0.21 (0.23) 0.12 (0.20) Photac-Fil Visiobond Pertac 1.59 (2.16) 0.61 (1.84) Photac-Fil Visiobond Visiodispers 0.38 (0.26)* 0.54 (1.53) Vitremer Scotchbond Z-IOO 4.05 (0.76) 1.16 (2.97) Vitremer Scotchbond Silux Plus 4.92 (1.84)*t 1.04 (1.61)t 'Significant difference (p<0.05). tsignificant difference (p<0.05). study. These were bonded to either microfilled resin composites Visiodispersf and Silux Plus, or hybrid resin composites, Pertacj and Z-100. Visiobondj and Scotchbond Multipurpose adhesive resin bonding agents were used in the bonding procedures. One hundred and sixty specimens were prepared using combinations of materials shown in Table 1. Specimens were prepared using 25 mm diameter phenolic rings] filled with mixed epoxy resin] and polished with 220, 320, 400 and 600 grit carbide polishing paper. A hole 6.5 mm in diameter and 2.5 mm deep was drilled into the centre of the polished surface and grooves added for increased retention. The hole was then filled with glass ionomer cement mixed according to manufacturer's instructions, and covered with a glass microscope slide to produce a smooth surface and to permit light curing. A 20 kg static load was immediately applied onto the specimens for 5 minutes, to compact the glass ionomer cement mass and minimize surface porosity. Upon removing the load, the self-cure glass ionomer cement Ketac-Bond had already set, since its setting time from the beginning of mixing is four minutes, whereas the resin-modified glass ionomer cements Photac-Fil and Vitremer were light cured for 20 and 40 seconds respectively, to produce a final set. A blue halogen light curing unit'[ was used for all light-curing procedures, and all triturating of glass ionomer cement capsules was done using a standard amalgamator.** The specimens were bench set at room temperature for 24 hours with the glass slides intact to attain maximum strength and prevent dehydration and contamination of the glass ionomer surfaces. The glass slides were carefully removed, ensuring that the smooth glass ionomer surface was not pitted. Singlesided masking tape with a 4 mm diameter hole was carefully applied to the centre of the glass ionomer cement surface. The exposed glass ionomer area within the masking template was then treated with a 'IBuehler, USA. '[Translux CL, Kulzer, Germany, "Ultramat SDI Amalgamator, Australia. 82 layer ofadhesive resin bonding agent, thinned with a gentle air blast and light-cured for 10 seconds. Immediately following this procedure, a transparent polycarbonate ring, 3 mm high with a 6.5 mm internal diameter, was centred over the resin treated glass ionomer cement in the template. Resin composite was then condensed into the polycarbonate ring and placed under a load of approximately 8 kgf. The composite plugs were light-cured vertically and also cured for 10 seconds horizontally at 90 angles to ensure complete curing of the material. All procedures were conducted at room temperature 23 e and manufacturers' instructions were followed precisely. Eighty bonded specimens were stored in distilled water at (36±l) e for 15 days while the other 80 specimens were thermally cycled for 1079 cycles at (6± l) e and (55± l) e for 15 days, with a 10 minute dwell time in each bath, and a 15 second interval between baths. Shear testing of bonded specimens was performed on a universal testing machine,tt using a crosshead speed of5 mmlminute and a 100 kg load cell. A shearing apparatus was constructed to grip the phenolic rings and a wedge blade system was designed to apply a shearing force approximately 0.1 mm from the adhesive interface (Fig. 1). Means and standard deviations were calculated, and ANOVA statistical analysis was carried out on the data using Minitab statistical software.ff The sheared surfaces were examined visually, and bonding failure recorded as adhesive or cohesive. Surface analysis of the sheared specimens was also undertaken using an environmental scanning electron microscope to observe the interfaces of the glass ionomer and composite surfaces. Surface porosity of glass ionomer cement was studied separately on unbonded Ketac-Bond specimens using a light microscopej] and videomicrometer.flf Two groups of ten specimens each were evaluated. Specimens were constructed in ttshimadzu DSS-5000, Shimadzu Corporation, Japan. HMinitab Windows Release 10.5, Minitab Inc., USA. E3 Electro-Scan, Electro Scan, USA. III INikon Microphot V Series, Nippon KK, Japan. ~[~IColorado Video Inc., USA. Australian Dental Journal 1998;43:2.

