FIXED PROSTHETICS
GENERALITY'
The generic term of fixed prostheses refers to medical devices that replace natural teeth that are inserted into the mouth permanently and that, in other words, cannot be removed and re-inserted at will.
These devices need anchor points on which to be "glued" and these points can be made up of teeth or implants. If the fixing point is a natural tooth of the patient, its shape must be modified in such a way as to be able to accommodate the prosthesis element to be inserted. It is said in a simplified way that the tooth is "filed". The filing consists roughly in a reduction of the tooth from all sides, and tends to create what is called a "pillar" of prostheses.
The fixed prosthesis, in whatever material it is made, is nothing more than the missing dental volume, inserted to replace the parts to be replaced.
THE FILING
This operative act aims at the reduction of abutment teeth to constitute what in prosthesis is called "abutment". Basically, the tooth is reduced in size from all sides, keeping only the central part: this roughly corresponds to the removal of a thickness equal to the tooth enamel or a little more. Along the base diameter of the abutment, what will be the edge of the fixed prosthesis is outlined. This edge is sculpted by the burs quite close to the gingival margin, or in some cases even slightly below, in order to aesthetically hide the future prosthetic edge.
The method of milling the edge and the shape given to it are linked to variable prosthetic theories, which the operator chooses according to preferences, the materials he will use, or the specific situation. The most commonly used shape is the so-called "chamfer" or "rounded shoulder", which consists of a kind of support curb carved at the base of the tooth, having as depth the measure necessary to contain the material of the prosthesis, without it protruding from the general profile of the resulting veneered tooth. An alternative is the so-called "knife blade" or "finishing preparation", where the finishing line is marked by the dentist with a change in the inclination of the filings. This second hypothesis is more used in particularly long or elongated teeth following periodontal surgery, because it is more conservative for the tooth.
Whatever the finish of the filing, this line must be clear and clear, to allow the dental laboratory to seal the abutment according to a clear and defined line. This will ensure the reduced susceptibility of the tooth to caries infiltration.
It should be noted that filing still constitutes damage to the tooth, and should only be performed if there are no alternative solutions. In general, every dental procedure must be weighed according to a criterion of biological cost / final benefit. Some vital teeth may require devitalization following filing, due to an excessive and uncontrollable sensitivity: for this reason the dentist always works with the utmost caution and circumspection.
3D DIGITAL SCANNING
The traditional physical silicone impression is now supplanted by a revolutionary technique: 3D digital scanning. The methodology is so new and interesting that we have dedicated a special page of this section to it, which can be consulted by clicking on the image ---------------->
THE PHYSICAL ("ANALOG") FOOTPRINT
This operation allows the dentist to detect in the mouth and transfer the work done on the tooth after its transformation into an abutment on a laboratory model.
The fixed prosthesis, whatever it is, is manufactured in a laboratory by a dental technician, who works on a reproduction of the patient's original mouth made in plaster cast in the impression taken by the dentist directly on the patient.
a) THE SPOON
This device is in practice a usually metallic tray with a roughly arched shape and available in various sizes which must contain the impression paste and lead it into the mouth. There are perforated or solid ones, depending on the use made and the material or the preferred impression technique.
In particular cases, you may prefer the construction of a so-called "individual" spoon which will be custom made in the laboratory and is usually made of resin.
b) THE FOOTPRINT PASTE
In fixed prostheses, two types of impression paste are mainly used for the relief of the main cast: silicone, or polyether. Both systems can be used with a single or double impression technique.
The professional chooses the preferred material based on his experience and on the basis of good agreement with his reference laboratory, to optimize the various steps and reduce the margin of error.
c) THE RETRACTION THREAD
It is a small fiber that is inserted between the tooth and the gum that surrounds it to the collar, and which serves to temporarily remove the gum from the stump, widening the natural gingival sulcus. This allows the impression paste to fit into the space thus created, clearly detecting the prosthetic preparation edges. It can be used in single or double turns: in the first case the wire is removed just before inserting the impression tray, in the second case one of the two wires could remain in place even during the impression and be removed afterwards.
d) THE TECHNIQUE
It simply consists in inserting the spoon loaded with impression paste in the mouth and dipping the dental arch with it. Holding it appropriately, the dentist waits for the material to harden and then carefully removes the spoon from his mouth. A silicone or a polyether have a hardening time that varies between 2 and 5 minutes, depending on the consistency and the temperature.
At this stage, especially if it is an upper aracata imprint, the patient may have the unpleasant sensation of the gag reflex. The dentist will recommend that you breathe calmly through the nose and can sometimes help the patient by placing a cotton swab dipped in alcohol under the nose.
