basic principles of tooth preparation

Needs & Objectives The needs for preparing the tooth structure and the objectives of

these preparations should be discussed in details to draw the rules, requirements and criteria of proposed preparations. This should clarify that operative dentistry is not a drill and fill policy. Biology & Mechanics The student should be aware that we are dealing with a respectful living structure that should be biologically considered. Also, we are working in a complex mechanical environment of the oral cavity that should be considered in different restorative treatments of the tooth structure. Variations

A complete understanding of the variations of the characters of different preparations and properties of available restorations should be developed.

Chapter V BASIC PRINCIPLES OF TOOTH PREPARATION

Learning Objectives

By DR. YASSER ALI AL-MORTADA AL-WASIFI

LECTURER OF OPERATIVE DENTISTRYAIN SHAMS UNIVERSITY

BASIC PRINCIPLES OF TOOTH PREPARATION. AL-WASIFI, Y.A. __________________________________________________________________________________________

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he displine of operative dentistry harbors the essential knowledge of basic

tooth restoration, which is of utmost importance to dental practitioner.

This basic knowledge must not be constructed as simply treating a tooth, but rather

in context of treating a person. The physiologic and psychologic aspects of the

patient must be given proper consideration. In the sense of local treatment, biological

and mechanical factors regarding care of the tooth tissues and contiguous oral tissues

are paramount.

In the past, most restorative treatment was due to caries (decay), and the term

“cavity” was used to describe a carious lesion in a tooth that had progressed to the

point that part of the tooth structure had been destroyed. Thus, the tooth was

cavitated (a breach in the surface integrity of the tooth) and was referred to as a

“cavity”. Likewise, when the affected tooth was repaired, the cutting or preparation

of the remaining tooth structure to best receive a restorative material was referred to

as a “cavity preparation”.

Now, many indications for treatment for teeth are not due to caries and

therefore, the preparation of the tooth is no longer referred to as “cavity preparation”

but as “tooth preparation”, and the term “cavity” is used only as a historical

reference.

DEFINITION Tooth preparation is defined as, any alteration of the defective, injured or

diseased tooth in order to best receive a restorative material which will re-establish a

healthy state for the tooth including esthetic correction where indicated, along with

normal form and function.

T


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NEEDS AND OBJECTIVES OF RESTORATIVE INTERVENTION Teeth need restorative intervention for a variety of reasons:

1. Repair a tooth after destruction from carious lesion.

2. Another often-occurring need is the replacement or repair of restorations with

serious defects.

3. As previously mentioned, esthetic demands of patients is a reason for placing

and replacing restorations.

4. Restorations are also required to restore form and function.

5. For restoration of occlusion.

6. Repair of fractured tooth.

7. Lastly, a tooth may be restored in a preventive sense.

In general, the objectives of tooth preparation are:

1. To remove all defects and give the necessary protection to the pulp.

2. Locate the margins of the restoration as conservatively as possible.

3. Form the cavity so that the tooth or the restoration will not fracture under forces

of mastication and the restoration will not be displaced.

4. Allow for the esthetic and functional placement of the restorative material.

PRINCIPLES OF TOOTH PREPARATION There are some general and fundamental principles, which must be realized in

preparing cavities for the reception of restorative materials. These principles are

essentially discussed under biologic and mechanical forms.

The biological concept aims to preserve the tooth vitality and function and

protect the supporting structures. This is performed by following definite steps

during cavity preparation to minimize irritation to vital tooth structure. These steps

are:


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a) Pulp protection against mechanical, thermal and chemical irritation during cavity

preparation.

b) Prevention of caries recurrence.

c) Working in a completely aseptic field.

The mechanical concept is primarily concerned with the preservation of the

structural integrity of both the tooth and restoration and retaining the restoration

inside its corresponding preparation. This can be achieved through a correct

mechanical cavity design as follows:

1. The cavity should be designed to decrease the magnitude of the destructive

stresses acting on the remaining tooth and the restoration.

2. The cavity design should decrease the deleterious and damaging effect of tensile

stresses created within the tooth as a result of defect.

3. Provision of adequate means of retention to prevent displacement of restoration

under functional forces.

As stated earlier by G.V. Black, the mechanical concept is performed by

following tooth preparation procedure which is divided into several steps. Each

should be thoroughly understood and each step should be accomplished as perfectly

as possible. There are occasions, however, when the sequence is altered, but this is

the exception and not the general rule.

