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Abstract: This study evaluated the wearcharacteristics and bonding to silver-palladium-copper-gold (Ag-Pd-Cu-Au) alloy of an acrylic resin that wasfilled with pre-polymerized composite particles andinitiated with tri-n-butylborane (TBB) derivative(Bondfill). Three methyl methacrylate (MMA)-basedresins (Bondfill, Super-Bond, and Multi-Bond II) anda microfilled composite restorative material (MetafilC) were assessed. Disk specimens were cast from thealloy and were air-abraded with alumina. The diskswere bonded with nine bonding systems selected fromtwo priming and three luting agents. Shear bonds trengths were measured be fore and a f t erthermocycling. Bond strength varied from 2.2 MPa to28.2 MPa. Three systems based on thione primers(Metaltite and V-Primer) and TBB-initiated resins(Bondfill and Super-Bond) had the highest bondstrength after thermocycling (15.9-20.4 MPa). Thetoothbrush-dentifrice abrasion test showed that theMetafil C material was the most wear-resistant, followedby Bondfill and Super-Bond. In conclusion, Bondfillresin is an alternative to Super-Bond resin for lutingmetallic restorations and for restoring tooth defects.However, care is required in selecting appropriateclinical cases. (J Oral Sci 53, 109-116, 2011)

Keywords: Ag-Pd-Cu-Au alloy; bonding; thione;toothbrush; tri-n-butylborane; wear.

IntroductionDuring the most recent decade, the use of adhesives in

bonding restorations has increased substantially. Thistrend is due to the development of adhesive functionalmonomers that are compatible with tooth structure, castingalloys, and ceramic materials. Currently available adhesivescontain functional monomers designed for intraoralapplications. Among these, acidic monomers are used forbonding tooth structure (1,2), whereas thiol or thionemonomers are used for priming noble metal alloys (3-5).An adhesive resin initiated with tri-n-butylborane (TBB)derivative, in combination with thiol or thione primer, hasbeen used for bonding between tooth structure and noblemetal alloy (6,7), between indirect composite materialand noble metal alloy (4,8-10), and for metal-to-metalbonding (11-14). In addition, TBB resin has been used indirect restoration (15,16).

Tooth wear, attrition, and formation of wedge-shapeddefects gradually progress due to occlusal contact,mastication, grinding, tooth brushing, and other factors.It would likely be beneficial for both tooth structure anddental material if interface integrity and wear resistancewere equivalent, particularly within the molar occlusalplane. One problem associated with TBB-initiated adhesiveresin has been insufficient wear resistance (15,16), whichresults from the fact that TBB resin contains no inorganicfillers, except for opaque or radiopaque components, andconsists mainly of thermoplastic acrylic resin matrix.

Recently, a modified TBB resin was developed for bothluting and restorative agents, and, unlike the original TBB

Journal of Oral Science, Vol. 53, No. 1, 109-116, 2011

Correspondence to Dr. Koji Naito, c/o Professor HideoMatsumura, Department of Fixed Prosthodontics, NihonUniversity School of Dentistry 1-8-13 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8310, JapanTel: +81-3-3219-8145Fax: +81-3-3219-8351E-mail: matsumura@dent.nihon-u.ac.jp

Bonding and wear characteristics of a tri-n-butylboraneinitiated adhesive resin filled with pre-polymerized

composite particles

Koji Naito

Division of Applied Oral Sciences, Nihon University Graduate School of Dentistry, Tokyo, Japan

(Received 8 January and accepted 14 February 2011)

Original

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resin, it contains pre-polymerized filler particles. Althoughthe manufacturer claims that the bonding characteristicsand wear resistance of the filled TBB resin have improved,there is no published information on the properties of thisnew material. The purpose of the present study was toevaluate the bonding and wear characteristics of TBB-initiated adhesive resin filled with pre-polymerizedcomposite particles. The working hypothesis was thatbonding to casting alloy and the wear resistance of a TBB-initiated resin are improved by incorporating pre-polymerized composite filler into the powder composition.

