Tuesday, November 2, 2010

How have crown restorations changed over the years?

When a tooth has been damaged in some way that results in a loss of tooth structure, such as a fracture, large cavity, or after a root canal, the indicated treatment is often a crown, or cap. A crown is an indirect restoration, or one that is fabricated outside of the mouth and then cemented over the tooth with a bonding cement. Although they require preparation or reduction of the remaining tooth to place, crown restorations are good for the overall health of the tooth because they strengthen and reinforce the underlying damaged tooth structure.

Back in the 1950's - 1960's, most crowns were fabricated from gold alloys. Although gold is very biocompatible, strong, and long lasting, these crowns were not highly esthetic and were not well accepted in the front of the mouth. Doctors tried to improve their appearance by veneering the front of the crowns with resin materials that were tooth colored, but the results were often unfavorable.

By the 1970's, Ceramic materials were being developed that could be fused to white gold alloys in a furnace. This technology vastly improved crown and bridge esthetics, but limitations still existed. These crowns were more opaque than natural teeth, because the dark gold needed to be blocked out by an opaque porcelain layer, which limited the crowns ability to transmit light. The light would enter the crown, hit the opaquer, and bounce back out, rather than being transmitted through the tooth like a fiberoptic light. This resulted in not only increased opacity, but a loss of color value and a greyish shadow in the gumline beyond the crown margin. In addition, the gold margin where the porcelain ends at the gumline often resulted in the appearance of a dark line at the gumline above or below the crown.

In the 1980's, porcelain technology was taken a step further, and all-ceramic restorations were developed. While this technology eliminated the previously discussed concerns, further problems arose. These early all-ceramic crowns were very weak. They had a high incidence of fracture, and could not be used for bridgework. Also, they needed to be bonded to the underlying tooth for any strength at all and the available bonding agents at the time were very limited. So even though these materials were available, the standard porcelain-fused-to-metal crowns and bridges were the most widely used.

By the 1990's porcelain technology was improving as was the strength and availability of resin bonding cements. Over the next 20 years, several generations of all ceramic restorations were developed, including products of sufficient strength that they could be used for bridgework of small spans in the anterior of the mouth. With these advancements in technology, we were finally able to achieve the esthetic results we had always strived for but fell short, but strength was still an issue. The most recent development in our quest for beauty and strength has been the development of porcelain layered over zirconia restorations. Although these crowns and bridges are more opaque than some of the previous all ceramic restorations, they are much stronger and light years ahead of porcelain-fused-to-metal in terms of esthetics. These days, much of my ceramic work focuses on zirconia. It is the only material so far that allows acceptable esthetics with the strength we need for function.

If you are in need of a crown or bridge, come in to see me and we will discuss all of your esthetic needs and porcelain options as we develop a treatment plan that is right for you.