Biomechanics of Tooth Preparation

7/29/2019 Biomechanics of Tooth Preparation

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INSTITUTE OF DENTAL SCIENCES

PRINCIPLES OF TOOTH PREPRATION


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Girish Yadav

MDS 1st Year

Dept. of prosthodontics


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* INTRODUCTION

* BIOLOGIC CONSIDERATIONS

* MECHANICAL CONSIDERATIONS

* ESTHETIC CONSIDERATIONS

* CONCLUSION

* REFERENCES


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INTRODUCTIONINTRODUCTION

Teeth do not have the regenerating capacityas most other

tissues have, once when enamel / dentin are lost as a resultofcaries, trauma etc, it requires a restorative material torestore the form & function

Teeth requirepreparation to receive restorations and thesepreparations are based on fundamental principles whichdetermines the success ofprosthodontic treatment


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BIOLOGIC CONSIDERATIONSBIOLOGIC CONSIDERATIONS

* PREVENTION OF DAMAGE DURING TOOTH PREPARATION

* Adjacent Tooth

* Iatrogenic damage or nicking of adjacent toothremoves fluoride rich superficial enamel layerandcreates a rough surface, which has every possibilityto accumulateplaque and eventually leads to dentalcaries

* This can be prevented with the help of a matrix band

placed in the interdental region duringproximaltooth preparation

*The best way to protect adjacent teeth is by usingthin tapered diamond through interproximal contactarea to leave a slight lip of enamel or fin of enamelwithout causing excessive tooth reduction orundesirable angulation ofrotary instrument


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* Soft Tissues

* Movable soft tissues like lips, cheeks and tongue arekept away from the site of preparation with the helpofmouth mirror, aspirator tip and saliva ejector

* Pulp

* Preventingpulpal injuryis very vital to save the tooth

* Extremes of temperature, chemicals andmicroorganisms can cause irreversible pulpitis

* CAUSES OF INJURY

* Temperature

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

* Excessive pressure, condition of the bur, higherrotational speeds all increase the heat generated


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*Air-water spraywhen accurately directedreduces the heat generated,prevents cloggingand increases the cutting efficiencyof the bur

* Special care should be taken while preparinggrooves or pin holes, as the coolant cannotreach the cutting edge of the bur, heatgeneration can be prevented by using lowrotational speed


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* Chemical Action

*The chemical action of certain dental materialslike bases, restorative resins, solvents & luting

agents when applied to freshly cut dentin cancausepulpal damage

* Cavity varnish & dentin bonding agents form aneffective barrier in most cases but they canaffect on the retention ofcemented restorations

* Bacterial Action

* Bacteria those are left behind or those whichgain access to the dentin because ofmicroleakage can lead topulpal damage

* Dental materials likezinc phosphate cementhave an antibacterial effectbut, because of

property ofvital dentin to resist infection theroutine use ofantimicrobials is not advocated

* CONSERVATION OF TOOTH STRUCTURE

*One of the basic tenets of restorative dentistry is theconservation of tooth structure as much as possiblewithin mechanical and esthetic principles oftooth

preparation


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* Tooth structure is conserved by employing the followingguidelines

* Use of partial coverage rather than completecoverage

* Preparation with minimal convergence angle

* Uniform occlusal reduction following anatomicalinclined planes

* Even preparation of axial surfaces

* Conservative margin selection

* Avoid unnecessary apical extension

* CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH

* Structural Durability

* The restoration must contain adequate bulktowithstand forces of mastication

* Occlusal Reduction

* Functional Cusp Bevel

* Axial Reduction


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* Occlusal Reduction

* Occlusal clearance provides adequate strengthand bulkof metal

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


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* For metal ceramic crowns in non-functional cusps1.0 1.5 mm and in functional cusps 1.5 2.0 mm

* For all ceramic2.0 mm throughout

*The basic inclined planes of the occlusal surfaceshould be followed for adequate clearance andwithout over shortening

*The amount ofocclusal reduction is not always thesame as the clearance needed

* Often part of a tipped tooth is already shortof theideal occlusal plane & will require less reductionthan would a tooth in ideal occlusion

* Avoid creating steep planes with sharp angles, since thesecan increase stress & hinder complete seating of thecasting

* For diminishing stresses rounding of the angles &

avoidance of deep grooves in the centre of the occlusalsurface & keeping the angulation of the occlusal planesshallow


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* Functional Cusp Bevel

* A bevel increases the surface area of metalcovering the cusp and hence provides adequate

bulk and strength for the restoration

* When adequate bevel is not given, the thin metalresulting over the functional cusps is easilydamaged because of weakness and createsdeflective occlusal contacts


