PRINCIPLES OF TOOTH PREPARATION

Dr.RAKESH R NAIR

PG STUDENT

DEPT.OF CONS & ENDO

KVG DC ,SULLIA

• Tooth preparation may be defined as the mechanical treatment of dental disease or injury to hard tissue that restores a tooth to the original form (Tylman).

• The mechanical preparation or the chemical treatment of the remaining tooth structure, which enables it to accommodate a restorative material without incurring mechanical or biological failure.(Marzouk)

• The current focus is on conservative tooth preparation that isnoninvasive and that minimally involves dentin.

• This trend is rational in the light of the reduction of the caries rateby fluorides, nutritional counseling and oral hygiene programs.Black’s principles of cavity preparation and Tylman’s principles oftooth preparation are both presently being modified toaccommodate imaginative approaches i.e., acid etching withminimum reduction.

• Dentistry is changing from macro tooth preparation to anenvironment of molecular chemistry i.e., esthetic bonding. Thesetechniques are not presently supported by any longitudinalstudies, but are exciting and promising.

Despite these advances, traditional crowns are still indicated for majority of patients. The classic design of the preparation must be visualized so that modifications if required can be instituted. Diagnosis and disciplined tooth preparation are essential to successful fixed prosthetics.

OBJECTIVES OF TOOTH PREPARATION

The objectives of preparation remain clearly defined, but

the methods of securing these goals are constantly being

revised.

1.Reduction of the tooth in miniature to provide

retention.

2.Preservation of healthy tooth structure to

secure resistance form.

3.Provision for acceptable finish line.

4.Performing pragmatic axial tooth reduction to

encourage favorable tissue responses from

artificial crown contour.

BIOLOGIC

Conservation of tooth

structure

Avoidance of over contouring

Supra gingival margins

Harmonious occlusion

Protection against tooth

fracture

MECHANICAL

Retention form

Resistance form

ESTHETIC

Minimum display of metal

Maximum thickness of

porcelain

Porcelain occlusal surfaces

Subgingival margins

PRINCIPLES OF TOOTH PREPARATION

(According to Rosenstiel)

Optimal restoration

Damage to adjacent teeth is prevented by positioning thediamond so a thin lip of enamel is retained between the burand the adjacent tooth.

Note that the orientation of the diamond parallels the long axis.

The enamel should be maintained mesial to the path of the diamond as the reduction progresses.

Soft tissuesDamage to the soft tissues of the tongue and cheeks

can be prevented by careful retraction with an aspirator

tip, mouth mirror , or flanged saliva ejector.

Great care is needed to protect the tongue

when the lingual surfaces of mandibular molars are

being prepared.

PulpGreat care also is needed to prevent pulpal injuries

during fixed prosthodontic procedures,

Especially complete crown preparation.

Pulpal degeneration that occurs many years after tooth preparation has

been documented.

Extreme temperatures, chemical irritation, or microorganisms can cause an

irreversible pulpitis particularly when they occur on

freshly sectioned dentinal tubules.

• Temperature• Chemical action • Bacterial action

PRINCIPLES OF TOOTH PREPARATION

According to Shillingburg, the design of a

preparation for a cast restorations and the execution of

that design are governed by five principles:

1.Preservation of tooth structure

2.Retention and resistance

3.Structural durability

4.Marginal integrity

5.Preservation of the periodontium.

Preservation of Tooth Structure

• In addition to replacing lost tooth structure, a restoration mustpreserve remaining tooth structure.

• Intact surfaces of tooth structure that can be maintained whileproducing a strong, retentive restoration should be saved if patientacceptance and retention requirements will permit it.

• Preservation of tooth structure in some cases mayrequire that limited amounts of sound tooth structure beremoved to prevent subsequent uncontrolled loss of largerquantities of tooth structure.

This is the rationale for the removal of 1 to 1.5 mm of occlusaltooth structure when preparing a tooth for an MOD onlay.

