Abutment evaluation / cosmetic dentistry training

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Abutment Evaluation in Abutment Evaluation in FPDFPD

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Contents:Contents:Introduction.Definitions.Ideal requirements.Assessment of abutment:

Clinical examination.Diagnostic casts.Radiographic evaluation.

Factors influencing abutment selection:Crown length.Crown form.Crown root ratio.Periodontal ligament area.Root configuration.


Root proximity.Periodontal disease.Long axis relation.Span length.Arch curvature.Occlusal anatomy.Endodontically treated tooth.Age of the patient.

Special problems:Pier abutment.Tilted molar abutment.Abutment for cantilever FPD.

Questionable abutments.Summary.References.


IntroductionIntroductionEvery restoration must be able to withstand the constant occlusal forces to which it is subjected.

In FPD,

Abutments bear the stresses of mastication and the choice of abutment influences the prognosis of treatment.


Successful selection of abutments for fixed partial dentures requires sensitive diagnostic ability.

Thorough knowledge of anatomy, ceramics, the chemistry and physics of dental materials,

metallurgy, Periodontics, radiology and the mechanics of oral function is fundamental.


Definitions Definitions (GPT–8)(GPT–8)

Abutment : A tooth, A portion of a tooth, or that portion of dental

implant that serves to support/or retain a prosthesis.

Primary abutment:

Abutment adjacent to edentulous space.

Secondary abutment:

Abutment remote from edentulous space.

Intermediate abutment: A natural tooth located between abutments that serve to

support a fixed or removable dental prosthesis.


Ideal requirementsIdeal requirements

Vitality: Vital tooth: Preferred. Pulp capped teeth: Endodontically treated before selecting

as abutment. Endodontically treated teeth: Asymptomatic. Radiograph evidence of good seal and

complete obturation.

Coronal tooth structure: Adequate occluso-gingival length to achieve retention. Teeth with short cinical crown: Crown lengthening . Tapered crown interferes with preparation parallelism.


Health of surrounding tissues: Healthy, free from inflammation. No mobility of abutment.

Periodontal ligament area: Periodontal ligament attachment of the root to the bone. Larger teeth have a greater surface area and better able

to withstand or bear added forces.

Compensation for lost tooth structure:1. Dowel core.2. Pin-retained amalgum.3. Composite resin core.


Assessment of Abutment:Assessment of Abutment:


Periodontal Examination:

Clinical Examination:Clinical Examination:


Presence or absence of Inflammation.Width of band of attached gingiva: Lack of attached gingiva (< 2mm): Greater chances of

chronic inflammation due to minute irregularities in the crown.

30-60% Mandibular 3rd molars: Do not have attached gingiva around distal segment.

Periodontium: Pockets. Tooth mobility. Furcation involvement. Clinical Attachment level.

Recession Location of FMG Epithelial Attachment

Negative On clinical crown On CEJ

Zero At CEJ On root

Positive On root On rootwww.indiandentalacademy.com

Condition of existing teeth:

Missing teeth: Have impact on arch integrity.

Extent of dental caries. Condition and type of existing

restoration. Wear facets: Indicative of

parafunctional activity. Fractures, malformation,

abrasion and erosion. Occlusion: Amount and direction

of slide from MI to CR. General alignment: Crowding, rotation, supra-eruption,

spacing, malocclusion, and vertical and horizontal overlap.www.indiandentalacademy.com

Diagnostic Cast:Diagnostic Cast:

Articulated diagnostic casts are integral part of diagnostic procedure.

Information…Unobstructed view of edentulous spaces and accurate assessment of span length and curvature.The length and true inclination of the abutment tooth to determine the preparation designs.Analysis of occlusion: Wear facets, occlusal discrepancy, supra-occlusion, etc.Mesiodistal drifting, rotation and faciolingual displacement of prospective abutment teeth.


Radiographic Evaluation:Radiographic Evaluation:Radiograph of interest: OPG and IOPAR

Information revealed:Signs of caries.Presence of periapical lesions and quality of previous endodontic treatment. General alveolar bone levels, with particular emphasis on prospective abutment teeth. Crown root ratio of the abutment. Length, configuration and direction of abutment roots.Widening of PDL ligament.Thickness of the cortical plate of bone around the teeth and trabeculation of bone. Presence of retained root tips beside abutment tooth.


Factors influencing abutment selection:Factors influencing abutment selection:


Crown lengthCrown length Adequate occluso-cervical crown length to achieve sufficient retention. Min of 4mm interproximal height from marginal ridge to gingival attachment. Short clinical crowns:

1. Full coverage preparations. 2. Additional length through

a) Periodontal surgery.b) Forced eruption.

