Esthetic Dentistry

Effects of home bleaching products on composite resinsShawn J. Bailey*/Edward J. Swift, Jr*

A number of home bleaching prodttcts have recently been introduced. This in vitrosttidy evaluated the effect of three representative bleaching products on the microhard-ness and stirface texture of hybrid and microfilled composite resins. Scanning electronmicroscopic examination revealed cracking of die microfilled specimens treated withcarbamide peroxide gels (Proxigel and White & Brite). White & Brite also catisedslight roughening of the hybrid composite resin. Microhardness tests indicated that thetreated composite resins became somewhat softer. However, the only statisticallysignificant softening was associated with White á Btite.(Ouintessence Int 1992¡23:489^94.)

Inlroduction

One of the main reasons that patients seek estheticdental treatment is a real or perceived discoloration ofthe anterior teeth. Discolored teeth ean be treatedwith various restorative techniques, such as directcomposite resin veneers, porcelain veneers, or ceramiccrowns.'

Various methods of vital bleaching have been de-veloped and used over the years. Until recently, mostbleaching methods have used a strong oxidizing agent(30% to 35% hydrogen peroxide) in combination witha heat or hght source. While such procedures are stillin use and are frequently effective, they require tediousrubber dam isolation and multiple appointments.""" Inaddition, they can cause tooth sensitivity and may notprovide a permanent lightening effect.'''̂

An alternative method of vital bleaching is nowavailable. Referred to as home bleaching or motithgtmrdbleachitig, this technique allows the patient to apply a

* Gênerai Practice Residenl, Yak-New Haven Hospital. NewHaven, Connecticut 06504.

*' Associate Professor, Department of Operative Dentistry, iJni-versity of towa. College of Dentistry, Iowa City, towa ^lliVl.

Address aii correspondence to Dr E. J. Swifr, Jr.

bleaching solution to the teeth in a custom-fitted plasticstent for a few hours each day. Early reports haveshown not only a high degree of patient interest, butalso excellent clinical effectiveness.''"•'•* A plethora ofhome bleaching products, most eontaining 10% car-bamide peroxide, are currently available." '•* Unfortu-nately, these products have been introduced with almostno documented in vitro or clinical research. Little isknown about their effeets on soft tissue, tooth sirueture,or restorative materials. One study has suggested,however, that carbamide peroxide bleaching gels causeslight roughening of hybrid composite resins.""'̂

The purpose of this study was to determine whetherhome bleaching products cause cbemical softening ofmicrofilled and hybrid composite resins. In addition,scanning electron microscopy was used to evaluate tbeeffeets of these products on the surface texture ofeomposite resins.

Three representative bleaching products wereselected for this study. Proxigel (Reed & Carnriek) hasbeen sold as an oral antiseptie gel for a number ofyears. It contains 10% carbamide peroxide and acarbopol resin, which is a high molecular weight,water-soluble polymer tbat increases viscosity andpotentiates the bleaching effect.'̂ •'•* White & Brite(Omnii International) is a 10% carbamide peroxidegel without carbopol. Natural White (AestheteLaboratories) is a completely different, three-stepbleaching system consi.sting of an acetic acid rinse, a

Quintessence international Voiume 23, Number 7/1992

Esthetic Dentistry

Table I Experimental and control groups

Group

12345678

Composite resin

Silux PlusSilux PlusSilux PlusSilu\FlusHereulite XRHereulite XRHereulite XRHerculife XR

Bleaching agent

None (control)White & BriteProxigelNatural WhiteNone (control)White & BriteProxigelNatural White

tions. The eomposite resin disks were rinsed withdeionized water after treatment and were stored indeionizcd water at all other times. The storage tismper-ature was 37°C, both during and between the bleachingtreatments.

The bleaching treatments were continued for 1month, Microhardness measurements {three on thetop surface of each sample) were repeated at 1, 2, and4 weeks. These measurements were converted intoKnoop hardness numbers (KHN), Representativespecimens from each group were also evaluated usingscanning electron microseopy.

6% hydrogen peroxide gel, and a titanium dioxidepolishing cream,''*

Method and materials

Composite resin was injected into round Teflon molds(10 mm in diameter and 2.5 mm deep), covered withMylar strips, and pressed between glass plates. Eachspeeimen was polymerized for 80 seconds with a wide-tipped Prismetics (LD Caulk/Dentsply International)visible light-curing unit. Two bisphcnol glycidyl meth-acrylate (bis - GMA) eomposite resin restorative materialswere used—Hereulite XR (Kerr/Sybron Corp), a sub-micron hybrid, and Silux Plus (3M Dental Products Div),a mierofilled resin. Twenty specimens of eaeh eompo-site resin were made. The specimens were polishedwitb medium, fine, and superfine Sof-Lex disks (3MDental Products Div) on a slow-speed handpieee.

