Shear bond strength of metal-ceramic repair systems
By Isabella Gagliardi Haneda, DDS, MSc
Renata Garcia Fonseca, DDS, MSc, PhD
Juliana Gomes Dos Santos Paes De Almeida, DDS, MSc
Carlos Alberto Dos Santos Cruz, DDS, MSc, PhD
Gelson Luis Adabo, DDS, MSc, PhD
Featured in General Dentistry, November/December 2009
Pg. 644-651
Posted on Friday, November 06, 2009
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This study evaluated the shear bond strength of different repair systems to nickel-chromium (NiCr) alloy after storage in water for either 24 hours or six months. One hundred cylindrical specimens (3 mm thick x 9 mm diameter) were fabricated using an NiCr alloy and embedded in a PVC ring, where they received one of five bonding and resin composite repair treatments (n = 20): Clearfil SE Bond/Clearfil AP-X (Group 1), Bistite II DC/Palfique (Group 2), CoJet/Z100 (Group 3), Scotchbond Multi-Purpose Plus/Z100 (Group 4; reference group), and CoJet Sand plus Scotchbond Multi-Purpose Plus/Z100 (Group 5). The specimens were stored in distilled water for 24 hours at 37°C, thermocycled for 1,000 cycles in two baths (at 5°C and 55°C) with a dwell time of 30 seconds for each bath, and stored in distilled water at 37°C for either 24 hours or six months.
At 24 hours, Group 3 presented the highest mean values (P < 0.001); at six months, Group 5 demonstrated the highest mean values (P < 0.001). At six months, Group 5 presented a significant increase in shear bond strength (P < 0.001), while Groups 1 and 3 showed a significant reduction (P < 0.001). There were no significant differences between the storage times for Group 2 (P = 0.064) or Group 4 (P = 0.490).
Received: March 5, 2009
Accepted: May 15, 2009
Metal-ceramic restorations are used commonly in daily dental practice because they combine porcelain esthetics and metal resistance, offering patients greater functional strength, even with loaded reconstructions in the molar region.1-4 Despite the excellent mechanical properties of metal-ceramic restorations, however, fractures remain an unfortunate reality in restorative dentistry.5,6 Fractures can result from trauma, inadequate occlusal adjustment, parafunctional habits, flexural fatigue of the metal substructure, incompatibility of the thermal expansion coefficient between the porcelain and the metal structure, adhesive bonding failures, inadequate tooth reduction during dental preparation, porosities in the porcelain, and inappropriate coping design.5,7-16 These failures may be categorized as simple (that is, involving only the porcelain body), mixed (involving the exposure of both metal and porcelain), or complex (with substantial metal exposure).12,17 When a fractured restoration continues to fulfill the requirements for preserving dental/periodontal health and replacement is not feasible, repair is a good treatment alternative that can increase the clinical longevity of the failed restoration.6,14
A critical aspect regarding intraoral repairs is the strength and durability of the bond between repair materials and the surface to be repaired.3,18 There are specific treatments that promote satisfactory adhesion between the repair materials and the surfaces that must be repaired. The treatment selected depends on the type of restoration failure and the surface that is to be repaired (that is, porcelain or metal).12,17 Although there are specific treatments designed for metal surfaces (such as metal primers), repairing the surface can become a clinical challenge when the metal substructure is exposed and it is necessary to bond resin to metal alloy.6,19-24
Newer, commercially available repair systems such as CoJet (3M ESPE), Bistite II DC (Tokuyama America Inc.), and Clearfil SE Bond (Kuraray Dental) are indicated for both porcelain and metal substrates. The CoJet system involves applying a silica layer by means of airborne particle abrasion to promote a mechanical and chemical bond between repair material and substrate. Bistite II DC contains MAC-10 (11-methacryloyloxundecan 1.1-dicarboxylic acid) monomers, while Clearfil SE II systems contain MDP (10-methacryloyl-oxydecryl dihydrogen phosphate) monomers, both of which have the ability to bond chemically to metal surfaces.25 However, few studies offer evidence concerning the clinical effectiveness of these materials, particularly in terms of complex failures when a large amount of metal is exposed.6,10
The oral environment also must be considered when making repairs, since temperature changes, pH variation, masticatory loads, and humidity may promote material degradation and decrease the durability of the repair.13,26,27 Evaluating the initial strength does not determine long-term success because some materials (such as silane, which is prone to hydrolytic degradation) are unstable in wet environments.13,28 According to Quaas et al, bonding systems are influenced differently by oral conditions.29
This study evaluated the shear bond strength between several metal-ceramic repair systems and nickel-chromium (NiCr) alloy and how long-term water storage affected the bond durability of these repair systems. The null hypotheses were that there was no significant difference among repair systems and that water storage did not influence the shear bond strength of the experimental groups.
