Marginal Adaptation of Class II CompositeFiilings: Guided Polymerization vs

Reduced Light intensity

Götz M. Lösche^

Purpose: To improve the marginal adaptation ot Ciass II composite fillings, a three-sited light-cjring tech-nique was proposed by Lutz et al in 1986 and has preven to be effective in several studies. This study in-vestigated whether this effect, attributed to guided poiymerization towards the cavity margins, might aiscbe due to the decreased polymerization stress resulting from reduced curing light intensity caused bytransdental curing.

Materials and Methods: Forty extracted human molars were mounted in casts and 40 Class II slot-typecavities were cut (cervical margins 0.5 mm above the CEJ). To measure the light being transmitted from ahaiogen curing light onto a composite layer within a cavity, miniature photodetectors were used and irradi-ance measured for a three-layer technique (1st: oen/ical-horizontai: 2nd and 3rd: lateral-diagonal] with oc-ciusai or three-sited curing. To evaluate the infiuence of iight intensity on marginai adaptation, theprepared cavities were treated as toiiows: after base placement and acid etching, the cavities were fiiiedwith Herculite XR (Kerr, Romulus, Ml, USA) in three increments. Aii cavities were fiiied with the iayeringtechnique described above. In two groups (n=10|, curing was carried out only from ODclusal. in the remain-ing groups, the cervical composite layer was cured through a iight-trahsmitting, iaterai reflective wedge.and the foiiowing iayers transdentally with a proximal wand positioning. Using the photodetectors, the out-put power of an argon iaser was adjusted so that the composite was cured with the equivaient irradiancecalculated for occlusal (high-mtensity groups) and three-sited curing occlusal (low-intensity groups). Repli-cas were produced after storing in water for 21 days and thermocycling (2000>;, 5°0 to Sö^C). The mar-gins cf the filiings were quantitatively assessed using a SEM at 20ÜX. The results were statisticallyanalyzed (ANOVA, Bonferroni/Dunn; p < 0.05).

Results: The summed values for the criteria "restoration margin fracture", "enamel margin fracture," and"marginal opening" at the cervical margins were 11.3 ±11.5 (three-sited curing, iow intensity), 14.1 ±15.7(occiusai curing, low intensity). 25.4 ± 21,2 (three-sited curing, high intensity), and 27.8 ± 30,6 (occlusaicuring, high intensity).

Conclusion: No significant difference was found between curing techniques at equal energy levels; the ef-

fect of the three-sited light curing technique should therefore not be attributed to guided polymerization

but to the reduction of light intensity.

J Mtiesive Dent 1999:1:31-39. Submitted for publication: 03.11,98; accepted for publication: 16.11.98.

Polymerization shrinkage is probably the greatestproblem associated with composite resins if

' Associate Professor, Department of Operative Dentistry, Preventive used as filling materials in large cavities. The rea-Dentistry, and Endodontics, School of Dental Medicine, Humboldt son for Shr inkage is t h e dec reas ing b ind ing dis-university BeriiniCharité), Berlin, Germany. ^^^^^ between monomer molecules and the

growing polymer chain. The amount of shrinkage at

Z!::^r^o^o^: Me¿S%ro?SÍ^; ;^tS - y t i-^ is related to the degree of polymerization.Preventive Dentistry, and Endodontics, Föhrer Strasse 15, 1335.3 j e, the conversion Of C = C bonds tO C-0 bondS.Berlin, Germany. E-mail: ioe5che@charite.de j^^^^^ f^^ g gj^g,,, material. Shrinkage must be corre-

lated to the degree of conversion. Depending on

31

Fig l Three-sited light curing technique as äescriQed by Lutz etal

the selection and combination of resin monomersand the type and amount of filler used, currentlyavailable composite fiiling materiais show a freevoiumetric shrinkage of 2.6 to 4.8 vol.-%.9

