PRINCIPLES OF TOOTH PREPARATION

Dr. Dipal MawaniPG

Contents Introduction Objectives of tooth preparation Principles of tooth preparation

• Biologic : Prevention of damage during tooth preparation Conservation of tooth structure Considerations affecting future dental health

• Mechanical : Retention form Resistance form Structural durability

• Esthetics : Metal ceramic Partial coverage restoration

Introduction What is tooth preparation?

Mechanical treatment of dental disease or injury to hard tissues that restore a tooth to original form

The process of removal of diseased and/or healthy enamel and dentin and cementum to shape a tooth to receive a restoration

OBJECTIVES OF TOOTH PREPARATION

Reduction of a tooth in miniature to provide retainer support.

Preservation of healthy tooth to secure resistance form.

Provision for acceptable finish lines. Performing pragmatic axial tooth reduction

to encourage favourable tissue responses from artificial crown contours.

Principles Of Tooth Preparation• Biologic :

• Mechanical :

• Esthetics :

Prevention of damage during tooth preparationConservation of tooth structureConsiderations affecting future dental health

Retention formResistance formStructural durability

Metal ceramicPartial coverage restoration

Biologic Considerations Prevention of damage during tooth

preparation

1. Adjacent Teeth-damaged tooth even if reshaped and recontoured is always more susceptible to caries

Less fluoride content More plaque retention

• AVOIDED BY:• Placing a matrix band interproximally• leaving thin lip/ fin of enamel intact

Interproximally.

2.Soft Tissues :Aspirator tip , mouth mirror.

3.Pulp

Extreme TemperatureChemical IrritationMicro-organisms AVOIDED BY:Using correct technique of tooth preparation

Selecting correct materials (cements)

Pre-op Radiographs

Irreversible Pulpitis

Causes of injury :• Temperature : Friction – heat

• Zach &Cohen – rise of 5.5 o C - 15% necrosis rise of 11.1o C - 60% necrosis rise of 16.6o C - 100% necrosis

AVOIDED BY: Feather edge touch Water –air spray coolant Slow-speed handpiece

Special Care While Preparing Grooves/ Pinholes.

• Chemical Action -Bases, Restorative Resins, Solvents, and Luting

Agents.

AVOIDED BY: Cavity Varnish / Dentin Bonding Agents

• Bacterial Action -Due to bacteria left behind after preparation or

having gained access to dentin due to microleakage.

AVOIDED BY: Removal of all carious dentin before tooth preparation.

Conservation Of Tooth Structure

Partial coverage.

Minimum convergence angle.

Anatomic reduction

Axial surface reduction. Apical extension.

Conservative margin

Considerations Affecting Future Dental Health

CAUSE

Insufficient Tooth Reduction

Inadequate Occlusal Reduction

Poor Margin Location

EFFECT

Overcontoured Restoration

Occlusal Dysfunction

Chipping of Enamel

Axial Reduction   

-Sufficient space for Good axial contours.

-Must Duplicate the contours and profile of the original tooth.

Margin geometry • Preparation without unsupported enamel• Ease of identification• Distinct boundary • Bulk of material • Conservation of tooth

Title

Margin Placement:

-A Supra-gingival Margin should be preferred over a Sub-gingival Margin.

Advantages of a Supra-gingival Margin are: Easily finished Easily cleansible Ease of impressions Easy Evaluation No trauma to the soft tissues.

Indications of Subgingival preparation • Dental caries , cervical erosion• Contact areas extended to gingival crest• Retention• Margin has to be hidden• Root sensitivity

A

C B

Margin Adaptation – • Recurrent caries – dissolution of cement• Accurate adaptation• Irregular or stepped junctions

BD

BIOLOGIC WIDTH

The distance from the epithelial attachment to the crest of the alveolar bone is called as the “biologic width.” It is normally about 2.04mm WIDE, INCLUDING THE EPITHELIAL ATTACHMENT ( 0.97mm) AND THE CONNECTIVE TISSUE ATTACHMENT (1.07mm).

When the margin of a restoration intrudes into the biologic width, inflammatory and osteoclastic activity are stimulated.

Bone resorption will continue until the alveolar crest is at least 2.0mm FROM THE RESTORATION MARGIN. The best outcome that can be expected is that the epithelial and connective tissue attachments will reestablish themselves at a more apical level. Continued inflammation with pocket formation is likely.

Preventing Tooth Fracture – • Minimize - destructive stresses – inlay -

wedge - opposing walls • Providing a cuspal coverage restoration -

complete crown.