Materials and methods
One hundred cylindrical specimens were fabricated (3.0 mm thickness x 9.0 mm diameter) from NiCr base metal alloy (VeraBond II, Aalba Dent, Inc.). Cylindrical patterns were fabricated from wax (Excelsior, S.S. White) using a circular metal matrix with an opening of 3.0 mm thickness x 9.0 mm in diameter. The wax pattern was placed in a silicone casting ring (OGP Produtos Odontologicos) using a prefabricated wax sprue (Pasom) and embedded in phosphate-bonded investment (Heat Shock, Polidental). Twenty minutes after investing, the mold was removed from the ring and placed in an oven that had been warmed to 800°C (EDGCON 3P, EDG). The mold remained inside the oven until the temperature reached 950°C (heating rate = 30°C/minute) and held at this temperature for 10 minutes, according to the manufacturer’s instructions.
Next, the mold was removed from the oven and cast using a centrifugal casting machine (C1, EDG), which was wound three times. The alloy was cast using an oxygen-propane flame (gas torch). The metal specimens were divested; at that point, the surfaces were cleaned using airborne particle abrasion (Basic Classic Blaster, Renfert USA Inc.) and 100 µm aluminum oxide (Bio-Art Equipamentos Odontologicos Ltda) to clean the surfaces. Using polymethyl methacrylate resin (Clas-Mold, Classico Odontological Goods Ltd.), the NiCr cylinders were embedded in a PVC ring (diameter = 2.5 mm; height = 27.0 mm). A metallographic polisher (Metaserv 2000, Buehler Ltd.) was used with silicon carbide papers (120, 220, and 320 grit) (3M ESPE) to smooth all specimen bonding surfaces. The rotation rate was 300 rpm and the surfaces were polished during 10 minutes per paper under continuous coolant water irrigation. Airborne particle abrasion was performed for 20 seconds, with pressure of 2.42 bars and a distance of 10.0 mm from the specimen surface.
The composite resin for each group was applied using a custom-made metal matrix (4.0 mm internal diameter x 2.0 mm thickness) (Fig. 2), which was placed on the surface of the specimen using the centralizing ring. The fluid resins of the adhesive systems, opacifying agents, and dual-polymerizing cement were photoactivated for 20 seconds each. The hybrid composite resin was polymerized for 60 seconds: first, for 40 seconds with the metal matrix in position and then for 20 seconds after the metal matrix was removed. The photoactivation was performed with a visible light curing unit (XL3000, 3M ESPE) at an intensity of ~650 mW/cm2 and a distance of 5.0 mm from the specimen surface. The intensity of light output from the polymerization unit was assessed with the same radiometer (DMC Equipment) prior to each use.
All specimens were stored in 37°C distilled water for 24 hours before thermocycling, which occurred between 5°C and 55°C for 1,000 cycles, with a 30-second dwell time for each temperature. After thermocycling, 10 specimens from each group were stored in 37°C distilled water for 24 hours before the shear bond strengths were determined. The remaining 10 specimens in each group were stored under the same conditions for six months.
