Cantilever Fixed Partial Denture

Cantilever Fixed Partial Denture

Pier Abutment

Pier Abutment Synonyms: Intermediary abutment/Intermediate abutment.

Pier Abutment Definition: Pier abutment is a natural tooth located between terminal abutments that serve to support a fixed or removable prosthesis.

Disadvantages of placing a rigid connector in pier abutment

Movements in tooth

• A tooth moves buccolingually, faciolingually and intrudes to a minor extent (buccolingual movement from 56μ to 108μ and intrusion is 28μ)

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• Anterior tooth moves more in a faciolingual direction and posterior tooth moves more in buccolingual direction
• These movements increase stresses transmitted onto the abutments.

Fulcrum-like effect
In pier abutments with rigid connectors, the independent direction and magnitude of movements of the abutment teeth cause a fulcrum-like effect on pier abutment.

Weakening of terminal retainers
The forces transmitted to the terminal retainers due to the pier abutment’s fulcrum-like effect can cause failure to the weaker retainer.

Microleakage and caries
The loose retainer will have leakage about the margin and can cause extensive caries leading to failure of abutment tooth.

This commonly occurs with the small anterior retainer than the posterior retainer.

Ideal design for pier abutment

To protect pier abutment, non-rigid connectors are designed.

Pier Abutment Indications

• In short span bridges
• In single tooth replacements.

Contraindications of non-rigid connector

• When pier abutment is mobile (then rigid connector is to be used)
• When posterior abutment opposes an edentulous space (supraeruption of abutment tooth and displacement of key can occur).

Advantages of non-rigid connector

• The movement in the connector prevents the middle abutment from acting as a fulcrum in buccolingual or occlusocervical direction
• Less stress to the terminal abutments
• Long life of prosthesis as chances of abutment failure is less.

Design of non-rigid connector

The non-rigid connector is a broken-stress mechanical union of retainer and pontic instead of the usual rigid solder joint.

Components

• Non-rigid connector consists of a key and keyway
• A T-shaped key that is attached to the pontic
• A dovetail key-way placed within a retainer
• Male component or the key is attached to mesial surface of the mesial pontic of distal edentulous spaces.

Location of non-rigid connector

• It must be placed on the middle abutment
• The keyway of the connector is placed in the distal side of the middle abutment and the key is placed on the mesial side of the first molar pontic.

Keyway

• When occlusal forces are applied to posterior tooth it leans mesially as the long axis of posterior tooth slants mesially hence, placement of the keyway in the distal side of the pier abutment seats the key into the keyway during mesial movement
• If keyway is placed on mesial aspect it gets unseated during mesial movements leading to mobility in abutments in the long run.

Examples of pier abutment design

Using non-rigid connector

Missing
Maxillary central incisor and opposite side lateral incisor.

Abutments
Lateral incisor, central incisor (pier) and canine.

Retainers

• Pin-retained partial veneer crown on lateral incisor
• Three-quarter crowns on the central incisor and canine.

Pontics
Porcelain fused to metal.

Design
A keyway is placed in the distal of the central incisor retainer to accommodate a key on the mesial of the lateral incisor pontic.

Using rigid connector

Missing
Mandibular central incisor and opposite side lateral incisor.

Abutments
Lateral incisor, central incisor (pier), and canine.

Retainers

• Pin-retained partial veneer crown on lateral incisor
• Three-quarter crowns on central incisor and canine.

Pontics
Porcelain fused to metal.

Design

• A completely rigid bridge is used in this situation because of short span length and small teeth
• If bone support is poor, the central incisor is extracted and rigid connector bridge placed from canine to canine.

 Causes of Abutment Failure

Failures in abutment can occur at different stages, from case selection to postinsertion problems.

Failures at different stages

• Case selection
• Mouth preparation
• Tooth preparation
• Inaccurate impression
• Temporary restorations
• Errors in wax pattern and laboratory procedures
• During insertion
• Postinsertion failures.

Case selection

• Failures occur when the patient has unrealistic expectations of the esthetics, function and life expectancy of prosthesis.
• Inadequate medical and dental history.
• Inadequate radiographic and intraoral examinations.
• Wrong selection of abutment tooth, for example selection of malaligned, mobile tooth as abutment.
• Patients with temporomandibular joint problems.
• Improper ridge contours—Bulbous ridge, healing ridge or excessively resorbed ridge can hamper esthetic outcome of prosthesis.
• Patients with poor oral hygiene.

