Fixed Prosthodontics and Crown & Bridge
What this domain tests
Preparation design, framework geometry and the fit and occlusion checks that decide whether a crown seats and stays seated.
Core rules
- Finish lines. Chamfer for metal and metal-ceramic; shoulder or deep chamfer for all-ceramic; avoid feather edges. Axial reduction typically 1.0–1.5 mm, occlusal 1.5–2.0 mm, metal-only occlusal 0.5 mm.
- Retention and resistance. Ideal total occlusal convergence near 6 degrees; excess taper destroys retention faster than any cement can compensate.
- Fit. Marginal gap of roughly 25–40 µm is the usual laboratory target. Die spacer of 25–50 µm provides cement space, stopping 0.5–1.0 mm short of the margin so the finish line stays in contact.
- Connectors. Zirconia bridges need generous cross-section: about 9 mm² anterior and 12–16 mm² posterior. Undersized connectors are a common fracture origin.
- Pontics. Modified ridge lap or ovate for aesthetics and cleanability; sanitary pontic where tissue contact is contraindicated.
- Occlusion. Light, even centric contacts; no heavy contacts on inclines; check excursive contacts on the remount, not by eye in the mouth.
Worked example
A three-unit metal-ceramic bridge returns with the distal retainer seating 200 µm high. Reasoning: the internal fit is the suspect, not the margin, because the margin visibly closes on one side. Check for a die-spacer void, a sharp axial angle, or a solder joint that pulled the framework. Verify by seating on the solid cast with fit-checker spray, relieve the internal hot spot only, then re-verify the margin before adjusting occlusion.
Traps that cost marks
- Relieving the margin instead of the internal surface.
- Designing connectors by appearance rather than cross-sectional area.
- Checking occlusion before the framework is seated and margins verified.