This document outlines principles of tooth preparation for restorations. It discusses biological considerations like preventing damage to adjacent teeth and soft tissues and conserving tooth structure. Mechanical considerations aim to provide adequate retention, resistance, and prevent deformation. Retention is increased by factors like taper, surface area, and roughness. Resistance is provided through geometry and luting agents. Deformation is prevented by alloy selection, adequate reduction, and margin design.

1. PRINCIPLES OF TOOTH PREPARATION
1. biological considerations
2. mechanical considerations
3. esthetic considerations
1. Biological considerations
A) prevention of damage during tooth preparation of :
a. adjacent teeth
- use metal matrix band
- use thin, tapered diamond
- common clinical error – tipping diamond away from adjacent proximal surface
b. soft tissue
- eg ; tongue, cheek, oral mucosa, peeriodontal ligament, gingiva
- use ; aspirator tip, mouth mirror, flanged saliva ejector, tongue depressor, rubber dent
- great care needed to protect tongue ; especially when ling surface of mand molars are
prepared
- for pulp ; prevent pulpal injuries
; causes - temperature, chemical irritation, Mos
(Causes of Injury)
1. temperature
- friction from heat generated
- rotatory instrument
- excessive pressure
- type, shape, condition of cutting instrument
- how to prevent ;
a. use water spray - to remove debris
b. make shoulder & chamfer finish line
c. slow speed handpiece
d. avoid high pressure
2. chemical action
- bases, restorative resins, solvents, luting agents
- these cause pulpal damage
- how to prevent ;
a. use cavity varnish
b. use dentin bonding agent
adv : effective barrier
disadv : effect retention
3. bacterial action
- from microleakage
- how to prevent ;
a. zn phosphat cement
b. chlorhexidine gluconate disinfecting solutions (consepsis)
c. remove all carious dentin before placement of restoration
d. not recommended ! - indirect pulp capping

2. B) conservation of tooth structure
why we have to conserve ;
- thickness of dentin inversely proportional to pulpal response
- damage to odontoblastic processes affect cell nucleus at dentin-pulp interface
how to conserve ;
1. use partial-coverage crown
2. min convergence angle between axial walls (taper)
3. preparation of occlusal surface
- reduction follow anatomical planes
- give uniform thickness
4. preparation of axial surfaces
- there's enough thickness of residual tooth struc surrounding pulpal tissue
- teeth should orthodontically repositioned
5. select margin geometry
- must be conservative
- compatible with tooth preparation
6. avoid unnecessary apical extension
C) consideration affecting future dental health
axial reduction
– indications :
1. to avoid plaque control
2. avoid pdl disease
3. to avoid poor form & subsequent occlusal dysfunction
4. avoid chipping of enamel/cusp fracture
5. avoid gingival inflammation
– how :
1. tooth preparation must provide sufficient space for development of good axial contours
- enable jucntion between restoration * tooth
- to make smooth & free preparation from ledges
2. crown should duplicate contours of original tooth
- except for malaliged/malformed tooth
3. slightly undercontoured flat restoration is better
- when error is made
- to keep plaque free
4. increase proximal contour on anterior crown
- to maintain interproximal papilla
5. sufficient tooth structure must be removed
margin placement
– whenever possible should be SUPRAGINGIVAL!
– advantages of supragingival
1. easily finished w/o associated with soft tissue trauma
2. kept plaque free
3. impressions are more easily made
4. restoration can be easily evaluated at the time of placement & at recall appointment
5. situated on hard enamel
– should use subgingival margin placement if ;
1. no indication of dental caries, cervical erosion
2. proximal contact area extends to gingival crest
3. additional retention/resistance is needed
4. margin of PFM crown is to be hidden behind labiogingival crest