The composite resin specimens were stored in 100%humidity for 24 hours to allow for postirradiation hard-ening,""" The surfaee hardness of each specimen wasthen measured with a Micromet II Microhardness Tester(Buehler Ltd) using a 10-g load and 12-sccond loadtime. Three tneasurenients were made on the top sur-face of each disk. The baseline hardness values allowedeach sample to serve as its own eontrol. Five samplesof each composite resin were left untreated as addi-tional controls.

The remaining specimens were divided into sixgroups (n = 5) for treatment with the different bleaeh-ing systems (Table 1), In the earbamide peroxidegroups (Proxigel and White & Brite), composite resinSpecimens were immersed for 4 hours daily in freshbleaching gel. The Natural White specimens weretreated each day following the manufacturer's direc-

Results

The results of the microhardness testing are shown inFig 1 and 2, The hardness data were analyzed usingthe general linear tnodels procedure of the SAS statis-tical software paekage (SAS Institute}. The data weresubjected to a two-way analysis of variance, whichshowed statistically significant differences by type ofbleaching agent (P < ,0001) and time (P < ,0002).Duncan's multiple range test (a = ,05) was used todetect specific differences.

The bleaching agents appeared to have a greatersoftening effect on Silux Plus than on Hereulite XR.All three bleaching agents caused some softening ofSilux Plus, while the hardness of control Silux Plusspecimens remained fairly eonstant. However, theonly statistically significant differenees were betweenthe eontrol and White & Brite groups after 14 and 2Sdays (Tables 2 and 3),

For Hereulite XR, there were no statistically signifi-eant differences between treatment and eontrol groupsat any given measurement time. However, the speci-mens treated with White & Brite did become signifi-cantly softer over the course of the experiment (Table 4).

Proxigel and White & Brite caused extensive crackingin the surface of all Silux Plus (microfilled) specimens(Fig 3), Tlie number and size of cracks appeared similarfor both bleaching gels. Although some eracks wereloeated at the interface of prepolymerized partielesand resin matrix (Fig 4), there was no eonclusive evi-denee that craeks occurred only at interfaces. No largecracks or other surfaee defects occurred in either theuntreated samples or those treated with Natural White(Fig 5),

None of the bieaching products caused cracking ofthe hybrid composite resin, Hereulite XR, However,White & Brite did cause very slight surface roughening,which was apparent only at relatively high magnifica-

490 Quintessence International Volume 23 imber 7/1992

Esthetic Dentistry

Fig 1 Microhardness of Silux Plussamples treated with home bleach-ing agents for 28 days.

Fig 2 Microhardness of HercjiiteXR samples treated with homebleaching agents for 28 days.

SILUXCorlral

Win» & Bnle

^

TIME OF MEASUREMENT [daysj

HERCULITE

TIME OF MEASUREMENT (days)

Table 2 Hardness of Silux Pltis specimens at day 14 ofbleaching treatment

Hardness DuncanGroup Treatment (KHN) grouping*

1 Control 28.7 ± 0.9 A3 Proxigel 24.9 ±2.2 A B4 NaturalWhite 23.8 + 0.7 A B2 White&Brite 23.1 ± 1.8 B

* Means with thesameletter are not signifieantly different ( i i= .05).

Table 3 Hardness of Silux Plus specimens at day 28 ofbleaching treatment

Hardness DuncanGroup Treatment (KHN) grouping*

1 Control 27.4 ± 0.8 A3 Proxigel 24.6 ±1.6 A B4 NaturalWhite 24.0 ± 1.3 A B2 White&Brite 22.4 ± 0.7 B

* Means with the satne letter are not significantly different (o = .05).

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Esthetic Dentistry

Fig 3 Scanning electron micrograph of Siiux surfacetreated with White & Brite. Specimens treated with Proxigelhad similar craciis. (Original magnitication x 250.¡

Fig 4 Scanning electron micrograph showing (arrow) aprepoiymerized Siiux patiicie separated fnjm the resin matrix.The specirnen was treated with White & Brite. (Originalmagnification x 3,000.)

Rg 5 Scanning electron micrograph of untreated Siiux.(Original magnification x 250.)

Fig 6 Siight surface roughening of Herculite specimenexposed to White & Brite. (Original magnification x 4,000.)