These steps are:

a) Obtaining the outline form.

b) Performing resistance and retention forms.

c) Convenience form.

d) Removal of any remaining infected dentin.

e) Finishing of enamel and external cavity walls.

f) Cleaning and toileting.


5

Remember that

The biological concept aims to preserve the tooth vitality and function and protect the supporting structures.

The mechanical concept is primarily concerned with the preservation of the structural integrity of both the tooth and restoration and retaining the restoration inside its corresponding preparation.

The steps followed to perform the biological concept are:

a) Pulp protection against mechanical,

thermal and chemical irritation during

cavity preparation.

b) Prevention of caries recurrence.

c) Working in a completely aseptic field.

According to G.V. Black, the steps followed to perform the mechanical concept are:

a) Obtaining the outline form. b) Performing resistance and retention

forms. c) Convenience form. d) Removal of any remaining infected

dentin. e) Finishing of enamel and external cavity

walls. f) Cleaning and toileting.

I. BIOLOGICAL CONCEPT OF TOOTH PREPARATION

I-A) PULP PROTECTION:

1) Against mechanical irritation:

To perform pulp protection against mechanical irritation during cavity

preparation, the following precautions should be considered:

Avoid direct traumatic injury of the pulp.

Avoid unnecessary pressure and wrong direction of instruments.

Avoid cutting across the recessional lines of the pulp chamber.

Avoid over cutting of dentin and weakening of the tooth structure.

Avoid sharp line angles within the cavity, which act as stress concentration

areas.


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2) Against thermal irritation:

To perform pulp protection against thermal irritation during cavity


Avoid heat generation during cavity preparation.

Avoid working without coolant.

Avoid long time working.

3) Against chemical irritation:

To perform pulp protection against chemical irritation during cavity


Avoid using of chemicals and caustics for toilet of the cavity.

Avoid using air jet for long time.

I-B) PREVENTION OF CARIES RECURRENCE:

To decrease the incidence of caries recurrence after cavity preparation, the

following precautions should be considered:

Removal of all carious enamel and dentin.

The cavity outline should be extended to include all pits, fissures and vulnerable

areas to caries

Proper extension of cavity margins to self-cleansable areas.

Removal of all undermined enamel. Undermined enamel is that not supported

by sound dentin, which may fragment under force leaving a marginal ditch that

leads to food accumulation and recurrent caries.

Proper inclination of CSA suitable with the type of restoration to provide

support to the restoration and enamel at cavity margins that prevents their

fragmentation and recurrence of caries.


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I-C) WORKING IN A COMPLETELY ASEPTIC FIELD:

It is unwise to treat a tooth and infect the patient with a dangerous systemic

disease that could be transmitted via the dental office. This constitutes a horrible

issue to most of the dental patients allover the world. To reduce the risk of cross

infection, the followings should be considered:

Use sterile instruments.

Use disposable tools as much as possible.

Application of rubber dam to keep the fields clean and dry, and decrease the

chance of droplet infection.

Follow the instructions of ADA and WHO to control infection in dental office.

II. MECHANICAL CONCEPT OF TOOTH PREPARATION

II-A) OBTAINING THE OUTLINE FORM:

Definition:

Outline form of prepared cavity is defined as the external shape of the

completed cavity boundaries, i.e. the shape or pattern of CSA of the prepared cavity.

Fundamentals of outline form:

To obtain ideal outline form, the following cardinal rules should be considered

(Fig. 5-1):

1. All the circumference of carious lesion must be included within the outline.

2. All pits, fissures, grooves and retentive areas must be included within the outline.

3. The cavity margins should be extended to sound tooth structure without

undermined enamel and in a self-cleansable area.

4. All carious and undermined enamel must be included in the prepared cavity.

5. The outline must be in the form of harmonious sweeping curves, in order to:

a) Prevent caries recurrence.

b) Avoid stress concentration areas.


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Fig. 5-1: Designing the outline form for pits and fissures lesion. A, The carious lesion should be included. B, Also the weakened unsupported enamel is included. C, The outline form includes all defective pits, fissures and grooves. D, The outline of the cavity should be extended to area self-cleansable. E, The final shape of the outline form for Class I carious lesion.

c) Obtain better esthetics.

