Definitions and Descriptions

With the generic term of fixed prosthetics, we indicate medical devices substituting natural teeth, permanently inserted into the mouth and which, in other words, cannot be removed and reinserted at pleasure. These devices need some anchoring points on which to be "glued" and these can be teeth or implants. If the fixing point is a patient's natural tooth, its shape must be modified in order to receive the prosthetic element to be inserted. In a simple way, it's common talk to say that the tooth must be "filed".

The filing of the tooth consists roughly in a reduction of its volume from all sides, aiming to create what is called a prosthetic "pillar". Regardless of the materials used to do so, fixed prosthetics is nothing other than the replacment of the lacking dental volume inserted as a sobstitution of the missing parts.

This operative act aims at the reduction of pillar teeth in order to establish what in prosthetics is called a "stump". Practically, the tooth is reduced from all sides only preserving the central part: this roughly correspond to the subtraction of a layer as thick as the enamel or little more. Along the baseline diameter of the stump, the future edge of the fixed prosthetics is designed.
This edge is sculpted by the burs very near to the gingival line or in some cases even slightly below it, to hide the future border of the manufact. The filing technique and the shape that is given to the border are bound to various prosthetic theories that the operator chooses according to his preferences, and the materials that are to be used or the specific situation.
The form generally used is the "chamfer" or "rounded shoulder", consisting in a sort of resting curb carved at the tooth's base and having the appropriate depth as to hold the prosthetic material without it thrusting out of the finished and covered prosthetic tooth's profile shape.
An alternative is the so called "knife blade" or "thinning finish", in which the finishing line is drawn by the dentist with a change of inclination of the filing direction. This second solution is particularly used in very long teeth or in teeth that have been elongated after periodontal surgery, mainly because it's a more conservative technique for the tooth. Whatever the finishing style of the filing, the line has to be clear and neat to allow the dental lab to seal the stump following a well defined and clean line. This will assure a reduced susceptibility of the tooth to caries.
It must be warned that the filing is always a damage for the tooth and it must be executed only if there are no alternatives. In general, every dental procedure must be weighed following a final biolgical cost/benefit criteria. Some vital teeth may require the devitalization after having been filed, due to an uncontrollable sensitivity outcome: for this reason, the dentist always operates with the maximum of caution.

This operation allows the dentist to make a record of the mouth and to transfer on a lab model the work that's been done on the tooth after the filing.

As a matter of facts, a fixed prosthetics is built in a lab by a technician who is working on a patient's mouth copy made from chalk cast in the silicon impression taken by the dentist.

A) The tray

This is usually a metal container, grossly shaped like an arch and designed in various sizes to contain the impression paste and carry it into the mouth. They can have holes or not, depending on preferences in technique or on the impression materials used.

In special cases, custom trays can be built to better fit the patients' anatomy: in this case they are made of resin.

B) The impression material

In fixed prosthetics, mainly two kinds of impression materials are used: silicone (polivynilsiloxane) and poliether. Both can be used with a single or with a double impression technique.

The professional chooses the preferred material from his experience and according to the agreements taken with the lab, to optimize the various steps and to reduce the error margin.

C) The retraction thread

It's a small fiber that is introduced between the tooth and the gum surrounding it at the collar and its aim is to keep temporarily the gum displaced from the pillar, enlarging the gingival sulcus and allowing the impression material to penetrate in the thus created space, neatly recording the prosthetic filing (preparation) that has been done. A single or a double thread can be used: in the first case it must be removed a moment before introducing the impression tray, in the second case one of the two threads may stay in place during the impression and be removed only afterwards.

D) The technique

It simply consists in introducing the tray loaded with impression material into the mouth and submerge the dental arch completely. Holding the tray appropriately, the dentist waits for the material to harden and then carefully lifts it out of the mouth. A silicon or a poliether approximately have a hardening time variable between 2 and 5 minutes, depending on consistency and temperature.

During this phase, especially if taking an impression of the upper denture, the patient may experience the uneasy sensation of vomit. The dentist will suggest to calmly breathe through the nose and sometimes will help the patient by putting an alcohol soaked cotton wad under the nose.

This is the phase that the patient doesn't see because it doesn't involve him in a physical and direct way.

