An update on discoloured teeth and bleaching part 2: mechanism of action of bleaching agents and management of discoloured teeth

’In-office’ bleaching

‘In-office’ bleaching techniques are those which are applied by the clinician to the teeth within the dental office. Such techniques can be associated with the use of ‘lasers, additional lights or heating lamps’, which aim to energize the oxidizing agent further and provide a more pronounced and rapid lightening in tooth shade. Historically, such techniques have employed the use of stronger bleaching concentrations, for shorter bursts of time, with the aim of achieving a more noticeable result, in a shorter timeframe. Despite the allure of such techniques, using stronger bleaches is illegal and increases the risk of post-operative sensitivity, pulpitis and/or pain.^8,9 Precautions must also be taken to use a light-cured or rubber dam, in order to protect the soft tissues from the effects of the bleach, as this may result in undesirable iatrogenic damage to the gingival and other soft tissues (Figure 1).¹⁰ There is also a greater risk of lightening due to dehydration of the teeth and, as a result of this, the risk of a patient leaving with whiter teeth and re-attending with a relapsing shade in a short period of time is high.¹¹ As a result of the need for surgery time, the costs for such treatments can also be high in comparison to ‘at-home’ bleaching solutions.

’At-home’ bleaching

‘At-home’ bleaching techniques employ the use of a bleaching tray which is constructed from a dental cast, which is derived from an alginate impression. The trays are often constructed from ethylene-vinyl acetate, which is a thermoplastic elastomeric material (Figure 2). The trays can be constructed with or without reservoirs. In a study by Matis et al, the clinical efficacy of a bleaching agent was assessed when using trays with reservoirs in comparison with trays without reservoirs. The trays with reservoirs yielded significantly lighter teeth when assessed using a colorimeter. However, no differences could be discerned when shade guides and photographs were used to assess outcome.¹² The ‘at-home’ bleaching technique entails the patient applying one drop of bleach into the tray, in alignment with each tooth. The tray is inserted into the mouth and any excess bleaching gel on the gingival tissues is removed with the aim of avoiding any iatrogenic blanching or damage to the tissues. The bleaching agents are often weaker than those employed in ‘in-office’ bleaching and, as a function of this, the incidence of adverse events is lower. The adage of ‘slow and steady wins the race’ comes to mind. Patients are instructed to use the bleach in the trays for 2 to 4 hours daily, until the desired shade is achieved. Patient satisfaction has been noted with bleaching with 10% carbamide peroxide for 2 hours daily after 4 to 6 weeks.¹³ It is recommended that the patient avoid colourful foods and drinks, as well as any agent which may stain the teeth while they are bleaching their teeth. It is also recommended that the patient uses a desensitizing toothpaste, and this can also be delivered in the same tray, having cleaned it of the bleaching agent. The clinician is wise to record clinical photographs with a matching tooth shade guide tab prior to beginning bleaching, as this will act as a baseline record – all too often, the patient and/or clinician forget the starting point, and this can lead to challenging conversations at the end of treatment. Clinical photographs with shade tabs should also be recorded at subsequent review appointments. Patients should be warned that, like all dental treatment, nothing is permanent and the effects of the bleaching can be variable. In light of this, patients should be advised that they will, in all likelihood, require periodic top-up bleaching in the future, and there is a risk that improvement in shade may not occur, although this is unlikely.

Presence of restorative material within the coronal portion of the tooth

Bleaching agents are ineffective in lightening the colour of teeth when a discoloured bulk of filling material or gutta percha is present within the pulp chamber or supra-crestal intra-dental space. To avoid such problems, it is recommended that all dark or discoloured restorative material be removed from the supra-crestal intra-dental tooth space, to a level of 2 mm below the adjacent bone crest. Removal of this material will aid in allowing a more appropriate absorption and transmission of light through the tooth.^14,15Figures 6 and 7 show an endodontically treated UL1 tooth which has gutta percha and a layer of material to seal the GP in, passing into the pulp canal space supra-crestally. Some of this staining may also be a result of blood breakdown products.

Restorative material or gutta percha (GP) in the pulp chamber of the tooth

In the case of gutta percha, it is recommended that endodontic treatments are completed and the gutta percha is cut back to 2 mm below the level of the adjacent crestal bone (Figure 8). The gutta percha should then be sealed into the canals using a material with excellent sealing capacity, such as Cavit or IRM. The base of the pulp chamber should then be filled with an additional layer of sealing material, such as glass ionomer cement, and the remaining pulp chamber space should be filled with a light shade of material with good marginal integrity. This can include glass ionomer cement or resin composite.