Compared to chemioaliy cured composites, thesetting reaction of iight-cured materials takes piacevery rapidiy and stress reiief by fiow is restricted.^iHigh curing-light intensity, whioh is desirabie for op-timal polymerization, results in a faster polymeriza-tion reaction^^, 24 thereby further restricting stressrelief. The benefioisi effect of lower curing-iight in-tensity on marginai adaptation has been demon-strated for cavities with restricted bonding areas,I e, Class V oomposite fillings in dentin,12.34,35 Class|3o or V22 fillings in enamei without circular beveis,or restoration margins of Class ill fiiiings close tothe CEJ.20

The iarge volume and the often clinicaiiy re-stricted area for safe bonding cervically^^ are prob-lems associated with ciass II composite fillings. To

overcome the problem of shrinkage, a multitude offilling techniques has been developed in the at-tempt to reduce shrinkage-espeoiaily cervicaliy-byplacing the composite in increments, ailowing theutmost unrestricted shrinkage of the filling mater-iai,

Probabiy the most sophistioated technique wasintroduced in 1986 by Lutz et ai.^s based on theidea of guiding polymerization shrinkage vectors to-wards the cavity margins using a iaterai light refiec-tive wedge at the cervioal margin and transdentaipoiymerization at the axial waiis of the proximal box(Fig 1). This so-called three-sited light-curing tech-nique has proven to be effective not oniy by the de-veioper but aiso by other authors,5.i9 A substantiaidecrease of curing light intensity, however, has tobe taken into account with this technique, as thelight intensity being transmitted through the wedgewas found to be less than 8%,6 Reflection of lighton the tooth surface^' and reduction of curing light

32 jve Dentistry

Fig 2 Position of miniature photodetectors and resuits of intracavity irradiance measurements.

intensity by transdental polymerization^e may beother important factors.

PURPOSE OF THE STUDY

The purpose of this study was to investigatewhether the good resuits achieved with the three-sited polymerization technique are attributablesolely to the effect of directed polymerization or ifthe reduction of curing light intensity also plays animportant role.

MATERIALS AND METHODS

To determine the amount of light reaching the sur-face of the uncured composite in a Oiass il cavity,extracted human moiars were mounted in castswith neighboring teeth. To reproduce gingival condi-

tions, Vestogum (Espe, Seefeld, Germany), a pinkpolyether, was used, Oiass II siot-type cavities(n=10) were cut so that the vertical cavity marginsextended on the buccai and iingual sides to within0.5 mm of the neighboring tooth. The cervical mar-gins were piaced 0,5 mm above the OEJ. After cav-ity preparation, the casts were stored in water for24 h prior to measurement of irradiance.

Before matrix placement, the cavities were biowndry. For evaluation of the three-sited iight-curingtechnique, a myiar matrix was adapted with a Luci-wedge'- (Hawe Neos Dental, Bioggio, Switzerland);for the Gcclusai curing technique, a metal matrixwith a sycamore wedge (Hawe Neos Dentai) was ap-plied.

The sensitive area (1 x 2,5 mm) of miniature sili-con photodeteotors (OSD 5-5T, Oentronic, Oroyden,Great Britain) was covered with a piece of No. 45plus No, 47 B geiatin fiiter (Kodak Wratten GeiatinFiiter. Eastman Kodak, Rochester, NY, USA), The

Vol 1, No l_1222_ 33

Fig 3 Expérimentai set-up for iow and high cunng iight intensity.

photodetectors were calibrated with a precision ra-diometer and irradiance detector (iL 1700 + SED033, International Light, Newburyport, MA, USA).The irradiance of a standard halogen light source(Kerr, Romuius, Mi, USA) was controiied with thesame equipment prior to irradiance measurementwithin each cavity (612+17 mW/cm').

For each curing technique, the detectors werepositioned at those locations where light wouid beexpected to hit the composite surface first. For oc-clusal curing, the sensitive area pointed upwards;for three-sited curing, two detectors were used foreaoh location, one sensitive area pointing towardsthe transparent matrix, one towards the oervioai oraxiai cavity walls. Both curing techniques were eval-uated in eaoh cavity. The measurements were re-peated five times after intermittent water storagefor 24 h. The positions of the detectors within thecavities are illustrated and the results given in Fig

For evaiuation of marginal adaptation, 40 ex-tracted human moiars, which were randomly as-signed to four groups (n=10), were mounted incasts as described above. Untii cavity preparationand filling of the cavities, the casts were stored inwater at room temperature.

