Dental Prostheses Health Article

Definition

Dental prostheses, artificially made devices resembling natural teeth, are used to replace missing or damaged teeth. These devices include inlays/onlays, crowns, bridges, dentures, partial dentures, and dental implants.

Purpose

Crowns and inlays/onlays are intended to repair damage to individual teeth. They replace tooth structure lost by decay or injury, protect the part of the tooth that remains, and restore the tooth's shape and function. Bridges, dentures, and partial dentures fill in a space in the jaw left by a missing tooth or teeth. They protect the shape of the mouth and restore function of the teeth and jaw.

Precautions

Some patients are allergic to the constituents in local or general anesthesic agents. In addition, many people are afraid of dental work and therefore may experience stress-related symptoms, even fainting, while in the dental office. Most dentists can help patients with this specific fear. Also, the dentist and dental assistant will need to be aware of any pre-existing conditions in the patient's history, e.g. diabetes, high blood pressure, heart disease, hemophilia, or HIV/AIDS.

Inlays/onlays

An inlay resembles a filling in that it fills the space remaining after the decayed portion of a tooth has been removed. The difference is that an inlay is shaped outside the patient's mouth and then cemented into place. After the decay is removed and the cavity walls are shaped, the dentist makes a wax pattern of the space. A mold is cast from the wax pattern. An inlay is made from this mold and sealed into the tooth with dental cement.

Inlays and their counterparts, onlays, are conservative alternatives to crowns. They don't require as much tooth preparation and often are more durable than amalgam fillings. Inlays cover the grooves on the surface of the molar. Onlays wrap over the tooth, covering more of its surface.

Inlays used to be made entirely of gold for durability. New inlay alloys of palladium, nickel, or chromium are now frequently used. Metals are the dentists' choice for inlays in molars. When inlays are required for visible anterior teeth, tooth-colored composites and porcelains are used. Reinforced porcelain and Lucite porcelain are durable but may not be suitable for patients who grind their teeth. Composites are also used in fillings.

Crowns

A prosthetic crown replaces the outer portion of the tooth to protect and strengthen it. This protection becomes necessary when a tooth cracks, has its entire structure weakened by decay, or becomes brittle after a root canal. Crowns can also cover dental implants or abutment (adjacent) teeth when fitting a bridge.

Crowns can also cover discolored or otherwise aesthetically displeasing teeth. Cosmetic dentistry does not use crowns as much as it once did. Crowns, though aesthetically pleasing, require more radical dental techniques. Dentists are opting for more conservative methods such as bleaching, bonding, or veneers.

The dentist first removes the decay, and the tooth is then prepared for a crown. It may be tapered on the outside edges to a peg, reinforced with a cast metal core, or rebuilt with both a cast metal core and a post. An impression of the prepared tooth and the teeth next to it is made. A retraction cord is placed around the tooth in order to get the impression medium under the gum where the crown will be fitted.

The dental technician will create a new crown, using a cast made from this impression. The technique the technician uses is called lost wax casting. A wax model is made of the crown. Another mold is made around the wax model and both are fired in a kiln. The wax melts, leaving an opening into which a restorative material can be poured. The crown may be made of gold or stainless steel alone, metal with a veneer of tooth-colored porcelain or resin, or of porcelain or resin alone. The finished crown is then placed over the prepared tooth, adjusted, and cemented into place.

When a tooth has had a root canal and the root has been filled, the tooth may not be strong. Post crowns are used in these cases. The tooth is leveled at the gum line and a stainless steel or gold post is fitted into the root canal. This post can then receive the new crown and hold it in place.

For other patients, it may be desirable to implant the crown. In this case, a steel post is embedded in the patient's jawbone. It is left in place until the bone adheres to the post. The post is exposed and the crown is made and fitted.

New computerized techniques are making the restoration process faster and more accurate. Chairside Economical Restoration of Esthetic Ceramics (CEREC) uses a computer system that allows the dentist to create ceramic crowns, inlays, and onlays, in one sitting. The tooth is prepared as usual but impressions are made digitally, using a hand-held camera. These photographs are converted to 3-D images on the computer screen, thus eliminating the need to take a physical impression of the patient's teeth. The dentist uses special 3-D CAD/CAM software to design the crown. A milling system attached to the CEREC machine is able to make a ceramic crown in 10 to 15 minutes.

Provided they are well taken care of, crowns can last 5–15 years or more.