6. Cavities approaching each other must be connected to avoid leaving a weak ridge

between them which is liable to fracture.

7. Extension for prevention or cutting for immunity.

Extension for prevention (Cutting for immunity):

Definition:

Extension of cavity margins to a self-cleansable area to decrease the possibility

of caries recurrence.

Factors affecting extension for prevention:

1. The extent of surface involvement in enamel.

2. The lateral spread of caries at DEJ, i.e. degree of undermining.

3. Esthetic demands necessitate the use of special outline with minimal extension.


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4. Oral hygiene; the better the oral hygiene, the lesser will be the extension.

5. Patient age; the older the patient, the lesser will be the extension due to tooth

attrition.

6. Force of mastication; the more to be ideal, the lesser will be the extension.

7. Restorative material as different extensions is used with different restorative

materials.

Technique:

1. Extending the occlusal outline up to 2/3 of the cusp inclined plane.

2. Extending the proximal outline up to the axial line angles of the tooth in direction

bucco-lingual.

3. Extending the gingival floor below the health gingival margins.

4. Extending the outline of cervical buccal and lingual cavities (Class V) up to above

the maximum height of contour and below the healthy gingival margin.

Adverse effects of extension for prevention:

1. Weakening of the sound tooth structure.

2. Increased irritation to the pulp.

3. Increased liability for gingival and periodontal problems in compound cavities.

4. Increased liability to recurrent caries.

Factors modifying the outline form:

1. Contact area in proximal cavities:

Location, dimensions and tightness of contact area determine the isthmus

outline of compound class II cavities to ensure the cavity outline is located in area

self-cleansable, i.e. the embrasures.

The isthmus outline will follow one of the Ingerham′s lines according to the

width of the contact area.


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Fig. 5-2: To ensure proper freeing of the contact area and placing the cavity walls in the embrasures, the outline form will follow one of the Ingrham’s lines. A, Straight line of small contact area. B, Uniform curve in case of moderately sized contact area. C, Reverse curve in case of wide contact area.

These lines may be (Fig. 5-2):

Straight; for cases with open or too small contact between teeth

Uniform or universal; for cases with moderate touch or plus contact between

teeth.

Reverse curve; for cases with tight and broad contact between teeth.

Remember that

• The reverse curve outline is always required in the buccal wall rather than the lingual

as the contact area is always shifted toward the buccal rather than lingual.

• The reverse curve outline could only be performed in the lingual wall when the caries

extension is far lingual at the isthmus portion.

• The reverse curve outline provides the adjusted required CSA at 90º and also conserves

much more tooth structure.


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2. Age of the patient:

Young aged patient shows increased liability for caries recurrence. So, the

outline should be well extended to insure all carious lesion and retentive area. At the

same time conservative due to increased liability to remake the cavity as a result of

caries recurrence.

3. Oral hygiene of the patient:

Patients with good oral hygiene, ultra conservative outline form could be

performed. While in cases with bad oral hygiene, all caries susceptible areas must be

included in the cavity outline and the cavity should be extended to area self-

cleansable.

4. Physical properties of the restorative material:

Brittle materials are in need for depth bulk with minimal width to decrease the

surface of restoration exposed to occlusal force and to increase their strength. So, the

outline width should be minimal as much as possible. In case of ductile materials, no

bulk depth or width specifications are needed.

5. Technique of construction of restoration:

Indirectly constructed restorations, such as cast gold restoration or esthetic

inlays, require further widening of the outline to allow easy and accurate

manipulation of the impression, wax pattern and casting of the metal.

6. Esthetic needs:

The outline extension may be affected by the location of the cavity. In anterior

teeth ultra conservative outline form should be performed in order to preserve the

natural esthetic appearance of the tooth structure as much as possible. So,

undermined labial enamel wall of Class III cavities could be left to preserve the

natural tooth appearance.


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For esthetic demands, the gingival margin of Class V cavity preparation should

be hidden subgingivally and the incisal wall is just limited to the defect in case of

tooth colored restorations in the anterior region and the incisal wall is given the

shape of a graceful curve for maximum esthetics. Graceful curve is performed by

cutting the incisal wall parallel to the curvature of the labial height of contours.

7. Convenience:

Areas of inaccessibility do hinder proper instrumentation and restoration. So,

slight extension of the cavity outline is indicated.