.40 --,------------------- Specimen HoldingJig Shearing Blade.35 D = Hybrid = Microfilled Resin Composite Ii.30 C. e os.25 Cl c E en.20 'C co T = Thermocycled Phenolic Ring Polycartxlnale Ring Glassronomer Cement... I'll CIl.c ID.15 en.10 SHEAR TEST APPARATUS WITH BONDED SPECIMEN Fig. I.-Schematic representation of the apparatus constructed for shearing the bonded composite-glass ionomer cement specimens. phenolic ring moulds placed in a template, as described earlier. The glass ionomer was covered with a glass microscope slide before applying a load to compact the GIC mass. Group I consisted of Ketac-Bond specimens compacted under finger pressure (approximately 8 kgf) for 5 minutes, while specimens in Group 2 were compacted using a static load of 20 kg for 5 minutes. The specimens were completely set after 5 minutes, and allowed to bench set intact for 24 hours. The glass slides were subsequently removed and specimens were stored in distilled water until analysed. Each specimen was studied at five random sites under the light microscope. Porosities were counted within the defined area on the video micrometer, and diameters were measured. Porosity was quantified as a mean per cent of the surface area for the five sites analysed, for each of the 20 specimens studied. Means and standard deviations were calculated for each group, and ANOVA statistical analysis was carried out to determine significant differences. Results The results ofthe shearbond testing are presented in Table 1. Means and standard deviations were calculated for each test condition (N= 10) and compared for significant differences using ANOVA (u=0.05). This study has found that the bond strength for unetched non-thermocyc1ed Ketac Bond bonded to resin composites ranged from 0.04 0.21 MPa (Table 1). There was no significant difference (p<0.05) between hybrid or microfilled resin composite bonded to Ketac-Bond, and no significant difference (p<0.05) due to thermal stressing. Ketac-Bond/Visiobond/Silux Plus showed the greatest shear bond strength (0.21 MPa) of all self-cured cement combinations tested (Fig. 2). Resin-modified glass ionomer cements, Photac Fil and Vitremer, showed significantly higher shear.05 0.00 --'------'_---'---L----'--- Pertac VisiDdispers Z-100 Silux Plus Fig. 2.-Shear bond strength (MPa) of self-cure glass ionomer cement Ketac-Bond, bonded to hybrid and microfilled resin composites (n= 10). Bars show means and vertical lines represent standard deviations. No significant difference detected. bond strengths than conventional Ketac-Bond (p<0.05). Glass ionomer-resin composite combinations, Photac-FillVisiobond/Pertac and VitremerlScotchbond/Silux Plus, showed mean shear bond strengths of 1.59 and 4.92 MPa respectively (Fig. 3). There was no significant difference in adhesive shear bond strength with respect to composite type (microfilled vs hybrid), for both resin-modified glass ionomer cements. There was, however, a significant difference in bond strengths (p<0.05) between Photac-Fil and Vitremer bonded to microfilled (Visiodispers vs Silux Plus) resin 8 Ii 6 c. ~.c 5....Cl c e 4 en 'C c 3 0 ṃ.. I'll CIl 2.c en 7 =Photac-Fil 0= Vitremer o T =Thermocycied Pertac Visiodispers Z-100 SiluxPlus Fig. 3.-Shear bond strength (MPa) of resin-modified glass ionomer cements Photac-Fil and Vitrerner, bonded to hybrid and microfilled resin composites (n=io). Bars show means and vertical lines represent standard deviations. Australian Dental Journal 1998;43:2. 83

6,----------------~ 5 D = Photac-Fil II = Vitremer ;; 4 T =Thermocycled Cl c e-ii) 'g 3 o.c... III CII ii 2 e III CII :E O--l.-------"----'---'-----'--- Photac-Fil Vitremer Fig. 4.-Mean shear bond strength (MPa) of resin-modified glass ionomer cements Photac-Fil and Vitremer bonded to resin composites (n=20). Bars show means and vertical lines represent standard deviations. composites, in the non-thermocycled group (Fig. 3). There was also a significant difference (p<0.05) in the mean bond strength (N=20) between Vitremer (4.48 MPa) and Photac-Fil (0.98 MPa) in the nonthermocycled group (Fig. 4). Thermal cycling had a significant effect (p<0.05) on reducing the bond strength in the Vitremer group but no detectable effect on Photac-Fil (Fig. 3,4). Mean surface porosity (N=10) for Ketac-Bond was 28 per cent of the total surface area in Group 1 (glass ionomer cement compacted under finger pressure) with pore diameters averaging 100 11m, and only 5 per cent for that of Group 2 (glass ionomer cement compacted using a static load of 20 kg) with pore diameters ranging between 12-30 11m. The scanning electron micrograph (Fig. 5) shows the fine grain structure of the glass ionomer cement, with no apparent porosity. Surface analysis of the debonded surfaces revealed that 55.6 per cent of the specimens failed adhesively along the GIC/composite interface whereas 44.4 per cent (mostly light-cured specimens) failed cohesively with composite tags located centrally on the GIC surface (Fig. 6). Discussion The materials selected for this study are shown in Table 1. Combinations of the various materials were selected based on the same manufacturer's products. The 'sandwich' technique is a system involving a glass ionomer, an adhesive resin, and a dental composite; therefore, the manufacturers' recommendations to use their specified materials within a given system were followed in this study. 