Management of the single discoloured tooth part 1: aetiology, prevention and minimally invasive restorative options

Extrinsic discoloration

Extrinsic discoloration results from chromogens derived from the diet or environment accumulating on the tooth surface in the pellicle layer (‘direct staining’). Common causes of direct staining are tobacco and coffee. Indirect extrinsic staining results from the chemical interaction of a compound (eg cationic mouthrinse such as chlorhexidine) with another compound producing a stain at the tooth surface. In most cases, extrinsic staining gives rise to generalized discoloration of multiple teeth, unless there is a local factor, such as poor alignment of the teeth, making oral hygiene measures difficult surrounding one tooth.

Intrinsic discoloration

Intrinsic discoloration results from a change to the normal structural composition or thickness of the dental hard tissues and has been subdivided into local causes and systemic causes.6 The local causes of intrinsic discoloration are the most common causes of the single discoloured tooth.6 These are summarized in Table 1. An example of localized tooth discoloration (enamel opacity) is seen in Figure 1.

Systemic causes of intrinsic discoloration include6 drug-related (eg tetracycline), metabolic (eg dystrophic calcification/fluorosis) and genetic causes (eg congenital erythropoietic porphyria, cystic fibrosis of the pancreas, hyperbilirubinemia, amelogenesis imperfecta and dentinogenesis imperfecta). These systemic causes often present with generalized discoloration. Depending on the timing of events in relation to tooth development, metabolic or drug-related systemic causes of intrinsic discoloration, such as fluorosis, may affect just a few teeth, leading to a more localized discoloration. Molar incisor hypomineralization7 is a condition of unknown aetiology characterized by severe hypomineralized enamel affecting incisors and first permanent molars, which may lead to localized discoloration in the upper anterior region.

Management options

A wide variety of management options are available for the single discoloured tooth as illustrated in Figure 2. Minimally invasive restorative options will be covered in the first part of this two part series. The Royal College of Surgeons of England produced UK National Clinical Guidelines in Paediatric Dentistry: Treatment of intrinsic discoloration in permanent anterior teeth in children and adolescents.8 These guidelines suggest that a hierarchy of treatment options should be pursued in a logical order until a satisfactory cosmetic outcome is achieved and describes microabrasion, non-vital/vital bleaching, composite resin veneers and porcelain veneers. This concept of starting with minimally invasive therapies, giving priority to preservation of tooth structure and only moving to more complex and invasive therapies when more conservative measures have been exhausted, is a sound concept when dealing with discoloured teeth. It has been described in the literature as the ‘Progressive approach concept’.9 In a similar manner, Magne et al10 eloquently described the range of aesthetic restorative solutions as a ‘Crescendo’ of methods ranging from the very conservative (such as tooth whitening) to the more invasive and sophisticated (such as porcelain laminate veneers).

One cause of localized tooth discoloration that doesn't fit the above concept is that of tooth resorption presenting as ‘pink spot’ lesions. As described earlier, the establishment of a correct diagnosis will lead to the most appropriate management. The appropriate management options for external invasive cervical resorption lesions which can lead to the ‘pink spot’ appearance have been described previously11 and may involve monitoring, non-surgical and surgical therapy, as well as extraction of the affected tooth and prosthodontic replacement. Figure 3 demonstrates a case of external invasive cervical resorption presenting as a pink spot that was managed surgically.

Case assessment and consent

The correct diagnosis will be made from a combination of a careful history, patient assessment and special tests. The endodontic status of the tooth should be considered and, if the clinician is suspicious that the discoloration is due to a necrotic pulp, or previous root canal therapy, then a peri-apical radiograph is essential. The quality of any existing root canal filling should be scrutinized; the European Society of Endodontology quality guidelines for endodontic treatment12 suggest that an indication for root canal retreatment is when the coronal dental tissue is to be bleached and the root canal filling is inadequate.

