The principles of cavity preparation

The clinical form of cavity preparation:

The principles of modern cavity preparation were first defined in 1896

by Dr. G.V. Black, a man considered by many to be the "father of modern

restorative dentistry". When his original sequence of cavity preparation was

described, "extension for prevention" of recurrent decay was one of the most

important considerations of cavity design. However, the degree of cavity

extension is considerably less today, because of the realization that caries,

primary or secondary, can be more effectively controlled and prevented by

greater emphasis on oral hygiene by the patient.

Although techniques have been refined and outlines have been

modified, Black's Principles are still used as the basic steps in the process of

designing and preparing a cavity. The adherence to these principles will

facilitate the procedure of cavity preparation and decay removal.

Thus, this systematic approach is still recommended to the operative

dentist as reliable guidelines for today.

The principles as suggested by Dr. Black are:

1. Obtain the required outline form.

2. Obtain the required resistance form.

3. Obtain the required retention form.

4. Obtain the required convenience form.

5. Remove any remaining carious dentin.

6. Finish the enamel wall.

7. Cleanse and medicate the cavity preparation.

Normally, these principles of cavity preparation are followed in the

sequence as outlined above.

MECHANICAL PRINCIPLES

This shows a definite procedure of thought and instrumentation to

produce a lasting restoration that will not fail under normal circumstances.

The mechanical factor included in cavity preparation is basically

concerned with the retention of the restoration and the continued vitality and

function of the tooth.

I- Outline form (Margins of the preparation)

Obtaining or establishing the outline form means carrying the margin

of the cavity to the position it will occupy upon completion of the cavity.

The outline should encompass the carious lesion and may include portions of

caries-susceptible areas on the surface being restored. The outline form

should follow a gently, sweeping curve, especially on the occlusal surface.

Many factors govern the outline form of the cavity.

1- Location and extent of carious lesion.

2- Position of pits & fissures dictate outline form.

3- Relationship of self cleansable areas to outline form.

4- Preservation of healthy and vital tooth structure.

5- Restorative material.

6- Esthetics.

7- Functional requirements of the restoration,or improving occlusal

relationships, even when the involved tooth structure is not faulty.

8- The desired cavosurface marginal configuration of the proposed

restoration.

1- Location and extent of carious lesion.

The cavity preparation must be extended sufficiently to include the

entire carious area and the enamel which is not supported by sound dentin.

Enamel which is unsupported is subject to fracture. This conservation of

tooth structure is of paramount importance for the preservation of esthetics,

prevention of irritation to the dental pulp, and the maintenance of the natural

relationship between the tooth and the supporting tissues.

Conservation of tooth structure is of almost importance and must be

considered repeatedly during the design of a cavity preparation.

Two separate cavities should not be united unless the separating ridge

is less than 0.5mm. (e.g. oblique ridge in upper molars or transverse Ridge

in lower 1st premolars).

Enamel which has been undermined by caries must be removed to

insure that the cavo-surface enamel (that enamel at the cavity margins) will

be supported by sound dentin.

Unsupported enamel (Undermined Enamel) should be removed during

cavity preparation, since it is liable to fracture creating open marginal areas

which may lead to secondary decay. An exception to this may be noted at

this time. With the introduction of the acid etch technique for composite

resins, unsupported enamel is not always removed but may be retained for

esthetic reasons.

Marginal ridges should be encroached upon as minimally as possible

in order that their strength is preserved.

Occlusal outline of upper and lower molars with marginal ridges

parallel to outer tooth surface to preserve enough thickness to these marginal

ridges.

2- Position of pits & fissures dictate outline form.

The areas of tooth with caries susceptibility are usually included in the

outline form. This principle is known as "extension for prevention". The

importance of the concept of conservation of tooth structure must be

considered. An occlusal outline form on posterior teeth may include the

fissured primary (developmental) grooves, and in some cases secondary

grooves, as well as buccal and lingual extensions of these grooves.

Cavity preparation should not automatically extend into all the

developmental and supplementary grooves, especially if: 1) they are not

carious, 2) they are not fissured and are unlikely to become carious.

3- Relationship of self cleansable areas to outline form.

The dentist must design the cavity so that all margins are located in

areas easily cleansed by the patient. These areas are found on smooth

surfaces above the height of contour, occlusal cusp inclines, incisal edges

and cusp tips.

4- Preservation of healthy and vital tooth structure.

Healthy tooth structure should NOT be removed unless justified since

tooth structure removed cannot be put back. However, establishing the

margins of cavity preparations sometimes necessitates the removal of some

healthy tooth structure. This may conflict with locating the cavity margins in

easily-cleansable areas since it necessitates the removal of healthy tooth

structure. However, a compromise can be reached.

The pulpal horns of posterior teeth are located beneath the cusp tips.

For example the pulpal horns of a lower first molar are located in the

approximate positions A, B, C, D, & E. The dentin surrounding the pulpal

horns should be conserved to insulate- and strengthen the restored tooth. The

outline is curved and flowing and is constricted as it curves around the

cusps.

5- Restorative material

Individual properties of each material utilized to restore teeth demand

certain design factors that must be incorporated into each cavity preparation.

