Special Features of Porcelain-Fused-to-Metal Crowns
Porcelain-fused-to-metal crowns offer a combination of strength and improved aesthetics that all-metal restorative counterparts don’t possess. PFM crowns have a long history of use in dentistry and have been the tried-and-true solution for dentists for more than 60 years.
More recently, all-ceramic crowns have emerged onto the scene. They have become more popular for patients and dentists alike. But they don’t come with a built-in track record of research to support their long-term use and benefits.
One of the biggest benefits that all-ceramic crowns offer is they require less good tooth structure to be removed for an ideal crown preparation. Because they contain no metal substructure, they also provide improved aesthetics, yielding a more enamel-like appearance. Porcelain-fused- to-metal crowns can still be a great choice for certain patients and specific areas of the mouth.
Drawbacks of Porcelain-Fused-to-Metal Crowns
Despite their numerous documented advantages, creating a highly lifelike porcelain-fused-to-metal crown can prove to be difficult. In the absence of a metal substructure, porcelain is fragile and lacks the strength necessary to withstand occlusal and shearing forces. With a substructure, the metal color can show through. It must be masked with opaque porcelain to obscure the underlying color before layering with Feldspathic porcelain.
Both metal and opaque porcelain prevent light from passing through the restoration as it does with a natural tooth, making the restored teeth look less lustrous when compared to the surrounding natural teeth.
An enemy of porcelain-fused-to-metal crowns is the recession of gum tissue. Over time, metal contact with the tissue may cause gum sensitivity and irritation, especially when base metal alloys are used. With recession, the metal margin begins to show, creating an ugly gray ring that isn’t aesthetically pleasing, particularly in the anterior area of the mouth. Dentists sometimes resort to using a porcelain margin during tooth preparation to overcome this drawback.
A PFM crown prep requires rather aggressive anatomical tooth reduction to avoid a bulky-looking appearance at delivery. Teeth must be shaped to account for the thickness of both the metal substructure and the layers of porcelain stacked on top of it in order for the ceramist to create a crown with good translucency and refractive properties.
PFM crown prep is of paramount importance from a clinical perspective. To achieve excellent aesthetics, follow general tooth preparation guidelines.
- Facial reduction for anterior teeth: 1.5 mm minimum
- Lingual reduction for anterior teeth: 1.0 mm minimum
- Buccal and lingual surfaces of posterior teeth: 1.5 mm minimum
- Incisal or occlusal reduction: 2.0 mm
- Interproximal reduction: 1.5 to 2.0 mm
- Smooth shoulder margins: Especially necessary for porcelain margins