Defect-orientated onlay with cavity design optimization and cervical margin relocation: case report

Introduction

The decision to restore a vital tooth with severe carious lesions and the workflow involved depend on several factors, including the treatment options (direct or indirect restorations), the material choice for indirect restorations (composite resin or ceramic-based materials), the preparation (defect-oriented, tissue-conservative), and the detailed clinical protocol (cavity design, luting, etc) 1,2.

There has been a growing tendency over the last few decades toward minimally invasive restorations and the preservation of valuable tooth structure, which is supported today by the rapid development of high-strength restorative materials as well as advanced, durable luting agents. Several techniques and a variety of materials are available for the fabrication of inlays and onlays; however, the decision about which technique and materials to use should be based on the individual clinical situation in respect to evidence- based concepts. In extensive carious lesions, as in the case of severely damaged or decayed teeth, especially those with thin walls, indirect restorations are recommended rather than direct fillings because the latter can induce unfavorable stresses on the remaining tooth structure due to polymerization shrinkage 1-7.

Regarding indirect restorations for inlays and onlays, composite resin-based CAD/CAM materials are considered to have several advantages over ceramics such as easy manufacturing, lower cost, and simpler repairability 3. One clinical issue for such indirect restorations is finding the balance between tissue conservation and a stable, appropriate preparation design. Therefore, certain treatment concepts such as immediate dentin sealing (IDS), cavity design optimization (CDO), and cervical margin relocation (CMR) can be very helpful to the practitioner in order to avoid the unnecessary sacrifice of tooth structure. For instance, a slight cavity convergence is required for the insertion of a restoration; therefore, in case of undercuts, tooth structure needs to be removed to achieve this convergence.

Such undercuts can be filled using the CDO technique, which means that no removal of healthy tooth structure is required. These comprehensive clinical techniques were introduced originally by Dietschi and Spreafico 8,9 and are described in detail in several publications 1-4. The clinical case presented in this article employs some of these concepts and demonstrates how to utilize them in challenging clinical situations. To date, IDS has been extensively studied, with several clinical advantages reported in the literature 8-12. Immediate application of an adhesive dentin bonding agent to the freshly cut dentin prior to impression taking has several advantages such as reduced dentin sensitivity, improved bond strength, fewer gap formations, and less consequential bacterial leakage 10,11. Additionally, IDS was shown to stabilize the restoration, especially in the case of thin occlusal veneers 13.

The technique of CDO is implemented by applying a composite liner (mainly flowable composite resin) directly after IDS to fill any undercuts in the cavity and optimize the cavity geometry 1-3,8. Generally, highly filled flowable composites are recommended instead of conventional restorative composites due to their ease of use 2,3. Nowadays, these composites are available with different viscosities, so practitioners can choose according to their personal handling preferences. In the clinical case presented here, two viscosities of flowable composites were used.

In severely damaged teeth, the margins of the cavity can extend subgingivally, which complicates the impression-taking procedure. It also complicates proper adhesive luting, which is necessary for minimally invasive partial-coverage restorations. In such cases, therefore, CMR can be very useful. After proper positioning and adaptation of the matrix, CMR is implemented through the elevation of deep proximal cavity margins with composite resin 2,3,8,9. In the present case, the treatment plan and protocol combine multiple evidence-based clinical concepts and techniques 2-5,14-16 for restoring a maxillary right first molar with an indirect CAD/CAM onlay.

Case presentation

A 32-year-old male patient presented with an endodontically treated maxillary right first molar that was restored with a temporary restoration. The overall periodontal status was stable, and the patient had good oral hygiene with no other carious lesions. Therefore, according to a synoptic treatment concept, no pretreatment was deemed necessary. The patient was presented with all possible treatment options, starting with a conventional full-coverage crown. However, he was advised to choose, and decided upon, a minimally invasive defect-oriented overlay. After removal of the temporary filling, it was established that the palatodistal cusp and both buccal cusps were severely undermined, with a wall thickness of < 1 mm. After excavation of some remaining caries, the mesial cavity margins were located subgingivally (Fig 1).

Fig 1 Situation after removal of the temporary filling and remaining caries lesions showing a deep subgingival mesial cavity margin.

Clinical procedure

During the first session, the remaining caries were removed. Then, rubber dam was placed and positioned with the help of ligatures, a metal matrix band, and a wedge, to include the deep subgingival mesial margin (Fig 2). All exposed dentin was sealed through IDS using a universal one-bottle adhesive system (Clearfil Universal Bond Quick; Kuraray Noritake Dental, Japan) after an etch-and-rinse technique using phosphoric acid (K-Etchant Syringe; Kuraray Noritake Dental) on the enamel for 30 s and on the dentin for 10 s (Fig 3). The universal adhesive was applied and rubbed into the tooth structure for 3 s, according to the manufacturer’s instructions, followed by gentle air drying and light curing (Elipar 2500; 3M ESPE, Neuss, Germany) for 10 s. Directly after the IDS procedure, a flowable composite resin (Clearfil Majesty ES Flow Low A3; Kuraray Noritake Dental) was used to elevate the deep mesial margin to a supragingival level, and was carefully adapted and subsequently light cured (Elipar 2500) for 20 s. Then, CDO was performed to seal all sharp margins and fill the undercuts. This procedure was carried out using a more viscous, highly filled, flowable composite resin (Clearfil Majesty ES Flow Super Low A3; Kuraray Noritake Dental) that has less flowability and can therefore be easily adapted in one increment, followed by light curing (Fig 4).

