Principles of Tooth Preparation
Vinay Pavan Kumar K 2 nd year MDS student
Department of ProsthodonticsAECS Maaruti College of Dental Sciences
Principles of tooth preparationPreservation 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 forces
Luting agent used
Occlusal reduction
Axial reduction
Preservation of periodontium
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
Tooth preparation
The process of removal of diseased and/or healthy enamel, dentin and
cementum to shape a tooth to receive a restoration
Requirements of tooth preparation
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
Guidelines for tooth preparation
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.
Preservation of tooth structure
Retention & Resistance
Structural durability
Marginal integrity
Preservation of the periodontium
Principles of tooth preparation
Preservation of tooth structure
Preserve the remaining tooth structure
Conservation guidelines-
Coverage: Partial v/s complete Margin: Supragingival v/s subgingival
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.
Retention & Resistance form
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 form
Dislodging forces
Geometry of the tooth preparation
Roughness of the fitting surface of the restoration
Materials being cemented
Luting agent being used
Dislodging forces
Forces that tend to remove a cemented restoration along its path of withdrawal
FPD subject to dislodging forces- Flossing under the connectors Sticky food
Geometry of the tooth preparation
Restrained movement (eg. Nut and bolt )
Sliding pair – two cylindrical surfaces constrained to slide along one another
Geometry of the tooth preparation
Taper / Total Occlusal Convergence (TOC) Substitution of internal features Path of insertion Freedom of displacement Length and Surface area Stress concentration Type of preparation
Taper
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)
Parallel walls – maximum retention
Taper visualize preparation walls prevent undercuts permit more nearly complete seating
of restorations during cementation
Ideal taper: 6°
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.
Total occlusal convergence
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.
Total occlusal convergence gauge
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.
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.
Substitution of internal features
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
Path of insertion
Imaginary line along which the restoration will be placed
onto and removed from the preparation Paths of all FPD abutments must parallel each other
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
In patient’s mouth – mouth mirror is held at an angle approximately ½ inch above the preparation
Image viewed with one eye
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
Path of insertion considered in 2 dimensions- mesiodistally and faciolingually.
Mesiodistal inclination - parallel to contact areas of adjacent teeth
Faciolingual orientation - affects esthetics of metal ceramic 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
Posterior ¾ crown parallel to long axis of the tooth
Anterior ¾ crown parallel to incisal ½ of the labial surface
Freedom of displacement
Numbers of paths along which a restoration can be removed from the tooth preparation
Only one path – maximum retention
Length and Surface area
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
Smaller tooth - short rotation radius
Grooves in the axial walls- reduce the rotation radius
Stress concentration
Retentive failure occurs - cohesive failure in cement
Stress concentration- around the junction of axial and occlusal surfaces
Rounding the internal line angles
Type of preparation
Complete crown> partial coverage crowns
Adding groove/ boxes increases retention
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
Roughness of the fitting surface of restoration
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
Materials being cemented
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
Luting agent being used
Adhesive cements- most retentive
Film thickness of luting agent- effect not certain
Adhesive resin> Glass ionomer> Zinc Phosphate= Polycarboxylate> ZnO-E
Factors influencing retention of cemented restorations
Resistance form
Dislodging forces
Luting agent being used
Geometry of the tooth preparation
Dislodging forces
Mastication and parafunctional activity - substantial horizontal or oblique forces
Lateral forces displace the restoration by causing rotation around the gingival margin
Luting agent being used
Resistance to deformation affected by compressive strength and modulus of elasticity
Adhesive resin> Glass ionomer> Zinc Phosphate> Polycarboxylate> ZnO-E
Geometry of the tooth preparation
Type of preparation Freedom of displacement Occlusocervical/incisocervical dimension Ratio of OC and FL dimension Circumferential form of the prepared tooth
Type of preparation
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)
Freedom of displacement
GROOVE Lingual wall
perpendicular to the direction of forceOblique angleV-shaped groove
PROXIMAL BOX Buccal and lingual walls
must meet the pulpal wall at 90°Oblique angle
Occlusocervical / incisocervical dimension
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.
Occlusocervical dimension
Total occlusal convergence
1mm <6°2mm <12°3mm <17°
Ratio of occlusocervical to faciolingual dimension
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 occlusal convergence
0.1 <6°0.2 <12°0.3 <18°0.4 <24°
Circumference form of prepared tooth
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.
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.
Structural durability 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
Occlusal reduction 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
Adequate reduction Inadequate clearance Overpreparation
Functional cusp bevel 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
Lack of functional cusp bevel:
Thin area in casting Overcontouring Overinclination
Axial reduction
Thin walls of casting– subject to distortion Overcontouring- disastrous effect on the
periodontium
Marginal integrity 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
Finish line configurations
Chamfer Heavy chamfer Shoulder Sloped shoulder Radial shoulder Shoulder with a bevel Knife edge
Chamfer
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
Heavy chamfer 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
Shoulder 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
Destruction of more tooth structure
Sharp 90° internal line angle
concentrates stress on tooth
Coronal fracture
Not used for cast metal restorations
Sloped shoulder 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
Radial shoulder
Modified shoulder Cavosurface 90° Shoulder width lessened with rounded internal angles Lesser stress concentration Good support for porcelain
Shoulder with a bevel
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)
Bevel: allows the cast metal margin to be bent or
burnished against the prepared tooth structure minimizes the marginal discrepancy removes unsupported enamel
Knife edge 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
Reduction depths
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.
Reduction uniformity
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.
Line angle form
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.
Preservation of the periodontium
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
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
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
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
Margin adaptation 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
Prevention of Damage During Tooth Preparation
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)
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
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.
A research tool for determination of tooth structure loss both in vitro and in vivo studies
References 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
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.