Biomechanics of Tooth Preparation

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INSTITUTE OF DENTAL SCIENCES

PRINCIPLES OF TOOTH PREPRATION

Girish Yadav MDS 1st Year Dept. of prosthodontics

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INTRODUCTION

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BIOLOGIC CONSIDERATIONS

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MECHANICAL CONSIDERATIONS

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ESTHETIC CONSIDERATIONS

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CONCLUSION

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REFERENCES

INTRODUCTION Teeth do not have the regenerating capacity as most other tissues have, once when enamel / dentin are lost as a result of caries, trauma etc, it requires a restorative material to restore the form & function Teeth require preparation to receive restorations and these preparations are based on fundamental principles which determines the success of prosthodontic treatment

BIOLOGIC CONSIDERATIONS

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PREVENTION OF DAMAGE DURING TOOTH PREPARATION

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Adjacent Tooth

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Iatrogenic damage or nicking of adjacent tooth removes fluoride rich superficial enamel layer and creates a rough surface, which has every possibility to accumulate plaque and eventually leads to dental caries

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This can be prevented with the help of a matrix band placed in the interdental region during proximal tooth preparation

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The best way to protect adjacent teeth is by using thin tapered diamond through interproximal contact area to leave a slight lip of enamel or fin of enamel without causing excessive tooth reduction or undesirable angulation of rotary instrument

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Soft Tissues

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Movable soft tissues like lips, cheeks and tongue are kept away from the site of preparation with the help of mouth mirror, aspirator tip and saliva ejector

Pulp

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Preventing pulpal injury is very vital to save the tooth

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Extremes of temperature, chemicals and microorganisms can cause irreversible pulpitis

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CAUSES OF INJURY

* Temperature * Heat is generated by the friction between the rotary instrument & the tooth being prepared

* Excessive pressure, condition of the bur, higher

rotational speeds all increase the heat generated

* Air-water spray when accurately directed

reduces the heat generated, prevents clogging and increases the cutting efficiency of the bur

* Special care should be taken while preparing grooves or pin holes, as the coolant cannot reach the cutting edge of the bur, heat generation can be prevented by using low rotational speed

* Chemical Action * The chemical action of certain dental materials like bases, restorative resins, solvents & luting agents when applied to freshly cut dentin can cause pulpal damage

* Cavity varnish & dentin bonding agents form an

effective barrier in most cases but they can affect on the retention of cemented restorations

* Bacterial Action * Bacteria those are left behind or those which gain access to the dentin because of microleakage can lead to pulpal damage

* Dental materials like zinc phosphate cement

have an antibacterial effect but, because of property of vital dentin to resist infection the routine use of antimicrobials is not advocated

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CONSERVATION OF TOOTH STRUCTURE

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One of the basic tenets of restorative dentistry is the conservation of tooth structure as much as possible within mechanical and esthetic principles of tooth preparation

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Tooth structure is conserved by employing the following guidelines

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Use of partial coverage rather than complete coverage

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Preparation with minimal convergence angle

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Uniform occlusal reduction following anatomical inclined planes

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Even preparation of axial surfaces

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Conservative margin selection

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Avoid unnecessary apical extension

CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH

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Structural Durability

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The restoration must contain adequate bulk to withstand forces of mastication

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Occlusal Reduction

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Functional Cusp Bevel

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Axial Reduction

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Occlusal Reduction

* Occlusal clearance provides adequate strength and bulk of metal

* For gold alloys occlusal clearance in non-functional cusps – 1.0 mm and for functional cusps – 1.5 mm

* For metal ceramic crowns in non-functional cusps

1.0 – 1.5 mm and in functional cusps 1.5 – 2.0 mm

* For all ceramic 2.0 mm throughout * The basic inclined planes of the occlusal surface should be followed for adequate clearance and without over shortening

* The amount of occlusal reduction is not always the same as the clearance needed

* Often part of a tipped tooth is already short of the ideal occlusal plane & will require less reduction than would a tooth in ideal occlusion

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Avoid creating steep planes with sharp angles, since these can increase stress & hinder complete seating of the casting For diminishing stresses rounding of the angles & avoidance of deep grooves in the centre of the occlusal surface & keeping the angulation of the occlusal planes shallow

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Functional Cusp Bevel

* A bevel increases the surface area of metal

covering the cusp and hence provides adequate bulk and strength for the restoration

* When adequate bevel is not given, the thin metal resulting over the functional cusps is easily damaged because of weakness and creates deflective occlusal contacts

