The effect of bead attachment systems on casting patterns and resultant tensile bond strength of composite resin veneer cast restorations Chee F. Lee, BDS(Hons), LDSRCS, Edmund R. Strickler, DDSC University

DDS,* Hugh P. Pierpont,

of North Carolina, School of Dentistry,

DDS,b and

Chapel Hill, N.C.

This study compared the difference in tensile bond strength between the composite resin veneer and the cast Ni-Cr disk when different bead adhesives were used to make the laboratory patterns. Visio-Gem, cyanoacrylate, and shellac were the adhesives tested. Fifty-six composite resin bonded Ni-Cr disks were prepared and tested to tensile failure with the Kemper-Kilian device. All tested samples showed a complex failure pattern. The results showed that the mean tensile bond strength of the cyanoacrylate group was significantly higher than the other two groups. No significant difference in the mean tensile bond strength was observed between the Visio-Gem and shellac groups. The higher tensile bond strength in the cyanoacrylate group is thought to be attributed to the low rheological property of the adhesive that allowed greater exposure of the bead for retention. Using different adhesives in the fabrication of composite resin veneered-castings may affect the bond strength in the composite resin-metal interface. (J PROSTAET DENT 1991;66:623-30.)

R

esin veneers have been used in fixed prosthodontics since the advent of poly(methy1 methacrylate) (PMMA) as a dental restorative material. Inhomogeneous miSupported by U.S. Public Health Service grant No. 08S7RR05970B2SO-7RRO5970-02. *Resident, Department of Orthodontics. bAssociate Professor and Chairman, Department of Occlusion and Fixed Prosthodontics, University of Texas Health Science Center, Dental Branch, Houston, Tex. CAssociate Professor, Department of Occlusion and Fixed Prosthodontics, University of Texas Health Science Center, Dental Branch, Houston, Tex. 10/l/31363

crofilled composite resinle3 (Visio-Gem and entacolor) veneers on crowns and fixed partial dentures have been alternatives to ceramometal restorations since the early 1980s. Microfilled composite resin is used as a veneer material because of its good esthetics,4, 5 color stability,5p 6 and biocompatibility.5 The resin is compatible with most castable metal foundations,5 easy to make and repair,5,7-9 and unlikely to abrade the natural dentition.3s 5,lo The resin can be retained on the casting by mechanical, chemical or a combination of both methods. Beads,“l nail heads,12 gauze,12 peripheral undercuts, wire 10ops,“~ nylon bristles, dovetails, ear loops, stubs, serrations, and

Fig. 1. SEM photomicrograph of unsieved spherical microbeads. (M), Microbead. (Original magnification X30.) THE

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LEE, PHERPONT,

water reservoir.

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F

I

: L I

I

L

CURING

PORT

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0.35 mm MICROBEAD

p

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MACHINED MICROBEAD DISC SAMPLE

!

Fig. 4. Diagram of cast machined microbead disk sample with corresponding composite resin placement jig.

ervoir of distilled water. The reservoir of distilled water acted as a lubricant, coolant, and debri-collecting medium during the sectioning process. The flatness of the ground surface of each resin pattern was checked with a Talysurf 10 instrument (Taylor-Hobson Mfg. Co., Leicester, England) surface tester. A thin layer of adhesive was painted with a sable brush on the ground surface of each Duralay resin pattern. Excess adhesive was removed with a gentle stream of oil-free compressed air. The Duralay resin pattern was lowered onto the microbeads in the petri dish by use of the alignment shaft of the Kemper/Kilian3s device. An even distribution of closely packed microbeads was obtained by allowing 5 seconds of securing time. Excess microbeads collected at the periphery or overlapped on the Duralay resin pattern were removed under a X25 power stereomicroscope (Model SCB-125, Bausch & Lomb Co., Rochester, N.Y.) to ensure a single uniform layer of microbeads. The Visio-Gem adhesive was polymerized for 5 seconds with the Visio-Alpha light (wavelength 460 nm) before collection of the microbeads. The Visio-Gem adhesive was cured for an additional 5 seconds after microbead application to complete the cure of the adhesive. The cyanoshellac systems were both self-curing adhesives. A thin layer of debubblizing agent, Mizzy (Mizzy, Inc., Clifton Forge, Va.), was sprayed onto the microbeadcoated Duralay resin pattern before investment. The debubblizer3g was used to reduce the surface tension on the microbeads during the investing procedure and to ensure good wettability of the investment in the undercuts beneath the microbeads. Any excess debubblizing agent was removed with a gentle stream of oil-free compressed air to avoid having a rough casting and to maximize the strength of the investment. Ceramigold investment (Whip Mix Co., Louisville, Ky.) was vacuum-mixed and invested according

