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Vinay Pavan Kumar K
2 nd year MDS student
Department of Prosthodontics
AECS Maaruti College of Dental Sciences
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Principles of tooth preparation
Preservation of
tooth structure
Retention &
resistance form
Structural
durability
Marginal
integrity
Geometry
-taper
-freedom of
displacement
-path of insertion
-length
-stress
-preparation type
Materials
cemented
Roughness of
fitting surfaces
Dislodging
forcesLuting
agent used
Occlusal
reduction
Axial reduction
Preservation of
periodontium
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Al-Fouzan etal quantified the volume of reduction of tooth
structure associated with different commonly used
preparation designs using microcomputed tomography
The all-ceramic crown preparation design for the
mandibular central incisors had the highest percentage
(65.26% ± 4.14%) of tooth structure reduction, while the
lowest percentage of tooth structure reduction was
associated with the ceramic veneer preparation design for
maxillary central incisors (30.28% ± 5.54%)
Al-Fouzan A.F Volumetric measurements of removed tooth structure
associated with various preparation designs Int J Prosthodont 2013;26:545–8
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Tooth preparation
The process of removal of diseased
and/or healthy enamel, dentin and
cementum to shape a tooth to receive a
restoration
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Biological -maintenance of pulp vitality,adjacent teeth & soft tissues
-conservation of tooth structure
Mechanical - retention & resistance
Esthetic - minimal display of metal- adequate thickness of porcelain- proper shade matching
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Total occlusal convergence
Occlusocervical/incisocervical dimension
Ratio of OC and FL dimension
Circumferential form of the prepared tooth
Reduction uniformity
Reduction depths
Finish line location
Line angle form
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Preservation of tooth structure
Retention & Resistance
Structural durability
Marginal integrity
Preservation of the periodontium
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Preserve the remaining tooth structure
Conservation guidelines-
Coverage: Partial v/s complete
Margin: Supragingival v/s subgingival
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Preparation of teeth with the minimum practical
convergence angle between axial walls
Occlusal surface reduction: follow anatomic planes
Axial surfaces : if necessary, teeth should be
orthodontically repositioned.
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Retention prevents removal of therestoration along the path ofinsertion or long axis of the toothpreparation.
Resistance prevents dislodgmentof the restoration by forcesdirected in an apical or obliquedirection and prevents anymovement of the restoration underocclusal forces.
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Dislodging forces
Geometry of the tooth preparation
Roughness of the fitting surface of the restoration
Materials being cemented
Luting agent being used
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Forces that tend to remove a cemented
restoration along its path of withdrawal
FPD subject to dislodging forces-
Flossing under the connectors
Sticky food
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Restrained movement (eg. Nut and bolt )
Sliding pair – two cylindrical surfaces constrained to slide along one another
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Taper / Total Occlusal Convergence (TOC)
Substitution of internal features
Path of insertion
Freedom of displacement
Length and Surface area
Stress concentration
Type of preparation
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Inclination - relationship of one wall of a preparation to
the long axis of that preparation
Tapered diamond bur: 2-3° inclination
Opposing surfaces with 3° inclination= 6° taper
External walls (converge)
Internal walls (diverge)
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Parallel walls – maximum retention
Taper
visualize preparation walls
prevent undercuts
permit more nearly complete seating of restorations during cementation
Ideal taper: 6°
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More the taper, lesser the retention
Retention
Jorgenson KD. The relationship between retention and convergence angle in
cemented veneer crowns. Acta Odontol Scand 1955 Feb;59(2):94-8.
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Angle between two opposing prepared axial surfaces
Historically TOC : 2°-6°
Clinical goal : 10°-22°
TOC beyond 10-22° – auxilliary features needed
Resistance testing was found to be more sensitive to
changes in the TOC than retention testing
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Esteves HJ, Costa N, Esteves IS, Clinical determination of angle convergence in a
tooth preparation for a complete crown. Int J Prosthodont. 2014 Sep-Oct;27(5):472-4.