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*The functional cusp bevel is usually placed on thefacial cusps of the mandibular teeth & on thelingual cusps of the maxillary teeth,paralleling theinclination of the cusp plane it opposes

* In a cross bite occlusal relationship, the functional cuspsare reversed & the functional cusp bevel is placed on thefacial cusps of the maxillary teeth & lingual cusps ofmandibular teeth

* Axial Reduction

* Inadequate axial reduction is commonly associatedwith gingival inflammation, probably because it ismore difficult for the patient to maintainplaquecontrol around the gingival margin

*The crown should duplicate the contours & profileof the original tooth unless it is malformed ormalpositioned

* If an error is made, a slightly under contoured flat

restoration is better as it is easier to keep free ofplaque


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* Margin Placement

* Whenever possible the margin of a preparationshould be Supragingival as Subgingival margins ofcemented restorations have been identified as amajor factor inperiodontal disease

* Supragingival margins are easier to prepareaccurately without trauma to the soft tissues

* They can be situated on hard enamel, whereasSubgingival margins are often on dentin orcementum

*Advantages ofsupragingival margins:-

*They can be easily finished

*They are more easily kept clean

* Impressions are more easily made, with lesspotential for soft tissue damage

* Restorations can be easily evaluated at recallappointments

* Indications for subgingival margins:-


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* Dental caries, cervical erosion, or restorationsextending subgingivally.

*The proximal contact area extends to the gingival

crest

*Additional retention is required

*The margin of a metal ceramic crown is to behidden behind the labiogingival crest

* Margin Adaptation

* The junction between a cemented restoration andtooth is always a potential site for recurrent caries

* The more accuratelythe margins are adapted to thetooth, the less chance for recurrent caries

* A well designed preparation has a smooth & evenmargin

* Rough, irregular or stepped junctions greatlyincrease the length of the margin & substantiallyreduce the adaptation of the restoration

* For a properly prepared tooth a skilled technician canmake a casting that fits to within 10 m & a

porcelain margin that fits to within 50 m


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* Marginal Integrity

* For survival of a restoration in the biologicalenvironmentof the oral cavity the margins should beclosely adapted to the cavosurface finish line of thepreparation

* The configuration of the preparation finish linedictates the shape & bulkofrestorative material inthe margin of the restoration

* Historically, the bevel was used as a device forcompensating for the solidification shrinking of alloys

used in fabricating cast restorations

* Margins should be acute in cross-section rather thanright angled to facilitate a closer fit

* To accomplish this, preparation finish lines shouldtake forms thatpermit acute edges in the restorationmargins


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* Configurations

* Chamfer

* Heavy chamfer

* Shoulder

* Radial shoulder

* Shoulder with bevel

*Knife edge

CHAMFERCHAMFER

* Preferred for veneer metal restorations

* Exhibits least stress, so that cement under it will have lesslikelihood of failure

* Care needed to remove unsupported enamel

HEAVY CHAMFERHEAVY CHAMFER

* Rounded internal angle

* Provides better supportthan chamfer for ceramicrestorations

* Not as good as shoulder


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SHOULDERSHOULDER

*Margin of choice for all ceramic

* More destruction of tooth but space minimizes stress thatmight lead to fracture

* 90 degree internal line angle concentrates stress in thetooth and is conducive to coronal fracture

RADIAL SHOULDERRADIAL SHOULDER

* Internal line angle is rounded by modified bin-angle chisel

* Stress concentration is less in the tooth structure

SHOULDER WITH BEVELSHOULDER WITH BEVEL

*Gingival finish line on the proximal box for inlays and onlays

* 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 EDGEKNIFE EDGE

* Lingual surface of mandibular posterior teeth

* Tilted teeth

Effect Of Finish Line On Marginal Seal & Marginal Seat OfEffect Of Finish Line On Marginal Seal & Marginal Seat OfFull Crown PreparationFull Crown Preparation

* Feather edge &parallel bevel demonstrated the bestmarginal seal

* 900demonstrated the best seating of restoration

* DISTORTION RELATED TO MARGIN DESIGN IN PFMDISTORTION RELATED TO MARGIN DESIGN IN PFMRESTORATIONSRESTORATIONS

* Three margin designs were compared


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* Chamfer

* Shoulder

* Shoulder with bevel

* Chamferexhibited more distortion than shoulder orshoulder with bevel

* Type of distortion ofshoulder and shoulder with bevel werealmost similar

* Study supports the theory thatplacing of additional metalat the gingival margin reinforces the margin and inhibits

marginal distortion

ABUTMENT EVALUATIONABUTMENT EVALUATION

Evaluated for 3 factors:

