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In vitro shear bond strength of two self-adhesive resin cements to zirconia
In vitro shear bond strength of two self-adhesive resin cements to zirconia Dana M. Qeblawi, DDS, MS,a Marc Campillo-Funollet, PhD,b and Carlos A. Muñoz, DDS, MSDc School of Dental Medicine, State University of New York at Buffalo, Buffalo, NY Statement of problem. Although the use of anatomic-contour zirconia restorations has expanded in the recent past, disagreement still exists as to reliable cementation techniques and materials. Purpose. The purpose of this in vitro study was to compare the immediate and artificially aged shear bond strength of 2 commercially available self-adhesive resin cements to zirconia: one with silica coating and silanation as a zirconia surface treatment and the other contained a phosphate monomer, which eliminated the need for a separate primer. Material and methods. Sixty composite resin rods (2.5 mm in diameter and 3 mm in length) were fabricated from a nano-optimized composite resin by using a polypropylene mold, then light polymerized with a light-emitting diode. zirconia plates (10104mm) were sectioned from an yttrium-stabilized zirconia puck, sintered, and then mounted in autopolymerizing acrylic resin custom tray material. Composite resin rods were cemented to the zirconia plates with 2 different cements. The surface treatment of zirconia followed the manufacturers’ instructions for each cement. The specimens were tested for shear bond strength at 3 aging conditions: immediate, after 24 hours of moist storage, and after 30 days of moist storage with 10 000 thermocycles. Specimens were loaded to failure in a universal testing machine, and the data were analyzed with 2-way ANOVA (a¼.05). Weibull parameters (modulus and characteristic strength) also were calculated for each group. Results. Two-way ANOVA revealed that only the aging condition significantly affected the bond strength to zirconia. The cement and the interaction of the cement and aging did not significantly affect the shear bond strength to zirconia. The highest bond strength for both cements was achieved at 24 hours, whereas the lowest bond strength values were recorded in the immediate groups. Conclusions. No significant differences in bond strength to zirconia were observed between a cement with a silane priming step and an methacryloxydecyl dihydrogen phosphate-containing cement without a separate primer. Aging had a significant effect on the shear bond strength of the 2 self-adhesive resin cements to zirconia. (J Prosthet Dent 2014;-:---)
Clinical Implications A cement that contains methacryloxydecyl dihydrogen phosphate may eliminate the need for a separate primer while still achieving a durable resin bond to zirconia restorations.
Yttria partially stabilized tetragonal zirconia has been widely used as a core material for fixed restorations because of its superior mechanical properties
compared with other ceramic materials.1-4 Recent clinical reports discussed chipping of the veneering ceramic as a concern with the
performance of veneered zirconia restorations,5-8 and anatomic contour zirconia (ACZ) has been introduced to overcome that concern. The industry
Presented at the American Association of Dental Research annual meeting, Tampa, Fla, March 2012. Supported in part by Ivoclar Vivadent Inc. a
Clinical Assistant Professor, Department of Restorative Dentistry. Senior Research Scientist, Department of Restorative Dentistry. c Professor, Department of Restorative Dentistry. b
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Volume markets ACZ as a suitable restorative option for patients with parafunctional habits for which other ceramic restorative materials are contraindicated. Zirconia-based restorations are fabricated with computer-assisted design and computer-assisted manufacturing (CAD/CAM) technology, and ease of design and fabrication has rendered these restorations more appealing than metal and metal ceramic options. Recent zirconia formulations have aimed at reducing impurities and grain boundaries to enhance translucency in ACZ restorations. However, although the optical properties of zirconia have improved, zirconia’s high strength remains the main reason for its expanding use. Zirconia has a high flexural strength, in the range of 900 to 1500 MPa as reported by the manufacturers. According to the International Organization for Standardization standard for Dental Ceramics, conventional cements are suitable for luting zirconia restorations when considering zirconia’s high flexural strength.9 Although an ideal crown preparation provides adequate retention and resistance to cast and pressed restorations, milled restorations have an arbitrary cement space of approximately 100 mm, which results in less retentive restorations. Furthermore, auxiliary resistance features commonly used in metal-based restorations are not routinely used with milled restorations. A restoration with reduced retention and resistance may benefit from a luting agent that has a durable bond to the restorative material. Self-adhesive resin cements form a new cement category that falls between conventional cements and adhesive resin cements. They were introduced to provide the ease of application of conventional cements while offering some of the advantages of adhesive resin cements, as higher shear bond strength (SBS),10 compressive strength,11 and good marginal seal.12,13 Self-adhesive resin cements may be considered a reasonable cement choice for ACZ restorations if reliable adhesion to zirconia can be achieved.
Adhesion to zirconia-based restorations has been investigated in the past decade. Because zirconia lacks a silica phase, surface treatment implemented for silica-based ceramics will not achieve a reliable resin bond to zirconia.14,15 Alternatively, various surface treatments such as airborne-particle abrasion, tribochemical silica-coating, selective infiltration etching, and phosphate monomer-containing primers have been investigated for enhancing the resin bond to zirconia-based restorations.15-30 The durability of the resin bond to zirconia also has been investigated through various regimens of artificial aging. Long-term storage and thermocycling in an aqueous environment are commonly used to simulate mechanical fatigue in the wet oral environment.16-19,21 Most studies reported a reduction in resin bond strength to zirconia after artificial aging.16,18,19,21,28,29,31 The aim of this in vitro study was to compare the immediate and artificially aged SBS values of zirconia to 2 commercially available self-adhesive resin cements: one with silica-coating and silanation as a surface treatment (RelyX Unicem Automix [RU]; 3M ESPE), and one that contains a phosphate monomer, methacryloxydecyl dihydrogen phosphate (MDP), which eliminates the need for a separate primer (SpeedCem [SC]; Ivoclar Vivadent AG). The first null hypothesis stated that no significant difference would be found in the SBS of the 2 cements to zirconia. The second null hypothesis stated that artificial aging would not affect the SBS of either cement to zirconia.
