- Email: [email protected]
Shear peel bond strength of compomers veneered to amalgam
Shear peel bond strength of compomers veneered to amalgam Yousra Hussein Al-Jazairya College of Dentistry, King Saud University, Riyadh, Saudi Arabia Statement of problem. The practice of veneering compomers to amalgam restorations has not been studied.
Purpose. This in vitro study was designed to assess the shear peel bond strength and fracture pattern of 3 currently available compomers veneered to amalgam. Material and methods. Sixty cylindrical preparations were filled with amalgam. Half (30) had no surface treatment, whereas the other half were air abraded. In both groups, 10 specimens each were veneered with Dyract AP, Hytac, and F2000 according to the manufacturers’ instructions. All samples were kept at 37°C in 100% relative humidity for 48 hours. SPBS was assessed with a universal testing machine and fracture patterns with a stereomicroscope. The results were analyzed with 2-way ANOVA and Tukey’s multiple comparison test. Results. Dyract AP veneered to air-abraded amalgam had the highest SPBS (5.78 ± 1.30 MPa); F2000 veneered to non–air-abraded amalgam had the lowest (2.99 ± 1.4 MPa). Sandblasting significantly influenced SPBS in the case of Hytac (P<.02) and F2000 (P<.01). Within the non–air-abraded group, Dyract AP had significantly higher SPBS than Hytac (P<.03) and F2000 (P<.015). F2000 air-abraded specimens exhibited adhesive bond failure only, whereas all other groups showed both adhesive and combined bond failures. Conclusion. Of the 3 compomers tested for veneering to amalgam, Dyract AP showed the highest SPBS. Air-abrading the amalgam surface was found to improve the SPBS of Dyract AP, though not significantly. Low SPBS and poor adhesion indicated that F2000 is unsuitable for veneering amalgam. (J Prosthet Dent 2001;85:396-400.)
CLINICAL IMPLICATIONS Of the 3 compomer materials tested in this in vitro study, Dyract AP showed the highest bond strength and appeared to be the material of choice for veneering amalgam restorations.
F
or more than a century, dental amalgam has remained a restorative material with superior handling and physicochemical properties. However, in some visible areas of the mouth, such as the buccal aspect of maxillary premolars, an amalgam restoration can pose an esthetic problem. Composites, prepared directly and indirectly, provide an alternative restoration for posterior teeth. Significant improvements have been made in their physical properties since their introduction.1 However, bonding to dentin in the cervical regions, moisture control, and the creation of a contact point still pose problems. Other tooth-colored restorative materials, such as porcelain and ceramic materials for the fabrication of inlays and onlays, have been introduced. However, their routine use has been discouraged by the need for additional removal of tooth structure, sophisticated equipment, and techniques as well as a dramatic increase in equipment costs.
aLecturer,
Department of Restorative Dentistry.
396 THE JOURNAL OF PROSTHETIC DENTISTRY
Concurrent with improvements in composites, new types of restorative materials have been developed that combine the chemistry of both glass ionomer cement and light-polymerized resin composite.2 These new materials include polyacid-modified resin composites (compomers) and resin-modified glass ionomers. In the latter, the esthetic qualities and toughness of conventional glass ionomer cements have been enhanced and their favorable physical properties retained. Compomers have gained wide acceptance among practitioners because of their handling properties, esthetics, ability to chemically bond to tooth structure, fluoride release, and reasonable cost.3,4 Despite the introduction of these new materials, dental amalgam remains the material of choice in certain situations, especially for badly broken-down posterior teeth. Recently, veneering amalgam restorations with esthetic materials have been attempted to solve the problem of esthetics. Although this concept is not new to restorative dentistry, no study has reported on veneering compomers to amalgam restorations. VOLUME 85 NUMBER 4
AL-JAZAIRY
Liatukas5 was the first to describe the procedure of masking the metallic color of an existing amalgam restoration with the use of silicate cement. Thereafter, other clinical studies6-11 described different techniques for veneering amalgam restorations with the use of composite resins. Salama and el-Mallakh12 developed a technique of producing esthetic stainless steel crowns by veneering a compomer material to metallic crowns. Air abrasion has been used in dentistry to modify the metal surface. Several studies12-15 have shown that it increases the surface roughness of most metals, including dental amalgams, and enlarges the surface area for bonding resin to metal. If the durability of dental amalgam could be combined with the esthetic appearance of composites and the fluoride release of glass ionomers, the result would not only maintain the durability of amalgam restorations but also achieve esthetics. The purpose of this study was to assess, in vitro, the shear peel bond strength (SPBS) and fracture pattern of some currently available compomers veneered to amalgam surfaces.
