Tensile Bond Strength of Composite toAir-Abraded Enamei

Klaus-R. JahnyBirgit GeitelVEckehard KostkaVRegine Wischnewski*"/Jean-François Roulet^

Purpose: The purpose of this study was to test the tensile bond strength of composite resin to enamel sur-faces treated with kinetic cavity preparation (KCP| and acid etching, respectively.

Materials and Methods: Plane labial surfaces of 280 human maxiiiary centrai incisors were treated witheither KCP by varying treatment conditions or phosphoric acid gel. Composite sampies were bonded to thepretreated surfaces with a bonding resin. After 24 hours specimens were leaded in tension in an Instrontesting machine until bond failure, A two-way anaiysis of variance was used to determine significant differ-ences. The failure sites were qualitativeiy evaluated by SEM,

Resuits: The determined tensile bond strengths and obsen/ed failure modes demonstrated that acid-etched

enamei has a significantly higher bond strength tc composite resih than KCP-treated enamei surfaces,

Conciusion: KCP-prepared enamei has to be acid etched before bondingto composite resin,

J Adhesive Denl 1999:1:25-30 . Submitted for pubiication: 20.07.98: accepted for publication: 31.08.98.

AS a rule, cavities are irregular in shape becausebefore and after caries excavation, the shape of

the cavity has to be changed to meet the require-ments of the filling materiai. This is usually carriedout with rotary instruments. However, their use isnot without probiems,^ Alternatives to rotary instru-ments are oscillating diamond burs" or air-abrasivetechnology, calied kinetic cavity preparation (KCP),This technology uses a narrowly focused particie

•i Professor, Department of Operative and Preventive Dentistry andEndodontics. Humboldt-University Beriin (Charité). Germany.

" Assistant Professor. Department of Operative and Preventive Den-tistry and EnOodontics. Humboldt-University Berlin ¡Charité). Ger-many.

'^ Associate Professor. Department of Operative and Preventive Den-tistry and Endodontics, Huniboldt-University Beriin (Charité). Ger-many.

" Dentist, private practice. Beriin, Germany.

^ Professor and Chairman. Department of Operative and PreventiveDentistry and Endodontics. Hurnboidt-University Berlin (Charité),Germany.

Reprint requests: Prof Dr K.-R. Jahn. Department of Operative andPreventive Dentistry and Endodontics. Humboldt-University Berlin(Charité). Föhrer Str. 15, D-13353 Berlin. Germany.

Stream that abrades tooth structures in proportionto the particle size, air pressure, nozzle distance,and angle between nozzle and prepared surfaceemployed. According to many authors, KOP's advan-tages include less stress for the patient and lesstrauma to the pulpal-dentin system,s Some authorseven claim that the KCP technique may be used tocondition the enamel surface instead of enameletching with phosphoric acid^'' '^' and that therestorative procedure wiii thus be easier if the KCPtechnique is used. One way to determine if this istrue is to measure the bond strength of compositeto such conditioned enamei surfaces.

MATERIALS AND METHODS

Two hundred eighty human maxillary central in-cisors, stored in 20% alcohol, were cieaned andembedded with the iabiai surface facing downwardwith a PMMA material (Paiadur R, Kulzer, Wehr-heim, Germany) into molds 30 mm in diameter.After poiymerization ofthe resin, the surfaces wereground with sandpaper in a polishing machine(Struers DP-U4, Copenhagen) to expose an enamel

Vol 1, No 1, 1999 25

Fig 1 Sample with a half of the moid (part A¡ and the screwholder (part B).

surface of approximately 30 mm^ in the center ofthe resm cylinder. The last step was performed withsandpaper of 1200 grit.

Next, the 28C teeth were randomly divided into20 groups of 14 mcisors. Sixteen groups weretreated with KCP (Whisperjet, American DentalTechnologies, San Carlos, CA) using different para-meters (Table 1), but the blasting time was consis-tent (10 seconds). In three groups the enamel wasetched with a phosphoric acid gel (De Trey etch,Dentsply, Konstanz, Germany) for 2C, 40, and 60seconds (positive control), and in one group theenamel was left untreated (negative control). Inorder to obtain almost flat surfaces, the sampleswere steadily moved manually in the KCP particlestream for 10 seconds at a constant distance fromthe nozzle of 1 and 3 mm. respectively. The powderand the enamel debris were continually removedwith a dental aspirator. Final cleaning was per-formed with a blast of dry air.