Cavity preparations in teeth have been more or less designed to meet

the needs of amalgam, with block-shaped cavities, edges with butt joints

(The marginal strength (also called "edge-strength") of dental cement and

amalgam is relatively weak, so their cavosurface margins must be prepared

at an angle of approximately 90° with the tooth surface provide maximum

edge strength for both the restorative material and the enamel), and

undercuts to lock into the cavity. Because amalgam as a metal is an excellent

thermal conductor, cavity preparations should be made shallow. A

restoration that is too shallow, however, has tendency to fracture because

amalgam is a material that is quite brittle. Preparations therefore are made so

the amalgam will be in the range of 2mm thick. When carious dentin

penetrates beyond this depth, a liner or cement base may be placed.

To compensate for the brittleness of the material, all cavities are more

or less mortised into the tooth. Flat walls parallel with or perpendicular to

the tooth surface compose the form of these box-like preparations.

Anchorage of the material is achieved by parallelism of opposing walls or by

slight undercuts in dentin.

Placing a bevel and etching the enamel at the cavosurface angle of

composite resin preparations improve the marginal seal. In some cases, the

bevel may also aid in improving the contour and esthetic qualities of the

composite resin restorations.

Cohesive gold restorations require a beveled cavosurface angle in the

cavity preparation because the malleting forces needed to place the cohesive

gold may fracture enamel rods from the unbeveled cavosurface angle, a

beveled margin also allows for the best marginal adaptation and finish.

Cast gold restorations require even more beveling than cohesive gold

restorations, since this imparts a better marginal seal. Cast gold has

sufficient strength to be used to veneer or cap the tooth, while amalgam

cannot be used in this way,

The physical properties of the restorative material will, therefore,

determine the position and configuration of the cavosurface margin.

6- Esthetics.

It is important to maximize the conservation of tooth structure,

because the natural tooth is the most esthetic. Should esthetics be

compromised, then an esthetic material must be used.

7- Functional requirements of the restoration

A restoration cannot be considered successful unless it fulfills the

functional requirements of the tooth. Occasionally, a restoration may be

placed to alter the contour of the tooth, restore an open contact, or to

improve occlusal function. The outline may have to be altered to

accommodate the desired objective.

REQUIRED FEATURES

Generally the specific,typical features of establishing proper outline

form are:

Preserving cuspal strength.

Minimizing faciolingual extentions.

Preserving marginal ridge strength.

Connecting two close faults,cavities,or restorations.

Restricting the depth of the preparation into dentin to a maximum of

0.2 mm for pit &fissure caries and 0.2 to 0.8 mm for the axial wall of

smooth surface caries.

RULES FOR ESTABLISHING OUTLINE FORM FOR PIT

&FISSURE TOOTH PREPARATION:

1-Extend the preparation margin until sound tooth structure,i.e. all

friable &/or weakended enamel should be removed.

2-Avoid terminating the margin on extreme eminences such as cusp

heights or ridge crests. All margins should be placed in a position to afford

proper tooth preparation,restoration placement,& good finishing of the

restoration’s margins.Follow smooth curves that preserve as much strong

cusp structure as possible.

3-All faults should be included. If the extension from a primary

groove includes ½ or more of the cusp incline,consideration should be given

to capping the cusp. If the extension is 2/3, the cusp-capping procedure is

most often the proper procedure, which removes the margin from the area of

masticatory stresses.

4-Restrict the pulpal depth of the preparation to a maximum of 0.2mm

into dentin. To be as conservative as possible,the preparation for a occlusal

surface pit&fissure lesion is first prepared to a depth of 1.5mm,as measured

at the central fissure.

5-When 2 pit &fissure preparations have less than 0.5m of sound

tooth structure beween them, they should be joined to eliminate a

weakenamel wall between them.

SMOOTH-SURFACE LESIONS occur in 2 locations:

Proximal surfaces.

The gingival portion of the facial and lingual surfaces.

When the proximal surface of a tooth is involved and requires

restoration, the proximal outline of the cavity preparation is influenced by: -

� Class II, III & IV smooth surface lesions

1. Extent of carious involvement.

2. The type of restorative material to be used.

3. Area of contact with adjacent tooth/teeth.

4. Accessibility of the proximal margins for placement and finishing of

the restoration and for subsequent cleansing by the patient.

5. Height of the gingival tissue.

6. Occlusion with teeth of the opposite arch.

Sickle probe testing for correct lateral extension

Regardless of the restorative material to be used, the gingival point-

angles (points X & Y) will be located in the same position facially &

lingually on the proximal surface being restored. Variation in proximal

outline due to the restorative material is in the direction which the planes of

the facial & lingual proximal walls follow as they proceed occlusally from

points X & Y.

On the proximal surface, the facial and lingual outlines for amalgam

restorations are parallel to the long axis of the tooth and parallel to each

other occluso-gingivally. For the gold inlay, the facial and "lingual outlines

diverge as they proceed occlusally from points X & Y to allow withdrawal

of the wax pattern.