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Basic unit of retention-opposing walls with minimal taper
Opposing walls not available for use-
Destroyed previously (severe attrition)
Partial veneer restorations
Greater than desirable inclination
Groove Box Pinhole
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Imaginary line along which the restoration will be placed
onto and removed from the preparation
Paths of all FPD abutments must parallel each other
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Visual survey - ensures preparation is
neither undercut or overtapered
Center of the occlusal surface of the
preparation is viewed with one eye
from a distance of 30 cm (12”)
Binocular vision avoided- undercut
preparation can appear to have an
acceptable taper
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In patient’s mouth – mouth mirror is held at an angle
approximately ½ inch above the preparation
Image viewed with one eye
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FPD abutments– common path of insertion
Firm finger rest established – mirror maneuvered until
one preparation is centered– mirror moved by pivoting
on the finger rest without change in angulation till the 2nd
preparation is centered
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Path of insertion considered in 2 dimensions-mesiodistally and faciolingually.
Mesiodistal inclination - parallel to contact areas ofadjacent teeth
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Faciolingual orientation - affects esthetics of metalceramic and partial veneer crowns
Facially inclined path of insertion
prominent facio-occlusal line angle
overcontouring or opaque show-through
For full veneer crowns
parallel to long axis of the tooth
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Posterior ¾ crown
parallel to long axis of the tooth
Anterior ¾ crown
parallel to incisal ½ of the labial surface
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Numbers of paths along which a restoration can be
removed from the tooth preparation
Only one path – maximum retention
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Longer preparation – more surface area – more retentive
Length must be great enough to interfere with the arc of
the casting pivoting about a point on margin on opposite
side of restoration
Short preparations – inclination critical
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Smaller tooth - short
rotation radius
Grooves in the axial
walls- reduce the
rotation radius
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Retentive failure occurs - cohesive failure
in cement
Stress concentration- around the junction
of axial and occlusal surfaces
Rounding the internal line angles
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Complete crown> partial coverage crowns
Adding groove/ boxes increases retention
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Potts RG, Shillingburg HT Jr, Duncanson MG Jr,Retention and resistance of
preparations for cast restorations. J Prosthet Dent. 1980 Mar;43(3):303-8
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Roughening/grooving the restoration -retention increased
Prepared by air-abrading the fitting surface with 50 µm of alumina
Airborne particle abrasion - increase in vitro retention by 64%
Roughening the tooth preparation- not recommended
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Retention affected both by the casting alloy and
the core build-up material
The more reactive the alloy is, the more
adhesion there will be with certain luting agents
Type I and II gold alloys- intracoronal
restorations
Type III and IV gold alloys- crowns and FPD
Ni-Cr alloys- long span FPD
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Adhesive cements- most retentive
Film thickness of luting agent- effect not
certain
Adhesive resin> Glass ionomer> Zinc
Phosphate= Polycarboxylate> ZnO-E
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Dislodging forces
Luting agent being used
Geometry of the tooth preparation
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Mastication and parafunctional activity - substantial
horizontal or oblique forces
Lateral forces displace the restoration by causing
rotation around the gingival margin
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Resistance to deformation affected by
compressive strength and modulus of
elasticity
Adhesive resin> Glass ionomer> Zinc
Phosphate> Polycarboxylate> ZnO-E
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Type of preparation
Freedom of displacement
Occlusocervical/incisocervical dimension
Ratio of OC and FL dimension
Circumferential form of the prepared tooth
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Partial coverage restoration< complete crown
(no buccal resistance areas in partial coverage)
Adding groove/ boxes increases resistance