THE CONSTRUCTION IN THE LABORATORY
Whatever the technique used to detect the teeth to be prostheticed, the preparation of the prosthetic margin and also what dentists call "the beyond fine preparation", that is the part of the gingival sulcus that had been opened with the retraction cord: this is the best guarantee to ensure that the prosthesis seals the edges properly.
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In the case of a physical silicone impression, this reaches the dental laboratory where it is filled with plaster. After hardening, the technician obtains a plaster copy of the patient's mouth which logically also includes the filed teeth and all related details.
The next steps would be too complex to describe. Suffice it to say that the technician builds the supporting structure in wax and then melts it in metal with the lost wax technique also used in jewelry, if it is a ceramic metal crown.
Once the structure has been obtained, it moves on to the layering of the ceramic, through various stages of firing up to the final glazing which contains the last touches of aesthetic finishing.
Finally, once the chewing parameters have been checked, the laboratory delivers the prosthesis to the dental office for the trial or cementation phase.
THE MATERIALS
ZIRCONIA OR INTEGRAL CERAMIC
It is the most advanced solution in existence: the tooth is in fact covered with a coating that has no metal parts. The entire mass of the crown is made up of an industrial-grade ceramic with exceptional mechanical resistance characteristics. It has now taken hold as a material of choice for posterior teeth in its simplest form of full zirconia. For aesthetic applications such as in anterior teeth, a zirconia cap is fabricated which is then coated with porcelain, to give the restoration a greater translucency.
METAL CERAMIC
It is still a very popular solution, especially due to its high reliability. It consists of a metal cap in steel or gold alloy that completely covers the dental abutment sealing the edges of the filing line, and a total aesthetic veneer in dental ceramic (porcelain). It offers maximum aesthetics combined with proven robustness. In its milled metal form, it remains a great solution for joining multiple teeth in longer three-piece bridges.
GOLD RESIN
It is a less used solution nowadays, but it retains some advantages. The covering of the tooth is entrusted to a gold alloy coating, while the aesthetics of the artifact is found only in the part exposed and visible from the outside (the veneer). This aesthetic is achieved with a plastic (resin) coating. This solution has the advantage that it offers entirely metal chewing surfaces, and therefore more resistant to fracture. On the other hand, the aesthetic part can sometimes peel off and is subject to color deterioration over time.
ARMED RESIN
It is a solution that includes a metal cap entirely coated in resin. In appearance it may resemble the ceramic crown, being entirely white, although it does not have the brilliance and depth of color of porcelain. Being an economical solution, it is sometimes preferred in all those cases in which the support tooth is not completely reliable and, despite fearing for the stability of its root in the mouth, one does not want to sacrifice the tooth.
Another reason for adopting this crown is that the resin is a more elastic and soft material in chewing and this can be advantageous in all those cases where you do not want to create too rigid surfaces.
THE CROWN (CAPSULE)
This term indicates the simplest of fixed prostheses that completely cover a tooth. The indications for the execution of a crown are many:
a) The structure of the tooth is too compromised in a mechanical sense, due to a very extensive caries.
b) The tooth has broken due to the mechanical stresses of chewing and can no longer be reconstructed with alternative techniques.
c) Often (but not always) a devitalized tooth requires a covering with a crown, because it is very compromised from a mechanical and static point of view, and also because its residual natural structure is usually dehydrated due to the lack of pulp. The consequence is that the tooth is more prone to fracture.
THE BRIDGE
When despite every effort to maintain it, a tooth is eventually lost, there is an empty space in the mouth in the position it previously occupied. This space constitutes a breach.
The gap can actually be more or less large, because the missing teeth can be more than one, and one next to the other. Certainly there are nowadays extremely refined ways of replacing lost teeth (implants), however in many cases, especially where implantation is not possible, more traditional prosthetic solutions are applied: bridges.
A bridge is a fixed prosthetic structure that replaces one or more missing teeth, leaning on at least two or more bridge pillars.
The pillars are nothing more than teeth reduced to abutments (filed), which will in turn be covered by the structure and connected together in order to fill the gap.
Obviously, the necessary condition for making a bridge is the presence of at least one pillar tooth before the breach and of at least one tooth after it. The number of pillars involved, i.e. of teeth that will have to be filed, ultimately depends on the mechanical condition of the mouth:
gap width (number of teeth to be replaced)
quality and firmness of the roots of the abutment teeth
Once the design of the bridge has been established, we proceed with the filing and subsequent construction phases, more or less the same as for the crowns, except that the bridge generally needs a series of additional checks concerning the structure (corrected positioning, chewing, etc.)