MATERIAL AND METHODS Sixty Teflon circular blocks (E. I. du Pont de Nemours and Co, Wilmington, Del.) were used. Cylindrical preparations 4 × 4 mm were made at their centers. Dispersalloy amalgam (Johnson & Johnson, East Windsor, N.J.) was condensed and carved with a Hollenbeck carver flush with the Teflon surface. The samples were stored in saline solution at room temperature for 5 days, then divided randomly into 2 groups of 30 each. The first group with no treatment of amalgam surfaces served as the control. The second group received surface treatment by texturing amalgam surfaces with 50 µm aluminum oxide particles with the use of an intraoral Microetcher Modler Precision Sandblaster (Danville Engineering Inc, Danville, Calif.). Microetching was carried out for 10 seconds vertically against the amalgam surfaces at 10 mm distance, by using a continuous circular motion under 80 psi air pressure. Thereafter, the samples were rinsed for 10 seconds and air-dried for 5 seconds. Each group then was subdivided into 3 sets of 10 samples each to receive the 3 restorative materials. Each compomer was packed directly on the amalgam surface in a split Teflon mold (2 × 4 mm) supported by a snugly fitting Teflon cylinder. The packing of each compomer was according to the manufacturer’s instructions. To standardize the curing distance, the tip of the polymerization unit was applied in contact with the surface of the plastic split mold. Verification of the unit light intensity output was checked for every 10 samples before polymerizing them with the digital read-out light meter available with the unit. The first set of 10 non–air-abraded amalgam samples was veneered with Dyract AP (DeTrey, Dentsply, APRIL 2001
THE JOURNAL OF PROSTHETIC DENTISTRY
Konstanz, Germany). A thin layer of adhesion promoting monomer of Dyract PSA primer/adhesive was brushed on the air-abraded amalgam surface, left undisturbed for 30 seconds, air-dried for 5 seconds, and then light polymerized for 10 seconds. A second layer of PSA primer/adhesive was applied in the same manner. Dyract AP material then was packed in the split mold in 2 increments and light polymerized for 40 seconds with the use of a Cotolus light-polymerizing unit (Coltene, Alstatten, Switzerland) with an 8-mm curing tip. Additional polymerizing was performed for 40 seconds after removal of the mold. The next 10 non–air-abraded amalgam samples were veneered with Hytac (ESPE, GmbH Seefeld, Germany). Hytac OSB primer/adhesive was applied with a brush on the amalgam surface and rubbed for 30 seconds, then air-dried for 5 seconds and light polymerized for 10 seconds. The second layer of the primer/adhesive was applied in the same manner but without rubbing. Hytac material was packed into the mold in 2 increments and polymerized for 40 seconds before and after removal of the mold. The remaining set of 10 non–air-abraded amalgam samples was veneered with F2000 (3M Dental Products, St. Paul, Minn.). A single thin layer of F2000 primer/adhesive was brushed on the amalgam surface, left undisturbed for 30 seconds, air-dried for 10 seconds, and then light polymerized for 10 seconds. F2000 material was packed into the mold in 2 increments and light polymerized for 40 seconds before and after the removal of the mold. The 3 sets of air-abraded amalgam samples were veneered with Dyract AP, Hytac, and F2000 compomer materials according to the previously described procedures. After their fabrication, all amalgam-compomer bonded samples were kept in 100% relative humidity at 37°C in a Memmert Universal Oven (Memmert GmbH, Schwabach, Germany).
Shear peel testing After 48 hours, each bonded sample was fitted into a specially constructed jig (Fig. 1) and subjected to shear peel testing with an Instron 8500 Digital Control Universal Testing machine (Instron Corp, Canton, Mass.) at a crosshead speed of 0.5 mm/min. A shear peel force test was used in this study because it has been found to be a severe test of adhesion, which is most likely to correlate with clinical performance.16,17 The bond strength at failure was calculated as the recorded failure load divided by the surface area of the bonded surface (12.6 mm2) and expressed in megapascals (MPa). After the SPBS test, the amalgam-compomer interfaces were examined twice with a Polyvar Reichert-Jung stereomicroscope (Leica Reichert, Wien, Austria) at ×32 397
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AL-JAZAIRY
Fig. 1. Shear peel test apparatus.
