Introduction
The outline form is the shape and position of the cavity margins on the tooth surface. Set first in every preparation, it decides how much sound tooth is sacrificed and where tooth and filling meet – and so the strength of the remaining tooth and the life of the margins. This note covers its rules, the factors that decide it, and the move from Black's extension for prevention to minimal intervention.
Outline form and its general rules
Establishing the outline form means placing the cavosurface margins where they will lie in the finished preparation and cutting an initial depth just inside the dentino-enamel junction (DEJ). The initial depth is usually about 0.2 mm into dentin under pits and fissures (a pulpal depth of about 1.5–2 mm) and about 0.5 mm into dentin on the axial wall of smooth-surface preparations.
The outline is decided at the level of the DEJ. Caries that spreads sideways along the DEJ widens the outline; caries that lies deeper than the initial depth is removed afterwards (topic 4) and does not.
Three principles
- All friable (weakened) and undermined enamel is removed.
- All faults next to the lesion are dealt with: carious or deep pits and fissures, defects and the margins of old restorations are included – or the fissures are sealed or smoothed by enameloplasty.
- All margins are placed where they can be finished, inspected and cleaned, and where they are not under heavy load.
The outline is drawn in smooth curves and straight lines, without sharp corners: a sharp angle concentrates stress in tooth and material and is hard to finish and fill.