MATERIAL AND METHOD A pilot study preceded this study with an effect size of 2, power of .8, and a significance level of .05 for sample size determination. Sixty composite resin rods that measured approximately 2.5 mm in diameter and 3 mm in length were fabricated from a nano-optimized composite resin material (Tetric EvoCeram; Ivoclar Vivadent AG) by using a
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polypropylene mold (Ultradent Products Inc), then light polymerized with a light-emitting diode (Bluephase G2; Ivoclar Vivadent AG) at 1200 mW/cm2 for 20 seconds. After the specimens were recovered from the mold, they were exposed to 10 minutes of additional light (400 to 580 nm) in a light furnace (Lumamat 100, P2; Ivoclar Vivadent, AG). Sixty 10104-mm plates were prepared from yttrium-stabilized zirconia ceramic pucks (DiaZir; Wieland Precision Technology) by using a lowspeed diamond saw (IsoMet 1000 Precision Saw; Buehler Ltd) with a diamond wafering blade (4-inch wafering blade, High Concentration, M412H; MetLab Corp) with water cooling. The zirconia plates were finished manually with 240-grit silicon carbide paper (Premium abrasive grinding discs, M106-240, Grit 240; MetLab Corp) to simulate the surface finish of milled restorations. The specimens then were sintered in a zirconia sintering furnace (Sintramat; Ivoclar Vivadent AG) at 1500 C for 7 hours. Each specimen was mounted in an autopolymerizing acrylic resin custom tray material (Bosworth Fastray; Bosworth Co) with a 15-hole mold (Ultradent Products Inc), with each hole measuring 25 mm in diameter and 26 mm in height. The molds were immersed in cold water during polymerization of the acrylic resin to prevent overheating of the specimens. The specimens were recovered, ground flat on both sides using a model trimmer, and divided into 2 groups (n¼30) based on surface treatment before bonding. Surface treatment followed the manufacturers’ instructions for the cements investigated. For RU, zirconia specimens were silicoated with 30-mm silica-modified aluminum oxide particles (Cojet; 3M ESPE), followed by the application of a silane coupling agent (RelyX ceramic Primer; 3M ESPE). For SC, the specimens were airborne-particle abraded with 50-mm aluminum oxide particles at 100 kPa pressure. No chemical primer was applied onto the specimens in this group.
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The bonding surface of the composite resin rods was treated with an unfilled resin (Heliobond; Ivoclar Vivadent AG) to enhance the adhesion between the cement and the rod and to avoid possible adhesive failures at that interface. The cement was dispensed onto the rod, the rod was positioned on the zirconia surface with cotton pliers, and the specimen was placed in a bonding clamp (Ultradent Products Inc) and tightened with light finger pressure. Excess cement was removed with a microbrush. Specimens from each cement group then were divided into 3 subgroups based on the aging conditions (n¼10). Group IM specimens were light polymerized for 20 seconds on both sides of the clamp (Bluephase G2) at 1200 mW/cm2 and tested immediately for SBS. Group 24h specimens were light polymerized for 20 seconds (Bluephase G2) on both sides of the clamp at 1200 mW/cm2, then stored in an incubator at 37 C and 100% humidity for 24 hours before testing. Group TC specimens were light polymerized for 20 seconds (Bluephase G2) on both sides of the clamp at 1200 mW/cm2, then stored in an incubator at 37 C and 100% humidity for 30 days. On the third day of storage, the specimens received 10 000 thermocycles31 (5 C to 55 C, 15 seconds dwell time). SBS values were collected at the end of the storage period. The SBS for the specimens was evaluated by using the notched SBS test method in a universal loading apparatus (model 33R4204; Instron Corp) set up for compression testing with a 5-kN load cell at 1 mm/ min crosshead speed. After debonding, all the specimens were examined under a light microscope (Olympus SZX12; Olympus America Inc) at 40 magnification to determine the mode of failure. Two-way ANOVA followed by the Tukey honestly significant difference test for cell means were used to analyze the data for significance (a¼.05). To calculate the Weibull parameters, the measured bond strength data for each group were ordered from lowest to highest, and the estimated probability
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3 of failure Pf for each datum was calculated as Pf ¼ i 0:5 n; where i is the ith datum and n is the total number of data points in the group. Next, the double natural logarithm of [1/(1 e Pf)] was calculated with lnln 1 1 Pf : Then lnln[1/(1 e Pf)] was regressed onto the natural logarithm of the fracture stress. The Weibull modulus was estimated as the slope of the regressed line. The characteristic strength sq was calculated as the value of stress for which lnln[1/(1 e Pf)] is zero. The stresses for 10% chance of failure and 90% chance of failure (s10 and s90) were calculated as the stresses that correspond to Pf¼.1 and Pf¼.9.
RESULTS The mean SBS values, standard deviations, and Weibull parameters for the 6 groups are listed in Table I. The normality of the results was verified by the Kolmogorov-Smirnov test (P¼.075) and the assumption of equal variances was tested by using the Levene Median test (P¼.096). No pretest failures were observed in any of the groups. The results of the statistical analysis (Table II) indicate that the cement had no significant effect on SBS (P¼.101).
Table I.