Specimens were subjected to shear load using a testing machine (Material Test System 810, MTS Systems Corp.) with a 10 kN load cell and a crosshead speed of 0.5 mm/minute. A stainless steel unibevel chisel apparatus was used to direct a parallel shearing force as close as possible to the resin/substrate interface (Fig. 3). The value of the load (N) was divided by the bonding surface area and the results were recorded in MPa.
Each specimen was examined under a stereoscopic lens (Carl Zeiss Inc.) at 30x magnification and the images were captured by a computer program. A single calibrated observer recorded the mode of failure as either adhesive (that is, failure at the substrate-resin interface), cohesive (failure within the substrate or within the restorative material), or a combination (areas of adhesive and cohesive failure). The adhesive area was divided into quadrants and the prevailing fracture in each quadrant was observed. The fracture was classified as adhesive or cohesive whenever one of these fracture types prevailed in at least three quadrants. When the four quadrants were divided equally between adhesive and cohesive failures, the fracture was categorized as mixed.25
Data were analyzed using two-way ANOVA, with shear bond strength as the dependent variable and the repair system and the storage time as the independent factors. Post hoc comparisons were made by Tukey’s HSD test. Significance was set at α = 0.05.
Results
The results of the two-way ANOVA showed that the repair system (P < 0.001), storage time (P < 0.05), and their interactions (P < 0.001) all were significant (see Table 2).
The statistical groupings identified with Tukey’s HSD test are shown in Table 3. At 24 hours, the CoJet system (Group 3) had the highest shear bond strength and Bistite II DC (Group 2) had the lowest shear bond strength (P < 0.001). The samples from Groups 1, 4, and 5 were not statistically different (P = 0.252).
At six months, the Group 5 samples showed statistical superiority compared with the other groups (P < 0.001). There was no significant difference between Groups 3 and 4 (P = 0.687), although both were statistically superior to Groups 1 and 2 (P < 0.001). There was no significant difference between Groups 1 and 2 (P = 0.875).
Discussion
At 24 hours, Group 3 specimens demonstrated significantly higher shear bond strength than the other groups and showed a higher percentage of cohesive failure in the opaque layer. This type of failure indicates that the adhesive bond strength at the interface between the substrate and the repair material is higher than the cohesive strength of the opacifying agent.27 The high bond strength at the substrate/repair material interface observed in Group 3 may be explained by the effectiveness of the bonding mechanism, which consists of micromechanical retention and chemical bond, provided by silica coating followed by silanization. Using the same repair systems used in the present study, Santos et al compared shear bond strength to NiCr after eight days of water storage, with significantly greater bond strength when the CoJet system was used.25 No other studies concerning CoJet’s bond strength to NiCr alloy have been identified in the literature.
Ozcan et al observed a trend toward higher fracture load values when CoJet Sand was used on cobalt-chromium (CoCr) alloy, although no significant difference was found in comparison to the other evaluated surface treatments (that is, hydrofluoridric acid etching or airborne particle abrasion with aluminum oxide).6 Other studies have reported that treating the surface with CoJet Sand particles significantly increased the bond strength between composite resin and semiprecious alloys.17,22,24
In the present study, Groups 1, 4, and 5 were not significantly different and showed intermediate mean bond strength at 24 hours. The predominant type of failure in these systems was adhesive (Fig. 4). Although no significant differences were found between the three systems, each one utilizes different surface treatments. In addition to micromechanical retention, Clearfil utilizes a chemical bond between the repair material and the metal oxides of the alloy surface. The chemical bond is provided by MDP monomer on both the Alloy Primer (Kuraray Dental) and the Cesead Opaque Primer (Kuraray Dental).