3. 5. indication of modification of axial contour
6. short crown – min height 3.5mm
margin adaptation
1. consider the junction between restoration & tooth
- potential site for recurrent caries
2. increase adaptation will decrease caries & decrease pdl disease
3. smooth & even margin
4. preparing smooth margin cannot be overemphasized
margin geometry
– guidelines should be considered ;
1. to ease of preparation w/o overextension or undermined enamel
2. ease of identification in impression/die
3. distinct boundary for easy finishing of wax pattern
4. sufficient bulk of material
- enable wax patterm to be handles w/o distortion
- give restoration strength
- give esthetics when porcelain is used
5. conservation of tooth geometry
6. refer page 220 (table 7-2)
occlusial reductions
1. need considerable reduction to compensate supraeruption of abutment teeth
2. if too much reduction will shorten axial wall and reduce the retension & resistance
preventing tooth structure
1. cuspal fracture occur from bruxism
2. tooth prepartion is designed to min destructive stresses
- complete cast better than onlay & inlay
2. Mechanical considerations
aim – to prevent dislodgement or distortion or fracture of the restoration during service
3 categories :
a. retention form
b. resistance form
c. prevent deformation of restoration
A) Retention form
def : resistance that prevents restoration from becoming dislodged by such forces parallel to path of
placement (vertical direction)
5 factors :
1. magnitude of dislodging forces
def : forces that tend to remove a cemented restoration along its p.o.p
: disclose forces > seating force
causes : pulling with floss under connectors
: eat sticky food
factors affecting : stickiness of food
: surface area of restoration
: surface texture of restoration

4. 2. geometry of tooth preparation
- cement is effective if restoration has single p.o.p
- the best shape is cylindrical
- if overtapered : cemented restoration not constrained by preparation
: restoration has multiple paths of withdrawal
- factors affecting
1. taper (decrease taper, increase retention)
- def : convergence of 2 opposite external walls of tooth preparation
- theory : parallel walls – max retention
- mechanically : 6 degrees – optimum for max retention
- too small degree – undercuts
- too large degree – lack of retention
- recommended to use grooves to decrease displacement
2. surface area (increase s.a, increase retention)
- depends on length of this p.o.p
- occlusal surface not include
- tall crown > retentive than short crown
- molar crown > retentive than premolar crown
3. stress concentration (decrease s.c, inccrease retention)
- lead to cohesive & adhesive failure
- causes : sharp occlusoaxial line angle
: stress not uniform & concentrated
- should make roundation of all line angles
4. type of preparation
- complete crown > retentive than partial crown
3. roughness of surfaces being cemented
- increase roughness, increase retention
- use : air abrasion, tin plating, acid etching
4. materials
- increase the reactivity of alloy, increase adhesion, increase retention
- base metal alloy (high gold content) > retentive than less reactive metal
- complete resin > retentive than amalgam
5. film thickness
- 25mm
6. type of luting agent
- adhesive resin cement (most retentive)
B) Resistance form
def : features that enhances the stability of a restoration & resist horizontal/lateral dislodgement
3 factors :
1. magitude & direction of dislodging forces
- mastication – cause horizontal/oblique/lateral dislodgement
- lateral force tend to displace restoration by rotation around gingiva margin
- individual biting force – 4340N (443kg)

5. - complete cast crown can withstand more than 13500N (1400kg)
2. geometry of tooth preparation
- increase taper/increase diameter/decrease height lead to decrease resistace
how to increase resistance :
- acceptable degree of convergence (taper) : 5-22 degress
- min wall height : 3.5-4mm
- make pyramidal preparation
- make grooves/boxes
- pinholes
- complete crown
- u-shaped grooved/flared boxes
3. physical properties of luting agent
- highest compressive strength : Glass Ionomer
- highest MOE : Zinc Phosphate
C) Preventing deformation
1. alloy selection
- type I & II gold alloy : intracoronal cast restorations
- type III & IV gold alloy : crown
- metal-ceramic alloys with high noble metal content = type IV
- nickel-chromium alloys (harder)
2. adequate tooth reduction
- to get sufficient bulk of tooth to withsatnd occlusal forces
- make func cusp (1.5mm) : mand – buccal
: max – palatal/lingual
- make non func cusp (1mm) : mand – palatal/lingual
: max – buccal
– for malaligned/overerupted/supraerupted tooth (reduction > 1.5mm)
3. margin design
- prevent distortion by :
a. design outline form
b. avoid occlusal contact
c. keep preparation margin 1-1.5mm away from occlusal contact areas
d. maintain adequate dentin thickness
e. make grooves/ledged/ant pinledge retainers