CROWNS & BRIDGES

What is a crown?

A crown or "cap" is a covering for the top portion if the tooth. It looks and functions like a natural tooth. The crown is cemented on to the remaining tooth structure and is therefore referred to as a "fixed" non removable restoration. The purpose of a crown is to hold the remaining tooth structure together, helping to prevent further destruction of tooth and in some cases repairing teeth that are otherwise non repairable.

What are indications for a crown?
 

• Large fillings involving half to three quarters the tooth or more, resulting in a weakened tooth increasing likelihood for fracture.
• Endodontic therapy (root canal treatment), this weakens the tooth increasing likelihood of fracture, this is especially true for posterior teeth (premolars and molars).
• Cracked teeth in which the crack is deep enough and in a direction which can lead to pulpal death or tooth loss.
• To improve the contours and appearance of existing teeth.
• Teeth that are unfillable with little or no tooth structure to hold a filling above the gum line.
• To correct the occlusal plane (chewing surface alignment).
• To provide contours to receive removable appliances.

What is the procedure for a crown?

The process consist of two appointments.

The first appointment is to prepare the crown, and generally takes 1 to 1-1/2 hours to complete. This appointment requires anesthesia. The tooth is carefully reduced to give adequate reduction for the crown (cap). Reduction amounts vary depending on the type of crown and materials to be used. Next an impression of the reduced tooth is made, a bite registration is taken,  tooth shade is selected and last a temporary crown is fabricated and cemented to the tooth for function, protection and to preserve the tooth's location.

The second appointment is to try in and cement the permanent crown. Anesthesia is generally not required, and is preferably not used to facilitate patient feed back about how the bite feels. This appointment generally is scheduled for 45 minutes.

What are crowns made of?

 Today's dental materials are remarkable and have lead to many different materials for today's crown and bridge work.

• Gold crown: generally this is cast from yellow gold but can be silver in color depending on the alloys properties. Gold crowns tend to fit the best and have the best wear characteristics on the opposing teeth much like natural tooth structure.
• Porcelain fused to metal: this is perhaps the most common form, and is used where aesthetics and durability are required. PFM crowns give you good fit and esthetics, the porcelain can be abrasive.
• Porcelain: this crown has no metal substructure and therefore is very esthetic. With today's materials their strength is approaching PFM crowns. Today's porcelain crown materials have improved abrasive properties but tend to be harder then tooth structure.
• Composite with porcelain fillers: This is the latest trend in dentistry and is improving all the time. The advantage is the abrasive properties of the material is reduced giving better wear of the opposing tooth.

How long do crowns last?

This is a complex question due to the many factors involved. One thing to remember is that crowns are a mechanical device and will need replaced at some time. Here are some factors when considering the longevity of a crown.

• The amount of tooth structure remaining to retain the crown. The less tooth the worse. This is a consideration when selecting the type of crown, some types of crowns require more tooth reduction/removal then others.
• The oral hygiene of the patient. If a patient has poor oral hygiene the prognosis is poor and in some cases crowns should not even be considered.
• The oral habits of the patient. Is the patient an ice chewier, nail biter, bruxer (tooth grinder), clincher.
• The patients skeletal pattern (jaw relationship). Some patients can generate incredible amounts of  biting (compressive forces) increasing the likelihood for porcelain fracture.
• The quality of the tooth preparation.
•  The average life span of a crown falls between ten to sixteen years. I seen crowns that have lasted 40+ and others that fail in less then ten years.

The thing to remember is why the crown is indicated. The benefits outweigh the risk.
 

 

BRIDGE's

A bridge is a way to replace missing teeth by crowning the teeth adjacent to where the missing teeth are. The missing teeth are replaced with something called a pontic and the crowns connect the pontic(s). This is all one cemented unit and therefore is a fixed non removable restoration. Bridges are indicated where teeth are missing and the adjacent teeth are in need of  crowns to strengthen them. Appointment times are slightly more then crowns since more work is involved but consist of basically the same procedures.