The restoration of endodontically threated posterior teeth: the full-adhesive strategy

Giovanni Tommaso Rocca
MD. Chef en clinique, Lecturer. Unite de Terapie Conservatrice et Endodontie, Section de Medecine Dentaire, Universite de Geneve.

Ivo Krejci
DMD. Professeur et Responsable de l’Unite de Terapie Conservatrice et Endodontie, Section de Medecine Dentaire, Universite de Geneve.

In the past, during millions of years, lifetime expectation of man was 30 to 40 years and one could think that evolution designed teeth for this limited period of time. However, during the last century, the lifetime expectation has drastically increased and it is now surpassing 80 years in highly developed countries. Besides challenging tooth structure as such in terms of resistance against attrition, abrasion and erosion, this development has an important impact on conservative dentistry, the goal of which is to keep teeth in the mouth of the patient during his whole life: In case a restoration is realized at the age of six or seven in a permanent tooth, it must serve for about seven to eight decades. Unfortunately, even with the best material and by using the best restorative technique, a median longevity of 70 to 80 years is not realizable today¹. Thus the replacement and repair of restorations must be implemented into a modern conservative approach². Because every replacement and even many repairs sacrifice a certain amount of nonrecoverable tooth structure, it is imperative to limit the number of retreatments to a minimum by increasing the longevity of every single restoration by highest quality and to proceed according to the rules of minimal invasivity, thus avoiding unnecessary removal of sound tooth structure. Up to now minimally invasive dentistry focused on primary restorations such as preventive resin restorations and small adhesive composite restorations. It must be stressed that the principle of minimally invasive treatment in connection with highest quality implies not only for primary restorations, but for the entire field of re-dentistry as well, thus touching large restorations on vital and, in particular, on devital teeth.

1.a) Initial view of the devitalized 1.5. The first premolar was extracted for orthodontic reasons. b) The cavity is cleaned. The distal part of the pre-endodontic resin composite is conserved to avoid a sub-gingival preparation. c) On the rx the thickness of the mesial wall is checked. d) The shiny aspect of the cavity after the application of the adhesive system.

Minimal invasivity can only be achieved by using adhesive techniques, because adhesion assures sufficient retention without the need of tooth-destructive macroretentive and fricative elements. In addition, adhesive systems may seal the cavity thus minimizing bacterial penetration. In this perspective, the adhesive restoration of devital teeth is an excellent example how adhesion may completely change the restorative approach. With conventional techniques based on macroretention and friction, crowning of devital teeth was a must to protect the remaining tooth substance and to avoid crown fractures. To achieve sufficient macromechanical retention of the crown, a post-retained core build up had to be made. The high bonding performances reached by modern adhesive systems have gradually changed this dogma and it is now time to reconsider real indications for post and crowns.

The following cases illustrate the change in paradigm in the field of restorations of devital teeth by using direct and indirect adhesive composite techniques without radicular posts, thus avoiding more destructive restorative procedures.

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Case 1. Direct composite technique

The first case is a 20 years-old woman who presented a devitalized maxillary right second premolar (tooth 1.5) with a large occluso-distal provisional restoration (Figure 1a). During the preliminary steps of the endodontic phase, the cavity was first cleaned and the pulpal tissue was removed. Then, a pre-endodontic composite resin wall was bonded on the distal aspect in order to seal the cavity during the provisional phase. A special care was put in all these preliminary adhesive procedures because the subgingival portion of that resin wall is often left in place for the definitive restoration. Once filled the root canal with gutta-percha and cement, the cavity is cleaned with a diamond bur (Figure 1b). The total loss of substance and cavity configuration are now evaluated. In a correct occlusal context, the width of the remaining walls is generally considered safe when it exceeds 1.0 to 1.5 mm, especially in the cervical part. Also the presence of weakening fissures should be detected. In the specific case of the premolar presented, the presence of a large and sound mesial wall, which protects the tooth from dramatic mesio-distal failures, allows the entire conservation of the vestibular and palatal cusps (Table 1).

A radiographic examination must always confirm the clinical diagnosis (Figure 1c). According to the minimally invasive philosophy, the restoration by a full adhesive approach is the best choice to reinforce the system and to avoid a further loss of substance. As their quality and longevity are not essentially different, the choice of a direct or indirect technique is above all dictated by clinical feasibility. From an esthetic point of view, the possibility to conserve entirely the vestibular wall of a maxillary premolar is a great advantage. In case of pre-existing dentin discolorations, an internal bleaching with a mixture of perborate and hydrogen peroxide can be performed before the definitive restoration (Figure 4). Once the cavity isolated with a rubber-dam and a metallic matrix, adhesive procedures can be effectuated (Table 2). The distal resin composite wall is cleaned and conditioned by use of 27 microns Al₂O₃ sandblasting. Then, enamel and dentin are both etched with 35-37% orthophosphoric acid 10 and 30 seconds, respectively. The cavity is abundantly rinsed with water and gently dried. Dentin is then wet with an amphiphilic primer solution and well dried. Organic silane application on composite is omitted to avoid contamination of the dentinal hybrid layer. A thin layer of bonding resin is spread into the cavity, excess is removed and the resin is polymerized with a powerful LED device for 20 seconds as close as possible to the surface of the bonding layer. The next step is the application of the resin composite. The goal is to limit the resin polymerization shrinkage. To that purpose, the resin composite is progressively stratified into the cavity by 2-3 mm oblique increments, starting from the interproximal distal wall (Figures 2a-2b).

