principles of tooth preparation in fixed partial dentures

Vinay Pavan Kumar K

2 nd year MDS student

Department of Prosthodontics

AECS Maaruti College of Dental Sciences

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

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

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

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

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 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.

Dislodging forces

Geometry of the tooth preparation

Roughness of the fitting surface of the restoration

Materials being cemented

Luting agent being used

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

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.

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


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.

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

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 ofadjacent teeth

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

Posterior ¾ crown

parallel to long axis of the tooth

Anterior ¾ crown

parallel to incisal ½ of the labial surface

Numbers of paths along which a restoration can be

removed from the tooth preparation

Only one path – maximum retention

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

Retentive failure occurs - cohesive failure

in cement

Stress concentration- around the junction

of axial and occlusal surfaces

Rounding the internal line angles

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

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

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

Dislodging forces

Luting agent being used

Geometry of the tooth preparation

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

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)

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

Minimal OC dimension:

Anteriors - 3mm

Premolars - 3mm

Molars - 4mm


Occlusocervicaldimension

Total occlusalconvergence

1mm <6°

2mm <12°

3mm <17°

Should be 0.4 or higher for all teeth


OC/FL ratio Total occlusalconvergence

0.1 <6°

0.2 <12°

0.3 <18°

0.4 <24°

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


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

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

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

Thin walls of casting– subject to distortion

Overcontouring- disastrous effect on the

periodontium

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

Chamfer

Heavy chamfer

Shoulder

Sloped shoulder

Radial shoulder

Shoulder with a bevel

Knife edge

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

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

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

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

Modified shoulder

Cavosurface 90°

Shoulder width lessened with rounded internal angles

Lesser stress concentration

Good support for porcelain

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

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

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


Uniformly reduced :

normal crown form

improved aesthetic

Makes easier for laboratory technician to create

esthetic restorations

Best achieved by placing depth grooves


Should be rounded (increases crown strength)

Sharp line angles – stress concentration

Facilitates laboratory fabrication and fit

Ease to pour impressions


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

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

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




Chhatwal N. Effect of tooth preparation and coolants on temperature within the pulp chamber. TPDI 2010;1(2):45-48.

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.

principles of tooth preparation in fixed partial dentures

Health & Medicine

tooth preparations

tooth preparationre

tooth preparation roughness

used preparation designs

occlusal forces

cemented restoration

volume of reduction

restoration materials