Clinical Significance in

Practice of Operative Dentistry and

Treatment plane

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

lingual surface of mandibular molars are prepared - for pulp ;

prevent pulpal injuries

; causes - temperature, chemical irritation,

(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

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 structure 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 and 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 and 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.

5. indication of modification of axial contour.

6. short crown – min height 3.5mm.

margin adaptation

1. Consider the junction between restoration and tooth- potential site

for recurrent caries.

2. Increase adaptation will decrease caries and decrease pdl disease.

3. Smooth and 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.

occlusal reductions

1. need considerable reduction to compensate supraeruption of

abutment teeth.

2. if too much reduction will shorten axial wall and reduce the

retention and resistance.

preventing tooth structure

1. cuspal fracture occur from bruxism.

2. tooth preparation is designed to min destructive stresses- complete

cast better than onlay and 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) five

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

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 tow 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 and 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 and resist

horizontal/lateral dislodgement three factors :

1. magitude and 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)

- 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 and II gold alloy : intracoronal cast restorations

- Type III and 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