Table I. Shear peel bond strength and frequency of bond failure of compomers veneered to amalgam Bond failure Amalgam groups
No. of samples
Non–air-abraded Dyract AP Hytac F2000 Air-abraded Dyract AP Hytac F2000
Bond strength (MPa*) mean ± SD
Adhesive
Cohesive
Combined
10 10 10
5.09 ± 1.60 3.21 ± 1.63 2.99 ± 1.40
6 6 3
0 0 0
4 4 7
10 10 10
5.78 ± 1.30 4.72 ± 1.57 4.72 ± 1.53
7 6 10
0 0 0
3 4 0
MPa = Mega Newton/m2.
magnification to determine the fracture modes. The bond failure was recorded as adhesive (no signs of amalgam fracture or compomer remnants on the amalgam surface), cohesive (exhibiting complete fracture of either amalgam or compomer), or combined (showing both adhesive and cohesive failures).18
Statistical analysis A 2-way analysis of variance (ANOVA) test was performed to analyze the differences between the air-abraded and non–air-abraded amalgam groups. Tukey’s multiple comparison test was used to determine the statistical significance of the differences within each tested group.
RESULTS The means and standard deviations (SD) of the SPBS in MPa are presented in Table I. Two-way ANOVA showed a highly significant difference 398
(P<.002) between SPBS of compomers veneered to air-abraded and non–air-abraded amalgam surfaces. The highest SPBS (5.78 ± 1.30 MPa) was recorded for Dyract AP bonded to air-abraded amalgam surfaces; the lowest (2.99 ± 1.4 MPa) was for F2000 bonded to non–air-abraded amalgam surfaces. Tukey’s multiple comparisons on the non– air-abraded groups revealed statistically significant differences between the SPBS of Dyract AP compared to Hytac (P<.03) and F2000 (P<.015); there was no statistically significant dif ference between the SPBS of Hytac and F2000 (P>.94). In the air-abraded group, there was no significant difference between the SPBS of the compomers (P>.25). Each of the 3 compomers showed a higher SPBS when veneered to an air-abraded amalgam surface than to a non–air-abraded amalgam surface. This difference was not statistically significant (P>.1) for VOLUME 85 NUMBER 4
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Dyract AP. However, Hytac bonded to air-abraded and non–air-abraded amalgam surfaces a showed statistically significant difference (P<.02) in SPBS, as did F2000 (P<.01). Stereomicroscopic examination revealed adhesive and combined bond failures (Table I). For the F2000 air-abraded amalgam group, only adhesive bond failure was observed in all the specimens with no breakage or chipping of compomer material. In the remaining groups, all specimens showed adhesive and combined bond failures.
DISCUSSION If sufficient bond strength could be achieved between compomer materials and dental amalgam, then compomers, with their advantages of fluoride release, ease of application, and reasonable cost, would be a better choice for veneering than composites. Surface morphology can affect attachment by improving the initial interaction, offering an increased surface area, and facilitating mechanical interlocking of the adhesive.15 It is therefore expected that air abrading an amalgam surface before veneering with compomer would lead to an improved attachment. This investigation tested the possibility of successfully bonding compomers to amalgam surfaces as well as the impact of air abrasion on the bond. McConnell et al14 found aluminum oxide with a particle size of 50 µm to be the most suitable abrasive agent for metal treatment. The current study used this approach for texturing the amalgam surface. It was found that, with air abrasion, the SPBS of the 3 compomers tested improved. This surface treatment significantly enhanced the SPBS for Hytac (P<.02) and F2000 (P<.01) (Table I). In the case of Dyract AP, although the SPBS increased with air abrasion, this effect was not statistically significant (P>.1). Improvement in bond strength after air abrasion was reported in an earlier study. Nahass15 evaluated the shear bond strength (SBS) of composite bonded to different amalgam surface treatments; she reported higher bond strengths when air abrasion was used in addition to carbide or diamond burs than when carbide or diamond burs were used alone. However, no study in the literature has reported the effect of air abrasion on the bond strengths of compomers veneered to amalgam. El-Kalla and Garcia-Godoy19 reported Dyract to have a significantly higher SBS than Hytac and Compoglass when bonded to primary and permanent teeth. In the current study, Dyract AP veneered to an air-abraded amalgam surface was found to have the highest SPBS (5.78 ± 1.30 MPa) among the 3 compomers tested (Table I). This value was lower than that of the SBS (9.52 ± 2.46 MPa) reported by APRIL 2001
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Salama and el-Mallakh12 for Dyract AP bonded to air-abraded stainless steel crowns. This variance may be due to the difference in materials veneered to, the type of bond strength evaluated, and the method of testing. In the non–air-abraded group, Dyract AP had a significantly higher SPBS than Hytac and F2000. The differences noticed between the SPBS of the 3 compomers could be attributed partly to the ability of the bonding agent used with each system to adhere to the amalgam surface and partly to differences in the properties of the compomer materials. The bond between compomers and amalgam is mainly micromechanical; some chemical bonding also may occur between the oxide layer on the air-abraded amalgam surface and the polycarboxylate and phosphate groups in compomer materials. Such adhesion can be attributed to the formation of ionic bonds with metal oxides or with the active metal compounds of the amalgam.20 In the current study, the failure pattern of F2000 veneered to the air-abraded amalgam surface was completely adhesive; the specimen separated at the compomer-amalgam interface. This, along with its low SPBS, makes F2000 unsuitable for veneering amalgams. On the other hand, Dyract AP and Hytac showed a combined failure pattern, with many compomer tags on the air-abraded surfaces mechanically interlocked to the amalgam surface. To confirm the superiority of Dyract AP as the compomer of choice, in vivo studies on the performance of compomers when veneered to amalgam are needed. Some clinical studies8,11 on bonding composites to amalgam restorations used an opaque light-polymerized glass-ionomer liner as an intermediary. One of the advantages of this liner is its ability to bond to set amalgam and mask its unesthetic color. Further research should focus on assessing the esthetic quality of compomers veneered to dental amalgam, the need for an intermediate opaquer or liner, and the optimal thickness of that liner.