(greatest if walls are perpendicular to direction
of force)
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GROOVE
Lingual wall
perpendicular to the
direction of force
Oblique angle
V-shaped groove
PROXIMAL BOX
Buccal and lingual walls
must meet the pulpal wall
at 90°
Oblique angle
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Minimal OC dimension:
Anteriors - 3mm
Premolars - 3mm
Molars - 4mm
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
Occlusocervicaldimension
Total occlusalconvergence
1mm <6°
2mm <12°
3mm <17°
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Should be 0.4 or higher for all teeth
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
OC/FL ratio Total occlusalconvergence
0.1 <6°
0.2 <12°
0.3 <18°
0.4 <24°
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Should possess circumferential irregularity
Maxillary molars – rhomboidal form
Mandibular molars – rectangular form
Premolars and anteriors – oval form
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Preserve corners of a tooth preparation
No axial grooves, boxes should be provided in corners
Chewing and parafunctional habits
Dislodging forces largely faciolingual
So, grooves and boxes on the proximal surfaces
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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A restoration must contain a bulk of material that is
adequate to withstand the forces of occlusion
Bulk should be confined to the space created by the
tooth preparation
To provide adequate bulk:
▪ Occlusal reduction
▪ Functional cusp bevel
▪ Axial reduction
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Full metal restoration:
1.5 mm – functional cusp
1mm – non functional cusp
Metal-ceramic crowns :
1.5 to 2mm – functional cusp
1 to 1.5mm – non functional cusp
All ceramic crowns :
2mm over all
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Adequate reduction Inadequate clearance Overpreparation
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Wide bevel on-
Lingual inclines of the maxillary lingual cusps
Buccal inclines of mandibular buccal cusps
Adequate bulk of metal in area of heavy occlusal contact
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Lack of functional cusp bevel:
Thin area in casting Overcontouring Overinclination
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Thin walls of casting– subject to distortion
Overcontouring- disastrous effect on the
periodontium
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Closely adapted margins to finish lines of preparation-
survival of restoration in the oral environment
Configuration of finish line-
dictates the shape and bulk of metal at the margins
affects the marginal adaptation
affects degree of seating
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Chamfer
Heavy chamfer
Shoulder
Sloped shoulder
Radial shoulder
Shoulder with a bevel
Knife edge
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Indications-
Cast metal crowns
Metal-only portion of PFM crowns
Distinct, easily identified
Least stress
Round end tapered diamond
Half the tip of the diamond
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Indicated for all-ceramic crowns
90 degree cavosurface angle with a large radius
rounded internal angle
Round end tapered diamond
Better than conventional chamfer but not shoulder
Bevel added - to use with metal restoration
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All-ceramic crowns
Facial margin of PFM crowns
Wide ledge-
resistance to occlusal forces
minimizes stresses which leads to fracture of porcelain
Flat-end tapered bur
Healthy contours
Maximum esthetics
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Destruction of more tooth structure
Sharp 90° internal line angle
concentrates stress on tooth
Coronal fracture
Not used for cast metal restorations
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120° sloped shoulder margin
Facial margin of a metal-ceramic crown
No unsupported enamel, yet sufficient bulk to allow
thinning of the metal framework to a knife-edge for
acceptable esthetics
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Modified shoulder
Cavosurface 90°
Shoulder width lessened with rounded internal angles
Lesser stress concentration
Good support for porcelain
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Indications:
Proximal box of inlays, onlays
Occlusal shoulder of onlays and mandibular ¾ crowns
Facial finish line of metal-ceramic restorations (gingival
esthetics not critical)
Situations where a shoulder is already present
(destruction by caries, previous restorations)
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Bevel:
allows the cast metal margin to be bent or
burnished against the prepared tooth structure
minimizes the marginal discrepancy
removes unsupported enamel
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Permit acute margin of metal
