The ultimate aesthetic challenge in dentistry: a single crown on a maxillary central incisor

Case presentation

A 22-year-old female patient presented in the dental school clinic with a discoloured left upper central incisor (Figure 1). The young woman was self conscious of this when smiling, so this aesthetic impairment was the main reason for her consultation (Figure 2 a–c). Patient assessment revealed a negative response to thermal stimulus and a negative response to percussion of the respective tooth. On inquiry, the patient reported a recently finished endodontic treatment with subsequent apical surgery and retrograde filling. A recent radiograph showed incomplete bone consolidation of the apical resection area.

Different therapeutic approaches and their costs were explained to the patient. The patient decided in favour of an adhesively luted glass ceramic crown, because a previous bleaching therapy did not result in the desired whitening. A less invasive ceramic veneer was not considered sufficient because of a large composite restoration sealing the endodontic access cavity at the palatal surface, which significantly reduced the stiffness of the tooth.

Because the patient was worried by the aesthetic impairment of the discoloured tooth (Figure 3 a, b), she did not want to wait for the complete osseous reconsolidation after apectomy but requested immediate prosthodontic treatment. To minimize risks, a slightly modified treatment concept was selected: after final tooth preparation, an aesthetic, laboratory-fabricated, long-term provisional was to be placed until replacement by the final ceramic crown after complete osseous healing.

All-ceramic restorations obtain their strength from the mechanical stability of the employed ceramic material. Furthermore, the fracture strength is determined by the geometry of the restoration and thus by the shape of the cavity or crown preparation. The basic principle of preparation design for all-ceramic restorations avoids tensile stresses in the material and loads the restoration primarily in compression mode by an adequate preparation geometry.^28,29 Fracture strength of the restorations is determined by size, volume, shape and surface characteristics of the ceramic material and additionally by structural inhomogeneities introduced during the manufacturing process.³⁰

The method of cementation of the crown to the prepared tooth was by adhesive luting, as opposed to conventional cementation. This gives a positive effect on the overall strength of the restoration, in particular for glass ceramic materials, which are more prone to bulk fracture and chipping effects than zirconia. Ceramic materials with fracture strength less than 350 MPa are not indicated for conventional cementation.³¹ Among those are feldspathic porcelains and leucite-reinforced glass ceramics that have to be placed adhesively using bonding agents and luting resins. Due to the adhesive bond between the ceramic restoration and enamel or dentine, a considerable increase in strength can be obtained because the inner surface of the ceramic restoration no longer acts as a mechanical boundary line at which cracks can initiate due to tensile stresses.³²

The following steps describe the sequence of preparation for a glass ceramic crown (Figure 4 a–t). The prepared tooth should exhibit an optimal retention and resistance form of a glass ceramic crown:^33,34,35

The correct preparation design of a tooth influences the success of a restoration.^33,36 In the first step of the crown preparation, proximal tooth surfaces are separated (Figure 4 a, b). To protect the integrity of the neighbouring teeth, it is recommended to preserve a thin layer of proximal-lateral enamel during separation initially (Figure 4c). This fragile enamel structure can easily be removed after finishing the separation procedure (Figure 4d). To display the position of the marginal gingiva at rest, in the next step this line is marked with a round diamond bur at the labial aspect of the tooth (Figure 4e). Afterwards, the necessary axial preparation depth at the labial aspect is marked, taking into account the geometry of the labial curvature of the tooth from cervical to incisal direction (Figure 4f, g). In the following, the remaining tooth structure between the depth cuts is removed to that level. In the proximal region, tooth structure that presents danger to the neighbouring teeth when being prepared with rotary burs can be removed by employing oscillating preparation instruments with one diamond-coated active surface and one inactive polished surface using the sono-abrasive technique (Figure 4h).^37,38,39 Prior to completing the preparation and determining the definitive position of the labial finish line, approximately 0.5 mm intrasulcular, the adjacent marginal gingiva, is displaced in apical-lateral direction with a retraction cord to prevent injuries by rotary burs (Figure 4 i–k). A slightly subgingivally placed margin is prepared labially to allow a complete masking of the discoloured tooth structure. For a better identification of the remaining substance to be removed, the labial depth cuts are marked with a waterproofed pen (Figure 4l) so that the necessary axial reduction can be achieved (Figure 4m).⁴⁰ To avoid cross contamination, a new pen is used for every patient. In the next step, the circular finish line is finalized with regard to the position of the gingiva at rest that had been marked previously (before retraction) (Figure 4n). After incisal reduction of the tooth (Figure 4 o, p), the palatal surface is prepared using a rugby-ball-shaped diamond bur (Figure 4q). The crown preparation procedure is completed by smoothing sharp line and point angles using a composite polisher (Figure 4r). Figures 4 s and t show the finished preparation before and after removal of the retraction cord; the effect and the amount of gingival displacement by the retraction cord is clearly visible. Figure 5 shows the final preparation from the incisal view. Tooth substance removal is controlled in all dimensions using a silicone index that has been fabricated on a gypsum duplicate of the wax-up (Figure 6 a, b).