Next, all enamel surfaces were treated with Opti-bond XR adhesive (Kerr, Glendora, CA) for 20 sec-onds. The adhesive was thinned out with a blast ofair, and a split mold (accordingto ISO/TC 106/SCl /W G l l but with an inner diameter of 4 mm, made ofbrass) was placed on top of the sample (Fig 1, partA). Eirst, the inner wall of the mold and the surfaceof the enamel were covered with a thin layer ofcomposite (Herculite XR, Kerr) and the material waspolymerized for lC seconds with a plasma light-cur-ing unit (ADT 1000 Plasma ARC Curing System,American Dental Technologies). Then the mold wasfilled with a defined volume of composite. One half

of the mold was removed and, to insure the properrelation of the connector to the Instron fixture (90degrees to the enamel surface), a screw holder wasplaced on top ofthe mold (Eig 1, part B). After hav-ing set a screw into the composite, it was polymer-ized from the lateral aspect for 10 seconds. Thescrew was manually loaded against the top surfaceof the sample. After separation of the sample fromthe screw holder and the mold, composite excessesat the bottom of the composite cylinder were re-moved with a scalpel blade, and the sample wascured from two sides at an angle of 45 degrees for10 seconds each.

Twenty-four hours later, a tensile strength testwas performed using an Instron dynamic testingmachine (Model 8501, Canton, MA). The cross-headspeed was 0.5 mm/min. A 5 kN load cell was usedand the sampling rate was 0.02 kHz. After separa-tion, the fracture interfaces were investigated withloupes at a power of 2x. Furthermore, the enamelsurfaces were replicated with a polyvinylsiloxaneimpression material (Silagum, DMG, Hamburg, Ger-many; Siliplast, Detax, Ettlingen, Germany) and anepoxy resin (Stycast 1266, Emeerson & Cumming,Westerlo-Oevel, Belgium). The replicas and the resinsamples were mounted on SEM holders with a con-ducting glue (Leit-C-Plast, Neugebauer, Münster,Germany). After sputtering with a Balzers sputterdevice (Model SCD 030, Balzer Union, Liechteh-stein) for 2 minutes at 40 mA, C.05 bar, the sam-ples were investigated ¡n a scanning electronmicroscope (Model 1810 SEM, Amray, Bedford,MA).

RESULTS

The results are summarized in Tables 1 and 2. Themost important result is that regardless of the con-ditions used, the bond strength to etched enamelwas significantly higher than the bond to enameltreated by KCP (one-way ANOVA, Student-NewmahKeuls test). As shown in Table 1, the particle sizehad a more important influence than the air pres-sure on the bond strength, as was revealed by atwo-way ANOVA (by pooling the other factors, dis-tance and angle. Table 3). The 27-pm particlesyielded a higher bond strength than the 50-|jm par-ticles. Single comparisons between groups (f test,adjusted according to Bonferroni) revealed that the27-Mm particles yielded a higher bond strength withan air pressure of 0.97 MPa.

26 The Journal of Adhesive Dentistry

ahnet al

Table 1 Test parameters and tensile bond strength of composite toblasted enamei

Distanceof nozzle

1

111111111113333

- P £ 0.01

Angleof nozzle

( • >

SO8080808080808080SOSOSOSO

so4545

Airpressure

(MPal

0.280.550.690.830.971.100.280.550.690.830.971.100.S30.830.830.S3

Particlesize(pm)

27272727272750505050505027502750

(s

3.253.253,503,244,823.862.5-13.523.022.722.462.332.461.982.543,36

Strength (MPa)tandard deviation)

(1.37) n

(1.15) n(1,03)(1.72) n(1.45) J *

-

(1.86)(1.04)(1.66)(1.03)(0.82)(0.68)(1.54)(1.51)(0.90) - |(0.98)(0.99) J

Table 2 Tensiie bond strengtb of composite to acid-etcbed enamei atvarious etching times

Etching tirhe(s)

204060

untreated ehamel

- P 5 0.01

Tensile bohd strength [MPs|(stahdard deviation)

19,54 (3,96) n19,66 (3,38) ~| ' ~\14,44 (3.47) J " J -|_ •1,12 (0.99) J ' J

Tabie 3 Two-way ANOVA: influence of particie size and air pressure ontensiie bond strength

Bond strengthfhaih effects(combined)d

PTwo-way ihteractions, p*dModelResidualTotal

d - particle sife.p = arr pressure.

Sum ofsquares

31,09825.167

7.4S826.79957.897

213.846271.743

df

6155

11122133

Meansquare

5.1S325,167

1,4985.3605.2631,7532.043

F

2.95714.3580.8543,0583.003

Sig.