II-B) PERFORMING RESISTANCE & RETENTION FORMS:

Definitions:

Resistance form is defined as that form given to prepared cavity to prevent

fracture of remaining tooth structure or/and the restoration.

Retention form is defined as that form given to prepared cavity to prevent

displacement or dislodgement of the restoration out of the prepared cavity.

Remember that

• Resistance and retention forms are two distinct but yet, inseparable and interrelated

steps.

Factors affecting stress response of tooth structure and restoration:

For proper designing of the resistance features of the prepared cavity, a detailed

understanding of the stress response of both the remaining tooth structure and the

restoration should be obtained.

The stress response of the remaining tooth structure and the restoration is

affected by 3 major factors, which are:


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a) The occlusal loading force.

b) The cavity design.

c) The physical properties of the restorative material.

a) The occlusal loading force:

Occlusal loading force affects the stress response of tooth and restoration

through its magnitude, direction and character.

1. Magnitude: That differs from patient to another according to the action of the

mastication muscles, type of occlusion and inter-cuspation, type of food, age and

sex. Also, it varies from location to another in the same patient and also from

time to another.

2. Direction: It may be directed as compressive, tensile or shear and this depends on

the form and shape of the loaded surface as well as the inclination of the cavity

walls at which the stresses transmitted will be analyzed 3. Character: It may be a static force in centric occlusion, dynamic in eccentric lateral

movement and cyclic and repetitive during masticatory function.

We can conclude that cyclic force with different magnitudes and directions are

present during mastication producing fatigue of the restorative material and enhance

its fracture.

b) The cavity design:

1. Walls and floors: Should be either parallel or perpendicular to the long axis of the

tooth to decrease the analysis of force into destructive tensile components (Fig. 5-

3). Also, they should be flat and smooth to avoid stress concentration and to

provide equal distribution of occlusal stresses.


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Fig. 5-3: Flat pulpal floor. A, Correct angulation of pulpal floor parallel to the occlusal plane and at right angle to the occluding force. B, Flat pulpal floor but with incorrect angulation.

Fig. 5-4: Minimal cavity width 1/4 - 1/3 the intercuspal distance will decrease the exposed area of restoration to occlusal loading force.

2. Cavity width and depth: Both of them are responsible to provide bulk to the

restoration. Increasing the bulk of the restoration especially brittle ones, increase

the strength of the material and its resistance to fracture. But, it was proofed that

it is better to provide bulk to the restoration through depth (within limits to

avoid pulpal irritation) rather than width. Increasing the width will weaken the

remaining tooth structure and the restoration as it leads to increased surface area

of the restoration exposed to occlusal force. It is recommended to get a cavity

depth of 0.5 – 1mm beyond the DEJ and a cavity width of 1/4 - 1/3 the inter-

cuspal distance (Fig. 5-4).

3. Conservation: Maximum conservation of remaining sound tooth structure as

much as possible is recommended to avoid their fracture.

4. Line angles: All axial line angles of the prepared cavity should be rounded in the

form of sweeping curves to avoid stress concentration. For compound cavities,


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Fig. 4-5: Rounding the axio-pulpal line angle to reduce stress concentration on the restorative used.

Fig. 5-6: The cavo-surface angle with brittle restoratives should be adjusted to the best compromise between the tooth and restoration. A) An acute CSA strengthens the margins of brittle restoratives but undermines the inner ends of enamel rods and grossly weakens the enamel margin and therefore is absolutely contraindicated. B) An obtuse CSA protects the outer ends of enamel rods and strengthens the enamel margins but grossly weakens the margins of brittle restoratives. C) The best compromise is obtained with a 90° CSA, which results in a sound enamel margin with no undermined rods and a strong restoration margins.

rounding of the axio-pulpal line angle is recommended as it leads to decreased

stress concentration and adds bulk to the restoration (Fig. 5-5).

5. Cavo-surface angle: It should have a correct angulation suitable with the physical

properties of the restorative material and the direction of enamel rods. For brittle

materials such as amalgam CSA should have 90 º angles to get the strongest

enamel wall and provides strength to amalgam at margins. Any deviation from

correct angulation may lead to fracture or tooth structure or restoration at margin

(Fig. 4-6). For ductile materials such as gold, it should be 135º to allow burnishing

of the gold over the enamel margin that provides protection.