However, significant differences were detected among the 3 aging conditions within each cement (P<.001). The interaction between cement and aging condition was not significant (P¼.083). The highest SBS were achieved in the 24h groups for both cements and were significantly higher than the SBS in the IM and TC groups (P<.001). Significant differences also were detected between the IM and TC groups (P<.001), with higher SBS means for the TC groups. The characteristic strength ranged from 16.8 for RU-IM to 34.0 for RU-24h. The Weibull modulus ranged from 5.4 for SC-24h to 12.2 for RU-IM. The failure mode as assessed under light microscopy was predominantly adhesive to zirconia. All specimens from the RU-IM, SC-IM, RU-24h, SC-24h, and RU-TC groups had adhesive failures to zirconia (Fig. 1). For the SC-TC group, 60% of the specimens had adhesive failures to zirconia, and 40% exhibited a combined failure mode of adhesive to zirconia and cohesive within the cement (Fig. 2).
DISCUSSION Adhesion to tooth structure and restorative materials is among the desirable properties of dental luting agents. This study investigated the immediate and artificially aged bond strength of 2 self-adhesive resin cements to zirconia. The surface treatment of zirconia varied according to the
Shear bond strength means (SDs) and Weibull analysis
Mean (SD) sq Cement Time (MPa) (MPa) m
s10
m Range (95%)
r
s90
(MPa) (MPa)
SpeedCem
IM
20.8 2.1
21.7
11.7
9.6-13.8
0.98
17.9
23.3
Unicem
IM
16.8 3.1
16.8
12.2
10.3-14.0
0.99
14.0
18.0
SpeedCem 24h
30.3 6.6
32.9
5.4
3.7-7.1
0.93
21.7
38.4
Unicem
24h
31.9 4.6
34.0
7.4
5.5-9.2
0.96
25.0
38.1
SpeedCem TC
25.6 2.9
26.8
10.2
7.4-12.9
0.95
21.5
29.1
Unicem
22.8 3.5
24.2
7.6
6.4-8.8
0.98
18.0
27.0
TC
SD, standard deviation; sq, characteristic strength; m, Weibull modulus; s10, stress for 10% chance of failure; s90, stress for 90% chance of failure; IM, group IM (tested immediately for shear bond strength); 24h, group 24h (stored in an incubator at 37 C and 100% humidity for 24 h before testing); TC, group TC (stored in an incubator at 37 C and 100% humidity for 30 d with 10 000 thermocycles).
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Volume ANOVA for main effects of cement and aging condition, and their interaction on shear bond strength to zirconia
Table II.
Source of Variation
df
Sum of Squares
F
P
Cement
1
45.938
2.782
.101
Aging
2
1528.505
46.287
<.001
Cement aging
2
86.032
2.605
.083
Residual
54
891.607
Total
59
2552.082
1 Scanning electron microscope image of specimen, demonstrating adhesive failure to zirconia, 35 magnification.
2 Scanning electron microscope image of specimen, demonstrating combined failure mode, 35 magnification. manufacturers’ recommendation for each cement. The first null hypotheses was accepted; the cement had no significant effect on SBS values. The second null hypothesis was rejected because aging significantly affected the SBS to zirconia. The SBS means for the 2 cements were not significantly different when measured under the
same aging condition. However, significant differences existed among the 3 aging conditions within the same cement (see modulus confidence interval in Table I). The highest SBS for both cements was achieved after 24 hours of moist storage. The results of the ANOVA and Tukey test were in line with the findings from the Weibull analysis,
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which shows higher characteristic strength for the groups tested at 24 hours. The variation in the Weibull modulus indicates a larger spread of the measured bond strength at 24 hours compared with the initial values. Nevertheless, the characteristic strength at 24 hours is significantly higher than that measured initially, which revealed a stronger bond after 24 hours. Simulated aging seems to lower the bond strength compared with the values achieved at 24 hours, but both cements still maintain higher characteristic strengths at 24 hours and 30 days compared with the initial values. SpeedCem contains an adhesive monomer (MDP) that has a phosphonic acid functional group capable of forming a stable chemical bond to zirconia and to calcium ions in the tooth structure.32 The efficacy of this monomer in enhancing the durability of the resin bond strength to zirconia has been previously documented,28,29 particularly when preceded by airborne particle abrasion, as in the current study15,17-20,23-25 The use of RelyX Unicem followed the silica-coating and silanation of the zirconia surface. This treatment also has been proven successful in enhancing immediate and short-term resin bond strength to zirconia.15,17,24,26,27,29 However, disagreement exists regarding its longterm efficacy; some investigators reported a durable long-term bond,24,29 whereas other investigators reported a degradation of the bonded interface.15,17,21 The lower SBS values obtained in the IM groups may be concerning. Although this may imply that patients should avoid excessive occlusal forces on such restorations immediately after cementation, the in vitro nature of this investigation limits its clinical application in the absence of supporting clinical data. The higher bond strength values achieved after 24 hours suggest a possible maturation in the resin bond to zirconia. Results of previous studies also reported higher resin bond
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strength to various substrates after short-term storage and a low number of thermocycles,31,33,34 which may be attributed to the completion of the resin polymerization and stress relaxation in the cement after water imbibition.31 Artificial aging regimens are still considered arbitrary, with no known correlation with the duration of clinical service. Seto et al31 investigated the effect of different numbers of thermocycles on the fatigue behavior of 5 resin cements. At 10 000 thermocycles, most cements had lost at least half of their initial bond strength measured at 24 hours, and only 2 cements maintained an SBS greater than 10 MPa. In the current study, although artificial aging with 10 000 thermocycles resulted in a lower SBS than that in the 24h groups, the TC groups still maintained relatively high SBS values, which exceeded 20 MPa. The conflicting results can be attributed to differences in the materials used and the methodology. No specific target SBS value has been determined for success or failure based on laboratory bonding studies. Although satisfactory bond strength values of resin cement to high-strength ceramics are yet to be determined for clinically successful performance, a SBS value of 13 MPa has been suggested as adequate when bonding to feldspathic ceramics because higher bond values resulted in cohesive failure within the ceramic.35 The data obtained in this study can be used to compare the cements tested and other studies with similar methodology. Clinical data are needed to substantiate the findings of this in vitro study. The type of failure observed in the majority of the specimens was adhesive at the cement-zirconia interface. Combined failure modes, adhesive to zirconia and cohesive within the cement, were observed in one of the artificially aged groups (SC-TC). However, these failures were mainly adhesive at the cement-zirconia interface. Because the failures observed were predominantly adhesive to zirconia, the SBS values collected can be considered
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5 representative of the interface being investigated. Some of the limitations of this study relate to the nature of the SBS test itself.36-39 According to DeHoff et al,37 traditional shear protocols underestimate shear stresses at failure. Anunmana et al38 raised the concern that SBS tests ignore the stress distribution within the adherence zone, which can significantly affect the mode of failure. Bella Dona and Van Noort39 questioned the validity of the shear bond test for resin-ceramic bonding because of the frequency of cohesive failures in the resin or the ceramic. Bulk cohesive failures have been related to the nonuniform interfacial stress distribution during loading. Although tensile tests are advocated to eliminate nonuniform interfacial stresses, these tests are very technique sensitive. 40 Another limitation is the limited number of cements evaluated and the relatively short storage of 30 days. Long-term aging regimens and a larger variety of resin cements are future considerations.
