Abutment & Its Selection In Fixed Partial Denture

List the ideal requirements of an abutment tooth and describe the steps taken to maintain biological integrity while preparing teeth for fixed partial denture.

Requirements of an Abutment Tooth

• Abutment teeth need to be strong enough to withstand the forces directed to the missing teeth in addition to those usually applied to the abutments
• Abutment teeth should not exhibit mobility
• An endodontically treated tooth can be considered if it is asymptomatic and if it can withstand the forces transmitted to it. The supporting tissues surrounding the abutment teeth should be healthy and free from inflammation.

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Factors to be considered in abutment selection

• Crown–root ratio
• Tooth configuration
• Periodontal surface area
• The length of the pontic span
• Forces
• Oral hygiene measures.

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Crown–root ratio

• An abutment teeth should have a combined pericemental area equal to or greater in pericemental area than the tooth or teeth to be replaced (Antes’ law)
• The optimum crown–root ratio is 2:3. A root ratio of 1:1 at least needs to be present for a prospective abutment
• If coronal structure is less, core build up or crown lengthening needs to be done to fulfil the crown–root ratio.

Tooth configuration

Root shape and angulation

A molar with divergent roots, single-rooted tooth with an elliptic cross section (broader labiolingually than mesiodistally), a long root or multiple roots provide better support than a root surface area with circular cross section.

Periodontal surface area

• Root surface area
  • Larger teeth will have greater surface area and will handle stress better.
• Bone support
  • Teeth with vertical and horizontal resorption give less support.
• The length of the pontic span
  • Two abutment teeth can support two pontics
  • Failures due to abnormal stress have been attributed to increased length of the span.

Abutment Tooth Forces

Varying the occlusal scheme by altering the occlusal table or plane can decrease the load on abutment teeth.

Abutment Tooth Oral hygiene

Teeth with slight mobility can be used as abutments if stabilised and if oral hygiene is maintained.

Principles of Tooth Preparation

• Biologic considerations.
• Mechanical considerations.
• Esthetic considerations.

Biologic considerations

(Are factors that affect the health of the oral tissues)

• Prevention of damage during tooth preparation to:
  • Adjacent teeth
  • Soft tissues
  • Pulp of the tooth being prepared.
• Conservation of tooth structure.
• Margin placement.
• Finish lines.
• Occlusal consideration.

Prevention of damage during tooth preparation to:

Adjacent teeth

If the proximal contact area is damaged during preparation it needs to be reshaped and polished otherwise it is susceptible to dental caries.

To prevent damage:

• A metal matrix band needs to be used around the adjacent tooth
• A thin tapered diamond is used to break the inter-proximal contact.

Soft tissues

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

Pulp

Extreme temperatures, chemical irritation can cause pulpal damage.

Prevention

• Assess morphology of the dental pulp chamber before preparation with the help of a radiograph
• Use new and perfect abrasives while reducing the tooth. This reduces the heat that is generated
• Apply gentle pressure while preparation of tooth
• Use copious amount of water spray directed at the area of contact between tooth and bur. This removes clogging and prevents desiccation of the dentin
• All retention grooves and polishing need to be done with a slow-speed handpiece with adequate amount of water spray
• Avoid use of chemical agents for cleaning.

Conservation of tooth structure:

• Use of partial veneer crowns instead of full veneer crowns
• Minimum taper between axial walls
• Occlusal surface reduction following anatomic planes
• Tilted tooth to be repositioned, so that less tooth structure is removed during preparation
• A conservative margin finish
• Supra gingival or crest of the gingival margin finish line.

Failures due to improper preparation of tooth:

• Insufficient axial reduction with an over contoured restoration can cause periodontal disease or dental caries
• Inadequate occlusal reduction can cause occlusal dysfunction and poor margin placement
• Excessive axial contours can cause gingival inflammation.

Margin placement Requirements

• Ease of preparation without overextension
• Easy to identify in the impression and on the die
• Easy to finish on wax pattern
• Sufficient bulk of material
• Preserve tooth structure.

Margin placement Types

• Supragingival.
• Subgingival.
• At the crest of the gingiva.

Supragingival margins

• They can be easily finished
• They are more easily kept clean
• Impressions are more easily made with less potential for soft tissue damage
• Restorations can be easily evaluated at recall appointments.

Subgingival margins

• Subgingival margins are often on dentin or cementum
• They are done when cervical erosion or restorations extend subgingivally and when a crown-lengthening procedure cannot be carried out
• A well-designed preparation has a margin that is smooth and will provide the patient with a longer-lasting restoration.

Finish lines Types

• Featheredge or shoulderless crown preparations: Conservative but not to be used.
• Chisel edge: Only on tilted tooth.
• Chamfer: All metal restorations.
• Shoulder: All ceramic restorations and not conservative.
• Sloped shoulder: Reduces possibility of leaving unsupported enamel and leaves sufficient bulk to allow thinning of the metal framework to a knife-edge for good esthetics.
• Shoulder with bevel: In subgingivally extended finish lines due to cervical erosion. A bevelled shoulder margin is used for the facial surface of a metal–ceramic restoration where a metal collar is to be used.