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6. Amount of retention: Adequate amount of retention for each part of the cavity

increases the stability of the restoration under stresses. In compound prepared

cavities, each part should have its own ample independent retention to avoid

fatigue and fracture of the restoration at the isthmus area.

7. Weak cusp: Weak cusp is that which have its base smaller than its heights that

may lead to cusp splitting under loading force. Such a cusp should be reduced to

decrease the height in relation to its base. This is called cusp tipping in case of

using amalgam to cover this cusp and should be for at least of 2 mm to provide

strength to the amalgam. It is also called cusp coverage by performing counter

bevel in case of using cast gold restoration to protect such a cusp, i.e. In-onlay.

c) The physical properties of restorative material:

The tensile type stresses are the significant stresses for the brittle substances as

amalgam, cements and porcelain because they have high compressive but very low

tensile and shear strength values. These substances are especially sensitive to tensile

stresses, i.e. they cannot withstand high tensile stresses without fracture and cannot

be finished to thin margins otherwise ditching will occur.

On the contrary to the strong and ductile gold this is utilized for protection of

the weakened tooth structure.

Forms of resistance:

The design features of cavity preparation that enhance primary resistance form

are:

1. Relatively flat floors.

2. Box shape, which provides the following advantages:

The seat of the restoration (pulpal and gingival wall) is placed at a distinct

right angle to the direct of functional stresses.


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Fig. 5-7: Box form cavity will provide flat pulpal floor, A) that will prevent restoration movement, whereas rounded pulpal floor, B) is conductive to restoration rocking action producing a wedging force, resulting in splitting of tooth structure.

The tendency to split the buccal and lingual cusps of bicuspids and molars

by forces transmitted through the restoration is greatly diminished since

the inverted truncated cone shape prevents the wedging action of the

restoration inside the tooth (Fig. 5-7).

It provides retention by friction due to relative parallelism of the axial

walls

It allows access to and easy visualization of the interior of the cavity,

which allows for an easier and better instrumentation and filling.

Restorative materials tend to adapt better against its plane surfaces.

It allows the employment of retention features in dentin

3. Inclusion of weakened tooth structure

4. Preservation of cusps and marginal ridges

5. Rounded internal line angles

6. Adequate thickness of restorative material

7. Seats on sound dentin peripheral to excavations of infected dentin (creation of

dentin ledges).

8. Reduction of cusps for capping when indicated.


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Types of retentive features:

a) By utilizing dentin:

1. Mechanical undercuts; through converging the cavity walls occlusally (Fig. 4-8a).

2. Frictional wall retention; between parallel opposing cavity walls (Fig. 4-8b).

3. Gripping action of dentin; due to its visco-elasticity, stress relaxation of dentin

will grip the restoration. This will occur only with gold foil restoration.

4. Pin retention.

b) By modifying the cavity outline:

1. Dove tail lock; in compound proximal cavities (Fig. 5-9).

2. Occlusal lock; in compound proximal cavities.

3. Buccal and lingual extensions; to prevent proximal displacement.

4. Extension for retention; performed by extension to the other side of the cavity.

Fig. 5-8: A) Basic primary retention form in Class II cavity preparation for amalgam with longitudinal external walls of proximal and occlusal portions converging occlusally and B) for cast inlay with similar walls slightly diverging occlusally.

Fig. 5-9: Occlusal dove tail that adds retention and prevent tipping of the cast restoration.


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Fig. 5-10: Cutting proximal axial grooves to provide lateral retention should be in expense of buccal and lingual walls rather than axial wall to provide lateral retention and avoid pulp exposure.

Fig. 5-11: Reverse gingival bevel cut with GMT of tapered fissure.

c) By modifying cavity design:

1. Proximal axial grooves; in amalgam cavity, it extends from gingivo-axial line

angle up to the axio-pulpal line angle, along the axio-buccal and axio-lingual line

angle with undercut. In gold cavity, it extends up to CSA without undercut (Fig.

5-10).

2. Grooves in dentin line angles.

3. Reverse gingival bevel at the gingival floor with gold inlay. It prevents proximal

displacement and rotation around axio-pulpal line angles (Fig. 5-11). It could not

be used with amalgam cavities as it will lead to stress concentration and mercury

accumulation at axio-gingival line angle resulting in excessive weakening of

amalgam.