The metal on the occlusal surface can protect against dramaticfailures,

such as fracture of tooth structure, as well as the less obviousfailures that may be occasioned by the flexing of tooth structure

Conservation of tooth structure by using partial-coverage restorations. In this case, they are used as fixed dental prosthetic abutments to replace congenitally missing lateral incisors.

Retention and Resistance

• No cements that are compatible with living tooth structure and the biologic environment of the oral cavity possess adequate adhesive properties to hold a restoration in place solely through adhesion.(Shillingburg)

• The geometric configuration of the tooth preparation must place the cement in compression to provide the necessary retention and resistance.

Retention prevents removal of the restoration along the

path of insertion or long axis of the tooth preparation.

Resistance prevents dislodgment of the restoration by

forces directed in an apical or oblique direction and prevents

any movement of the restoration under occlusal forces.

Retention and resistance are interrelated and often inseparable

qualities.

The essential element of retention is two opposing vertical

surfaces in the same preparation.

Taper

The axial walls of the preparation must taper slightly to permit the

restoration to seat; ie,

two opposing external walls must gradually converge or

two opposing internal surfaces of tooth structure must

diverge occlusally.

The terms angle of convergence and angle of divergence can be used to

describe the respective relationships between the two opposing walls of

a preparation.

The relationship of one wall of a preparation to the long

axis of that preparation is the inclination of that wall.

• A tapered diamond or bur will impart an inclination of 2 to 3 degrees to any

surface it cuts if the shank of the instrument is held parallel to the intended

path of insertion of the preparation.

• Two opposing surfaces, each with a 3-degree inclination, would give the

preparation a 6-degree taper.

Prothero (1923) indicated that convergence of peripheral surfaces shouldrange from 20-50.

Jorgensen (1955) found that retention decreases as the taper increases. Arange of 2-6.50 has been considered to be optimal. He tested the retention ofcrowns at various angles by applying a tensile force on cemented crown.Maximum strength was obtained at 50.

Mack (1980) estimated that a minimum taper of 120 is necessary just toensure the absence of undercuts. The tendency to over taper preparationsmust be avoided in order to achieve maximum retention.

Ebnashi (1969) to minimize stress in the cement interface between thepreparation and restoration, a taper of 2.5 to 6.50 has been suggestedoptimum, but there is only a slight increase in stress as taper is increased from0 to 150. However at 200 stress concentration was found to increase sharply.

• Consciously attempting to cut a taper can easily result in an

overtapered and nonretentive preparation.

• A taper or total convergence of 16 degrees has been proposed

as being achievable clinically while still affording adequate

retention.

• This is probably an acceptable overall target. It can be as low as

10 degrees on preparations on anterior teeth and as high as 22

degrees on molars.

Freedom of Displacement

Maximum retention is achieved when there is only one path.

A full veneer preparation with long, parallel axial walls and grooves would produce such retention .On the opposite extreme, a short, overtapered preparation would be without retention because the restoration could be removed along an infinite number of paths.

Limiting the freedom of displacement from torquing or twisting forces in a horizontal plane increases the resistance of a restoration.

A groove whose walls meet the axial wall at an oblique angle does not provide the necessary resistance. V-shaped grooves produce roughly one-half as much resistance to lingual displacement as do grooves with a definite lingual wall.

Surface area :Greater the area of the cement film bound to the preparation and to the internal detail of the casting, greater the retention.

The total surface area of the preparation is influenced by the size of the tooth, the extent of the coverage by the restoration, and internal features such as grooves and boxes

Area under shear:

Most important for retention is the

area of cement that will experience

shearing rather than tensile stress.

For the shear strength of the cement

to be utilized, the preparation must

have the opposing walls nearly

parallel with each other.

The direction in which a restoration

can be removed must be limited to

one path. A severely over tapered

preparation has many paths along

which tensile force could remove a

crown.

