[DEntal Influence of Light Intensity on Polymerization Shrinkage and Integrity of Restoration-cavity Interface | Dental Composite

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  £iir J Oral Sci 1995: 103: 322-326Printed in Denmark  ,  All rights reservedCopyright  ©  Munksgaard 1995 EUROPE-APJ JOURNAL OF ORAL SCIENCES fSSN 0909-883 Influence  of  light intensity  on polymerization shrinkage  and integrity  of  restoration-cavityinterface A.J.Feilzer, L. H.Dooren, A. J.de Geeand  C.  L. Davidson Department of Dental Materials Science,Academic Center for Dentistry Amsterdam(ACTA), Amsterdam, the Netherlands Feibei- AJ, Dooren LH, de Gee AJ, Davidson CL: Influence of light intensity onpolymerization shrinkage and integrity of restoration-cavity interface. Eur J OralSci 1995; 103: 322-326.  © Munksgaard, 1995.The restJts of this study showed that the use of high intensity curing light unitsnegatively affected the integrity of the restoration-cavtty interface in class V re-storations. This is explained by the high reaction rates of light curing resincomposites. The interfacial integrity was better preserved with low light intensityas it extends the visco-elastic stage of the setting materials, thereby moderatingthe setting stress development. The ultimate polymerization shrinkages for bothconditions were equal, which suggested equal degrees of conversion and thusequal material properties. The results may alleviate the trend in using higherintensity light curing units and in particular the development of units with laserbeams in an attempt to further increase conversion rates. Key words: Adaptation; Setting stress: Flow;Light intensfty; Conversion rateA, J, Feilzer, ACTA, Department of Dental Mater-ials Science, Louwesweg 1, 1066 EAAmsterdam, the Netherlands, Telefax: +31-206692726, E-mail: a,feilzer(a;acla.niAccepted for pubiicafion March 1995 A high degree of conversion, which is primarilyrelated to curing light intensity and exposure time,is an important factor for the longevity of a resto-ration. However, the higher the conversion in resincomposites, the higher the shrinkage (1). In adhe-sive resin composite restorations, shrinltage bringsabout stress, which frequently leads to marginalleakage. Hence the objective of obtaining optimalmaterial properties of composite restorations con-flicts Vfcfith the objective of achieving optimal conti-nuity of integrity at the composite-cavity interface.A well-accepted approach to reduce this stressin direct resin composite restorations is the use ofelastic stress absorbing lining materials whichmay render sufficient strain to replace to the stress (2,  3). For heavily loaded restorations however, thismethod may negatively affect the stability and lon-gevity, as the restoration may flex in the low mod-ule support. Another approach for polymerizationstress reduction which does not affect the final me-chanical stability of the restoration can be foundin controlling the plasticity (flow capacity) of therestoration during curing (4). Based on studies ofchemically initiated composites, it has been sug-gested that a positive relation exists between set-ting time and flow characteristics of resin compos-ites (5). An increase in the setting time of light cur-ing composites can be achieved by decreasing thecuring light intensity The question is to what ex-tent the curing light intensity can be reduced with-out sacriflcing the ultimate material properties.The light intensity of commercially available lightunits varies from approximately 50 to 900 mW/cm^ (6,  7). RuEGGEBERG  et al  (8) have shown that thelight intensities should be above 233 mW/cm' toprovide sufficient energy for an adequate cure ofcomposite layers of I mm thickness. So, at thelowest acceptable value of 233 mW/cm^, and ap-plying the composite in separately cured layerswith a maximum thickness of 1.0 mm, the ulti-mate cure and thus the stiffness of the restorationmay not be influenced. Only with very low inten-sity curing light units, even with prolonged lightexposure times, are problems with curing to beexpected.The aim of this study was to determine the in-fluence of the intensity of the curing light on theintegrity