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d) Special retentive features:

1. Adhesive systems for composite restoration.

2. Dowel pin retention, i.e. post inside the root canal.

Factors affecting selection of retentive form:

1. Type of restorative material.

2. Available amount of remaining tooth structure.

3. Esthetic demands.

4. Amount of retention needed.

5. Pulp vitality; for non vital pulp will not provide gripping action of dentin due to

excessive dehydration.

6. Type of occlusion; abnormal occlusion increases the magnitude of stresses.

Remember that

• Stability is mean prevention of restoration displacement towards the center.

• It is gained by:

1. Definite cavity walls.

2. Flat pulpal floor.

3. Definite and slightly rounded line angles.

• Retention means prevention of restoration displacement towards the periphery.

• It is either:

1. Axial; against vertical displacement.

2. Lateral; against lateral displacement.


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II-C) CONVENIENCE FORM:

Definition:

Convenience form is defined as that shape given to the cavity to make it easily

seen, reached, instrumented and restored.

Convenience features:

1. Accentuation of point and line angles.

2. Slight extension of cavity outline to facilitate insertion and condensation of the

restorative material.

3. Roundation of axial line angles.

4. Beveling of enamel wall with gold restoration to provide room for burnishing the

gold to protect marginal enamel. It is also performed with composite resin cavity

preparation to increase surface area for acid etching.

5. Selection of smaller specially designed instruments that enable the operator to

prepare surfaces which are difficult to reach.

II-D) REMOVAL OF REMAINING CARIOUS DENTIN:

Definition

It is the process of removing decay and decalcified enamel and dentin; and

determining the prepared cavity depth.

Caries pattern:

Caries in enamel follows the direction of enamel rods in a triangular pattern

with its base toward the DEJ. The caries then shows a lateral spread at DEJ then

follows the direction of dentinal tubules of dentin.

The routine cavity depth:

The routine cavity depth should be extended 0.5 – 1mm beyond DEJ in order to:

1. Avoid cutting at this sensitive area; i.e. DEJ.


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2. To get sure that there is no undermined enamel resulted from the lateral spread

of caries at DEJ.

3. To detect lateral spread of caries at DEJ.

4. To provide sufficient bulk of the restoration.

5. To add retentive features.

Conditions at routine cavity depth:

Three conditions may be found at the routine cavity depth.

a) Hard sound viable dentin:

It is considered as the best condition. Finish enamel walls and apply either

varnish in case of amalgam or calcium hydroxide liner in case of composite.

b) Hard discolored dentin:

It is considered as sound dentin but the discoloration is due to the

chromogenicity of microorganisms. In posterior teeth, it could be left and the final

restoration could be placed. In anterior teeth, it must be removed as it appears from

enamel affecting esthetic.

c) Soft dentin:

Which is painful, denoting presence of viable protoplasmic processes, and

which may be discolored (chronic caries) or not (acute caries). This layer constitutes

the floor of deep and moderately deep cavities. Such soft dentin must be removed

since it is carious and if left will extend to involve the pulp.

If still soft, caries should be removed selectively forming dentin ledge and the

cavity will be deep that needs sub-base and base before placing the final restoration.

If there is still soft caries, it must be evaluated, either acute or chronic caries. If it

is an acute caries, it could be left and sub-base and base are placed before placing the


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final restoration. Only apply calcium hydroxide as indirect pulp capping, as the last

layer is sterile. If it is a chronic caries, it should be removed even pulp exposure

occurs, as the last later is infected.

Acute caries Chronic caries

Histology

Acid penetration occurs before

bacterial invasion, so the last

layer is sterile, it is just

decalcified; i.e. Affected and

not infected.

Acid penetration coincides with

bacterial invasion, so the last

layer is both infected and

affected.

Last layer

Could be left, only apply

calcium hydroxide to neutralize

acidity, i.e. indirect pulp

capping.

Could not be left and must be

removed even if leads to pulp

exposure.

Patient age Young, less than 20 years. Old, more than 40 years.

Site First molar teeth and lower

anterior teeth. Anywhere.

Duration Short by months. Long by years.

Color Yellow. Dark brown.

Consistency Soft and removed in flacks. Harder and removed in debris.

Pulp reaction Hyperemia. Degeneration and necrosis.

Instruments used to remove caries:

1. Hand instruments:

Excavators with different shapes and sizes could be used in a direction parallel

to the pulp horns from the cavity periphery to the center with scooping motion. It

provides no heat generation.