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BRASS

SHAW

COMPOSITE

MACHINED ~~CRO~~A~ DISC SAMPLE ASSEMBLY

THREAD

ADAPTER

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ALIGNMENT

SHAFT

Fig. 5. Assembled sample before testing on Instron chine.

ma-

to manufacturer’s instructions. AI1 of the Duralay resin patterns were vacuum-invested within 15 minutes. The investment was allowed to set overnight in a humidifier and was placed in a burnout oven at room temperature. The oven was raised to 1400’ F over an hour and the investment was allowed to heat-soak an additional 30 minutes. The Ni-Cr alloy (Bake-On NP alloy, Johnson & Johnson, East Windsor, N.J.) was melted with a gas-oxygen torch and the patterns were cast with a broken-arm centrifugal casting machine. Each metal disk was divested and sandblasted with 50 pm grit aluminum oxide at 35 lbs of air pressure, held at 1 cm for 20 seconds until the gross investment was removed. It was then cleansed ultrasonically in ‘70% hydrofluoric acid for 10 minutes and distilled water for an additional 5 minutes. The metal disk was sandblasted again with 25 pm

625

‘4’&‘&Je

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1,

Descriptive statistics for tensile bond strengths (psi) of different adhesives used

ig. 5) and tested to tensile failure with the Instron instrument jhstron Corp., Canton, Mass.) at a crosshead speed of 0.5 cm/min. The ~ernper~~i~~a~ device served to align the disk and composite resin assembly and also prevents any shear stress from developing using tensile bond st~emgt~

testing..

The d8erence in mean tensile bond strengths among the three adhesive systems was evaluated by use of one-way analysis of variance (ANOVA), foliowe inear contrasts betweem systems at a 0.05, and fractu ttesns that developed were investigated under ~25 magni~cat~o~. resentative samples from each group were prepared and examined under a scanning electron microscope (x65).

he data of 56 disks were recorded, sample of the shellac group was exduded from the data analysis because the dis resin were separated efore tension was applied e Pnstron instsument. The tensile bond strengths of the composite resinbonded disks are presented ix-2Table atiom of the mean tensile bond stren late group was sligbtly greater compares with the VisioGem amd shellac groups. The ANOVA showed a significant diEerence among the mean temsi2ebond stsengtbs of the tbree systems (VisioGem resin, ~yamoacrylate, and shellac adhesives; F 3.91; F 0.05) (Table II). The linear contrast showed mo significant diEerence between the mean tensile bond strengths of the Visio-Gem resin (347.3 psi) and the shellac groups (351.5 psi). There oweves,

~si~~~~~~~t~~ffe~e~~e

(463.9

psi)

wbenthe

nd strength of the ~yanoacry~ate gro he Visio-Gem resin amd the she&x (Table III). Viewed with the stereomic pe at X25 rnag~~fi~at~o~~ ail of the composite resin-bo disks showed a complex fracture pattern (Fig. 6). ‘Large surfaces af cohesive frac-

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Fig. 6. Diagram of representative sample from each of three adhesive groups. (V) VesioGem adhesive; (C,l cyanoacrylate adhesive; and (S) shellac adhesive. (Original magnification X2.63.)

ture were seen in the centers of the disk (Fig. 7) in 25 samples. In the periphery the fracture plane was seen to transverse on the top of the microbeads, with resin tags remaining in the undercuts. Small scattered areas of adhesive fractures were seen but no disk demonstrated fracture of the cast microbeads. The scanning electron photomicrographs of selected samples from the three groups revealed no difference in the amount of embedded and shear heights of the microbeads (Fig. 8).

