Adhesive Cementation of Chairside CAD/CAM Inlays and Onlays

Tooth-colored restorations for posterior teeth are gaining popularity due to aesthetic, biologic, and functional considerations.1-3 Chairside computer-assisted design/computer-assisted manufacture (CAD/CAM) systems (eg, CEREC 3D, Sirona Dental Systems) facilitate fabrication of indirect restorations in one clinical appointment. Adhesive cementation is a requirement for the long-term success of indirect, silica-based ceramic and composite restorations and provides various advantages even for high-strength ceramic materials such as aluminum-oxide and zirconium-oxide ceramics.4

This article discusses current materials and techniques for successful fabrication of chairside CAD/CAM inlays/onlays and provides a step-by-step guideline for their cementation with a new adhesive resin cement (Panavia F 2.0, Kuraray). This fluoride-releasing resin cement can be universally applied and provides excellent bond strengths between indirect restorative materials and tooth structure.5

TRADITIONAL MATERIALS FOR RESTORING POSTERIOR TEETH

The restorative dentist is faced with a variety of materials when restoring missing tooth structure in posterior teeth that do not require full-coverage crowns. Depending on the extent and location of a defect, amalgam has traditionally been the restorative material of choice for such cases.6 Amalgam has an exceptional record of clinical excellence, proven longevity, and good physical properties. Nevertheless, the use of amalgam does not allow the dentist to create a restoration that would meet the aesthetic expectations of many patients. Cast-gold inlays and/or onlays are another option for moderately large defects. Cast-gold restorations offer good clinical long-term success.7,8 This success, however, depends on indication, clinical setting, and experience of the clinician.2

TOOTH-COLORED MATERIALS FOR POSTERIOR TEETH

Patients’ and dentists’ demands for tooth-colored restorations that mimic the appearance of natural, unrestored teeth have led to the popularity of composite resins and dental ceramics.2,3 Wear characteristics and polymerization shrinkage of composite resin materials negatively affect their marginal integrity. This may be problematic for direct application in large cavities in premolar and molar teeth, which are susceptible to high occlusal loads.9 Highly filled, small particle size hybrid composites and advanced resin-bonding techniques minimize these problems,9,10 but their routine use for extensive posterior cavities cannot be recommended.

Indirect, laboratory-fabricated restorations are now gaining popularity for use in posterior teeth, since they overcome some of the physical limitations of direct composite resin restorations.1 In addition, indirect restorations allow better control over critical parameters including interproximal/occlusal contact areas and occlusal tooth morphology. Dental ceramics and indirect composite resins are preferred materials for laboratory-fabricated inlays and onlays.2,11-13 Indirect composite resin materials are polymerized under controlled conditions and with system-specific curing units outside of the oral cavity. This ensures a high degree of polymerization (conversion rate) and better physical properties. It also ensures that polymerization shrinkage of the restorative material occurs outside of the mouth before placement in the tooth preparation.

Extensive tooth destruction may require restoration with full-coverage crowns. Porcelain-fused-to-metal and all-ceramic crowns have proven to be very useful when restoring posterior teeth.2,3 Feldspathic porcelain is a silica-based ceramic and is often utilized for highly aesthetic restorations due to its excellent optical characteristics that are very similar to the natural tooth.2 These characteristics remain stable over long periods of time. The brittle, silica-based ceramics require reinforcing substructures and are typically used as veneering porcelains for metal or ceramic cores. If bonded with a composite resin cement, silica-based ceramics can be utilized without substructures for laminate veneers as well as for ceramic inlays and onlays. Therefore, resin bonding with adequate materials and proven techniques is key for clinical success for such treatment options.4,15,18,19

Ceramic inlays/onlays offer excellent aesthetics, biologic compatibility, and long-term durability.2 They are usually fabricated with silica-based ceramics, such as feldspathic porcelain and leucite-reinforced feldspathic porcelain, due to their optical and aesthetic properties. Silica-based ceramics can be easily bonded when properly treated. Reinforced silica-based ceramics (eg, IPS Empress, Ivoclar Vivadent) may even be used for single-tooth, full-coverage restorations. However, when full-coverage, all-ceramic restorations are selected for posterior teeth, reinforcing high-strength ceramic core materials such as aluminum-oxide or zirconium-oxide ceramics should be considered.3 High-strength ceramic materials are not the materials of choice for bonded restorations, since it is much more difficult to achieve strong and long-term stable resin bonds to high-strength ceramic materials.3

PREPARATION DESIGN FOR CERAMIC INLAYS/ONLAYS

In general, the design of preparations for indirect inlays and onlays depends on the physical properties of the restorative material and should follow manufacturers’ recommendations. Other factors in preparation design include the anticipated cementation mode and the need for a strong composite resin bond to the restorative material. A conventionally cemented cast-gold inlay/onlay requires a retentive preparation design. Adhesive resin bonding significantly increases retention, therefore a retentive preparation design is not a priority for bonded ceramic restorations. Internal preparation angles in the occlusal box may be approximately 15° to allow for easy try-in and insertion. Rounded external and internal line angles are recommended14 with deep chamfer or rounded shoulder margins, which should be located in enamel for optimal resin bonding. It is important that the required minimum thickness of material is ensured for the entire restoration. Insufficient tooth preparation, particularly in the areas of the central grooves or at the internal line angles (the ridge between the occlusal and interproximal box), may lead to fracture of the restoration.

