three basic principles of tooth preparation
biologic (conservation of tooth structure)
mechanical
esthetic
the poor appearance of some metal-ceramic restorations is often due to what
insufficient porcelain thickness
what are the parameters for the biologic width?
3mm btw restoration and bone
the more accurately the restoration is adapted to the tooth, the less the chance for what?
recurrent caries or periodontal disease
what is the recommended convergence angle of the axial walls?
6 degrees
the location of the finish line for #21 should be ____ and #19 should be
.75mm (bc of the shoulder)
.5mm (typical chamfer)
which POW should you choose when making an FPD?
always choose the molar POW
the first plane grooves should be what?
parallel to the long axis of the tooth
how many planes of reduction are there for a #21
3 (fxnal cusp, 1rst plane, second plane)
the shoulder should be how long?
1.2-1.5 mm
you should prepare all abutments at the same time to minimize what?
undercuts
retention form
forces parallel to long axis
resistance form
lateral forces (occlusal forces)
what angle should the shoulder be?
120 degrees
advantages of supra gingival margins
easily finished
impressions more easy
restoration can be more easily evaluated
more easily cleansed
indications of sub gingival margins
- presence of caries, cervical lesions, or restorations that extend subgingivally
- need for additional retention
- root sensitivity
- need to hide the margin of a metal-ceramic crown for esthetics on anteriors