The CoJet Sand-Scotchbond Multi-Purpose Plus system also utilizes a combination of mechanical and chemical bonding. The abrasion pressure embeds silica particles onto the metal surface, promoting micromechanical retention and making the surface chemically more reactive to repair material by forming bonds with the silane coupling agent.5
In the reference group (Group 4), the airborne particle abrasion with aluminum oxide provides micromechanical retention only. This retention may have been favored by the Scotchbond Multi-Purpose Plus adhesive (3M ESPE), which increased substrate wetting and, as a result, increased the contact between the opaque resin and the treated metal surface.
Santos et al reported no significant difference between the Clearfil and Scotchbond systems or between the Scotchbond and CoJet-Scotchbond systems; however, the Clearfil system demonstrated statistical superiority over the CoJet-Scotchbond system.25 These disparate results may be due to the fact that the present study did not use phosphoric acid for Group 5 and the Scotchbond Multi-Purpose Plus adhesive was applied directly on the silica-coated surface. The lack of a chemical bond in the study by Santos et al may have resulted from the application of phosphoric acid after airborne particle abrasion, which might have removed the silica from the metal.25 Other studies evaluated the Scotchbond system on NiCr alloy and reported mean bond strengths ranging from 9.94–10.2 MPa.3,11
In the present study, Group 2 demonstrated the lowest shear bond strength at 24 hours. The bond mechanism of this system involves micromechanical retention, provided by aluminum oxide airborne particle abrasion, and a chemical bond provided by the MAC-10 monomer; however, this mechanism was less effective than the treatments recommended by other systems. The reduced effectiveness of this system may be due to the lack of an intermediary component that would increase the surface wetting, improving the contact of the high-viscosity cement with the metal surface. Adhesive failure was prevalent in this system, in keeping with the results of a 2006 study by Santos et al.25 No other studies have evaluated the effectiveness of the Bistite II system for metal/ceramic repairs.
After six months, Group 5 demonstrated the highest shear bond strength (19.67 MPa), which was significantly different from the other systems. The bond provided by airborne particle abrasion with CoJet Sand consists of chemical adhesion and mechanical retention. Group 5’s high percentage of combination failures (41.2%) and cohesive failures in the opaque (35.3%) may have resulted from the effectiveness of this treatment. The mean shear bond strength of this system correlates with the mechanical strength of the opacifying agent. The statistical superiority of Group 5 compared to Groups 1 and 4 may be explained by the high mechanical strength of the opaque used in the Group 5 samples. Although the same opaque was used in the reference group, Group 4’s mode of failure was predominantly adhesive. Group 4’s bond mechanism was based solely on micromechanical retention and the bond strength at the repair material/substrate interface was lower than the opaque mechanical strength.
However, Group 3 (which received the same surface treatment as Group 5) showed a prevalence of cohesive failure in the opaque; as a result, the results for Group 3 at six months are related to the opaque mechanical strength. After long-term water storage, the opaque used in Group 5 was superior to the Sinfony opaque (3M EPSE) used in Group 3. It is likely that the degradation of the Sinfony opaque (due to water storage) promoted the decrease in bond strength for Group 3’s specimens, making this system statistically equal to the reference group after six months. No studies are available that report the bond strength of these three systems to NiCr alloy after long-term water storage.
At six months, Groups 1 and 2 showed the smallest bond strength values and were not significantly different. Samples in Group 2 reported 100% adhesive failure, indicating lower bond strength at the repair material/substrate interface, probably due to the lack of an intermediate component that would increase surface wetting.