Successively, the preparation was optimized under the continuous use of rubber dam by removing all walls of a thickness of < 1 mm and creating a proper path of insertion for the proximal boxes (Fig 5) 2. At the end of the first treatment session, an impression was taken and the tooth was isolated with Vaseline, except for a small central spot to create a semi-adhesion 2. The tooth was then temporarily restored using composite restorative material without adhesive (Clearfil Majesty ES-2; Kuraray Noritake Dental).

Fig 2 Adaptation of rubber dam and the matrix, allowing good accessibility for the CMR procedure.

Fig 3 Etch-and-rinse technique using phosphoric acid on the enamel (30 s) and the dentin (10 s).

Fig 4 CDO performed with two different types of flowable composites.

Fig 5 Preparation after the removal of the undermined thin walls and elevation of the mesial margin; the palatomesial cusp was not included in the preparation.

The restoration was then designed in the laboratory using exocad DentalCAD (Exocad, Darmstadt, Germany) and milled (inLab MC XL; Dentsply Sirona, Bensheim, Germany) from a novel nanoceramic-reinforced composite resin-based CAD/CAM block (Katana Avencia Block; Kuraray Noritake Dental). After 1 week and during the second treatment session, the restoration was tried in and checked for marginal adaptation, internal fit, static and dynamic occlusion, and esthetics. All the criteria were acceptable to both the practitioner and the patient. After the high-gloss polish (Clearfil Twist Dia; Kuraray Noritake Dental) of the restoration, the intaglio surface was air abraded using 50 μm Al2O3 particles at a pressure of 1 bar for 15 s, followed by cleaning with phosphoric acid gel. After rubber dam isolation and protection of the adjacent teeth with Teflon (Fig 6), the IDS and composite resin were air abraded using 50 μm Al2O3 particles at a pressure of 0.5 bar (Fig 7), followed by phosphoric acid gel, together with the enamel, for 30 s (Fig 8).

A novel cleaning agent (Katana Cleaner; Kuraray Noritake Dental) was used on the restoration (Fig 9a) and intraorally on the tooth (Fig 9b) to ensure that there was no residual contamination on either surface. To enhance bonding, a universal adhesive system (Clearfil Universal Bond Quick), which also contains a silane coupling agent, was applied to the tooth (Fig 10a) and the restoration (Fig 10b)17.

Then, a dual-cure self-adhesive composite resin cement (Panavia SA Cement Universal; Kuraray Noritake Dental) was applied to the restoration (Fig 11). The restoration was seated carefully into the final position and fixed using finger pressure for the complete curing process, followed by a short light curing of 3 s to facilitate the removal of the excess material, followed by light curing for 10 s. The occlusion was checked to ensure that no further adjustments were required, after which the restoration and the margins were polished (Clearfil Twist Dia). Figure 12 shows the final restoration in situ, and Table 1 presents an overview of the materials used.

Fig 6 Rubber dam isolation and protection of the adjacent teeth.

Fig 7 Air abrasion of the IDS and composite using 50 μm Al2O3 particles at a pressure of 0.5 bar.

Fig 8 Phosphoric acid etching of the enamel and composite for 30 s.

Fig 9 Cleaning and removal of any residuals on the restoration (a) and tooth (b).

Fig 10 Application of the universal adhesive system containing silane on the tooth (a) and restoration (b).

Fig 11 Application of a dual-cure self-adhesive composite resin cement to the restoration.

Fig 12 Final restoration in situ.

Discussion

In the presented case, the cavity design was intended to be defect-oriented and was adjusted in relation to the decayed tissues that were removed. Adjustment was also made to adhere to the minimum wall thickness recommended for achieving sufficient stability for the tooth, and to prevent possible non-repairable tooth fractures that could extend below the cementoenamel junction. Hence, all undermined cusps with a wall thickness of < 1mm were removed 2. The palatomesial cusp, although undermined, had a wall thickness of at least 1 to 1.5 mm and was therefore maintained and reinforced with composite resin. This provides a better distribution of the biting forces and protection of the remaining tooth structure 2. No extra macro-retentive cavity design was required, as it is well proven that using a proper luting technique is sufficient 1,2,4,18,19. The decision to use proximal boxes in this case

was based on the extent of the cavity after caries excavation and removal of the thin walls, which extended distally to within the interproximal contact point. Therefore, the decision was taken to move the distal margin cervically to a level below the contact point to facilitate the cleaning of the restoration margin, which is prone to plaque accumulation, by placing it in a more accessible area. To avoid unnecessary removal of healthy tooth structure, a proximal box was prepared instead of placing the entire preparation cervically.

