INTRODUCTION. Restorative Dentistry. Restorative Dentistry

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1 Restortive Dentistry Brz Orl Res 2005;19(1):30-5 Restortive Dentistry The effect of insertion nd photopolymeriztion techniques on microlekge of Clss V cvities quntittive evlution Efeito de diferentes técnics de inserção e fotopolimerizção n microinfiltrção de cviddes Clsse V vlição quntittiv Fbin Mntovni Gomes Frnç* Frederico Seidi Hori** Alex José Souz dos Sntos*** José Roberto Lovdino**** ABSTRACT: The im of this in vitro study ws to evlute by spectrophotometry the influence of the incrementl technique nd progressive light curing in the microlekge of Clss V cvities. Forty smples were prepred with clss V cylindricl cvities on the buccl root surfce of bovine incisive teeth nd filled with composite resin (Z250). The smples were divided into four groups: I: cvity ws bulk filled nd the composite ws light cured for 40 seconds; Group II: cvity ws bulk filled nd soft-strt polymeriztion ws used; Group III: cvity ws filled with the incrementl technique in two cots nd light cured for 40 seconds; Group IV: cvity ws filled with the incrementl technique in two cots nd light cured with soft-strt polymeriztion. After the restortion, the specimens were thermlly stressed for 3,000 cycles in bth t 5 ± 2 C nd 55 ± 2 C, protected with nil enmel, colored with 2% methylene blue nd cut into sections. These sections were triturted nd the dye ws recovered with PA ethnol nd the superntnt ws evluted. The dt were submitted to ANOVA nd the results showed the following verges: bulk filled nd conventionl photopolymeriztion (I) µg/ml; bulk filled nd progressive photopolymeriztion (II) µg/ml; incrementl insertion nd conventionl photopolymeriztion (III) µg/ml; incrementl insertion nd progressive photopolymeriztion (IV) µg/ml. No significnt sttistic differences were observed mong the men vlues. The Degulux soft-strt equipment probbly emits too high initil light intensity to promote progressive photopolymeriztion. DESCRIPTORS: Dentl lekge; Dentin-bonding gents; Composite resins. RESUMO: O objetivo deste estudo in vitro foi vlir trvés de espectrofotometri influênci ds técnics de inserção incrementl e de fotopolimerizção progressiv n microinfiltrção de cviddes Clsse V. Form preprds 40 cviddes cilíndrics n superfície rdiculr vestibulr de incisivos bovinos e preenchids com resin compost (Z250). Os espécimes form divididos em qutro grupos: I: resin foi inserid em cmd únic e fotopolimerizd por 40 segundos; II: resin foi inserid em cmd únic e fotopolimerizd progressivmente com um prelho soft-strt ; III: resin foi inserid em dus cmds e fotopolimerizd por 40 segundos; IV: resin foi inserid em dus cmds e fotopolimerizd progressivmente. Em seguid os espécimes form submetidos ciclos térmicos 5 ± 2 C e 55 ± 2 C, preprdos, cordos com zul de metileno 2%, secciondos, triturdos e vlidos em espectrofotometri pr se quntificr o cornte infiltrdo. Os ddos form submetidos ANOVA. Os resultdos demonstrrm s seguintes médis: inserção únic e fotopolimerizção convencionl (I): 0,06075 µg/ml; inserção únic e fotopolimerizção progressiv (II): 0,04030 µg/ml; inserção incrementl e fotopolimerizção convencionl (III): 0,04648 µg/ml; inserção incrementl e fotopolimerizção progressiv (IV): 0,04339 µg/ml. Não form encontrds diferençs esttístics significtivs entre s médis. Observou-se que o prelho Degulux soft-strt emite um intensidde de luz inicil muito lt pr promover fotopolimerizção progressiv. DESCRITORES: Infiltrção dentári; Adesivos dentinários; Resins composts. INTRODUCTION Light-ctivted resin composite restortive mteril hs revolutionized clinicl dentistry, nd modern dvnced technology continues to develop resin composites 15. As result, new resin composites hve become widely used for mny purposes in restortive dentistry 2. Light cured mterils offer * PhD, Professor, Deprtment of Restortive Dentistry, São Leopoldo Mndic Dentl Reserch Center. ** Reserch Assistnt; ****PhD, Hed Professor Deprtment of Restortive Dentistry, School of Dentistry of Pircicb, Stte University of Cmpins. *** PhD, Assistnt Professor, School of Dentistry, Potigur University. 30