Crown formCrown form Tapered crown: Interferes with preparation parallelism. Full coverage retainers: Retention & esthetics. Eg: 1) Anterior teeth: Poorly developed cingulum & short proximal walls. 2) Mandibular premolars: Poorly developed lingual cusps & short proximal surfaces.


Crown – root ratioCrown – root ratioDefinition:

Physical relationship between the portion of tooth within alveolar bone compared with the portion not within the alveolar bone, as determined by radiograph. (GPT-8)

Ratio of the length of tooth occlusal to the alveolar crest of bone compared with length of root embedded in bone.

a

b

CRR = a/b

a

b


Biomechanical concept:Represents Class I lever :

Crown Effort arm. (E)

Root Resistance arm. (R)

Centre of rotation of tooth Middle of root that is embedded in alveolar bone.

Loss of alveolar bone: Chance of harmful lateral force increases.

Crown portion (effort arm) : Increase.

Root portion (resistance arm) : Decrease.

Centre of rotation moves apically.

E=1 R=2

50 lb 25 lb


Value of CRR: Ideal- 1:2. Rarely observed in practice.

Exceptionally conservative & limits treatment. Optimum- 1:1.5. Minimum- 1:1. Considered if opposing occlusion is:

Prosthetic appliance. Mobile, periodontally involved teeth.

Teeth with loss of more than 1/3rd periodontal support: Questionable value.

CRR = 2/3 CRR = 1/1


CRR in clinical practice:Factors decreasing CRR: Tooth preparation.

Overdentures: Reduction of crown to 1 to 2mm above FGM

improves CRR from 1:1 to 1:2 or 1:3.

Factors increasing CRR: Increasing VDO. Surgical crown lengthening. Forced Eruption:

Approx 2mm/mnth.Allows time for periodontal ligament to repair and alveolar

bone to remodel.Preserves biologic width and provides better CRR.


Shortcomings:Based on linear measurements only thus does not express the actual area of bone support.

Since most roots conical in shape, Root length an 1 dimensional linear measurement, Other criteria should be used to evaluate total alveolar support.

In multi-rooted teeth, relatively little area of root trunk provides more extended surface area than 3 individual roots for fiber attachment.

CRR definition does not recommend the preferred radiographic technique.


Periodontal ligament areaPeriodontal ligament area

Root surface area or the area of periodontal ligament attachment of the root to the bone.

Large teeth Greater surface area Better ability to bear added stress.

Periodontal disease Loss of supporting bone Lesser capacity to serve as

abutment.

Loss of periodontal support from root resorption is only 1/3rd to ½ as critical as the loss of alveolar crestal bone.


ANTE’S LAW (1926)ANTE’S LAW (1926)Irwin H.Ante (Toronto, Ontario Canada)

Eponym in FPD, for the observation that the combined pericemental area of all abutment teeth

supporting a fixed dental prosthesis should be equal to or greater in pericemental area than the tooth or

teeth to be replaced.(GPT-8)

This has been adopted and reinforced by other authors (Johnston, Dykema, Shillingburg, Tylman) as Ante’s law.


A1m< A2p+ A2m

(A1m + A2p) = (A1p + A2m) (A1p+A2p+A1m) > (Ac+A2m)

Poor prognosis FPD


Average root surface area (Jepsen 1963)

Smallest tooth:Maxillary arch: Lateral incisor.Mandibular arch: Central incisor.


Condition existingProbable

modification (Abutment no.)

Bone loss from PDL disease. Increase

Mesial or distal tipping or changes in axial inclination Increase

Migration of abutment teeth decreasing mesio-distal length of edentulous area

Decrease

Less than favourable opposing arch relationships producing increased occlusal load.

Increase

Endodontically restored teeth as abutments with root resection.

Increase

Arch from situations creating greater leverage factors Increase

Tooth mobility created after osseous surgery Increase (Splinting)

Factors modifying Ante ‘s law:


Validity of Ante’s Law:

Support: Better prognosis of short span FPDs then excessively long

span. Reasons:

Overstressing of PDL. Leverage and torque.

Contradictions: Study by Newman and Ericsson

Majority of treatments presented had abutment root surface area less than half that of replaced teeth.

Results: Successful prosthesis with no loss of attachment after 8-10 years.

Attributed success to meticulous root planing during active phase of treatment, proper plaque control during observed period and occlusal design of the prosthesis. www.indiandentalacademy.com

Cross arch bridges: Successful since 1970s. Does not fulfill Ante’s law.

Rationale:Severe periodontal disease

Loss of teeth.Loss of periodontal support for remaining teeth.

