Dental Crowns and Margin Placement

Study Notes

Finishing line is the final margin that separates prepared tooth structure from unprepared tooth structure.
It must be clear, smooth, and well-defined.
It should be continuous from one surface to another.
Must lie on sound tooth structure to prevent interference with crown seating.

Supragingival: margin above the gingival tissue.
- Advantages: Easy preparation and finishing, easy impression taking, easy cleaning, less destructive, typically on hard enamel, easy to evaluate.
Subgingival: margin below the gingival tissue, but not exceeding 2mm from the free gingival margin.
- Indicated when: Aesthetic restoration margin needs to be hidden, extra retention is needed for short clinical crowns, dental caries, cervical erosion, or restorations extend subgingivally, proximal contact area extends apically to gingival crest, root sensitivity cannot be controlled by conservative methods, axial contour modification is needed.
Margin placement depends on patient lip lines (smile line).

The junction between a cemented restoration and tooth is a potential site for recurrent caries due to luting agent dissolution and roughness.
A well-designed preparation has a smooth and even margin.
Rough, irregular, or "stepped" junctions increase margin length and reduce restoration adaptation.

The cross-sectional configuration of the margin (finish line) is the line of demarcation between prepared and unprepared tooth structure.
Margin design should be considered:
- Chamfer: smooth, bevelled margin, widely used, versatile, good adaptation.
- Shoulder: 90° angle, easier to prepare, less adaptable.
- Feather edge: very thin, delicate, requires precise preparation.
- Rounded shoulder: blends shoulder and chamfer, provides smooth transition.
- Bevel: shallow, angled margin, enhances bonding.

Tooth preparation should allow enough space for a functional occlusal scheme in the finished restoration.
Disrupted occlusion (tilting, supra-eruption) requires occlusal analysis and reduction to compensate for the supra-erupted tooth.
In severe occlusal disruption, violating the principle of tooth structure conservation is preferable to potential harm from a traumatic occlusal scheme.

Tooth preparation should prevent fracture of restored teeth.
Cuspal coverage restoration (onlay) is preferred over inlay to reduce fracture risk.
Complete crown provides the best protection against fracture, but can lead to periodontal disease and poor esthetics.

Retention: ability of the preparation to resist crown removal along its path of withdrawal.
Path of insertion: imaginary line for restoration insertion and removal without lateral forces.
Factors affecting Path of Insertion:
- Axial surface reduction
- Rounding of axial line angles
- Proximal grooves
- Ledges and indentations
- Pinholes
Surface area: long axial walls are more retentive than short walls.
Stress concentration: cohesive failure occurs when cement adheres to both tooth and restoration, cement strength is exceeded.
Type of preparation: complete crown has twice the retention of partial-coverage restorations.
Roughness of the fitting surface: smooth surface leads to failure at the cement-restoration interface, roughness increases retention.
Material being cemented: cement adheres better to amalgam than composite or cast gold.
Type of luting agent: adhesive resin cements are the most retentive.
Film thickness of luting agent: ideal dimension is 20-40 µm per wall.

Features that enhance restoration stability and resist dislodgement along an axis other than the path of withdrawal.
Tipping path: path along which restoration can be displaced under dislodging occlusal force.
Rotation is prevented by resistance areas that are placed in compression.
Adequate resistance depends on:
- Magnitude and direction of dislodging forces
- Geometry of tooth preparation
- Physical properties of the luting agent

Appearance Zone: anterior oral area visible on smiling, usually extends from maxillary premolar to premolar (can include first molars).
Options for esthetic restorations include:
- Partial veneer crowns
- Metal-ceramic restorations
- All-ceramic restorations
Conservative preparations are necessary for esthetics, requiring 1-1.2 mm material thickness for optimal results. This limits their use on thin teeth and those with large pulps.