All following steps, from preparation to comple-tion ofthe restoration, were done under 4.5x mag-nification (Zeiss, Oberkochen, Germany). Afterinitial preparation for proper extension of the verti-cai margins (0.5 mm distance to neighboring tooth),the adjacent typodont tooth was removed for opti-mal access during extension of the cervical marginto 0.5 mm above the CEJ. The axial depth of thecavity at the Ievei of the cervicai floor was 1.5 mm.Occlusal and vertical margins were beveied with afine-grit diamond (Composhape H 40, Intensiv, Vi-ganelio-Lugano, Switzerland). Only a short bevelwas prepared at the cervioal enamel. Ail exposeddentin was covered with a glass ionomer base

34 TheJournai of Adhesiva Dentistry

Lösche

Fig 4 Resuits at occlusal margins (the bars represent the summed vaiues ofthe criteria restoration margin fracture, enameimargin fracture and marginal opening; nonsignificant differences are underiined; p < 0,05),

(Ketac Bond, Espe, Germany). The enamel marginswere acid etched with a 35% phosphoric etchinggei. After 60 s, the cavity was rinsed for 60 s andthoroughly dried.

After remounting and fixing the adjacent ty-podont tooth, mylar matrices with Luciwedge'-(groups 1 and 4) or steel matrices in combinationwith a sycamore wedge {groups 2 and 3) were ap-plied. An experimental argon ion laser (Model 2010with fiberoptic 316. Spectra Physics, MountainView, CA, USA) was used which had been adjustedin wavelength and curing area to the halogen curinglight. Before composite piacement, the power out-put of the iaser was adjusted for each cavity withhelp of the photodetectors mentioned above: whenthe low intensity for the three-sited curing technique(50 mW/cm2 at the cervicai and 150 mW/cm^ atthe vertical walls) was used as a standard, the laserwas adjusted for occlusal curingtothe same values[group 2). When the values measured for occlusai

cur ing {cervicai 250 mW/cm^; middle 400mW/cm^, and occiusal 500 mW/cm^) were used asa standard, the values of the three-sited curing(group 4) were adjusted to the same high-intensityvalues (Fig 3).

A bonding agent (Bondiite; Kerr, Romulus, Ml,USA] was applied, biown thin, and cured from oc-ciusai for 20 s (intensity at occiusal margin = 500mW/cm2). Curing time for each increment of com-posite (Herculite LY, Kerr) was 60 s. After poiymer-ization, the teeth were removed from the casts andthe fillings finished and polished under direct visionwith fine grit diamonds (Composhape H 40 and H15, Intensiv) and aluminum-oxide-coated fiexiblediscs (SoFlex, 3M, St Paul, MN, USA),

The teeth were stored in water for 21 days atroom temperature and thereafter thermocycled(2000x, 5°C to 55°0), Repiicas were produced afterwater storage and thermocyoling, A quantitativemarginai anaiysis in the SEM3i at a magnification of

Vol 1, No 1 ,199935

vertical margins• 3-sited - standard Q occlusal - low intensity • occlusal - standard |R| 3-sited - high intensity

vertical margin before TC vertical margin after TC

intensity p > 11.05 liiyerin}; ifcliriiqut p > 11.115 lií;ht inl,- iiiv.k'ch. p > 11.1)5

Fig 5 Results at verticai margins (the bars represent the summed vaiues of the cntena restoration margin fracture, enamelmargin fracture and marginai opening: nonsignificant differences are underlined; p < 0.05).

