Management of the single discoloured tooth part 2: restorative options

Masking

This is where one or several shades of composite resin are used to camouflage the underlying darkened tooth shade. For severe discolorations, a resin-based opaque layer placed over the discoloured areas of tooth may be required.7 This is then covered with dentine and enamel shades to match the adjacent teeth. Various stains and special effect composite shades giving fluorescence, translucency and opalescence to resin restorations provide opportunities to harmonize discoloured teeth with adjacent ones. When applying composite in the cervical region, placing a retraction cord improves the visibility and provides better isolation, making it easier to perform the procedure.8 A clinical case demonstrating masking of discoloration using a direct composite resin veneer is demonstrated in Figure 2. The upper right lateral incisor tooth was discoloured secondary to a root treatment and a corroded amalgam restoration in the palatal access cavity. The patient, over 70 years of age, declined any form of bleaching or tooth preparation and wanted a rapid solution in one visit. In any case, bleaching is known to be less effective in cases whereby the cause of discoloration is due to metallic compounds, as in the case of amalgam.9 The palatal amalgam access restoration was replaced with a high value composite shade. Owing to the severity of the discoloration, and the need for an opaque layer, as well as enamel and dentine shades, the result is slightly over contoured. Nevertheless, the patient was pleased with the simple and quick solution.

Figure 3 demonstrates a case of a young female patient with fluorosis and discoloration of the upper right lateral and central incisor teeth. The areas of dark discoloration were disguised with direct composite resin, including an opaque layer (masking) and some superficial opaque white flecks (simulation) to harmonize the teeth with the generalized mottled appearance.

Masking with a direct composite veneer may also prove useful in cases where a form of non-vital tooth whitening is considered desirable, however, access to the pulp chamber is unavailable owing to an existing indirect adhesive restoration such as a resin-bonded bridge or indirect palatal veneer.

Simulation

This is where one or several shades of composite resin are used to mimic the discoloration of a tooth in an adjacent tooth in order to improve overall aesthetics and symmetry. It is less commonly used since most patients wish to have the discoloration disguised (masking) rather than more areas of discoloration artificially created. Setien et al8 demonstrated a case where a patient with yellow discoloration cervically at the upper right central incisor and a midline diastema had the diastema closed and the yellow cervical discoloration mimicked in the upper left central incisor to improve the symmetry of the smile.

Indications

Ceramic veneers have a higher resistance to wear15^,16 than their composite counterparts. Ceramics are also considered to have a greater ability to mask discoloration than resins.17 For an aged tooth with mild discoloration and lots of character, a ceramic veneer may be a reasonable option. For aged and mildly discoloured teeth, areas of intense coloration, spots and craze lines can be incorporated into 0.5–1 mm ceramic veneers, facilitating their integration despite a low thickness. Ceramic veneers may also be appropriate when the teeth or pontics adjacent to the discoloured tooth have ceramic restorations. Using similar materials can help in integrating the appearance of the teeth. Also, if the discoloured tooth is lingually or palatally displaced in relation to the adjacent teeth, then the depth of tooth reduction can be reduced. Figure 5 demonstrates the integration of a minimal preparation ceramic veneer on a displaced tooth with an adjacent metal-ceramic prosthesis.

Limitations

For severe discolorations of single teeth the use of a ceramic veneer is more problematic and is unlikely to provide a satisfactory aesthetic result.18 In order to mask severe discolorations successfully, a thickness of up to 2 mm of glass-ceramic will be required.19 This is particularly destructive if tooth preparation up to 2 mm is required in order to avoid over contouring the veneer. Such tooth reduction would remove the entire layer of enamel in most areas of incisor teeth, particularly cervically, leaving a dentine surface for bonding procedures which is less predictable and more technique sensitive than a bond to enamel.20 If feldspathic porcelains are used, an opacious layer of ceramic is required to mask the underlying discoloration followed by dentine and enamel shades that will integrate with the adjacent teeth, adding to the required depth of preparation. For severely discoloured teeth, it has been suggested to construct veneers with high density alumina cores,21 which are known to exhibit a low transmittance of light,22 using a computer-assisted design/computer-assisted manufacture system. For such veneers, the high density alumina core cannot be etched, unlike feldspathic porcelain and glass ceramic veneers. Accordingly, the bond to the luting resin is less predictable.

For patients with a thin tissue biotype and high lip line, ceramic veneers on discoloured teeth are at risk of aesthetic failure due to gingival recession. In teeth discoloured due to pulp breakdown products, exposure of the tooth root will uncover the darkened roots that will contrast against the bright veneer and give a poor appearance. For patients with thin gingival tissues, failure to address the underlying tooth discoloration through bleaching can also mean a greyish hue can appear apical to the gingival margin. Another reason for this greying of the gingival margin and interdental papillae, in cases where opacious veneers, opacious luting resins or crowns with opaque cores have been used, is related to the lack of light transmission through the crown into the root and out into the surrounding tissues and the shadow created by the upper lip. This has been described as the ‘umbrella effect’.23

Relative contra-indications

For young patients with highly translucent teeth adjacent to a severely discoloured one, creating a thin veneer with an opaque core/layer that will harmonize well with the adjacent teeth is extremely difficult to achieve. For teeth that are discolored owing to pulp breakdown products in the dentine, the paradox is that the more tooth reduction that is carried out to give a greater thickness of porcelain (to mask the tooth's discoloration), the greater the intensity of discoloration the tooth will have (since the tooth surface is closer to the source of discoloration-pulpal breakdown products).