Types of preparation :

Different types of preparations have different retentive valuesand these correspond fairly closely to the surface area of the axialwalls, provided other factors (e.g. taper) are kept constant. Thus theretention of full veneer crowns are almost double that of partialcoverage restorations.

Adding grooves or boxes to a preparation with a limited path ofwithdrawal does not markedly affect its retention because the surfacearea is not increased significantly(Rosenstiel). However other authorshave reported that, where the addition of grooves or boxes limits thepath of withdrawal, retention is increased.

For the grooves to be effective, the

lingual wall of the groove must be

distinct and perpendicular to adjoining

axial wall

The walls of a groove that meet the

axial wall at an oblique angle do not

provide necessary resistance (A). The

walls of a groove must be

perpendicular to rotating forces to

resist displacement (B).

Length of the preparation is important factor in retention

Longer preparation has a greater retention than does a short preparation, due to greater surface area.

The preparation with longer walls interferes with the tipping displacement of the restoration better than the short preparation. Because of greater surface area preparation with larger diameter will have greater retention than with narrow preparation of same length

Surface roughness:

The adhesion of the dental

cements primarily depends on

microscopic irregularities and the

recesses on the surfaces being

joined the prepared tooth

surface should not be highly

polished

Resistance:

Resistance prevents dislodgement of restoration by forces directed in the apical or oblique direction

Resistance to sliding or tipping must be designed into the preparation

Leverage And Resistance:

Leverage, is the predominant factor in the dislodgement of the cemented restoration, occur when line of action of force passes outside the supporting tooth structure.Length of preparation has strong influence on its resistanceShortening a preparation will produce a proportionally greater diminution of the resisting area

A short restoration on a short

preparation is less likely to fail

through tipping than is a long

restoration on same preparation as

the force on it acts through a longer

lever arm.

A preparation on a tooth with a

smaller diameter resists pivoting

movements better than a preparation

of equal length on a tooth of larger

diameter because smaller teeth will

have a short rotational radius for the

arc of displacement and the incisal

portion of the axial wall will resist

displacement

When relatively long crown must be made on a short preparation additional resistance form, in form of pin retained core must be created.

Resistance and Tooth Width

A wider preparation has greater retention than narrower one of equal height. Under some circumstances a crown on the narrow tooth can have greater resistance to tipping than one on wider tooth. This is because crown on narrower tooth has a shorter radius of rotation resulting in larger resisting area

Resistance of a preparation on a short, wide tooth can be enhanced by addition of grooves

• The path of insertion is an imaginary line along which the• restoration will be placed onto or removed from the• preparation.

• It is determined mentally by the dentist• before the preparation is begun, and all features of the• preparation are cut to coincide with that line.

Path of insertion

If the center of the occlusal surface of a preparationis viewed with one eye from a distance of approximately30 cm (12 inches), it is possible to sigh! down theaxial walls of a preparation with a minimum taper

For a preparation to be surveyed in the mouth, wheredirect vision is rarely possible, a mouth mirror is used.It is held at an angle approximately 1/2 inch above the preparation, and

the image is viewed with one eye.

If fixed partial denture abutment preparations arebeing evaluated for a common path of insertion, a firmfinger rest is established and the mirror is maneuvereduntil one preparation is centered.

Then, pivoting on the finger rest, the mirror is moved, without changing its angulation, until it is centered over the second preparation

Structural Durability

• A restoration must contain a bulk of material that is adequate towithstand the forces of occlusion.

• This bulk must be confined to the space created by the toothpreparation.

• Only in this way can the occlusion on the restoration be harmoniousand the axial contours normal, preventing periodontal problemsaround the restoration

• There are three preparation features that contribute to the durabilityof the restoration that is (1)Occlusal reduction (2)axial reduction (3)provision for reinforcing struts

Occlusal Reduction

One of the most important features for providing adequate

bulk of metal and strength to the restoration is Occlusal Clearance.