Materials and MethodsShear bond test

A silver-palladium-copper-gold (Ag-Pd-Cu-Au) alloy(Castwell M.C.12, GC Corp., Tokyo, Japan) was selectedas the adherend material. Two priming agents – Metaltite(Tokuyama Dental Corp., Tokyo, Japan) and V-Primer(Sun Medical Co., Ltd., Moriyama, Japan) – were assessedas bonding promoter. Two single-liquid primers containingthe adhesive functional monomer 6-methacryloyloxyhexyl2-thiouracil-5-carboxylate (MTU-6) or 6-(4-vinylbenzyl-

n-propyl) amino-1,3,5-triazine-2,4-dithione (VTD) wereused (Table 1). Three auto-polymerizing methylmethacrylate (MMA)-based acrylic resins (Super-BondC&B Opaque Ivory and Bondfill, Sun Medical Co., Ltd.:Multi-Bond II, Tokuyama Dental Corp.) were used asluting materials. Both the Super-Bond and Bondfill resinscomprise three components: TBB derivative, monomerliquid, and powder. They differ in that the latter containspre-polymerized composite filler particles. The Multi-Bond II resin is a powder-liquid composition. The liquidcontains borate compound and MTU-6. Information on thematerials is summarized in Table 1.

The Ag-Pd-Cu-Au alloy was cast in a cristobalite mold(Cristoquick 20, GC Corp.) using a high-frequencyinduction heating machine (Argoncaster-C, Shofu Inc.,Kyoto, Japan). A total of 162 disk specimens (10 mm indiameter; 2.5 mm in thickness) were prepared from the alloyand divided into nine sets of 18 disks. All disks were wet-ground with 800-grit silicon carbide abrasive paper(WetorDry Tri-M-ite, 3M Corp., St. Paul, MN, USA).The alloy surfaces were subsequently air-abraded withalumina (50-70 µm; Hi-Aluminas, Shofu Inc.) by means

Table 1 Materials assessed

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of an airborne-particle abrader (Jet Blast II, J. MoritaCorp., Suita, Japan) for 10 s at an air pressure of 0.6 MPa.The distance of the abrader orifice from the disk surfacewas approximately 10 mm.

After air-abrasion, a piece of double-coated tape witha circular hole, 5 mm in diameter, was positioned on themetal specimen to define the bond area. The disks, exceptfor those included in unprimed groups, were primed withone of the two priming agents. An SUS 303 stainless steelring (6 mm in inside diameter by 2 mm deep with a 1-mm-thick wall) was placed around the opening area. A brush-dip technique was used to fill the ring with one of the threeMMA-based acrylic resins. Thirty minutes after prep-aration, the specimens were stored in 37°C water for 24h. This state was defined as 0 thermocycles. One half ofthe specimens (nine sets of nine specimens) were testedat 0 thermocycles. The remaining specimens (nine sets ofnine specimens) were subsequently thermocycled between5°C cold water and 55°C hot water for 20,000 cycles with60 s dwell time per bath (Thermal Shock Tester TTS-1 LM,Thomas Kagaku Co., Ltd., Tokyo, Japan). Each specimenwas placed in a steel mold and seated in a shear-testingjig. Shear bond strengths were then determined with amechanical testing machine (Type 5567, Instron Corp.,Canton, MA, USA) at a crosshead speed of 0.5 mm/min.

Toothbrush-dentifrice abrasion testTwenty-eight cuboid specimens (Fig. 1) were prepared

from four resin-based materials: three MMA resins (Super-Bond Polymer Brush-Dip Clear and Bondfill, Sun MedicalCo., Ltd.; Multi-Bond II, Tokuyama Dental Corp.) and adirect composite restorative material (Metafil C, SunMedical Co., Ltd.). The MMA resins were packed into astainless steel mold (16.0 × 2.0 × 2.0 mm) with brush-diptechnique and covered with a piece of glass plate (1.3mm thickness, Micro Slide Glass, Matsunami Glass,Osaka, Japan). The Metafil C composite was packed intothe mold, covered with the glass plate, and light-exposedfor 120 s. Thirty minutes after polymerization, the topsurface of each specimen was wet-ground with a series ofsilicon carbide papers (#800-2000, WetorDry Tri-M-ite).All specimens were stored in distilled water at 37°C for24 h. Each specimen was fixed in a specimen holderattached to a toothbrush abrasion testing machine (K236,Tokyo-Giken Co., Ltd., Tokyo, Japan). A dentifricecontaining abrasives of conventional silica (Crest TartarProtection Regular Paste, RDA-Value 136, Procter &Gamble Co., Cincinnati, OH, USA) was used as an abrasiveslurry with a paste-to-water ratio of 1:1. The vessel of themachine was loaded with 150 g of slurry. A toothbrush(Bee King, Bee Brand Medico Dental Co., Ltd., Osaka,

Japan) with nylon filaments (Tynex, Dupont Japan, Tokyo,Japan) was fixed in the toothbrush holder of the machineand moved back and forth on the specimen at 140 strokesper minute. A consistent load of 3.4 N was applied to thetop of the toothbrush holder with a steel weight. Thespecimens were abraded with a total of 20,000 reciprocalstrokes. The toothbrush and slurry were replaced for thetesting of each specimen. Seven specimens were tested foreach material. After the testing, specimens were removedfrom the machine, ultrasonically cleaned in distilled waterfor 10 min, and gently air-dried.

