pros 1 chapter 3

PRINCIPLES OF TOOTH PREPARATION

3 PRINCIPLES OF TOOTH PREPARATION

The principles of tooth preparation may be divided into three broad categories:

1. Mechanical considerations, which affect the integrity and durability of the restoration

2. Biologic considerations, which affect the health of the oral tissues

3. Esthetic considerations, which affect the appearance of the patient

I. MECHANICAL PRINCIPLES

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MECHANICAL PRINCIPLES

Forces can be directed against the prosthesis during function.

Significance?


Types of Occlusal Forces1. Tipping force2. Twisting or rotational force3. Path of insertion force


Types of Occlusal Forces

1. Tipping force- Can occur in

buccolingual or mesiodistal directions.


Types of Occlusal Forces1. Tipping force

2. Twisting or rotational force- May cause a restoration to start to

move circumferentially around the prepared tooth

MECHANICAL PRINCIPLES Types of Occlusal Forces

1. Tipping force

2. Twisting or rotational force

3. Path of insertion force- can be apically or occlusally directed- depending on whether the mandible is closing into a bolus of food or opening with sticky food interposed between the prosthesis and opposing teeth.


MECHANICAL PRINCIPLESFactors Preventing Restoration

Dislodgement 1. Axial Wall Height

Factors that will affect the length of occlusocervical height of the abutment

1.1. magnitude of occluding force1.2. span length1.3. type of preparation1.4. length of the lever arm1.5. bone support



- MINIMAL ACCEPTABLE HEIGHTHeight which allows the tooth structure

to interfere with the arc of rotation as tipping forces attempt to cause rotation a fulcrum located at the finish line on the opposite side of the tooth



On short teeth, adequate axial wall height may only be achieved by extending the finish line

- subgingivally- onto the root surface- not desirable or advanageous

Alternative - prepare tooth with less taper


Dislodgement 2. Taper of the Preparation

Opposing walls must converge occlusally

Divergent walls produce undercuts and prevents seating of restoration


Dislodgement 2. Taper of the

PreparationIncreased taper

reduces the ability of the restoration to

- resist occlusally directed dislodging forces

- lessens its

ability to interfere with the arc of rotation as tipping forces act to unseat the restoration


Dislodgement 2. Taper of the Preparation

A total convergence of 3-5 degrees is considered ideal


Dislodgement 3. Ratio of Preparation Diameter to

Axial Wall HeightIt is often mistakenly assumed that a

large diameter tooth will yield a more retentive preparation than a smaller diameter ones.


Dislodgement 3. Ratio of Preparation Diameter to

Axial Wall HeightIf the axial wall height and taper are

the same for both teeth, the smaller diameter tooth interferes more effectively with the arc of rotation because the smaller radius of curvature allows the preparation to better resist the dislodgement.


Dislodgement 4. Circumferential Irregularity

Circumference of the tooth is usually irregular therefore uniform reduction will create an irregular shaped abutment.

- resists tipping and twisting forces.


Dislodgement 4. Circumferential Irregularity

In round, short and/or overtapered abutment- intentional placement irregularities are done

- forms: boxes grooves

- placement: middle of proximal surface


Dislodgement 5. Occlusal Irregularity

Occlusal reduction following the anatomic form produces an irregular surface which aids in retention.

Irregularities can be used to enhance resistance to dislodgement

- example: pinholes- occlusal, cingulum, incisal


Dislodgement 6. Rigidity

Prosthesis must be thick enough to resist flexure and loosening.

Occlusal reduction – minimum of 1-1.5 mm

Axial reductionocclusal area- minimum of 1 mm cervical area – minimum of 0.3-0.5 mm



Dislodgement 7. Adaptation

Small amount of space is required between a restoration and the prepared tooth to allow complete seating during cementation.

Excessive space reduces resistance to dislodgement by placing to much dependence on the physical properties of the luting agent.


Dislodgement 8. Surface area

Increased surface area is most significant when the additional area results in greater axial wall length.

MECHANICAL PRINCIPLESFinish Line

The point at which a preparation

terminates on the tooth.

MECHANICAL PRINCIPLESFunctions of a Finish

Line 1. During visual evaluation of tooth

preparation, it is a measure of the amount tooth structure already removed.


Line 2. One of the features that can be used

to evaluate the accuracy of the impression made for the indirect procedures.


Line

3. On the die, a distinct finish line helps in the evaluation of the quality of the die and aids in trimming it accurately.

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Line 4. The correct

marginal adaptation of the wax pattern depends on an obvious finish line.


Line 5. The evaluation of the restoration is

also aided by the proper finish line.


Line

6. At cementation, a sharp finish line aids in determining whether the restoration is fully seated.

MECHANICAL PRINCIPLESForms of Finish Line

1.Chamfer

- preferred cervical finish line for fixed prosthodontics.- should be utilized whenever possible because it is easily developed and visually distinct.