Table 4 Hardness of Herculite specimens treated withWhite & Brite at different measurement intervals

Time(d)

17

1428

Hardness(KHN)

45.5 ± 4.138.9 ± 3.136.3 ± 2.432.7 ± 3.1

Duncangrouping-^

AA BA B

B

Mca[is with the same letter are no I significantly different (a = .05).

Fig 7 Scanning electron micrograph of untreated Herculitespecimen. (Ohginal magnification x 4,000.)

492 Quintessence Internafionai Volume 23, Number 7/1992

Esthetic Dentistry

tion (Fig 6). Roughening appeared to be dtie to loss ofmatrix material rather than to etching of the barium-glass Slier particles. Untreated Herculite XR specimensand those exposed to Proxigel or Natural White hadsmoother surfaces (Fig 7).

Discussion

Proxigel and Natural White caused some softening ofhoth composite resins, particularly the microfilledcomposite resin. More softening would be expectedwith microfilled resins, which have a higher resin con-tent than do hybrids."^ Nevertheless, the softeningassociated with these products was minimal and notstatistically significant. White & Brite, however, didcanse statistically significant softening of both SiluxPlus and Herculite XR. At 14 and 28 days after speci-men preparation, Siltix Plus treated with White & Britewas significantly softer than the Silux Plus controlgroup. The Herculite XR specimens treated withWhite & Brite were significantly softer at the end ofthe experiment.

Chemical softening of composite resins is believedto occur in vivo, contributing to wear of the resin inhoth stress-bearing and non-stress-bearing areas.'̂ '̂ ^Softening is can sed by chemicals with solubilityparameters similar to that of the resin matrix. Thebis - GMA resin polymer can be softened by chemi-cals with soluhiiity parameters in the range of 18.2to 29.7 (MPa)"^.^' A wide variety of solvents havesolubility parameters within that range." Studies withfood-simulating liquids suggest that some food sub-stances (eg, alcoholic beverages) cause softening andaccelerated wear of composite resins.̂ ^

Proxigel contains a variety of chemical agents: car-bamide peroxide, carbopol, glycerin, phcnacetin,phosphoric acid, trolamine (or triethanolamine), andflavorings. White & Brite has a similar composition;the most notable difference is the absence of the car-bopol thickening agent. None of these agents is listedin the solvent tables of the Polymer Handbook,^ so itis unclear whether any have solubility parameters simi-lar to that of bis - GMA, Furthermore, the surfacechanges could be caused by complex interactionswithin these multicomponent bleaching products,rather than by one specific chemical component.

Acetic acid, supplied as a rinse in the Natural Whitesystem, has a solubility parameter of 20.7 and there-fore can soften bis - GMA resins. However, NaturalWhite caused no significant softening in the present

study, perhaps because of the short exposure timesused.

Carbamide peroxide breaks down into urea and hy-drogen peroxide. Hydrogen peroxide in turn breaksdown into free radicals, which eventually combine toform molecular oxygen and water. Some aspect of thischetnical process might accelerate the hydrolytic de-gradation of composite resins described by Soderholmet al. The presence of the thickening agent, carbopol,which is said to combine with the free radical inter-mediates,"* could alter the process in some way, account-ing for the differences between Proxigel and White &Brite.

The clinical relevance of the present findings is un-certain. More research on the effect of home bleach-ing agents on dental materials is clearly needed. Con-trolled clinical studies would be particularly valuablein enhancing the profession's understanding of theseeffects. The softening and other surface degradation(roughening or cracking) of composite resins causedby home bleaching agents appears to be ratherslight. In fact, similar surface deterioration may becaused by certain foodstuffs. Furthermore, existingcomposite resin restorations often are replaced aftersuccessful bleaching therapy to improve the shadematch.

Nevertheless, home bleaching agents sbould not heused indiscriminately when composite resin restora-tions are present. The patient should be informed thatbleaching may accelerate the natural "aging" processof composite resin restorations. The patient must alsorealize that restorations frequently must be replaced toensure proper shade matching if bleaching is successful.Finally, if posterior composite resin restorations arepresent, the patient should be instructed to be carefulto confine the bleacbing gel to the anterior segment ofthe tray.

Summary

The bleaching agents evaluated in this study causedonly slight changes in the surface of composite resins.Silux Plus, a microfilled composite resin, had extensivecracking after treatment with either Proxigel or White& Brite, both of which are 10% carbamide peroxidegels. Also, the surface of Herculite XR, a hybrid com-posite resin, was slightly rougher after treatment withWhite & Brite.