Mouth preparation

Soft tissue preparation

• Inadequate periodontal procedures.
• Failure to correct high and attached frenums.
• Failure to restore the health of the prospective abutment tooth by adequate mouth preparations.

Hard tissue preparation

• If present, overhanging restorations, supraerupted tooth, mesially drifted tooth occupying pontic space, un-restored caries tooth, all need to be corrected before tooth preparation for fixed partial denture.
• Extensively damaged tooth need to be endodontically treated.

Tooth preparation

• Excessive tooth reduction.
• Improper path of removal and placement (lack of parallelism).
• Over reduction leading to decreased height of abutment tooth decreases resistance form.
• Inaccurate, irregular finish lines—Poor marginal adaptation.
• Reduced labial reduction—Lack of bulk for porcelain material leads to failure of porcelain or an overcontoured restoration leads to periodontal problems.
• Inadequate cooling of tooth during preparation leads to loss of vitality of tooth.
• If anatomical morphology of tooth preparation is not followed during occlusal preparation, inaccurate finish lines, over reduction with more taper leads to decreased retention and resistance of the prosthesis leading to failure.

Inaccurate impression
Inaccurate, distorted final impressions can lead to distorted dies that form inaccurate wax patterns leading to failure of prosthesis.

Temporary restorations

• Over extension of temporary restoration below gingival margin can cause recession of gingiva.
• Lack of occlusion in temporary restoration can lead to supraeruption of opposing tooth.

Errors in wax pattern and laboratory procedures

• Materials used for dies and wax patterns should be dimensionally stable.
• Technique for wax pattern fabrication and marginal adaptation should be correct.
• Improper spruing, weak investment materials, incomplete wax burn out with wax residues, reused alloys, alloys that have not flown completely into the mould due to faulty machine setting all lead to failure of prosthesis.

During insertion

• Restoration with poor marginal adaptation.
• Lack of proximal contact can lead to food impaction.
• Lack of proper occlusion.

Postinsertion failures

• Recurrent caries or secondary caries under the retainers.
• Failure of luting agent – consistency of luting agent while luting is significant.
• Over reduced tooth can fracture leading to failure.
• Fracture of facings.
• Soldered connector joints that fracture.
• Poor pontic design can cause soft tissue response, such as inflammation.
• Periodontal involvement of tooth due to bad oral hygiene measures.

Mesial Half Crown

Mesial Half Crown Synonym

One half crown

Mesial Half Crown Definition

Mesial half crown restores the occlusal and mesial surfaces as well as portions of the facial and lingual surfaces.

Mesial Half Crown Indication

In mesially tilted molars (second mandibular tilted into the space of first mandibular).

Mesial Half Crown Contraindications

• In caries or decalcified tooth extending distally
• In caries prone mouth
• In poor oral hygiene maintenance
• If there is severe marginal ridge height difference between the distal of the second molar and the mesial of the third molar
• The molar tooth should not converge more than 25°–30°.

Mesial Half Crown Preparation

• A mesial half crown is a three-quarter crown rotated 90° with the distal surface uncovered
• As the second molar is generally over-tapered it needs additional grooves on buccal and lingual aspect.

Steps in preparation

• The path of insertion is established and many lines are scribed on base of cast parallel to the established path.
• Occlusal or incisal reduction is done first parallel to the planned occlusal plane.
• Depth guides are placed on the buccal and lingual surfaces of abutment teeth and are prepared parallel to the scribe lines on the base of the diagnostic cast.
• A chamfer diamond with a 2°–3° taper is used to prepare the inaccessible areas first, such as the distal and lingual aspect, then the facial and mesial aspects.
• After the reduction parallelism is checked with intraoral mirrors, a trial cast is made of the preparation and evaluated for parallelism on dental surveyor.
• All sharp margins are rounded, and finish lines are evaluated for smoothness.

Cantilever Fixed Partial Denture

Cantilever Fixed Partial Denture Synonym

Cantilever bridge

Cantilever Fixed Partial Denture Definition

This is a fixed partial denture in which the pontic is retained and supported only on one end by one or more abutments.

Selection of cantilever abutments

• Good bone support should be present more than the average
• Adequate clinical crown height should be present
• Should be able to develop an harmonious occlusion
• Extensively damaged teeth, maligned teeth, endodontically treated teeth and mobile teeth are contraindicated from being used as cantilever abutments.