Mechanical Considerations Retention Form

Resistance Form

Structural Durability

Title

Retention : A+B

Resistance: B+C+D

Retention Form :

Magnitude of the dislodging forces Geometry of the tooth preparationRoughness of the fitting surface of the restorationMaterials being cementedFilm thickness of the luting agent

Magnitude of the dislodging forcesDepends on the stickiness of food

Geometry of the tooth preparation – prostheses depend on the geometric form rather than - cements Cement is effective only if the restoration has a single path of withdrawal

Closed lower pair of kinematic elements - Formed by two cylindrical surfaces - curve of a complete crown - closed - grooves - partial crown - prevent movement at right angles - complete crown - over tapered - no longer be cylindrical,

Taper - Theoretically, maximum retention - parallel walls – undercutsTaper small – limited path of withdrawalWard - first to recommend taper of 3 to 12 ° Jorgensen and Kaufman - 2.5.to 6.5 ° - optimum

Freedom of Displacement. Maximum retention - when there is only one path ….long parallel axial wallsDefinite wall perpendicular to the direction of the force ….Proximal box - Buccal and lingual wall…meet the pulpal wall …near 90°

LengthMore surface area - more retentive Length - enough to interfere with the arc of the casting The shorter the wall - more important its inclination. The shorter walls - little taper

Path of insertion Imaginary line along which the restoration will be placed.Survey a preparation …..12 inches ….preparation to be surveyed in the mouth - ½ inch above the preparation

For metal ceramic crowns, the path - parallel - long axis of the teeth.

Partial Coverage Restorations

• Roughness of the Surfaces Being Cemented • If Internal surface of a restoration is very smooth-

retentive failure occurs at the cement restoration interface

• Restoration is roughened or grooved.• Air-abrasion - with 50 µm of alumina – 64% retention

Materials Being Cemented• More reactive the alloy is, the more adhesion • …Base metal alloys are better retained

Type Of Luting Agent

Adhesive resin cements are the most adhesive followed by cements which bond with the tooth and zinc phosphate cement.

Adhesive resin cements-long term deterioration-resin dentin interface-NANOLEAKAGE

Resistance form:

Depends on• Magnitude and direction of the dislodging

forces• Geometry of the tooth preparation • Physical properties of the luting agent

Magnitude and Direction of the Dislodging Forces.

Properly designed occlusion, the load should be well distributed …

Geometry of the Tooth Preparation

Concept of resistance area(RA)

Length Width

Hegdahl and Silness - Increased preparation taper and rounding of axial angles tend to reduce resistance Molar teeth require more parallel preparation

Partial-coverageResistance must be provided by grooves , boxes or pinholes

Tylman suggests - proximal grooves just buccal to the junction of the buccal and middle third of the proximal.

Johnston and associates and Vale - the groove as buccal as possible but still within the original contact area.

Bassett and associates, Jones, Baum, and Shillinburg - groove placement as buccal as possible.

Anthony H.L. Tjan, Gary D. Miller… J.P.D. 1981 discussed about two groove flare designs referred to as Type I and Type II.

Type I groove-flare : Fishhook design Encroachment on pulp

Type II groove-flare : placed into the dentin paralleling a line tangent

Biologic factors which influence the choice of groove location

size and location of the pulp chamberthickness of the enameldirection of the enamel rodsalignment of the tooth involved physical properties of the dental structure.

Maximum length - 0.5 mm short of the gingival margin.

Bowley and Lai ......Both grooves and boxes provided significant improvement of total surface area for both the 3- and 4-mm vertical preparation heights.

Cambagni , Bernal, Goodacre and Kim .....The most effective method of enhancing resistance form in a tooth preparation that lacks resistance is to decrease the total occlusal convergence of the cervical portion of the prepared axial walls.

Physical Properties Of Luting Agents

Compressive strength of re-inforced ZnOE is halved at mouth temperature.

Zinc Phosphate has high modulus of elasticity so retention depends less on taper when compared to Zn Polycarboxylate

Adhesive resin>Resin>Glass Ionomer>Zinc Phosphate>Polycarboxylate

Structural durability

A restoration must contain a bulk of material that is adequate to with stand the forces of occlusion. This bulk must be confined to the space created by the tooth preparation.

Structural durabilityOcclusal reduction.Functional cusp

bevel.Axial reduction.

Occlusal reduction One of the most important feature for providing adequate bulk of metal and strength to the restoration is occlusal clearance.

Occlusal reduction Functional

cusp Non

functional cusp

All metal 1.5 1.0

Metal ceramic

1.5-2.0 1.0-1.5

All ceramic 2.0 2.0

FUNCTIONAL CUSP AND NON FUNCTIONAL CUSP

AXIAL REDUCTION Plays an important role in securing space for an adequate thickness of the restorative material.

Esthetic Considerations Metal-ceramic restorations :

Facial tooth reduction : Adequate reduction of the facial surface –

Color depth and translucency. Minimum reduction of 1.5 mm

Incisal Reduction incisal edge – no metal backing - translucency –

2mm reduction.