Axial reduction may fade out
Thin margin - difficult to wax and cast
Susceptible to distortion
Indications:
Mandibular posterior teeth with very convex axial
surfaces
Lingually tilted lower molars
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All metal crowns –
Chamfer depth: 0.3-0.5 mm
Axial surface reduction: 0.5 -0.8 mm
Occlusal reduction: 1- 1.5 mm
Metal ceramic crowns –
Finish line depth: 1-1.5 mm
Occlusal reduction: 2mm
All ceramic crowns–
Finish line and facial reduction depth: 1mm
Incisal/occlusal reduction: 2mm
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Uniformly reduced :
normal crown form
improved aesthetic
Makes easier for laboratory technician to create
esthetic restorations
Best achieved by placing depth grooves
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Should be rounded (increases crown strength)
Sharp line angles – stress concentration
Facilitates laboratory fabrication and fit
Ease to pour impressions
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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Margin placement
Direct effect on ultimate success of restoration
Margins should be as smooth as possible
Placed in area that can be finished well by the dentist
and kept clean by the patient
Placed in enamel whenever possible
Should be supragingival whenever possible
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Supragingival margins
Less potential for soft tissue damage
Easily prepared and finished
More easily kept clean
Impressions are more easily made
Restorations easily evaluated at recall
appointments
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Subgingival margins: Esthetics
Existing caries, cervical erosion, or restorations extend
subgingivally, and crown-lengthening is not indicated
Proximal contact area extends to the gingival crest
Additional retention is needed
Margin of a metal-ceramic crown is to be hidden behind
the labiogingival crest
Root sensitivity cannot be controlled by more
conservative procedures, such as the application of
dentin bonding agents
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Finish line should not be closer than 2mm to the alveolar
crest
Placement in this area –
gingival inflammation
loss of alveolar crest height
pocket formation
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Junction between a cemented restoration and
the tooth - potential site for recurrent caries
Casting- fits within 10 µm
Porcelain margin- 50 µm
Stepped irregular margin- poor adaptation
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Adjacent teeth :
Iatrogenic damage
Metal matrix band
Leave a slight lip or fin of proximal enamel
Soft tissues:
Careful retraction of lips, cheeks
Care to protect tongue when lingual surfaces of mandibular molars prepared
Pulp
Temperature
Chemical action of cements
Bacterial action (microleakage)
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Borelli etal In vitro analysis of residual tooth structure of maxillary anterior teeth
after different prosthetic finish line preparations for full-coverage single crowns
Journal of Oral Science, Vol. 55, No. 1, 79-84, 2013
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Different preparation depths
With/without coolants
Rise in temperature was noted without coolants
1mm depth – 0.540 C
2mm depth – 10 C
3 mm depth - 1.840 C
Drop in temperature was noted with coolants
1mm depth – 0.400 C
2mm depth – 0.820 C
3mm depth – 1.130 C
Chhatwal N. Effect of tooth preparation and coolants on temperature within the pulp chamber. TPDI 2010;1(2):45-48.
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Shillingburg HT, Fundamentals of Fixed Prosthodontics, 4th
edition, USA, Quintessence publications,2012, pp119-137.
Rosenstiel SF, Contemporary Fixed Prosthodontics, 4th
edition, USA, Mosby, 2006, pp 166-201.
Goodacre C J. Designing tooth preparations for optimal
success. Dent Clin N Am 2004; 48: 359-85.
Borelli etal In vitro analysis of residual tooth structure of
maxillary anterior teeth after different prosthetic finish line
preparations for full-coverage single crowns Journal of Oral
Science, Vol. 55, No. 1, 79-84, 2013
Al-Fouzan A.F Volumetric measurements of removed tooth
structureassociated with various preparation designs Int J
Prosthodont 2013;26:545–8
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Parker MH. Resistance form in tooth preparations. Dent
Clin N Am 2004; 48: 387-96.
Owen CP, Retention and resistance in preparations for
extracoronal restorations. Part II: Practical and clinical
studies, J Prosthet Dent 1986;56(2):148-153.
Gilboe DB, Teteruck WR. Fundamentals of extracoronal
tooth preparation. Part I-Retention and resistance form.
J Prosthet Dent 2005;94:105-7.
Chhatwal N. Effect of tooth preparation and coolants on
temperature within the pulp chamber. TPDI
2010;1(2):45-48.