Porcelain Veneer Preparation
THE NEXT DDS
Although bonded porcelain laminate veneer restorations (PLVs) have increased in popularity, they are not an entirely new concept to the field of dentistry. In 1938, Pincus described a technique used in the film industry to enhance actors’ and actresses’ smiles. This technique utilized tooth-shaped ‘veneers’ retained by a denture adhesive during filming.1 These veneers were not permanently bonded to the tooth structure and therefore served a short-term transient need.
Research to develop suitable bonding agents began in the early 1950s. The first documented attempt to develop an adhesive system was by Hagger. The system utilized was based on glycerophosphoric acid dimethacrylate, which could be catalytically polymerized by the action of sulphinic acid in 5 to 30 minutes at 20°C. A 1952 investigation by Kramer and Mc Lean showed that this acid increased adhesion to dentin by penetrating the surface and forming what is now known as the hybrid layer. In 1955, Buonocore’s landmark study on adhesion used a 30-second conditioning protocol with 85% phosphoric acid to achieve enamel decalcification. Having exposed the organic enamel that allowed self-curing resin attachment, he observed an increase in the surface area available for bonding after acid etching.
Rochette later proposed a technique that consisted of a silane-treated ceramic restoration for fractured incisors; this restoration was bonded to acid-etched enamel without tooth preparation. Simonsen and Calamia examined the bond strengths between etched porcelain and composite resin. Their results showed that bond strengths were in the magnitude of 7.5 MPa (1100psi), which was then thought to be sufficient for the retention of etched porcelain veneers. After the initial study, a follow-up of over 200 bonded porcelain veneers revealed that no veneers were dislodged or fractured after two years.
Dunne and Millar evaluated the clinical performance of 315 veneers placed on 96 patients in teaching hospitals after 5.25 years. A total of 262 (83%) veneers remained problem-free at the end of the study. Of the 17% with problems, 8% were repairable; 11% were considered failures. Age, gender, and fabrication technique (ie, platinum foil or refractory die), use of rubber dam, and year of bonding were not significant factors. Dumfahrt and Schaffer in 2000 evaluated the clinical performance of 191 porcelain laminate veneers (PLVs) and evaluated the gingival response in a long-term survey. They concluded that PLVs offered a predictable and successful treatment modality that preserved maximum sound tooth structure. Interestingly they reported an increased rate of failure when veneers where partially bonded to dentin. The survival of porcelain veneers followed over a period of 10 years was 91% in this study.
During the 1980s, PLVs began to increase dramatically in popularity due to the conservative nature of the technique. The ability to alter tooth morphology and color without the greater preparation needed for conventional preparations (ie, full-coverage restorations) offered a new approach in treatment planning. With the improvement in bonding systems to enamel and dentin substrate, more long-term predictable results are expected.
The strength of PLVs relies on the adaptation of the remaining tooth structure, the luting cement, and the porcelain restoration interface. The success of the restoration is largely determined by the strength and durability of the bond between the three different substrates. Once the restoration is bonded to the tooth, the porcelain is supported by the underlying tooth structure under compression.