After taking an impression, intermaxillary registration is carried out fabricating an interocclusal record in maximal intercuspal position and a facebow record is performed.⁴¹ The shade of the prepared tooth is documented with a special shade guide (IPS Natural Die Material Shade Guide, Vivadent, Schaan, Leichtenstein) by digital photography.

A diagnostic template made from a gypsum duplicate of the analytical wax-up using a transparent polyethylene matrix allows the chairside fabrication of a direct provisional restoration with correct dimensions and alignment (Figure 7 a, b). To ensure a high-quality functional and aesthetic restoration until incorporation of the final ceramic crown, after waiting for complete re-ossification of the apicectomy site, a laboratory-fabricated, long-term provisional with optimal marginal adaptation is produced (Figure 8).

One week after impression taking, the gingiva presents in good condition after removal of the chairside provisional (Figure 9). The new lab-made provisional is seated using a eugenol-free temporary cement. It blends well into the surrounding teeth (Figure 10) and supports the soft tissues and the lip profile (Figure 11).

One year after the start of treatment, periapical radiographic examination was repeated and demonstrated healing of the osseous structures. At this stage, the final ceramic crown was fabricated. A core corresponding to the anatomically correct shape of the respective tooth was pressed using lithium-disilicate glass ceramic and finalized with veneering porcelain (Figure 12 a, b).

After removal of the long-term provisional and cleaning of the tooth with a rotating brush and fluoride-free prophylaxis paste, the gingiva presented in healthy condition again at the final appointment (Figure 13). Using coloured, glycerin-based, try-in pastes, the aesthetics of the ceramic crown was checked intra-orally with reference to hydrated adjacent teeth and the correct shade of the luting resin was determined. Subsequently, the precise fit of the crown on the prepared tooth and quality of proximal contacts were checked before minor functional interferences during protrusive and laterotrusive movement paths were eliminated.

Afterwards, the inner surfaces of the lithium-disilicate glass ceramic crown were etched with hydrofluoric acid for 20 seconds (Figure 14). After thoroughly rinsing and drying the crown, the fitting surfaces were silanized (Figure 15).^42,43,44,45 After adhesive pretreatment of the prepared tooth by conditioning enamel and dentine with 37% phosphoric acid and applying an Etch-and-Rinse adhesive, the ceramic crown was adhesively luted using a dual-curing resin cement.

The restoration exhibited optimal functional and aesthetic integration into the neighbouring teeth (Figure 16 a–c), with aesthetic improvement achieved (Figure 17 a–c). Background illumination demonstrates the excellent light transmittance capacity of the ceramic crown, by having virtually the same optical properties as the surrounding natural dentition (Figure 18). Ultraviolet light activates the inherent fluorescence properties of the restoration, which are equal to natural tooth structure (Figure 19). The patient's smile was no longer compromised by the discoloured central incisor (Figure 20 a–c).

Conclusion

All-ceramic restorations are an indispensable means for modern conservative and prosthetic dental treatment procedures.⁴⁶ Aesthetics and biocompatibility characterize these restorations. Clinical trials exhibit an excellent longevity for all-ceramic restorations if a correct indication is selected and material- and patient-related limitations are observed.