SPECIAL CONDITIONS THAT NEED

Reduced or restricted extensions for smooth-surface tooth

preparations are: proximal contours and root proximity, esthetic

requirements, and the use of some tooth preparations for composite

restorations.

Increased extensions for smooth-surface tooth preparations are:

mental or physical handicaps, advanced pt. age, partial dentures abutments,

extra means of retention &resistance form and the need to adjust tooth

contours.

Resistance form:

Primary resistance form may be defined as that shape and placement

of the preparation walls that best enable both the restoration and the tooth to

withstand, with out fracture, masticatory forces delivered principally in the

long axis of the tooth.

PRINCIPLES

1- Use the box shape with a relatively flat floor,which helps the

tooth resist occlusal loading by virtue of being at right angles

to those forces of mastication that are directed in the long

axis of the tooth.

2- To restrict the extension of the external walls(keep as small

as possible) to allow strong cusp and ridge areas to remain

with enough dentin support

3- To have a slight rounding of internal line angles to reduce

stress concentrations in tooth structure.

4- To cap week cusps and envelope enough week tooth structure

to resist fracture of the tooth by forces directed both in the

long axis and obliquely

5- To provide enough thickness of restorative materials to

prevent their fracture under load.

6- To bond the material to tooth structure whenever indicated.

The factors that contribute to resistance form include:

1. Areas included within the outline form.

2. Cavity wall angulations.

3. Cavity preparation depth.

4. Internal line angles.

5. Type of restorative material used.

1- Areas included:

Features of resistance form and how to achieve it:

The more minimal the surface area of the restoration exposed to

occlusal stresses, the less liable it will be to deterioration and displacement.

Excessive facio-lingual width weakens the cusps and exposes unnecessarily

increased surface area of the restoration to stress.

Surface enamel becomes undermined when caries reaches the dentino-

enamel junction and extends laterally along the junction before penetrating

deeper into the dentin. Enamel unsupported by sound dentin tends to fracture

(cleave) under occlusal stresses and must therefore be removed. A basic rule

guides the reduction of the cusps during initial tooth preparation is: cusp

reduction should be considered when the out line extends half the distance

from a primary groove to a cusp tip, and is mandatory if it extends two thirds

this distance.

2- Cavity wall angulations.

Masticatory forces are directed primarily along the long axis of the

teeth. Since the teeth and/restorative materials resist compressive forces best,

the cavity preparation should "be designed to minimize tensile and shear

stresses. This is done by preparing the internal walls of the cavity

perpendicular or parallel to the direction of the force, which is along, the

long axis. Thus, pulpal and gingival walls should be horizontal or

perpendicular to the long axis. The other walls, including the axial, are

vertical or parallel to the long axis.

Cavity walls should be flat and should be directed either parallel or

perpendicular to the line of occlusal force. The line of occlusal force is

normally parallel to the long axis of the tooth. "Cup-shaped" cavity

preparations are to be avoided, as they poorly resist dislodging of

restorations.

Cup shaped cavity

(Micro movement of the restoration during occlusal loading leading to cusp flexing & tooth

fracture)

3- Cavity preparation depth.

The depth of the pulpal and axial walls must be sufficient for the

restorative material to have enough bulk to resist fracture. Placing these

walls just within dentin is usually adequate for this purpose. Increasing the

width of a cavity will not significantly increase the resistance of the

restoration to fracture but it will decrease the strength of the tooth. The

restorative material is more likely to fail if the cavity is too shallow rather

than if it is too narrow.

It is important that the pulpal and axial walls be maintained at their

ideal depth just within the dentin if at all possible. If they are prepared too

deeply, the pulp will be endangered and the tooth will be weakened. The

interjacent dentin between the ideal positions of the pulpal and axial walls

and the pulp not only protects the pulp, it adequate thickness of restorative

material is generally obtained when the axial and pulpal walls within a

cavity preparation are located into the dentin beyond the dentino-enamel

junction. At this depth, sufficient dentin remains overlying the pulp to afford

it proper protection and enough thickness of amalgam (about 1.5mm) is

present to resist fracture under occlusal stresses.

4- Internal line angles.

Well defined internal line angles delineate the walls of the cavity

preparation, aid in establishing uniform depth, and contribute to retention

form by preventing rotation of the restoration.

Resistance to rotational displacement of a restoration is increased

by definite line angles. However, present, dental research indicates that

occlusal stresses are not dissipated within the restorative material and are

concentrated at the line angles of the cavity preparation; therefore rounding

(coving) internal line angles reduce stress concentration.

Fracture resistance of the tooth: Photo elastic stress investigations

indicate that the form of the buccopulpal and lingopulpal line angles in a

class II cavity preparation is of considerable importance. Concentration of

stress at sharp line angle when an opposing cusp applies force on the

restoration and the tooth will lead to a definite possibility of fracture through

the cusp.

Line angles such as the pulpo-axial should be slightly rounded to

minimize stress concentration in the restoration.

Sharp line angles

Rounded line angles

Incorrect treatment of line angles Correct treatment of line angles

Fracture resistance of the restoration:

In class II amalgam restoration if the axiopulpal line angle is left as a

sharp right angle it creates a concentration of stress in the amalgam when

occlusal force is applied.

