l1 - principles of tooth preparation

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Principles of tooth preparation



Introduction

Teeth do not possess the regenerative ability found in most other tissues.

Teeth require preparation to receive restorations.

Careful attention to every detail is imperative during tooth preparation.



The principles of tooth preparation may be

divided into three broad categories:



BIOLOGIC CONSIDERATIONS

The adjacent teeth, soft tissues, and the pulp of the tooth being prepared

are easily damaged in tooth preparation

Adjacent teeth:

Iatrogenic damage to an adjacent tooth is a common error in dentistry.



A metal matrix band around the adjacent tooth for protection may behelpful; however, the thin band can be perforated and the underlying

enamel damaged.

The preferred method is to use the proximal enamel of the tooth that isbeing prepared for protection of the adjacent structures.

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Teeth are 1.5 to 2 mm wider at the contact area than at the cementoenameljunction..



Pulp

Pulp size, which can be evaluated on a radiograph, decreases with age. Up

about age 50, it decreases more so occlusocervically than faciolingually



Causes of injury to pulp:

Temperature

Chemical action

Bacterial action



Temperature

Considerable heat is generated by friction between a rotary instrument and t

surface being prepared.



Excessive pressure, higher rotational speeds, and the type, shape, and condition the cutting instrument may all increase generated heat.

With a high-speed handpiece, a feather light touch allows efficient removal tooth material with minimal heat generation

Even with the lightest touch, the tooth overheats unless a water spray is used.



The spray also removes debris-which is important because clogging reduces cuttingefficiency and prevents desiccation of the dentin.

Relying on air cooling with a high- speed handpiece is dangerous, because it caneasily overheat a tooth and damage the pulp.



Chemical action:

The chemical action of certain dental materials (restorative resins, solvents,and luting agents) can cause pulpal damage especially when they are applied to

freshly cut dentin.

Cavity varnish or dentin bonding agents form an effective barrier in most

instances.



Bacterial action:

Pulpal damage under restorations has been attributed to bacteria that either were

left behind or gained access to the dentin because of microleakage.

Many dentists now use an antimicrobial agent, such as chlorhexidine gluconate

disinfecting solution after tooth preparation.



One of the basic tenets of restorative dentistry is to conserve as much tooth

structure as possible while preparation design remains consistent with themechanical and esthetic principles of tooth preparation.

The thickness of remaining dentin has been shown to be inversely proportional

to the pulpal response, and tooth preparations extending in close proximity tothe pulp should be avoided



How to be conservative?

Tooth structure is conserved through adherence to the following guidelines:

1. Use of partial-coverage rather than complete-coverage restorations.



2. Preparation of teeth with the minimum practical convergence angle (taper)between axial walls



4. Preparation of the axial surfaces so that a maximal thickness of residual tooth

structure surrounding pulpal tissues is retained; if necessary, teeth should beorthodontically repositioned



5. Selection of a margin geometry that is conservative and yet compatible with theother principles of tooth preparation.



6. Avoidance of unnecessary apical extension of the preparation.

C id ti Aff ti F t D t l H lth



Considerations Affecting Future Dental Health

Insufficient axial reduction will result in overcontoured restorations that hamper

plaque control. This may cause periodontal disease or dental caries.

Under reduction Bulky crown Periodontal disease



Inadequate occlusal reduction may result in poor form and subsequent occlusal

dysfunction.

Poor choice of margin location, such as in the area of occlusal contact, may

cause chipping of enamel or cusp fracture.

Margin placement:



Margin placement:

Whenever possible, the margin of the preparation should be supragingival.

subgingival margins of cemented restorations have been identified as a majo

etiologic factor in periodontal disease.

Advantages of supragingival margins



Advantages of supragingival margins

include the following:

1. They can be easily finished without soft-tissue trauma.

2. They are more easily kept plaque free.

3. Impressions are more easily made.

4. Restorations can be easily evaluated.

5. They can be situated on hard enamel, whereas subgingival margins are often odentin or cementum.

A subgingival margin is acceptable if any of the



A subgingival margin is acceptable if any of the

following pertain:

1. Dental caries, cervical erosion, or restorations extend subgingivally,

2. The proximal contact area extends to the gingival crest.

3. Additional retention and/or resistance is needed.



4. The margin of a metal-ceramic crown is to be hidden behind the labiogingivalcrest.

5. Root sensitivity cannot be controlled by more conservative procedures.

6. Modification of the axial contour is indicated, such as to provide an undercut

to provide retention for the clasp of a partial removable dental prosthesis



Margin geometry



Margin geometry

For evaluation the following guidelines for margin design should be considered:

1. Ease of preparation without overextension or unsupported enamel.

2. Ease of identification in the impression and the die.

3. distinct border to which the wax pattern can be finished.

4. Sufficient bulk of material (to give the restoration strength and when porcelain is

used, esthetics).

5. Conservation of tooth structure (if the other criteria are met).

margin designs



margin designs

A: Feather edge.

B: Bevel.C: Chamfer.

D: Shoulder.E: Beveled shoulder.

margin designs



margin designs



A comprehensive 2001 literature review suggests that margin design selectionshould be based on the type of crown, applicable esthetic requirements, ease of

formation, and operator experience.

Occlusal considerations:



Occlusal considerations:

P i h f



Preventing tooth fracture:

a complete crown is often a better solution, because it offers the greatest

protection against tooth fracture, tending to "hold" the cusps of the tooth together.

l1 - principles of tooth preparation

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