Feldspathic Porcelain

Dental porcelains are partially crystalline minerals (feldspar, silica, alumina) dispersed in a glass matrix.³⁶ Porcelain restorations are made from finely ground ceramic powders that are mixed with distilled water or a special liquid, shaped into the desired form, and then fired and fused together to form a translucent material that looks like tooth structure. Currently, some ceramic inlays and onlays are fabricated in the dental laboratory by firing dental porcelains on refractory dies, but more are fabricated by pressing or milling methods, which are described later. The fabrication steps for fired ceramic inlays and onlays are summarized as follows:

 After tooth preparation, an impression is made, and a die-stone master working cast is poured (Fig. 11-3, A).

 The die is duplicated and poured with a refractory investment capable of withstanding porcelain firing temperatures. The duplication method must result in the master die and the refractory die being accurately interchangeable (see Fig. 11-3, B).

 Porcelain is added into the preparation area of the refractory die and fired in an oven. Multiple increments and firings are necessary to compensate for sintering shrinkage (see Fig. 11-3, C).

 The ceramic restoration is recovered from the refractory die, cleaned of all investment, and seated on the master die and working cast for final adjustments and finishing (see Fig. 11-3, D).

Although feldspathic porcelain inlays and onlays are less popular than in the past, some dental laboratories continue to use this method because of its low startup cost. The ceramic powders and investments are relatively inexpensive, and the technique is compatible with most existing ceramic laboratory equipment such as firing furnaces. The major disadvantage is its technique sensitivity, both for the technician and the dentist. Inlays and onlays fabricated with this technique must be handled gently during try-in and bonding to avoid fracture. Feldspathic porcelains are weak, so even after bonding, the incidence of fracture can be relatively high.³⁶

Pressed Glass-Ceramics

Over 40 years ago, it was discovered that certain glasses could be modified with nucleating agents and, on heat treatment, be changed into ceramics with organized crystalline forms. Such “glass-ceramics” were stronger, had a higher melting point than noncrystalline glass, and had variable coefficients of thermal expansion.³⁷ At first, these glass-ceramics were developed primarily for cookware and other heat-resistant products. However, in 1984, the glass-ceramic material Dicor (DENTSPLY International, York, PA) was patented and became a popular ceramic for dental restorations. A major disadvantage of Dicor was its translucency, which necessitated external application of all shading.

Dicor restorations were made using a lost-wax, centrifugal casting process. Newer leucite-reinforced glass-ceramic systems (e.g., IPS Empress, Ivoclar Vivadent, Amherst, NY) also use the lost-wax method, but the material is heated to a high temperature and pneumatically pressed, rather than centrifuged, into a mold. Although some studies indicate that hot pressed ceramics are not substantially stronger than fired feldspathic porcelains, they do provide better clinical service.28,29,38,39 The fabrication steps for one type of leucite-reinforced pressed ceramic restoration (IPS Empress) are summarized as follows:

1. After tooth preparation, an impression is made, and a working cast is poured in die-stone. A wax pattern of the restoration is made using conventional techniques (Fig. 11-4, A).

2. After spruing (see Fig. 11-4, B), investing, and wax pattern burnout, a shaded ceramic ingot and aluminum oxide plunger are placed into a special furnace (see Fig. 11-4, C). The shade and opacity of the selected ingot (see Fig 11-4, D) are based on the information provided by the clinician, specifically the desired shade of the final restoration and the shade of the prepared tooth.

3. At approximately 2012°F (1100°C), the ceramic ingot becomes plastic and is slowly pressed into the mold by an automated mechanism.

4. After being separated from the mold, the restoration is seated on the master die and working cast for final adjustments and finishing.

5. To reproduce the tooth shade accurately, a heavily pigmented surface stain is typically applied. The ceramic ingots are relatively translucent and available in a variety of shades, so staining for hot pressed ceramic inlay and onlay restorations is typically minimal.

The advantages of leucite-reinforced pressed ceramics are their (1) similarity to traditional “wax-up” processes, (2) excellent marginal fit, (3) moderately high strength, and (4) surface hardness similar to that of enamel.40,41 Although pressed ceramic inlays and onlays are stronger than porcelain inlays made on refractory dies, they are still somewhat fragile during try-in and must be bonded rather than conventionally cemented. IPS Empress inlays and onlays have performed well in clinical trials ranging up to 12 years in duration.^{22,24,28,29,42}

Lithium Disilicate

A newer type of ceramic, lithium disilicate (e.max, Ivoclar Vivadent Inc., Amherst, NY), is available in both pressed (IPS e.max Press) and machinable (IPS e.max CAD) forms, and either can be used to fabricate inlays and onlays.⁴³ The two forms of e.max are slightly different in composition, but lithium disilicate is a moderately high-strength glass ceramic that also can be used for full crowns or ultra-thin veneers. In vitro testing of this ceramic material has shown very positive results, and it has become a highly popular alternative for inlays and onlays. However, because the material is relatively new, long-term clinical studies to demonstrate superior performance are lacking.

CAD/CAM

Improvements in technology have spawned increasingly sophisticated computerized devices that can fabricate restorations from high-quality ceramics in a matter of minutes. Some CAD/CAM systems are expensive laboratory-based units requiring the submission of an elastomeric or digital impression of the prepared tooth. The CEREC system was the first commercially available CAD/CAM system developed for the rapid chairside design and fabrication of ceramic restorations. The most popular dental CAD/CAM systems in use today are the CEREC 3D (Sirona Dental Systems, LLC, USA, Charlotte, NC) and E4D (D4D Technologies, LLC, Richardson, TX) (Fig. 11-5).

Generation of a chairside CAD/CAM restoration begins after the dentist prepares the tooth and uses a scanning device to collect information about the shape of the preparation and its relationship with the surrounding structures (Fig. 11-6). This step is termed optical impression. The system projects an image of the preparation and surrounding structures on a monitor, allowing the dentist or the auxiliary personnel to use the CAD portion of the system to design the restoration. The operator must input or confirm some of the restoration design such as the position of the gingival margins (Fig. 11-7).