Principles of tooth preparation in Fixed Partial Dentures

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


Restrained movement (eg. Nut and bolt )

Sliding pair – two cylindrical surfaces constrained to slide along one another


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


Total occlusal convergence gauge


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


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



Dislodging forces

Mastication and parafunctional activity - substantial horizontal or oblique forces

Lateral forces displace the restoration by causing rotation around the gingival margin


Resistance to deformation affected by compressive strength and modulus of elasticity

Adhesive resin> Glass ionomer> Zinc Phosphate> Polycarboxylate> ZnO-E


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


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


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


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


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


Reduction uniformity

Uniformly reduced : normal crown form improved aesthetic

Makes easier for laboratory technician to create esthetic restorations

Best achieved by placing depth grooves


Line angle form

Should be rounded (increases crown strength)

Sharp line angles – stress concentration

Facilitates laboratory fabrication and fit

Ease to pour impressions


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.

Health & Medicine

Principles of tooth preparation in Fixed Partial Dentures