Principles of tooth preparation

If there’s a missing tooth è Decay, overeruption, tilting, food impaction and gum diseases.

Principles of tooth preparation:

a-Biological consideration.

b-Mechanical consideration.

c-Esthetic consideration.

 

a-Biological Consideration:

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.

# Prevention:

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.

 

2-Conservation

Partial coverage

Indications	Contraindications
Intact facial surface, minimal carries, average and greater tooth length and good oral hygine.	Extensive tooth destruction and need for maximum

 

3-ناقص

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.

Supragingivally	Subgingivally
# 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.

 

Marginal geometry:

a-easy to do.

B-not over-extended.

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.*

Margin design:

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.

Factors:

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:

offset

Esthetic consideration:

-looks natural.

-in metal ceramic rest è sufficient.

-facial reduction.

-Incisal Reduction 2mm, avoid excessive reduction.

Labial Margin:

Supragingival: root not discolored/low lip line

Metal not shown during normal function.

Metal margin

Subgingival: high lip line.

 

Instruments:

Burs	Instruments
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.
7-flame-shaped

Category: Endodontic, Endodontics, Prosthodontics, Prosthodontics