The vertical loss of each specimen was determined witha confocal scanning laser microscope (1LM21W, LasertecCorp., Yokohama, Japan) equipped with a He-Ne(wavelength: 633 nm) laser light source. The resolutionperformance of the microscope was approximately 0.03µm. The distance from the original specimen surface tothe deepest abraded point was defined as wear depth. Themethod for measuring wear depth is illustrated in Fig. 2.

Scanning electron microscopic observationAfter surface preparation, debonding, and wear testing,

typical specimens were mounted on stubs, dried in avacuum desiccator for 24 h, and sputtered with osmium(HPC-1S, Vacuum Device Inc., Mito, Japan) for 30 s. Thespecimens were then observed with a scanning electronmicroscope (S-4300, Hitachi High-Technologies Corp.,Tokyo, Japan) operated at 15 kV.

Statistical analysisStatistical analyses were performed with the statistical

software package SPSS version 19.0 (IBM Corp., Somers,NY, USA). For the two tests, medians, interquartile ranges,means, and standard deviations were calculated. The values

Fig. 1 A representative photograph of a cuboid specimenand metal jig before wear testing.

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for shear bond strength and wear depth were primarilyanalyzed by the Levene test for evaluation of equality ofvariance. When the results of the Levene test did notdemonstrate homoscedasticity in all categories, the resultswere analyzed by the Kruskal-Wallis test (IBM SPSS19.0). Based on the results of Kruskal-Wallis test, thenonparametric multiple comparison Steel-Dwass test wasused to compare differences among the nine adhesivesystems for each thermocycling condition and among thefour dental materials for wear depth (Kyplot 5.0, KyensLabInc., Tokyo, Japan). The difference between pre-thermocycling and post-thermocycling bond strengthswas analyzed with the Mann-Whitney U test. A P valueless than 0.05 was considered to be statistically significantin all analyses.

ResultsOn the Levene test, the results of shear bond strength

testing were not homoscedastic for several groups, and theresults of the nine adhesive systems were thus analyzedby the Kruskal-Wallis test and multiple comparison Steel-Dwass test. The vertical columns in Table 2 show thebond strengths of the nine adhesive systems before and afterthermocycling. Mean pre-thermocycling bond strengthvaried from 10.9 MPa to 28.2 MPa, and the values werecategorized as a through e. Before thermocycling,Metaltite/Bondfill had the highest bond strength (categorya) and V-Primer/Multi-Bond II had the lowest bondstrength. After thermocycling, mean bond strength variedfrom 2.2 MPa to 20.2 MPa and was classified into fourcategories (f-i). After thermocycling, the three systems withMetaltite/Super-Bond had the highest bond strength(category f), and the three groups with the two unprimedsystems had the lowest bond strength (category i). TheMann-Whitney U test revealed that the reduction in bondstrength after thermocycling was significant in all nine

Fig. 2 Specimen design and location of laser microscope scanner during evaluation of specimen wear depth.

Table 2 Shear bond strength of nine adhesive systems to Ag-Pd-Cu-Au alloy (MPa), n = 9

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systems (P < 0.05 for all comparisons). The greatestdecrease in bond strength was for unprimed Bondfill

(89.4%), while Metaltite/Super-Bond had the lowestdecrease (22.0%).

Figure 3 shows the roughened relief pattern after surfaceair-abrasion of the casting alloy with alumina. Figures4a-4d show the debonded alloy surfaces. Although thesespecimens were thermocycled, remnant resin materialwas detected in the groups with high bond strengths (Figs.4a and 4b; bond strength categories f and g).

Table 3 shows the results of wear testing. After

Fig. 3 Scanning electron micrograph of a cast Ag-Pd-Cu-Au alloy disk air-abraded with alumina.Original magnification, ×500.

Table 3 Vertical loss after toothbrush-dentifrice abrasion(µm), n = 7

Fig. 4 Scanning electron micrographs of the debonded surface of thermocycled Ag-Pd-Cu-Au specimens.The specimens were originally bonded with (a) Metaltite primer and Super-Bond (categories f and g),(b) V-Primer and Bondfill (category g), (c) Metaltite and Multi-Bond II (category h), and (d) Bondfill(category i). Original magnification, ×500.

