The success of all-ceramic crowns depend highly upon the luting agent chosen to retain the restoration. The main purpose of any cement is to create a durable bond between the crown and the prep, by filling in the micro-gap between the crown and the tooth. Luting agents are also responsible for ensuring good marginal integrity. A successful luting agent must be biocompatible, have low solubility, low radiopacity and maintain good aesthetics. It is important to remember that cement cannot compensate if the tooth has not been correctly prepared. The proper retention forms and resistance of the preparation are still important factors for success.
Clinicians must consider the selected ceramic restoration to decide which luting agent will yield the best results. The restorations may be etchable or non-etchable, depending on the core material. Etchable restorations include silica-based ceramics, leucite-reinforced feldspathic ceramic such as IPS Empress and lithium disilicate glass ceramics, and IPS e.max. Non-etchable ceramics include non-silica-based material, including zirconium oxide and aluminum oxide. Both etchable and non-etchable types must be primed to create a resin-loving, hydrophobic substrate surface and ensure a strong bond between the substrate and the luting cement. The luting agent and techniques for each of these restorations will differ.
Leucite-reinforced feldspathic ceramics must be bonded. Lithium disilicate crowns can be cemented using conventional cementation materials and techniques. They can also be chemically bonded.
When bonding, the surface of the restoration is etched with 5% hydrofluoric acid for 20 seconds before being thoroughly rinsed. A primer is applied for one minute, enabling the resin-based cement to bond to the all-ceramic crown. Sometimes it is necessary to precondition the tooth, but this is dependant on the luting agent used, so it is important to follow the manufacturer’s instructions.
The surface should be etched using 4% to 10% hydrofluoric acid between one and three minutes before rinsing thoroughly. Once etched, ceramic residues may form on the fitting surface, which can compromise the bond strength. To remove any residues, the crown should be placed in an ultrasonic bath of distilled water for four minutes and must be carefully evaluated to ensure the surface is free from residues. After silane application, an adhesive resin is applied to the fitting surface of the crown.
Clinicians can choose from dual-cured or light-cured resin cement. Both have their advantages and disadvantages. With light-cured cement, working time is longer, facilitating the removal of excess cement, which helps to reduce finishing time. Dual-cured cement tends to be used for thicker ceramic crowns and times where there is insufficient light penetration for a maximum cure. It is worth bearing in mind that dual-cured cement offers less working time and possible colour instability due to amine degradation.
Appropriate luting agents for aluminium oxide crowns with a high aluminium oxide content include hybrid glass ionomer cement and resin cement. Sandblasting alters the fit surface more effectively than conventional acid etching, helping to increase bond strength. Appropriate luting agents for high-strength ceramics such as zirconium oxide include resin-modified glass ionomer cement, conventional resin cement, zinc-phosphate cement, and self-etching adhesive cement has also been developed. The success rate of high-strength ceramic restorations isn’t reliant on the resin bond to the crown, but to successfully support an all-ceramic crown, adhesive cementation procedures are generally necessary.
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References
https://www.dentalaegis.com/id/2013/02/selecting-luting-agents
https://www.cda-adc.ca/jcda/vol-75/issue-5/379.pdf
http://www.dentaleconomics.com/articles/print/volume-105/issue-9/science-tech/self-adhesive-cements-simplified.html
https://www.oralhealthgroup.com/features/a-review-of-luting-agents-cornelis-h-pameijer-dmd-dsc-phd/