Effect of luting cement on the overall shade

Opacious luting cements are unable to mask severe tooth discolorations when used with translucent restorations. When cementing the veneer, the thickness of the cement lute is nearly impossible to control accurately. Vichi et al,19 referring to the cement lute, suggested that this parameter can be controlled by the operator only to a certain extent; it cannot be considered as a procedure to correct colour. The authors went on to state that the availability of different cement shades allows only minor aesthetic corrections, which might be instrumentally detectable but are clinically not relevant.19

Survival rates

Porcelain veneers have a relatively good survival rate with a ten-year prospective clinical trial25 showing that, for a single operator, the percentage of veneers that were clinically acceptable without the need for intervention decreased from an average of 92% (95 CI: 90–94%) at 5 years to 64% (95 CI: 51–77%) at 10 years. Similarly, a study that used data from over 80,000 different adult patients treated in NHS general dental practices found that only 53% of veneers survived without re-intervention after 10 years.26 Porcelain fractures and large marginal defects, often associated with discoloration, are commonly reported as reasons for failure of veneers.25

When a veneer fails and is replaced, it has been shown that in circa 20% of cases a crown is the next restoration that the tooth will receive.27 Crowns are clearly more invasive and carry the risk of pulpal death28 and tooth fracture and move the tooth further down the restorative spiral towards extraction.

Translucent crowns

Crowns made from a translucent shade of ceramic, such as the lithium disilicate material used in pressed glass-ceramic crowns (Emax, Ivoclar Vivadent, Lichtenstein), can be used to restore discoloured teeth. Such crowns have the advantages of the need for minimal (0.5–1.5 mm) tooth preparation and the ability to be etched and adhesively cemented to unretentive preparations, such as the crown forms found in obliquely fractured or severely worn teeth.

Such translucent crowns can work well for mildly discoloured teeth. However, for teeth with localized areas of more intense discoloration, it has been suggested to perform selective deeper (additional 0.3 mm) tooth preparation, followed by masking the most intensely discoloured regions, using an opaque shade of composite resin blended into the adjacent crown preparation,29 in conjunction with a translucent crown. Etchable, translucent glass-ceramics can be used with supra-gingival chamfered margins, where the cervical portion of the tooth is not unacceptably discoloured. This is advantageous for periodontal health, since it is known that a relationship exists between marginal discrepancy and periodontal tissue inflammation for sub-gingivally located crown margins.30

For more intensely discoloured teeth, translucent crowns are an unpredictable option for achieving an aesthetic crown that will harmonize with adjacent teeth. This is for the same reasons as have been discussed for ceramic veneers; the underlying discoloration will shine through the translucent ceramic. This shine-through of the discoloration is reduced most effectively by increasing the ceramic thickness up to 2 mm,19 which corresponds to aggressive tooth reduction. The use of varying shades of cement has little effect on the final colour at 1.5 mm and none at 2 mm thickness.19 A laboratory study examining 0.8 mm thickness specimens of IPS Empress ceramic found again that cement shade had no effect on the final colour of the restoration.31 Barath et al,32 referring to laboratory testing of Empress 2 (Ivoclar Vivadent AG, Schaan, Liechtenstein) and In-Ceram Alumina (Vita Zahnfabrik, Bad Säckingen, Germany), concluded that no method exists to predict the outcome of an all-ceramic restoration, based on consideration of the luting agent and the background colour.

Opaque crowns

Opaque crowns, whether made of a metal or an all-ceramic material, have the advantage of greater predictability in final colour of the restoration and are better suited to severely discoloured teeth.29 A case of zirconia opaque core all-ceramic crowns being used to mask discoloration of root-treated incisor teeth is demonstrated in Figure 9.

The drawbacks of such crowns, compared to translucent crowns, include the need for equi-gingival or sub-gingival margins and the risk of periodontal inflammation and gingival recession and slightly deeper tooth preparations, to allow for the thickness of the opaque coping and the veneering ceramic. True resin bonding of such restorations is not possible owing to the inability of metal and highly sintered ceramics to be etched, although tribochemical treatments and adhesive resin-based cements can be used for cementation.33 Zirconia is a popular material choice for an all-ceramic crown core and is available in a range of shades, thicknesses and translucencies, depending on the coping manufacture system used. Although early zirconia copings were very opaque, more modern, thin copings have up to 70% of the translucency value of lithium disilicate glass-ceramic materials,34 and so careful selection of an appropriately opaque coping should be made for the severely discoloured tooth. Figure 10 demonstrates how crowns placed to mask underlying discoloration can face aesthetic failure owing to marginal discoloration and exposure of discoloured root following gingival recession.

The decision to place a crown restoration should not be made lightly, since the negative sequelae of crowns are well known. A study considering the treatment instituted when re-intervention is necessary on a tooth which has previously been crowned, found that 19% of crowned teeth were extracted and/or replaced by a denture as the next intervention.27