CONCLUSIONS Of the 3 compomer materials tested in this in vitro study for veneering to amalgam, Dyract AP showed the highest SPBS, followed by Hytac. Air abrading the amalgam surface did not significantly improve the bond strength of Dyract AP. However, in the case of Hytac and F2000, it produced a significant increase in SPBS. All Dyract AP and Hytac specimens exhibited adhesive and combined fracture patterns. F2000 specimens veneered to air-abraded amalgam surface showed only adhesive bond failure. Its poor adhesion to amalgam after air abrasion, along with its low SPBS, makes F2000 unsuitable for veneering amalgam. 399
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I thank Dr W. Awliya and Professors E. S. Akpata and H. Al Tahawi of the King Saud University at Riyadh for their valuable suggestions.
REFERENCES 1. Wilder AD, Bayne SC, Heymann HO. Long term clinical performance of direct posterior composites. Academy of dental materials proceedings of conference on clinically appropriate alternatives to amalgam: biophysical factors in restorative decision-making. Acad Dent Mater Proc 1996;9:151-69. 2. McLean JW, Nicholson JW, Wilson AD. Proposed nomenclature for glass-ionomer dental cements and related materials. Quintessence Int 1994;25:587-9. 3. Blackwell G, Kase R. Technical characteristics of light curing glassionomers and compomers. Academy of dental materials proceedings of conference on clinically appropriate alternatives to amalgam: biophysical factors in restorative decision-making. Acad Dent Mater Proc 1996;9:77-88. 4. Marks LA, Weerheijm KL, van Amerongen WE, Groen HJ, Martens LC. Dyract versus Tytin Class II restorations in primary molars: 36 months evaluation. Caries Res 1999;33:387-92. 5. Liatukas EL. Amalgam restorations with silicate cement facings for anterior teeth. J Prosthet Dent 1970;23:560-1. 6. Lambert RL, Scrabeck JG, Robinson FB. Esthetic composite resin facings for amalgam restorations. Gen Dent 1983;31:222-4. 7. Gordon M, Laufer BZ, Metzger Z. Composite-veneered amalgam restorations. J Prosthet Dent 1985;54:759-62. 8. Quiroz L, Swift EJ Jr. A technique for esthetic veneering of amalgam. Compend Contin Educ Dent 1986;7:350, 352-4. 9. Cooley RL, McCourt JW, Train TE. Bond strength of resin to amalgam as affected by surface finish. Quintessence Int 1989;20:237-9. 10. Cardash HS, Bichacho N, Imber S, Liberman R. A combined amalgam and composite resin restoration. J Prosthet Dent 1990;63:502-5. 11. Plasmans PJ, Reukers EA. Esthetic veneering of amalgam restorations with composite resin—combining the best of both worlds? Oper Dent 1993;18:66-71.
