Restoration of mutilated

teeth

BY

Dr. Manal Rafei Hassan

Associate prof.of fixed prosthodontics , Minia University

Indirect restorations are frequently placed on teeth which have lost substantial amounts of tooth structure.

Retention and resistance form are lost as the height of the tooth preparation is reduced in relation to the intended occlusal surface position of the final restoration.

A foundation or core build-up restoration may be required to supplement retention and resistance form.

A foundation restoration, or core, is used to build a damaged tooth to ideal anatomic form before it is prepared for a crown.

The strength required of a foundation restoration will vary, depending on the location of the tooth in the dental arch, as well as on the design of the surrounding tooth preparation.

A core build-up restoration must withstand crown preparation and impression taking and contribute to the retention and support of a provisional crown before the definitive crown restoration is placed.

When retention and resistance depend significantly on the core build-up, the strength of the foundation restoration and its retention to the underlying tooth structure will directly influence the survival of the restoration;

o Posterior teeth are exposed to greater forces than anterior teeth, and the direction of load differs.

o Abutments for fixed or removable prostheses are subject to increased stresses.

The exact nature of any particular core will depend on the degree to which the tooth in question is broken down and how much coronal dentine remains

Structural core space filling core

Space filling core:

When much coronal dentine remains, the role of a core is simply to fill out any undercuts and give an appropriate shape that will provide adequate retentive and resistance form.

The functional demands and stresses encountered by the core material will be minimal.

The mechanical properties of the core material are not critical, and the material of choice may be determined by other factors such as; ability to bond to tooth structure, cariostatic properties and ease of handling.

Structural core:

When a large amount of coronal dentine has been lost, the core material will replace a substantial part of the clinical crown, and will form the bulk of the final preparation.

The core material will be subjected to significant functional demands and stresses, particularly in molar teeth, and must therefore have adequate mechanical properties and sufficient retentive features.

Desirable properties for a core material:

Compressive strength to resist intraoral forces.

Flexural strength to prevent core dislodgement during function.

Biocompatibility with surrounding tissues.

Ease of manipulation.

Ability to bond to tooth structure, pins and posts

Capacity for bonding with luting cement.

Coefficient of thermal expansion similar to dentine.

Dimensional stability.

Minimal water sorption.

Short setting time to allow core placement and tooth preparation to be carried out during the same visit.

No adverse reaction with temporary crown materials or luting cements.

Cariostatic potential.

Low cost.

Contrasting colour to tooth tissue unless being used for anterior cores.

Choice of core material:

The choice of core material depends on several clinical variables.

The role of core material with regard to space filling or functional role is critical.

Accordingly, the degree to which the core will be subjected to stress, and the amount of bracing provided by remaining coronal dentine should be considered.

Amalgam alloy should not be used beneath anterior full-veneer crown restoration as corrosion products may stain the dentine peripheral to the restoration.

Core build-up materials for direct placement include:Amalgam:

Amalgam has adequate mechanical properties for many core build-up situations. It may have the best record when used for substantial posterior cores.

It has good contrast with tooth substance and is easy to prepare.

It is radio-opaque, has good resistance to microleakage and can be shaped to ideal restoration form.

However: It has high thermal conductivity and

coefficient of thermal expansion. Not adhesive to tooth structure,

although methods of bonding amalgam using resin adhesives are available so retention can be provided by undercuts, pins or slots.

Conventional amalgams set too slowly to allow tooth preparation during the same visit. Modern fast-setting spherical alloys may allow preparation 20-30 min after placement.

Amalgam requires an absolutely rigid matrix for proper condensation.

Resin composite:

Composite core materials are becoming increasingly popular for core build-ups.

Provided adequate moisture control is obtained, these materials may be reliably bonded to tooth structure.

Immediate tooth preparation is possible.

An incremental technique is required to ensure complete polymerization.

Some resin composite core materials possess similar compressive and tensile strengths to amalgam cores.

It does not require condensation and formulations are available that release fluoride.

Disadvantages:

The high coefficient of thermal expansion of composite cores, and their greater potential for water uptake.

Eugenol-based temporary cements may soften their surface or impede bonding of resin-based luting cements.

Conventional tooth colored composite is not recommended as a foundation material. Special colored core materials should be used.

Glass ionomers:

Traditional glass ionomer cements are only suitable for use as a space-filling core.

Several glass-ionomer materials are marketed specifically for use as a core build-up material.