3M resin composites were only bonded to 3M glass ionomers using a 3M adhesive, and similarly for ESPE products. The only exception was bonding 3M composites to Ketac-Bond. This provided a better means for comparison of the two systems, and a clearer perspective when applied to a clinical setting. Previous studies have shown that shear bond strength of self-cure glass ionomer cement bonded to resin composite ranged between 3.6-8.0 MPa after a 30 second etching time with 37 per cent phosphoric acid.' Smith et al. 5 reported a mean bond strength of 2.7 MPa for unetched Ketac-Bond, This study has found that the bond strength for unetched non-thermocycled Ketac-Bond, a self-cure glass ionomer cement, ranged from 0.04-0.21 MPa (Table 1). Chemical bonding between self-cure glass ionomer cement, Ketac-Bond, and resin composites, as demonstrated in this study, was minimal and etching is possibly indicated.v' although it may affect the integrity of the glass ionomer cement." Resin-modified glass ionomer cements, Photac Fil and Vitremer, showed significantly higher shear bond strengths than Ketac-Bond, when bonded to the various resin composites. There was no detectable difference in bond strengths ofphotac-fil and Vitremer bonded to either hybrid or microfilled resins made by the same manufacturer. There was however, a significant difference in bond strengths between Photac-Fil and Vitremer bonded to the microfilled composites, Visiodispers and Silux Plus, respectively. Vitremer/Scotchbond/Silux Plus (3M) performed significantly better than Photac FillVisiobondlVisiodispers (ESPE). The higher bond strengths of Vitremer to resin composite could be the result of the curing process which according to the manufacturer is three-fold:*** firstly, an acid/base reaction identical to that of conventional glass ionomer cements; secondly, a light-activated free radical polymerization of methacrylate groups of the polymer and HEMA initiated by visible light; and thirdly, a water-activated redox catalyst reaction which allows the methacrylate cure to proceed in the dark. It is also possible that the composition of Vitremer lends itself to better chemical bonding with resin composites, especially those made by the same manufacturer. The clinical significance of this observation and that of the bond strength values (Table 1), would be the recommendation to use a resin-modified glass ionomer rather than a conventional one as a base for composite facings. Mount" observed the presence ofvoids and porosities in both hand and machine mixed cements and found that porosites in triturated specimens were smaller and more regular in size than those in hand mixed specimens, but there was an occasional large void present presumably incorporated during syringing. Kerby et al.' also reported that the shear bond strength of unetched Vitrebond,' the predecessor of ***Vitremer tri-cure glass ionomer system, 3M Dental Products Division, Technical Product Profile, 1992. 84 Australian Dental Journal 1998;43:2.

Fig. 5-Scanning electron micrograph showing the compacted, fine grain Vitremer glass ionomer surface with no visible porosity. X230. Vitremer, was 14.5 MPa but the unetched GIC surface was prepared with 320 and 400 grit SiC wet and dry paper. Compacting the glass ionomer mass with a 20 kg load, as discussed earlier, reduced the porosity to only 5 per cent of the surface area and provided a smoother bonding surface, thus limiting the effect of micro-retention and providing a unique indicator of the extent of chemical adhesion. The high values that previous investigators have reported for self-cure and resin-modified glass ionomer cements indicate that the majority of the 'bond' has been attributed to micro-retention either as a result of etching the glass ionomer cement or inherent irregularities, porosities and surface morphology roughness. Several mechanisms are thought to be involved in the chemical adhesive bond between resin-modified glass ionomers and resin composites. Increased availability ofunsaturated double bonds in the air inhibited layer of the resin-modified glass ionomer cements may assist in the chemical bonding to the resin bonding agent and resin composite. l Unpolymerized hydroxyethyl methacrylate (HEMA) Fig. 6-Scanning electron micrograph demonstrating resin composite tag on Vitremer glass ionomer surface. X235. Australian Dental Journal 1998;43:2. 85