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toothbrush-dentifrice abrasion, the greatest wear wasobserved in the Multi-Bond II (category l) specimens,followed by the Bondfill and Super-Bond (category k)specimens. The Metafil C composite was the most wear-resistant material among the four materials assessed(category j). Figures 5a-5d show scanning electronmicrographs of abraded material surfaces. Parallel rutscaused by use of the toothbrush-dentifrice were visible inthree acrylic materials (Figs. 5a-5c). The splintered fillerparticles in the Bondfill resin appeared to be more wear-resistant than the resin matrix (Fig. 5b). In contrast,detachment of spherical particles (probably polymethylmethacrylate) from the abraded surface was observed onthe Multi-Bond II surface (Fig. 5c). The surface of theMetafil C composite was smooth and wear-resistant underconditions of abrasion (Fig. 5d).

DiscussionThis study evaluated bonding to noble metal alloy and

wear resistance of a modified TBB resin. Evaluation of thebond strength of nine adhesive systems revealed thecharacteristics of priming agents and luting agents, as

well as their combined performance. Comparison of post-thermocycling bond strength is especially useful inevaluating the bonding durability of adhesive systems.Regarding the two priming agents, Metaltite was superiorto V-Primer when combined with Bondfill (categories fand g) and Multi-Bond II (categories h and i). Theseresults are in partial agreement with those of a previousstudy that evaluated combinations of thione primers andacrylic resins (17). The superiority of Metaltite may be dueto the difference in monomer structure between MTU-6in Metaltite and VTD in V-Primer. Specifically, MTU-6is a thione-methacrylate that is soluble in ethanol andmethacrylic monomers, whereas VTD is a vinyl-thione thatis insoluble in most alcohols and methacrylates. Perhapsthese differences had a negative effect on co-polymerizationof VTD and methacrylic monomers, particularly in aperoxide-tertiary amine redox initiation system. Thedifference in bond strength between Super-Bond andBondfill resins was not significant when the alloy wasprimed with Metaltite (category f) or V-Primer (categoryg), possible because the composition of these resins isalmost identical. The present results also suggest that

Fig. 5 Scanning electron micrographs of the worn surface of (a) Super-Bond, (b) Bondfill, (c) Multi-BondII, and (d) Metafil C specimens. Original magnification ×500.

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incorporation of pre-polymerized fillers does not affect thebonding performance of Super-Bond resin. In addition, thebonding durability of Multi-Bond II to Ag-Pd-Cu-Aualloy was inferior to that of Super-Bond and Bondfill(categories f and g; categories h and i), which is probablydue to the difference in initiation systems, i.e., TBB andperoxide-amine redox initiators (17).

Wear testing revealed the surface characteristics of thefour materials. As shown in Fig. 5c, detachment of polymerparticles was frequently observed in Multi-Bond resinafter wear testing. In the current methodology, wear wasdefined as the depth of the worn surface, and detachmentof the macro-filler considerably increased the wear depthof the specimen. This is the principal reason why the wearresistance of Multi-Bond II resin was inferior to that ofother resins. Testing revealed that the wear resistance ofSuper-Bond and Bondfill resins was comparable. As shownin Figs. 5a and 5b, pre-polymerized filler appeared to bemore wear-resistant than the surrounding resin matrix.However, in the present study, wear was defined as thedistance from the material surface to the deepest point. Asthe result, the wear resistance of the Super-Bond andBondfill resins was equivalent. Moreover, when surfaceroughness values were calculated, the surface of filledresin was rougher than that of unfilled resin. As comparedwith the three acrylic resins, the Metafil C compositematerial had a smoother surface and lower wear value,findings which are clearly due to the properties of thethermosetting matrix and to increased loading of pre-polymerized fillers composed of sub-micron and nano-sizedsilicon oxides.

The working hypothesis, that the incorporation of pre-polymerized composite filler into the powder compositionimproves bonding to casting alloy and wear resistance ofTBB-initiated resin, was rejected under the experimentalconditions of the present study. However, incorporation ofa small amount of pre-polymerized fillers did not negativelyaffect the wear resistance of TBB-based Super-Bond resinor its ability to bond to Ag-Pd-Cu-Au alloy. Also, theproperties of the two TBB-based resins were superior tothose of conventional acrylic resin. In conclusion, thepresent results suggest that both types of TBB resin canbe used for luting metallic restorations to tooth structureand for direct restorations.

AcknowledgmentsThis work was supported in part by Special Research

Grant for the Development of Distinctive Education fromthe Promotion and Mutual Aid Corporation for PrivateSchool of Japan (2009 and 2010).

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