Tooth-Bleaching: A Review of the Efficacy and Adverse Effects

of Various Tooth Whitening Products

ABSTRACT

Tooth bleaching (whitening) is one of the most common and inexpensive method for treating discolouration of teeth. Dental

aesthetics, especially tooth colour, is of great importance to majority of the people; and discolouration of even a single

tooth can negatively influence the quality of life. Therefore, a review of the literature was carried out (limited to aesthetic

tooth-bleaching) to provide a broad overview of the efficacy and adverse effects of various tooth whitening products on

soft and hard oral tissues.

INTRODUCTIONDental aesthetics, including tooth colour, is of greatimportance for majority of the people and anydiscolouration or staining can impact their quality of lifenegatively. The colour of teeth reflects a combination ofits intrinsic colour and the presence of extrinsic stainsdue to various factors such as smoking, intake of tannin-rich foods and drinks (e.g. red wine), and the use ofchlorhexidine or metal salts such as tin and iron.1-3 Anumber of methods can be used to remove staining likeprofessional cleaning and polishing, whitening tooth-pastes, internal bleaching of non-vital teeth, externalbleaching of vital teeth, and micro-abrasion of enamel.Severe stains can be covered with crowns or veneers,but this is a more invasive and costly option.4,5The increasing demand for a better appearance andwhiter smile, has made vital tooth-bleaching (alsoreferred to as tooth-whitening) a popular dentalprocedure. It has developed into one of the fastestgrowing areas of aesthetic dentistry. It provides a moreconservative treatment approach for discoloured teethas compared to other restorative treatment modalitiessuch as composite fillings, veneers or crowns.1Commonly used methods for tooth-whitening includein-office or power bleaching,6dentist-supervised homebleaching or nightguard vital bleaching,7and easilyavailable over-the-counter (OTC) whitening products forself-application.METHODOLOGYThis literature review was limited to aesthetic tooth-bleaching and aimed to provide a broad overview ofbleaching techniques, their efficacy, and adverse effectson soft and hard tissues as well as the management oftooth sensitivity and gingival irritation. In formulatingthis review, only English-language articles availableelectronically were selected. The PubMed database andGoogle scholar search engine were explored withkeywords which included: tooth-whitening, tooth-bleaching, carbamide peroxide, hydrogen peroxide,bleaching and dentistry, home-bleaching, and vitalbleaching. Over 200 articles were initially reviewed and82 articles were shortlisted on the basis of theirapplicability to the present topic of review and thenstudied in detail.Efficacy of different types of tooth-whiteningproducts: Nightguard vital bleaching using 10% CP isthe most widely used and extensively researched tooth-bleaching technique. The American Dental Associationhas awarded its seal of acceptance to a number ofdentist-supervised home bleaching products containing10% CP.8Nightguard vital bleaching techniqueshave been effective for bleaching teeth stained by aging,mild fluorosis, trauma, inherent discoloration andtetracycline.9,10 According to the American DentalAssociation guidelines for the acceptance of peroxide-containing oral hygiene products, the clinical efficacymay be demonstrated by a change of two value orientedshade increments and a perceptible colour must bemaintained in 50% of the recall population at 6 monthscompared to the control, to reflect the duration ofefficacy.11 In a long-term clinical trial, Leonard et al.,12reported whitening of teeth in 98% of the participants by10% CP and 82% of the participants retained thewhitening effect upto 47 months post-treatment. A meta-analysis of the clinical trials from 1989-1999 on dentist-supervised home bleaching products using 10% CP suggested that only 73% of the population will show acolour change of two units or greater and 50% retaincolour at 6 months postbleaching.13 Higher CPconcentrations (15% and 20%) available for home-bleaching may whiten teeth slightly quicker than 10% CPduring the early phase of treatment. However, thewhitening effect shows some relapse after the cessationof active bleaching treatment before the colour isstabilized. Teeth treated with 10% CP, stabilize in colourfor 2 weeks following the cessation of the treatment butthe higher-concentration products last much longer.14However, it is claimed that rapid whitening shown by thehigher-concentration products is temporary andfollowing rebound, there will be no difference.15HP and CP tooth-bleaching products with equivalentperoxide concentrations demonstrate similar whiteningefficacy with few side effects.16,17A large number of OTC whitening