Fixed Prosthodontics
Fixed prosthodontics tests whether you can design a preparation and restoration that survives occlusal load without biologic harm. The board favors principles over recipes: retention, resistance, marginal seal, and tissue health.
Core rules
- Ideal axial convergence is about 6–12°; beyond roughly 20° retention falls sharply.
- Reduce functional cusps 1.5–2.0 mm and nonfunctional cusps 1.0–1.5 mm; verify with a silicone index.
- Use a chamfer (0.3–0.5 mm) for metal margins and a shoulder or deep chamfer (1.0–1.5 mm) for ceramic.
- Preserve 1.5–2.0 mm of axial dentin ferrule on endodontically treated teeth.
- Keep margins supragingival where possible and round all line angles.
Worked example
A mandibular first molar needs a PFM crown. The functional cusp is reduced 2.0 mm, nonfunctional 1.5 mm, axial walls 1.5 mm with 10° convergence, and a 0.5 mm chamfer is placed. A 2 mm ferrule remains. Reasoning: the functional cusp needs more porcelain support; the chamfer suits the metal collar; the ferrule resists lateral force.
Traps
Under-reduction leaves thin porcelain that fractures. Overtapering destroys retention. A knife-edge margin prevents a stable ceramic seal. Placing a margin into plaque stagnation causes gingival inflammation. A sharp internal line angle creates a stress concentrator.