5- Type of restorative material used.

The physical properties of the type of restorative material to be used

directly influence the amount and type of resistance form to be used in

Cavity design.

Caries Pattern

Brittle materials that have low tensile strengths, such as amalgam and

cements, require good box form and strict adherence to proper preparation.

Less brittle materials such as composite resins can be used in shallower or

rounded preparations when other means of retention are used in addition to

wall friction. Cast gold restorations also utilize box form, but the vertical

walls must be divergent enough to allow insertion and withdrawal of the

restoration.

Resistance to proximal displacement of amalgam restorations is

sometimes obtained by placing grooves into the facial and lingual proximal

walls at the depth of the axial wall. Grooves are sometimes countersunk into

the dentin of other cavity walls where additional resistance form is required.

A dovetail is formed by flaring a portion of a cavity preparation so

that it is wider than the isthmus of the preparation and provides mechanical

resistance to proximal displacement of two-surface restorations (i.e. Class II

on posterior teeth and lingual-proximal restorations on anterior teeth).

Occlusal lock (dove tail)

Retentive grooves

III- Retention Form:

Retention form refers to the features of a cavity preparation that

prevent dislodgement of the restoration.

In many cases resistance and retention forms are often established

during the same procedure in cavity preparation. Together, they prevent

displacement of the restoration and fracture of both restoration and tooth

structure under occlusal stresses.

Retention forms influenced by the following factors:-

1- Relationship of cavity walls to one another:

Friction between the restorative material and the walls of the cavity

preparation acts retention is improved by increasing the area of the walls

(cavity depth) and making opposing walls parallel (proper box form). Walls

are not intentionally roughened to increase retention, since normal

instrumentation sufficiently rough.

2- Undercut points and grooves:

Additional retention may be gained by placing points and grooves in

the dentinal walls of the preparation. The grooves are perpendicular to the

path of withdrawal and they prevent displacement of the restoration from the

cavity. Undercuts are confined to the dentin to avoid undermining the

enamel. They are usually placed in the facial and lingual walls of occlusal

preparations, the buccal and lingual proximal walls of Class II preparations,

and the incisal and gingival walls of Class V preparations. They are not

normally placed in the pulpal or axial walls in order to avoid approaching

the pulp. Undercut retention is used with amalgam, composite, and gold foil

restorations.

Axial retentive groove depth as influenced by buccolingual extension, A. small cavity with minimal

extension, no axial groove required, B. cavity begin to wrap around the tooth, proximal retentive grooves

indicated to lock the restoration into position, C. large cavity extending around to cover a portion of the

buccal and lingual surface. Note; any further extension or increased buccolingual angle would justify

placement of a pin.

3- Auxiliary:

Dovetails, parallel grooves, pins and etching of enamel are additional

methods that may be used to enhance retention with certain types of

restorative materials.

A dovetail is a constriction in the outline form of a preparation that

prevents displacement of the restoration toward a proximal surface of the

tooth. Dovetails are mainly used on the occlusal surfaces of posterior teeth to

prevent mesial or distal displacement of amalgam and cast restorations.

Parallel grooves are used to increase retention of cast gold

restorations, The grooves must be parallel to the path of insertion of the

casting. They act by increasing the surface area of the preparation and by

undercutting some of the surfaces that are parallel to the path of insertion.

Occasionally, additional retention may be obtained through the use of

pins carefully placed into the dentin within a cavity preparation. Pins are

usually reserved for use in extensive cavity preparations.

Pins are small diameter screws and wires that are used to attach a

restorative material directly to dentin. Nonparallel pins are used with

amalgam restorations. Parallel pins are sometimes a part of cast restorations

and must parallel the path of insertion.

Retentive devices, such as points, grooves and pins are placed in the

dentin just inside the dentino-enamel junction. They should not be placed in

the enamel or at the DEJ which will undermine the enamel and seriously

weaken it. The internal boundary for these retentive devices is the ideal

position of the pulpal and axial walls. Retention should not be placed in

interjacent dentin. When the pulpal and axial walls do not have their ideal

depths and contours due to extensive carious destruction or fracture of the

tooth, retention is, nevertheless, placed just within dentin and with respect to

the ideal position of the pulpal and axial walls. This placement prevents

undue weakening of the tooth and the possibility of exposing the pulp.

The recent trends in conservative restorative dentistry reduce the

dependence on the mechanical interlocking (retention) of the restorative

material into the cavity because it leads to unnecessary tooth cutting and

destruction to create dove tails, undercuts, grooves, etc. Instead they reduce

the cavity size and tooth cutting as much as possible (just remove caries) and

depends on the new generation of adhesive materials capable of boding all

restorative materials to tooth structure (enamel and dentin) as for example

"Bonded Amalgam Restorations" and "Composite Bonded Restorations" or

even "Glass ionomer Restorations (GIC)".

"Adhesion form instead of resistance form and retention form"

By these new concepts the restorative material becomes an integral

part of the tooth increasing its overall strength and fracture resistance to

masticatory forces.