*CROWN-ROOT RATIO


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* ROOT CONFIGURATION

*PERIODONTAL LIGAMENT AREA

* CROWN-ROOT RATIO

* 1:1 minimum

* 2:3 optimum

* >1:1 when opposing tooth is artificial

* ROOT CONFIGURATION


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* Roots broader labio-linguallyare preferred than theyare mesio-distally

*Irregularpreferred than conical

ROOT SURFACE AREAROOT SURFACE AREA

* ANTES LAW

* Irvin H Ante.1928

*Dykema et al

* Tylman

* Shillingburg

* Nyman & Erikson, J Clin Periodontol 1982

* Nyman & Lindhe. J Clin Periodontol 1976

* Cast doubt on the validity ofAntes Law by demonstratingthat teeth with considerably reduced bone support can besuccessfully used as FPD abutments


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* No loss of attachment after 8 to 11 yrs because of

* Meticulous root planing

* Proper plaque control

* Occlusal design of prosthesis

CANTILEVER F.P.DsCANTILEVER F.P.Ds

* Long termprognosis is poor

* Cantilever designs may be preferred since re-adhesion afterfailure is greatly facilitated and often leads to predictablelong term success


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RESIN BONDED F.P.DsRESIN BONDED F.P.Ds

* Bonded pontics

* Cast perforated

* Resin retained F.P.Ds

* Rochette type

* Etched cast resin retained F.P.Ds

* Maryland type

* Virginia bridge

* Chemical bonded resin F.P.Ds

MECHANICAL CONSIDERATIONSMECHANICAL CONSIDERATIONS

* RETENTION FORM:

* Quality of preparation that prevents the restoration from

becoming dislodged by forces parallel to the path ofwithdrawal


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* Dental caries and porcelain failure outrank lack ofretention as a cause of failure

* RETENTION DEPENDS ON:

* Magnitude of dislodging force

* Geometry of tooth preparation

* Roughness of fitting surface

* Materials being cemented

* Film thickness of luting cement

MAGNITUDE OF DISLODGING FORCESMAGNITUDE OF DISLODGING FORCES

* Greatest removal forces generally arise when exceptionallysticky food is taken


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* Also depends on surface area and texture of restorationbeing pulled

GEOMETRY OF TOOTH PREPARATIONGEOMETRY OF TOOTH PREPARATION

* Taper

* Surface area

* Stress concentration

TAPERTAPER

* Parallel walls were advocated for inlay restorations

* 3 to 5 degrees


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* 6 degrees

* 10 to 14 degrees

* Overall 2.5 to 6.5 degrees has been suggested as optimum

* This is based on an inclination of approximately 30beingproduced, on both surfaces by the sides of the taperedinstrument

* The result would be an overall taper or an angle ofconvergence of60

*


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SURFACE AREASURFACE AREA

* More the surface area covered more the retention

* Long axial walls are more retentive than short walls

* Features such as grooves & boxes that are placed in thepreparation increase the surface area thereby increasingretention

STRESS CONCENTRATIONSTRESS CONCENTRATION

* More important for retention than the total surface area isthe area of cement that will experience shearing ratherthan tensile stress when the restoration is subjected toforces along thepath of insertion

* To decrease failure potential, it is essential to minimizetensile stress


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* Stress is more concentrated at thejunction of axial andocclusal surfaces

* Rounding of margins will decrease stress concentration

* To obtain greatest area of cement under shear, thedirections in which a restoration can be removed must berestricted to essentially one path

* Iffeatures are added to the preparation so that only a forcein one direction can move a restoration withoutcompressing the cement film against one or more surfaces,the retention is enhanced

* A full veneer crown preparation has an excellent retentionbecause of mesial, distal, & facial walls limit thepath ofinsertions to a narrow range


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

* To create a more retentive form, grooves, boxes, orpinholes 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 shearthan tension

* This greater surface area would lead to a preparation withlarger diameter, which will have greater retention than will

a narrow preparation

ROUGHNESS OF SURFACES BEING CEMENTEDROUGHNESS OF SURFACES BEING CEMENTED


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* The adhesion of the dental cements depends primarily ontheprojections of the cement into microscopic irregularities& recesses on the surfaces being joined

* Theprepared tooth surfaces therefore should not be highlypolished

RESISTANCE FORMRESISTANCE FORM

* Prevents dislodgement of the restoration by forces directedin an apical or oblique direction and prevents anymovement of restoration under occlusal forces


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RESISTANCE FORMRESISTANCE FORM

* Depends on:

* Magnitude & direction of dislodging forces

* Geometry of tooth preparation

* Physical properties of luting agent

* Deformation of material

GEOMETRY OF TOOTH PREPARATIONGEOMETRY OF TOOTH PREPARATION

* Increased preparation taperand rounding of axial anglestend to reduce resistance

* Molars require moreparallel preparation than Premolars orAnterior Teeth to achieve adequate resistance form