CONCLUSIONS Within the limitation of this in vitro study, the following conclusions can be drawn. No significant differences in bond strength to zirconia were observed between a cement with a silane priming step and an MDP-containing cement without a primer. Aging condition had a significant effect on the SBS of the 2 self-adhesive resin cements to zirconia.
REFERENCES 1. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24: 299-307. 2. Conrad HJ, Seong WJ, Pesun IJ. Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98:389-404. 3. Manicone PF, Rossi Iommetti P, Raffaelli L. An overview of zirconia ceramics: basic properties and clinical applications. J Dent 2007;35:819-26. 4. Filser F, Kocher P, Weibel F, Luthy H, Scharer P, Gauckler LJ. Reliability and strength of all ceramic dental restorations fabricated by direct ceramic machining (DCM). Int J Comput Dent 2001;4:89-106.
5. Larsson C, Vult von Steyern P. Five-year follow-up of implant-supported Y-TZP and ZTA fixed dental prostheses: a randomized, prospective clinical trial comparing two different material systems. Int J Prosthodont 2010;23:555-61. 6. Raigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed SE, Billiot S, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prostheses: a prospective clinical pilot study. J Prosthet Dent 2006;6:237-44. 7. Tinschert J, Schulze KA, Natt G, Latzke P, Heussen N, Spiekermann H. Clinical behavior of zirconia-based fixed partial dentures made of DC-Zirkon: 3-year results. Int J Prosthodont 2008;21:217-22. 8. Sailer I, Gottnerb J, Kanelb S, Hammerle CH. Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. Int J Prosthodont 2009;22:553-60. 9. International Standard ISO 6872: 2008 Dental ceramics. International Organization for Standardization, Genève, Switzerland. Available at: http://www.iso.org/iso/catalogue_detail. htm?csnumber=41385. Last accessed October 14, 2014. 10. Peutzfeldt A, Sahafi A, Flury S. Bonding of restorative materials to dentin with various luting agents. Oper Dent 2011;36:266-73. 11. Piwowarczyk A, Lauer HC. Mechanical properties of luting cements after water storage. Oper Dent 2003;28:535-42. 12. Schenke F, Hiller KA, Schmalz G, Federlin M. Marginal integrity of partial ceramic crowns within dentin with different luting techniques and materials. Oper Dent 2008;33:516-25. 13. Aschenbrenner CM, Lang R, Handel G, Behr M. Analysis of marginal adaptation and sealing to enamel and dentin of four selfadhesive resin cements. Clin Oral Investig 2011;16:191-200. 14. Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-74. 15. Qeblawi DM, Muñoz CA, Brewer JD, Monaco EA Jr. The effect of zirconia surface treatment on flexural strength and shear bond strength to a resin cement. J Prosthet Dent 2010;103:210-20. 16. Phark JH, Duarte S Jr, Blatz M, Sadan A. An in vitro evaluation of the long-term resin bond to a new densely sintered high-purity zirconium-oxide ceramic surface. J Prosthet Dent 2009;101:29-38. 17. Kern M, Wegner SM. Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 1998;14:64-71. 18. Blatz MB, Sadan A, Martin J, Lang B. In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling. J Prosthet Dent 2004;91:356-62. 19. Wolfart M, Lehmann F, Wolfart S, Kern M. Durability of the resin bond strength to zirconia ceramic after using different surface conditioning methods. Dent Mater 2007;23:45-50. 20. Kern M, Barloi A, Yang B. Surface conditioning influences zirconia ceramic bonding. J Dent Res 2009;88:817-22.
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Volume 21. Valandro LF, Ozcan M, Amaral R, Leite FP, Bottino MA. Microtensile bond strength of a resin cement to silica-coated and silanized Inceram zirconia before and after aging. Int J Prosthodont 2007;20:70-2. 22. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Selective infiltration-etching technique for a strong and durable bond of resin cements to zirconia-based materials. J Prosthet Dent 2007;98:379-88. 23. Magne P, Paranhos MP, Burnett LH Jr. New zirconia primer improves bond strength of resin-based cements. Dent Mater 2010;26: 345-52. 24. Passos SP, May LG, Barca DC, Özcan M, Bottino MA, Valandro LF. Adhesive quality of self-adhesive and conventional adhesive resin cement to Y-TZP ceramic before and after aging conditions. Oper Dent 2010;35: 689-96. 25. Attia A, Lehmann F, Kern M. Influence of surface conditioning and cleaning methods on resin bonding to zirconia ceramic. Dent Mater 2011;27:207-13. 26. Lin J, Shinya A, Gomi H, Shinya A. Effect of self-adhesive resin cement and tribochemical treatment on bond strength to zirconia. Int J Oral Sci 2010;2:28-34. 27. Miragaya L, Maia LC, Sabrosa CE, De Goes MF, Da Silva EM. Evaluation of selfadhesive resin cement bond strength to yttria-stabilized zirconia ceramic (Y-TZP) using four surface treatments. J Adhes Dent 2011;13:473-80.