Glass ionomers :

o Bond to dentine

o Release fluoride

o Have comparable thermal behaviour to dentine.

o Can be made a contrasting colour to tooth

o Easy to prepare.

Disadvantages:

They are relatively slow setting.

Their early resistance to moisture is poor.

Many products are not radio-opaque.

Flexural strength and fracture toughness are low.

The long term behaviour of these materials is not well documented.

Resin ionomers and compomers:

These materials are considered to offer the advantages of both glass ionomers and resin composites.

They offer improved flexural strength in comparison to glass ionomers.

The high coefficient of thermal expansion of composite has been reduced.

Light activation offers speed of set.

However:

Glass ionomers, resin-reinforced glass ionomers and most compomers are significantly weaker than tooth structure. They should be limited to situations where only minimal tooth structure is missing.

Preoperative Assessment:Restorability of tooth:

The extent of caries and existing restorations should be assessed.

Any restorations remaining in proximal boxes, class V areas and other regions should be removed, unless recently placed by the operator.

If more than 50% ---- bonded compomer

resin ionomer base

If less than 50% ---- high strength core

(amalgam or composite)

Mechanical retention may be increased by:

o Grooves

o Boxes

o Dovetails

o Converting sloping surfaces into vertical and horizontal components.

o Reducing and covering undermined cusps.

o The use of pins should only be considered as a last resort as they will further weaken already compromised teeth.

Pulpal/endodontic status:

Prior to core build-up, an assessment should be made of the pulpal status of the tooth in question.

If the pulp is exposed, or there are signs of irreversible pulpitis, endodontic treatment should be performed.

In case of an endodontically treated tooth, the quality of the treatment should be assessed radiographically.

Periodontal/occlusal assessment:

The periodontal status of the abutment should be assessed.

If there is inadequate sound tooth structure apical to the preparation margin for satisfactory retention and resistance form, surgical crown lengthening may be considered.

Greater strength is required for crowns in the following cases:

o Bruxism or clenching habits.

o Teeth that support crowns with heavy canine or incisal guidance.

o Abutments for fixed and removable prostheses.

Limited resistance and retention:

When a preparation has limited retentive and resistance form, there are a number of steps that may be taken to reduce the potential loss of the restoration.

These include limiting influence of lateral forces with:

Additional preparation features, increasing crown height, and utilizing adhesives.

Additional preparation features:Grooves:

Placing a groove in the axial wall of a preparation will reduce the moment arm of the force that acts to dislodge a restoration about a point of rotation.

Multiple grooves will significantly limit the path of withdrawal, and increase both resistance and retention of the final restoration.

Pin retention for core foundations:

Pins may be useful for providing retention to the core, although they should be used carefully.

Incorrect placement can lead to pulpal or root surface perforation. Failure of the pin to seat fully may lead to looseness or fracture of the pin.

Guidelines for the use of pins:

o Use one pin per missing cusp or marginal ridge, up to a maximum of 4.

o Use large diameter pins whenever possible

o Use the minimum number of pins compatible with adequate retention.

o Pins should extend 2 mm into dentine and restorative material.

o Keep 1 mm of dentine between the pin and enamel-dentine junction.

o Pins should be placed away from furcation areas and parallel to the external tooth surface.

o Coating of pins with adhesion promoters such as panavia and 4-META materials improves fracture resistance of composite and amalgam cores.

Increasing crown height:

By placing the margins more gingivally, or even subgingivally. This may predispose to periodontal problems.

Margins should ideally be no more than 0.5 mm subgingivally in order to keep clear of the attachment complex.

When there is a need to increase the clinical crown height, then surgical crown lengthening may be considered.

Adhesively retained restorations:

Advances in adhesive techniques have added another mode of treatment in addition to more conventional techniques.

Resin-based composite materials may be reliably bonded with similar resin-based luting cements, to ceramic restorations following etching and silane treatment of the ceramic fitting surface.

Base metals may be bonded to tooth structure with bi-functional polymeric resins.

Gold may be bonded to tooth structure following heat-treatment.

Adhesively bonded restorations may permit preservation of tooth structure when compared with more destructive traditional options.

Clinical experience and laboratory studies have shown that extension of the crown at least 2 mm apical to the junction of the core with the remaining tooth structure provides extracoronal bracing and prevents fracture of tooth structure. This is described as the ferrule effect

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