If there’s a missing tooth è Decay, overeruption, tilting, food impaction and gum diseases.
Principles of tooth preparation:
1-damaging during tooth preparation
– Prevention of damaging of adjacent tooth by a metal matrix, and proximal enamel of the tooth being prepared (lip or fin).
– prevention of damaging soft tissues by using a mirror, flanged saliva ejector or aspirator tip.
– Protection of the pulp:
# Pulp size evaluation with radiographs, it decreases with age.
# Causes of injury; friction, excessive pressure, decrease speed, type and size of cutting instrument.
1- high speed.
2-water spray for: cooling and remove debris.
3-use of partial coverage restoration.
4-decrease the convergence angle.
5-follow the anatomic plans give a uniform thickness in the restoration.
6-selection of a conservation margin supragingival whenever possible.
|Intact facial surface, minimal carries, average and greater tooth length and good oral hygine.||Extensive tooth destruction and need for maximum|
4-axial reduction: ideal crown duplicate the original tooth contour.
– sufficient tooth reduction è a-accurate contour. b- same color.
5- Marginal placement:
Open margin: space between crown and tooth.
-Marginal placement supragingivally whenever possible.
|# Easy without trauma for soft tisuues.
# On hard enamel.
# Easily finished and kept clean.
# Easy impression.
|# Metal ceramic restoration.
# Add retention.
# caries, erosion or restoration extended subgingivally.
# Uncontrolled root sensitivity by conservative procedure.
– If anterior teeth mostly èsubgingivally, while If posterior teeth mostly è supragingivally
Margin Adaptation: a-Cemented junction.
B-junction between *cemented* rest and tooth finished line.
C-margin is smooth.
– If it’s not well adapted è a-recurrent carries. b-discoloration.
a-easy to do.
C-conservation without unsupported enamel.
d-easy to identify (impression and dye).
*e- accurate boundry for wax pattern.
f- sufficient bulk to handle the wax pattern.*
1- Feather edge è not accurate, with round end diamond bur.
2-Knife edge èusually with tilted teeth, with a round end diamond bur.
3-Charmfer èwith cast metal and metal ceramic crown.
N.B. Buccal is more prepared because we will put the metal ceramic buccally for esthetics.
4-Shoulder 90degree è all ceramic restoration, facial, less conservative, bulk.
5-shoulder with bevel è* stress and breaks.*
b-Mechanical consideration. (Durability of restoration.)
Retention form: prevent the removal of the restorationin a vertical(axial) direction.
1-Taper: 6degree-10degree, -Avoid undercuts.
– increase taper è decrease retentionèmore pathway.
-Parallel preparation è more retentive, 6degree-10degreeè less retentive,
20degree taperè less retentive.
2-Surface area: -Increase widthè increase retention.
-Molars retention > Premolars retention > Anterior teeth.
Roundation of sharp edges.
-Shorted tooth è complete coverage, so shorted tooth èintraradicular.
Short tooth(what to do to have more retention): a-more parallel. b-grooves or boxes.
3-Roughness α Retention èincrease roughness -> increase retention.
4-Material of cement.
-stress, roundation of sharp angles.
Resistance Form: Prevent the dislodgement or removal of the restoration, depends on: force of dislodgement magnitude and direction.
-biting force distributed all over the teeth.
1-increase tapering è decrease tapering, increase taper è decrease friction è increase dislodgement è decrease retention.
2-Increase lengthè increase retention. 3-Increase diameterè increase retention.
-Decrease diameterèIncrease frictionèdecrease resistance.
3-grooves enhance the resistance, *increase boxes form grooves*è Increase resistance* (U-shaped is better than v-shaped)
4-Partial coverage è decrease the resistance.
-wide teeth are more resistant, bevel is more resistant.
Deformation of restoration:
-in metal ceramic rest è sufficient.
-Incisal Reduction 2mm, avoid excessive reduction.
Supragingival: root not discolored/low lip line
Metal not shown during normal function.
Subgingival: high lip line.
|1-Tapered fissure||a-cusp bevel.
b-grooves (in box). c-offset.
|2-round end tapered diamond bur||a-depth orientation grooves.
b-occlusal and axial reduction.
c-chamfer finish line.
|3-Torbedo bur||Axial finishing.|
|4-flat end tapered diamond bur||a-axial reduction.
b-shoulder finish line.
|5-thin round end tapered diamond||Proximal axial reduction.|
|6-small wheel diamond foot ball bur||Reduction in lingual of anterior teeth.|