For gold alloys, there should be 1.5 mm of clearance on the functional

cusps (lingual of maxillary molars and premolars and buccal of

mandibular molars and premolars)

• Metal-ceramic crowns will require 1.5 to 2.0 mm onfunctional cusps that will be veneered with porcelain and

• 1.0 to 1.5 mm on nonfunctional cusps to receive ceramic coverage.

• There should be 2.0 mm of clearance on preparations for all-ceramic crowns.

• Malposed teeth may have occlusal surfaces that are not parallel with the occlusal table. Therefore, it may not be necessary to reduce the occlusal surface by 1.0 mm to achieve 1.0 mm of clearance.

The basic inclined plane pattern of the occlusal surfaceshould be duplicated to produce adequate clearancewithout overshortening the preparation

Functional Cusp Bevel

An integral part of the occlusal reduction is the functionalcusp bevel . A wide bevel on the lingualinclines of the maxillary lingual cusps and the buccalinclines of mandibular buccal cusps provides space foran adequate bulk of metal in an area of heavy occlusalcontact.

If a wide bevel is not placed on the functional cusp,several problems may occur

To prevent a thin castingwhen there is no functional cusp bevel, an attempt maybe made to wax the crown io optimal thickness in thisarea. An overcontoured restoration will result and adeflective occlusal contact is likely to occur unless theopposing tooth is reduced

Axial reduction:Second prerequisite for structural durability is axial reduction. When it is sufficient, restoration walls have satisfactory thicknesses with out over contouring

Reinforcing struts: the features that serve to provide space for the metal that will improve the durability and the rigidity of the restoration: Offset the occlusal shoulder, the isthmus, the proximal grooves, and the box. Isthmus connects the boxes, and the offset ties the grooves together to enhance the reinforcing “truss effect”.

Marginal Integrity

The restoration can survive in the biological environmentof the oral cavity only if the margins are closely adaptedto the cavosurface finish line of the preparation. The configurationof the preparation finish line dictates the shapeand bulk of restorative material in the margin of therestoration It also can affect both marginal adaptationand the degree of seating of the restoration

FINISH LINE REQUIREMENTS

Definition:The point at which a preparation terminates on the

tooth is called the finish line. It is also defined as the peripheral extension of a tooth preparation (GPT).

There are three requirements for successful restoration margins.

Fundamentals of tooth preparation:Shillingburg,Jacobi,Bracket

Functions :

The finish line serves the following functions :

•During visual evaluation of the tooth preparation, it isa measure of the amount of tooth structure alreadyremoved. It also delineates the extent of the cut in anapical direction. The more distinct it is, the better itserves these purposes.

•The finish line is one of the features that can be usedto evaluate the accuracy of the impression made forindirect procedures.

•In the die, a distinct finish line helps to evaluate thequality of the die and helps in accurate die trimming.

•The correct marginal adaptation of the wax patterndepends on an obvious finish line.

•The evaluation of the restoration is also aided by aproper finish line.

•At cementation, a sharp finish line aids indetermining whether the restoration is fully seated.

• Historically a bevel was introduced to compensate for the casting shrinkage of alloys used to fabricate crowns.

• Metal margins should be ideally acute in cross section rather than right-angled to facilitate a closer fit.

• D is the distance by which the crown fails to seat

• d is the shortest distance between the tooth structure and the restoration• If the inner angle of the metal margin forms an angle m ,of less than 90

degrees with the path of insertion ,as does a bevel or a chamfer ,d will be smaller than D.

• The shortest distance from the casting margin to tooth structure ,d, can be stated as a function of D and the sine of the angle m or the cosine of the angle p,which is the angle between the surface of the bevel and the path of insertion.

d = D sin m

or

d = D cos p

As the angle m is reduced its sine value also reduces and so does the value of d…thus reducing the marginal discrepancy.

An angle of 30 to 45 degrees is considered optimal

• Angles above 50 degrees will not reduce the value of d.• Angles below 45 degrees will result in too thin a casting.

