Prosthodontics is a discipline in dentistry related to the use of prostheses to replace missing teeth and restore badly broken down teeth. Indirect restorative materials such as crowns and onlays are also used to restore badly broken down teeth. Missing teeth are replaced with full or partial dentures. Dental implants are used to replace teeth, either single teeth or multiple teeth on a bridge prosthesis.

Why should missing teeth be replaced?

Not all lost teeth need replacing. Wisdom teeth for instance don’t need replacing unless this happens to be the only molar left in that area. When teeth are removed the bone in the jaw starts to break down.  This can cause problems for the adjacent teeth because the common bone wall which is shared between the teeth recedes and leads to some loss of support for the remaining tooth.

When teeth are left unsupported by other neighboring teeth, they tend to tilt and move. This can lead to the tooth not meeting the opposing tooth  properly and over time can lead to chipping and breaking of the tooth. When a tooth is on an acute angle the surface facing the space tends to be difficult to clean. This often leads to gum disease and in some cases dental decay.

The lack of chewing units or teeth can mean overloading other teeth. These teeth break down from the increased work load on the tooth. This can be a problem when multiple teeth in one quadrant are missing. The pressure on teeth are not unlike stumps supporting the floor of a house. If there are insufficient stumps for a particular load then the existing stumps will be burdened with more forces. In the case of teeth, cracks and fractures will occur over time. The speed at which this happens is very much dependant on how hard you chew, what you chew and if you grind your teeth at night or during the day.

Why are crowns, onlays and dental implants costly?

Indirect restorations are costly because they have a laboratory component involved in the fabrication of the prosthesis. A dental technician who is an expert at fabricating these prostheses is involved with manufacturing these. The materials used to make the prostheses are costly and so are the technology used in making these prostheses.

The clinical knowledge and skill necessary to prepare the tooth or in the case of dental implants, the implant base, is substantially greater. The process involved to provide an indirect restoration is exacting and time consuming.

Dental ImplantsGenerally, crowns on natural teeth can cost anywhere from $1400-$2400 per crown. Further costs may be incurred if the tooth needs to be built up for a firm base with other materials prior to the crown being fitted. This treatment often involves 2 or more appointments if a laboratory is involved. However if CAD CAM is used the crown may be fabricated onsite within an hour. Dental implants can cost $4500-$6000 per tooth which includes the placement of the dental implant in the bone and taking impressions for the crown to seat on the implant fixture. Sometimes the bone may need to be bulked up with a graft material. This is an added step which will often lead to more cost. The cost for veneers start from $600  and up to $2000 or more per tooth. The variation to the cost depends on the case and the type of material used to make the veneer.

The costs to provide an indirect prosthesis depends very much on the material type used ie, gold or full ceramic and also the case on presentation. Some individuals need more clinical time to achieve the same high quality result compared to others. And therefore for the same procedure it will cost more. The best way to determine the exact cost is to have a consultation prior to the procedure.

How long should these restorations last?

This is a very difficult question to answer. The duration any restoration will last in any given mouth depends very much on the person. Factors which impact on the prostheses include:

1)    Oral hygiene. Keeping the surrounding teeth and prosthesis clean is very important.

2)   Grinding of teeth. If natural teeth can break from grinding of teeth then man made materials don’t stand much of a chance in outlasting natural teeth under such conditions.

3)   Individuals bite. The load placed on the prostheses depends on the position and location of the tooth. If the tooth is crooked the forces placed on the prostheses may be exaggerated on specific points of the prostheses. This can lead to material fatigue and early failure.

4)   What you eat and how hard you chew. Some individuals have a stronger bite than others. What you chew and the frequency you chew on the particular foods will also impact on the longevity of the prostheses. Some like to chew on ice cubes or crushed ice. This is terribly damaging to natural teeth and will also damage dental crowns and bridges. An acidic diet can also impact on the surface of the prostheses.

5)   Oral habits. Chewing on hard items or using teeth as tools for cutting wire, string, etc will damage dental prostheses. Use scissors or pliers instead of your teeth for such heavy duty activities.

6)   Material properties. Depending on the material used to make the prostheses the longevity of the prostheses can vary considerably. Gold is very tough and durable and will outlast most other materials. It is also very gentle on the opposing teeth as it does not wear the opposing tooth down like roughened ceramic. Every material on the planet has a breaking point. When pushed to their limits all will fail. Porcelain or ceramic cracks naturally under function when placed in a moist environment. This is an inherent property of the material and unfortunately the mouth is warm and moist so eventually the ceramic crown will fail from cracking or fracturing.

7)   Condition of the tooth. If the tooth is very broken down prior to the crown being placed or if it has had root canal treatment the tooth may fail earlier than the prosthesis.This has nothing to do with the procedure but the weak tooth structure to begin with.

8)  Medical condition and lifestyle. Smoking, uncontrolled gum disease and diabetes lowers the longevity of dental implants. This is from the bone not being able to maintain it’s position around the dental implant. As a result the bone breaks down leading to loss of the dental implant.

Essentially nothing in the mouth last forever. Even natural teeth don’t last forever without some level of breakdown. But unfortunately we have not come up with a material like natural enamel and dentine (this is what teeth are made of ). So if I had to give you a figure I would say that most crowns and bridges engineered well should last 6-10 years at the very least. This figure will change depending on the many factors already discussed above.

Missing Teeth

When teeth are removed or in some cases do not develop, aesthetics and function are compromised. The options available to the individual to replace these missing teeth include the use of prostheses which are either removable or fixed in the mouth. The choice to have one or the other is mainly dependent on costs as the latter is more expensive than the former.

  1. Removable prostheses: Full dentures or partial dentures.
  2. Fixed prostheses: Bridges supported by natural teeth or implant fixtures in bone.

