Types of Crowns
There are many different methods of crown fabrication, each using a different material. Some methods are quite similar, and utilize either very similar or identical materials.
Full Gold Crowns
Full gold crowns (FGCs) consist entirely of a single piece of alloy. Although referred to as a gold crown, this type of crown is actually comprised of many different types of elements, including but not limited to gold, platinum, palladium, silver, copper and tin. The first three elements listed are noble metals, while the last three listed are base metals. Full gold crowns are of better quality when they are high in noble content. According to the American Dental Association, full gold crown alloys can only be labeled as high noble when they contain at least 75% noble metal.
Full gold crowns are cast metal restorations that are made using the lost-wax technique. After the dentist prepares a tooth for a crown, he or she will take an impression of the prepared tooth, the adjacent teeth in the same arch and the opposing teeth in the opposing arch. With all of the necessary boundaries of the future cast crown defined in three dimensions within the impression material (i.e. the necessary height, width and depth of the crown is now recorded in impression material), the impression(s) are sent to a dental laboratory where they will be poured up in various types of dental stone or plaster. After the stone models are formed, they are ditched, died and articulated so that the laboratory technician can see how the two arches meet and properly access the tooth replicates to perform his tasks. The lab technician will then apply wax to the die (analog of the prepared tooth) and manipulate and craft the wax until he or she has built it up into what appears like and conforms to the specific dimensions of the tooth being restored. Prior to applying the wax, though, a die spacer is applied to the die. This is a thin coat of material that is painted onto the die to provide a space between the gold crown and the actual tooth structure to be filled with cement upon final cementation. A lubricant is also applied so that the wax pattern, as the wax-up of the crown is referred to, can be easily removed when completed.
The wax pattern, is removed from the die and invested in a sort of plaster while connected to a short plastic stick, called a "sprue former", which will stick out of the investing plaster. The investment, as it is called now, is placed in a furnace, which will completely burn off the wax and plastic that formed the wax pattern/sprue complex. What is left is a hollow within the investment material, known as an "investment pattern". Because the sprue former stuck out a little bit from the investment material, there is a communication between the outside and the investment pattern. The investment pattern is then placed in a sort of simple centrifuge where pennyweights of gold are melted down and rapidly shot through the communication in the investment pattern, through the sprue that was formed by the sprue former, and into the hollow that used to be inhabited by the wax pattern of the crown waxed-up by the technician, thus called the lost-wax technique. After properly cooling, the single piece crown-and-sprue of gold is sectioned, and the sprue can be recycled in another casting. The crown is touched-up in the location of the sprue attachment, finished and polished to a high shine, and delivered to the dentist so that he or she can try it in the mouth, make certain it has all of the proper contacts with the adjacent and opposing teeth, and cement it to the prepared tooth.
Porcelain-fused-to-metal (PFM) crowns consist of a metal coping that is made with the same lost-wax technique used for a full gold crown, that then is primed for an application of feldspathic porcelain. The coping may conform to the proper dimensions of the restored tooth except for the facial surface of the tooth; it may be a thin shell on the prepared tooth designed to be covered entirely with porcelain; it may also be somewhere in the middle, such as cut back so as to provide space for porcelain only on the facial and occlusal surfaces of the crown.
The alloy used for PFMs is of a different variety for those used for FGCs. This is because the alloy for a PFM must be able to withstand the intense temperatures of the ceramic furnace necessary to properly bake the porcelain without distorting or sagging, which would alter the structural architecture of the metal coping, effectively ruining it.
Sometimes the dentist sees if the coping fits on the tooth prior to having the porcelain applied, while other times, the lab continues to complete the PFM and returns it to the dentist only after applying the porcelain. A coping try-in is more common in situations of multiple attached units or long-span bridges, where there might be a need to section the single-unit coping if the fit does not properly match the prepared teeth.
The strength afforded by cast metal restorations is a tremendous advantage. Whether the restoration is fabricated completely out of metal or has some porcelain affixed to it, the surfaces consisting entirely of metal can be as thin as 0.5 mm, which aids significantly in conservation of tooth structure. When porcelain is to added to the occlusal surfaces of posterior teeth or proximal surfaces of anterior teeth, at least 1 mm must be left for porcelain, in addition to the .5 mm for the coping thickness. The only reason to incorporate porcelain into a restoration is for aesthetics; its involvement otherwise serves only to undermine the strength and endurance of the crown.
Introduced in 1989, In-ceram, by Vita, infused the fragile new "all-ceramic crown" with glass to produce what was then thought to be a superior product.
Procera AllCeram, owned by Nobel Biocare, is a CAD/CAM based method which produces a crown by overlaying a very durable ceramic coping of either alumina or zirconia, referred to as a "core," with Vitadur Alpha porcelain. Introduced in 1991, Procera can now be used to produce crowns, bridges and veneers.
The Empress system is superficially similar to a lost-wax technique in that a hollow investment pattern is made, but the similarities stop there. A specially designed pressure-injected leucite-reinforced ceramic is then pressed into the mold, as though the final all-ceramic restoration has been "cast." The Empress crown can be utilized for anything the lost-wax technique can be used for, in addition to veneers (which would not be made of cast metal).
The CEREC method of fabricating all-ceramic crown is by electronically capturing and storing a photographic image of the prepared tooth and, using computer technology, crafting a 3D restoration design that conforms to all the necessary specifications of the proposed inlay, onlay or single-unit crown; there is no impression. After selecting the proper features and making various decisions on the computerized model, the dentist directs the computer to send the information to a local milling machine. This machine will then use its specially designed diamond burs to mill the crown from a solid ingot of a ceramic of pre-determined shade to match the patient's tooth. After about 20 minutes, the restoration is complete, and the dentist sections it from the remainder of the unmilled ingot and tries it in the mouth. If the restoration fits well, the dentist can cement the restoration immediately. A CEREC machine costs roughly $100,000, with continued purchase of ceramic ingots and milling burs.