TYPES OF FINISH LINES

Chamfer finish line :

The preferred finish line for the veneer metal

restorations is the chamfer. This finish line has been

shown experimentally to exhibit the least stress, so that

the cement underlying it will less likelihood of failure. It

can be cut with the tip of a round end diamond, while the

axial reduction is being done with the side of that

instrument. However, a torpedo diamond is less likely to

produce a butt joint. The margin of the cast restoration

that fits against it combines an acute edge with a nearby

bulk of metal.

Heavy chamfer finish line :

A heavy chamfer is used to provide a

90-degree cavosurface angle with a large radius

rounded internal angle. It is created with a

round end tapered diamond. In the hands of an

unskilled operator, this instrument can create an

undesirable fragile lip of enamel at the

cavosurface. The heavy chamfer provides better

support for a ceramic crown than does a

conventional chamfer, but it is as good as a

shoulder. A bevel can be added to the heavy

chamfer for use with a metal ceramic

restoration.

Shoulder :

The shoulder has long been the finish

line of choice for the all-ceramic crowns. The

wide ledge provides resistance to occlusal

forces and minimizes stresses that might lead to

fracture of the porcelain. It produces the space

for healthy restoration contours and maximum

esthetics. However, it does require destruction

of more tooth structure than any other finish

line, the sharp 90-degree internal line angle

associated with the classic variety of this finish

line concentrates stress in the tooth and is

conductive to coronal fracture. The shoulder

generally is not used as a finish line for cast

metal restorations.

Shoulder with bevel :

The shoulder with a bevel is a used as a

finish line in a variety of situations. It is utilized

as the gingival finish line on the proximal box

of inlays and onlays, and for the occlusal

shoulder of onlays and mandibular three quarter

crowns. This design can also be used for the

facial finish line of metal ceramic restorations

where gingival esthetics is not critical. It can be

used in those situations where a shoulder is

already present, either because of destruction by

caries or the presence of previous restorations.

It is also a good finish line for preparations with

extremely short walls, since it facilities axial

walls that are nearly parallel.

Knife-edge :

The knife-edge margin provides for an

acute margin of metal. But its use can create

problems. Unless it is carefully prepared, the

axial reduction may fade out instead of

terminating in a definite finish line. The thin

margin of the restoration that fits this finish line

may be difficult to accurately wax and cast. It is

also more susceptible to distortion in the mouth

when the casing is subject to occlusal forces.

Featheredge :

A featheredge finish line is unacceptable

because it is not sufficiently distinct and results

in so little cervical tooth reduction that the

restoration must be over contoured to possess

adequate rigidity. Also, since a feather edge is

more difficult to see visually, occlusocervical

undulations and irregularities in the finish line

are more likely to be present, making it much

more difficult to fabricate a restoration that fits

accurately.

Instrumentation

The advent of hand piece capable of speeds in excess of 100,000 rpm made possible efficient cutting with smaller instruments,which made more sophisticated preparations practical.

With high speed instruments the problem of over heating the tooth during preparation is critical.

Cutting dry can cause three times more dentinal burning and thermal changes leading to pulpal inflammation and necrosis than with adequate air water spray.

Brown et` al calculated the temperature of high speed dry cutting to be 118 degree celcius.The seriousness of which can be understood from Zach`s contentions that even an increase of 20 degree Fahrenheit will lead to pulpal death in 60% of the teeth.

Dry cutting of non-vital teeth also should be avoided as it can lead to micro fractures in the enamel.

There are basically three main rotary instruments used in tooth preparation.

1. Diamond stone2. Tungsten carbide bur3. Twist drill

Diamond stonesNumerous small ,irregularly placed sharp diamond chips are electroplated with a nickel or chromium bonding medium to steel instrument blanks whose head is machined to the desired final shape of the instrument.They most effective against cutting enamel and porcelain.An ideal diamond instrument should have diamond stones evenly placed with intimate contact between the chips and the binding material.