DISCUSSION Steps were taken in this study to ensure that the microbeads were of uniform size. Variation in the size of the microbeads affects the amount of composite resin veneer retained.e Smaller beads are preferred because they are more retentive than larger beads. Microbeads of 0.18 mm in diameter have been shown to give the best retention.15 The volume of composite resin retained by a spherical microbead was determined by the shear height (H), embedded height (H’), and radius of the microbead (r) (Fig. 9) when tensile load was tested. Mathematically this is represented as follows: Pr2(H+H’)-%

*r3

In this experiment, the microbeads were one layer thick and arranged randomly on the Duralay resin surface. This arrangement was chosen over a uniformly spaced arrangement because the latter arrangement would be impractical for clinical applications. Moreover, the effect of different microbead arrangements on the tensile bond strength of composite resin veneer cast restorations is inconclusive. Nicholls and Shue43 demonstrated that microretention beads separated by a space of one bead diameter apart gave a statistically higher tensile bond strength compared with beads arranged randomly or beads separated by 2 to 3 bead diameters apart. In a later study, however, Shue et a1.g found no statistical difference in the tensile bond strengths when the microbeads were placed randomly or spaced one

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Fig. 7. Diagram showing complex fracture failure pattern. (C) Cohesive fracture; (A) adhesive fracture; (T) fracture transversing at top of microbeads. (Original magnification X5.5.)

bead apart. These conflicting results may be due to the small sample sizes used in the two studies, Further investigations are needed to determine whether t differences exist and are clinically significant. A high compressive-strength investment (phosphate) investment) was used to prevent fracturing of the investment material in the undercut volume beneath the microbeads during the casting process to maximize the surface volume for retention (Fig. 1O).3QFurthermore, the investment material had to be suitable for the bigb casting temperature used.

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aph of cross-sectioned representative sample from Visi5icrobead and CT,!composite resin. (Original rn~gni~~at~o~ X65.) ross-sectioned representative sample from cyanoacrylate rcrobead and (r) composite resin. ~~~i~~~a~ rna~~~~cat~~~x65.) C, SE t3rn~~~~gr~~~of cross-sectioned representative sample from shellac group. cm) Microbead :ic $,j composite resin. (Original rn~~~~~~~t~~~X65.)

9, Diagram fhstrates amount of availairPleretention in microbead under tensile load .EST

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m VOLUME

,

)

EMBEDDED

HEIGHT

OF AVAILABLE RETENTION : T r * ( H - H’ ) - */a in r 3 r = radius of bead H = shear height H’ = embedded height

Fig. 10. Diagram shows possible investment microfractures that may occur during casting process.

In the experiment, each cast disk was machined to maintain a constant surface area for several reasons: to allow accurate placement of the disk into the composite resin placement jig, to eliminate any pooling of adhesives at the periphery, and to minimize shearing force during testing. A crosshead speed of 0.5 cm/min was used as in other similar studies9943s44to allow comparisons. Speeds up to 50 cm/min had been shown not to influence the resistance to tensile failure.s When the composite resin veneered metal disk was tested to tensile failure, a complex fracture pattern was obtained. Cohesive fracture of the composite resin occurred when its diametric strength was exceeded by the retentive force created by the undercuts. Photocuring of the composite resin veneer resulted in polymerization shrinkage, the direction of which was from the periphery to the center of the veneer. Such contraction of the composite resin may have weakened the adhesive bond at the periphery and resulted in a fracture pattern transversing the top of the microbeads. Small isolated areas of adhesive fracture could result either from adhesive blocking out the undercuts, incomplete penetration of the composite resin into the undercuts, or incomplete casting. Further tests are needed to accurately determine the nature of the fracture pattern. The similarities and differences in the mean tensile bond strengths between different adhesive systems may be explained by the viscosity and film thickness of the adhesive fluids. The Visio-Gem adhesive was a very viscous fluid. The shellac adhesive was more fluid but required a thicker layer to secure the microbeads on the Duralay resin pattern, The viscous Visio-Gem adhesive and a thick layer of shellac adhesive yielded similar mean tensile bond strengths that were of no statistical significant difference. On the contrary, the cyanoacrylate adhesive was more fluid and gave a thin and even coating. This resulted in a large shear heigbt and a small embedded height. The cyanoacry-

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late group showed a significantly higher mean tensile bond strength when compared with the Visio-Gem adhesive and shellac groups. Further tests are needed to confirm the hypothesis. Clinically, the Visio-Gem adhesive had the advantage of an unlimited working time but it was the most expensive of the three systems. Shellac and cyanoacrylate were inexpensive, easily available, and had a long shelf life. Tbe cyanoacrylate set quickly, but the working time could be extended by refrigerating the adhesive before use. CONCLUSION No statistical significant difference was found in the mean tensile bond strength of the composite resin veneer between the Visio-Gem resin (347.3 psi) and shellac (351.7 psi) groups. There was statistical difference when the cyanoacrylate group was compared with the shellac and Visio-Gem resin groups. The result demonstrated tbat different adhesive systems used in securing the retentive microbeads during the fabrication of composite resinveneered restorations could be a factor in affecting the bond strength in the microbeads-resin interface. We thank Dr. Ceib Phillips for her help with biostatistics, Dr. William von der Lehr, Dr. Dorothy T.Y. Pang, and Mr. Claude Davis for their assistance with the project.