When considering traumatized teeth, Andreasen et al13 noted that a persistent or late appearance of a grey discoloration of the crown following trauma may occur in the absence of radiographic changes and often represents pulpal necrosis. The authoritative traumatology text13 states that, for aesthetic reasons, a severe discoloration per se can be considered an indication for endodontic treatment in order to make bleaching of the crown or other treatment options possible.

When discussing intrinsic discoloration in adolescents Wray and Welbury8 suggested pre-treatment photographs, shade-taking and sensibility tests were recommended in all cases. This would equally apply to adults with discoloured teeth in combination with appropriate radiographs.

Since treatment of the discoloured tooth will be an elective, aesthetic procedure, the consent process must be robust and the treatment provided have more benefits than harmful effects. There is a higher onus of disclosure on the dentist than when dealing with an emergency situation of a patient in pain.14 Any attendant risks must be explained to the patient, such as the possibility of loss of tooth vitality if more invasive treatments are planned, such as certain porcelain veneer preparations or full coverage crowns.

Monitor and review

Traumatized teeth can become discoloured pink, bluish or grey following pulpal haemorrhage or necrosis. Pink discoloration that occurs 2–3 days following injury can be reversible.13^,15 Discoloration can take 2–3 months to resolve in traumatized teeth following survival or revascularization of the pulp.16 Therefore, in certain trauma cases, monitoring and review may be the only course of action required.

Scaling and polishing and air polishing

For single discoloured teeth, a good starting point in management is simple scaling and polishing procedures. This will remove any extrinsic stains from the tooth and allow the true underlying tooth colour to be assessed.

Air polishing17 is where a compound, such as sodium bicarbonate, calcium carbonate or aluminium tri-hydrate, is expelled through a mixing nozzle with air and water and used in a controlled jet against tooth enamel. An air polishing handpiece is demonstrated in Figure 4. Air polishing is a way of rapidly removing extrinsic staining from tooth enamel and can be useful on stubborn stains, such as those found in heavy smokers. Air polishing with sodium bicarbonate on cementum and dentine should be avoided since it will abrade these surfaces, although on enamel it is no more abrasive than rubber cup and pumice.18

Air polishing is operator-sensitive and, when sodium bicarbonate is used, it can lead to abrasion of exposed dentine surfaces, often resulting in patients experiencing increased dental sensitivity post-operatively.19 An unpleasant taste and stinging of the soft tissues may be concerns by patients to air polishing with sodium bicarbonate and a novel bioactive glass material used for air polishing may improve patient acceptability, as well as having a desensitizing effect.19

Vital tooth whitening

A wide variety of home-based gels can be used to whiten teeth successfully.20 Vital tooth whitening using 10% carbamide peroxide gel in a tray is a popular method which minimizes the risks of tooth sensitivity and gingival irritation compared to more concentrated solutions.21 Vital tooth whitening is a useful non-invasive technique that is often used in combination with other techniques in order to achieve aesthetic results. A recent Council of the European Union Directive22 on tooth whitening allows dentists to supply products legally which release or contain up to 6% hydrogen peroxide (equivalent to 20% carbamide peroxide) to patients aged 18 and above for tooth whitening. The UK government is obliged to amend regulations to reflect this once the new European Directive22 is in place and so, hopefully, the long history of confusion over the legality of tooth whitening for dentists could be coming to a close.

Vital bleaching is useful in the management of white and brown discoloration from mild fluorosis which may affect single teeth, depending on the chronology of the fluoride exposure. This can often be used in conjunction with microabrasion to give good results. Equally, localized yellow and brown hypoplastic lesions, which sometimes feature surface defects within the enamel as a result of disturbance in amelogenesis from infection or trauma, can be initially managed with vital tooth whitening.23 This aims to eliminate the yellow-brown discoloration prior to further aesthetic management, such as the use of megabrasion and/or camouflage using direct composite. Figure 5 demonstrates the use of vital bleaching to manage localized brown discoloration due to fluorosis.