28. de Souza G, Hennig D, Aggarwal A, Tam LE. The use of MDP-based materials for bonding to zirconia. J Prosthet Dent 2014;112: 859-902. 29. da Silva EM, Miragaya L, Sabrosa CE, Maia LC. Stability of the bond between two resin cements and an yttria-stabilized zirconia ceramic after six months of aging in water. J Prosthet Dent 2014;112:568-75. 30. Menani LR, Farhat IA, Tiossi R, Ribeiro RF, Guastaldi AC. Effect of surface treatment on the bond strength between yttria partially stabilized zirconia ceramics and resin cement. J Prosthet Dent 2014;112:357-64. 31. Seto KB, McLaren EA, Caputo AA, White SN. Fatigue behavior of the resinous cement to zirconia bond. J Prosthodontics 2013;22: 523-8. 32. Moszner N, Salz U, Zimmermann J. Chemical aspects of self-etching enameledentin adhesives: a systematic review. Dent Mater 2005;21:895-910. 33. Piwowarczyk A, Lauer HC, Sorensen JA. In vitro shear bond strength of cementing agents to fixed prosthodontic restorative materials. J Prosthet Dent 2004;92:265-73. 34. White SN, Golshanara A. Fatigue of resin cement-base metal alloy bond strength. J Prosthodont 1996;5:253-8. 35. Özcan M, Vallittu PK. Effect of surface conditioning methods on the bond strength of luting cement to ceramics. Dent Mater 2003;19:725-31.
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36. Scherrer S, Cesar PF, Swain MV. Direct comparison of the bond strength results of the different test methods: a critical literature review. Dent Mater 2010;26:e78-93. 37. DeHoff PH, Anusavice KJ, Wang Z. Threedimensional finite element analysis of the shear bond test. Dent Mater 1995;11: 126-31. 38. Anunmana C, Anusavice KJ, Mecholsky J. Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-notch test. Dent Mater 2010;26:111-7. 39. Della Bona A, Van Noort R. Shear vs. tensile bond strength of resin composite bonded to ceramic. J Dent Res 1995;79:1591-6. 40. Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho B, et al. Relationship between surface area for adhesion and tensile bond strength: evaluation of a micro-tensile bond test. Dent Mater 1994;10:236-40. Corresponding author: Dr Dana Qeblawi State University of NY at Buffalo 225E Squire Hall Buffalo, NY 14214 E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications A cement that contains methacryloxydecyl dihydrogen phosphate may eliminate the need for a separate primer while still achieving a durable resin bond to zirconia restorations.
Yttria partially stabilized tetragonal zirconia has been widely used as a core material for fixed restorations because of its superior mechanical properties
compared with other ceramic materials.1-4 Recent clinical reports discussed chipping of the veneering ceramic as a concern with the
performance of veneered zirconia restorations,5-8 and anatomic contour zirconia (ACZ) has been introduced to overcome that concern. The industry
Presented at the American Association of Dental Research annual meeting, Tampa, Fla, March 2012. Supported in part by Ivoclar Vivadent Inc. a
Clinical Assistant Professor, Department of Restorative Dentistry. Senior Research Scientist, Department of Restorative Dentistry. c Professor, Department of Restorative Dentistry. b
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Volume markets ACZ as a suitable restorative option for patients with parafunctional habits for which other ceramic restorative materials are contraindicated. Zirconia-based restorations are fabricated with computer-assisted design and computer-assisted manufacturing (CAD/CAM) technology, and ease of design and fabrication has rendered these restorations more appealing than metal and metal ceramic options. Recent zirconia formulations have aimed at reducing impurities and grain boundaries to enhance translucency in ACZ restorations. However, although the optical properties of zirconia have improved, zirconia’s high strength remains the main reason for its expanding use. Zirconia has a high flexural strength, in the range of 900 to 1500 MPa as reported by the manufacturers. According to the International Organization for Standardization standard for Dental Ceramics, conventional cements are suitable for luting zirconia restorations when considering zirconia’s high flexural strength.9 Although an ideal crown preparation provides adequate retention and resistance to cast and pressed restorations, milled restorations have an arbitrary cement space of approximately 100 mm, which results in less retentive restorations. Furthermore, auxiliary resistance features commonly used in metal-based restorations are not routinely used with milled restorations. A restoration with reduced retention and resistance may benefit from a luting agent that has a durable bond to the restorative material. Self-adhesive resin cements form a new cement category that falls between conventional cements and adhesive resin cements. They were introduced to provide the ease of application of conventional cements while offering some of the advantages of adhesive resin cements, as higher shear bond strength (SBS),10 compressive strength,11 and good marginal seal.12,13 Self-adhesive resin cements may be considered a reasonable cement choice for ACZ restorations if reliable adhesion to zirconia can be achieved.
