Veneer Preparation Limits: From Feldspathic to CAD/CAM
The concept of minimal-preparation veneers is not new; it goes back to the early 1980s. At that time it was shown that if the inner surface of porcelain is etched with hydrofluoric acid, it can be bonded to etched enamel with resin composite. From then on, a veneer's retention came from the bond and no longer depended on the shape of the preparation. That is why feldspathic veneers were used to rebuild tooth form with as little reduction as possible.
Today a wider range of ceramic materials is available. Each of them has a minimum thickness below which the risk of the veneer fracturing rises. That minimum thickness is what determines how much reduction is needed.
∆ The basic principle
The reduction must be at least equal to the minimum thickness of the chosen material. If the reduction is smaller, the veneer stands out beyond the natural form of the tooth (overcontour), and a ledge forms at the margin, where the edge of the veneer meets the tooth. That ledge collects plaque and leads to gingival inflammation.
Reduction depth is measured from the surface of the mock-up. The mock-up is a temporary model of the final tooth form, made from the wax-up with temporary composite directly on the teeth. When the tooth form is not going to change, the mock-up surface is the tooth's current surface.
∆ Feldspathic veneers
Feldspathic porcelain is the oldest dental ceramic and, optically, the closest to natural enamel. These veneers are made by hand. The technician mixes the ceramic powder with a special liquid and fires it layer by layer on a refractory die (a heat-resistant model) or on platinum foil.
Feldspathic veneers achieve esthetics on the level of a natural tooth. Their thickness starts at about 0.5 mm at the thickest part and reaches its minimum at the margins. Technically, their average thickness can be taken as about 0.3 mm. Feldspathic is therefore the thinnest option among ceramic veneers. However, the ceramic itself is weak on its own, and its durability depends on a strong bond to enamel.
∆ Pressed veneers
The most common material for this method is lithium disilicate, whose best-known example is e.max Press. It is stronger than feldspathic porcelain. Pressed veneers are made with the lost-wax technique. First the veneer is shaped in wax, and the wax is then burned out with heat. Next, a ceramic ingot is softened in the furnace and pressed into the space the wax left behind.
After this step there are two ways to complete the veneer:
- Layering: the pressed ceramic serves as a core, and porcelain is layered on top of it. This gives the best esthetic result, but needs more reduction than a thin veneer.
- Monolithic with surface characterization: the veneer is made from a single piece of ceramic, and color details are added with stains on its surface. The esthetic result does not reach that of layering, and the value (the lightness of the tooth) usually drops. In addition, surface stain may be lost during later adjustments, such as occlusal adjustment, or through wear in the mouth.
∆ CAD/CAM veneers
In this method the veneer is designed in software and milled from a prefabricated ceramic block. Common blocks for veneers are e.max CAD (lithium disilicate) and Celtra Duo (zirconia-reinforced lithium silicate).
Like pressed veneers, CAD/CAM veneers can be made monolithic or layered, with similar esthetic limitations. Even in monolithic form, however, they need more reduction than feldspathic veneers, because very thin edges chip during milling.
∆ Minimum reduction by material (mm, from the mock-up surface)
| Material | Cervical | Labial | Incisal |
|---|---|---|---|
| Feldspathic | 0.3 | 0.3–0.5 | 1–1.5 |
| e.max Press – thin veneer | 0.3 | 0.3 | 0.4 |
| e.max Press – regular or layered veneer | 0.6 | 0.6 | 0.7 |
| e.max CAD – thin veneer | 0.4 | 0.4 | 0.5 |
| e.max CAD – regular veneer | 0.6 | 0.6 | 0.7 |
| Celtra Duo | 0.4 | 0.6 | 1–1.5 |
The e.max and Celtra Duo values are taken from the manufacturers' instructions for use, and the feldspathic values from reference texts.
∆ Thin veneer versus regular veneer
A thin veneer and a regular veneer are made from the same material; they differ only in the minimum thickness allowed. The manufacturer permits a thinner thin veneer on three conditions: the preparation is in enamel, the incisal margin does not lie on worn surfaces or in the path of jaw movements, and the veneer is made only monolithic with surface stain. Layering is therefore not permitted for a thin veneer. A regular veneer has the material's standard thickness and allows layering and better masking of the underlying color. In layered e.max veneers, at least half of the total thickness must be pressed ceramic.
∆ Color change and reduction depth
The values in the table are the minimum thickness for cases where the tooth's color does not change much. The larger the gap between the tooth's current color and the final color, the thicker the ceramic needs to be.
For feldspathic veneers, about 0.2 mm of thickness is needed for each shade step, but the first step is covered by the same 0.3 mm minimum. A shade step is the distance between two consecutive shades within one group of the VITA shade guide. For example, going from A3 to A1 counts as two steps: A3 to A2 and A2 to A1. Feldspathic therefore needs about 0.5 mm (0.3 + 0.2) of labial reduction. This is a rule of thumb, not an exact figure.
This rule cannot be applied directly to e.max, because masking in e.max depends mostly on the translucency of the ingot (for press) or block (for CAD). HT (High Translucency) ingots are the most translucent, LT (Low Translucency) are less translucent, and MO (Medium Opacity) are used to mask darker substrates. For a color change such as A3 to A1, the table's minimum thickness (0.6 mm labially) is usually enough, provided the prepared tooth's shade is sent to the lab.
Note that 0.6 mm in the cervical third is more than the enamel thickness in that area, and the preparation may reach dentin. For this reason, with mild color changes, feldspathic veneers preserve more enamel.
∆ Clinical notes
- A thin veneer suits only teeth with a good underlying color, because ceramic this thin cannot mask a dark color.
- To control reduction depth, grooves are first cut in the mock-up with 0.3 and 0.5 mm depth-cutting burs, and the groove floors are marked with a pencil. The mock-up is then removed and the tooth surface is reduced evenly down to the marks. Finally, the space created is checked with a silicone index taken from the wax-up. This check matters most in the cervical third, where enamel is thinnest.
- The cervical margin is prepared as a light chamfer with a depth equal to the cervical column of the table. A shallower depth leaves a ledge at the margin.
∆ References
- Horn HR. Porcelain laminate veneers bonded to etched enamel. Dent Clin North Am. 1983;27(4):671–684.
- Calamia JR. Etched porcelain facial veneers: a new treatment modality based on scientific and clinical evidence. N Y J Dent. 1983;53(6):255–259.
- Gürel G. The Science and Art of Porcelain Laminate Veneers. Quintessence; 2003.
- Ivoclar. IPS e.max Press — Instructions for Use.
- Ivoclar. IPS e.max CAD — Monolithic Solutions, Instructions for Use.
- Dentsply Sirona. Celtra Duo — Directions for Use.