CHAIRSIDE CAD/CAM INLAYS AND ONLAYS

Traditionally, fabrication of indirect restorations requires multiple appointments and includes the following clinical steps: tooth preparation, making the impression, try-in, and final insertion. The number of steps can be reduced by using CAD/CAM technology, especially systems that offer chairside fabrication of restorations (eg, CEREC 3D). While earlier generations of this system were exclusively used for inlays and onlays, CEREC 3D and CEREC inLab (Sirona Dental Systems) systems offer a wide variety of restorative and material options. With CEREC, an optical impression of the prepared cavity is scanned chairside, and a computer transfers the information to the milling component of the unit, where the restoration is fabricated. The milling of the ingots takes only a few minutes and enables the dentist to deliver indirect restorations in a single appointment.

MATERIAL OPTIONS FOR CHAIRSIDE CAD/CAM INLAYS AND ONLAYS

Silica-based feldspathic ceramics such as Vita Mark II and Vita Esthetic line (Vita) are materials available as ingots for CEREC. These are considered the standard materials for ceramic inlays and onlays. High-strength ceramics should be considered whenever increased fracture resistance or ceramic cores are needed (eg, very large restorations and full-coverage crowns). High-strength ceramic materials for CEREC include Vita In-Ceram Spinell, Alumina, and Zirconia blocks (Vita). Vitablocs alumina are made from glass-infiltrated aluminum-oxide ceramic, which may be used as a core material. In-Ceram Spinell includes a spinell core (an oxide of magnesium and aluminum), which is slightly weaker but more translucent than the alumina core. It is recommended for aesthetic, single-unit anterior restorations. The latest material, Zirconia, is a partially stabilized zirconium-oxide ceramic material with high flexural strength, fracture toughness, and fatigue resistance.3 Generation e and ProCAD (Ivoclar Vivadent) offer leucite-reinforced, silica-based ceramic materials from the IPS Empress (Ivoclar Vivadent) family of materials. Indirect composite resin materials are also available (eg, 3M Paradigm MZ100 Block for CEREC, 3M ESPE) and have become increasingly popular for inlays and onlays.

ADHESIVE CEMENTATION OF SILICA-BASED CERAMIC INLAYS/ONLAYS

Silica-based ceramic inlays and onlays require adhesive bonding, which increases retention, improves marginal seal, and significantly strengthens the restoration as well as the supporting tooth.2,4,15 The clinical success and fracture resistance of silica-based ceramic restorations increase dramatically when adhesive bonding techniques are applied in combination with composite resin cements.16,17

A strong and durable resin bond to ceramics relies on a true chemical bond and micromechanical interlocking.18 Acid-etching silica-based ceramics with hydrofluoric acid (HF) yields a preferable surface texture and roughness and exposes silica-containing crystalline structures.2,19,20 Available products offer HF solutions from 4% to 9.8%, which are usually applied for 1 to 2 minutes, depending on manufacturers’ recommendations, and then rinsed with water. Reinforced, silica-based ceramics with an increased crystalline content require shorter etching times. Application of a silane coupling agent on the acid-etched ceramic wets the surface and provides a chemical covalent bond to silica-based ceramics.21,22 Silane coupling agents are available as single-bottle or multiple-bottle applications. Single-bottle products are applied in a separate step, which is followed by the application of a bonding agent. High contents of solvents are susceptible to evaporation and limit the shelf life of single-bottle silane coupling agents.23 Other systems require mixing of a silane coupler with other components of the bonding system. This activates the coupling agent, which is immediately applied in a single step and does not require subsequent application of a bonding agent. As a result, the terms silane coupler and bonding agent may refer to  formulations that are completely different chemically. It is, therefore, mandatory to follow manufacturers’ recommendations closely and to avoid interchanging products from different manufacturers even though they may have similar or identical names.