Tunsten carbide bursThey are best suited for making precise preparation features and smooth surfaces on enamel and dentin.They can also be used to cut metal.The metal in the head of the tungsten carbide bur is formed by sintering tungsten carbide powder and cobalt powder under heat and vacuum.These are then cut into desired lengths and attached to steel rods using soldering or welding.Most burs have six and occasionally eight blades.Finishing burs will have 12 blades.The finer the finish more the number of blades.

The angle at which the face of the blade meets a line extending from the cutting edge to the bur axis is known as the rake angle.The more positive the rake angle.

The twist deill is made up of steelIt cuts only at its tip as it is pushed into the tooth in the direction of the long axis of the instrumentIt has deep twin heliocal flutes that wind around the shaft in a tight spiral,helping to remove chips from the hole.Used to make small,uniform diameter,parallel-sided holes in dentin to receive retentive pins for restorations.The drill diameter is slightly larger than the pins that are incorporated into cast restorations to allow for a small cement space.The working portion of this type of drill should be 3 – 5 mm long.a shallow pilot hole is made with no.1/2 round bur on a

narrowhorizontal ledge to ensure that the hole will be drilled precisely in its intended position.

0.6 mm twist drill,pin holes for parallel pins for cast restorations0.5 mm Kodex drill used for creating pinholes for minim threaded pins,which retain amalgam and compositecores.

Dual instrumentation

Diamond burs cut through enamel more efficiently than carbide burs but they leave micro scratces on the surface reducing the finish of the preparation.

Leading to rough cavosurface and marginal preparation with diamond burs.

Hence for preparation of grooves ,box forms , isthmus etcHere both diamond and carbide burs are used of the same length and diameter which are configured

Clearance angle

Rake angle

Common errors of tooth preparation :

1.Insufficient occlusal or incisal reduction.

2.Lack of uniform reduction of labial or buccal surfaces compromising esthetics.

3.Minimum axial reduction on the buccal and lingual surface of the posterior teeth,which increases the incidence of working prematurities. The distinction betweenreduction and clearance is crucial.

4.Inappropriate proximal reduction, which prevents having a cleanable embrasurespace.

5.Over reduction of teeth and/or violation of the biologic width.

6.Insufficient gingival reduction to accommodate a definite finish line.

7.Undercuts on the distolingual surface of the preparation and/or lack of parallelismof the FPD abutments.

8.Failure to contour proximal surfaces of adjacent teeth to allow seating of arestorations.

ESTHETIC CONSIDERATIONS

The restorative dentist should develop skill in determining the esthetic expectations of the patient. Patients prefer their dental restorations to look as natural as possible. However, care must be taken that the esthetic considerations are not preserved at the expense of the patient’s long term oral health or functional efficiency.

Whenever possible, accomplishment of an esthetically acceptable result without the use of metal-ceramic crowns is preferred, not only because tooth structure is conserved but also because no restorative material can approach the appearance of intact tooth enamel.

CONCLUSION

The principles of tooth preparation can be categorized into biologic,

mechanical and esthetic considerations. Often those principles conflict, and the

clinician must decide how the restoration should be designed. Each tooth

preparation must be measured by clearly defined criteria, which can be used to

identify and correct problems. It is important to understand the pertinent theories

underlying each step. Successful preparations can be obtained by systematically

following these steps. It is important to critically evaluate each step before

proceeding to the next step to ensure an optimal quality final restoration, which

will serve the patient for a long time.

Fundamentals of Fixed prosthodontics Herbert T.

Shillingburg, Hobo, Whitself, Jacobi, and Brackett. Third

Edition

Contemporary fixed prosthodontics. Stephen F.

Rosensteil, Martin F. Land and Junhei fjujimoto, Second

Edition.

Tylman’s theory and practice of fixed prosthodoontics.

William F.P. Malone and David L. Koth. Fifth Edition.

Principles of operative dentistry-Marzouk,3rd edition.

References

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