0,0100.0000.5140.0120,001

27

Air pranur« piPa] n^. «tching tim« \^

Fig 2 Tensile bond strength of composites bonded to enamei treated with KCP applied with different conditions (particle size, airpressure, beam angle) at a constant distance of 1 mrr and a constant time (10 seconds). As controis, tensile bond strength ofcomoosites on etched enamei (positive) and untreated enamei (negative) are shown (* P < 0,01),

No main effects were found for the angie, Oniywith 50-|jm partioies a higher bond strength wasfound with a 45-degree angle between the surfaceofthe sample and the direction ofthe nozzle.

The sampies in which the enamel was etohed for60 seconds shov/ed significantly iower bondstrength than those with etching times of 20 and40 seconds (Tabie 2, Fig 2). The negative control(untreated enamel) showed the iowest bondstrength (P < 0,01), which was lower than etchedenamel, but aiso lower than enamei treated withKCP (Fig 2),

As a rule, adhesive faiiure modes were observedin the samples where the enamel was conditionedwith KCP, A typicai example is shown in Fig 3, Thesame was true for the samples with untreatedenamei. Ail samples in which the enamel wasetched for 20 seconds showed cohesive fractures

of the composite (Fig 4), whereas 40 seconds ofetching time also produced mostly cohesive com-posite fractures except in two cases: in one case anenamel fracture was observed (Fig 5) and in theother sampie there was a mixed fracture. The sam-pies in which the enamei was etched for 60 sec-onds showed the foiiowing fracture modes: 64%mixed, 29% cohesive, and 7% adhesive fractures.

DISCUSSION

Based on our results, it is obvious that the bondstrength of composite to etched enamel is muchhigher than to enamei treated with KOP, As com-pared to untreated enamel, a significant but smallincrease of bond strength could be observed fortheKCP sampies. It is more than questionabie whether

28 The Journal cf Adhesive Dentistry

Fig 3 Adhesive fracture at the enamel-composite interface.Enamel was conditioned with KCP, (a) Enamel after separa-tion from adhesive and composite material, (b) Enamel notbonded. (SEM, 25x)

Fig 4 Cohesive fracture within the composite. The hole inthe center was created by the separation from the screw tip,enamel etched for 20 seconds, (a) Enamei. (b) Composite(SEM, 25x)

Fig 5 Cohesive fracture within the enamei (enamel etchedfor 60 seconds}, (a) Enamei prisms. [b¡ Enamel surface, ¡c)Exposed dentin due to enamel fracture. (SEM, 23x¡

the tensile bond values ofthe KCP samples are suf-ficient to withstand the polymerization shrinkagestress^ in a clinical situation. Our results are con-gruent with those of Wiedemann et al,12 whoshowed with profiiometry and tensile bond tests aiow, but as compared with etched enamei, insuffi-cient increase ofthe composite-enamei bond afterKCP treatment. Berry and Ward,i Nikado et al,^Roeder et al,io and Valentino and Nathanson^i-have demonstrated that conditioning the enameiwith KCP IS not sufficient to obtain a good bond tocomposites. The bond strength values reported byValentino and Nathanson,ii who used maleic andorthophosphoric acid as a conditioning agent, weremuch lower than those in the present study. Thiscouid be explained by either the thermocyciing orthe different test used (shear bond], Roeder et al^oreported simiiar bond strength values also usingOptibond and Herculite. However, no details aboutthe air pressure used for biasting were given. Berryand Ward^ reported values between 8.5 and 13.8MPa based on tensiie tests. This is substantiailyhigher than the tensile strength reported in the pres-ent study. However, they did not specify the particiesize used for blasting.

In contradiction to our results, Laureli et a l ' andKeen et al^ reported equal shear bond strength val-

29

ues for etched and KCP-treated enamel. However,

these results are only partly comparable because of

the different methodology (shear bond test). Also,

the details of their investigations are unknown for a

vaiid comparison,5 Furthermore, their values are

higher than the accepted standard in the literature

for shear bond strength on etched enamel.

The results of the fracture interface evaluation

reported in the present study support the superior-

ity of the etching technoiogy. All sampies in which

the enamel was conditioned with KCP showed ad-

hesive fracture modes, whereas the majority of the

etched samples showed cohesive fractures indicat-

ing that the bond strength at the interface is higher

than either the strength of enamel or composite.

CONCLUSION

Since the bond strength of composite bonded to

KCP-treated enamel is considerabiy lower than that

of composite bonded to etched enamel, KCP can-

not be used as a substitute for etching technoiogy.

As a consequence, if the KOP technique were used

for cavity preparation, the enamel margins should

be etched before the insertion of the composite.

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30 The Journal of Adhesive Dentistry