2 the prctitioner the freedom to time the initition of polymeriztion, so tht the restortion cn be plced nd shped with ese 4. The most commonly used monomer for both nterior nd posterior resins is Bis-GMA, which hs higher moleculr weight thn methyl methcrylte, nd helps to reduce the polymeriztion shrinkge. Two physicl phenomen invribly ccompny the polymeriztion of resins of dentl interest such s: the development of het nd the volumetric shrinkge of the cured mteril compred with the prepolymer 18. The polymeriztion shrinkge of resin composite cn crete contrction forces tht my disrupt the bond to cvity wlls. This competition between the mechnicl stress in polymerizing resin composites nd the bonds of dhesive resins to the wlls of restortions is one of the min cuses of mrginl filure nd subsequent microlekge, leding to secondry cries 2,22. The shrinkge of the cured resin cn be usefully divided into two prts which re: the pre-gel shrinkge (in which volumetric chnge cn be compensted by continued flow of the mteril) nd the post-gel shrinkge (in which the shrinkge of the polymer is ccompnied by the development of modulus of elsticity) 18. As result, the stresses begin to grow nd cn cuse dhesion filure 10 or deformtion of the surrounding tooth structure resulting in microcrcks in the cervicl enmel tht predispose the tooth to frcture 22. Dvidson et l. 3 (1984) reported the influence of contrction stresses, generted during polymeriztion shrinkge, on dhesion of light-cured nd chemiclly cured resin composite to dentin, in both two-dimensionl nd three-dimensionl models. In the two-dimensionl model, the bond strength could withstnd the contrction forces becuse the dhesion of the resin composite ws performed on flt dentin surfce. This configurtion llowed lrge, free, nd unbounded surfce, which permitted the flow of the resin cross the free surfce during its polymeriztion shrinkge, thereby minimizing stresses t the bonded surfce. In the three-dimensionl cvity models, the composite ws bonded to two or more cvity wlls 2. In such situtions, the resin flow ws limited or restricted, leding to n increse in the stress generted t bonded surfces. Techniques re used to minimize the effect of shrinkge polymeriztion, like progressive photopolymeriztion nd incrementl insertion technique. In the progressive photopolymeriztion, the composite is irrdited by low initil light intensity followed by norml light intensity. With low initil light intensity, the resin stys for longer period in the pre-gel stge of contrction, in which volumetric chnge cn be compensted by the continuous flow of the mteril 18. The im of soft-strt polymeriztion is to prolong the time spn before reching the gel point by low light-curing intensities nd to increse the flow cpbility of the mteril. Afterwrds, high light intensities re necessry for complete polymeriztion nd optiml mechnicl properties 12. The purpose of the incrementl techniques is to minimize the stress generted by polymeriztion contrction, inserting resin lyers in the cvity nd reducing the bonded res. As result, we hve lower C-fctor, which llows the resin to flow t the free surfces 2. It is necessry to verify if the vrition methods of photopolymeriztion intensity nd incrementl technique re being effective in preventing mrginl microlekge; therefore, the im of this study ws to evlute in quntity by spectrophotometry the influence of the incrementl technique nd progressive light curing in the microlekge of clss V cvities. MATERIAL AND METHODS In this study, forty bovine incisors were used. After being extrcted, the teeth were clened of gross debris, polished nd exmined under microscope (4 X) (Crl Zeiss, Gottingen, Germny) in order to discrd dmged teeth. All teeth were stored in distilled wter t 5 C for three months. Cylindricl clss V cvities of 1.85 ± 0.05 mm in dimeter nd 1.5 mm in depth were prepred 4 mm below the cemento-enmel junction t the buccl root surfce with specil dimond bur (KG Sorensen Ind. Com. Ltd., Brueri, São Pulo, Brzil) t high speed nd dequte wter cooling. Ech dimond bur ws replced fter every five cvity preprtions. The cvities were restored with Single Bond (3M ESPE, St. Pul, MN, USA) nd microhybrid resin (Z250, 3M ESPE, St. Pul, MN, USA), nd light cured with Degulux soft-strt Curing Light unit (Deguss-Hüls AG, Hnu, Germny). Before beginning the experiment, the light intensity of the light-curing unit ws ssessed with rdiometer (Demetron Reserch Corportion, Dnbury, CT, USA); the initil low light intensity ws 400 mw/cm 2 for 10 seconds nd the high intensity ws 600 mw/cm 2 for 30 seconds. The light intensity for continuous photopolymeriztion ws 600 mw/cm 2. The root pices were removed with dimond disc (KG Sorensen Ind. e Com. Ltd., B- 31

