Standardized Bonding Protocol for Posterior Porcelain Restorations

INTRODUCTION
When properly performed and precisely followed, the following protocol for bonding of indirect porcelain restorations will provide the clinician with the highest bond strengths possible with minimal occurrence of postoperative sensitivity. Though many alternative techniques and products exist with the aim of simplifying the process, none, in the author’s opinion, approach the consistency and adaptability of the procedures outlined below.
When bonding to the natural tooth with the aim of creating a final restoration of the highest durability, there are (again, in the opinion of the author), 3 key principles for success: isolation from moisture, attention to detail, and material selection. As with most dental procedures, strict adherence to best practices will pay great dividends in the long-term success of the procedure.

PROTOCOL
Based upon the best currently available data, the following protocol has been developed with the aim of bonding posterior porcelain restorations with the highest long-term bond strengths. Slight deviations from the protocol will be required if alternative materials are utilized.

Figure 1. Porcelain onlay preparations isolated with a medium weight rubber dam.	Figure 2. E.Max porcelain inlays (Ivoclar Vivadent) with microbrush carriers tacked on with flowable composite.
Figure 3. Intaglio of porcelain onlays being cleaned with phosphoric acid following trial fit in the mouth.	Figure 4. A properly silanated porcelain inlay will have a satin finish.
Figure 5. Differential etching of the preparation allows the enamel to be fully etched while minimizing the potential for over etching the dentin surfaces (30 seconds enamel, 15 seconds dentin).	Figure 6. Cotton pellets saturated with 2% chlorhexidine are placed in the preparations for 30 seconds; this serves to increase the long-term bond strengths to dentin.