Removal of excess intra-pulpal material, whether it be restorative material or gutta percha, should be executed under rubber dam, in order to ensure an uncontaminated field and to avoid compromising the endodontic treatment present. In cases where the endodontic treatment has been previously completed, and in the absence of apical pathology, the gutta percha can similarly be accessed under rubber, and cut back to 2 mm below the adjacent crestal bone and similarly sealed. Removal of endodontic material from within the pulp space and subseqent selective bleaching can result in considerable improvements in tooth shade (Figures 8, 9, 10).

Blood breakdown products

If the restorative material has been completely removed, and the gutta percha is appropriately cut back, resultant discoloration of the tooth may be as a result of either blood breakdown products, in particular iron sulphide passing into the dentinal tubules when the tooth became non-vital, the presence of breakdown products from the restorative material or the presence of other chromogenic materials within the dentinal tubules. In such cases, a bleaching agent can be used to lighten the tooth conservatively and predictably. This can be performed either by external bleaching or an inside/outside bleaching technique (Figures 3, 4 and 5).

Hypersensitivity

Dental hypersensitivity is a frequently reported side-effect of bleaching,^{48,50,51,52,53,54,55,56} which occurs as a likely combination of opening up of the dentinal tubules following removal of the stain, whilst also eliciting a mild reversible pulpitis reaction. Bruzell et al reported a prevalence of 50.3% for ‘at-home’ bleaching in comparison to 39.3% for ‘in-office’ bleaching.⁵⁷ In other studies, prevalence of sensitivity has been reported between 55%⁵⁵ and 70%⁵⁸ of the study population. Despite these rates, patient satisfaction was found to be high,⁵⁷ and discontinuation of bleaching in ‘at-home’ bleaching has been found to occur in less than 20% of patients.^55,59 Hypersensitivity is usually a transient occurrence,⁶⁰ and can be managed by desensitizing toothpastes, fluoride preparations and 5% potassium nitrate gels.⁵⁸ The avoidance of extremes of temperature during the period of bleaching can provide symptomatic relief.⁵⁸

Predictors of sensitivity include:⁵⁰

Despite these predictive factors, all patients should be warned of bleaching-related sensitivity given that studies, including the use of placebo gel, have shown comparable prevalence of symptoms and a greater intensity of symptoms with placebo gels.^55,61

Uneven results

Whilst the bleaching agent can be applied to all teeth in even measure, it is difficult to guarantee that all teeth will bleach unequivocally to the same extent and at the same rate. It is important to inform the patient that the rate and extent of lightening is variable, although efficacy is high, and this is supported by the literature. Definite completion dates and time frames are given at the dentist's peril.

It is important for the patient to exercise due care and attention as, in the case of bleaching a single tooth, the risk exists of overshooting the shade and having a residual tooth which is considerably lighter than the other teeth. This would necessitate bleaching of the remaining dentition, and it may be challenging to get a homogeneous end result (Figure 15).

Shade regression

Patients must be made aware that, despite lightening of the teeth occurring, some relapse of the bleaching effect is likely over time. Patients are advised to keep their bleaching trays, and bleaching syringes can be requested at a later stage, if and when the shade deteriorates. A randomized clinical trial investigating degree of colour change, rebound effect and sensitivity found that ‘in-office’ bleaching resulted in greater colour regression in comparison to ‘at-home’ bleaching at six months. At the 2 week, 1 month and 3 month follow-up intervals, no difference in effect was noted.⁶² In contrast to this, another study comparing efficacy and shade regression with ‘in-office’ and ‘at-home’ bleaching found no difference in efficacy or shade regression between the two at 9 months. This study, however, only examined bleached maxillary first pre-molars in participants aged 20–25 years old.⁶³ In a 9–12 year retrospective case series study, Ritter et al found that, at a mean observation time of 118 months, colour stability was observed by 43% of participants.⁶⁴ A double-blind randomized clinical trial of two carbamide peroxide-containing bleaching agents reviewed colour stability at 2 years following intervention. This study found that tooth shade remained lighter than baseline at 2 years and, despite high patient satisfaction, 66% of participants gauged their shade relapse as mild to moderate.⁶⁵ The presence or absence of a tray reservoir does not appear to have an impact on shade regression.¹²

The literature would suggest that shade stability following bleaching is high and can range from 2 to 10 years or more. Relapse to the original pre-bleaching shade is seldom reported.