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12. Salama FS, el-Mallakh BF. An in vitro comparison of four surface preparation techniques for veneering a compomer to stainless steel. Pediatr Dent 1997;19:267-72. 13. Lubow RM, Cooley RL. Effect of air-powder abrasive instrument on restorative materials. J Prosthet Dent 1986;55:462-5. 14. McConnell RJ, Taylor D, Moriarity K. Bonding strengths of resin to noble and base metal alloys. J Dent Res 1989;69:955-60. 15. Nahass MA. Evaluation of shear bond strength and microleakage between amalgam and veneering composite resin. [MDS thesis.] Riyadh: King Saud University; 1997. p. 200. 16. Tavas MA, Watts DC. Bonding of orthodontic brackets by transillumination of a light activated composite: an in vitro study. Br J Orthod 1979;6:207-8. 17. Harris AM, Joseph VP, Rossouw PE. Shear peel bond strengths of esthetic orthodontic brackets. Am J Orthod Dentofacial Orthop 1992;102:215-9. 18. Chappell RP, Eick JD, Mixson JM, Theisen FC. Shear bond strength and scanning electron microscope observation of four dentinal adhesives. Quintessence Int 1990;21:303-10. 19. el-Kalla IH, Garcia-Godoy F. Bond strength and interfacial micromorphology of compomers in primary and permanent teeth. Int J Paediatr Dent 1998;8:103-14. 20. McConnell RJ. Metal resin bonding. J Calif Dent 1994;40:2-6. Reprint requests to: DR YOUSRA HUSSEIN AL-JAZAIRY COLLEGE OF DENTISTRY KING SAUD UNIVERSITY PO BOX 58461 RIYADH 11594 SAUDI ARABIA FAX: (966)1-252-0095 E-MAIL: [email protected] Copyright © 2001 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2001/$35.00 + 0. 10/1/114824 doi:10.1067/mpr.2001.114824
VOLUME 85 NUMBER 4
Purpose. This in vitro study was designed to assess the shear peel bond strength and fracture pattern of 3 currently available compomers veneered to amalgam. Material and methods. Sixty cylindrical preparations were filled with amalgam. Half (30) had no surface treatment, whereas the other half were air abraded. In both groups, 10 specimens each were veneered with Dyract AP, Hytac, and F2000 according to the manufacturers’ instructions. All samples were kept at 37°C in 100% relative humidity for 48 hours. SPBS was assessed with a universal testing machine and fracture patterns with a stereomicroscope. The results were analyzed with 2-way ANOVA and Tukey’s multiple comparison test. Results. Dyract AP veneered to air-abraded amalgam had the highest SPBS (5.78 ± 1.30 MPa); F2000 veneered to non–air-abraded amalgam had the lowest (2.99 ± 1.4 MPa). Sandblasting significantly influenced SPBS in the case of Hytac (P<.02) and F2000 (P<.01). Within the non–air-abraded group, Dyract AP had significantly higher SPBS than Hytac (P<.03) and F2000 (P<.015). F2000 air-abraded specimens exhibited adhesive bond failure only, whereas all other groups showed both adhesive and combined bond failures. Conclusion. Of the 3 compomers tested for veneering to amalgam, Dyract AP showed the highest SPBS. Air-abrading the amalgam surface was found to improve the SPBS of Dyract AP, though not significantly. Low SPBS and poor adhesion indicated that F2000 is unsuitable for veneering amalgam. (J Prosthet Dent 2001;85:396-400.)
CLINICAL IMPLICATIONS Of the 3 compomer materials tested in this in vitro study, Dyract AP showed the highest bond strength and appeared to be the material of choice for veneering amalgam restorations.
F
or more than a century, dental amalgam has remained a restorative material with superior handling and physicochemical properties. However, in some visible areas of the mouth, such as the buccal aspect of maxillary premolars, an amalgam restoration can pose an esthetic problem. Composites, prepared directly and indirectly, provide an alternative restoration for posterior teeth. Significant improvements have been made in their physical properties since their introduction.1 However, bonding to dentin in the cervical regions, moisture control, and the creation of a contact point still pose problems. Other tooth-colored restorative materials, such as porcelain and ceramic materials for the fabrication of inlays and onlays, have been introduced. However, their routine use has been discouraged by the need for additional removal of tooth structure, sophisticated equipment, and techniques as well as a dramatic increase in equipment costs.
aLecturer,
Department of Restorative Dentistry.