Ideal cantilever situations

• Replacement of lateral incisor using canine as cantilever.
• Replacement of first premolar using second premolar as cantilever.
• A cantilever bridge should ideally have lengthy roots with a favorable configuration, good clinical crown height, good crown–root ratios and healthy periodontium.

Cantilever Advantages

• Conservative design with preservation of tooth structure
• Secondary abutments used can be prepared easily with parallelism.

Cantilever Disadvantages

• Produces torquing and lateral forces
• Cannot restore long-span edentulous space
• Lateral forces can tip rotate or drift the abutment tooth.

Modifications

Spring cantilever

• For replacing maxillary incisors
• Support is from splinted premolars or molars; a long resilient bar connects both.

Advantages

• In diastema cases
• Preservation of tooth structure as only all-metal restoration is required for molar.

Disadvantages

• Interference in speech
• Distortion of connector can displace pontic
• Food entrapment is common.

Design examples

• Can replace maxillary lateral incisor with canine as abutment
• Can replace mandibular first premolar with second premolar and first molar as abutments
• In some instances can replace molars with no distal abutments to prevent super-eruption of opposing teeth. The replaced pontic should be premolar sized, good occlusogingival height and minimal occlusal contact.

Cementation of all Ceramic Restorations

With the introduction of all ceramics and CAD–CAM restorations, the types of cementation have evolved into adhesive cementation.

Adhesive cementation is the bonding of the restoration (crown/veneer) to the tooth. The tooth is being sealed as well.

Classification of dental cements

Based on chemistry and application, they are of three types:

• Water-based cements
• Resin-based cements
• Oil-based cements.

Water-based cements

• Undergo acid–base setting reaction and are acidic during cementation
• Are non-adhesive or have low bond strength with tooth [E.g. glass ionomer (Fuji I, Ketac Cem), resin-modified glass ionomer (RelyX luting, FujiCEM), zinc phosphate (Harvard, D-Tech) and zinc polyacrylate (Poly F)].

Resin-based cements

• Can be light cure or dual cure type
• If the crown is >3 mm thick autopolymerizing resin cement is used as light cannot penetrate to the depths; otherwise a dual cure resin cement is used
• Are chemically similar to composites
• They have higher bond strengths with tooth structure
• The tooth needs to be etched and rinsed
• Some of these cements are self-adhesive to dentin
• The restoration should be surface treated and primer is applied before cementation
• These cements have stronger physical properties but are technique sensitive. [E.g. Resin luting agents, etch and rinse composites (RelyX ARC, C & B cement) and self-etch and self-adhesive composites (Multilink, RelyX Unicem), compomers (Principle)] .

Oil-based cements

• Used for provisional cementation
• Eugenol containing or eugenol free [E.g. are zinc oxide-eugenol and non–eugenol zinc oxide (Provicol)].

Commonly used luting agents for all ceramic restorations

• Zinc phosphate (for zirconia restorations)
• Glass ionomer cement (for zirconia restorations)
• Resin luting agents, mostly dual cure (for any all-ceramic restoration).

Resin-modified glass ionomer is not used for cementing all ceramic restorations as they undergo expansion due to water absorption following cementation. The most popular cements are resin cements and glass ionomers.

Step-by-step procedure

• Isolate the tooth and clean it.
• Place plastic strips interproximally.
• Etch the enamel with phosphoric acid, clean and dry the tooth.
• Seal any open dentinal tubules using a bonding agent.
• The fitting surface of the ceramic restoration is etched using hydrofluoric acid gel.
• Apply silane coupling agent to the ceramic restoration.
• Select the shade of the resin luting agent and mix the components using a plastic spatula
• Load the crown and carefully seat over the prepared tooth.
• Do not give any excessive seating force.
• Remove any excess cement after tack curing.
• Final curing using light is done after removal of the excess luting agent.
• Adjust the premature contacts.
• Finish the cement margins with carbide finishing burs, fine paper discs or rubber cups.

Cantilever Conclusion

• For good esthetics of the final ceramic restoration, the color of the underlying luting agent must be faintly colored and translucent enough to bring out the natural color of the dentin and enamel
• If the cement is opaque, the esthetic quality declines and the margins begin to be visible. Color can be controlled by varying the proportions of opaque and translucent catalyst.