* The functional cusp bevel is usually placed on the

facial cusps of the mandibular teeth & on the lingual cusps of the maxillary teeth, paralleling the inclination of the cusp plane it opposes

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In a cross – bite occlusal relationship, the functional cusps are reversed & the functional cusp bevel is placed on the facial cusps of the maxillary teeth & lingual cusps of mandibular teeth

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Axial Reduction

* Inadequate axial reduction is commonly associated with gingival inflammation, probably because it is more difficult for the patient to maintain plaque control around the gingival margin

* The crown should duplicate the contours & profile of the original tooth unless it is malformed or malpositioned

* If an error is made, a slightly under contoured flat restoration is better as it is easier to keep free of plaque

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Margin Placement

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Whenever possible the margin of a preparation should be Supragingival as Subgingival margins of cemented restorations have been identified as a major factor in periodontal disease

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Supragingival margins are easier to prepare accurately without trauma to the soft tissues

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They can be situated on hard enamel, whereas Subgingival margins are often on dentin or cementum

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Advantages of supragingival margins:-

* They can be easily finished * They are more easily kept clean * Impressions are more easily made, with less potential for soft tissue damage

* Restorations can be easily evaluated at recall appointments

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Indications for subgingival margins:-

* Dental caries, cervical erosion, or restorations extending subgingivally.

* The proximal contact area extends to the gingival crest

* Additional retention is required * The margin of a metal – ceramic crown is to be hidden behind the labiogingival crest

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Margin Adaptation

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The junction between a cemented restoration and tooth is always a potential site for recurrent caries

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The more accurately the margins are adapted to the tooth, the less chance for recurrent caries

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A well designed preparation has a smooth & even margin

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Rough, irregular or stepped junctions greatly increase the length of the margin & substantially reduce the adaptation of the restoration

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For a properly prepared tooth a skilled technician can make a casting that fits to within 10 µm & a porcelain margin that fits to within 50 µm

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Marginal Integrity

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For survival of a restoration in the biological environment of the oral cavity the margins should be closely adapted to the cavosurface finish line of the preparation

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The configuration of the preparation finish line dictates the shape & bulk of restorative material in the margin of the restoration

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Historically, the bevel was used as a device for compensating for the solidification shrinking of alloys used in fabricating cast restorations

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Margins should be acute in cross-section rather than right – angled to facilitate a closer fit

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To accomplish this, preparation finish lines should take forms that permit acute edges in the restoration margins

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Configurations

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Chamfer

* Heavy chamfer *

Shoulder

* Radial shoulder * Shoulder *

with bevel

Knife edge

CHAMFER

* * *

Preferred for veneer metal restorations Exhibits least stress, so that cement under it will have less likelihood of failure Care needed to remove unsupported enamel HEAVY CHAMFER

* * *

Rounded internal angle Provides better support than chamfer for ceramic restorations Not as good as shoulder

SHOULDER

* * *

Margin of choice for all ceramic More destruction of tooth but space minimizes stress that might lead to fracture 90 degree internal line angle concentrates stress in the tooth and is conducive to coronal fracture

RADIAL SHOULDER

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Internal line angle is rounded by modified bin-angle chisel

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Stress concentration is less in the tooth structure

SHOULDER WITH BEVEL

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Gingival finish line on the proximal box for inlays and onlays

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Facial finish line where esthetics is not critical

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Good finish line for preparation with extremely short walls, since it facilitates axial walls that are nearly parallel

KNIFE EDGE

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Lingual surface of mandibular posterior teeth

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Tilted teeth

Effect Of Finish Line On Marginal Seal & Marginal Seat Of Full Crown Preparation

* * * *

Feather edge & parallel bevel demonstrated the best marginal seal 900 demonstrated the best seating of restoration DISTORTION RELATED TO MARGIN DESIGN IN PFM RESTORATIONS Three margin designs were compared

* * *

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Chamfer

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Shoulder

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Shoulder with bevel

Chamfer exhibited more distortion than shoulder or shoulder with bevel Type of distortion of shoulder and shoulder with bevel were almost similar Study supports the theory that placing of additional metal at the gingival margin reinforces the margin and inhibits marginal distortion

ABUTMENT EVALUATION Evaluated for 3 factors:

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CROWN-ROOT RATIO

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*

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ROOT CONFIGURATION

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PERIODONTAL LIGAMENT AREA

CROWN-ROOT RATIO

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1:1

minimum

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2:3

optimum

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>1:1 when opposing tooth is artificial