IV- Convenience form:

Convenience form means that the dentist must create sufficient access

to the lesion to facilitate visibility and instrumentation in the preparation of

the cavity and the insertion of a restorative material.

This is influenced by:

1. Armamentarium to be utilized.

2. Location of cavity margins.

3. Accessibility for proper caries removal and pulp protection.

4. Accessibility for proper restoration placement.

a) Proper finishing of margins of cavity preparation.

b) Retentive areas.

c) Convenience (starting) points.

5. Type of restoration to be placed.

Instruments used:

Utilization of rubber dam to effect tissue retraction, cleanliness of the

operating field, and for color contrast during cavity preparation, together

with use of suitably selected burs and instruments for cavity preparation, are

determinants for the convenience form required for a specific restorative

procedure.

The use of a "proxitector" for protection of adjacent tooth during proximal cavity preparation.

(also a matrix band can be used for the same purpose)

Location of cavity margins:

Cavity margins must be located where they can be properly restored

and finished. Their location must allow also for subsequent polishing and

cleansing of the restoration.

V- Removal of remaining carious dentin

Cavity preparations are designed to remove carious tooth structure.

Preparations that are designed according to Black's principles will

automatically include those lesions that have just penetrated the den-tin.

Moderate or gross amounts of carious tooth structure, however, are not

removed by ideal cavity designs and therefore caries removal in these

instances becomes a separate and significant step.

To describe the process of caries removal, it is necessary to categorize

the clinical lesions normally encountered:

1) Small lesions: the caries has just penetrated the dentino-enamel

junction and will be entirely removed in the ideal cavity

preparation.

2) Moderate lesions: the caries extends beyond the boundaries of the

ideal preparation in localized areas.

3) Large lesions: the caries extends beyond the ideal preparation to

the extent that some of the usual preparation landmarks become

obliterated.

4) Gross caries: so much of the tooth is curiously involved that most,

or all, of the normal landmarks have been destroyed.

Small lesions

The removal of small carious lesions should present no problem to the

operator as they lie within the area included by the ideal preparation.

Moderate lesions

The removal of moderate sized carious lesions is initiated by utilizing

Black's principles of outline, resistance and retention form. This will usually

complete the preparation except for localized areas of decay. The removal of

the remaining decay is best accomplished with a round bur in the slow speed

hand piece. The largest round bur that will fit into the carious area should be

selected. The blades of a slowly rotating large round bur act as many spoon

excavators and the carious material can be carefully planed away. The larger

burs also have fewer tendencies to penetrate tooth structure because of their

greater bearing area. This technique is utilized until all the soft, crumbly

material has been removed. Check the area with an explorer or spoon

excavator to establish that all the remaining tooth structure is hard.

Dental caries in dentin and enamel progresses in several "zones". Not

all of the dentinal "zones" warrant removal. Clinically, the criterion followed

is hardness. If the dentin feels hard or crisp to the touch of an explorer or

spoon excavator, then it need not be removed. Only those areas which are

soft should be removed. The entire wall or floor should not be reduced to

include one localized carious area.

Sequence of Decay Removal

Entry is made into tooth in conventional manner

with a high speed fissure bur.

Ideal depth and width are established, ignoring the

carious tooth structure.

Decay extending beyond the limits of the ideal

preparation is removed with the largest round bur

that will fit into the area.

The caries removal process should begin

peripherally in the DEJ areas. As the carious dentin

is removed peripherally, the bur is worked into the

deeper areas. Often it is necessary to enlarge the

occlusal opening to gain both visual and

'mechanical access

Decay in areas involving potential exposures, such

as the axial and pulpal walls, should be removed

last.

After all decay has been removed, the preparation is

reeval-ated for undermined enamel, resistance form,

and retention form.

All undermined enamel areas should be removed

with the high speed fissure bur and an attempt made

to reestablish lost retention and resistance form.

The pulpal should be flattened only at ideal depth.

Any pulpal floor destruction beyond ideal depth

should be left and not smoothed.

Large lesions:

The removal of decay from large carious lesions can usually be

accomplished within the sequence of Black1s principles. The basic cavity

outline is established utilizing the high speed handpiece with a carbide bur.

Any undermined enamel that remains is removed at this time. In addition to

removing weakened tooth structure, this procedure will also afford access

for the next step which is decay removal.

Caries removal begins in those areas where there is the least chance of

pulp exposure, (peripherally) and continues centrally until all the affected

dentin has been removed. Should an exposure then occur, it will in all

probability be after all or nearly all the decay has been removed. This is

advantageous for two reasons: 1) the exposure site will be relatively clean

with no carious material remaining, and 2) it will be unnecessary to subject

the tooth to the additional trauma of decay material after treatment of the

exposure site.

Following caries removal some of the steps of cavity preparation may

have to be repeated to ensure proper cavity preparation design and assure

smooth cavity margins.

Gross lesions:

The removal of decay becomes the first of Black's steps to be

performed in the gross caries situation. The situation becomes more

complicated, since the amount of destruction often leaves the operator

without anatomical landmarks, and the chances for an exposure are much

greater. Caries removal again starts peripherally. As sound dentin is reached

laterally, it followed to the central areas. The affected dentin in the pulp horn

areas is removed last, for the reasons described earlier.