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2. Rotary instruments:

Large round bur with low speed and without pressure.

Remember that

• Dentin ledge is a three-dimensional form in a level pulpal to the cavity. It impairs the

resistance form, so it should be lined with sub-base and base to the proper level of the

pulpal floor.

• Dentin bridge is the thickness of dentin protecting the pulp.

II-E) FINISHING OF ENAMEL WALL:

Objectives:

1. To give the CSA its correct inclination.

2. To remove any undermined enamel.

3. To produce smooth enamel walls that increases adaptation of the restoration.

4. To keep the rounded and convenient outline with proper cusp contours.

Requirements of enamel wall:

According to NOY’S rules for keeping strong enamel wall at margins:

1. Enamel wall must rest on sound dentin.

2. Enamel rods which form the CSA must have their inner ends resting on sound

dentin.

3. The outer ends of enamel rods must be covered by the restorative material and

inner ends rest on sound dentin. This establishes the strongest required enamel

wall. This had done by beveling the enamel rods and using a strong ductile

restorative material, e.g. Gold inlays.

4. The enamel walls and margins should be finished smooth, free from short, loose,

friable or undermined enamel rods.

5. The enamel wall must take the same inclination of enamel rods.

6. The enamel wall must have an inclination suitable with the physical properties of

the restorative material.


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7. The enamel wall must be beveled in case of using high strength restorative

material.

Factors affecting inclination of CSA

1. Direction of enamel rods.

2. Location of the cavity wall.

3. Friability of enamel.

4. Degree of marginal strength of the restorative material. CSA 90° for brittle

material as amalgam and CSA 135° for ductile material as gold.

Remember that

• Undermined enamel is only left in the labial surface of class III because of:

Esthetic needs.

It is subjected to minimal force produced from lip musculature.

The destructive force is away from it as it is in a palato-labial direction.

Beveling of enamel wall:

Definition:

Beveling means increasing CSA inclination more than 90°.

Types:

1. Short bevel involving part of enamel thickness.

2. Long bevel involving the full enamel thickness up to DEJ.

3. Full bevel including enamel and dentin up to the pulpal floor.

4. Counter bevel made against enamel rods of the cusp to make inlay with cusp

coverage, i.e. in-onlay.

There is only one bevel made in dentin, which is the reverse gingival bevel for

retention of Class II gold inlay.


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Functions of bevel:

1. Protection of weak enamel rods.

2. Facilitate burnishing of ductile materials.

3. Protection of cement line from solubility.

4. Protection of weak cusps → counter bevel.

5. Making the enamel wall parallel to the direction of enamel rods.

6. Increasing surface area for acid etching needed for retention of composite

restoration.

7. Better esthetic and decreased demarcation of composite restoration.

8. Add retention → reverse gingival bevel.

Instruments used for finishing of enamel wall:

1. Hand instruments; Chisel and hatchets are the instruments of choice with no heat

generation.

2. Rotary instruments; Cutting fissure bur.

II-E) TOILET OF THE CAVITY:

Definition:

The process of removing all debris from the prepared cavity, e.g. cut chips,

blood, saliva and bacteria.

Objectives:

1. Increasing adaptation of the restoration to cavity walls.

2. Prevents contamination of the restorative material.

3. Enables the operator to examine properly all steps.

4. The cavity should be clean and dry before insertion of the filling material.


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Technique:

1. Phenol; but it leads to pulp necrosis.

2. Silver nitrate; but it leads to:

Discoloration of tooth structure.

Irritation to the pulp.

Tarnish of amalgam.

3. Alcohol; leads to:

Dehydration of dentin.

Pulp irritation.

4. Hot air blast; leads to dehydration of dentin.

5. Hydrogen peroxide 3%:

Highly effective through its effervescent action.

Should be washed immediately with warm water to avoid thermal pulp

shock.

6. Water spray:

The best as it is not a medicament.

Cotton pellet to dry the cavity to reduce the use of air stream.

Removal of remaining water by air for short time.

Remember that

• Adaptation is the maximum degree of proximity between the restoration and the tooth

structure.

• For proper adaptation:

The tooth surface should be smooth, dry and clean.

The restoration should be properly constructed.

basic principles of tooth preparation

Documents

tooth tissues

tooth vitality

diseased tooth

affected tooth

cavity preparation

remaining tooth structure

vital tooth structure

repair of fractured