1. Remove the provisional—The provisional restoration needs to be gently removed ensuring that the preparation margins are not damaged in the process.
2. Ensure adequate isolation—It is the strong preference of the author to utilize a rubber dam (such as medium weight Latex Dental Dams [Hygenic] or medium weight DermaDam [Ultradent Products]) for all procedures related to posterior porcelain. The rubber dam allows the creation of an unequaled clear field in which to operate. The ability to maintain a clean, dry field throughout the duration of the preparation and cementation procedure is of utmost importance.¹ Furthermore, it allows the operator and the assistant to concentrate on the bonding procedure without the need to constantly ensure that the field is clear of contaminants (Figure 1).
3. Air abrasion—Before try-in of the restoration, it is imperative that the preparation be completely cleaned and free of debris. Residual provisional material or provisional cement will impede the ability to trial fit the restorations and will negatively affect bond strength. The use of 27 µm aluminum oxide at 40 psi (such as MicroEtcher IIA [Danville Materials] or RONDOflex Plus 360 [KaVo Dental]) is well proven to ideally clean the preparation surfaces without significant risk of removing previously placed dental bonding agents, dentin, or enamel. When utilizing immediate dentin sealing, the use of air abrasion also serves to “reactivate” the bonded dentin surface at the cementation appointment.^2,3
4. Trial fit of the restoration—The restoration is checked for integrity of fit and interproximal contacts. The restoration should be carried to the mouth with a nonwax pick-up stick (such as Micro-Stix Original Hold [Microbrush International] or OptraStick [Ivoclar Vivadent]). With small porcelain restorations, the waxed tip pick-up stick has the possibility of contaminating the internal surface and compromising the bond. An alternative method (developed by Dr. Ed McLaren) is to tack a microbrush to the occlusal surface with flowable composite to use as a carrier (Figure 2).
5. Clean the restoration—Before the delivery of the restoration, the intaglio of the restoration should be appropriately etched with hydrofluoric (HF) acid by the laboratory (different porcelain materials and different HF acid concentrations require variable etch times). A properly etched porcelain surface should resemble that of etched enamel. If it has a snowy white appearance, it has been over etched (this can be removed with ethanol in an ultrasonic bath). After the trial fitting, the intaglio must be cleaned (though not re-etched). A 15-second aplication of 35% phosphoric acid gel will serve to cleanse the porcelain surfaces, without further etching, and is easily rinsed off (Figure 3).
6. Apply silane to the restoration—Utilizing a 2-component silane (such as BIS-SILANE [BISCO]) allows for the creation of a highly active porcelain surface. Parts A and B of the silane are mixed just prior to application to ensure fresh chemistry. Two coats of silane are then applied to the dry intaglio surface of the restoration; after 30 seconds, the remaining solvent is evaporated with dry and oil-free air (using nitrogen or the Warm Air Tooth Dryer [A-dec]). The surface of a properly etched and silanated restoration will have a satin finish (Figure 4).
7. Etch the tooth—Utilizing a high viscosity 35% phosphoric acid gel (such as Ultra-Etch [Ultradent Products] or Scotchbond Etchant [3M ESPE]), the preparation is differentially etched. The etchant is initially placed primarily on the enamel for 15 seconds (Figure 5), followed by placement on the dentin surface for 15 seconds more. This results in thorough etching of the enamel without a significant risk of over etching the dentin.⁴ The risk of over etching the dentin is also minimized through the use of immediate dentin sealing at the preparation appointment. It is imperative that all steps are precisely timed, not merely estimated. The etchant is then thoroughly rinsed with an air-water syringe and carefully dried to ensure that the dentin is not desiccated, but no standing moisture remains.
8. Apply 2% chlorhexidine—The use of a 2% chlorhexidine (CAVITY CLEANSER [BISCO] or Consepsis [Ultradent Products]) solution is well proven^5,6 to enhance the long-term in-vivo bond to dentin through the inhibition of enzymes (matrix metalloproteinase-2 [MMP-2, MMP-8, MMP-9]) that degrade the collagen scaffolding below and within the hybrid layer over time. It also serves as a rewetting agent if the dentin has been desiccated at the previous step. Cotton pellets, saturated with 2% chlorhexidine, are placed in the preparations for 30 seconds (Figure 6), and the preparations are then gently dried with cotton pellets, not rinsed. When utilizing immediate dentin sealing, this step is most effective when performed at the preparation appointment following the etching of the dentin.
9. Apply primer—The primer contains hydroxyethylmethacrylate, which is responsible for creation of the hybrid layer in the demineralized dentin. Primers need sufficient time (see manufacturer instructions) to infiltrate the collagen network and agitation to ensure that all surfaces are properly coated (Figure 7). It is the preference of the author to utilize a total-etch, 2-bottle system that is filled (such as ALL-BOND 3 [BISCO] or Optibond Fl [Kerr]).
10. Apply adhesive—The adhesive is applied to the restoration intaglio and to the preparation surfaces. This layer acts as a wetting agent for the resin cement. It is generally advisable to postpone curing of the adhesive until the restoration is fully seated. This will minimize the chance of inadequate seating of the restoration due to the film thickness of the cured adhesive layer.
11. Apply resin-based cement—All currently available posterior porcelains benefit from the use of a resin cement. Zirconia is the rare exception, though resin cements are still advisable in preparations of low macro-mechanical retention. There are 2 general classes of resin cements: light-cured and self/ dual-cured. The light-cured resins are beneficial primarily because of their long-term color stability. The dual-cured cements will shift slightly to a yellow shade over time. It is the author’s preference to utilize the light-cured resin cements (such as CHOICE 2 [BISCO], Rely X Veneer [3M ESPE], Variolink II [Ivoclar Vivadent], or NX3 [Kerr]) for most anterior units to avoid complications with shade shifting. With thicker or more opaque restorations, dual-cure resin cements (such as DUO-LINK [BISCO], Variolink Dual Cure [Ivoclar Vivadent], NX3) are utilized. This ensures maximum polymerization of the cement with inlays, onlays, and zirconia crowns. The slight color shift in these thicker posterior restorations is nearly undetectable. The resin cement is generally applied to both the restoration and the preparation.
12. Seating—The restoration is carried to the mouth with the carrier oriented such that the restoration is properly aligned with the preparation. The restoration is gently inserted in the preparation and held in place with an appropriately-sized ball burnisher while the carrier is removed. The gross excess of cement is removed with a brush. A small amount of excess cement should be retained at the margins. Utilizing the ball burnisher, the restoration is seated with a strong, but consistent, force. It is imperative that the force not be lifted until the initial cure is started. If pressure is prematurely lifted, gaps in the cement will form at the margins.

