Current Adhesive Protocols for Indirect Ceramic Restorations

The substrate or bonding surface to which the indirect ceramic restoration will be adhered plays a key role in deciding the optimal cementation protocol. Long-term clinical success of any adhesive interface will always depend on its weakest link, demanding from clinicians proper understanding of how to treat both interfaces (ceramic and abutment) to enhance the bond strength of resin cement systems.

When a chairside CAD/CAM workflow is intended for elaboration of indirect ceramic restorations, the ceramic material options can be divided into three main groups: resin matrix ceramics, silicate ceramics, and oxide ceramics. Each of these requires specific mechanical and chemical surface treatments to enhance their bond strength values.

Resin Matrix Ceramics

Resin matrix ceramics have become quite popular because of the speed and simplicity of their elaboration process. This group of materials has great machinability and dimensional stability, providing excellent marginal adaptation, which is desired for any indirect restoration. Additionally, a simple manual polishing or light-curing stain/glaze protocol can facilitate the delivery of these restorations in one visit.

Most resin-based ceramic blocks are made under a high-pressure/high-temperature process, which increases homogeneity but can reduce bond strength if the surface treatment is not followed according to the manufacturer's recommendations. Multiple types of fillers are added to these resin blocks to improve physical and esthetic properties, but the absence of a glassy matrix makes them non-etchable. Air-particle abrasion with 50-µm aluminum oxide particles is required to augment the surface roughness and area, which is desirable to achieve maximum mechanical retention and interlocking. The application of a silane coupling agent will promote chemical bonding, and when these two types of retention methods are present, the bond strength values increase significantly.¹

Another type of resin matrix ceramic material is the hybrid ceramic or polymer-infiltrated ceramic network, whose porous ceramic structure (86%) is infiltrated with a polymer (14%). This percentage of added polymer provides better milling properties when compared to traditional ceramics and a modulus of elasticity in between human dentin and enamel. Hybrid ceramics are etchable and require 5% hydrofluoric acid etching for 60 seconds to achieve ideal surface roughness for optimized micromechanical retention.² A silane application will promote chemical bonding, adding to their overall bond strength.

In general, resin matrix ceramics have a lower modulus of elasticity compared to traditional ceramics, making them an ideal option for intracoronal restorations like inlays.³

Silicate Ceramics

Implemented since the beginning of the chairside CAD/CAM era as feldspathic ceramic blocks, silicate ceramics have been continuously reformulated from leucite-reinforced ceramics to lithium disilicates and zirconia-reinforced silicate ceramics to improve their physical properties. All silicate ceramics are etchable materials; etching times vary depending on the material's composition.

Feldspathic ceramics come in polychromatic blocks, making them a desirable option for laminate veneers or monolithic anterior crowns when a natural color of the abutment tooth is present. As an etchable material, these ceramics require an application of 5% hydrofluoric acid for 60 seconds to achieve ideal surface roughness and micromechanical retention. A silane coupling agent is always needed before the adhesive cementation process. For thin anterior feldspathic ceramic restorations, a light-curing resin cement system is indicated to facilitate the procedure, as working time is longer when compared to dual-curing resin cement systems.⁴

Leucite-reinforced ceramics also have a multichromatic distribution within the block, making them ideal for high-esthetic anterior restorations. Also an etchable material, they require 60 seconds of hydrofluoric acid application, followed by a silane coupling agent. Resin bonding is necessary for both feldspathic ceramics and leucite-reinforced ceramics, augmenting their flexural strength and long-term survival rates.

Lithium-disilicate ceramics are popular for various indications, specifically monolithic crowns, inlays, onlays, and screw-retained implant crowns. Their high flexural strength makes them the strongest glass-ceramic material.⁵ They need to be crystallized in a ceramic furnace after milling and pre-polishing to obtain the restoration's final color and physical properties. A combination firing applying stain/glaze during the crystallization firing is possible, saving valuable chairtime when same-day delivery of the restorations is intended. Lithium-disilicate restorations achieve a sound bond to resin cements after hydrofluoric acid etching for 20 seconds, followed by silanization of the etched surface. Due to their higher flexural strength, conventional cementation is also possible when a retentive preparation is present.

Novel zirconia-reinforced lithium-silicate ceramics have been introduced in a fully crystallized version, which saves the time spent during the crystallization process (approximately 25 minutes) of traditional lithium disilicate. Finishing by manual polishing only or with a stain/glazing technique can be done with this newer ceramic material type, while published clinical success data is still limited.⁶ The internal surfaces of the restorations should be etched with hydrofluoric acid for 20 seconds, followed by silane application. Traditional cementation techniques are also possible with zirconia-reinforced lithium-silicate ceramics, but the overall strength is still higher when an adhesive cementation process is used.

