Bio-Mechanical Principles and Tooth Preparation

Study Notes

Improper tooth preparation can negatively affect long-term dental health
Axial reduction is crucial for creating space for restorative materials with proper thickness
Margin placement significantly impacts restoration fabrication and cleanliness
Supragingival margins are generally preferred for ease of preparation, finishing, and impression taking, as well as for easy restoration evaluation
Subgingival margins are necessary in certain cases, such as when caries, erosion, or previous restorations extend subgingivally, or when additional retention is needed
Crown margins should be placed at least 2.0 mm away from the alveolar crest to avoid bone resorption

Marginal adaptation is crucial for preventing recurrent caries and periodontal disease
Smooth and even margins minimize chances of recurrent caries and periodontal disease
A well-designed preparation ensures a smooth and even margin
Featheredge margins (shoulderless) should be avoided due to lack of bulk at the margin, which can lead to over contoured restorations
Chisel-edge margins are associated with excessively tapered preparations and can be difficult to control
Chamfer margins are ideal for cast metal crowns and metal portions of metal-ceramic crowns, providing adequate bulk and a distinct finish line
Beveled margins are suitable for cast restorations when a ledge or shoulder exists, providing a smooth transition between the restoration and unprepared tooth structure
Shoulder margins are recommended for all-ceramic crowns and facial portions of metal-ceramic crowns, providing adequate bulk for porcelain
Radial shoulder margins offer a modified shoulder design with a rounded internal angle, reducing stress concentration in the tooth structure
Sloped shoulder margins are a modification of the classic shoulder design, reducing the possibility of leaving unsupported enamel
Shoulder with bevel margins are used for proximal boxes of inlays and onlays, occlusal shoulder of onlays and mandibular three-quarter crowns, and facial finish line of metal-ceramic restorations where gingival esthetics are not critical

Feather Edge: Conservation of tooth structure, Can lead to over-contoured restorations
Chisel Edge: Conservation of tooth structure, Can lead to over-contoured restorations, Difficult to control margin location
Chamfer: Distinct, well-defined finish line, Care during preparation to avoid unsupported enamel, Suitable for cast metal restorations, lingual margins of labial veneer metal-ceramic restorations
Shoulder: Adequate bulk of restorative material, Less conservative than chamfer finish line, Sharp internal line angle, Suitable for full veneer metal-ceramic restorations, all-ceramic crowns
Shoulder with Bevel: Adequate bulk of restorative material, Less conservative to tooth structure, Bevel removes unsupported enamel, Bevel needs to be placed apically, Suitable for metal-ceramic restorations, previous caries or restorations at finish line area
Beveled: Removes unsupported enamel, Less conservative, Requires subgingival extension for optimal esthetics, Suitable for ledges due to previous caries or erosion at the cervical margin