81. Model cast RPD. Principles of model cast framework construction.

Introduction

A one-piece cast framework partial denture is designed on the surveyed study cast before anything is made. The design has to answer one question: how will the forces be carried? The answer depends on whether the gaps are bounded by teeth or end freely.

Mouth preparation

Before the final impression, the teeth are modified so the denture can be supported and retained without harmful forces. Six steps, in order:

1. Correct the occlusal plane (e.g. shorten over-erupted teeth).
2. Recontour proximal surfaces of back teeth: guide planes parallel to the path of insertion.
3. Recontour proximal surfaces of front teeth.
4. Recontour facial and lingual surfaces to create or reduce undercuts.
5. Prepare rest seats (occlusal rests about 1.5 mm deep, cingulum rests).
6. Make crowns where teeth need new contours.

Static and biodynamic aspects

Teeth move very little in their sockets; the mucosa over the ridge compresses much more. So a base resting on mucosa and a clasp on a tooth move differently under load, and the framework must manage this difference.