* The resisting area of a cylindrical preparation would includehalf of its axial surface

* As the degree of taper increases, the resisting areadecreases


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* A long narrow preparation can have a greater taperthan ashort & wide one withoutjeopardizing resistance

* Conversely the walls of a short wide preparation must bekept nearlyparallel to achieve adequate resistance form

* Thepermissible taperof a preparation is directlyproportional to the height/width ratio

* Thepreparation taperthat will still permit an effectiveresisting area, for a preparation in which the height equalsthe width, is double than permissible in a preparation inwhich the height is only one half the width

MAGNITUDE & DIRECTION OF DISLODGING FORCESMAGNITUDE & DIRECTION OF DISLODGING FORCES

* The strongest forces encountered during function are

apically directed & can produce tension & shearin thecement film only through leverage


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* This leverage, which is probably the predominant factor indislodgement ofcemented restorations, occurs when theline 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 willbe no tipping of the restoration

* The margin on all sides of the restoration is supported bythe preparation

* The torque produced merely tends to seat the crownfurther

* If the occlusal table of the restoration is wide, even avertical force can pass outside the supported margin &produce a destructive torque

* This can also occur in crowns on tipped teeth & retainersfor cantilever bridges

* A force applied to a cemented crown at an oblique anglecan produce a line of action which will pass outside thesupporting tooth structure


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* The point on the margin that lies closest to the line ofaction is the fulcrum pointor the centre of rotation


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LENGTH OF THE PREPARATIONLENGTH OF THE PREPARATION

* Length of a preparation has a strong influence on itsresistance

* Shortening a preparation will produce a proportionatelygreater diminution of the resisting area

* The ability of a restoration to resist tipping depends notonly on the preparation, but also on the magnitude oftorque

* If two crowns ofunequal length on 2 preparations ofequallength are subjected to identical forces, the longer crown ismore likely to fail because the force on it acts through alonger lever arm

* When a relatively long crown must be made on a shortpreparation, additional resistance form, usually in the formof apin retained core, must be created before the castrestoration can be made

WIDTH OF THE PREPARATIONWIDTH OF THE PREPARATION

* A wide preparation has greater retention than a narrowerone ofequal height


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* Under some circumstances a crown on the narrow tooth canhave greater resistance to tipping than one on the widertooth

* The resistance of a preparation on a wide, short tooth canbe greatly enhanced by the addition ofgrooves

PATH OF INSERTIONPATH OF INSERTION

* Before any tooth structure is cutthe path of placementshould be decided keeping in mind theprinciples of tooth

preparation

* A path must be selected that will allow the margins of theretainers to fit against their respective preparation finishlines with the removal ofminimum ofsound tooth structure

* This path should not encroach upon thepulp or theadjacent teeth

* Thepath of insertion forposterior full & partial veneercrowns is usually parallel with the long axis of the tooth


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* On the other hand thepath 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 almostno metal visible on the facial surface

*

* For a full crown to have structural durability, with propercontours, itspath of insertion should beparallel to the longaxis of the tooth

* In case of a tilted tooth, apath of insertion paralleling thelong axis of the tooth may be blocked by theproximalcontours of the adjacent tooth

* In such cases thepath of insertion is madeperpendicular to

the occlusal plane

* A long standing loss ofproximal contactis usuallyaccompanied by tipping of the adjacent tooth into thespace

* In such cases thepath of insertion parallel with the longaxis of the tooth might not allow a crown to seat even if theits distal wall is grossly under contoured


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* The space between the adjacent tooth must be madegreaterthan the mesiodistal diameterof the prepared toothat the gingival finish line

* This can be achieved by inclining the path of insertion sothat removal of equal amounts of enamel from each of theadjacent teeth will allow a crown to seat on the preparedtooth

* In cases where more than 50% of the enamel thickness hasto be removed from either adjacent tooth, or if there isntadequate space for gingival embrasures then, teeth shouldbe separated & uprighted orthodontically


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* A negative taperor undercutmust be eliminated or it willprevent the seating of the restoration

* Preparation tapercan be evaluated by viewing it with oneeye from a distance of approximately 30 cm or 12 inches

* In this way it is possible to see all the axial walls with anideal taper of60

* An undercut as great as 80 can be overlooked if both theeyes are used

*

* A mouth mirrorcan be used when it is difficult to survey thepreparation under direct vision

* The entire finish line should be visible to one eye from onefixed position with no obstruction by any part of theprepared tooth

* To verify theparallel paths of insertion the image should becentered in the mirror

* Shillingburg HT,. Fundamentals of fixed prosthodontics


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* Rosenstiel. Contemporary fixed prosthodontics.

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Biomechanics of Tooth Preparation

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