Irritation or iatrogenic soft tissue damage

Soft tissue and, in particular, gingival irritation may be caused by the tray itself or by the bleaching material within the tray. It is recommended that well-fitting trays formed from high quality impressions are constructed to carry the bleaching agent. Some authors have recommended a scalloped margin to the tray to minimize soft tissue contact and thus irritation.⁵⁵ In the case of ‘at-home’ bleaching, patients should be appropriately instructed on how much bleach to deliver into the tray. Instructions should be given on the removal of excess bleaching agent which may seep out through the side of the tray and on to the soft tissues. Similarly, a poorly applied and maintained gingival barrier during ‘in-office’ bleaching can result in a significant gingival inflammatory response. Greater gingival irritation has been reported in ‘in-office’ bleaching procedures than ‘at-home’ techniques, and this may be as a result of the higher concentrations of bleaching agent used.⁶⁶ There is a risk of greater iatrogenic damage to the gingival and other soft tissues if this material is allowed to remain in contact with the soft tissues for a protracted duration. Adverse effects include burning or blanching of the tissues, which can result in a whiter/lighter appearance to the soft tissues. Studies involving exposure of human tissue equivalents to tooth-whitening agents resulted in altered tissue morphology, induced proliferation of basal keratinocytes, and caused apoptosis of cells in all epithelial strata. An increased expression of cytokines that are linked to inflammation was also observed.⁶⁶ Whilst exposure of soft tissues to bleaching agents may be asymptomatic or sore, such findings would support a preventive and mindful approach.

External cervical resorption

External cervical resorption (ECR) is an inflammatory-mediated breakdown of the external cervical portion of the tooth. It has been reported to be a potential serious sequela of internal bleaching.^67,68 Isolated cases of resorption defects have been reported in the literature in relation to bleaching, the first of which were reported by Harrington and Natkin in 1979.⁶⁹ Much of the literature available regarding ECR pertains to traumatized or sub-optimally root-filled and poorly sealed teeth. In many cases, such teeth can be observed to undergo ECR without bleaching being carried out. In an analysis of 257 teeth exhibiting ECR, Heithersay found that orthodontics was the most common single predisposing factor, being associated with 24.1% of the teeth. Trauma was the second most common sole factor (15.1% of teeth). Trauma, in combination with intracoronal bleaching, orthodontics, or delayed eruption was associated with a further 11.2% of teeth. Intra-coronal bleaching was found to be the only potential predisposing factor in 3.9% of the teeth. Surgery involving the cemento-enamel junction area was a potential predisposing factor in 5.4% of teeth. Interestingly, 16.4% of teeth had no identifiable predisposing factor. Heithersay concluded that there was a strong correlation of ECR with orthodontic treatment, trauma, and intracoronal bleaching, either alone or in combination.⁷⁰ The combination of internal bleaching with a history of trauma has been cited to be the most important predisposing factor for ECR.⁷¹ Several long-term follow-up studies have demonstrated an association between ECR and non-vital internal bleaching.^{69,72,73,74,75,76,77}

A greater risk of cervical resorption has also been reported when the internal surface of the tooth is etched prior to bleaching and a high concentration of hydrogen peroxide^75,77 is heated within the tooth. It has been suggested that heating of the bleaching agent^78,79 and etching of the internal tooth surface promotes deeper penetration of the bleach, allowing it to reach and damage the cementum and periodontal tissues, and in a more energetic state denature the dentine also, eventually eliciting a foreign body reaction, resulting in resorption.⁷⁵ The literature is strongly in support of the association of ECR and teeth which are endodontically treated as a result of trauma.^{74,78, 80,81,82,83} It has been suggested that the damaged cementum layer in the presence of high concentrations of bleaching agent initiates the resorptive process,⁸⁴ although it may be that a damaged cementum layer alone is the aetiological factor for ECR initiation. There is support for a lower concentration of bleaching agent to be used, as the final achievable effect is comparable, although it may take longer to achieve.⁸⁵

The efficacy of internal non-vital bleaching is nonetheless high, and a robust protocol is necessary to minimize the risk of ECR occurring. It is key that the gutta percha is removed to 2 mm below the cemento-enamel junction and an effective barrier between the endodontic treatment and bleaching agent is placed. The sealing material should extend to the CEJ to prevent passage of the bleaching gel into the endodontic space and into the periodontal ligament space via local dentinal tubules.⁸⁶ A number of materials have been proposed for this purpose. Cavit has been found to have better sealing capacity than IRM,⁸⁷ although following bleaching this will need to be removed. Alternatively, a glass ionomer cement (GIC) may be used. It has been demonstrated that a 2 mm thickness of GIC is effective in preventing the penetration of a 30% hydrogen peroxide solution into a root canal.⁸⁸

While the risk of ECR from internal bleaching appears to be low, it is important for the clinician to recognize the association with previous orthodontics and endodontic treatment as a consequence of trauma. High concentrations and heating of the bleaching agent is not recommended