of the restoration-cavity interface of  invitro  Class V composite restorations and on thepolymerization shrinkage as a measure for the  Light Intensity  versus  Integrity of Adhesion  323 Table 1 Materials used in this study BrandManufacturerBatchClearfil Liner Bond System -  CA Conditioner- SA Primer II- Photo Bond III- Protect LinerComposite- Clearfil Lustre (US)Cavex Holland BV, Haarlem, NLCavex Holland BV41119 0012A degree of conversion. In addition, the effect of astress absorbing elastic intermediate was investi-gated. Materials and methods Determination of the integrity of the restoration-cavity  interface  -  The adhesive Clearfil Liner Bondsystem in combination with the composite restor-ative material Clearfil Lustre was used in this study(Table 1). The Clearfil Liner Bond system consistsof a surface conditioner (CA agent); a primer (SAprimer); a bonding agent (Clearfil Photo Bond);and an elastic liner (Protect Liner). The use of Pro-tect Liner is advised by the manufacturer for allclinical cases with a vital pulp. The CA agent is acitric acid - calcium chloride based tooth surfaceconditioner to be used for both dentin and enamel.In 24 extracted human teeth (premolars and ca- nines),  V-shaped Class V cavities were prepared(Fig. 1) in the labial and lingual surfaces with thecervical margin approximately 1 mm below theenamel-cement junction. Cavities were made witha 109/010 diamond bur at high speed with watercoolant as uniform as possible with a depth of 1.5±0.1  mm in the dentin. The teeth were random-ly divided into three groups. The groups differedfrom each other with respect to the elasticity ofthe materials used to fill the cavity. In group I therestorative procedure was according to the manu-facturer's instructions: CA agent / SA primer /Photo Bond / Protect Liner / Clearfil Lustre. Ingroup II, the teeth were restored as group I,, butwithout the use of Protect Liner. In group III, thecavities were filled as group I, but the compositeClearfil Lustre was completely replaced by ProtectLiner. For each group, half of the samples werecured with a Translux CL curing unit (Kulzer,Wehrheim, Germany) at a light intensity of 250mW/cm^ (measured with the Curing Radiometer,Demetron Research, Danbury, CT, USA) and theother half  was  cured with the Elipar II light curingunit (ESPE, Seefeld, Germany) with an intensityof 650 mW/cm^. The restorations were finishedusing a finishing diamond bur and Sofex discs(3M, St Paul, MN, USA). After thermocycling  400  X, 15-65°C) the specimens were sectionedbucco-lingually through the restoration with a dia-mond saw (Isomet, Buehler, Evanston, IL, USA)at slow speed. Impressions were made with a poly-vinylsiloxane impression material (Type I wash,,KERR (Europe), Basel, Switzerland) after dryingthe sectioned surface by way of wiping off the sur-face with a tissue. This superficial drying was doneto avoid the risk of creating artifacts by dehydra-tion. Araldite (Ciba-Geigy NV, Arnhem, the Neth-erlands) casts were made which, after gold sputter-ing (Sputter coater S150B; Crawley, Edwards, WestSussex, UK) were examined in the ScanningElectron Microscope (SEM) (Philips XL20, Eind- Table 2 Integrity of the restoration-cavity interface mean scores as percentage of perfect integrity) Group IGroup IIGroup III250 mW/cm^650 mW/cm^250 mW/cm^650 mW/cm^250 mW/cm=650 mW/cm- 1 100% 88% 100%100%100% 88% 9 100%iOO%100%100%100%100% Zone 3 100% 69%88%69% 100% 94%4 100% 94% 100%100%100%100% 5 100%100%100% 94% 100%100% Group I restorative procedure according to manufacturer: CA agent/SA primer/P'hoto Bond/Protect Liner/Clearfil Lustre restor-ative.Group II as groiip I, but without Protect Liner,Group III as group 1, but instead of OeaTfll Lustre, Protect Liner was used as the restorative material.  324 Eeilzer et al. Eig. 1.  Schematical representation of the experimental Class Vrestoration and the division of the restoration-cavity interfaceinto the five zones in which the integrity was scored. hoven, the Netherlands) at magnifications to 1000  X. To judge the extension of the integrity ofthe restoration-cavity interface, the interface wasdivided into five zones of equal length (Fig.l). Theinterfacial condition in each zone was scored on atwo-way scale: open or closed . For example, ifinterfacial separation was observed in zone 1 andin zones  ?