396 THE JOURNAL OF PROSTHETIC DENTISTRY
Concurrent with improvements in composites, new types of restorative materials have been developed that combine the chemistry of both glass ionomer cement and light-polymerized resin composite.2 These new materials include polyacid-modified resin composites (compomers) and resin-modified glass ionomers. In the latter, the esthetic qualities and toughness of conventional glass ionomer cements have been enhanced and their favorable physical properties retained. Compomers have gained wide acceptance among practitioners because of their handling properties, esthetics, ability to chemically bond to tooth structure, fluoride release, and reasonable cost.3,4 Despite the introduction of these new materials, dental amalgam remains the material of choice in certain situations, especially for badly broken-down posterior teeth. Recently, veneering amalgam restorations with esthetic materials have been attempted to solve the problem of esthetics. Although this concept is not new to restorative dentistry, no study has reported on veneering compomers to amalgam restorations. VOLUME 85 NUMBER 4
AL-JAZAIRY
Liatukas5 was the first to describe the procedure of masking the metallic color of an existing amalgam restoration with the use of silicate cement. Thereafter, other clinical studies6-11 described different techniques for veneering amalgam restorations with the use of composite resins. Salama and el-Mallakh12 developed a technique of producing esthetic stainless steel crowns by veneering a compomer material to metallic crowns. Air abrasion has been used in dentistry to modify the metal surface. Several studies12-15 have shown that it increases the surface roughness of most metals, including dental amalgams, and enlarges the surface area for bonding resin to metal. If the durability of dental amalgam could be combined with the esthetic appearance of composites and the fluoride release of glass ionomers, the result would not only maintain the durability of amalgam restorations but also achieve esthetics. The purpose of this study was to assess, in vitro, the shear peel bond strength (SPBS) and fracture pattern of some currently available compomers veneered to amalgam surfaces.
MATERIAL AND METHODS Sixty Teflon circular blocks (E. I. du Pont de Nemours and Co, Wilmington, Del.) were used. Cylindrical preparations 4 × 4 mm were made at their centers. Dispersalloy amalgam (Johnson & Johnson, East Windsor, N.J.) was condensed and carved with a Hollenbeck carver flush with the Teflon surface. The samples were stored in saline solution at room temperature for 5 days, then divided randomly into 2 groups of 30 each. The first group with no treatment of amalgam surfaces served as the control. The second group received surface treatment by texturing amalgam surfaces with 50 µm aluminum oxide particles with the use of an intraoral Microetcher Modler Precision Sandblaster (Danville Engineering Inc, Danville, Calif.). Microetching was carried out for 10 seconds vertically against the amalgam surfaces at 10 mm distance, by using a continuous circular motion under 80 psi air pressure. Thereafter, the samples were rinsed for 10 seconds and air-dried for 5 seconds. Each group then was subdivided into 3 sets of 10 samples each to receive the 3 restorative materials. Each compomer was packed directly on the amalgam surface in a split Teflon mold (2 × 4 mm) supported by a snugly fitting Teflon cylinder. The packing of each compomer was according to the manufacturer’s instructions. To standardize the curing distance, the tip of the polymerization unit was applied in contact with the surface of the plastic split mold. Verification of the unit light intensity output was checked for every 10 samples before polymerizing them with the digital read-out light meter available with the unit. The first set of 10 non–air-abraded amalgam samples was veneered with Dyract AP (DeTrey, Dentsply, APRIL 2001
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Konstanz, Germany). A thin layer of adhesion promoting monomer of Dyract PSA primer/adhesive was brushed on the air-abraded amalgam surface, left undisturbed for 30 seconds, air-dried for 5 seconds, and then light polymerized for 10 seconds. A second layer of PSA primer/adhesive was applied in the same manner. Dyract AP material then was packed in the split mold in 2 increments and light polymerized for 40 seconds with the use of a Cotolus light-polymerizing unit (Coltene, Alstatten, Switzerland) with an 8-mm curing tip. Additional polymerizing was performed for 40 seconds after removal of the mold. The next 10 non–air-abraded amalgam samples were veneered with Hytac (ESPE, GmbH Seefeld, Germany). Hytac OSB primer/adhesive was applied with a brush on the amalgam surface and rubbed for 30 seconds, then air-dried for 5 seconds and light polymerized for 10 seconds. The second layer of the primer/adhesive was applied in the same manner but without rubbing. Hytac material was packed into the mold in 2 increments and polymerized for 40 seconds before and after removal of the mold. The remaining set of 10 non–air-abraded amalgam samples was veneered with F2000 (3M Dental Products, St. Paul, Minn.). A single thin layer of F2000 primer/adhesive was brushed on the amalgam surface, left undisturbed for 30 seconds, air-dried for 10 seconds, and then light polymerized for 10 seconds. F2000 material was packed into the mold in 2 increments and light polymerized for 40 seconds before and after the removal of the mold. The 3 sets of air-abraded amalgam samples were veneered with Dyract AP, Hytac, and F2000 compomer materials according to the previously described procedures. After their fabrication, all amalgam-compomer bonded samples were kept in 100% relative humidity at 37°C in a Memmert Universal Oven (Memmert GmbH, Schwabach, Germany).