Adhesion to zirconia-based restorations has been investigated in the past decade. Because zirconia lacks a silica phase, surface treatment implemented for silica-based ceramics will not achieve a reliable resin bond to zirconia.14,15 Alternatively, various surface treatments such as airborne-particle abrasion, tribochemical silica-coating, selective infiltration etching, and phosphate monomer-containing primers have been investigated for enhancing the resin bond to zirconia-based restorations.15-30 The durability of the resin bond to zirconia also has been investigated through various regimens of artificial aging. Long-term storage and thermocycling in an aqueous environment are commonly used to simulate mechanical fatigue in the wet oral environment.16-19,21 Most studies reported a reduction in resin bond strength to zirconia after artificial aging.16,18,19,21,28,29,31 The aim of this in vitro study was to compare the immediate and artificially aged SBS values of zirconia to 2 commercially available self-adhesive resin cements: one with silica-coating and silanation as a surface treatment (RelyX Unicem Automix [RU]; 3M ESPE), and one that contains a phosphate monomer, methacryloxydecyl dihydrogen phosphate (MDP), which eliminates the need for a separate primer (SpeedCem [SC]; Ivoclar Vivadent AG). The first null hypothesis stated that no significant difference would be found in the SBS of the 2 cements to zirconia. The second null hypothesis stated that artificial aging would not affect the SBS of either cement to zirconia.
MATERIAL AND METHOD A pilot study preceded this study with an effect size of 2, power of .8, and a significance level of .05 for sample size determination. Sixty composite resin rods that measured approximately 2.5 mm in diameter and 3 mm in length were fabricated from a nano-optimized composite resin material (Tetric EvoCeram; Ivoclar Vivadent AG) by using a
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polypropylene mold (Ultradent Products Inc), then light polymerized with a light-emitting diode (Bluephase G2; Ivoclar Vivadent AG) at 1200 mW/cm2 for 20 seconds. After the specimens were recovered from the mold, they were exposed to 10 minutes of additional light (400 to 580 nm) in a light furnace (Lumamat 100, P2; Ivoclar Vivadent, AG). Sixty 10104-mm plates were prepared from yttrium-stabilized zirconia ceramic pucks (DiaZir; Wieland Precision Technology) by using a lowspeed diamond saw (IsoMet 1000 Precision Saw; Buehler Ltd) with a diamond wafering blade (4-inch wafering blade, High Concentration, M412H; MetLab Corp) with water cooling. The zirconia plates were finished manually with 240-grit silicon carbide paper (Premium abrasive grinding discs, M106-240, Grit 240; MetLab Corp) to simulate the surface finish of milled restorations. The specimens then were sintered in a zirconia sintering furnace (Sintramat; Ivoclar Vivadent AG) at 1500 C for 7 hours. Each specimen was mounted in an autopolymerizing acrylic resin custom tray material (Bosworth Fastray; Bosworth Co) with a 15-hole mold (Ultradent Products Inc), with each hole measuring 25 mm in diameter and 26 mm in height. The molds were immersed in cold water during polymerization of the acrylic resin to prevent overheating of the specimens. The specimens were recovered, ground flat on both sides using a model trimmer, and divided into 2 groups (n¼30) based on surface treatment before bonding. Surface treatment followed the manufacturers’ instructions for the cements investigated. For RU, zirconia specimens were silicoated with 30-mm silica-modified aluminum oxide particles (Cojet; 3M ESPE), followed by the application of a silane coupling agent (RelyX ceramic Primer; 3M ESPE). For SC, the specimens were airborne-particle abraded with 50-mm aluminum oxide particles at 100 kPa pressure. No chemical primer was applied onto the specimens in this group.
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The bonding surface of the composite resin rods was treated with an unfilled resin (Heliobond; Ivoclar Vivadent AG) to enhance the adhesion between the cement and the rod and to avoid possible adhesive failures at that interface. The cement was dispensed onto the rod, the rod was positioned on the zirconia surface with cotton pliers, and the specimen was placed in a bonding clamp (Ultradent Products Inc) and tightened with light finger pressure. Excess cement was removed with a microbrush. Specimens from each cement group then were divided into 3 subgroups based on the aging conditions (n¼10). Group IM specimens were light polymerized for 20 seconds on both sides of the clamp (Bluephase G2) at 1200 mW/cm2 and tested immediately for SBS. Group 24h specimens were light polymerized for 20 seconds (Bluephase G2) on both sides of the clamp at 1200 mW/cm2, then stored in an incubator at 37 C and 100% humidity for 24 hours before testing. Group TC specimens were light polymerized for 20 seconds (Bluephase G2) on both sides of the clamp at 1200 mW/cm2, then stored in an incubator at 37 C and 100% humidity for 30 days. On the third day of storage, the specimens received 10 000 thermocycles31 (5 C to 55 C, 15 seconds dwell time). SBS values were collected at the end of the storage period. The SBS for the specimens was evaluated by using the notched SBS test method in a universal loading apparatus (model 33R4204; Instron Corp) set up for compression testing with a 5-kN load cell at 1 mm/ min crosshead speed. After debonding, all the specimens were examined under a light microscope (Olympus SZX12; Olympus America Inc) at 40 magnification to determine the mode of failure. Two-way ANOVA followed by the Tukey honestly significant difference test for cell means were used to analyze the data for significance (a¼.05). To calculate the Weibull parameters, the measured bond strength data for each group were ordered from lowest to highest, and the estimated probability
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3 of failure Pf for each datum was calculated as Pf ¼ i 0:5 n; where i is the ith datum and n is the total number of data points in the group. Next, the double natural logarithm of [1/(1 e Pf)] was calculated with lnln 1 1 Pf : Then lnln[1/(1 e Pf)] was regressed onto the natural logarithm of the fracture stress. The Weibull modulus was estimated as the slope of the regressed line. The characteristic strength sq was calculated as the value of stress for which lnln[1/(1 e Pf)] is zero. The stresses for 10% chance of failure and 90% chance of failure (s10 and s90) were calculated as the stresses that correspond to Pf¼.1 and Pf¼.9.
RESULTS The mean SBS values, standard deviations, and Weibull parameters for the 6 groups are listed in Table I. The normality of the results was verified by the Kolmogorov-Smirnov test (P¼.075) and the assumption of equal variances was tested by using the Levene Median test (P¼.096). No pretest failures were observed in any of the groups. The results of the statistical analysis (Table II) indicate that the cement had no significant effect on SBS (P¼.101).