Proximal Reduction extent is contingent on the location of

metal-ceramic junction.Proximal surface with no metal backing at

the incisal edge – looks most natural

Low lip line

Collarless Metal collar

Patient’s smile - initial examination

High lip line

Margins - not to be placed so far apically - encroach on the attachment.

Supragingival margin - Easier to keep clean Subgingival margins - Indicated for esthetic reasons - when the patient has a high lip line

Partial-coverage restorations :

Proximal margin : …place the margin just buccal to proximal contact area - metal - hidden by the distal line angle.

Tooth preparation angulation

Facial margin :

Just beyond the occlusofacial line angle. A short bevel is needed to prevent enamel chipping If buccal margin - correctly shaped, no reflection of light Mandibular partial cast crowns - metal display is unavoidable A chamfer, rather than a bevel, is recommended for the buccal margin

Current Concepts Tooth preparations for complete

crowns: An art form based on scientific principles

Charles Goodacre

1. The total occlusal convergence, ideally should range between 10 and 20 degrees.

2. 3 mm should be the minimal occlusocervical /incisocervical dimension of incisors and premolars prepared within the recommended 10 to 20 degrees of total occlusal convergence.

3. The minimal occlusocervical dimension of molars should be 4 mm when prepared with 10 to 20 degrees total occlusal convergence.

4. The ratio of the occlusocervical/incisocervical dimension of a prepared tooth to the faciolingual dimension should be at least 0.4 or higher for all teeth.

5. Whenever possible, teeth should be prepared so that the facioproximal and linguoproximal corners are preserved.

6. Teeth without natural circumferential morphology after tooth preparation (round teeth) or teeth that lack adequate resistance form should be modified with the creation of grooves/boxes.

7. Many molars need auxiliary grooves or boxes to enhance resistance form because of their short occlusocervical dimensions and the unfavorable ratio of the occlusocervical dimensions to the faciolingual dimensions.

8. Axial grooves/boxes should be used routinely when mandibular molars are prepared for fixed partial dentures, and they should be located on the proximal surfaces.

9. When tooth conditions and esthetics permit, finish lines should be located supragingivally.

10. When subgingival finish lines are required, they should not be extended to the epithelial attachment.

11. Chamfer finish lines approximately 0.3 mm deep are well suited for all-metal crowns.

12. Both shoulder and chamfer finish lines can be used with all-ceramic crowns if the crowns are bonded to the prepared teeth. Depths greater than 1 mm are not required when a semitranslucent type of allceramic crown is used.

13. Axial and occlusal reductions for all-metal crowns should be at least 0.5 mm deep and 1.0 mm deep, respectively.

For metal-ceramic crowns, Facial /axial reductions in excess of 1 mm can compromise the remaining tooth structure external to the pulp, whereas 2.0 mm of occlusal reduction is commonly achievable even on a young tooth.

With all-ceramic crowns, it is not necessary to exceed 1 mm of axial reduction with semitranslucent systems and higher value, lower chroma shades.

2 mm incisal/occlusal reduction for allceramic crowns.

14. Line angles should be rounded on all-ceramic tooth preparations to reduce stress in the definitive restoration.

With crowns that use metal, the primary purpose of line angle rounding is to facilitate pouring impressions and investing wax patterns without trapping air bubbles and to facilitate removing casting modules.

15. Smooth tooth preparation appears to enhance the fit of restorations. Surface roughness generally increases retention with zinc phosphate cement, but its effect with adhesive cements (polycarboxylate, glass ionomer, resin) has not been as definitely determined.

CONCLUSIONSuccessful restoration:

Accurate diagnosis

Thoughtful Rx planning

Preparation design

On tooth preparation dependsPulp vitalityPeriodontal healthGood estheticsProper occlusionProtection of remaining tooth Longevity of the restoration

Contemporary fixed prosthodontics ; Rosensteil, Lang,Fujimoto;3rd ed.

Fundamental of fixed prosthodontics ; Shillingburg et al,3rded.

Fundamental of Tooth preparation ,Shillingburg

Modern practice of Fixed prosthodontics – Johnston , 4th ed.

Theory & practice of fixed prosthodontics – Tylman

J.P.D. 1965 ; 15 : 129

J.P.D. 1976 ; 35 :538

J.P.D. 1979 oct.;42(4) : 405 – 10

J.P.D. 1981 ; 45 : 138

References

J.P.D. 1987 ; 57 : 411 J.P.D. 1989 ; 62 : 264 J.PD. 1991 ; 65 : 56 J.P.D. 1996 ; 75 : 129J.P.D. 1998 ; 79 : 671 DCNA 2004 Apr.; 48(2) : 387-97

J.P.D 2001;85:363-76.

J.P.D 2009;101:7-12.

J.P.D 2003;89:565-71

J.P.D 2004;91:33-41

J.P.D 2007;98:436-444

Thank You !