Shear peel testing After 48 hours, each bonded sample was fitted into a specially constructed jig (Fig. 1) and subjected to shear peel testing with an Instron 8500 Digital Control Universal Testing machine (Instron Corp, Canton, Mass.) at a crosshead speed of 0.5 mm/min. A shear peel force test was used in this study because it has been found to be a severe test of adhesion, which is most likely to correlate with clinical performance.16,17 The bond strength at failure was calculated as the recorded failure load divided by the surface area of the bonded surface (12.6 mm2) and expressed in megapascals (MPa). After the SPBS test, the amalgam-compomer interfaces were examined twice with a Polyvar Reichert-Jung stereomicroscope (Leica Reichert, Wien, Austria) at ×32 397
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AL-JAZAIRY
Fig. 1. Shear peel test apparatus.
Table I. Shear peel bond strength and frequency of bond failure of compomers veneered to amalgam Bond failure Amalgam groups
No. of samples
Non–air-abraded Dyract AP Hytac F2000 Air-abraded Dyract AP Hytac F2000
Bond strength (MPa*) mean ± SD
Adhesive
Cohesive
Combined
10 10 10
5.09 ± 1.60 3.21 ± 1.63 2.99 ± 1.40
6 6 3
0 0 0
4 4 7
10 10 10
5.78 ± 1.30 4.72 ± 1.57 4.72 ± 1.53
7 6 10
0 0 0
3 4 0
MPa = Mega Newton/m2.
magnification to determine the fracture modes. The bond failure was recorded as adhesive (no signs of amalgam fracture or compomer remnants on the amalgam surface), cohesive (exhibiting complete fracture of either amalgam or compomer), or combined (showing both adhesive and cohesive failures).18
Statistical analysis A 2-way analysis of variance (ANOVA) test was performed to analyze the differences between the air-abraded and non–air-abraded amalgam groups. Tukey’s multiple comparison test was used to determine the statistical significance of the differences within each tested group.
RESULTS The means and standard deviations (SD) of the SPBS in MPa are presented in Table I. Two-way ANOVA showed a highly significant difference 398
(P<.002) between SPBS of compomers veneered to air-abraded and non–air-abraded amalgam surfaces. The highest SPBS (5.78 ± 1.30 MPa) was recorded for Dyract AP bonded to air-abraded amalgam surfaces; the lowest (2.99 ± 1.4 MPa) was for F2000 bonded to non–air-abraded amalgam surfaces. Tukey’s multiple comparisons on the non– air-abraded groups revealed statistically significant differences between the SPBS of Dyract AP compared to Hytac (P<.03) and F2000 (P<.015); there was no statistically significant dif ference between the SPBS of Hytac and F2000 (P>.94). In the air-abraded group, there was no significant difference between the SPBS of the compomers (P>.25). Each of the 3 compomers showed a higher SPBS when veneered to an air-abraded amalgam surface than to a non–air-abraded amalgam surface. This difference was not statistically significant (P>.1) for VOLUME 85 NUMBER 4
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Dyract AP. However, Hytac bonded to air-abraded and non–air-abraded amalgam surfaces a showed statistically significant difference (P<.02) in SPBS, as did F2000 (P<.01). Stereomicroscopic examination revealed adhesive and combined bond failures (Table I). For the F2000 air-abraded amalgam group, only adhesive bond failure was observed in all the specimens with no breakage or chipping of compomer material. In the remaining groups, all specimens showed adhesive and combined bond failures.
DISCUSSION If sufficient bond strength could be achieved between compomer materials and dental amalgam, then compomers, with their advantages of fluoride release, ease of application, and reasonable cost, would be a better choice for veneering than composites. Surface morphology can affect attachment by improving the initial interaction, offering an increased surface area, and facilitating mechanical interlocking of the adhesive.15 It is therefore expected that air abrading an amalgam surface before veneering with compomer would lead to an improved attachment. This investigation tested the possibility of successfully bonding compomers to amalgam surfaces as well as the impact of air abrasion on the bond. McConnell et al14 found aluminum oxide with a particle size of 50 µm to be the most suitable abrasive agent for metal treatment. The current study used this approach for texturing the amalgam surface. It was found that, with air abrasion, the SPBS of the 3 compomers tested improved. This surface treatment significantly enhanced the SPBS for Hytac (P<.02) and F2000 (P<.01) (Table I). In the case of Dyract AP, although the SPBS increased with air abrasion, this effect was not statistically significant (P>.1). Improvement in bond strength after air abrasion was reported in an earlier study. Nahass15 evaluated the shear bond strength (SBS) of composite bonded to different amalgam surface treatments; she reported higher bond strengths when air abrasion was used in addition to carbide or diamond burs than when carbide or diamond burs were used alone. However, no study in the literature has reported the effect of air abrasion on the bond strengths of compomers veneered to amalgam. El-Kalla and Garcia-Godoy19 reported Dyract to have a significantly higher SBS than Hytac and Compoglass when bonded to primary and permanent teeth. In the current study, Dyract AP veneered to an air-abraded amalgam surface was found to have the highest SPBS (5.78 ± 1.30 MPa) among the 3 compomers tested (Table I). This value was lower than that of the SBS (9.52 ± 2.46 MPa) reported by APRIL 2001
