Prosthodontics and Dental Materials
Prosthodontics and dental materials: treatment planning for missing teeth plus the material science behind crowns, bridges, dentures and restorations. Items often give a clinical situation and ask which design or material choice follows logically from it.
Core rules
- Fixed prosthodontics: axial reduction for metal-ceramic about 1.2–1.5 mm, occlusal 1.5–2 mm. A chamfer suits metal and metal-ceramic margins; shoulder or knife-edge depends on the all-ceramic system.
- Ideal total convergence is roughly 6–10°. A very short crown needs grooves or pins for retention, not more taper.
- A ferrule of 1.5–2 mm of sound dentine improves fracture resistance of post-retained crowns.
- Kennedy classification depends on the most posterior edentulous area; modifications count extra edentulous areas and never change the class.
- Rests direct occlusal load along the long axis of the abutment; guide planes and correct clasp position on the survey line improve stability.
- Complete dentures: record vertical dimension at rest minus 2–4 mm freeway space; the posterior palatal seal provides retention.
- Materials: gamma-2 phase (Sn-Hg) is amalgam's weak, corrosion-prone phase, so high-copper mixes are preferred; zinc oxide-eugenol inhibits resin and composite polymerisation; glass-ionomer bonds chemically to tooth and releases fluoride; addition silicones are the most dimensionally stable impressions; zirconia toughens by stress-induced transformation.
Worked example
A patient is missing all posterior teeth on the right and a single premolar on the left. The most posterior edentulous area is on the right, so the case is Kennedy Class II; the left bounded space is a modification, giving Class II modification 1. Design follows: the distal extension needs an altered-cast or functional impression so the base rests on functional mucosa, while the bounded premolar space can be restored with a fixed partial denture.
Traps
- Counting a modification as a new Kennedy class.
- Using eugenol-containing temporary cement beneath an adhesive restoration.
- Blaming the cement for failure of a short, over-tapered crown preparation.
- Confusing the impression with the die stone - the stone die expands and is a replica of the preparation.