ROOT CONFIGURATION

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Roots broader labio-lingually are preferred than they are mesio-distally Irregular preferred than conical ROOT SURFACE AREA

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*

ANTE’S LAW

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Irvin H Ante.1928

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Dykema et al

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Tylman

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Shillingburg

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Nyman & Erikson, J Clin Periodontol 1982

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Nyman & Lindhe. J Clin Periodontol 1976

Cast doubt on the validity of Ante’s Law by demonstrating that teeth with considerably reduced bone support can be successfully used as FPD abutments

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No loss of attachment after 8 to 11 yrs because of

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Meticulous root planing

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Proper plaque control

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Occlusal design of prosthesis

CANTILEVER F.P.D’s

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Long term prognosis is poor

Cantilever designs may be preferred since re-adhesion after failure is greatly facilitated and often leads to predictable long term success

RESIN BONDED F.P.D’s

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Bonded pontics

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Cast perforated

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Resin retained F.P.D’s

*

*

Rochette type

Etched cast resin retained F.P.D’s

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Maryland type

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Virginia bridge

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Chemical bonded resin F.P.D’s

MECHANICAL CONSIDERATIONS

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RETENTION FORM:

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Quality of preparation that prevents the restoration from becoming dislodged by forces parallel to the path of withdrawal

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*

Dental caries and porcelain failure outrank lack of retention as a cause of failure

RETENTION DEPENDS ON:

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Magnitude of dislodging force

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Geometry of tooth preparation

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Roughness of fitting surface

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Materials being cemented

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Film thickness of luting cement

MAGNITUDE OF DISLODGING FORCES

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Greatest removal forces generally arise when exceptionally sticky food is taken

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Also depends on surface area and texture of restoration being pulled GEOMETRY OF TOOTH PREPARATION

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Taper

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Surface area

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Stress concentration

TAPER

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Parallel walls were advocated for inlay restorations

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3 to 5 degrees

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6 degrees

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10 to 14 degrees

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Overall 2.5 to 6.5 degrees has been suggested as optimum

* * *

This is based on an inclination of approximately 30 being produced, on both surfaces by the sides of the tapered instrument The result would be an overall taper or an angle of convergence of 60

SURFACE AREA

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More the surface area covered more the retention

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Long axial walls are more retentive than short walls

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Features such as grooves & boxes that are placed in the preparation increase the surface area thereby increasing retention STRESS CONCENTRATION

* *

More important for retention than the total surface area is the area of cement that will experience shearing rather than tensile stress when the restoration is subjected to forces along the path of insertion To decrease failure potential, it is essential to minimize tensile stress

*

* * *

*

Stress is more concentrated at the junction of axial and occlusal surfaces

Rounding of margins will decrease stress concentration

To obtain greatest area of cement under shear, the directions in which a restoration can be removed must be restricted to essentially one path If features are added to the preparation so that only a force in one direction can move a restoration without compressing the cement film against one or more surfaces, the retention is enhanced

A full veneer crown preparation has an excellent retention because of mesial, distal, & facial walls limit the path of insertions to a narrow range

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* *

* * * *

However, when the facial surface is left uncovered the crown on this preparation could be removed towards the lingual, the incisal, or any direction in between

To create a more retentive form, grooves, boxes, or pinholes are substituted for the missing axial wall These features are also useful for augmenting retention on severely damaged teeth

Length of a preparation is an important factor in retention A long preparation has greater retention than does a short preparation This is due to greater surface area of the longer preparation & to the fact that most of the additional area is under shear than tension This greater surface area would lead to a preparation with larger diameter, which will have greater retention than will a narrow preparation

ROUGHNESS OF SURFACES BEING CEMENTED

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The adhesion of the dental cements depends primarily on the projections of the cement into microscopic irregularities & recesses on the surfaces being joined The prepared tooth surfaces therefore should not be highly polished RESISTANCE FORM

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Prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of restoration under occlusal forces

RESISTANCE FORM

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Depends on:

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Magnitude & direction of dislodging forces

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Geometry of tooth preparation

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Physical properties of luting agent

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Deformation of material GEOMETRY OF TOOTH PREPARATION

* *

* *

Increased preparation taper and rounding of axial angles tend to reduce resistance

Molars require more parallel preparation than Premolars or Anterior Teeth to achieve adequate resistance form