One decisive factor influencing the material of choice for partial coverage restorations such as inlays, onlays, and overlays is the occlusal environment. When restoring extensive cavities, especially in endodontically treated teeth in posterior load-bearing regions, the occlusal load and its unfavorable context must be considered. Using a composite resin-based material can offer some stress-absorbing properties as well as simple reparability 2,20,21. At the same time, such material provides favorable mechanical properties for these types of restorations 4,20,22.

Applying the CDO technique helps to stabilize the tooth structure by reducing tissue removal 1-3,8. Inserting an indirect restoration as an inlay or onlay requires a cavity design with tapered or at least parallel cavity walls; therefore, in the case of undercuts, tooth structure removal is necessary to obtain such a cavity geometry. This can be avoided when the CDO technique is used.

Due to the presence of a subgingivally deep mesial cavity margin, a very thin enamel margin was available that rendered a ‘clean’ selective enamel etching without the accidental application of the gel on the dentin. The enamel was etched for only 10 s to prevent the over-etching of the dentin, which was another reason for choosing a universal adhesive system. The positioning and adaptation of the matrix band is critical to the success of the CMR technique in order to achieve good isolation and a proper seal and, at the same time, to avoid any overhanging margin. The matrix band was thus pre-formed and customized to perfectly adapt to the tooth surface. For CMR, it is recommended to use a flowable composite only up to 1 to 1.5 mm; therefore, a flowable composite of conventional flowability was used for this purpose 2. After CMR, and for successive CDO, another high-viscosity flowable composite was used to seal and fill the irregularities and undermined areas. The choice of the flowability of the composite resin to be used depends on the practitioner’s personal preference. For instance, both materials used in this case are from the same manufacturer and have the same composition, but differ slightly and solely in the weight percentage of the filler and thus in the degree of viscosity. Therefore, according to the manufacturer, the use of two layers with two different viscosities is comparable to the classic composite layering technique.

The techniques of CMR and CDO can be carried out using either flowable or restorative composites. A disadvantage of the conventional restorative composite is the need for additional finishing after application. The problem with excessive preparation and finishing after IDS and CDO is the possibility of re-exposure of the dentin 2. From the author’s point of view, a highly filled, flowable composite with low flowability can be a practical alternative. This material was therefore used in the presented case. Another advantage of flowable composites is their easy handling and adaption, especially in the undercuts and areas of low accessibility 3.

As a minimal thickness of composite is usually preferred for the CDO technique, the use of bulk-fill composites is not essential, since it does not offer added value to restorative composite for this specific indication 1-4. The use of an adhesive core buildup can be considered as another alternative technique; however, as the main purpose of CDO is to block the undercuts and flatten the cavity floor, and not to build up the tooth, restorative composite resin was used, as suggested in the literature 1-4.

Although most manufacturers of resin-based materials recommend using silane monomers for the adhesive luting, it has been shown that the use of methacrylate-containing resin primers enhances the bond strength to those materials 17,23. For this reason, a universal adhesive containing silane, methacryloyloxydecyl dihydrogen phosphate (MDP), and methacrylates was used, in conjunction with the self-adhesive composite resin. To ensure a clean surface and to avoid contamination, which negatively influences the bond strength 23,24, a universal cleaner was used that contains MDP salts. According to the manufacturer, the cleaner can adhere chemically to the protein, eg, from saliva and blood, and can be used intraorally (on the tooth structure) and extraorally (on the restoration). Ideally, the air abrasion procedure should be performed after the try-in and directly before luting to clean the surface of any contaminating material that may have adhered to it during the try-in. If this is not possible, for instance, due to the absence of an air-abrasion device in the dental office, the use of a cleaner after the try-in can improve the bond strength to the restorative material 25. In the present case, a self-etch, self-adhesive composite resin cement was used for the luting. However, etching with phosphoric acid was performed prior to the luting to enhance the bond strength to the enamel 26 and to remove the remnants of Al2O3 particles used for air abrasion of the IDS and the composite resin. As the IDS was already performed during the first visit, there was no exposed dentin during luting, and therefore no danger of over-etching the dentin by using phosphoric acid in combination with a self-etch adhesive or composite resin cement.

Table 1 Overview of the materials used and their main components

Conclusion and clinical relevance

Severely damaged teeth with extensive lesions present a challenging situation for the practitioner to preserve valuable tooth structure and ensure a proper impression-taking and luting procedure. With the advances in the field of adhesive technology and materials, as well as the use of techniques such as CMR and CDO, a minimally invasive, defect-oriented restoration can be performed either chairside in one visit, or through a conventional approach. The approaches described in this case presentation are evidence- based; nevertheless, the combination of the several techniques and materials can be different in every case, depending on the clinical situation and the preferences of the practitioner.

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