Some softening of both composite resins occurredafter treatment with home bleaching agents. However,

Quintessence International Volume 23, Nutnber 7/t992 493

Esthetic Dentistry

the only statistieally significant differences in hardnesswere associated with White & Brite.

References

1. Goidstein RE. Diagnostie diiemma: to bond, iaminule, orcrown? Im J Periodont Rest Dem t987;7(.S):9-29.

2. Boksman L, Jordan RE: Conservative treatment of thestained dentition: vital bleactiing. Aust Dent J t9SJ:2S:67-72.

3. Swift EJ: A method for bteactiing discolored vital tcctii. Qidn-tessence Int \9SS,\9.mi-f<l2.

4. McEvoy SA: Chemical agenti for removing extrinsic stainsfrom vital teetti. t. Teeiinique development. Quintessence Inl1989;20:323-328.

5. McEvoy SA: Chemical agents for removing eïtrinsie stainsfrom vitat teeth, tt. Current techniques and their ctinicaiapplication. Quintessence ¡nt t989;20:379-384.

6. Goidstein CE, Goldstein RE, Feinman RA, ct al: tiieachingvital teetb: state of the an. Quinlessence tnl l989;21):329-333.

7. Morrison SW: Vitat tootli bteaching .-Ttie patient's viewpoint.Gen Dent 198fi;34:238-240.

S. Feiglin B: A 6-year recati study of clinicalty cticmicattybteached leeth. Orai Surg Orai Med Oral Palliai t9R7:fi3:610-613.

9. Haywood VB, Hcyniann HO: Nightguard vital bleaching.Quintessence Int 1989;20:t73-t76,

to. Darnell DH, Moore WC: Vitat tooth bleaching: the Whiteand Britc teohnique. Compend Com Educ Denl 1990;n:86-94.

11. Ctiristensen GJ: Tooth bieaching, home-use products. ClinResAssoe Newsielter 19S9;t3(7):l-3.

t2. Christensen GJ: Tootti bleactiing, home-use products (updatereport). Clm Res Assoc Newsletter t989;t3(12):l.-3.

13. Haywood VB: Nightguard vital bleaching: current informationand research. Eslhet Dem Update 1990;t:20-25.

14. Albers HF(ed): Lightening teeth./IDE/TJic/iort 1990;2:l-24.

15. Hunsaker KJ. Christensen GC, Christensen RP: Toothbleaching chemicals —influence On teeth and restorations.J Dent Res t9yO:6y:303(abstr No. 1558).

16. Leung RL, Fan PL, .Tohniion WM: Post-irradiation tiardeningof visible light activated composite resin. / Dent Res t983;62:263-265.

17. Johnston WM. Leung RL, Ean PL: A mathematical modei forpost-irradiation hardening of photoactivated compositeresins. Dem Mater iy85;l:191.-t94.

18. Craig RG (ed): Restorative Dental Malerials, ed 8. St t.ouis,CV Mosby Co, t989, p 256.

19. Wu W. Toth EE. Moffa JF. et ai: Subsurface damage iayer ofin vivo worn dental composite restorations. J Denl Res 19S4;63:675-680.

20. van Groeningen G, Jongebloed W, Arends J: Composite de-gradation in vivo. Dem Maler 1986;2:225-227.

21. Wu W, McKinney JE: Influence of ctiemicals on wear of dentalcomposites. J Dent Res 1982;61:1180-1183.

22. Brandrup J, tmmergut EH (eds): Polymer Handbook, ed 3.New York, John Wiley and Sons, 1989, pp 5t9-559.

23. MeKinney JE. Wu W: Chemical softening and wear of dentaicomposites. J Dem Res iyK5;64:1326-t33].

24. Söderhoim K-J, Zigan M, Ragan M, et al: Hydrolylic degra.dation of dentai composites. J Dent Res 1984;63:1248-1254.

a

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ISBN 1-B5097-011-4;108pp,91 illus: US Î38

Rubber Dam in Clinical PracticeJ. S. Reid, P. D. Cailis, and C. J. W. Patterson

Although the rubber dam was first introduced intodentistry more than tOO years ago, it is stitl not widetyaccepted in general dental practice. Yet tbis technique isbasically simple to use once it tias been mastered and basmany advantages botb for ttie patient and the dentist.

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All dentists will benefit from baving this handbook readilyat hand so that they can put this tecbnique to regular useand benefit from its advantages of safety, belter workingconditions in the mouth, and better patient managemetit.

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