MOBILE PROSTHETICS
GENERALITIES
When (for any reason) it is no longer possible to replace the patient's teeth with a fixed solution, since there are no suitable teeth available to build a bridge and / or the bone is not enough to insert implants, there is no another option than the so-called mobile (removable) prosthesis.
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The mobile prosthesis is defined as such because it is possible to insert and remove it from the patient's mouth. Regardless of the kind, it has a general characteristic: at least part of the teeth it replaces have to 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's also the reason why such prosthesis is usually more problematic than the fixed one, both in terms of comfort for the patient and of management for the dentist.
PARTIAL PROSTHESIS
This type of mobile prosthesis needs the persistence of a sufficient number of natural teeth in the patient's mouth so as to anchor the mobile part. The number and position of these teeth may vary, but designing the prosthesis there has to be a correct mechanical balance between the pillar teeth and the added parts. Often, it is necessary to cover the residual teeth with fixed prostheses in order to offer more support to the mobile part, sometimes even joining more than one tooth with a bridge. In this case, the partial prosthesis is called 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, 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 built out of acrylic resin, a pink colored plastic.
On top of it, resin teeth are placed, performing the chewing function. More rare is the use of ceramic teeth.
Connector or Bar - This component of the mobile prosthesis has the function of connecting the right side with the left one. It can be made of acrylic resin or (more often) it may be a metal band.
This part of the prosthesis is unfortunately always necessary because balancing issues. There are prostheses that may occupy only one side of the mouth, but these are rather complicated to build, so the preferred solution is to anchor the teeth on one side with a reciprocating connector on the opposite side of the mouth.
Hook - The hook is the most common fastening element of the removable prosthesis. It consists of a small metal extension that, starting from the saddle or one of the connectors, protrudes towards the external part of the tooth, embracing the external portion of it. When the prosthesis is inserted, the hook theoretically should make a slight friction on the tooth and then slide into position with a click.
Of course, since these are flexible metal structures, given the high frequency of continuous wearing on and off, they tend to loosen, and the dentist has to periodically tighten the hooks again. It is never advisable to attempt this maneuver with improvised home tools due to the risk of snapping 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 has no natural teeth 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 must be clear that at this point, the main problem to be solved will be the stability of the device. The total prosthesis in fact, completely relies on the so-called "suction effect". The resin base resting on the gum is built in such a way as to adhere tightly to it and its edges follow the line of the mucosal parts in order to seal the covered area.
When putting the denture into place, air is forced out of 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 rebound the denture and unfortunately cause the artifact to become unstable.
It is understandable that at least in principle the upper arch prosthesis will be more easy to stabilize, because it rests on a bigger supporting surface, while the lower one has to be designed around the tongue's base. Furthermore, speaking and chewing, may also bring 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-off 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 prepared at chairside, placed on the surface of the saddle, and shaped on the gum by placing the prosthesis in the mouth. This technique is less refined, since the cold resin does not have the same characteristics as the one used in the dental lab and should be reserved for very small or non-permanent relining.
The same technique can be improved by processing the cold resin under heat and pressure.
b) Indirect relining. The dentist records the gap to be filled with an impression paste and then removes the prosthesis from the mouth with a complete impression embedding the prosthes. 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 it for 4-5 days. The paste adjusts with the variations in thickness and records all the functional movements induced by the mouth's musculature. After this registration phase, the prosthesis is returned to the laboratory to complete the base.