This final article in the series describes the modification of teeth to improve their shape for the support and retention of RPDs.
In this part, we will discuss
Correction of unfavourable survey lines
Creating retentive areas
New publications: All the parts which comprise this series (which will be published in the BDJ) have been included (together with a number of unpublished parts) in the book A Clinical Guide to Removable Partial Dentures (ISBN 0-904588-599) and A Clinical Guide to Removable Partial Denture Design (ISBN 0-904588-637). Available from Macmillan on 01256 302699
Preparation may be undertaken for a number of reasons.
Provide rest seats.
Establish guide surfaces.
Modify unfavourable survey lines.
Create retentive areas.
Tooth preparation for RPDs should be planned on articulated study casts after they have been surveyed and a denture design produced.
Shaping of enamel surfaces for any of the reasons listed is usually undertaken with rotary diamond instruments of appropriate size and shape. The resulting roughened enamel surface must always be smoothed and polished. Special burs, stones and abrasive-impregnated rubber wheels and points are available for this purpose. Subsequent application of a topical fluoride varnish, to reduce the chance of carious attack of the modified enamel surfaces, should be carried out routinely.
Rest seats may need to be prepared to:
produce a favourable tooth surface for support (Fig. 1);
prevent interference with the occlusion (Fig. 2);
Rest seats on anterior teeth
The advantages of guide surfaces
It is widely accepted on the basis of clinical observation that the use of guide surfaces confers a number of benefits in RPD construction. The benefits include the following:
Prevention of clasp deformation.
The preparation of guide surfaces
Guide surfaces are usually prepared, somewhat imprecisely, by eye. The position in which the handpiece must be held to prepare the required guide surfaces, so that they are all parallel to each other and to the path of insertion, should be established on the study cast.
As a check on the accuracy of the prepared guide surface, an alginate impression may be taken to produce a second study cast. This cast can then be placed on a surveyor and the parallelism of the guide surfaces checked using the analysing rod. If correction is found to be needed, further intra-oral adjustment can be undertaken.
A more precise approach to the preparation of guide surfaces can be achieved by the use of jigs constructed on a prepared study cast and then transferred to the mouth, either to control the positioning of the handpiece or to check on the location and amount of enamel reduction.Fig. 18, Fig. 19, Fig. 20
About this article
Analysis of the compressive strain below the removable and fixed prosthesis in the posterior mandible using a digital image correlation method
Biomechanics and Modeling in Mechanobiology (2012)