200x was performed. The criteria for evaluationwere: excellent margin, marginal irregularities, sub-margination, overhangs, restoration margin frac-ture , enamel margin f racture, and marginalopening. The values of the criteria restoration mar-gin fracture, enamel margin fracture, and marginalopening—being regarded as indicators for stress de-velopment-were summed and the results statisti-cally analyzed at a significance level of p = 0.05(ANOVA, Bonferroni/Dunn, Stat View 4.02, AbacusConcept, Berkeley, CA, USA).

RESULTS

The results found for the three margin locations areshown in Figs 4 to 6. Non-significant differences(p > 0.05] are underlined.At occlusal and vertical margins, adaptation was

very good and no significant differences were foundbetween groups (Figs 4 and 5). In the cervical area(Fig 6) before thermocycling, low summed marginalcriteria values were found for both curing tech-niques with low-light intensities, while the values forboth high-intensity groups were significantly higher.After thermocycling, the increase in these summedvalues for the low-intensity groups was more pro-nounced than for the high-intensity groups, againshowing significantly worse marginal adaptationcompared to the low-intensity groups. Curing lightintensity was found to be a significant factor, whilelayerihg technique had no significant influence.

DISCUSSION

The influence of curing light intensity on the poly-merization of composite resins has been shown ihmany studies, and up to a certain limit, a linear cor-

36 The Journal of Adhesive Dentistry

cervical margins

• 3-sited - standard • ocolusal - low intensity \J occiusal - standard • 3-siled - high intensity

cervical margin before TC cervical margin after TC

lii;hl inten.sity p < ().()5 liivfring ttcliiiicjiie p > 0.05 liijht int.

Fig G Results at cervical margins (the bars represent the summed values ofthe criteria restoration margin fracture, enamelmargin fracture and marginal opening: nonsignificant differences are underlined; p < 0.05|.

relation of curing light intensity and polymerizationis found.26 The maximal absorption wavelength ofcampheroquinone used as a photoinitiator in mostcomposites is 468 nm,^^ but waveiengths of 410 to500 nm, 454 to 495 nm,38 and 440 to 500 nmi"are regarded as effective for polymerization as well.To predict polymerization from light intensity mea-surements, only the relevant spectrum should beassessed.S36,37 AS proposed by McCabe and Car-rick,26 Kodak gelatin filters No. 45 (435 to 550 nm)and No. 47 B (370 to 500 nm) were used to limitthe sensitivity of the silicon photodetectors to 435to 500 nm. Because they were thin, they were pre-ferred over a more specific narrow bandpass filter(ideally 468 nm), which was too thick and couid notbe reduced to the size of the photodetectors.

For the occiusai and middie detector position,the values found for occiusal curing are in agree-ment with the results of Hansen and Asmussen, in-dicating that curing from ocolusal is efficient, and

iittle iight reduction has to be taken into account.^^The low vaiues found for the cervical iayer if curedfrom the occiusal can be attributed to the narrowconfiguration of a slot-type cavity, which restrictsthe access of iight. The irradiance measured is,however, still sufficient, as minimai curing energyshouid be 233 mW/cmV60 s.32

The vaiues found for the three-sited curing tech-nique can be attributed to severai factors. A consid-erabie decrease of irradiance occurred: themaximai iight intensity transmitted through thewedge was found to be less than 8%.^ Due to thedimensions and rigidity of the Luciwedge'-, it canseldom be compietely inserted into the proximalspace, thus increasing the exit distance of the lightwand to the tooth. This results in poor and inhomo-geneous polymerization of the composite in theproximal box.3.29 Cutting off the excess length ofthe wedge wouid improve the resuits,^^ but for ade-quate curing, additional polymerization from oc-

Vol 1, No 1 ,1999 37

clusal is necessary,^^ The reflection of light fromthe surface of the tooth, matrix, or restorative mater-iai 37 is sureiy one major factor infiuencing the mea-surement at the verticai waiis. The thickness of thetooth structure^^ and the rapidly increasing opacityof enamel due to dehydration^ are other clinicaiiyimportant factors, ail together resuiting in valuestoo low to ensure complete poiymerization.

