Case 1. Indirect technique: the endocrown
The second patient is a 30 years-old man who needs to restore an endodontically treated mandibular right first molar (4.6) with a large cavity including the mesiodistal interproximal walls and a big portion of both lingual and vestibular cusps (Figures 3a-3c). The simultaneous loss of both interproximal walls associated to an endodontic cavity has been always considered by the literature as the most inconvenient cavity configuration in term of fracture risk3,4.
Moreover, the occlusal anatomy of the neighboring 4.7 reveals a clenching occlusal context. The set of axial and shear forces would put the vestibular and the lingual walls in high risk of catastrophic failure. Thus, a full coverage of the cusps is considered necessary to balance the occlusal forces (Table 1). At the same time, the occlusal reduction of the vestibular cusp implicates an esthetic concern. From an esthetic point of view margins of the restoration should be put in the cervical third, close to the gingival line, but this cavity configuration would mean an ulterior loss of substance. In the present case, being the inferior molar esthetic needs moderate, margins of the restoration were placed in the middle of the vestibular face at 2-3 mm from the occlusal plane. An internal bleaching of the tooth was programmed before the treatment to reach a good esthetic aspect of the remaining vestibular wall (Figures 4a-4d).
In the clinical case presented, the large amount of tissues lost due to pathology and to the endodontic treatment supports the use of an “adhesive indirect overlay restoration for devitalized teeth” or “endocrown” instead of a full crown. This technique allows for the conservation of sound dentin and, above all, peripheral enamel, maintaining the possibility to bond margins of the future restorations to it, which is known to have a beneficial effect on marginal stability5. The adhesive procedure also prevents the use of a post and a core which would be necessary in a typical crown preparation. Moreover, the adhesive cavity configuration keeps all margins of the restoration away from the periodontium, which is beneficial for hygiene and periodontal health6,7. A conventional indirect composite technique is thus accomplished, by programming two appointments8,9. During the first appointment, cavity is cut under local anesthesia in order to reach an ideal geometry. The old resin composite is removed in the interproximal regions. The vestibular and the lingual cusps are reduced in order to leave 2-3 mm from the occlusal plane (Figure 5b). Once the cavity is properly isolated (Figure 5c) an adhesive system is applied on the entire dentin and on the mesio-distal thin sub-gingival portions of enamel margins and light cured. Then, a thin composite layer is applied on dentin and into the mesial box and light cured. The aim is to fill the pulp chamber, cover all dentin and to get an ideal geometry of the cavity: correct taper, minimal undercuts, cervical margins relocated supra-gingivally and adequate interocclusal space. For that purpose, a low shrinking micro- or nano-hybrid composite is applied. The fabrication of a concavity in the middle of the pulp chamber composite will help with the positioning of the restoration during luting and improve the adhesive surface available for the future endocrown. Finishing the enamel margins with fine diamonds instruments is the last step before impression (Figure 5d).
A soft lightcuring resin is applied as temporary restoration (eg Fermit, IvoclarVivadent AG, Schaan, Switzerland) (Figures 6b, 6c) The indirect restoration is then fabricated inlab. In that specific case the endocrown was milled from a CAD/CAM composite resin block (LAVA Ultimate, 3M ESPE AG, Seefeld, Germany) and then esthetically modified with a free-hand technique. During the second appointment the workpiece is tried in the mouth. The anatomy, the esthetic integration, the interproximal surface contacts and the fit of the margins are checked. Consequently, the internal surface of the indirect resin composite restoration is adhesively treated and then left under light protection.
(Table 2, Figures 7a-7c) The next step is the adhesive treatment of the cavity (Table 2, Figures 8a-8c). The presence of only enamel and resin composite, without exposure of dentin, facilitates the whole procedure. A conventional photopolymerizable hybrid resin composite is used as luting cement. Before the insertion into the cavity, this composite should be heated-up to a temperature of about 50°C to decrease its viscosity.
Immediately thereafter, the restoration is inserted into the cavity and forced in place manually with the finger. The use of metallic plugger is contraindicated when the thickness of the restoration is fine, because it may introduce fractures. Excesses tamof luting composite at margins are removed with a probe and interproximal floss. A final push with a plastic ultrasonic tip helps to seat the restoration in its final position (Figure 9a).
A first light polymerization with a high power LED unit which serves to fix the surface of the luting composite is performed for 5 s per surface. Then full polymerization in contact with the irradiated surface is achieved by light curing for at least 90 s per surface (Figure 9b). Any composite excess is subsequently removed with fine diamonds and re-polished with flexible discs or silicone points with slight pressure. A layer of glycerine gel is finally applied over the entire surface of the restored tooth and the luting composite is cured for 5 s per surface through this gel to eliminate the oxygen inhibition layer on the surface of the luting composite, if still present. Finally, rubber dam is removed and occlusion is checked (Figure 10).