Appraisal and Cementation of Porcelain Laminate Veneers
Christopher C.K. Ho
Appraisal and Try-In of Veneers
It is important to assess veneers on models to check marginal fit as well as to evaluate the integrity of the porcelain to ensure that there are no defects or fractures prior to cemenation. It is vital to have confirmation from the patient that they are happy prior to proceeding with the cementation procedures.
It is preferable not to use local anaesthetic for the patient to approve the final aesthetics prior to cementation. However, if local anaesthesia is required, an alternative is to use the AMSA local anaesthetic block technique, so that the injection achieves pulpal anaesthesia of the central incisors through the second premolar without collateral numbness of the face and facial muscles of expression. This is best achieved with the computer-controlled injection system – the Wand (Milestone Scientific, Livingston, NJ, USA) – which delivers a virtually painless palatal injection.
Correct preparation of the fitting surface of the veneer involves micro-mechanically roughening the surface by etching with hydrofluoric acid. This removes a layer of glass, leaving a roughened surface. There is a salt residue on the surface, which should be removed to enhance the final bond strength. The surface is then silanated and ready for cementation. Isolate carefully to enhance access and restrict moisture contamination.
The veneers are adhesively bonded with light-cure resin cement, which allows sufficient working time to seat the veneer and possesses better colour stability. There are various shades of cement that can be utilised, which have minimal influence on the final shade due to the low film thickness of the cements once luted. Using opaque cements may help to block out discoloration as well as increase the value of the final shade of the veneer. If opaque cement is used it should be applied sparingly, as too much will make the veneer distinct and not lifelike in appearance.
The provisional veneers can be carefully removed using a spoon excavator to lever them from the proximal walls. If this is unsuccessful, the provisional material can be sectioned with a vertical cut and a torquing movement applied with an instrument to remove separate fragments.
The tooth surface should be cleaned of any residual resin cement or provisional material, to ensure perfect adaptation of the veneers. If a spot etched temporary veneer was placed, then the etched area will need be prepared with a fine diamond to allow a clean surface to which to bond. This will also ensure that there is no resin present that would interfere with the seating of the veneer.
The tooth is then cleaned with fine pumice slurry or air abraded with 27 micron aluminium oxide, carefully avoiding the soft tissues to minimise any chance of gingival bleeding. Small finishing strips can be used interproximally to clean the contact areas.
Each veneer should be assessed to ensure that the marginal fit around the die is accurate. It is good practice to assess each veneer with transillumination to ensure there are no fractures within the porcelain. The veneers should then be appraised on the preparation individually to assess fit. This is best done dry (without water or try-in gels), as marginal adaptation is then better visualised. Do not apply excessive pressure while trialling the veneers, as they are brittle prior to bonding.
Incomplete seating is normally due to resin cement that has not been removed, remaining provisional material or tight contact points. Once each individual veneer has been assessed, then all the veneers should be assessed in place, evaluating the proximal contacts. It may be necessary to use the try-in gels at this stage to allow temporary seating of the veneers.
The veneers should be checked with the patient in relation to colour, form and length, as well as whether they are pleasing to the patient or may require modification. There are different water-soluble try-in gels that a clinician can use to alter the colour of the veneer, from lowering or raising the value to opaquing the restoration to mask discoloration.
At this stage the patient should not be asked to check occlusion, as this may cause fracture of an unbonded veneer.
Treating the Fitting Surface of the Veneer
Once the final aesthetics of the veneers are approved, the restorations are prepared for cementation.
The veneers (being silica-based restorations) must be etched with hydrofluoric acid, which allows a micro-mechanical bond when adhesively bonded. The fitting surface is etched with 9.5% hydrofluoric acid for 20 seconds with lithium disilicate (e.max) or 60 seconds for other silica-based ceramics. The use of hydrofluoric acid dissolves the glassy matrix surrounding the crystalline phase within the porcelain, leaving retentive areas between the acid-resistant crystals.