Main components of the cavity form. Requirements for the cavity design

A cavity form is the prepared shape of a tooth after the caries lesion has been opened and the infected tissue removed. It is built from walls, margins and angles, and it must meet three groups of requirements: biological (remove all infected dentin and keep as much sound tooth as possible), mechanical (give the restoration resistance and retention) and convenience (give access, visibility and easy placement of the material). The outline form is the first step that sets the shape of the cavity.

Every cavity form, whatever its class, is built from the same components. Walls are the prepared surfaces, margins are where the prepared surface meets the unprepared tooth, and angles are where two or three of these surfaces meet. A preparation is simple if only one tooth surface is involved, compound if two surfaces are involved and complex if three or more surfaces are involved.

The walls are the surfaces of the prepared cavity. They are divided into external and internal walls.

• External walls are prepared surfaces that extend to the external surface of the tooth. Each external wall takes the name of the tooth surface it is adjacent to, for example buccal, lingual, mesial, distal, occlusal or gingival.
• Internal walls are prepared surfaces that do not extend to the external surface of the tooth. They are the axial wall and the pulpal wall (floor).
• The axial wall is the internal wall parallel to the long axis of the tooth.
• The pulpal wall (floor) is the internal wall perpendicular to the long axis of the tooth, occlusal to the pulp.
• The floor (or seat) is the prepared wall that is reasonably horizontal and perpendicular to the occlusal forces directed occlusogingivally, generally parallel to the long axis of the tooth.
• The gingival floor is the floor of a proximal box that lies gingivally, near the papilla margin.
• Walls are formed from enamel or dentin. An enamel wall has enamel rods supported by sound dentin. A dentinal wall is formed entirely of dentin.

Each external wall ends in a margin that bears its name.

• The cavosurface margin is the junction of the prepared and the unprepared surface of the tooth. It is the edge of the cavity at the enamel or cementum surface.
• The cavosurface angle is the angle between the prepared wall and the unprepared tooth surface at the cavosurface margin. It may differ with the location on the tooth, the direction of the enamel rods on the prepared wall, or the type of restorative material to be used.
• A butt joint is a cavosurface angle of 90 to 100 degrees. It is used for brittle materials with low strength, such as amalgam and glass-ionomer restorations.
• A bevel is an inclination of a cavity wall or floor directed away from the cavity. Bevels extend the preparation to include surface defects, supplementary grooves or other areas of the tooth surface. A bevel produces an angle of more than 90 degrees with the cavity wall. Bevels are used for metal cast inlays and for composite restorations.

The lecture lists six bevels, shown in the figure below.

• Partial bevelnot used in cast restorations.
• Short bevelused with cast alloys, especially for types I and II.
• Long bevelused for class I, II and III cast materials.
• Full bevelused when no other form of bevel is possible.
• Counter bevelused for capping cusps.
• Hollow ground bevela bevel listed with the others in the lecture.

• Line angles are formed at the junction of two surfaces of different orientation.
• Point angles are formed at the junction of three surfaces of different orientation.
• A class I occlusal cavity has eight line angles: bucco-pulpal, linguo-pulpal, mesio-pulpal, disto-pulpal, mesio-buccal, mesio-lingual, disto-lingual and disto-buccal. It has four point angles: mesio-bucco-pulpal, mesio-linguo-pulpal, disto-bucco-pulpal and disto-linguo-pulpal.

The requirements for cavity preparation fall into three sets. Each has its own purpose, and all three must be met in one preparation.

The biological requirements aim to obtain a biologically sound form.

• Remove all infected dentin completely.
• Keep as much sound structure as possible: sound enamel, and the dentin in which the organic matrix is preserved (affected dentin).
• Exclude the possibility of new caries development (secondary caries).
• Preserve the basic functions of the dental pulp.
• Do not disturb the integrity and functions of the periodontal tissues.

The mechanical requirements create the resistance and retention form.

• Resistance form is the shape and placement of the walls that let the restoration withstand masticatory forces without fracture. It excludes wall and restoration fractures (the biomechanical aspect) and avoids secondary caries (the healthy aspect). Resistance is achieved with a flat floor in sound dentin, enamel supported by sound dentin, cavosurface angles of 90 degrees, and slightly rounded line and point angles, which prevent stress concentrating at sharp corners.
• Retention form is the shape of the cavity that keeps the restoration from being displaced. The restoration should not be pushed aside and should not fall under the influence of masticatory forces. Retention is achieved with flat pulpal floors, walls perpendicular to the masticatory force, rounded line and point angles, and grooves, coves or dovetails where needed. Retention features lock the restorative material into the tooth.

The convenience form gives adequate access and visibility to the operating site, so that the preparation and the restoration can be carried out easily. It allows easy application of the material, easy use of accessories such as matrices and wedges, and direct control of the cavity elements and of the restoration. Sturdevant's Art and Science of Operative Dentistry describes it in the same terms: sufficient access to the lesion for visibility and instrumentation, and room to insert the restorative material.

The outline form is set by carrying the margin of the cavity to the position it will occupy when the cavity is complete. It must enclose the carious lesion and may include parts of caries-susceptible areas on the surface being restored. The lecture states that the outline form should follow a gently sweeping curve, especially on the occlusal surface. Final outline form is not fixed until the carious dentin and overlying tissue have been removed.

• The mesial and distal walls should have a slight divergence from the pulpal floor, which avoids undermining the marginal ridges.
• The margins should join each other in a rounded fashion.
• The outline should include all defective pits and fissures.
• The outline should avoid undermining the marginal ridges.
• For a buccal or lingual wall, the inclination depends on the buccolingual width of the cavity measured against the intercuspal distance. When the width is up to one third of the intercuspal distance, the opposing external walls converge slightly pulpo-occlusally, following the direction of the enamel rods. When it is one third to one half, the walls are parallel to each other and perpendicular to the pulpal floor. When it is two thirds or more, each wall has two planes: one perpendicular to the ledge of the pulpal floor and formed entirely of dentin, and one following the enamel rods, which diverges occlusally.
• For amalgam, the lecture asks for a minimum occlusal depth of 2 mm, a cavosurface angle of 90 degrees, a box-shaped preparation with a flat pulpal floor, and rounding of all internal line and point angles.