The benefits of having something fixed are they feel and function better, especially when the teeth being replaced are at the front of the mouth. Speech is not impaired unlike when dentures are used. Also chewing is much more efficient and comfortable with the use of fixed prostheses.

What are bridges?

These are prostheses which are attached to two structures, natural teeth or implant fixtures, adjacent to the area of missing teeth. The artificial tooth or teeth replacing the missing structures are suspended and supported by the anchoring structures on either side of the gap by crowns. The crowns are often cemented onto the anchors but in cases of implant fixtures they can be screwed in place.

Unlike bridges, partial dentures which replace some missing teeth, can be removed by the individual to clean the teeth and denture. These prostheses are also supported and retained by the adjacent teeth to the gap where teeth are missing.

This is a 3 unit bridge. (Courtesy of East Hamilton dental)

When all teeth in the mouth are missing the only option is to have a full denture. However with the use of dental implants, fixed structures can be used instead of the conventional dentures. In this instance, the teeth are supported by the implant fixtures in bone by a metal structure and either ceramic or acrylic resin (denture material) is used to mimic the gums and teeth. The ceramic or acrylic resin material is baked onto the metal structure in the dental laboratory. Function and comfort is better with the fixed option but also the most expensive. They are also very costly to maintain over the long term.

Another option available to the edentulous person is the use of conventional dentures but supported by a few implant fixtures. This is a removable option which allows the patient to remove the prosthesis to clean. It is more comfortable than a conventional denture which only relies on suction to the gums for retention and support. The dental implants act to secure the prosthesis from moving when chewing.

In the upper arch the dental implants will allow for a denture which does not need to cover the whole palate. This adds to the comfort of a dental prosthesis as there is less plastic covering the palate.

What are dental implants?

Dental implants are titanium alloy screws which are placed in the jaw bone to support a framework upon which either a tooth or bridge or denture is placed. The dental implant fixture sitting in bone needs to fuse to bone before it can be loaded. This often takes 2 or more months .

A minor surgical procedure is performed in order to allow for the fixture to be placed in bone. First the tooth is removed and the bone allowed to heal. This will usually take 1 month. After 2 months the bone is exposed and drilled to create a space for the implant fixture. A healing cap is placed over the fixture head and the tissues allowed to heal. Once the fixture has fused to bone an impression is taken and the prosthesis is fabricated by the dental technician.

The final prosthesis is secured to the implant fixture by a screw.

Implant Abutment
Picture of a implant abutment and implant on a acrylic model.
Implant Fixture
View of the same model with the implant to the right and the abutment to the left.
Crown
This shows the abutment covered with a acrylic crown on the acrylic model.

Tooth Support

When teeth breakdown and large amounts of tooth structure go missing, direct restoratives cannot reliably restore these teeth over the long term. In such cases crowns or onlays are used instead.

What are crowns?

These are caps which are cemented onto the remaining tooth after the tooth has been cut back by the requisite amount to house the crown. In most cases the missing tooth structure needs to be built up with a material which will act as a base to support the crown. After an impression is taken of the remaining tooth stump it is sent to the dental laboratory for crown fabrication.

There are two main types of materials which can be used to make a crown.

Zirconia crown
A Zirconia crown.
  1. Metal alloys: Gold or non-precious metal.
  2. Tooth coloured: Composite resin, ceramic, zirconia.

The gold material is the most reliable and long lasting material available. It is kind to the opposing tooth when in function and will last a very long time. However it is not very aesthetic. The other great benefit of the gold material is only minimal tooth structure needs removal as the material is strong in thin sections. This will protect the tooth from fracturing as more tooth structure is left behind.

Zirconia crown
Zirconia crown on the stone model.

The ceramic based materials are good for their aesthetics but more importantly they can be bonded to the tooth structure. This allows these materials to be well secured on the underlying tooth and will afford more support to the remaining tooth because of the bonding technology available.

A “new kid in town” is the zirconia material. This is very strong and has been given the title of ‘white steel’. Its strength allows the crown to be thin and therefore, like gold, can be conservative on tooth structure. Unlike gold the added advantage of this material is its aesthetic appeal. The newer products are very tooth like in appearance. The down side to this material is it is relatively new and we don’t have much long term data on its use as yet.

 

What are onlays?

Onlays are part crowns. They cover the top portion of the tooth in question and will support the tooth by bonding to the remaining tooth structure. This is a very good restorative option as, unlike crowns, only replaces lost tooth structure. Hence they are very conservative on tooth structure as the tooth does not need to be cut down too much.

Most onlays today are ceramic based as they bond reliably to the remaining tooth structure but don’t last as long as crowns or gold onlays. A good gold onlay will last for many years before they come off as most of the retentive features are reliant on the shape of the tooth cavity rather than the cement.

The use of an onlay instead of a crown really comes down to the amount of tooth structure left. The more tooth structure we keep the better it is for the tooth. There is less chance of the pulp dying and the tooth is more resistant to fracturing. Onlays are used in preference to crowns when the compromise is accepted that the onlay will have a lower longevity in function compared to the crown.

Onlay
A ceramic onlay.
Onlay on ceramic
The same ceramic onlay on the stone model. Note the onlay only covering the chewing surface.

Another form of an onlay is a veneer.  A veneer is predominantly used on the front teeth when the appearances of teeth need to be improved or shortened teeth need to be lengthened. This cosmetic procedure involves removing some tooth structure to allow space for the veneer material. The veneer is made of porcelain material and is bonded to the tooth.

The composite resin material can also be used to veneer a tooth or teeth. It tends to be less robust as the porcelain material but if chipped or minimally broken can be repaired reliably and at a minimum cost.

All these options are available to patients at ProCare Family Dental. As is our practice, we will be in close consultation with you when the decision is being made with respect to which option best suit your needs.