Above you see a schematic of a cross section through a fixed bridge and the teeth that it sits on (called abutments). As you can see, the abutment teeth are prepared by “shaving” them down so that they are smaller, and their sides are slightly tapered and reasonably parallel to each other. This makes it possible to create a casting which will fit over the prepared abutment teeth. Usually the casting is a metal substructure which will eventually be covered with tooth-colored porcelain.
Note: Is it absolutely essential to replace all missing teeth? The answer is NO, but there are a number of consequences you may want to know about. Click the icon to the right to find out what they are
Each casting has at least two crown retainers. These are the hollow, tooth shaped “caps” at the ends of the bridge shown above. These retainers are shaped in the form of the original tooth and replace the original anatomy (shape) of the tooth. This does not harm the abutment teeth and most frequently improves their function and appearance since most crowns cover large unsightly fillings with solid tooth colored armor. (See my pages on crown preparations.)
The false teeth between the two abutments are called “pontics“, and these are solidly attached to the retainers on either end. Together with the retainers they form the structure of the fixed bridge. This appliance is cemented in place permanently and should remain in position for many years.
This is a photo of a three unit bridge that replaces a single missing tooth. This picture shows the finished bridge on a plaster model of the teeth that it will eventually fit on. Note that the bridge is covered in a thick layer of tooth colored porcelain. The color of the porcelain was chosen to match the surrounding teeth in the patient’s mouth. The margins of the crown retainers (where the porcelain meets the tooth structure, plainly seen here) are actually cut below the patient’s gum line so they are not seen in the mouth
This image shows the same bridge removed from the plaster model. It shows the shape of the abutment teeth after they have been prepared (the blue paint on the dies is called “die relief paint” and ensures that there is room under the retainers for a layer of cement). It also shows the ridge where the original missing tooth was extracted. In this case, I have taken advantage of the unhealed socket to allow the pontic to extend slightly into it so it will shape the healed ridge. Note that you cannot see any of the metal casting underneath the porcelain.
Here is a view of the same bridge from underneath. You can plainly see the metal casting that actually hugs each abutment tooth. The metal extends between the abutments inside of the pontic as well, but it is completely covered with porcelain for hygienic reasons as well as esthetics. These porcelain to metal structures are extremely strong and durable. It is possible to break the porcelain with a sharp blow from a heavy solid object, but it doesn’t happen very often because the porcelain to metal interface is stronger than the porcelain itself, and this generation of porcelain is twelve times stronger than the porcelain five years ago.
Not all bridges need to be built with a metal substructure. Certain porcelains have such improved characteristics that they are able to withstand the forces in the mouth without a metal framework. They are used mostly to replace front teeth which do not get quite as strenuous a workout as the back chewing teeth. All-porcelain bridges are frequently chosen for short spans between front teeth because of their superior esthetic qualities. Without the metal substructure they transmit light more naturally and are more difficult to distinguish from the natural teeth beside them.
Fixed bridges come in many configurations. The one above is a three unit bridge. The schematic at the head of this page depicts a four unit bridge. One of the most frequent configurations is known as a “roundhouse” restoration. This form goes from upper right canine tooth to the upper left canine tooth and has four pontics. It can replace all the top front teeth. In some instances, when the teeth are extremely crowded and crooked, an alternative to orthodontics would be removal of four of the crooked front teeth and replacement with a roundhouse bridge in which the teeth are made smaller and fit the space correctly.
In some situations, these cases can become so complex that the general dentist might refer the patient to a specialist (called a prosthodontist) in order to fabricate very large bridges with numerous abutment teeth, all of which must be kept parallel in order for the bridge to insert into place correctly. If even one abutment is not parallel to all the others, the bridge might not fit all the way down, leaving the margin of the crown retainers above the level of the margins of the teeth for which they were fabricated. This leads to “open margins” which allow leakage and consequent failure of the bridge.
Dental Implants vs. Fixed bridges
Dental implants are are becoming the tooth replacement of choice in dentistry. They are beginning to replace bridges in Europe and North America. They are titanium “screws” that are inserted into the jawbone in place of an extracted tooth. The operation is fairly easy and takes only about 30 to 60 minutes depending on the number of implants to be placed. The implant is allowed to heal for 3 to 6 months and then the dentist uncovers it and inserts an abutment followed by a crown. Like a bridge, the implant/abutment/crown combination is fixed in place and cannot be removed, but unlike the bridge, there is no need to shave down any adjacent natural teeth. The cost of doing a single implant with abutment and crown is about the same as the cost of doing a three unit fixed bridge. Implants have a number of serious advantages over the fixed bridge, and consequently, implants are becoming the tooth replacement of choice. The success rate of implants is, in fact greater than the success rate for a three unit bridge. Implants also prevent the loss of the bone that used to support the extracted tooth, while bone continues to disappear under a pontic leaving more and more space between the gums and the pontic.