2. Knife edge or

chisel edge finish- not as well defined as chamfer.- often used on tipped teeth when formation of a chamfer would result in excessive tooth reduction.


3. Feather edge

- unacceptable because –- not sufficiently distinct- results in so little cervical

tooth reduction - restoration must be over

contoured to possess adequate rigidity.

- since a feather edge is difficult to see visually, rregularities in the finish line are more likely to be present, making it more difficult to fabricate a restoration that fits accurately.


4. Shoulder and beveled

shoulder

- difficult to form

- produces a greatest depth of tooth reduction

- required with ceramic restorations because proper color is achievable only through material thickness.

MECHANICAL PRINCIPLESINSTRUMENTATION

MECHANICAL PRINCIPLESInstrumentation

Rotary instruments must be selected that allow the tooth to be reduced

1. according to the requirements of proper retention and resistance form and

2. finish line development.

MECHANICAL PRINCIPLESInstrumentation

Kinds of Rotary Instruments

1. Diamond cutting instruments.

- Have diamond particles attached to a concentric metal shaft.

- Available in coarse, medium and fine grit

2. Dental burs possessing carbide cutting blades.

MECHANICAL PRINCIPLESVisibility

Ways on achieving good visual access

1. Use of fiberoptic handpiece lights and numerous lights aimed at the oral cavity from different directions greatly aids in the visibility.

2. Removing of excess oral fluids.

3. Retraction of soft issues that interfere with vision.

II. BIOLOGIC PRINCIPLES

BIOLOGIC PRINCIPLES

Teeth must be prepared in a manner that creates the least amount of trauma to the pulp.

- retain as much tooth structure as possible

- proper use of rotary instruments and the application of surface coolants

Pulpal Consideration

BIOLOGIC PRINCIPLESPulpal Considerations

1. Conservation of tooth structure.2. Depth of reduction.3. Speed of reduction.4. Instrument age and use of

pressure.use of coolants.


1.Conservation of tooth structure.

Retain as much as tooth structure as practicable


1. Conservation of tooth structure.

2.Depth of reduction.

WAYS ON CONTROLLING THE AMOUNT OF REDUCTION

1.Best provided by placing strategically located depth cuts in the unprepared tooth surfaces and placed to the desired depth.





2. The intervening tooth structure is removed by using the base of the depth cut as a guide to proper reduction.





3. If caries removal will make the preparation excessively deep, place an insulating base material (minimum of 0.5mm) over the area in proximity to the pulp.

* For adequate protection against thermal shock



2. Depth of reduction.

3.Speed of reduction. Rapid continuous removal of tooth structure causes

rapid heat build-up that may cause irreversible pulpitis.

- reduction must be performed INTERMITTENTLY in a STEADY and CONTROLLED MANNER to avoid EXCESSIVE HEAT BUILD-UP.

4. Reducing the tooth for a period of 5-10 seconds

5. Then remove the instrument from the surface for a few seconds




3. Speed of reduction.

4.Instrument age and use of pressure.

Only sharp instruments should be used for bulk tooth reduction.

- Dull instruments create more friction , thus more heat is produced.

- Use of excessive pressure should be avoided because this will cause undue heat generation.

- Accentuated if worn instruments are used.




3. Speed of reduction.

4. Instrument age and use of pressure.5. Use of coolants. Delivery of water stream from handpieces during

reduction.

Disadvantage: interferes with vision

Periodontal Consideration


Supragingival location for the finish line:- allows good visual access for

evaluating finish line forms.- facilitates accurate impression of

prepared tooth.- allows more accurate assessment of

prosthesis fit and contour.- Provides access for marginal

refinement and polishing.- permits more accurate long term post

insertion evaluation of marginal integrity


- MOST important reason relates to the preservation of periodontal health.

NOTE: There is no junction of any restorative material and the tooth that is as smooth as intact tooth structure.

- marginal plaque acculmulation is inevitable and when this occurs subjingivally, it is not easy for the patient to remove it and the likelihood of adverse periodontal changes increases.

- if tooth preparation extended beyond the gingival margin for some reasons, care must be exercised to avoid excessive tissue trauma.

III. ESTHETIC PRINCIPLES

Achieving a color that matches the surrounding teeth necessitates a certain minimal thickness in the ceramic material.

This is accomplished by adequate and uniform reduction of the facial surface.

Use of Depth Guides1. Prevents excessive depth reduction.2. Ensures adequate uniform reduction.

pros 1 chapter 3

Education

mechanical principlesfactors

restoration dislodgement

mechanical considerations

mechanical principles

seating of restoration

mechanical principles

dislodgement example

tipping forces