The descriptions of various methods of making a crown using different materials are made based on the assumption that qualified, skilled technicians use the latest technology and microscopes. This ensures that, functionally, the crown being made has a perfect fit to the model and works well within all movements of the lower jaw in relation to the upper jaws on an articulator (a jaw simulator).
The dentist needs to be skilled and take very good impressions with special emphasis and time put aside to ensure that the impressions are accurate and clear. This can be quite a lengthy process.
The external part of a crown (the visible part) can be made of the following materials:-
· They can be made of composite. This is resin with porcelain suspended in it (see Composite).
· Crowns can be made of acrylic (which are usually temporary crowns/caps).
· They can be made completely of yellow gold which apart from aesthetics can last much longer than porcelain as it wears at the same rate as natural teeth. This does mean that the opposing teeth can wear holes in the gold. Gold crowns can be in the mouth longer than any other crown on average.
· Crowns can be made of porcelain (the best way to describe porcelain is to imagine trying to create a hologram inside a clear marble of a spiral in different colours that is 22% bigger than you want it to be: after firing it at 1000 degrees celsius it shrinks by 22%. This gives you a shrunken, cracked, rough tooth replica that needs a lot of work recontouring and adding to eventually give you the crown finished and polished.
· A really well made, full colour porcelain tooth is bespoke and hand-made. It can take up to a day to make. Porcelain is harder and stronger, but if not finished and polished properly, can act like 'sandpaper' to the opposing teeth in the mouth. This could wear them down over a long period of time but porcelain is strong in compressive strength and weak in tensile strength and so needs to be supported by some sort of structure to balance these forces.
· Some porcelain crowns don't have a sub-structure and due to this are inherently weaker. Over 2mm of thickness renders porcelain weak and under 2mm makes it strong e.g., veneers, as they are thin. These do allow the light through with minimal refraction indices, looking fantastic on the right dentine colour (see Veneers).
Tooth-coloured crowns are supported by a sub-structure for strength and accuracy of fit which can be made of metal, all-porcelain, all-ceramic and even glass fibre etc. Some sub-structures do not let light through:-
· Yellow gold sub-structures are much more biocompatible but more expensive. They give a warmer, more vital colour to porcelain teeth than other metals. The use of metals in the mouth can prove to be an advantage in cases where dead discoloured tooth stumps can be masked by metal, making the crowns look much better than they would, had they been all-ceramic.
· Palladium metal is a good substitute and is sometimes needed as it is stronger for long bridges but this is also expensive. It is important to understand that bridges can be more expensive as they need extra soldering to join the metal parts together. This, combined with normal manufacture of bridges results in a much more time-consuming process than a single crown.
· Titanium is the latest metal used that is very light, very strong, very biocompatible but can give a grey halo to the teeth, as it doesnt emit a warm colour in processing. It is very expensive to manufacture.
· The last metal alternative, being Chromium nickel-based alloys, may contain elements of nickel which can create an allergic reaction in patients. This is cheap and can be used a lot for this reason.
· Thin gold-plated thimble providing support and good colour characteristics to the porcelain. The porcelain is weaker as the metal is one thickness and does not compensate in areas where more support is needed. Popularity varies from one country to the next (Captek).
Some sub-structures do let light through to the root stump:-
Ceramic sub-structures are the latest technologies and are the future. These include materials like Zirconia and Alumina which are both very strong and let the light through. When a crown is made using these materials, they are much more realistic than metal-supported crowns, which do not transmit light. Disadvantages are that they can be quite white, which is a problem that can be overcome by a skilled dental laboratory.
These are biocompatible and in many cases show good tissue regeneration after the crowns are cemented. These are more expensive than metal sub-structures as manufacturing processes outside of laboratory control dictate costs.
