Material-related cementation procedures
In this chapter:
■ Adhesive cementation of silica-based ceramics (feldspathic ceramics, glass-ceramics)
■ Adhesive cementation of oxide ceramics (zirconia)
■ Adhesive cementation of hybrid materials (resin-nano ceramic, resin-infiltrated ceramic network)
■ Universal silanes/primers and universal resin cements
The stability of all-ceramic and hybrid material restorations is low, and for good clinical long-term outcomes it needs to be improved by means of the adhesive cementation with resin cements. In contrast to traditional cements, the resin cements chemically bond to both the restorative material and the tooth substance, and the adhesive cementation reinforces the tooth-restoration complex. Additional benefits of the resin cements include high translucency and tooth-resembling color, both leading to good esthetic results with translucent ceramic or hybrid materials1–6.
One important prerequisite for good adhesive bond between the restoration and the tooth substance is moisture control in the intraoral environment. The bonding agents and the resin cements are hydrophobic and only adhere to dry surfaces. The application of rubber dam is, therefore, highly recommended for all adhesive cementation procedures where applicable.
Another crucial factor is the adequate pretreatment of both substrates, the restoration, and the tooth substance. The pretreatment comprises an increase in surface area for the adhesive fixation by roughening the surface of the restoration, and by chemically pretreating the restoration and tooth surfaces for the establishment of the chemical link, the adhesion, between the restoration material and the tooth. The roughness of the restoration surface leads to a retention of the cement additional to the chemical link, and thereby supports the bond strength.
The presented restorative materials are chemically different and require different pretreatment and different types of resin cements for the adhesive fixation.
1.9.2 Adhesive cementation of silica-based ceramics (feldspathic ceramics, glass-ceramics)
Silica-based ceramics, such as feldspathic ceramics and the leucite- and lithium-disilicate reinforced glassceramics, need to be etched with hydrofluoric acid to increase the surface roughness (Fig 1-9-1) (see also Part I, Chapter 1).
Fig 1-9-1a Scanning electron microscopic (SEM) image of a leucite-reinforced glass-ceramic after the etching with hydrofluoric acid. Note the round appearance of the leucite crystals and the retentive zones resulting out of the etching procedure.
Fig 1-9-1b Scanning electron microscopic (SEM) image of a lithium-disilicate-reinforced glass-ceramic after the etching with hydrofluoric acid. Note the rod-like appearance of the lithium-disilicate crystals and the retentive zones resulting from the etching procedure.
For the different ceramics, differently concentrated hydrofluoric acids and different etching times are recommended. With the feldspathic and leucite-reinforced ceramics, a 9.5% buffered hydrofluoric acid is recommended to be applied for 60–90 seconds, and with the lithium-disilicate glass-ceramics, a stronger, 5% non-buffered hydrofluoric acid is applied for 20 seconds. The different acid concentrations and etching times are important for the ideal surface roughness and texture. The ideal surface properties after the etching significantly depend on the chemical structure of the material. Hence, the recommendations of the manufacturer of the material for the etching are crucial.
The chemical link between the silica-based ceramic and the resin cement is, thereafter, established by the application of a coupling agent, the silane. The silane binds to the silica present in the anorganic glass-matrix and polymerizes with the organic matrix of the resin cement. For chemical activation, the silane needs to be hydrolyzed to silanol. Two types of silanes exist today: two-component silanes, which induce the hydrolyzation of the silane by mixing of the two components, and one-component silanes, which are already hydrolyzed, ie, in the silanol state. The one-component silanes are less technique sensitive at their application, yet since they are chemically activated they are more sensitive to storage and handling, and their efficacy is dependent on the expiration date. It is, therefore, crucial to handle and store the one-component silanes as recommended by the respective manufacturer.
With thin ceramic restorations (<1.5 mm of thickness)7, like veneers a bonding agent is applied prior to the application of the light-curing or dual-curing Bis-GMA resin cement, and both the bonding and the resin cement are light-cured at the same time. For restorations with >1.5 mm of thickness, the bonding layer needs to be light-cured prior to the application of the resin cement (Note: only in situations with large internal gaps, the fit of the restoration may be impaired). In case of precise fit of the restoration, no bonding agent is applied prior to the application of the resin cement.
The tooth substance is pretreated according to the well-established procedures for Bis-GMA resins, ie, enamel etching with phosphoric acid and application of dentin primers (also see Part II for clinical case examples). The application of the bonding is performed with ceramic restorations of <1.5 mm of thickness. For ceramic restorations of >1.5 mm thickness, no bonding agent is applied prior to the cementation procedure (step-by-step procedure, Figs 1-9-2 to 1-9-17).
Fig 1-9-2 Some manufacturers of resin cements offer analogously colored glycerin gels for the try-in of the restorations.
Figs 1-9-3a to 1-9-3c (a and b) Pretreatment of the feldspathic veneer with 9.5% hydrofluoric acid for the etching, and thereafter with silane. (c) Application of the silane to the etched, roughened ceramic surface, for the establishment of the chemical bond of the resin cement to the ceramic.
Fig 1-9-4a Etching of the enamel with phosphoric acid. In the present case, no protection of the neighboring teeth was needed as they were restored with crowns.
Fig 1-9-4b Rough etched enamel surface, note the matt appearance.
Fig 1-9-5 Application of a light-curing bonding agent to the etched enamel, no light-curing. Note: protection from the surrounding light is needed after this step.
Fig 1-9-6 Application of bonding agent to the silanized ceramic surface, no light-curing. Note: protection from the surrounding light is needed after this step.
Figs 1-9-7a to 1-9-7e The cementation of the veneer was performed with a light-curing filling composite (Tetric Classic, Ivoclar Vivadent, Schaan, Liechtenstein), heated up to 68°C (Calset, Addent, Danbury, CT, USA) to reduce the viscosity while maintaining the high filler content, and the excellent physical properties. Note the glossy soft appearance of the heated composite (d) as compared to unheated composite (e). The composite was distributed with a spatula to the entire veneer surface prior to application to the abutment tooth.
Fig 1-9-8 Application of the veneer to the pretreated abutment tooth. Gentle and constant finger pressure allows for careful stepwise removal of the composite excess. Finishing of the margins is performed at this stage, followed by light-curing starting from the palatal region, thereafter buccally and incisally, until polymerization of the composite resin and the bonding agent is achieved.
Fig 1-9-9
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