Table I.
However, significant differences were detected among the 3 aging conditions within each cement (P<.001). The interaction between cement and aging condition was not significant (P¼.083). The highest SBS were achieved in the 24h groups for both cements and were significantly higher than the SBS in the IM and TC groups (P<.001). Significant differences also were detected between the IM and TC groups (P<.001), with higher SBS means for the TC groups. The characteristic strength ranged from 16.8 for RU-IM to 34.0 for RU-24h. The Weibull modulus ranged from 5.4 for SC-24h to 12.2 for RU-IM. The failure mode as assessed under light microscopy was predominantly adhesive to zirconia. All specimens from the RU-IM, SC-IM, RU-24h, SC-24h, and RU-TC groups had adhesive failures to zirconia (Fig. 1). For the SC-TC group, 60% of the specimens had adhesive failures to zirconia, and 40% exhibited a combined failure mode of adhesive to zirconia and cohesive within the cement (Fig. 2).
DISCUSSION Adhesion to tooth structure and restorative materials is among the desirable properties of dental luting agents. This study investigated the immediate and artificially aged bond strength of 2 self-adhesive resin cements to zirconia. The surface treatment of zirconia varied according to the
Shear bond strength means (SDs) and Weibull analysis
Mean (SD) sq Cement Time (MPa) (MPa) m
s10
m Range (95%)
r
s90
(MPa) (MPa)
SpeedCem
IM
20.8 2.1
21.7
11.7
9.6-13.8
0.98
17.9
23.3
Unicem
IM
16.8 3.1
16.8
12.2
10.3-14.0
0.99
14.0
18.0
SpeedCem 24h
30.3 6.6
32.9
5.4
3.7-7.1
0.93
21.7
38.4
Unicem
24h
31.9 4.6
34.0
7.4
5.5-9.2
0.96
25.0
38.1
SpeedCem TC
25.6 2.9
26.8
10.2
7.4-12.9
0.95
21.5
29.1
Unicem
22.8 3.5
24.2
7.6
6.4-8.8
0.98
18.0
27.0
TC
SD, standard deviation; sq, characteristic strength; m, Weibull modulus; s10, stress for 10% chance of failure; s90, stress for 90% chance of failure; IM, group IM (tested immediately for shear bond strength); 24h, group 24h (stored in an incubator at 37 C and 100% humidity for 24 h before testing); TC, group TC (stored in an incubator at 37 C and 100% humidity for 30 d with 10 000 thermocycles).
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Volume ANOVA for main effects of cement and aging condition, and their interaction on shear bond strength to zirconia
Table II.
Source of Variation
df
Sum of Squares
F
P
Cement
1
45.938
2.782
.101
Aging
2
1528.505
46.287
<.001
Cement aging
2
86.032
2.605
.083
Residual
54
891.607
Total
59
2552.082
1 Scanning electron microscope image of specimen, demonstrating adhesive failure to zirconia, 35 magnification.
2 Scanning electron microscope image of specimen, demonstrating combined failure mode, 35 magnification. manufacturers’ recommendation for each cement. The first null hypotheses was accepted; the cement had no significant effect on SBS values. The second null hypothesis was rejected because aging significantly affected the SBS to zirconia. The SBS means for the 2 cements were not significantly different when measured under the
same aging condition. However, significant differences existed among the 3 aging conditions within the same cement (see modulus confidence interval in Table I). The highest SBS for both cements was achieved after 24 hours of moist storage. The results of the ANOVA and Tukey test were in line with the findings from the Weibull analysis,
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which shows higher characteristic strength for the groups tested at 24 hours. The variation in the Weibull modulus indicates a larger spread of the measured bond strength at 24 hours compared with the initial values. Nevertheless, the characteristic strength at 24 hours is significantly higher than that measured initially, which revealed a stronger bond after 24 hours. Simulated aging seems to lower the bond strength compared with the values achieved at 24 hours, but both cements still maintain higher characteristic strengths at 24 hours and 30 days compared with the initial values. SpeedCem contains an adhesive monomer (MDP) that has a phosphonic acid functional group capable of forming a stable chemical bond to zirconia and to calcium ions in the tooth structure.32 The efficacy of this monomer in enhancing the durability of the resin bond strength to zirconia has been previously documented,28,29 particularly when preceded by airborne particle abrasion, as in the current study15,17-20,23-25 The use of RelyX Unicem followed the silica-coating and silanation of the zirconia surface. This treatment also has been proven successful in enhancing immediate and short-term resin bond strength to zirconia.15,17,24,26,27,29 However, disagreement exists regarding its longterm efficacy; some investigators reported a durable long-term bond,24,29 whereas other investigators reported a degradation of the bonded interface.15,17,21 The lower SBS values obtained in the IM groups may be concerning. Although this may imply that patients should avoid excessive occlusal forces on such restorations immediately after cementation, the in vitro nature of this investigation limits its clinical application in the absence of supporting clinical data. The higher bond strength values achieved after 24 hours suggest a possible maturation in the resin bond to zirconia. Results of previous studies also reported higher resin bond
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strength to various substrates after short-term storage and a low number of thermocycles,31,33,34 which may be attributed to the completion of the resin polymerization and stress relaxation in the cement after water imbibition.31 Artificial aging regimens are still considered arbitrary, with no known correlation with the duration of clinical service. Seto et al31 investigated the effect of different numbers of thermocycles on the fatigue behavior of 5 resin cements. At 10 000 thermocycles, most cements had lost at least half of their initial bond strength measured at 24 hours, and only 2 cements maintained an SBS greater than 10 MPa. In the current study, although artificial aging with 10 000 thermocycles resulted in a lower SBS than that in the 24h groups, the TC groups still maintained relatively high SBS values, which exceeded 20 MPa. The conflicting results can be attributed to differences in the materials used and the methodology. No specific target SBS value has been determined for success or failure based on laboratory bonding studies. Although satisfactory bond strength values of resin cement to high-strength ceramics are yet to be determined for clinically successful performance, a SBS value of 13 MPa has been suggested as adequate when bonding to feldspathic ceramics because higher bond values resulted in cohesive failure within the ceramic.35 The data obtained in this study can be used to compare the cements tested and other studies with similar methodology. Clinical data are needed to substantiate the findings of this in vitro study. The type of failure observed in the majority of the specimens was adhesive at the cement-zirconia interface. Combined failure modes, adhesive to zirconia and cohesive within the cement, were observed in one of the artificially aged groups (SC-TC). However, these failures were mainly adhesive at the cement-zirconia interface. Because the failures observed were predominantly adhesive to zirconia, the SBS values collected can be considered
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5 representative of the interface being investigated. Some of the limitations of this study relate to the nature of the SBS test itself.36-39 According to DeHoff et al,37 traditional shear protocols underestimate shear stresses at failure. Anunmana et al38 raised the concern that SBS tests ignore the stress distribution within the adherence zone, which can significantly affect the mode of failure. Bella Dona and Van Noort39 questioned the validity of the shear bond test for resin-ceramic bonding because of the frequency of cohesive failures in the resin or the ceramic. Bulk cohesive failures have been related to the nonuniform interfacial stress distribution during loading. Although tensile tests are advocated to eliminate nonuniform interfacial stresses, these tests are very technique sensitive. 40 Another limitation is the limited number of cements evaluated and the relatively short storage of 30 days. Long-term aging regimens and a larger variety of resin cements are future considerations.