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Salama and el-Mallakh12 for Dyract AP bonded to air-abraded stainless steel crowns. This variance may be due to the difference in materials veneered to, the type of bond strength evaluated, and the method of testing. In the non–air-abraded group, Dyract AP had a significantly higher SPBS than Hytac and F2000. The differences noticed between the SPBS of the 3 compomers could be attributed partly to the ability of the bonding agent used with each system to adhere to the amalgam surface and partly to differences in the properties of the compomer materials. The bond between compomers and amalgam is mainly micromechanical; some chemical bonding also may occur between the oxide layer on the air-abraded amalgam surface and the polycarboxylate and phosphate groups in compomer materials. Such adhesion can be attributed to the formation of ionic bonds with metal oxides or with the active metal compounds of the amalgam.20 In the current study, the failure pattern of F2000 veneered to the air-abraded amalgam surface was completely adhesive; the specimen separated at the compomer-amalgam interface. This, along with its low SPBS, makes F2000 unsuitable for veneering amalgams. On the other hand, Dyract AP and Hytac showed a combined failure pattern, with many compomer tags on the air-abraded surfaces mechanically interlocked to the amalgam surface. To confirm the superiority of Dyract AP as the compomer of choice, in vivo studies on the performance of compomers when veneered to amalgam are needed. Some clinical studies8,11 on bonding composites to amalgam restorations used an opaque light-polymerized glass-ionomer liner as an intermediary. One of the advantages of this liner is its ability to bond to set amalgam and mask its unesthetic color. Further research should focus on assessing the esthetic quality of compomers veneered to dental amalgam, the need for an intermediate opaquer or liner, and the optimal thickness of that liner.
CONCLUSIONS Of the 3 compomer materials tested in this in vitro study for veneering to amalgam, Dyract AP showed the highest SPBS, followed by Hytac. Air abrading the amalgam surface did not significantly improve the bond strength of Dyract AP. However, in the case of Hytac and F2000, it produced a significant increase in SPBS. All Dyract AP and Hytac specimens exhibited adhesive and combined fracture patterns. F2000 specimens veneered to air-abraded amalgam surface showed only adhesive bond failure. Its poor adhesion to amalgam after air abrasion, along with its low SPBS, makes F2000 unsuitable for veneering amalgam. 399
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I thank Dr W. Awliya and Professors E. S. Akpata and H. Al Tahawi of the King Saud University at Riyadh for their valuable suggestions.
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12. Salama FS, el-Mallakh BF. An in vitro comparison of four surface preparation techniques for veneering a compomer to stainless steel. Pediatr Dent 1997;19:267-72. 13. Lubow RM, Cooley RL. Effect of air-powder abrasive instrument on restorative materials. J Prosthet Dent 1986;55:462-5. 14. McConnell RJ, Taylor D, Moriarity K. Bonding strengths of resin to noble and base metal alloys. J Dent Res 1989;69:955-60. 15. Nahass MA. Evaluation of shear bond strength and microleakage between amalgam and veneering composite resin. [MDS thesis.] Riyadh: King Saud University; 1997. p. 200. 16. Tavas MA, Watts DC. Bonding of orthodontic brackets by transillumination of a light activated composite: an in vitro study. Br J Orthod 1979;6:207-8. 17. Harris AM, Joseph VP, Rossouw PE. Shear peel bond strengths of esthetic orthodontic brackets. Am J Orthod Dentofacial Orthop 1992;102:215-9. 18. Chappell RP, Eick JD, Mixson JM, Theisen FC. Shear bond strength and scanning electron microscope observation of four dentinal adhesives. Quintessence Int 1990;21:303-10. 19. el-Kalla IH, Garcia-Godoy F. Bond strength and interfacial micromorphology of compomers in primary and permanent teeth. Int J Paediatr Dent 1998;8:103-14. 20. McConnell RJ. Metal resin bonding. J Calif Dent 1994;40:2-6. Reprint requests to: DR YOUSRA HUSSEIN AL-JAZAIRY COLLEGE OF DENTISTRY KING SAUD UNIVERSITY PO BOX 58461 RIYADH 11594 SAUDI ARABIA FAX: (966)1-252-0095 E-MAIL: [email protected] Copyright © 2001 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2001/$35.00 + 0. 10/1/114824 doi:10.1067/mpr.2001.114824
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