REFERENCES 1. Lutz F, Setcos JC, Phillips RW, Roulet JF. Dental restorative resins: types and characteristics. Dent Clin North Am 1983;27:697-712. 2. Lutz F, Phillips RW. A classification and evaluation of composite resin systems. J PROSTHET DENT 1983;50:480-8. 3. Strohaver RA, Mattie DR. A scanning electron microscope comparison of microfilled fixed prosthodontic resins. J PROSTHET DENT 1987; X:559-65. 4. Greenberg JR, Rafetto RF Jr. Laboratory light-cured composite resins: a clinical study. Part 1. Compend Contin Educ Dent 1985;6:402-12. 5. Barzilay I, Myers ML, Cooper LB, Graser GN. Mechanical and chem-

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28. Yu Xl’, Xu JW, Kui CC, Ming XH. Further study of chemically coupled resin bonded restorations. Quintessence Int 1987;10:717-21. 29. Yamashita A. The clinical application of new adhesive resin (MMA-4META-‘TBB-0) to an adhesion bridge (adhesion splint). Shikai Tenbo 1982;55:671-82. 30. Get&man L, Vrijhoef MMA, Uchiyama. Proceedings of the international symposium on adhesive prosthodontics. Nijmegen, Netherlands: Eurosound Brukkerij BV, 1988. 31. Uchiyama Y. Clinical study on retentive force of adhesive resins. Nippon Matetsu Shika Gakkai Zasshai 1986;30:198-204. 32. Tanaka T. Fujiyama E, Shin&u H, Takaki A, Aisuta M. Surface treatment of nonpreciuus alloys for adhesion-fixed partial dentures. d PR(~sTHET DENT 1986;55:456-62. 33. Hansson 0. The silicoater technique for resin-bonded prostheses: clinical and laboratory procedures. Quintessence Int 1989;20:85-99. 34. Laufer BZ, Nicholls JI. Time delay effects on the tensile bond strength developed by the silicoater. Quintessence Dent Technol 1987;11:199203. 35. PeutzfeeEdr, A, Asmussen E. Silicoating: evaluation of a new method of bonding ccxnpwite resin to metal. Stand J Dent Res 1988;96:171-6. 36. Caeg C, Leinfelder KF, Lacefield WR, Bell W. Effectiveness of a method used in bonding resins to metal. J PRO~THET DENT ?990;64:37-41. 37. Van der Veen JH, Krayenbrink TG, Bronsdijk AE, Van de Poe1 ACM. Resin-bonding of tin-electroplated precious metal fixed partial dentures: one-year clinical results. Quintessence Tnt 1986;1?:299-301. 38. Kemper R, Kilian R. New test system for tensile bond strength testing [Abstractj. J Dent Res 1976;55:308. 39. Phillips RW. Skinner’s Science of dental materials. Pb~ladeIpb~a: WB Saunders Co. 1982;406-IQ, 425, 443. 40. Duke ES, Norling BK. Vacuum curing of a light activated composite veneering resin [Abstract]. J Dent Res 1985;64:32. 41. Leung RL, Fan PL, Johnston WM. Post-irradiation polymerization of visible light-activated composite resin. J Dent Res 1983;62:363-5. 42. Braden M, Causton BE, Clark RI. Diffusion of water in composite filling materials. J Dent Res 1976;55:730-2. 43. Nicholls JI, Shue SL. Effect of bead spacing on the tensile bond strength of resin veneers to cast alloys. Quintessence Dent Tech& 1986;10:5115. 44. Nicholis Jl, Nakanishi DR. Tensile bond strength of veneering resins to opaque systems. Quintessence Dent Technol 1936;10:35-8. Reprint recjuest~s to: DR.CWEE FAI LEE UNWERSITY OFNORTH CAROLINA ATCHAPEL fIlLL SCHOOLOFDENTISTRY CB# ~~~O,BRAUERWALL CHAPEL HILL, NC 27599-7450

The effect of bead attachment systems on casting patterns and resultant tensile bond strength of composite resin veneer cast restorations.

This study compared the difference in tensile bond strength between the composite resin veneer and the cast Ni-Cr disk when different bead adhesives w...
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