Fixed Prosthodontics: Crowns, Bridges, Veneers and Implants
What is examined
Design and fabrication of indirect restorations - preparation geometry, die work, material minimum thickness, connectors, pontic design, implant prosthetics and bonding protocols.
Preparation and die work
- Axial reduction 0.8–1.5 mm, occlusal reduction 1.5–2.0 mm for metal-ceramic; convergence ideally 6°, acceptable 6–10°.
- Margin: chamfer for metal and metal-ceramic, shoulder or deep chamfer for ceramics.
- Die preparation: trim and ditch the margin, then apply die spacer - typically 25 µm per coat, two to three coats (50–75 µm). Excess spacer gives a loose fit and an exposed margin.
Material minimums
- Lithium disilicate: 1.0 mm minimum thickness; connector cross-section about 12 mm² for molars, 9 mm² elsewhere.
- Zirconia: 0.5 mm as a framework, 0.8–1.0 mm monolithic.
- Feldspathic veneers: about 0.5 mm, bonded to enamel.
Pontics and implants
- Modified ridge lap for anterior aesthetics, ovate for tissue contact, sanitary for posterior cleaning access.
- Screw-retained implant crowns need the access channel angled clear of the incisal or occlusal contact; cement-retained crowns need a margin the clinician can fully clean.
Worked example
A zirconia crown debonds and its intaglio surface is glassy smooth. Zirconia cannot be etched, so retention is mechanical: 50 µm alumina abrasion at about 2 bar, a 10-MDP primer, then resin cement. Without that sequence the cement simply shears away.
Traps
- Under-reducing the occlusion and blaming the material when it fractures.
- Seating a crown on an unditched, rounded die margin.
- Etching zirconia with hydrofluoric acid, which does nothing but contaminate it.
- Ignoring connector dimensions on a long-span bridge.