Vital bleaching is also useful in the management of single discoloured teeth due to pulp canal obliteration or calcific metamorphosis. Figures 6 (a–f) demonstrate the management of a 29-year-old lady who presented complaining of a yellow discoloration affecting both her upper central incisors. She had traumatized both upper centrals over 15 years ago. Since then they had been asymptomatic apart from recent progressive discoloration. The teeth were completely sound and unrestored. A periapical radiograph confirmed pulp canal obliteration to be the cause of the discoloration. Vital tooth whitening using 10% carbamide peroxide at night-time in a two-tooth bleaching tray was used to manage the case successfully. The tray was cut back from the adjacent teeth in order to avoid them whitening. Since the lateral incisors were not discoloured, any shade lightening would have made matching the shade of the four incisor teeth more problematic. The result was stable at 6 months. Management with external vital bleaching in such cases avoids the need for attempting elective root canal therapy and internal bleaching which would be unnecessarily destructive.

Non-vital whitening

Non-vital tooth whitening is the most desirable conservative way of managing minimally restored single teeth that have discoloured secondary to residual necrotic pulpal remnants in the pulp chamber following root canal therapy or pulpal haemorrhage into the dentine following trauma.24 Discoloration due to metallic compounds, such as those found in dental amalgam and corrosion products of silver points, may be less responsive to bleaching than discoloration of pulpal origin6 and so, in such cases, consideration should be given to other management options. As for all forms of bleaching, patients should be informed that existing restorations will not whiten with the treatment and further restorative work may be required.

There are three different ways of performing non-vital bleaching:25

A variety of materials have been suggested for each technique (Table 3).

Using thermocatalytic bleaching, cervical root resorption has been reported as a risk and so this technique is not recommended.26 In-office bleaching often produces short-term success, based largely on the dehydration of the tooth25 and so is generally not favoured for non-vital teeth.

There have been no cases of cervical root resorption associated with the use of 10% carbamide peroxide gel using the inside/outside technique. Clinical experience indicates that this is a rapid, convenient technique for patients. A clinical study27 showed this technique to be more rapid in producing whitening of the tooth compared to the walking bleach technique at two weeks, although after six months there was no difference in outcome following the use of the two techniques.

Settembrini et al28 described the inside/outside bleaching technique. The palatal access of the discoloured non-vital tooth is opened and an apical barrier of glass ionomer (GIC) placed to maintain a coronal seal to the root filling, as well as protecting the periodontium from the bleaching agent (Figure 7). Carbamide peroxide gel (10–15%) is used internally as well as externally; being held in a customized tray (Figure 8). The patient wears the gel and bleaching tray either overnight or for at least two hours before changing the bleaching gel.

Results can be obtained rapidly (Figures 9a and b) and the in-surgery chair time can be reduced compared to the walking bleach technique, which requires the dentist to replenish the bleaching agent perhaps 3–4 times with the removal and replacement of a temporary filling in the palatal access cavity.

For non-vital bleaching, it has been suggested29 that the shape of the glass ionomer barrier should be 2 mm thick and scalloped like a ‘bobsled tunnel’ to reflect the scalloped contour of the cemento-enamel junction (CEJ). The rationale behind this is the suggestion that the inflammatory process involved in cervical invasive resorption can be triggered by the bleaching agent reaching the periodontium through the dentinal tubules and a defect in the CEJ.30

Personal clinical experience suggests that this scalloped barrier can be easily created by initially creating a flat GIC barrier with a Machtou plugger and then selectively removing excess material with a goose neck round bur or endodontic ultrasonic instrument. Figure 10 demonstrates a clinical example of the inside/outside bleaching technique and associated scalloped barrier used in an upper left central incisor.