The impression recorded by the dentist reaches the dental lab where it is filled with chalk. when the hardening is complete, the technician obtains a solid chalk copy of the patient's mouth, obviously including the filed teeth and all the connected details.

In particular the most clearly visible part must be the prosthetic margin and the part that clinicial call the "beyond preparation's end", the part of the root that had been covered by the retraction thread: this is the best guarantee for the prosthesis to appropriately seal the borders.

The phases that follow would be too complex to be described. It suffices to know that the technician builds the supportive structure with wax and then casts it with metal, using the "lost wax" technique also used for jewellery, in the case of a metal-porcelain manufact.

Once the structure is obtained, he does the porcelain stratification, passing through various cooking steps, the last one of which being the glasur that contains the finishing and the final aesthetic touches.

Finally, after controlling the mastication parameters, the lab sends the job to the dentist for a test or for the cementation.

In the side pictures, an integral porcelain crown (with no metal parts)

METAL SUPPORTED PORCELAIN It's by far the most common and used solution and it consists in a metal cap, made of a special steel or gold alloy completely covering the filed tooth pillar, sealing it along the finishing line and an aesthetic porcelain coating. It offers the maximum aesthetic result together with a proved structure reliability.

INTEGRAL PORCELAIN Aesthetically it is by far the most performing solution: the tooth is covered with a full porcelain coating which has no metal parts. The whole mass of the crown is made of ceramic material. Since it is a relatively recent technology, the dentist applies carefully in the most indicated cases, after a precise analyisis of all the chewing parameters.

GOLD - ACRYLIC RESIN Nowadays it's a less used solution, but still bearing some advantages. The covering of the tooth is done by a golden alloy while the easthetic part of the manufact is only located in the exposed and externally visible part (facet). This effect is obtained by means of a resin (acrylic or composite) covering the outsdie facing part. This solution has the advantage of offering only metal part to be exposed to biting and chewing and is therefore more fracture resistant. On the other side, the resin part is subject to chip off or to slow pigmentation or colour degeneration in time.

ARMED RESIN In this solution, a metal cap is completely coated with a resin instead of a porcelain. It looks like a porcelain at a first glance being totally white nonetheless it does not have the same brilliancy and depth of colour that a ceramic material has. Being it a non expensive solution, it's mostly adopted in all of those cases where the supporting tooth is not totally reliable and even if there are doubts about its root stability, the tooth must not be sacrified. Another reason for adopting this solution is that resin is a softer surface when biting and this can be an advantage in all those cases where this kind of chewing surface is desirable.

With the word "crown", the most simple fixed prosthesis fully covering one tooth is meant. Indications to execute a crown can be many:
a) The tooth's structure is mechanically too compromised because of a caries. b) The tooth cracked under the mechanical stresses induced by chewing and is no more reconstructable with any alternative method.
c) Very often (but not always), a devitalized tooth requires to be covered with a crown because it's very compromised from a mechanical and static point of view and also because the remaining structure is dehydratated for the lacking of the dental pulp.
As a consequence, the tooth is more exposed to the risk of fracture. The crown is nothing other than the covering of the tooth stump with the chosen structure and materials. It represents the restoring of the original situation by means of the substitution of the missing volume, giving back aesthetics and function. The crown only applies for a single tooth.

When, despite every effort to keep a tooth in the mouth, it unfortunately gets lost, a leak in the denture appears in the position where the tooth itself used to be. This space is called a breach. The breach can be more or less wide because the missing teeth can be more than one and one after the other.

Of course nowadays very refined methods of replacing missing teeth are not scarce (implants), however in many cases, in particular where implants cannot be done, more traditional prosthetic solutions come to stage: bridges.

A bridge is a prosthetic structure replacing one or more teeth and resting over two or more pillars. Pillars are filed teeth which are thus reduced to be stumps and will be covered by the structure and connected to fill the breach.

Obviously the necessary condition to build a bridge is the presence of at least two pillar teeth, one bfore the breach and one after it. The number of teeth to be involved to become pillars depends in the end by the mouth's mechanical conditions:

  • width of the breach (number of teeth to be replaced)
  • quality and solidity of the roots of the pillar teeth

  • Once the bridge's project is done, the filing follows and the construction phases, more or less as for the crowns, except for the fact that the bridge generally needs a series of supplementary controls regarding the structure (fitting, biting etc.)