Adhesive failures were prevalent in Group 1 (52.9%) but high numbers of cohesive (29.4%) and combination (17.7%) failures were observed as well, indicating that the opaque degradation influenced the results after six months. The fact that adhesive failures were predominant suggests that the water affected the bond between the MDP monomer and the metal oxides of the alloy surface, thus decreasing the bond strength between repair material and substrate. A 2000 study by Kelsey et al noted a significant decrease in the shear bond strength of the Clearfil system between 24 hours and 30 days of water storage. However, cohesive failures in the porcelain were prevalent, leading the authors to attribute the reduced bond strength to the progression of crack formations in the porcelain.4
In the present study, there were no significant differences between Groups 2 and 4, based on storage times. Group 5 samples showed a significant increase in shear bond strength after six months. The increase in bond strength values after long-term water storage may be explained by the improved polymer conversion in the composite materials or because hygroscopic expansion of resin composites can reduce the effects of residual stress due to polymerization shrinkage at the interface.21,30
By contrast, Groups 1 and 3 showed a significant reduction in shear bond strength after six months. Group 3 samples demonstrated prevalent cohesive failure in the opaque layer, indicating that the decrease in bond strength was due to the opaque’s hydrolytic degradation.
Adhesive failure was prevalent in Group 1 samples but the percentages of cohesive and combination failures increased after six months. Based on these results, it was determined that the reduction in shear bond strength was due to degradation in the bond between MDP monomer and the metal oxides of the alloy surface as well as the degradation of the opaque by water.
Despite the superiority of the Group 5 specimens observed in this study, the authors recommend additional in vitro studies that subject the specimens to cyclical loading and to a longer storage period in water, to simulate the oral environment more closely and achieve more reliable parameters for clinical practice.
Conclusion
Within the limitations of this study, it was concluded that among the systems tested, Bistite II DC and Clearfil SE Bond are the least appropriate choices of material when complex failure occurs and NiCr alloy is exposed, due to Bistite’s lower bond strength values during both evaluation periods and Clearfil SE Bond’s unstable behavior during the long water storage period, which reduced its strength by half. Of the materials tested, the CoJet Sand-Scotchbond combination (Group 5) was effective after both storage periods.
Disclaimer
The authors have no commercial interest in any products or manufacturers mentioned in this article.
Author information
Dr. Haneda is a postgraduate student, Department of Dental Materials and Prosthodontics, Araraquara Dental School, Sao Paulo State University (UNESP), Araraquara, SP, Brazil, where Dr. Fonseca is an assistant professor and Drs. Cruz and Adabo are associate professors. Dr. Almeida is a postgraduate student in Health Sciences, University of Brasilia, Federal District, Brazil.
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Manufacturers
Aalba Dent, Inc., Cordelia, CA; 800.227.1332, www.aalbadent.com
Bio-Art Equipamentos Odontologicos Ltda, Sao Carlos, SP, Brazil; 55.16.3371.6502, www.bioart.com.br
Buehler Ltd., Lake Bluff, IL; 800.283.4537, www.buehler.com
Carl Zeiss Inc., Thornwood, NY; 800.223.2343, www.zeiss.com
Classico Odontological Goods Ltd., Sao Paulo, SP, Brazil; 55.11.3022.2588, www.classico.com.br
DMC Equipment, Sao Carlos, SP, Brazil; 11.4432.0232
EDG, Sao Carlos, SP, Brazil; 55.16.3377.9600, www.edg.com.br
Kuraray Dental, New York, NY; 800.879.1676, www.kuraraydental.com
MTS Systems Corp., Eden Prairie, MN; 800.328.2255, www.mts.com
OGP Produtos Odontologicos, Sao Paulo, SP, Brazil; 11.6951.4773
Pasom, Sao Paulo, SP, Brazil; 55.11.2213.7016, www.pason.com.br
Polidental, Cotia, SP, Brazil; 55.11.4613.6133, www.polidental.com.br
Renfert USA Inc., St. Charles, IL; 630.762.9787, www.renfert.com
S.S. White, Rio de Janeiro, RJ, Brazil; 55.21.2122.9323
Tokuyama America Inc., Encinitas, CA; 877.378.3548, www.tokuyama-us.com
3M EPSE, St. Paul, MN; 888.364.3577, www.3mespe.com
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General Dentistry, November/December 2009 , Volume 57 , Issue 6
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