Advantages of a bevel

• Easy burnishing of the cast metal margin
• Decreases marginal discrepancy
• Protects unprepared tooth structure from chipping.

Occlusal considerations

If occlusion is disrupted by supra-erupted or tilted teeth either uprighting of tooth or a modified restoration should be considered after endodontic treatment.

Adequate occlusal clearance is required for an optimal occlusion.

Mechanical Considerations

• Providing retention form.
• Providing resistance form.
• Preventing deformation of the restoration.

Providing Retention form:

This is the quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of withdrawal.

Factors affecting retention:

• Magnitude of the dislodging forces
• Geometry of the tooth preparation
• Roughness of the fitting surface of the restoration
• Materials being cemented
• Type of luting agent
• Film thickness of the luting agent.

Magnitude of the dislodging forces

Great removal forces occur with sticky food. The magnitude of the dislodging forces depends on the stickiness of the food and the surface area and texture of the restoration being pulled.

Geometry of the tooth preparation

Fixed prostheses depend on the geometric form rather than on adhesion for retention as majority of the luting agents are non-adhesive like zinc phosphate.

Factors

• Taper
• Surface area
• Stress concentration
• Type of preparation.

Taper

• Maximum retention is obtained if a tooth preparation has parallel walls
• Smaller degrees of taper have more retention
• As the taper increases, however, so does the free movement of the restoration andretention will be reduced
• The recommended convergence between opposing walls is 6 degrees.

Surface area

Provided the restoration has a limited path of withdrawal, its retention is dependent on the length of this path in sliding contact. Therefore, crowns with long axial walls are more retentive than those with short axial walls.

Stress concentration

Round margins may reduce stress concentrations and hence, increase the retention of the restoration.

Type of preparation

Retention is increased by adding grooves and boxes to a preparation with a limited path of withdrawal.

Roughness of the surfaces

Retention is increased if the restoration is roughened or grooved by air-abrading the fitting surface with alumina.

Materials being cemented

• More reactive the alloy, more adhesion with certain luting agents
• Base-metal alloys are better retained than less reactive high-gold content metals
• Cement adheres better to amalgam than to composite resin or cast gold
• Crowns adhere better with composite resin than with amalgam cores.

Type of luting agent

Adhesive resin cements are the most retentive, with less bond during long-term.

Providing Resistance form

Lateral forces tend to displace the restoration by causing rotation around the gingival margin which is prevented by any areas of the tooth preparation that are placed in compression called resistance area.

Resistance depends on:

• Magnitude and direction of the dislodging forces
• Geometry of the tooth preparation: Increased taper and rounding of axial angles, short tooth preparations with large diameters reduce resistance
• Physical properties of the luting agent: Zinc phosphate cements have a higher modulus of elasticity than polycarboxylate cements, hence polycarboxylate cement depends more on the taper of the preparation than zinc phosphate cement.

Preventing deformation of the restoration

Restoration must have sufficient strength to prevent it from being permanently deformed during function.

Factors

• Alloy selection
• Adequate tooth reduction
• Margin design.

Alloys

Type 3 or Type 4 gold alloys. High-noble metal content metal–ceramic alloys have a hardness equivalent to Type 4 golds and nickel–chromium alloys are more hard and can be used in long-span fixed partial dentures (FPD).

Adequate tooth reduction

A minimum alloy thickness of about 1.5 mm over centric cusps (buccal in the mandible and lingual in the maxillae) and 1 mm in non-functional cusps. Occlusal reduction should follow the morphology of tooth.

Margin design

Depending on the type of restoration the ideal margin design needs to be followed.

Esthetic considerations

• Complete examination and assessment of teeth during smiling, talking, and laughing.
• Find the patient’s esthetic requirements.
• Type of restoration selected.

Partial-coverage restorations

Proximal margin

Place proximal margin just buccal to the maximal contact area where metal will be hidden by the distal line angle of the neighbouring tooth.

Facial margin

The facial margin of a maxillary partial-coverage restoration should be extended just beyond the occlusofacial line angle. A short bevel is needed to prevent enamel chipping. A chamfer can be placed where appearance is less important (e. g., on molars) because this will provide greater bulk of metal for strength.

Metal–ceramic restorations

Facial tooth reduction

A minimum reduction of 1.5 mm is required for sufficient bulk of porcelain and metal for strength.

Labial margin placement

Supragingival margin placements are easier to prepare properly and easier to keep clean.
After observing the patient’s smile, margins should be placed.
Margins should not be placed subgingivally encroaching on the attachment. If it extends within 1.5 mm of the alveolar crest bone resorption tends to occur.