Cross arch bridgesSplints the teeth together Increasing life span.Provides rigidity.Prevents overloading of abutment by favorable distribution of load.

If presumptive abutments are well distributed and periodontal infection is under control, 20-30% of original periodontal tissue support can be

sufficient for fixed cross arch bridges.www.indiandentalacademy.com

Root configuration:Root configuration:

Irregularly shaped, multiple, divergent roots offer better prognosis.

Broader labiolingually than mesiodistal dimension are

preferred to the round ones.

Multirooted teeth with widely separated roots than that converge, fuse.


Root proximities:Root proximities:

There must be adequate clearance between the roots of proposed abutments to permit the development of physiologic embrasures in completed prosthesis.

Malpositioned anterior teeth and the mesiobuccal roots of maxillary molars often present unfavorable root proximities where desired embrasure form is not possible.

Solution to root proximity: Selective extraction or root resection procedures.


Periodontal disease: Periodontal disease:

Healthy periodontal tissue is prerequisite for all fixed restorations. Occasional lapse in plaque removal by patient is unlikely to

affect the long term prognosis.

Abutment with bone loss needs careful assessment. Conical shape of roots: with 1/3rd of root length exposed, ½ of

the supporting are is lost. Lengthened clinical crown leads to greater leverage force.

Successful fixed prosthesis with severely reduced periodontal support, is assured when periodontal tissues have been returned to excellent health, and long term

maintenance has been ensured, otherwise results will be disastrous.www.indiandentalacademy.com

Mobility:Mobility:

Miller’s classification:Grade I: Slight movement. Acceptable.Grade II: Excessive side to side movement.

Assessment of cause and no. of teeth to be replaced.Grade III: Both lateral and vertical movement. Not suitable.

Splinting

Immobilization of teeth by joining them to one another.

Careful assessment is essential , other wise it creates more problems leading to break down of the healthy tooth itself.

Do not prevent periodontitis and may actually increase chances of inflammatory disease, since home care will be inhibited.

When in doubt do not splint.


Long axis relation:Long axis relation:

Periodontal ligament can better withstand the forces directed along the long axis of the tooth.A severely inclined tooth will not withstand forces as well as one that is erect.

Inclined tooth as abutment: Shorter edentulous span with less occlusal force.

Common path of insertion for all retainers: Conventional FPD: Less then 25° inclination. Resin-bonded FPD: Less then 15° inclination mesio-

distally and same plane facio-lingually.

Evaluation:Diagnostic casts with a dental surveyor. Radiographs.


Span Length: Span Length: Bending or deflection α (Length of edentulous span)3

1/α(occluso-gingival thickness of pontic)3


Disadvantages of Longer pontic span: Potential for producing more torquing forces abutment. Less rigidity.

To minimize flexing caused by long and/or thin spans, pontic designs with a greater occluso-gingival dimension should be selected. The prosthesis may also be fabricated of an

alloy with higher yield strength, such as nickel-chromium.


Secondary abutment:

Overcomes problems created by unfavorable crown-root ratios and long spans.

Criteria in selection:

Must have at least as much root surface area and as favorable a crown-root ratio as the primary abutment it is intended to bolster.

Sufficient crown length for adequate retention of retainer. Space between adjacent abutments to prevent impingement

on the gingiva under the connector.


Retainers on secondary abutments must be at least as retentive as the retainers on the primary abutments. As the retainer on 2ndry abutments will be placed in tension when

the pontic flexes, with primary abutment acting as fulcrum.


Arch curvature:Arch curvature:

When pontics lie outside the interabutment axis line, the pontics act as a lever arm, which can produce a torquing movement. Common problem in replacing all four maxillary

incisors.

Solution: Additional retention in opposite direction from the lever

arm and at a distance from the inter-abutment axis equal to the length of the lever arm.

The first premolars sometimes are used as secondary abutments for a maxillary four pontic canine-to-canine FPD.

P R


Occlusal anatomy:Occlusal anatomy:

Natures own anatomy and contour should be recreated in all restorations.

Has an indirect influence on the loads transmitted ..

Ridges and grooves increase the sharpness and shearing action of teeth and reduce friction between opposing surfaces by keeping the contacting area to minimum.

Permits the most efficient mastication of food, thus reducing the load transmitted.

Attrited teeth need more muscular power and longer and more masticatory strokes in order to chew food enough.

Stallard.www.indiandentalacademy.com

Factors affecting occlusal forces: Degree of muscular activity. Habits such as bruxism. Number of teeth being replaced. Leverage on the bridge. Adequacy of bone support.