If the removal of decay exposes more undermined enamel, it should

be removed to satisfy the requirement of resistance form. After complete

caries removal, the proper outline form is established with all margins in

easily cleansable areas. It may be necessary to achieve retention form

through dovetails, grooves, pins, etc.

VI- Finish the enamel wall:

It is necessary to refine the enamel walls of a cavity preparation to

ensure the best possible adaptation between the restorative material and

enamel. Another objective of this procedure is the removal of any

unsupported enamel which may fracture later, and lead to secondary decay.

Microleakage must be kept to a minimum.

Special attention should be given to the design of the cavosurface

margin, since it should complement the physical properties of the restorative

material. Adequate planning of all enamel walls will result in a smooth

flowing and harmonious cavo-surface margin which enhances the

condensability & adaptability of the restorative material, thereby increasing

the strength of the restoration.

Loose, friable enamel rods should be removed from the cavo-surface

margins of the cavity preparation to avoid defective margins on the

completed restoration. Cavo-surface margins are usually planed with hand

instruments.

Diagram showing Cavo-surface angle > 270°: (B) which gives a strong enamel margin. (A) restoration

margin angle.

A 90° cavo-surface angle (butt joint) provides maximum strength for

enamel margin and amalgam.

A 45° cavo-surface angle (bevel) provides best structure of enamel

rods at margins for acid etching, bonding and composite restoration.

VII- Cleansing of the cavity

The final procedure in cavity preparation is the removal of all debris

(i.e. blood, saliva, tooth particles � smear layer) with warm water and

chlorohexidme solution (antiseptic) applied on a cotton pledget and followed

by drying with short blasts of warm air. The cavity preparation should then

be finally inspected and the explorer tip carried into all line-angles and

auxiliary retentive areas.

DO NOT DESICCATE DENTIN

More recently a modified concept of the stages or steps of cavity

preparation has been proposed. In an academic setting, the student should be

instructed to accomplish tooth preparation in two stages which are:

A- Initial stage:

I- Outline form:

Step 1: outline form and initial depth:

Establishing the outline form means:

i. Placing the preparation margins in the positions they will

occupy in the final preparation except for finishing enamel

walls and margins.

ii. Preparing an initial depth of 0.2 to 0.5mm pulpally of the DEJ

position or 1.5mm cavity depth at central groove. Depth of the

axial wall in smooth surface lesions shouldn't exceed 0.2 to

0.8mm deep into dentin.

iii. The outline form must be traced before any mechanical

alteration to the tooth is begun.

Step 2: Primary resistance form:

Primary resistance form may be defined as that shape and placement

of the preparation walls that best enable both the restoration and the tooth to

withstand, with out fracture, masticatory forces delivered principally in the

long axis of the tooth.

The relatively flat pulpal and gingival walls prepared perpendicular to

the tooth's long axis help resist forces in the long axis of the tooth and

prevent tooth fracture from wedging effects.

The primary resistance is thus obtained by:

1. To use the box shape

2. To restrict the extension of the external walls.

3. To have a slight rounding (coving) of internal line angles to

reduce stress concentrations.

4. In extensive tooth preparations, to cap weak cusps and envelope

or include enough of a weakened tooth.

5. To provide enough thickness of restorative material to prevent

its fracture under load.

6. To bond the material to tooth structure when appropriate (any

bonded restoration results in increased resistance form).

Step 3: Primary Retention Form:

Primary retention form is that shape or form of the conventional

preparation that resists displacement or removal of the restoration from

tipping or lifting forces.

Because retention needs are related to the restorative material used,

the principles of primary retention form vary depending on the material.

1. Amalgam restoration in most class I and all class II conventional

preparations, the material is retained in the tooth by developing

external tooth walls that converge occlusally.

The facial and lingual walls of the occlusal portion of the preparation,

as well as the proximal portion, converge toward the occlusal surface.

The occlusal convergence of the proximal portion has several

advantages in addition to producing retention. It allows slight facial and/or

lingual extension of the proximal portion of the preparation in the gingival

area while conserving the marginal ridge, thus reducing the forces of

mastication on critical areas of the restoration.

2. Adhesion systems provide some retention by micromechanically

bonding amalgam to tooth structure and also reducing or

eliminating microleakage. However, until longevity studies

demonstrate that bonding systems provide complete retention

form, traditional retention features should be provided for

amalgam restorations.

Composite restorations primarily are retained in the tooth by a

microchemical bond that develops between the material and etched and

primed tooth structure. In such restorations, the enamel and dentin are etched

by an acid and the dentin is primed with a dentin bonding agent. These

procedures are discussed in a subsequent.

3. Glass ionomer cement is retained by chemical adhesion to tooth

structure.

Step 4: Convenience Form:

Convenience form is that shape or form of the preparation that

provides for adequate observation, accessibility, and ease of operation in

preparing and restoring the tooth. On occasion, obtaining this form may

necessitate extension of distal, mesial, facial or lingual walls to gain

adequate access to the deeper portion of the preparation.