The resisting area of a cylindrical preparation would include half of its axial surface As the degree of taper increases, the resisting area decreases

* *

* *

A long narrow preparation can have a greater taper than a short & wide one without jeopardizing resistance Conversely the walls of a short wide preparation must be kept nearly parallel to achieve adequate resistance form

The permissible taper of a preparation is directly proportional to the height/width ratio The preparation taper that will still permit an effective resisting area, for a preparation in which the height equals the width, is double than permissible in a preparation in which the height is only one half the width

MAGNITUDE & DIRECTION OF DISLODGING FORCES

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The strongest forces encountered during function are apically directed & can produce tension & shear in the cement film only through leverage

*

* * *

* *

*

This leverage, which is probably the predominant factor in dislodgement of cemented restorations, occurs when the line of action of a force passes outside the tooth structure, or when the structures flex

If the force passes within the margin of a crown, there will be no tipping of the restoration The margin on all sides of the restoration is supported by the preparation The torque produced merely tends to seat the crown further

If the occlusal table of the restoration is wide, even a vertical force can pass outside the supported margin & produce a destructive torque This can also occur in crowns on tipped teeth & retainers for cantilever bridges

A force applied to a cemented crown at an oblique angle can produce a line of action which will pass outside the supporting tooth structure

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The point on the margin that lies closest to the line of action is the fulcrum point or the centre of rotation

LENGTH OF THE PREPARATION

* * *

*

*

Length of a preparation has a strong influence on its resistance Shortening a preparation will produce a proportionately greater diminution of the resisting area The ability of a restoration to resist tipping depends not only on the preparation, but also on the magnitude of torque

If two crowns of unequal length on 2 preparations of equal length are subjected to identical forces, the longer crown is more likely to fail because the force on it acts through a longer lever arm

When a relatively long crown must be made on a short preparation, additional resistance form, usually in the form of a pin – retained core, must be created before the cast restoration can be made WIDTH OF THE PREPARATION

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A wide preparation has greater retention than a narrower one of equal height

*

*

Under some circumstances a crown on the narrow tooth can have greater resistance to tipping than one on the wider tooth

The resistance of a preparation on a wide, short tooth can be greatly enhanced by the addition of grooves PATH OF INSERTION

* * *

*

Before any tooth structure is cut the path of placement should be decided keeping in mind the principles of tooth preparation A path must be selected that will allow the margins of the retainers to fit against their respective preparation finish lines with the removal of minimum of sound tooth structure This path should not encroach upon the pulp or the adjacent teeth

The path of insertion for posterior full & partial veneer crowns is usually parallel with the long axis of the tooth

*

On the other hand the path of insertion for an anterior 3 – quarter crown should be inclined to parallel the incisal 2/3rds of the facial surface enabling the restoration to have almost no metal visible on the facial surface

* *

* *

* *

For a full crown to have structural durability, with proper contours, its path of insertion should be parallel to the long axis of the tooth

In case of a tilted tooth, a path of insertion paralleling the long axis of the tooth may be blocked by the proximal contours of the adjacent tooth In such cases the path of insertion is made perpendicular to the occlusal plane

A long standing loss of proximal contact is usually accompanied by tipping of the adjacent tooth into the space In such cases the path of insertion parallel with the long axis of the tooth might not allow a crown to seat even if the its distal wall is grossly under contoured

* *

*

The space between the adjacent tooth must be made greater than the mesiodistal diameter of the prepared tooth at the gingival finish line This can be achieved by inclining the path of insertion so that removal of equal amounts of enamel from each of the adjacent teeth will allow a crown to seat on the prepared tooth

In cases where more than 50% of the enamel thickness has to be removed from either adjacent tooth, or if there isn’t adequate space for gingival embrasures then, teeth should be separated & uprighted orthodontically

* *

* *

A negative taper or undercut must be eliminated or it will prevent the seating of the restoration Preparation taper can be evaluated by viewing it with one eye from a distance of approximately 30 cm or 12 inches

In this way it is possible to see all the axial walls with an ideal taper of 60 An undercut as great as 80 can be overlooked if both the eyes are used

* * * *

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A mouth mirror can be used when it is difficult to survey the preparation under direct vision The entire finish line should be visible to one eye from one fixed position with no obstruction by any part of the prepared tooth To verify the parallel paths of insertion the image should be centered in the mirror

Shillingburg HT,. Fundamentals of fixed prosthodontics

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Rosenstiel. Contemporary fixed prosthodontics.

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