Relapse of discoloration in non-vital teeth following bleaching is relatively common and it is important that this is discussed with patients as part of the consent process. Reported rates of relapse/recurrence of discoloration after two years are 10%, after five years 20–25%, and after eight years 49%.31^,34

In a long-term follow-up of 35 patients, 63% presented satisfactory results 16 years after bleaching treatment (combined in-office followed by walking bleach).35 In 37% of patients the result was no longer satisfactory and obvious darkening had taken place. No root resorptions occurred. Some studies have found that dentists are more critical of dental appearance than patients. Glockner et al34 found that, when using the walking bleach technique (sodium perborate and 30% hydgrogen peroxide with a phosphate cement barrier), after a 5-year observation period, 75% of the cases were judged as successful by the dentist, while 98% of the patients were satisfied with the result.

Microabrasion

Microabrasion36 is where a microscopic layer of enamel is simultaneously eroded and abraded leaving an intact surface; an amorphous, prismless layer that appears smooth and lustrous.

Microabrasion can be a useful management strategy for developmental mineralization defects within the superficial enamel layer (including fluorosis) and superficial internalized intrinsic stains. The technique is ineffective against discoloration of dentinal or pulpal origin. Discoloration affecting the entire thickness of enamel (eg some forms of amelogenesis imperfecta) will not respond to microabrasion. Microabrasion removes 50–200 microns of enamel and so the repetition of its use should be limited.

Commercially available kits can be used to microabrade teeth or, alternatively, readily available materials including 37% phosphoric acid gel and pumice37 can be used. Microabrasion will create a smooth enamel surface which will decrease the value and increase the chroma of the affected tooth.23 Accordingly, the technique is often used in combination with vital tooth whitening to improve the appearance of teeth. Equally, for some cases that don't respond to microabrasion, possibly in combination with bleaching, the use of direct composite resin for masking severe discoloration may be necessary.

Air abrasion

Banerjee and Watson38 described air abrasion as a pseudomechanical, non-rotary method of cutting dental hard tissues using the kinetic energy of a stream of desiccated abrasive particles to bombard the tooth surface at high velocity. Commonly, alumina particles with an average size of 27.5 microns are used for cutting teeth. The technique may be used to remove extrinsic stains from discoloured teeth, however, it is less conservative of tooth tissue than air polishing. Air abrasion has been documented38 in use for conservative enamel preparation prior to direct composite veneers in a case of discoloration due to fluorosis. When alumina is used to abrade discoloured teeth, areas of healthy tooth tissue adjacent to discoloured areas will be removed indiscriminately, depending on the aim of the nozzle.

Megabrasion

Megabrasion was described by Magne.23 It is a simple procedure that does not depend extensively on the artistic skills of the operator. It is useful for managing idiopathic white enamel opacities and intrinsic developmental yellow-brown enamel discolorations. For yellow-brown discolorations, a preliminary single tooth bleaching treatment may be indicated to reduce or remove the discoloration. Essentially, megabrasion involves complete mechanical eradication of the lesion and subsequent restoration with a neutral and translucent direct composite resin. The rationale for megabrasion23 recognizes that the intact underlying dentine provides the natural optical effects of the tooth (colour, intense dentine lobes and fluorescence), and the simple freehand application of a neutral, translucent and slightly fluorescent composite allows the recreation of the tooth's surface morphology and natural appearance without over contouring.

The clinical procedure involves the use of a coarse diamond instrument (Figure 11a) at low speeds (400–2000 rpm) to allow a safe and well controlled elimination of the discoloured enamel. Coarse flexible discs can be used to remove any sharp angles. Appropriate neutral and translucent enamel shades of direct composite are then placed to restore the defect and transmit the natural chroma from the underlying dentine to give an aesthetic appearance. A clinical case is demonstrated in Figure 12. The term ‘Enamel biopsy technique’39^,40 has been used to described the process of using a tungsten carbide burr (Figure 11b) to remove localized intrinsic enamel discolorations. The authors prefer the use of a white stone for the megabrasion technique (Figure 11c). Macroabrasion41 is another term used for the technique.