Class li slot-type cavities were cut, and wheresufficient enamei was available, a bevel was pre-pared to provide optimal prerequisites for adhesivebonding to enamei.̂ ^ The enamei thickness ciose tothe CEJ does not allow a bevei of sufficient width.i^Nevertheless, a short bevel was prepared to reducethe amount of enamel marginal fractures foundwith butt joint preparations in that area," Althoughrather effective dentin bonding agents are currentlyavaiiabie which aliow augmentation of the adhesionarea in the critical cervicai region, aii exposeddentin was covered with a giass ionomer base tolimit the bonding area and reveal the maximal infiu-ence of stress during shrinkage and thermocyciing.

To aliow adjustment of intensity without wave-length shifts and to achieve the high irradiance val-ues measured for occiusai curing as well as fortransdental poiymerization, an experimental argonion iaser was used for polymerization, Waveiengthswere limited to 457 to 501 nm,3S The beam of thelaser was widened and applied through a standardlight rod combined with the dichroic fiiter used inthe halogen curing unit, making the curing charac-teristics simiiar to those of the light source usedduring irradiance measurements, Intracavity mea-surements revealed equivalent results to the Op-tiiux 400,

Thermocyoiing is the common procedure to in-duce stress, based on the difference in coefficientsof thermal expansion of composite and tooth struc-ture,^ Together with the quantitative margin analy-sis of cavity margins of adhesive restorations, itallows prediction of the success of adhesiverestoration techniques to a certain degree,3i [n pre-vious studies, the criteria restoration margin frao-ture, enamel margin fracture, and marginalopening—especially for cavity margins ciose to theCEJ—were found to be indicators for stress deveiop-ment and faiiure of the restoration.1618.20,21 jf^gywere therefore summed for better statistical com-parison of the expérimentai groups.

In a previous study, good marginal adaptationand no statisticaiiy significant difference was foundbetween occiusal and three-sited curing, because a

bevel of sufficient width could be prepared and ahybrid composite was placed and cured in incre-ments.i9 Since occiusal and vertical enamel thick-ness allowed the preparation of an adequate bevel,similar results were found in this study and neithercuring iight intensity nor iayering technique had asignificant impact on marginal adaptation.

The results at the cervicai margins showed theinfiuence of increased and faster polymerizationshrinkage with higher intensities,12.24 jf^g fast de-velopment of the polymerization reaction of light-curing composites impairs stress reiief by flow,i" re-sulting in signifioantly worse results for restorationscured with higher iight intensities. The more pro-nounced increase of marginal deficiencies afterthermocyciing for the two groups cured with low in-tensity can be explained by postcuring contraction,̂This is lower in the high-intensity groups, becausethe composite is more completely polymerized bythe first curing. Although post-irradiation poiymer-ization is obvious in both groups cured with iow in-tensity, complete polymerization is questionable.

The procedure proposed by Radke^̂ to cure thecervical layer not only through the Luciwedge butadditionaily from occiusal would in effect be similarto the soft-start polymerization described by Mehlet aP'̂ for Class V cavities. This technique has notyet been finaiiy evaiuated for Ciass il cavities withrestricted bonding area, but preliminary resultsshow that at equal energy levels, marginai discrep-ancies are significantly reduced by step-wise poly-merization (unpublished data).

CONCLUSION

The good results achieved with the three-sited light-curing technique should not be attributed to the ef-fect of guided polymerization, but to polymerizationwith iower light intensity. The following steps mightensure good marginal adaptation and adequatecure: 1) increasing the cervical bonding area by useof efficient dentin bonding agents; 2) iayering ofseveral smaii increments at the cervicai floor (eg,using iow- viscosity resins); or 3) using different cur-ing techniques, such as precuring through the lat-eral refiective wedge and postcuring from occiusalor using soft-start polymerization in combinationwith occiusai curing.

38 The Journal of Adhesive Dentistry

ACKNOWLEDGMENTS

This study was supported by the University Research Fund / Char-i té/ HU Berlin, Project No. 98-181.

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