>-A,  then zones 1, 3 and 4 scored open while zones 2 and 5 scored closed . For eachgroup, mean scores per zone were calculated aspercentages of perfect integrity. Statistical compar-ison of the results was done using a loglinearmodel analysis with SPSS-pc package (9). Determination of polymerization shrinkage -  Theinfluence of light intensity on the polymerizationshrinkage of Clearfil Lustre was determined with alinometer (10) on 1.50-mm-thick samples for lightintensities of 250 mW/cm- and 650 mW/cm^. Thelinear polymerization contraction (lin%) was regis-tered continuously with a computer for a periodof  60  min at 23±  1°C.  The data were converted intovolumetric values (vol%=31in% - 0.03(hn%)2) (10).For each light intensity, five measurements werecarried out and averaged, to obtain one meanshrinkage curve per intensity Statistical differencesbetween both conditions were revealed by way of ar-test. Results Table 2 shows the mean scores as percentages ofperfect integrity of the restoration-cavity interfacefrom the SEM observations. Table 3 gives the re-sults of the statistical analysis. On a P=0.05 level,no significant effect of the restorative method(Group I vs II vs III) was found. Inspection of theparameter estimates showed that the restorationscured with the low intensity light curing unit (250mW/cm-), had a significantly lower amount of de-fects at the interface than with the high intensityunit (650 mW/cm-). No significant difference perzone could be found. Closed zones (perfect integri-ty) were significantly scored more than open zones(interfacial separation).Figs. 2 and 3 show the mean polymerizationshrinkage curves of Clearfil Lustre cured with 250mW/cm' and 650 mW/cm- for the first 2 min andfor 60 min, respectively. The standard error of thepolymerization shrinkage determinations grad- -6S0 mW/cm -250mW/cni=time (mini Fig, 2 Polymerization shrinkage  n = 5)  in relation to curinglight intensity of Clearfil Lustre (1,5 mtn thickness) for the first2 min of cure. The curves differ significantly frotn 0.05 minonward (P<0,05).Table 3 Results of  statistical  analysis l=zone (1 to 5)2=group (1 to 3)3=m'ft'/cm- ('250 mW/cm^'4=score ('open' or 'closed')Model#1 [123j[4]#2 [123j[141#3 [I23][24]#4 [123][34]or '650 mW/cm^'G^52.9233.2749.7938.75df23202122 SEi, Difference model #i and #2 G^= 19.65Difference model #1 and #3 G^=3,13Difference model #1 and #4 G^=14,17 -650 mW/cm -250  mVltonf 30lime (man) Eig. 3.  Polymerization shritikage (n=5) in relation to curinglight intensity of Clearfil Lustre (1.5 mm thickness) for the firsthour of curing. The curves do not differ significantly frotn ap-proximately 15 tninutes onvfard (P<0.05).  Light Intensity  versus  Integrity of  dhesion  3 5 ually increased from 0.01 vol% for the early partof the determination to 0.05 vol% for the laterpart. The shrinkages of Clearfil Lustre resultingfrom activation at 250 mW/cm^ and 650 mW/cm-showed significant differences between 3 s and 15min. Past 15 min, the shrinkage values becameequal. Discussion The bond strength of the latest generation dentinbonding agents is often superior to the cohesivestrength of the substrate (11-13). However, mar-ginal leakage studies with the same materials alsoreported separation of some sort at the interface (11,  13, 14). Apparently, adhesion of the restora-tion to the entire cavity wall is not solved only bystrong bonding. Shrinkage stress and not inade-quate bond strength is to blame for adhesive fail-ures in composite resin restorations. The results ofthe SEM observations (Table 2) showed, in gen-eral, acceptable continuity of integrity of the resto-ration-cavity interface for both intensities. Thenumber of defects per zone was low. However, theintegrity of the restoration—cavity interface of therestorations cured at low intensity was significantlybetter than the integrity after curing at high inten-sity. This agrees with the findings of  UNO  & As-MUSSEN (4), who also found a significant improve-ment of the marginal adaptation at reduced