products, includingwhitening strips or tray less whitening systems, paint-on-gels, gels with pre-fabricated trays and whiteningtoothpastes, have become increasingly popular in recentyears because of their low cost to the consumer, andoverwhelming marketing by manufacturing companies.Whitening strips usually contain 6 – 14% HP in gel form.An integrated clinical summary of nine randomizedclinical trials reported the efficacy of whitening stripscontaining 14% HP similar to popular tray-basedbleaching systems.18 A clinical comparison of two brush-applied whitening systems showed that a 19% sodiumpercarbonate system, that dries to form an adherent film,provided significant improvement in tooth colourcompared to 18% CP gel.19 Zantner et al.20 reported thata new bleaching lacquer, containing 8% CP for self-application without the use of a mouth guard, producedtwo shade improvements in tooth colour.A recent systematic review8of home-based chemically-induced whitening of teeth demonstrated that dentist-supervised home bleaching systems and OTC products(paint-on gels and whitening strips) are effective whencompared with placebo or no treatment and the efficacyvaries because of different levels of active ingredients.However, the majority of the studies are eithersponsored or conducted by the manufacturers and areof shorter term.8Furthermore, tooth-whitening productsare not regulated in many countries and most of theseproducts have not undergone clinical evaluation forsafety and effectiveness. Therefore, there is a greatneed for independent laboratory and clinical trials whichcould provide a good indication of what could beexpected in practice.In-office bleaching procedures are performed usinghigher HP (30 – 38%) concentrations at chair-side underthe close supervision of a dentist. A number of clinicalstudies have demonstrated the effectiveness of in-officebleaching alone21-23 or in combination with further use oftake-home bleaching products.24,25 Auschill et al.,26 in arandomized clinical trial comparing the efficacy of at-home, OTC and in-office bleaching techniques, reportedthat all treatment methods were able to achieve sixgrades of whitening but the time factor involved in thetreatment was significantly different with the in-officebleaching technique requiring the least time. However,the most accepted method amongst the patients was theat-home bleaching technique. In contrast to theseresults, another study showed that treatment with anin-office bleaching (35% HP) product was less effectivecompared to a 14-day application of 10% CP in a tray.27Special lights and heat-generation devices are alsomarketed by several companies as a necessary tool forin-office bleaching to expedite the bleaching efficacy. Afew studies have reported the acceleration or enhancingeffect of different light or laser sources on in-officebleaching treatments,28-30 while other studies reportedno effect of light-activation on the final outcome ofin-office bleaching with HP.31,32 Hein et al.,33investigated the contribution of three bleaching lights(Luma Arch, Optilux 500, and Zoom!) to act as catalystsfor whitening teeth in a split-arch clinical study. Hereported that neither the heat produced by the lights northe light outputs per se were responsible for catalyticactivity and the tested lights did not lighten teeth morethan their irrespective bleaching gels alone. Inspite ofcontradictory reports in the literature, to date there is noconcrete evidence to show that these devices improvethe final outcome of in-office bleaching treatment.34,35In-office bleaching products are accepted by theAmerican Dental Association but due to thediscontinuation of the professional component of theSeal Program on December 31, 2007, these bleachingproducts are not eligible for the ADA Seal.36Adverse effects: Adverse effects of vital toothbleaching procedures on hard and soft tissues of theoral cavity have been reported in the literature.37 Toothsensitivity and gingival or mucosal irritation are the mostcommon side effects of vital tooth-bleaching. Othereffects include minor orthodontic tooth movement,temporomandibular dysfunction due to long-term trayuse, and sore throat.38Tooth sensitivity: Tooth sensitivity occurs in two-third ofthe patients treated with home bleaching products. Themajority (55%) may experience mild sensitivity whereas10% experience moderate and only 4% may experiencesevere sensitivity.37 Symptoms are noticed early in thetreatment, usually after 2 – 3 days, and may persist 3 – 4hours following removal of the tray and disappear shortlyafter the treatment ends.39 The aetiology of toothsensitivity following bleaching treatment is multifactorialand is poorly understood.40 Sensitivity is thought to becaused by the diffusion of by-products produced duringHP and CP breakdown through dentinal tubules.41