3 rueri, SP, Brzil) nd seled with composite resin (Z250, 3M ESPE, St. Pul, MN, USA) nd Super Bonder cynocrylte dhesive (Henkel Loctite Adhesives Ltd., Itpevi, São Pulo, Brzil). After the preprtion, the teeth were rndomly ssigned to four test groups of 10 specimens ech (Tble 1). Group I: The cvity ws bulk filled nd the composite ws light cured for 40 seconds; Group II: The cvity ws bulk filled nd softstrt polymeriztion ws used with n pproprite light curing unit (Deguss-Hüls AG, Hnu, Germny); Group III: The cvity ws filled with the incrementl technique in two cots nd light cured for 40 seconds; Group IV: The cvity ws filled with the incrementl technique in two cots nd light cured with soft-strt polymeriztion. After the restortions, ll specimens were immersed in distilled wter for 24 hours; fter tht, the restortions were polished with SofLex luminum oxide discs (3M ESPE, St. Pul, MN, USA). The restored teeth were subjected to 3,000 temperture cycles between 5 C nd 55 C. The cycles consisted of 60 seconds in ech bth with n exchnge time of 15 seconds between bths. After cycling, the whole tooth surfce except 1 mm window round the restortion ws covered with two cots of nil vrnish (Risqué, Nisi, SP, Brzil), nd immersed in 2% methylene blue solution (Merck, Drmstdt, Germny) for 12 hours t 37 C. After this time, the smples were rinsed in tp wter nd dried. The specimens were cut with dimond discs nd 5 mm sections of the root including the restortions were removed. The nil vrnish nd superficil stin were removed from ech specimen with grded series of SofLex luminum oxide discs (3M ESPE, St. Pul, MN, USA). Ech specimen ws weighed nd ground into powder in mill for hrd tissues (Mrconi Equip. Ltd., Pircicb, São Pulo, Brzil). The powder specimen ws weighed gin, nd if the difference between the initil nd the finl weight ws TABLE 1 - Division of ech group ccording to photopolymeriztion nd insertion technique. Groups I II III IV Tretments Bulk insertion; conventionl polymeriztion Bulk insertion; soft-strt polymeriztion Incrementl insertion; conventionl polymeriztion Incrementl insertion; soft-strt polymeriztion greter thn 10%, the smple would be discrded. The powder of ech smple ws individully immersed in glss tube contining 4 ml of bsolute lcohol PA (Merck, Drmstdt, Germny), for 24 hours, in order to dilute the methylene blue solution. After this procedure, the solutions were centrifuged (Tomy IC 15NA, Tomy Ind., Tokyo, Jpn) t 3,000 rpm for 3 min. The superntnt ws nlyzed using spectrophotometer (Beckmn DU 65, Instruments, Inc., Fullerton, CA, USA) djusted with wvelength of 565 ηm. In order to determine the bsorbency, the spectrophotometer ws djusted with pproprite wvelength for the methylene blue, corresponding to the mximum bsorbency for the dye. To clibrte the spectrophotometer, the bsorbency of the stndrd solutions (contining from 0 to 4 µg of dye/ml) ws determined t wvelengths rnging from 400 to 700 ηm, nd the mximum vlue ws obtined t 565 ηm. In this wvelength, the bsorbencies for the stndrd solutions were obtined. With these vlues, coefficient of liner correltion (r = ) nd stright-line eqution (y = x ) were determined. To clculte the quntity of dye in the dye concentrtion (µg/ml) tht infiltrted between the tooth nd the restortion, the y ws chnged for the bsorbency vlue of ech smple in liner eqution. The microlekge of ech specimen ws expressed s µg of dye/ml, lower vlues indicting lower microlekge. Dt were sttisticlly nlyzed by two-wy ANOVA nd Tukey- Krmer test. RESULTS The results of the nlysis of vrince (two-wy ANOVA) expressed in Tble 2 nd Grph 1 reveled no significnt differences in men microlekge bsed on two independent vribles (p > 0.05). The incrementl technique nd the soft-strt polymeriztion did not influence the microlekge round clss V cvities in root surfces of bovine teeth. The generl vrition coefficient of the experiment (in percentge) ws No differences were detected when the mens of microlekge were compred between groups. DISCUSSION Secondry cries t the tooth/restortion interfce nd prticulrly t the dentin or cementum/restortion interfce is the min reson for the replcement of resin composite restortions 12. It hs been demonstrted tht the ingress of bc- 32