2. Stratification of the restorative resin composite. a) First, the distal interproximal wall is stratified in several increments. b) The metal matrix is removed and the occlusal part of the restoration is progressively builded-up. c) Occlusal view of the sector 1 week after.

Each layer is polymerized for 40 seconds with the LED lamp. Then, restoration is finished and polished with silicone points and fine abrasive disks. A final touch of polymerization under glycerin gel avoids any oxygen-inhibition layers. Finally, rubber-dam is removed and occlusion is checked and eventually adjusted (Figure 2c).

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Case 1. Indirect technique: the endocrown

3. Vestibular and occlusal aspect of the 4.6. When the patient smiles, the esthetic aspect of the devitalized tooth is lightly grayish in the cervical part.

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 risk^3,4.

4. Tooth bleaching before the treatment. a) e b) The vestibular gingival level defines the quantity of gutta-percha that has to be removed inside the roots in the cervical part. c) A mix of sodium perborate and 3% hydrogen peroxide is inserted in the cavity. Then, a bonding resin is directly applied on the margins and polymerized. d) The cavity is easily filled with a flowable composite resin as temporary restoration and polymerized.

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).

5. The preparation of the cavity for the indirect restoration. a) After 1 week the dental dyscromia is reduced. The tooth is ready to be prepared. b) Old composite is removed and the cusps are reduced 2-3 mm. c) Cavity is isolated and adhesively treated. A hybrid composite resin is applied in the mesial and distal boxes to relocate margins occlusally. The use of a transparent shade in the first part of the pulp chamber facilitates the detection of the root canals in case of endodontic reintervention. d) Lingual and vestibular enamel is refurbished. All dentin is covered by a thin layer of composite resin. The cavity is ready to be impressed.

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 health^6,7. A conventional indirect composite technique is thus accomplished, by programming two appointments^8,9. 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).

6. Impression and temporization of the cavity. a) An half bite tray records the cavity and the occlusion at the same time. b) The cavity is finally isolated with a layer forming glycerin gel and a soft resin temporary material is inserted into the cavity. Interproximal wedges minimize bleeding and material over-filling. c) The soft resin is then polymerized in correct occlusion.

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.

7. The adhesive treatment of the workpiece. a) The intaglio surface is first sandblasted. b) A silane coupling agent is applied and a bonding resin is successively spread into the cavity without being pre-cured (Table 2). c) The endocrown is ready to be luted.

(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.

8. The adhesive treatment of the cavity. a) Composite resin inside the cavity is sandblasted. b) Enamel is etched with H3PO4 acid. c) The bonding resin is applied and left uncured.

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).

9. Luting of the endocrown. a) A thin layer of a pre-heated restorative composite resin is used as luting cement and spread into the cavity. Then, the restoration is forced in place manually and then with the help of ultrasonic energy. Excess of luting composite at the margins are removed with a probe and interproximal floss. b) The full polymerization of the endocrown is achieved with a high-power (at least 1000 mW) LED lamp for 3X90sec. in contact with the restoration.

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).

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10. The radiographic (a) and clinical (b) aspect of the endocrown restoration one week after the luting.

Conclusions

Recently, the restoration of endodontically treated teeth with adhesive techniques has been advocated both in the root and in the crown to prevent further loss of sound tissues as adhesion ensures sufficient material retention without needing an aggressive macroretentive preparation^10-12. In particular, the use of bonded overlays like endocrowns for the coronal restoration of an ETT is growing against classical full-crown restorations. Reason for this change of paradigm is a more conservative approach, which preserves tooth tissues and allows re-intervention in case of failure. Furthermore, endocrowns eliminate many technical steps during the fabrication - such as post cementation, core fabrication, temporary crown and potential crown lengthening, which increase treatment time and costs. All these intermediary stages may also favor bacterial infiltration and cause endodontic re-infection. Different materials can be used to fabricate an endocrown like feldsphatic and glass-ceramic, hybrid composite and the newest CAD/CAM ceramic and composite blocks. Scientific literature is still not clear about which material is best indicated for such restorations. The authors prefer micro-hybrid composite resins - lab-made or in the form of CAD/CAM blocks - claiming their stress absorbing properties and their practical benefits like the possibility to modify and repair the surface easily¹³. Lithium-disilicate reinforced glass-ceramics (e.g. IPS e.max Press and CAD, Ivoclar, Schaan-Liechstenstein) may be alternatives to this concept above all for esthetic reasons. Several in-vitro studies and some in-vivo trials have confirmed the validity of this adhesive approach, especially for molars^10,14-19. By relying on adhesion, no radicular posts are necessary any more on molars and at the present moment it is difficult to decide if posts are necessary on premolars and front teeth. If yes, they may only be considered in case of extremely destructed teeth. Consequently, in many instances, restoration of devital teeth may follow the same principles as the restoration of vital teeth²⁰.

Photos copyright:
Figures 1 and 2 with the courtesy of GT. Rocca, I. Krejci, “Restaurations adhésives pour dent dépulpée. L’alternative au tout couronne”. Réalités Cliniques 2011(1)25-31.

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