Results of excessive occlusal forces: Loosening of prosthesis through flexure. Ceramic fracture. Tooth mobility (In presence of decreased bone support).


Replacement strategies: Buccolingual width of pontic should harmonize with

buccolingual dimension of natural unmutilated teeth, and recreate the normal buccal and lingual form to the height of contour.

The total meso-distal width of the cusps of abutment should be equal or exceed that of pontics.


Endodontically treated abutments:Endodontically treated abutments:

Vital tooth preferred.Pulp capped teeth: Should be endodontically treated before

selecting as abutment.Endodontically treated teeth: Asymptomatic, with radiographic evidence of good seal

and complete obturation. Should have post and core foundation for retention and

strength. Its better to remove badly damaged tooth rather than

attempting endodontic treatment. Can not be selected for cantilever FPD.


Age of the patient: Age of the patient:

FPD is usually contraindicated in adolescents

Teeth are not fully erupted. Excessively large pulp horns.

Treatment options:

Space maintainer: Holds abutment and opposing teeth in position.

Minimal tooth reduction: Prosthesis considered temporary and remade when pulp size

permits.


Special problems:Special problems:


Pier abutments/Intermediate AbutmentPier abutments/Intermediate Abutment

Definition:

A natural tooth located between terminal abutments that serve to support a fixed or removable partial denture.


Completely rigid restoration: Contraindicated..

Physiologic tooth movement:Faciolingual 56 to 108μm.Intrusion 28μm.Independent in direction and magnitude:

Tendency for prosthesis to flex.Stress concentration around abutments.

Arch position of abutment:Forces transmitted to terminal retainers as a result of middle

abutment acting as a fulcrum, causes failure of weaker retainer.

Disparity in retentive capacity:

Retention: Smaller anterior tooth < Larger posterior tooth.

Dislodgement of anterior retainer.www.indiandentalacademy.com

Non-rigid connector: Broken stress mechanical union of retainer and pontic.

Design: T-shaped key attached to pontic. Dovetail keyway placed within the retainer.


Advantages: Movement prevents the transfer of stress from segment

being loaded to the rest of the FPD. Transfers shear stress to supporting bone rather then

concentrating it in connector. Minimize mesio-distal torquing while permitting them

to move independently.

Disadvantages: Not preferred in teeth with decreased periodontal

attachment. Supraeruption of key and posterior unit when opposed

by RPD or no teeth and anterior three unit by natural teeth. www.indiandentalacademy.com

Location: On pier abutment. On terminal abutment Pontic act as lever arm. Keyway: Normal distal contour of pier abutment. Key: Mesial side of distal pontic.

Nonrigid connector on distal side of pier abutment.

Nonrigid connector on mesial side of pier abutment.


Problem: Discrepancy in long axis of molar

and premolar makes it impossible

to achieve common path of insertion.

3rd molar tipped with tilted 2nd molar

prevents complete seating of FPD.

Tilted molar abutmentsTilted molar abutments


Solutions:Encoachment from 3rd molar is slight: Restoring and recontouring the mesial surface of 3rd molar.

Excessive tilting:

Orthodontic correction: Up righting 2nd molar. Extraction of 3rd molar to facilitate

distal movement of 2nd molar. Temporary FPD post treatment

to prevent relapse.

Average treatment time 3 months.www.indiandentalacademy.com

For tilt < 25 to 30 degrees:

Proximal half crown on distal retainer:

3/4 crown i.e. 90° rotated so that distal surface is uncovered.

Indication: No caries and decalcification on distal surface. Low caries index.

Contraindication:

Severe marginal ridge height discrepancy 2nd and 3rd molar.


Telescopic crown and coping:Telescopic crown and coping: Inner coping: Fit the tooth preparation. Proximal half crown: Serve as retainer for FPD is fitted

over the coping.

Advantage: Allows for total coverage of the clinical crown while compensating for discrepancy between the paths of insertion of the abutment.


Nonrigid connector:Nonrigid connector:

Full crown on molar with its path of insertion parallel to long axis of that tilted tooth.

Box form placed in the distal surface of premolar to accommodate keyway.

Nonrigid connector on mesial surface of molar Greater tipping of the tooth.


Abutment for the cantilever FPDAbutment for the cantilever FPDDefinition:

A fixed dental prosthesis in which pontic is retained and supported only on one end by one or more abutments.


Biomechanics:

Requirement of abutment: Lengthy root with favorable configuration. Long clinical crowns. Good CRR. Healthy periodontium. Single missing teeth, at least two abutments.