4 TABLE 2 - Mens nd Stndrd Devitions (SD) of microlekge (µg/ml). Groups n Men nd SD Tukey I - Bulk filling/conventionl photopolymeriztion (± ) II - Bulk filling/ soft-strt photopolymeriztion (± ) III - Incrementl filling/conventionl photopolymeriztion (± ) IV - Incrementl filling/ soft-strt photopolymeriztion (± ) Sme letters indicte no significnt difference. Dye concentrtion ( g/ml) I II III IV Groups GRAPH 1 - Comprison between mens of dye concentrtion ccording to the different insertion nd photopolymeriztion techniques. Sme letters indicte no significnt difference. teri t the tooth/restortion interfce is the min reson for secondry cries nd relted pulpl rections 1,14. In most lbortory studies in which microlekge is evluted, liner mesurements of the extent of dye penetrtion t the tooth/restortion interfce re performed 5. These two-dimensionl liner mesurements do not tke into ccount the density of the lekge in three-dimensionl lekge-pttern. The spectrophotometric dye-recovery method used in the present study llows for the direct nd quntittive mesurement of lekge volumetriclly 19. The shrinkge tht occurs during the conversion of the monomer to the polymer of the dhesive nd of the composite resin certinly works ginst the formtion of n dhesive bond between the resin nd the dentin nd lso contributes to the brekdown of the composite resin itself in vivo over time 8. Mny fctors hve direct effect on the polymeriztion shrinkge of composite resin: size of the restortion, cvity configurtion, plcement technique (incrementl or bulk), nd curing method (chemicl or light-curing). The role of cvity configurtion on the development of polymeriztion stresses with resin composite ws demonstrted by Feilzer et l. 6 (1987). Dvidson et l. 3 (1984) reported tht when polymeriztion is restricted to one direction only, substntil mrginl bond tht withstnds contrction forces occurs, becuse the resin composite still flows. When the composite ws restricted to three dimensions rther thn one, lmost no bonds withstood the polymeriztion shrinkge. Generlly, the fewer the free, unbonded res present in cvity, the less the resin is ble to flow, nd therefore the greter the contrction stress t the bonded surfces 2. In this study, Clss V cvities were used with high C-fctor (4.8) to evlute the role of the incrementl technique nd soft-strt polymeriztion in diminishing the effect of cvity configurtion in composite polymeriztion shrinkge. Another importnt fctor tht influences the bond qulity is the light intensity. An idel light intensity (233 mw/cm 2 for 1 mm increments) is necessry to chieve deep nd complete polymeriztion of the mteril nd good mechnicl properties 8,16. However, previous studies hve shown tht the mrginl dpttion of light-curing composites cn be improved by light curing the mteril slowly with low intensity light 10. Pires et l. 13 (1993) described tht light intensity diminishes s the curing tip is moved frther from the composite resin restortive mteril. Dvidson-Kbn et l. 4 (1997) demonstrted tht exposure time nd light intensity positively ffects the degree of conversion, while smple thickness negtively influences the cure. When the conversion rte on the top surfce ws tken s reference, the curing t depth of 2 mm becme criticl 16. For tht reson, pproximtely 1.0 mm ws chosen s the incrementl lyer thickness nd lso to gurntee good mechnicl properties. However, cvity depth ws 1.5 mm; becuse of tht, the bulk-filled groups lso hd smll lyers, perhps cusing no differences mong mens of microlekge. 33