The occlusal divergence of vertical (longitudinal) walls of tooth

preparation for class II cast restorations also may be considered as

convenience form.

Extending proximal preparations beyond proximal contacts is another

convenience form procedure. Although exceptions may be made to such an

extension, preparing the proximal walls to obtain clearance with an adjacent

proximal surface affords better access to1 finish the preparation walls and

the restorative material. For cast restorations clearance with the adjacent

proximal surface is mandatory to finish the preparation walls, make an

accurate impression of the prepared tooth, and try in the casting.

B- Final tooth preparation stage:

Once the extensions and wall designs have fulfilled the objectives of

initial tooth preparation, the preparation should be expected carefully for

other needs.

Step 5: Removal of any remaining enamel pit or fissure infected

dentin and/or old restorative material if indicated.

In dentin, as caries progresses an area of decalcification precedes

the penetration of microorganisms. This area of decalcification often appears

discolored in comparison with undisturbed dentin, yet, it doesn’t exhibit the

soft texture of caries. This dentin condition may be termed "affected dentin"

and differs from "infected dentin" in that it has not been significantly

invaded by microorganisms. It is accepted and appropriate practice to allow

affected dentin to remain in a prepared tooth.

'"Caries-disclosing" dyes may aid that decision. Fortunately the

decision does not require exactness, for it is not necessary that all dentin

invaded by microorganisms be removed. In shallow or moderately deep

lesions, the removal of the masses of microorganisms and subsequent

sealing of the preparation by a restoration at best destroy those

comparatively few remaining microorganisms and at worst reduce them to

inactivity or dormancy.

When a pulpal or axial wall has been established at the proper initial

tooth preparation position and a small amount of infected carious material

remains, only this material should be removed, leaving a rounded, concave

area in the wall. The level or position of the wall peripheral to the caries

removal depression should not be altered.

It is more expedient to remove extensive caries early in the tooth

preparation before time and effort are spent in doing a tooth preparation for a

certain restorative material that is then deemed inadequately for satisfactory

restoration of the tooth.

In one appointment, infected dentin is removed from several teeth and

temporary restorations are placed. After all the teeth containing extensive

caries are so treated, then individual teeth are restored definitively. This

procedure stops the progress of caries and is often referred to as the caries

control technique.

Large areas of soft caries usually are best removed with spoon

excavators by flaking up the caries around the periphery of the infected mass

and peeling it off in layers. The bulk of this material is thus easily removed

in few large pieces.

i. Vertically oriented retention locks and retention grooves are

used to provide additional retention for proximal portions of

some tooth preparations.

ii. Horizontally oriented retention grooves are prepared in most

class V preparations for amalgam

iii. Pins, slots, steps and amalgam pins, when the need for

increased retention form is unusually great; several other

features may be incorporated into the preparation. The use of

pins and slots increase both retention and resistance forms,

while steps and amalgam pins do it to a lesser extent.

iv. Beveled enamel margins increase both the surface area of

etchable enamel and maximize the effectiveness of the bond by

etching more enamel rods.

5: Procedures for finishing the external walls of the tooth

preparation:

• Definition: Finishing the preparation walls is the further development,

when indicated; of a specific cavosurface design and degree of

smoothness or roughness that produces the maximum effectiveness of the

restorative material being used.

• Objectives:

1. Create the best marginal seal possible.

2. Afford a smooth marginal junction between cavity and restoration.

3. Provide maximum strength of both the tooth and the restorative

material at and near the margin.

• Finishing of external walls has two primary features:

1. The design of the cavosurface angle.

2. The degree of smoothness or roughness of the wall.

The design of the cavosurface angle:

Because of the low edge strength or friability of amalgam the tooth

preparation cavosurface angle of 90° produces maximal strength for both the

amalgam and the tooth; except removing unsupported enamel rods at

gingivocavosurface of a class II amalgam restoration (bevel).

Beveling can serve for useful purposes in the tooth preparation for a *

casting.

1. It produced a stronger enamel margin.

2. It permits a marginal seal in slightly undersized castings.

3. It provides marginal metal that is more easily burnished and

adapted.

4. It assists in adaptation of gingival margins of castings that fail to

seat by a very slight amount.

The bevel of the margin in a preparation for castings should produce a

cavosurface angle that results in 30 to 40 degrees marginal metal. The

marginal gold alloy will be too thin and weak if the angle of the gold bevel

is less than 30 degrees.

Ceramic materials belong to that category of materials that

contraindicated beveling the cavosurface margin.

Beveling enamel margins in composite preparations is primarily

indicated in larger restorations that have increased retention needs.

The potential for retention is increased by increasing the surface area

of enamel available for etch and having a more effective area of etch

obtained by etching the cut ends of the enamel rods. Other advantages of

beveling composites are:

1. Adjacent minor defects can be included with a bevel

2. Esthetic quality may be enhanced by a bevel creating an area of

gradual increase in composite thickness from the margin to the

bulk of the restoration.

3. The marginal seal may be enhanced.

The restorative material is the primary factor dictating the desired

smooth or rough enamel wall.

