Fixed Prosthodontics
Fixed prosthodontics tests the geometry of preparation, material choice by site, and the cement that matches that material.
Preparation principles
- Retention comes from surface area, small taper and a single path of insertion; total convergence near 6 degrees is ideal, and retention falls sharply above 10-12 degrees.
- Resistance form needs a flat occlusal floor and a margin that prevents rocking; axial reduction about 1.5 mm for metal-ceramic, 1.0-1.5 mm for zirconia and lithium disilicate.
- Occlusal reduction 1.5-2 mm for metal-ceramic, respecting anatomy rather than flattening the surface.
- Finish line: chamfer for metal and metal-ceramic margins, shoulder or deep chamfer for ceramics.
Materials and connectors
- Lithium disilicate is etched with hydrofluoric acid and silanated; zirconia is airborne-particle abraded and bonded with an MDP-containing primer.
- Connectors: at least 2-3 square millimetres anteriorly and 3-4 posteriorly for metal-ceramic; zirconia needs more bulk.
- Resin cement for ceramic and adhesive work; resin-modified glass ionomer where moisture control is imperfect.
Planning
- Mount casts in centric relation, check the envelope of function, and avoid heavy non-working contacts on cantilever units.
Worked example
A maxillary first premolar needs a metal-ceramic crown, but after the first preparation the clinical crown is only 3 mm tall and convergence is 20 degrees. Retention is inadequate. Adding a proximal groove for resistance and extending the margin apically, or building a core, is chosen rather than simply accepting the taper. The reasoning combines surface area with convergence rather than a memorised number alone.
Traps
Confusing taper with total convergence; forgetting ferrule when a post is planned; choosing a shoulder for a metal margin; cementing zirconia with zinc phosphate and expecting adhesion; ignoring minimum connector cross-section.