MOBILE PROSTHETICS
GENERALITIES
When for any reason it is no longer possible to replace the patient's teeth with a permanently fixed solution, and therefore there are no suitable teeth available to build a bridge and / or there is a shortage of bone to resort to implantology, there is no another option than the so-called removable prosthesis.
The mobile prosthesis in general is defined as such because it is possible (in fact) to insert and remove it from the patient's mouth. Of whatever type it is, it has a general characteristic: at least part of the teeth it replaces must rest on the gum.
This fact is obviously a biological compromise because the gum is not structured to directly support the chewing load, and it is also the reason why the removable prosthesis is usually considerably more problematic than the fixed one, both in terms of comfort and management by the dentist.
PARTIAL PROSTHESIS
This type of mobile prosthesis presupposes the persistence in the patient's mouth of a sufficient number of natural teeth to allow the mobile part to be anchored. The number and position of these teeth may vary, but in the design of the prosthesis there must be a correct mechanical balance between the abutment teeth and the added part. Often, to offer more support to the mobile part, it is necessary to cover the residual teeth with fixed prostheses, sometimes even joining more than one with a bridge. In this case, the partial prosthesis takes the name of Combined.
A removable prosthesis consists of various parts that must be perfectly matched to achieve the result.
Saddle - This term indicates the most delicate part of the prosthesis, that is, the one that comes into direct contact with the surface of the gum. Although some recent material innovations have been introduced, the vast majority of saddles are made of acrylic resin, a pink colored plastic.
Above it there are also resin teeth that perform the chewing function. More rare is the use of ceramic teeth fixed on the saddle.
Connector or Bar - This component of the mobile prosthesis has the function of connecting the right side with the left. It can be made of acrylic resin or it can be a metal element, such as in the so-called skeletonized prosthesis (also called simply "skeletonized").
This part of the prosthesis is unfortunately always necessary for balancing issues of the artifact. While there are prostheses that can occupy only one side of the mouth, these are rather complicated to build, so the preferred solution is to anchor the teeth on one side with a connector on the opposite side.
Hook - The hook is the most common fastening system of the removable prosthesis on natural teeth. In practice, it consists of a small metal extension that, starting from the saddle or one of the connectors, protrudes towards the external part of the teeth, embracing the external portion of the support tooth. When the prosthesis is inserted, the hook theoretically should make a slight friction, after which, the prosthesis is positioned with a click in the provided seat and the hook offers the necessary retention so that it does not move anymore.
Naturally, since these are flexible metal structures, with the greater frequency of fitting of the prosthesis, they tend to loosen, and the dentist must periodically tighten them again. It is never advisable to attempt this maneuver with improvised means due to the risk of breaking the hook itself.
Attachment - As an alternative to the hook, it is possible to use a more aesthetic solution. In essence, it is a male / female connector that precisely fits the moving part to a structure based on natural teeth. The only way to obtain the fixation is to build one of the two parts of the attachment on one or more teeth: to do this, you usually need a crown or a bridge on which to weld the attachment itself.
TOTAL PROSTHESIS
This type of prosthesis is basically the classic "denture". The patient does not have even a natural tooth available and will not install bone implants, so the only way to give him a set of teeth is to produce an arch completely resting on the gum.
It is clear that at this point, the main problem to be solved will be the tightness of the device. The total prosthesis in fact, completely relies on the so-called "suction effect". The resin base that rests on the gum is built in such a way as to adhere closely to it and its edges follow the course of the mucous parts in order to seal the covered area.
When placing the prosthesis, the air is compressed outside the covered area creating a vacuum effect, and the prosthesis stabilizes in place.
There are many factors that influence this simple principle. First of all, the gum does not have the same consistency in all its parts, so more elastic areas can reject the prosthetic support and unfortunately cause the artifact to become somewhat unstable.
It is understandable that at least in principle the upper arch prosthesis will be more easily stable, because it has a greater support area at its disposal, while the lower one must be satisfied with turning around the tongue. Furthermore, the language, with its movements, is a further source of instability.
MAINTENANCE
A periodic check, at least once a year, of any type of removable prosthesis is practically a necessity. The bone underneath the resin saddles tends to be reabsorbed due to the continuous chewing pressure, so an excessive space is slowly created between the saddle itself and its support surface.
In this situation, the prosthesis becomes unstable. In the case of a total prosthesis, it tends to fall or move too much, while in the case of a partial or combined prosthesis, the chewing effort is discharged excessively on the dental pillars, sometimes seriously damaging them.
Attachments and hooks are subject to wear and loosening, and some also have components that need to be replaced periodically.
The general health of the supporting teeth must be checked rigorously.
Relining - It consists in recovering the lost contact between the saddle or the base of the total prosthesis and the supporting gum, filling the space with new resin. The relining techniques are different.
a) Cold or direct relining. The dentist uses an acrylic resin that is mixed in the chair, placed on the surface of the saddle, and shaped on the gum by placing the prosthesis in the mouth. This technique is quite coarse, as the cold resin does not have the same characteristics as that used in the laboratory and should be reserved for very small or non-permanent relining.
The same technique can be improved by putting the resin just applied to heat under pressure.
b) Indirect relining. The dentist detects the difference in thickness to be filled with an impression paste suitable for the purpose and then removes the prosthesis from the mouth with a complete impression, in which the prosthesis remains embedded. The laboratory restores contact with the gum, filling the empty space.
A second variant of this technique consists in inserting a soft paste into the prosthesis and asking the patient to use the prosthesis for 4-5 days. The paste records not only the variations in thickness, but also all the functional movements induced by the musculature of the mouth. After this registration phase, the prosthesis is returned to the laboratory to complete the base.