CONCLUSIONS Within the limitation of this in vitro study, the following conclusions can be drawn. No significant differences in bond strength to zirconia were observed between a cement with a silane priming step and an MDP-containing cement without a primer. Aging condition had a significant effect on the SBS of the 2 self-adhesive resin cements to zirconia.
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Volume 21. Valandro LF, Ozcan M, Amaral R, Leite FP, Bottino MA. Microtensile bond strength of a resin cement to silica-coated and silanized Inceram zirconia before and after aging. Int J Prosthodont 2007;20:70-2. 22. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Selective infiltration-etching technique for a strong and durable bond of resin cements to zirconia-based materials. J Prosthet Dent 2007;98:379-88. 23. Magne P, Paranhos MP, Burnett LH Jr. New zirconia primer improves bond strength of resin-based cements. Dent Mater 2010;26: 345-52. 24. Passos SP, May LG, Barca DC, Özcan M, Bottino MA, Valandro LF. Adhesive quality of self-adhesive and conventional adhesive resin cement to Y-TZP ceramic before and after aging conditions. Oper Dent 2010;35: 689-96. 25. Attia A, Lehmann F, Kern M. Influence of surface conditioning and cleaning methods on resin bonding to zirconia ceramic. Dent Mater 2011;27:207-13. 26. Lin J, Shinya A, Gomi H, Shinya A. Effect of self-adhesive resin cement and tribochemical treatment on bond strength to zirconia. Int J Oral Sci 2010;2:28-34. 27. Miragaya L, Maia LC, Sabrosa CE, De Goes MF, Da Silva EM. Evaluation of selfadhesive resin cement bond strength to yttria-stabilized zirconia ceramic (Y-TZP) using four surface treatments. J Adhes Dent 2011;13:473-80.
28. de Souza G, Hennig D, Aggarwal A, Tam LE. The use of MDP-based materials for bonding to zirconia. J Prosthet Dent 2014;112: 859-902. 29. da Silva EM, Miragaya L, Sabrosa CE, Maia LC. Stability of the bond between two resin cements and an yttria-stabilized zirconia ceramic after six months of aging in water. J Prosthet Dent 2014;112:568-75. 30. Menani LR, Farhat IA, Tiossi R, Ribeiro RF, Guastaldi AC. Effect of surface treatment on the bond strength between yttria partially stabilized zirconia ceramics and resin cement. J Prosthet Dent 2014;112:357-64. 31. Seto KB, McLaren EA, Caputo AA, White SN. Fatigue behavior of the resinous cement to zirconia bond. J Prosthodontics 2013;22: 523-8. 32. Moszner N, Salz U, Zimmermann J. Chemical aspects of self-etching enameledentin adhesives: a systematic review. Dent Mater 2005;21:895-910. 33. Piwowarczyk A, Lauer HC, Sorensen JA. In vitro shear bond strength of cementing agents to fixed prosthodontic restorative materials. J Prosthet Dent 2004;92:265-73. 34. White SN, Golshanara A. Fatigue of resin cement-base metal alloy bond strength. J Prosthodont 1996;5:253-8. 35. Özcan M, Vallittu PK. Effect of surface conditioning methods on the bond strength of luting cement to ceramics. Dent Mater 2003;19:725-31.
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36. Scherrer S, Cesar PF, Swain MV. Direct comparison of the bond strength results of the different test methods: a critical literature review. Dent Mater 2010;26:e78-93. 37. DeHoff PH, Anusavice KJ, Wang Z. Threedimensional finite element analysis of the shear bond test. Dent Mater 1995;11: 126-31. 38. Anunmana C, Anusavice KJ, Mecholsky J. Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-notch test. Dent Mater 2010;26:111-7. 39. Della Bona A, Van Noort R. Shear vs. tensile bond strength of resin composite bonded to ceramic. J Dent Res 1995;79:1591-6. 40. Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho B, et al. Relationship between surface area for adhesion and tensile bond strength: evaluation of a micro-tensile bond test. Dent Mater 1994;10:236-40. Corresponding author: Dr Dana Qeblawi State University of NY at Buffalo 225E Squire Hall Buffalo, NY 14214 E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
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