Conventional FPD Cantilever FPD


Missing lateral incisor:Missing lateral incisor: Abutment: Canine Long root and good bone support. No occlusal contact on pontic in either centric or lateral

excursions. To prevent rotation of pontic and abutment:

Rest on mesial of pontic & rest preparation in inlay or other metallic restoration on distal of central incisor.

Mesial aspect of pontic “wrapped around” distal of uninvolved central incisor.


Missing first premolar:Missing first premolar:Full veneer retainers on second premolar and first molar abutments.

Missing molar with no distal abutment:Missing molar with no distal abutment: Full veneer retainers on 1st and 2nd

premolar. Pontic representing premolar &max

occluso-gingival height for rigidity. Light occlusal contact & no contact

in excursion.


Questionable abutmentsQuestionable abutments


Classification:Classification:General disorders:

Mineralization: 1) Amelogenesis Imperfecta. 2) Dentinogenesis Imperfecta. 3) Hypocalcification. 4) Ectodermal Dysplasia. 5) Discolouration due to drugs like Tetracycline. 6) Flouridosis. 7) Internal resorption .

Congenital & growth deformities: 1) Malformed dentition. 2) Malposed teeth. 3) Skeletal disparities of Maxillo-mandibular relationships. 4) Oligodontia.


Local problems:1) Polycarious tooth.2) Periodontally involved teeth.3) Occlusal plane correction.4) Endodontically treated teeth:

a) Previously treated teeth.b) Currently treated teeth.

5) Tilted teeth.6) Attrition, abrasion, erosion.


Treatment strategy:Treatment strategy:

Abutment with generalized mineral disturbance: Full coverage restoration. Success depends on supporting tissue response.

Congenital and growth deformities: 1st line of treatment:

Orthodontics. Interceptive periodontics. Restorative dentistry.

Malposed teeth: Judicious tooth reduction. Orthodontics for minor tooth movement: Requires

periodic occlusal adjustments. Telescopic crowns.


Polycarious tooth: No contraindication. DMF >3 Clinical approach to uncontrolled caries:

Caries control program. Endodontic and periodontic consultation. Cast metal restoration where indicated after

amalgum restoration. Recall visits strictly maintained.

Occlusal plane correction: Supra-erupted teeth. Intentional RCT. Reduction to satisfactory occlusal plane. Tooth preparation to receive retainer. Construction of opposing prosthesis.


Periodontally involved teeth: Review the reason for the condition. Periodontal treatment before caries control.Loss of periodontal support: Splinting may compensate.Mobility: Due to Primary TFO: Occlusal correction. Due to Secondary TFO: Splinting.

Furcation involvement: ( Class III) Open and closed root debridment. Filling the furca with polymeric ZOE cement

or GTR. Tunnel preparation . Root amputation and hemisection.www.indiandentalacademy.com

Tooth with root resection or hemisection as abutment: Short span FPD. Large remaining roots. Large natural clinical crown despite of

access opening or placement of post and core. Prosthesis contours encouraging plaque control. Occlusion managed to diminish load and lateral contacts. Posts tapered near apex to preserve strength of dentin. Professional maintenance is provided.


Summary:Summary:

Success of prosthesis depends on many foundational steps taken to prepare it. Proper handling of abutment teeth is one of these important foundational steps that either enhances or detracts from the eventual value of the prosthesis.

When conditions are proper like, crown contour, retention and criteria of good preparation techniques and design are met, sound abutment considerations will be a strong link in the success of the prosthesis.

Selecting a suitable abutment forms the preliminary treatment planning for FPD whose proper selection and preparation aids in long term durability of the restoration.


References:References:Shillingburg. Fundamentals of Fixed prosthodontics. 3rd ed.Tylman’s Theory and practice of fixed prosthodontics. 8th ed. Rosenstiel, Land, Fujimoto. Contemporary Fixed prosthodontics. 3rd ed.Jhonston’s modern practice in fixed prosthodontics. 4th ed. Colin R. Cowell. Inlays, crown and bridges. A clinical handbook. 4th ed.Glossary of Prosthodontic Terms. JPD 2005;94.Crown root ratio : Its significance in restorative dentistry. JPD 1979;42.The prosthodontic concept of crown-to-root ratio: A review of the literature. JPD 2005;93.Restoration of periodontally compromised dentitions using cross-arch bridges. Principles of perio-prosthetic patient management. BDJ 2007;203:4.Influence of occlusion on posterior cantilevers. JPD 1992;67.The pier abutment: A review of the literature and a suggested mathematical model. Quintessence Int 2006;37.Management of class III furcally involved abutments for fixed prosthodontic restorations. JPD 1988;60.


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Abutment evaluation / cosmetic dentistry training