principles of tooth preparation in FPD

1. Vinay Pavan Kumar K
2 nd year MDS student
Department of Prosthodontics
AECS Maaruti College of Dental Sciences

2. 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
forces
Luting
agent used
Occlusal
reduction
Axial
reduction
Preservation of
periodontium

3.  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

4. Tooth preparation
The process of removal of diseased
and/or healthy enamel, dentin and
cementum to shape a tooth to receive a
restoration

5.  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

7.  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 NAm 2004; 48: 359-85.

8.  Preservation of tooth structure
 Retention & Resistance
 Structural durability
 Marginal integrity
 Preservation of the periodontium

9.  Preserve the remaining tooth
structure
 Conservation guidelines-
 Coverage: Partial v/s complete
 Margin: Supragingival v/s
subgingival

10.  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.

11.  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.

12.  Dislodging forces
 Geometry of the tooth preparation
 Roughness of the fitting surface of the restoration
 Materials being cemented
 Luting agent being used

13.  Forces that tend to remove a cemented
restoration along its path of withdrawal
 FPD subject to dislodging forces-
 Flossing under the connectors
 Sticky food

14.  Restrained movement (eg. Nut and bolt )
 Sliding pair – two cylindrical surfaces constrained to
slide along one another

15.  Taper / Total Occlusal Convergence (TOC)
 Substitution of internal features
 Path of insertion
 Freedom of displacement
 Length and Surface area
 Stress concentration
 Type of preparation

16.  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)

17.  Parallel walls – maximum
retention
 Taper
 visualize preparation walls
 prevent undercuts
 permit more nearly complete
seating of restorations during
cementation
 Ideal taper: 6°

18.  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.

19.  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 NAm 2004; 48: 359-85.

20. Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

21. 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.

22.  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

23.  Imaginary line along which the restoration will be placed
onto and removed from the preparation
 Paths of all FPD abutments must parallel each other

24.  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

25.  In patient’s mouth – mouth mirror is held at an angle
approximately ½ inch above the preparation
 Image viewed with one eye

26.  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

27.  Path of insertion considered in 2 dimensions-
mesiodistally and faciolingually.
 Mesiodistal inclination - parallel to contact areas of
adjacent teeth

28.  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

29.  Posterior ¾ crown
 parallel to long axis of the tooth
 Anterior ¾ crown
 parallel to incisal ½ of the labial surface

30.  Numbers of paths along which a restoration can be
removed from the tooth preparation
 Only one path – maximum retention

31.  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

32.  Smaller tooth - short
rotation radius
 Grooves in the axial
walls- reduce the
rotation radius

33.  Retentive failure occurs - cohesive failure
in cement
 Stress concentration- around the junction
of axial and occlusal surfaces
 Rounding the internal line angles

34.  Complete crown> partial coverage crowns
 Adding groove/ boxes increases retention

35. 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

36.  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

37.  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

38.  Adhesive cements- most retentive
 Film thickness of luting agent- effect not
certain
 Adhesive resin> Glass ionomer> Zinc
Phosphate= Polycarboxylate> ZnO-E

40.  Dislodging forces
 Luting agent being used
 Geometry of the tooth preparation

41.  Mastication and parafunctional activity - substantial
horizontal or oblique forces
 Lateral forces displace the restoration by causing
rotation around the gingival margin

42.  Resistance to deformation affected by
compressive strength and modulus of
elasticity
 Adhesive resin> Glass ionomer> Zinc
Phosphate> Polycarboxylate> ZnO-E

43.  Type of preparation
 Freedom of displacement
 Occlusocervical/incisocervical dimension
 Ratio of OC and FL dimension
 Circumferential form of the prepared tooth

44.  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)

45. 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

46. Minimal OC dimension:
 Anteriors - 3mm
 Premolars - 3mm
 Molars - 4mm
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.
Occlusocervical
dimension
Total occlusal
convergence
1mm <6°
2mm <12°
3mm <17°

47.  Should be 0.4 or higher for all teeth
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.
OC/FL ratio Total occlusal
convergence
0.1 <6°
0.2 <12°
0.3 <18°
0.4 <24°

48.  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 NAm 2004; 48: 359-85.

49.  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 NAm 2004; 48: 359-85.

50.  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

51.  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

52. Adequate reduction Inadequate clearance Overpreparation

53.  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

54.  Lack of functional cusp bevel:
Thin area in casting Overcontouring Overinclination

55.  Thin walls of casting– subject to distortion
 Overcontouring- disastrous effect on the
periodontium

56.  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

57.  Chamfer
 Heavy chamfer
 Shoulder
 Sloped shoulder
 Radial shoulder
 Shoulder with a bevel
 Knife edge

58.  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

59.  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

60.  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

61.  Destruction of more tooth structure
Sharp 90° internal line angle
concentrates stress on tooth
Coronal fracture
 Not used for cast metal restorations

62.  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

63.  Modified shoulder
 Cavosurface 90°
 Shoulder width lessened with rounded internal angles
 Lesser stress concentration
 Good support for porcelain

64.  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)

65.  Bevel:
 allows the cast metal margin to be bent or
burnished against the prepared tooth structure
 minimizes the marginal discrepancy
 removes unsupported enamel

66.  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

67.  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 NAm 2004; 48: 359-85.

68.  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 NAm 2004; 48: 359-85.

69.  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 NAm 2004; 48: 359-85.

70. 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

71.  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

72.  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

73.  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

74.  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

75.  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)

76. 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

77.  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.

79.  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

80.  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.