Glycerine, used as a carrier in most bleaching agents, ishydrophilic and causes dehydration of tooth structureduring bleaching treatment. This can also result in toothsensitivity.42 The use of bleaching products with higherperoxide concentration also increases the risk of toothsensitivity.43Patients with existing sensitivity should be treatedbefore starting bleaching treatment: Desensitizingtoothpastes and fluoride gels can be used for 2 – 3weeks prior to the treatment or during treatment.A neutral sodium fluoride gel in a tray can be wornovernight or gels containing 3% to 5% potassium nitrateor fluoride and potassium nitrate in a tray before or afterbleaching for 10 – 30 minutes. Furthermore, thefrequency and / or duration of application can bereduced and the treatment can also be interrupted, ifnecessary.Gingival or mucosal irritation: Some patients mayexperience gingival or mucosal irritation during homebleaching procedures. Soft tissue irritation may becaused by an ill-fitting tray impinging on the gingivaand/or the use of excess material.39 Managementincludes simply adjusting and polishing the tray and orinstructing the patient to use less material. During anin-office bleaching procedure, a higher HP concentrationis usually used. HP is a caustic substance and cancause burns of the gingival or mucosal tissue.44Therefore, a rubber dam or light-cured resin, provided bythe manufacturer, should always be used to protect softtissues during in-office bleaching procedures.Effects on tooth structure: Bleaching of vital teethinvolves direct contact with the enamel surface for anextensive period of time which differs according toproducts. This fact increased concerns about thepossible adverse effects of such a strong oxidizing agenton the enamel or dentine. The available literature iscontradictory. Some scanning electron microscopestudies reported changes in surface morphology ofenamel following bleaching with CP45,46 and/or HPproducts47 while others reported no alterations in theenamel morphology.48,49 Hegedüs et al.,50 in an atomicforce microscopy study, demonstrated that CP and HPwere capable of causing alterations in enamel surface.In a recent study,51 it was found that all four differentkinds of opalescence teeth whiteners damaged enamel.The most damage was done by the 10% and 20% CPproducts because of the much longer exposure period of112 hours in comparison to only 7 hours for theOpalescence Quick PF 45% CP and Treswhite Supreme10% HP. Certain studies have also reported negativeeffects on enamel and dentine microhardness,52-56 whileothers reported no change in the microhardness ofenamel57,58 and dentine.59 Lewinstein et al.60 reportedthat in-office bleaching products, i.e. 35% HP and35% CP, reduced hardness of enamel and dentinesignificantly more than the home bleaching products, i.e.10% CP, but the application of 0.05% fluoride solution for5 minutes completely restored the softened toothstructure. In an in vitro study, Sulieman et al.61 reportedthat 35% HP did not damage enamel or dentine and theadverse effects reported in the literature may be relatedto the pH of the products used. A small reduction indentine surface microhardness following exposure to10% CP in situ was reported by Arcari et al.,62 but theyconcluded that this might be clinically insignificant.Current literature indicates that the experiments varygreatly in their methodology, the type of bleaching agentused, the duration of application, load applied and theposition of indents. However, human enamel exhibitslarge regional variations in structure related to thedifferences in local chemistry (varying levels ofmineralization, organic matter and water) andmicrostructure (fractions of inorganic crystals andorganic matrix).63,64 Therefore, enamel microhardnessmay vary from area to area. This may be the reason forcontroversies found in the literature. There is a greatneed to develop a standardized protocol to evaluate theeffects of tooth-bleaching products on microhardness ofenamel and dentine.Effects on restorative materials: Increasing use ofperoxide bleaching agents has raised concerns abouttheir effects on different restorative materials. Severalin vitro studies have evaluated the effects of CP (10 – 16%)and HP (30 – 35%) whitening products on the