The prepared walls of inlay or onlay preparations require a very

smooth surface to permit undistorted impressions and close adaptation of the

casting to the enamel margins (fine sand-paper disks).

Prepared walls and margins of composite restoration can be

roughened using coarse diamond stones to increase surface area for bonding.

When using amalgam restorative materials, a very smooth preparation

walls are not as desirable as for cast restorations (a rougher surface prepared

wall markedly improves resistance to marginal leakage).

High speed can lead to overextension of margins, grooved walls

and/or rounded cavosunacfe angles, especially on proximal margins. If this

method is used, plain-cut fissure burs produce the finest surface; these burs

produce a smoother surface than cross-cut burs, diamonds, or carborundum

stones.

The planning action of razor-sharp hand instrument can result in

smooth enamel wall, although it may not be as smooth as that achieved with

other instruments, Hand instruments such as enamel hatchets and marginal

trimmers may be used in planning enamel walls, cleaving enamel and

establishing enamel bevels.

Step- 9: Final procedures cleaning, in spacing & sealing:

Removing all chips and loose debris that have accumulated, and

making a final complete inspection of the preparation for any remaining

infected dentin, unsound enamel margins or any condition that renders the

preparation unacceptable to receive the restorative material.

The usual procedure in cleaning is warm water from the syringe and

then to remove the visible moisture with a few light surges of air from the air

syringe.

In some instances, debris will cling to the walls and angles despite the

above efforts and it may be necessary to loosen this material with an

explorer or small cotton pellet.

It is important not to dehydrate the tooth by overuse of air or by the

application of alcohol.

The act of sterilizing a preparation before inserting a restoration may

be a logical procedure (disinfection).

The dentin tubule lumen, varying from 1 to 4 µm in diameter at

varying distances between the DEJ and the pulp certainly presents sufficient

size for the entrance of microorganisms. Investigations have verified the

presence of microorganisms in dentin tubules beneath preparation walls.

The presence of reparative dentin deposited as a result of pulpal insult

constitutes a significant deterrent to bacterial progress. Another possible

answer as to why all teeth with carious involvement do not eventually have

pulpal infection is that bacteria may be in a dormant condition as the result

of the more sealed environment of a restored tooth or the-germicidal action

of the restorative material destroys any invading bacteria. The germicidal or

protective effect ranges from the fluoride content of some materials to the

deposition of corrosive products at the interface of the preparation wall in an

amalgam. Zinc oxide-eugenol cement has significant germicidal properties

over an extended period of time. Therefore some protection from further

carious action is afforded by some restorative materials (glass lonomer

cement GIC).

The routine use of specific sterilization medicaments should no longer

be a strong consideration. However, the use of dentin bonding agents (for

bonded restorations) and sealers (for non-bonded restorations) to effect a

dentin tubular seal is recognized.

Eliminating bacterial penetration is so important that the use of dentin

bonding agents or sealers will likely become unaffected.

New approach in decision making, cavity design and

tooth preparation:

Biological form of tooth preparation

Diagnostic Factors:

1. There must be a reason to place a restoration in the tooth. Caries

fractured teeth, esthetic needs for improved form or function. b.

2. An assessment of both pulpal and periodontal status will influence

the potential treatment.

3. Assessment of the occlusal relationships.

4. Patients concern for esthetics should be considered.

5. The relationship of a specific restorative procedure with other

treatment planned for the patient must be considered.

6. The risk potential of the patient to further dental disease should be

assessed. A high-risk patient may require altered treatment

planning initially until the risk factors are better controlled.

7.

Patient Factors;

1. The patient's knowledge and appreciation of good dental health.

2. The patient's economic status.

3. The patient's age.

4. Whether or not adequate isolation of the operating site can be

obtained.

5. Caries in this patient, is it reversible (incipient) or irreversible

(cavitated); active or not. (Caries risk assessment).

Conservative Factors:

1. While one of the primary objectives of operative dentistry is to

repair the damage from dental caries, the preservation of the

vitality and integrity of the tooth is paramount.

2. The pulp should not be subjected to unnecessary abuse.

3. The less tooth structure removed the less potential damage that

may occur to the pulp.

4. The smaller the tooth preparation, the easier it is to retain the

restorative material in the tooth.

5. Examples of conservative tooth preparation features:

• Minimal extensions of the tooth preparations, especially facio-

lingually and pulpally.

• Supragingival margins and Rounded internal line angles.

• Only uncoalescent fissures are excised, enameloplasty and

fissure sealant instead of extension for prevention.

Material Factors:

1. An amalgam restoration requires a specific mechanical tooth

preparation for resistance and retention.

2. An indirect cast metal restoration also requires a specific

tooth preparation form that provides:

• Draw or draft to provide seating.

• A beveled cavosurface configuration,

3. Adhesive composite restorations or GIC do not typically require

tooth preparations as precise as those for amalgam and cast metal

restorations.

4. Ceramic inlay restorations do require specific preparation depths

and wall designs but do not require complex cavosurface marginal

configurations.

5. Bonded amalgam restoration still requires the same tooth

preparation as for non-adhesive amalgam restoration.