on the surfaces of Photac-Fil and Vitremer increases the surface wetting capability of the bonding agent and could increase the bond strength when polymerized. J Unsaturated methacrylate pendants which are available on the polyacid chain within the polymerized resin-modified glass ionomer cement, may also form covalent bonds with the resin bonding agent. J.j Vitremer and Photac-Fil also contain modified polyacrylic acids (PAA) which polymerize to form cross linked PAA that could increase the strength of the cement and ultimately the adhesive bond strength to resin composite." If the 'sandwich' restoration is to succeed clinically, a chemical bond would be preferable to a micro-mechanical one, since an adhesive bond would be more stable over time, and less likely to separate under load. Analysis of the debonded surfaces revealed adhesive failure along the GIC/Composite interface in 55.6 per cent ofspecimens, and cohesive failure in the remaining 44.4 per cent, most of which were resin-modified ionomers. The sheared specimens that failed cohesively all exhibited resin composite tags located at the centre of the GIC surface, thus indicating that the cohesive strength of the resinmodified cements is greatly increased compared with that of the conventional cements.' This is in good correlation with the bond strength data, and indicates that a chemical bond does exist between the resin-modified cements and resin composites. The fact that the resin tags were all centrally located on the GIC mass may be the result ofinherent stress concentrations within the shear apparatus at the centre of the sheared composite ring. The cohesive strength of the composite may have been overcome only at these particular stress concentration points. Thermal stressing exhibited no significant effect on the bond strength of self-cure GIC Ketac-Bond. It did, however, cause a considerable decrease in the bond strength of Vitremer bonded to both hybrid and microfilled resin composites. This decrease in shear bond strength after thermocycling may be due to the discrepancy in the coefficients of thermal expansion of the glass ionomer and composite, and is perhaps more pronounced in the Vitremer group. Water sorption by cements and composites is known to be both time and temperature dependent. Nicholson et al. 10 found that light-cured glass ionomer cements, namely Vitrebond, increased in mass and volume after storage in water at 37 C, leading to greater plasticity and reduction in strength. They concluded this was principally caused by the presence of strongly hydrophilic functional groups in the lightly cross-linked polymer matrix formed by photochemical polymerization. The high temperature to which the specimens were subjected during thermocycling may have caused leaching of acid/base products from the ionomer mass, or altered the chemical composition or balance of the materials, thus leading to decreased bond strengths in the thermocycled group. 86 Summary Reducing the porosity of the glass ionomer cements to approximately 5 per cent of the surface area by compaction using a 20 kg static load limited micro-mechanical retention and provided a better indicator of the magnitude of chemical adhesion. Thermal stressing affected the bond strengths of resin-modified glass ionomer cements, but had no significant effect on conventional cements. The filler content of the resin composite (microfilled vs hybrid) did not affect the adhesive shear bond strength to both resin-modified and self-cured glass ionomer cements. Chemical bonding between selfcured glass ionomer cement, Ketac-Bond, and resin composite is minimal. Chemical bonding does exist between resin-modified glass ionomer cements and resin composites. Vitremer/Scotchbond/Silux Plus showed the highest adhesive shear bond strength of all combinations tested. Resin-modified glass ionomer cements showed a true adhesive bond to resin composites and are therefore recommended for use in the 'sandwich' technique, and as bases for composite restorations. Acknowledgements This project was supported by the Australian Dental Research Foundation Incorporated. Materials were kindly supplied by ESPE GmbH, Germany, and 3M Co., USA. Special thanks are due to Dr L.A. Dalton-Ecker for supporting this project. References 1. McLean JW, Powis DR, Prosser HJ, Wilson AD. The use of glass ionomer cements in bonding composite resins to dentine. Br DentJ 1985;158:410-4. 2. Li J, Liu Y, Liu Y, Soremark R, Sundstrom F. Flexure strength of resin-modified glass ionomer cements and their bond strength to dental composites. Acta Odontol Scand 1996;54:55-8. 3. Kerby RE. Knobloch L. The relative shear bond strength of visible light-curing and chemically curing glass-ionomer cement to composite resin. Quintessence Int 1992;23:641-4. 4. Wilson AD. Resin-modified glass ionomer cements. Int J Prosthodont 1990;3:425-9. 5. Smith GE, Soderholm KJM. The effect of surface morphology on the shear bond strength of glass ionomer to resin. Oper Dent 1988;13: 168-72. 6. Garcia-Godoy F, Malone W. The effect of acid etching on two glass ionomer lining cements. Quintessence Int 1986;17:621-3. 7. Rosen M, Cohen J, Becker PJ. Bond strength of glass ionomer cement to composite resin. J Dent Assoc S Afr 1991;46:511-3. 8. Papagiannoulis L, E1iades G, Lekka M. Etched glass ionomer liners: surface properties and interfacial profile with composite resin. J Oral Rehabil 1990;17:25-36. 9. Mount GJ. The tensile strength of the union between various glass ionomer cements and various composite resins. Aust Dent J 1989;34:136-46. 10. Nicholson JW, Anstice HM, McLean JW. A preliminary report on the storage in water on the properties of commercial lightcured glass-ionomer cements. Br Dent J 1992;173:98-101. Address for correspondence/reprints: Dr C. S. Farah, Department of Pathology, QEII Medical Centre, The University of Western Australia, Nedlands, Western Australia 6009. Australian Dental Journal 1998;43:2.