GOOD MORNING

PRINCIPLES OF TOOTH PREPARATION

PRAGY MALLIKROLL:59

BDS 2ND BATCH

CONTENTS

INTRODUCTION PRICIPLES

INTRODUCTION

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

GPT-8 Tooth preparation may be defined as the mechanical treatment of dental disease

or injury to hard tissue that restores a tooth to the original form. (Tylman) The mechanical preparation or the chemical treatment of the remaining tooth

structure, which enables it to accommodate a restorative material without incurring mechanical or biological failure. (Marzouk)

Principles

Preservation of tooth structure Retention and resistance Structural durability Marginal integrity Preservation of periodontium

By Shillingburg

According to Rosenstiel

Biologic principles Methods of prevention A metal matrix band placed around the adjacent tooth.

Damage to adjacent teeth is prevented by positioning the diamond so a thin lip of enamel is retained between the bur and the adjacent tooth.

Note that the orientation of the diamond parallels the long axis.

The enamel should be maintained mesial to the path of the diamond as the reduction progresses.

Soft tissues

Damage to the soft tissues of the tongue and cheeks can be prevented by careful retraction with an aspirator tip, mouth mirror , or flanged saliva ejector.

Great care is needed to protect the tongue when the lingual surfaces of mandibular molars are being prepared.

Pulp

Temperature Chemical action : base, luting agent Bacterial action: due to microleakage

Preservation of tooth structureCan be done by:• Using partial coverage than complete coverage

restoration when possible• Preparation of teeth with minimum practical taper.• Preparation of occlusal surface following the

anatomic planes.• Axial surfaces: if necessary, teeth should be

orthodontically repositioned.

Margin Integrity

Margin – The outer edge of a crown , inlay, onlay or other restoration.

GPT-8

The restoration can survive in the biological environment of the oral cavity only if the margins are closely adapted to the cavosurface finish line of the preparation.

The configuration of the preparation finish line dictates the shape and bulk of restorative material in the margin of the restoration It also can affect both marginal adaptation and the degree of seating of the restoration

Supragingival Margin At or above the gingival crest.

Margin should be placed supragingivally whenever possible.

ADVANTAGES- Place on enamel

- Easy to prepare without trauma to soft tissues.

- Can be easily finished

- Impression making is also less traumatic to soft issues & easy to reproduce.

- Fit of the restorations can be easily evaluated.

- Can be easily maintained by patient

Subgingival Margins

Major aetiologic factor in peridontitis.

Deeper the restorative margin is in the sulcus ,greater is the inflammatory response.

Place at least 2mm above the alveolar crest so that the biologic width is not encroached.

Indications Caries ,erosion or restorations extending subgingivally.

Aesthetics – when metal ceramic restorations are used.

Additional retention – in case of short crowns

Root sensitivity.

Modification of axial contour.

Proximal contact extending to gingival crest.

Finish Line

Definition: The point at which a preparation terminates on the

tooth is called the finish line. It is also defined as the peripheral extension of a tooth preparation (GPT-8).

The finishline serves the following functions:

During visual evaluation of the tooth preparation,it is a measure of the amount of tooth structure already removed.It also delineates the extent of the cut in an apical direction.The more distinctitis,the better it serves these purposes.

The finish line is one of the features that can be used to evaluate the accuracy of the impression made for indirect procedures.

In the die,a distinct finish line helps to evaluate the quality of the die and helps in accurate die trimming.

Finish line configuration Chamfer Finish Line

Shoulder

Shoulder With Bevel

Radial Shoulder

Slope Shoulder

Knife Edge

Chamfer Finish Line

It is an obtuse angled finish line. It is distinct. Exhibits less stress. Most conservative. Adequate seal Minimal tooth reduction Can be done with round ended diamond or torpedo diamond burs.

Indications- Cast metal crowns Metal-only portion of PFM crowns.

Contraindications Should not be indicated for porcelain restorations as the obtuse

angle produces shearing forces which is not tolerated by porcelain

Shoulder • Finish line is perpendicular to axial surfaces of teeth• All-ceramic crowns• Facial margin of PFM crowns where esthetics is more important.

Wide ledge- • resistance to occlusal forces• minimizes stresses which leads to fracture of porcelain

Flat-end tapered bur is used Less conservative Healthy contours Maximum esthetics

Radial shoulder Modified shoulder

Cavosurface 90°

Shoulder width lessened with rounded internal angles

Lesser stress concentration

Good support for porcelain Done with flat end tapered diamond and end cutting parallel sided carbide

finishing bur Finishing is done with modified special bin-angle chisel.

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

Shoulder with a bevel Similar to shoulder but external bevel is created on the

gingival margin of facial line. 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)

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• Full metal crown in young patient.

Marginal adaptation

The junction/space between a cemented restoration & tooth is always a potential site for recurrent caries or periodontal disease because of dissolution of luting agent & inherent roughness.

Hence ,preparing a smooth & even margin is the beginning of various steps tissue displacement,impression making ,die formation ,waxing finishing,casting ,involved in making a restoration fit better with least space.

Clinically acceptable marginal gap is 10 microns for cast metal & up to 50 microns for ceramic restorations. The discrepency in adaptation can have a horizontal & vertical component.

Retention form

Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation. Primary

Sleeve Wedge

Secondary Pin, box and grooves

Resistance form

Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces.

Factors influencing resistance

1. Dislogding forces

2. Taper

3. Diameter of tooth

4. Height

5. Type of preparation

6. Luting agent

Structural durability Restoration must contain bulk of material that is

adequate to withstand the forces of occlusion. This bulk must be confined to the space created by

the tooth preparation.

1) Occlusal reduction Provides adequate bulk of metal Should reproduce the Basic Inclined Plane pattern of

occlusal surface – planar reduction

functional cusp Non functional cusp

Gold alloys 1.5 mm 1 mm

Metal ceramic

1.5 to 2 mm 1 to 1.5 mm

All ceramic 2 mm 2 mm

Inadequate reduction makes restoration weaker, provides no space for functional morphology and the perforation may occur while finishing procedure.

A flat occlusal surface may over shorten the preparation.

2) Functional cusp bevelProvides space for an adequate bulk of metal in an

area of heavy occlusal contact.

Lack of functional cusp bevel may produce several problems :

A. Thin area or perforation. B. Over inclination of the buccal surface will destroy

excessive tooth structure reducing retention.C. Over contouring and poor occlusion.

3) Axial reduction securing space for an adequate thickness of the

restorative material. Inadequate reduction : Distortion, over contouring

the axial surface and consequently periodontal problems.