5 According to Versluis et l. 20 (1996), in finite element study, the totl mount of composite necessry to fill cvity becomes lower for n incrementl filling technique compred with the single bulk filling technique in the sme occlusl contour. This results in higher residul shrinkge stresses for the incrementl filling method. The polymeriztion contrction of ech individul filling increment will cuse some deformtion to the cvity, decresing its volume. Such sitution must result in higher stress stte of the toothrestortion complex. Also, the type of incrementl technique ffects the mount of pplied restortive mteril, nd therefore cusp displcement. These findings re in contrst with some studies tht stted tht the incrementl technique reduces the polymeriztion shrinkge effects 2,21. In spite of this study s results, the incrementl techniques should be used to chieve both good mechnicl properties ssocited with high rte of conversion nd close dpttion of the resin composites to the cvity wll. Previous studies hve demonstrted tht it is ctully possible to reduce mrginl gp of resin composite by strting the light polymeriztion rection grdully nd progressively 4,9. Mehl et l. 11 (1995) showed tht hrdness is not chnged by using pre-polymeriztion step with light intensity reduced to bout 50% of the finl curing intensity vlues, if the specimens re post cured for t lest 20 seconds with the full intensity. The initil light intensity reported in the relted literture rnges from 150 to 250 mw/ cm 2 ; nd the finl intensity is round 800 mw/ cm 2. 7,10 It is known tht the Degulux soft-strt, the equipment used in this study, emits initil light intensity of 400 mw/cm 2 nd finl light intensity of 600 mw/cm 2. Contrsting the vlues of light intensity emitted by the Degulux soft-strt with the vlues showed by Skguchi, Berge 17 (1998) (low intensity vlues vry from 116 to 241 mw/ cm 2 ) nd Friedl et l. 8 (2000) (150 mw/cm 2 - low intensity nd 800 mw/cm 2 - high intensity), we cn conclude tht the initil light intensity emitted by this equipment might be too high to promote progressive photopolymeriztion. A combintion of fctors such s shrinkge rte, flow cpcity, completeness of polymeriztion nd bond strength my ffect the resulting shrinkge nd its impct upon the integrity of the tooth-restortion complexes, nd influence the clinicl success of restortions. However, this study shows tht in smll Clss V cvities with high C-fctor the incrementl filling technique nd progressive photopolymeriztion (initil light intensity 400 mw/cm 2 nd finl 600 mw/cm 2 ) did not reduce the microlekge compred to the bulk filling technique nd conventionl light cure (600 mw/cm 2 ). CONCLUSIONS The incrementl technique nd the soft-strt polymeriztion did not influence the microlekge in clss V cvities in root surfces of bovine teeth. The Degulux soft-strt equipment probbly emits initil light intensity too high to promote progressive photopolymeriztion. ACKNOWLEDGMENTS The uthors re indebted to the Deprtment of Phrmcology, Anesthesiology nd Therpeutics, School of Dentistry of Pircicb, Stte University of Cmpins (UNICAMP) for the use of lbortory equipment. REFERENCES 1. Brnnstrom M, Nyborg H. Pulpl rection to composite resin restortions. J Prosthet Dent 1972;27(2): Crvlho RM, Pereir JC, Yoshiym M, Pshley DH. A review of polymeriztion contrction: the influence of stress development versus stress relief. Oper Dent 1996;21(1): Dvidson CL, de Gee AJ, Feilzer A. The competition between the composite dentin bond strength nd the polymeriztion contrction stress. J Dent Res 1984;63(12): Dvidson-Kbn SS, Dvidson CL, Feilzer AJ, de Gee AJ, Erdilek N. The effect of curing light vritions on bulk curing nd wll-to-wll qulity of two types nd vrious shdes of resin composites. Dent Mter 1997;13(6): Eick JD, Robinson SJ, Byerley TJ, Chppelow CC. Adhesives nd nonshrinking dentl resins of the future. Quintessence Int 1993;24(9): Feilzer A, De Gee AJ, Dvidson CL. Setting stress in resin composite in reltion to configurtion of the restortion. J Dent Res 1987;66(1): Feilzer AJ, Dooren LH, De Gee AJ, Dvidson CL. Influence of light intensity on polymeriztion shrinkge nd integrity of restortive-cvity interfce. Eur J Orl Sci 1995;103(5): Friedl KH, Schmlz G, Hiller KA, Mrkl A. Mrginl dption of Clss V restortions with nd without softstrtpolymeriztion. Oper Dent 2000;25(1):

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