Figure 7. A properly primed tooth surface will illustrate a satin finish similar to that of silanated porcelain.	Figure 8. The bulk of cement removal is performed while the cement is in a gel state.
Figure 9. The ET6F bur (Red Stripe [Brasseler USA]) is ideal for refining porcelain margins following cementation.	Figure 10. Lithium dislicate (e.max HT [Ivoclar Vivadent]) pressed onlays Nos. 30 and 31.
Figure 11. Lithium dislicate (e.max HT) pressed onlay No. 3.	Figure 12. Lithium dislicate (e.max HT) pressed onlay No. 3.

13. Tack cure—The resin cement is light-cured for approximately 2 seconds to achieve a gel state in the cement (Figure 8). During this stage, the restoration is continually held in place with the ball burnisher while interproximal areas are flossed and embrasures cleared of residual cement utilizing a No. 12 scalpel.
14. Cover with glycerin and post cure—The margins of the restoration are then covered with a layer of glycerin while the cure is completed. This ensures the prevention of an inadequately cured oxygen-inhibited layer in the cement.
15. Finalize the margins and adjust the occlusion—Most porcelain restorations will benefit from a final recontouring of the marginal areas. This ensures a flush transition from the tooth to the porcelain and will increase the longevity of the restoration. All adjustments to the porcelain should be performed with a fine diamond bur (such as an ET6F [Brasseler USA], F858-014 [Axis Dental], 8858-014 [KOMET], 858-014F [SS White Burs]) (Figure 9).
16. Occlusion check and adjustment—The rubber dam can now be removed and the occlusion checked and finalized. It is imperative that the strength, location, and direction of the contact are precisely controlled. Sequential utilization of a C-fold towel (to ensure dry surfaces), 80-µm Red (in excursive movements) (Madame Butterfly [Almore International]), and 20-µm Blue (in MIP) (AccuFilm II [Parkell]) will clearly show the details of the contacts. Adjustments should be made with a fine diamond bur (such as a 379F [Brasseler USA], F379 [Axis Dental], 8379 [KOMET], or a 379-023F [SS White Burs]) and brought back to full polish (using polishing kits such as Dialite [Brasseler USA], CeraGlaze [Axis Dental], Diamond Ceramic Polisher Kit [KOMET], or Jazz P3S [SS White Burs]).

SUMMARY
Strict adherence to a carefully designed protocol for the cementation of posterior porcelain restorations allows for a reliable and minimally invasive procedure. Though such a thorough protocol for porcelain cementation may initially seem time consuming or over-engineered, it is quickly learned and can be applied in nearly all situations. As with many practices in dentistry, the cementation is but the final step in the creation of a beautiful, minimally invasive, and durable restoration that lies at the end of a long line of procedures. All these procedures must also be completed at equivalent levels of excellence to ensure a successful result (Figures 10 to 12).

References

1. Eiriksson SO, Pereira PN, Swift EJ Jr, et al. Effects of saliva contamination on resin-resin bond strength. Dent Mater. 2004;20:37-44.
2. Chaiyabutr Y, Kois JC. The effects of tooth preparation cleansing protocols on the bond strength of self-adhesive resin luting cement to contaminated dentin. Oper Dent. 2008;33:556-563.
3. Magne P, So WS, Cascione D. Immediate dentin sealing supports delayed restoration placement. J Prosthet Dent. 2007;98:166-174.
4. Pioch T, Stotz S, Buff E, et al. Influence of different etching times on hybrid layer formation and tensile bond strength. Am J Dent. 1998;11:202-206.
5. Carrilho MR, Geraldeli S, Tay F, et al. In vivo preservation of the hybrid layer by chlorhexidine. J Dent Res. 2007;86:529-533.
6. Hebling J, Pashley DH, Tjäderhane L, et al. Chlorhexidine arrests subclinical degradation of dentin hybrid layers in vivo. J Dent Res. 2005;84:741-746.

    

Dr. Schoenbaum is assistant clinical professor in the University of California, Los Angeles (UCLA) Division of Restorative Dentistry and the assistant director of UCLA Continuing Education. He is faculty in the UCLA Center For Esthetic Dentistry, where he is the instructor for the aesthetic direct composite course, porcelain preparation course, implant prosthetics course, and the CAD/CAM digital impression course. He lectures extensively on dental technology, aesthetic dentistry, and dental photography. He maintains a private practice within the UCLA faculty group dental practice. He can be reached at tschoenb@ucla.edu or at toddschoenbaum.com.

Disclosure: Dr. Schoenbaum reports no disclosures.

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