physicalproperties, surface morphology and colour of differentrestorative materials.65 Haywood66 reported that anightguard vital bleaching technique had no significanteffect on the colour and physical properties of porcelain,amalgam and gold. An increase in the surfaceroughness of porcelain, microfilled composite andmodified glass ionomer following treatment with 10-16%CP was reported by Turker and Biskin.67 Modified glassionomer also showed increased surface porosity andcracks in certain areas. Controversy exists about theinfluence of external pre- and post-operative bleachingon microleakage of composite restorations. Crim68reported that pre-restorative bleaching with 10% CP didnot affect the marginal seal of subsequently placedrestorations. Ulukapi et al.69 reported that pre- and post-operative bleaching with CP increased marginal leakageof resin composite restorations at enamel and dentinemargins but amalgam restorations showed noalterations. In contrast, other studies did not reportincreased microleakage rate at enamel margins.70The oxidation of surface pigments and amine compoundsby bleaching agents can alter the colour of restorativematerials. The oxidizing effect on the polymer-matrixof resin-based materials also increases surfaceporosities.64 There is no clear evidence indicatingwhether the changes in tooth-coloured restorativeTooth-bleaching materials are superficial or deep. However, polishing ofresin composite fillings is advisable following bleachingprocedures to decrease the adherence of certaincariogenic micro-organisms.Bleaching agents also cause increased release ofmercury from amalgam restorations.71 Coating ofamalgam restorations with a protective varnish such asCopalite before bleaching procedure has been reportedto reduce release of mercury into the surroundingenvironment.72 The corrosion potential of amalgam isalso decreased if restorations are polished prior to thebleaching therapy.Effects on bond strength: The effect of variousbleaching procedures on shear or tensile bond strengthof composites to enamel and dentine has been studiedextensively. The majority of the studies reported that thebond strengths of composite restorative materials toenamel and dentine73-76 was significantly reduced whenapplied immediately after bleaching with HP or CP.Josey et al.77 reported no negative effects of 10% CPbleaching on composite-enamel bond strength.However, controversy exists about the effects of alcohol-or acetone-based bonding agents on the bond strengthsto enamel and dentine.64Several factors are responsible for the reduction incomposite bond strengths to enamel and dentine.Polymerization inhibition of the resin adhesive systems,due to the presence of oxygen released by the bleachingprocess on the enamel surface and within the dentinaltubules, is the likely mechanism for the reduction in bondstrength.78 Significant loss of enamel calcium andphosphorus content and morphological alterations of themajority of the crystals of the surface layer caused bythe peroxide-based bleaching agents also adverselyaffects the bond strength.79 Adebayo et al.,80 reportedthat the use of conditioners prior to bonding with selfetching adhesive system to bleached enamel maysignificantly improve bond strength. However, thereduction in bond strength is time-dependent andreturns to normal after a few days, when the residualoxygen is liberated. Recommended waiting time beforeperforming bonding procedures after tooth bleachingranges from 3 to 7 days,81 7 – 14 days78 to 3 weeks.82Therefore, it is advisable to wait for a while beforeperforming bonding procedures after bleaching.CONCLUSIONDifferent treatment modalities are available to the patientdesigning a whiter smile. Tooth sensitivity and gingival ormucosal irritation are the most common side effects ofvital tooth-bleaching. However, ADA recognisedproducts tend to include agents to minimize or preventthese side effects. Dentists should educate themselvesto be able to inform their patients about the benefits andrisks of different whitening methods based on the currentscientific evidence and to suggest the best treatmentoption based on a correct diagnosis.REFERENCES1. Kihn PW. Vital tooth whitening. Dent Clin N Am 2007; 51: 319-31.2. 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