Dual-curing and self-curing composite resin cements provide a sufficient degree of polymerization (conversion rate) underneath ceramic restorations. Adequate polymerization may not be achieved with strictly light-cured composite resins.24 This is especially important when the ceramic restoration is thicker than 1 mm.13,24 Very opaque and thick, especially high-strength ceramic materials may require dual-cure or self-cure composite resin cements.15,19

Adhesive Cementation Of Composite Resin And High-Strength Ceramic Materials

Conventional silane coupling agents and composite resin cements cannot provide long-term, durable resin bonds to high-strength ceramics.15,25-28 The modified composite resin cement Panavia F 2.0 contains the proprietary adhesive functional monomer 10-methacryloyloxydecyl dihydrogen phosphate, which chemically bonds to tooth structures and metal oxides. Resin cements and ceramic priming agents containing this adhesive phosphate monomer (eg, Panavia F 2.0 and Clearfil Silane Kit, Kuraray) not only provide reliable and long-term durable resin bonds to silica-based ceramics and metal alloys, but also provide these bonds to glass-infiltrated and densely sintered alumina as well as zirconia ceramic.15,25-30 Panavia is recommended for indirect composite resin and high-strength ceramic (eg, aluminum-oxide and zirconium-oxide ceramic) materials.15,25-28

CLINICAL LONG-TERM SUCCESS OF CHAIRSIDE INLAYS/ONLAYS

Clinical studies are considered “long-term” when the investigated materials/para-meters are followed for at least 5 years. The long-term success of inlays/onlays fabricated with the CEREC system is well documented, ranging from 84.9% to 100% success after observation periods of 5 to 11.8 years.31-43 Detailed results of these studies are listed in the Table. A number of recent short-term clinical studies support these trends and include a variety of materials and treatment options.44-49

CLINICAL APPLICATION AND CEMENTATION OF INLAYS/ONLAYS WITH A NEW ADHESIVE RESIN CEMENT

The universal, self-etching, self-adhesive, dual-cure, fluoride-releasing resin cement Panavia F 2.0 contains 2 photo initiators to provide a wide curing range. Thus, this material can be cured with any halogen, plasma ARC, or LED light. It was originally developed for metal and ceramic inlays/onlays, crowns, fixed partial dentures, and laminate veneers. Panavia F 2.0 polymerizes in the absence of oxygen. This “anaerobic” polymerization mode provides convenient working and setting times and offers easy removal of excess material before setting, which may be initiated by application of an oxygen blocking gel (Oxyguard II, Kuraray) or a curing light. In addition, chemical initiators ensure a high degree of polymerization even without oxygen blockers or light. Panavia F 2.0 is indicated for a wide range of materials and indications, and is available in 4 shades: TC (tooth color), white, opaque, and light. The clinical use of this material and its components for adhesive cementation of a CAD/CAM, silica-based ceramic inlay is described below.

Clearfil Silane Kit is a 1-step silane system, which can be universally applied to almost all types of ceramics. Most ceramic bonding systems require HF etching and rinsing of silica-based ceramics. With Clearfil Silane Kit, a phosphoric acid gel (K-Etchant Gel, Kuraray) is applied to the ceramic bonding surface of the inlay to pretreat and activate the surface. The gel is rinsed off with water after 5 seconds. Equal parts of Clearfil SE Bond Primer and Clearfil Porcelain Bond Activator (Kuraray) are mixed and applied on the restoration bonding surface for 5 seconds followed by air-drying. Tooth surfaces are pretreated with ED Primer II (Kuraray) and air-dried after 30 seconds. ED Primer II

provides effective and mild single-step conditioning of enamel and dentin without acid-etching and rinsing. Equal amounts of the 2 Panavia F 2.0 pastes are dispensed, mixed for 20 seconds, and applied to the bonding surface of the inlay. The inlay is seated on the tooth, and excess cement is removed. Panavia F 2.0 is light-cured for 20 seconds from each side for a total of 60 seconds. Alternatively, the oxygen blocking gel Oxyguard II may be applied on the margins for 3 minutes. In that case, light-curing is not required.

CASE PRESENTATIONS

Figure 1. Occlusal view of onlay preparation in maxillary left first molar tooth.	Figure 2. Postoperative view.

A failing restoration in the maxillary left first molar tooth of a 23-year-old female patient was removed. A base was applied, and the tooth was prepared for a ceramic onlay (Figure 1). Figure 2 depicts the postoperative clinical situation after adhesive cementation of a silica-based ceramic onlay.

Figure 3. Onlay preparation in maxillary left first molar.	Figure 4. CAD/CAM milled composite resin onlay (3M Paradigm MZ100 Block for CEREC, 3M ESPE).
Figure 5. Postoperative occlusal view of cemented onlay.	Figure 6. Postoperative lateral view of cemented onlay.

Figure 3 shows the preparation of the maxillary left first molar in a 27-year-old patient. The tooth was previously restored with a composite resin restoration, which later failed. An onlay was milled from indirect resin composite material (3M Paradigm MZ100 Block for CEREC [Figure 4]) and adhesively cemented following the above protocol. Figures 5 and 6 are the postoperative intraoral views from the occlusal and lateral aspects.

Summary

Chairside CAD/CAM restorations offer aesthetic, functional, biocompatible, and long-term successful alternatives to traditional materials and techniques, and can be fabricated in one appointment. Adhesive cementation is key for the long-term clinical success of CAD/CAM inlays and onlays. The clinical use of a newly developed composite resin cement for adhesive cementation of CAD/CAM inlays/onlays has been described.

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