Partial dentures are defined as restorations capable of being removed from the mouth by the patient.
The components of partial dentures may be listed as:
l. saddles
2. occlusal rests and similar components
3. direct retainers
4. indirect retainers
5. connectors
THE SABDLES
The saddle is the part of a partial denture that replaces lost alveolar tissue and carries artificial teeth. The fitting surface of a saddle can be made of acrylic resin or metal; both have advantages and disadvantages, but the majority is made in acrylic resin, and certainly this material should be used exclusively for free-end saddles.
Acrylic teeth are always fixed to the saddle through a bond with resin. The saddle may have a fitting surface of cast metal on to which acrylic resin base material is attached by retention tags. In these cases the union of tooth to the resin, being of a physic-chemical nature, is always sufficiently strong. In some cases of close anterior occlusion a small saddle may display a weakness of the union between metallic base and intervening resin.
OCCLUSAL RESTS AND SIMILAR COMPONENTS
An occlusal rest is a metal projection attached to a partial denture, extending over and bearing on the occlusal or prepared surface of a standing tooth. It must fit the surface accurately and must be cast and not made from wrought metal.
An occlusal rest serves these purposes:
l. It transmits vertical load to the tooth.
2. It transmits some lateral load to the tooth.
3. If deflects food.
4. It may improve occlusion.
5. It may act as an indirect retainer.
ONLAYS
When an occlusal rest is extended to cover the greater proportion of the occlusal surface of a tooth it is called an onlay. Onlays may be cast in gold or cobalt chromium alloy. But when made of gold the appliance is unnecessarily heavy and costly. It has been suggested that the hardness of cobalt chromium alloy would have an excessive abrasive action upon the enamel of opposing natural teeth, but in practice this does not seem to occur. It is possible that in some cases, there may be more marked faceting than usual, but the degree is slight.
Onlays serve the following functions:
l. They may support a partial denture.
2. They may connect an overclosed occlusion.
3. They may improve the occlusion.
EMBRASURE HOORS
These are metal attachments to a partial denture which are placed in the embrasure between two contiguous natural teeth. They cover the occlusal or incisal surfaces of the teeth and continue over to the labial or buccal surfaces, but never extend below the survey line, and consequently play no part in direct retention.
All embrasure hooks may serve the following purposes:
l. Support the denture.
2. Brace the denture.，英语论文，英语论文 |