lightintensities. This can be explained by the differencein polymerization rates resulting from the activa-tion with different light intensities. During curing,the visco-elastic behavior of the composite changesfrom viscous to visco-elastic to elastic. This processleads to stress development, as the restorative ma-terial is adhesively bonded to the cavity walls. Inthe viscous stage, no stress development will occur,whereas stresses can partly be relieved by flow andelastic strain in the visco-elastic stage and, finally,by compliance of the surrounding structures. Toohigh stress levels will eventually lead to interfacialseparation. As the rate of conversion determinesthe rate of shrinkage stress development and theultimate stress level, any retardation of the reactionwill contribute to slowing down the shrinkagestress development and its ultimate value. In addi-tion, the time period for viscous flow will be ex-tended. The polymerization rate of Clearfil Lustre,as reflected by the shrinkage rate (Fig. 2), wasslowed down significantly when the curing light in-tensity was reduced from 650 to 250 mW/cm^. Themain portion of stress reduction, favoring the in-terfacial integrity, is expected to occur during thefirst 10 s of light activation. During this period theconversion is less than 50% of that obtained withthe high intensity unit (Fig. 2). The moderation ofthe reaction may also contribute to the overallstress reduction by allowing more yielding of thefree surface of the restoration to the underlyingcontracting bulk, as a result of a slower stiffnessdevelopment.From Fig. 3 it can be seen that the shrinkagecurves of Clearfil Lustre resulting from activationat 250 and 650 mW/cm^ reached the same levelwithin 1 h. This may illustrate (1) that in the 1.5mm thick samples the same degree of conversioncould be obtained with both light intensities.Therefore, it is assumed that the final mechanicalproperties were not influenced in the particular res-toration geometry. The reduced light intensity asused by  UNO  &  ASMUSSEN  (4) did influence the me-chanical properties, but this may be due to a lightintensity which is considerably lower than the 250mW/cm^ used in this study.The influence on interfacial integrity from theelastic behavior of the various components of theLiner Bond system follows from a comparison ofscores between groups I, II and III. Whereas in-terfacial integrity of the restorations in groups I,II and III was significantly influenced by influen-cing the visco-elasticity during setting, no statisti-cal difference was found by changing the elasticityfrom a material choice. Group I comprised re-storations with Protect Bond applied as an elasticstress absorbing layer between cavity wall andClearfil Lustre. In group II, Clearfil Lustre wasused without Protect Bond, while in group III,Protect Bond was used as the restorative instead ofClearfil Lustre. An increase of elastic complianceto setting stresses was expected in the order: groupIK group I< group III, but no influence on integ-rity could be detected after statistical comparisonof the groups. Two main reasons may account forthe discrepancy with earlier findings that stress ab-sorbers favor the cavity wall-restoration integrity (2,  3). In the previous studies (2, 3) one of the earlybonding systems was used. The bond strength ofthe present generation, which includes the ClearfllLiner Bond system, has improved considerably. Inaddition, the choice of Clearfil Lustre as the fillingmaterial was somewhat unfortunate, as it subse-quently appeared that its Young's module was ofthe same order as that of Protect Bond (the 1-hvalue of Protect Bond was 4.9 GPa and of ClearfilLustre 6,1 GPa,, measured in a 3-point bending set- up).  The use of a stiffer composite might have re-vealed the beneficial effect of building in more elas-ticity. To statistically express the small differencesfrom the present choice of materials, larger samplesizes would have been required. However, it isquestionable whether these small differences willbe of any clinical relevance. A conclusion whichcan be drawn for the particular cavity preparation.
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