Oxide Ceramics

Oxide ceramics, such as zirconium dioxide (zirconia), are characterized by excellent mechanical properties, which are significantly superior to those of silica-based ceramics.⁷ Flexural strength values of conventional zirconia polycrystal range between 1000 MPa and 1500 MPa. Its inherent strength allows for conventional cementation of adequately dimensioned full-coverage restorations.

The latest zirconia generations offer significantly greater light transmission than previous generations. Pre-shaded multilayer high-translucent zirconia materials offer a range of esthetic treatment options and can be applied to anterior teeth. The higher translucency is achieved by slight changes of the yttrium oxide content (5 mol% or more instead of 3 mol%), resulting in a larger number of cubic-phase particles. More cubic zirconia offers significantly higher light transmission but lower flexural strength values than conventional zirconia, between 550 MPa and 800 MPa.⁸

High-translucent zirconia blocks for chairside CAD/CAM systems have recently entered the marketplace. The restorations are milled from presintered blocks with slightly enlarged dimensions, compensating for the 20% to 25% material shrinkage that occurs during the final sinter step after milling. With a special chairside furnace and speed sintering program, the sintering of a single crown can be accomplished within 20 minutes.²

Given the broad popularity of zirconia restorations, clinical application and cementation protocols are widely debated. In general, these restorations are typically considered cementable because of their high inherent flexural strength, which exceeds natural chewing forces. Therefore, zirconia-based crowns and bridges with adequate retention and ceramic material thickness can be cemented conventionally.

Resin-modified glass-ionomer or self-adhesive resin cements are prevalent and provide at least a certain level of adhesion to both teeth and ceramic without additional time-consuming and technique-sensitive bonding and priming steps. However, zirconia restorations that are less strong, thin, lack retention, or rely on resin bonding, such as resin-bonded fixed prostheses or bonded laminate veneers, require resin bonding with composite resin luting agents. To achieve high and long-term durable resin bond strengths to zirconia in a practical manner, the APC concept, a three-step approach, is recommended: (1) air-particle abrade the bonding surface with aluminum oxide (A); (2) apply special zirconia primer (P); (3) use dual-cure or self-cure composite resin cement (C).⁸

After restoration cleaning, zirconia should be air-particle abraded with alumina or silica-coated alumina particles. Small particles (50 mm to 60 mm) at a low pressure (<200 kPa [2 bar]) are sufficient. The subsequent step includes application of a special ceramic, which contains special adhesive phosphate monomers. The monomer MDP (10-methacryloyloxydecyl dihydrogen phosphate) has been shown to be particularly effective to bond to metal oxides. Dual-cure or self-cure composite resins should be used to ensure adequate polymerization. The APC concept is also indicated when adhesive cementation to prefabricated titanium abutments (ti-bases) is needed. If custom titanium abutments with retention and resistance form are preferred, conventional cementation of monolithic zirconia restorations with resin-modified glass-ionomer or self-adhesive resin cements is suggested.

Analyzing the abutment tooth to determine if enamel, dentin, or both are present is a key step during the treatment planning phase so that the optimal surface treatment for the tooth surface can be selected. Implementing the best possible isolation technique can facilitate the adhesive cementation protocol, providing a dry and controlled operatory field.

Conclusion

Understanding ceramic material properties, clinical indications, and specific surface treatment protocols will lead practitioners to higher clinical success rates with indirect restorations. Independent of the material type, whether resin matrix ceramics, silicate ceramics, or oxide ceramics, it is important to always analyze the abutment condition and select the best cementation technique. Current adhesive protocols and constant development of ceramic materials for fabrication of indirect restorations provide a promising future in the area of less invasive restorative dentistry.
 

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2. Conejo J, Ozer F, Mante F, et al. Effect of surface treatment and cleaning on the bond strength to polymer-infiltrated ceramic network CAD-CAM material. J Prosthet Dent. 2021;126(5):698-702.

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6. Demirel M, Diken Türksayar AA, Donmez MB. Translucency, color stability, and biaxial flexural strength of advanced lithium disilicate ceramic after coffee thermocycling. J Esthet Restor Dent. 2022. doi: 10.1111/jerd.12960.

7. Blatz MB, Vonderheide M, Conejo J. The effect of resin bonding on long-term success of high-strength ceramics. J Dent Res. 2018;97(2):132-139.

8. Blatz MB, Conejo J. Cementation and bonding of zirconia restorations. Compend Contin Educ Dent. 2018;39(suppl 4):9-13.
 

About the Author

Julian Conejo, DDS, MSc
Assistant Professor, Clinical Restorative Dentistry, and Director, Chairside CAD/CAM Dentistry, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania