fpd lecture 2009-2

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FIXED P RTI LDENTURESTreatment Planning andBiomechanics

Donna N. Deines, DDS, MS

Resources: Shillingburg, et alRosenstiel, et al

Components of the FPD

Abutment: tooth serving as attachment for FPD

Retainer: extracoronal restoration cemented to abutment

Pontic: artificial tooth suspended from abutments

Connector: rigid or non-rigid metal connecting pontics / retainers

Treatment of Tooth Loss

Caries

Periodontitis

Trauma, congenital Decision to remove tooth

Careful assessment Replacement decision

Consequences of tooth loss:

Supra-eruption

Tilt ing

Loss of proximal contact

Disruption of occlusion

Restoration of the Occlusal Plane

Occlusal interferences are produced when FPD is made to a

supraerupted opposing dentition.

Opposing tooth restored to correct occlusal plane

May require RCT; periodontal surgery; orthodontics; extraction

Relation of Tooth Loss to the Edentulous Ridge

Alveolar ridge resorption results vary due to individual

patient factors length of time, existence of periodontal

disease, trauma, arch, etc.

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Relation of Tooth Loss to the Edentulous Ridge

Knife-edge ridge

Loss of interdental papillae

Indications for a Fixed Partial Denture

Replace function of missing teeth

Stabilize occlusion (drifting, prematurities)

Improve stress distribution

Provide esthetics and phonetics

Comfort

Contraindications for FPD

Too great a span length

Long edentulous space at the end of an arch

Tipped abutments, divergent alignment

Non-restorable abutment teeth or periodontium

Severe loss of tissue in the edentulous ridge

(Limited financial ability / advanced age or

systemic (terminal) illnesses)

Stress Distribution in Fixed Partial Dentures

An FPD distributes forces favorably by directing

forces in the long axis of the abutment teeth.

Conventional Fixed Partial Denture

Abutment on each end

Periodontally sound abutments, straight alignment

No gross soft tissue defect

Dry mouth increases risk of failure

Resin-Bonded Fixed Partial Denture

Conservative, enamel preparation

Single missing tooth; slight - moderate tissue resorption

Good axial alignment and light occlusal stresses

Especially indicated for younger patients

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Posterior Resin-Bonded FPD

Occlusal rests; 180o encirclement of axial tooth structure.

Single molar replacement requires minimum occlusal load.

Implant-Supported Crown / Fixed Partial Denture

Indications: insufficient abutments / no distal abutment

Single tooth implant saves virgin adjacent teeth

Span length limited by availability of bone / ridge configuration

Implant-Supported Fixed Partial Dentures

Prosthesis is usually not attached to adjoining natural teeth.

Implant-supported fixed prosthesis placed in a totally edentulousmandible

Limitations of Implant Placement

Amount of bone may severely limit potential for

implant placement - maxillary sinus / mandibular canal

Precise abutment alignment and positioning for

occlusal forces

Implant-Supported Fixed Partial Dentures

Insufficient number of abutment teeth

Lack of distal abutment

Connection of implants / natural teeth can be compromised

Indications for Removable Partial Dentures

Periodontally involved teeth

Tilted molar abutments

Multiple edentulous spaces

Edentulous space with no distal abutment

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Treatment Options for Tooth Loss

Removable Partial Denture (RPD)

Gross soft tissue defects

Traumatic injury

Ablative surgery

Disadvantages of Removable Prostheses

Soft tissue irritation of edentulous ridge

Less comfortable than FPD

Esthetics often inferior to FPD

Fixed partial dentures are preferred for

comfort and estheticsCase Presentation

Present treatment options

Advantages / disadvantages

Patient input esthetics, finances

Agree on definitive treatment

plan Understanding of risks /

responsibilities

No prosthetic treatment Unrealistic expectations

Do no harm

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Abutment Evaluation

Caries

Existing restorations Endodontic assessment

Periodontal health

Orthodontic position

Occlusion

Abutment Evaluation: Remove all caries, oldrestorations, base; then evaluate.

Pulp exposure? Symptomatic? PA pathology?

Proximity of cavity depth to alveolar crest

Biologic width

Pulpal Health: Vital or Endodontically Treated -Asymptomatic with sound tooth structure remaining.

Questionable / pulpal exposure RCT before FPD

Evaluation of Diagnostic Casts:AccurateMounted on semi-adjustable articulator w/ facebow / CR

Edentulous spaces and span

length

Curvature of the arch

Occlusocervical dimension

Inclination of the abutment

teeth

M-D drifting, rotation, F-L

displacement of abutments

Interocclusal relationships

Abutment Alignment and Path of InsertionEvaluation of the path of insertion

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Discrepancies in the long axes of abutment teeth

Complicates the ability to prepare parallel paths of

insertion.

Facio-lingual and mesio-distal inclinations

Evaluation of Interocclusal Relations

Interocclusal space is necessary to re-establish a

proper occlusal plane.

The occlusion may be acceptable or changes may

necessary.

Diagnostic waxing: visualize problems and results Diagnostic Waxing and Case Planning

OR

Healthy periodontium: a prerequisite for all fixed

prosthodontic restorations

No mobility / zone of attached tissue / good oral hygiene

Additional abutment evaluation of the periodontium:

Crown-root ratio

Root configuration

Periodontal ligament area

Abutment Evaluation: Crown-Root Ratio

Crown - Root Ratio

Length of tooth occlusal to the alveolar crestcompared with the length of root embedded in bone

Optimum C:R is 2:3

Minimum C:R is 1:1

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Periodontal disease - Horizontal bone loss

dramatically reduces supported root surface area

Conical root shape diminishes actual area of support morethan expected from the height of bone.

The center of rotation (R) moves apically and the lever arm(L) increases, magnifying the forces on the supportivestructure.

Rosenstiel

A crown-root ratio 1:1 may be adequate if:

Opposing occlusal force is

diminished

Artificial teeth

Periodontally compromised

Abutment Evaluation: Root Configuration

Favorable: elliptical; widely

separated roots; curvature in

apical 1/3

Unfavorable: round; fused roots;

conical taper

Well aligned tooth provides

better support than a tilted one.

Root Morphology

2nd molar long, separated roots;

1st molar extensive caries and

positioned against adjacent tooth.

Abutment Evaluation:

Root Surface (Periodontal Ligament) Area

Antes Law: The root surface area of the abutment

teeth (embedded in bone) should equal or surpass that of

the teeth being replaced with pontics.

Generally successful

Antes Law: The root surface area of the abutment teeth should

equal or surpass that of the teeth being replaced with pontics.

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Deflection of the FPD relates to span length

The deflection is proportional

(varies directly) to the cube

of the length of its span.

Law of Beams

Bending also varies inversely with the cube of the

occlusogingival thickness of the pontic / connector

Design pontic/connector with adequate O-G thickness

Use alloy with high yield strength

BIOMECHANICAL CONSIDERATIONS

Bending or deflection of the FPD

Abutments and retainers receive greater

torque than a single crown

Modify preparations to increase retention and

resistance

Place boxes / grooves in response to direction of

anticipated torque

Dislodging forces on an FPD

Occlusal forces can act in a M-D direction on an FPD.

Forces at an oblique angle or outside the center of the

restoration cause F-L dislodgement .

FPD and Dislodging Forces

Grooves / boxes resistance to dislodgement.

Place boxes / grooves in response to direction of

anticipated torque.

Use retainer with appropriate retention / resistance.

Double abutments (splinting) can help problems

caused by poor crown-root ratio and long spans.

Double abutments help stabilize the prosthesis by

distributing forces over more teeth.

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Criteria for secondary abutments:

Root surface area and C:R must = 1o abutments

2o retainers must have retention of 1o retainers

Long crown length and adequate interproximalspace for connectors

Long-term periodontal splint

Bone loss and increased physiologic movement

Deflection / torque microleakage / debonding

Caries involvement of abutment teeth

Fracture of RCT abutment with large amount of missing toothstructure

Is splinting necessary here?Effect of Arch Curvature on FPD Deflection

Pontics lying outside the inter-abutment axis act as a leverarm torquing movement.

Additional resistance in opposite direction from lever arm;distance = to length of the lever arm

SPECIAL PROBLEMS: Pier Abutments

An edentulous space on both sides of a lone free-

standing abutment

Physiologic tooth movement

direction and amount varies from anterior to posterior

SPECIAL PROBLEMS: Pier Abutments

Cause of failure - loosened retainer

Prosthesis flexure / movement of teeth

Tensile stresses between terminal retainers

and abutments; intrusion of abutments under

loading

Differences in retentive capacities between

abutments (relative to size)

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FPD #4-6-8 (Pier abutment #6)

Rigid Connectors

Extensive caries through crown

resulting from #6 retainer

debonding from abutment.

Non-Rigid Connector Non-Rigid Connector / Pier Abutment

Criteria for use: Short span length Non-mobile abutments

Equal distribution of

occlusal force

Location:Location:

Within distal surface of pier retainerWithin distal surface of pier retainer

((mesialmesial seating action of posteriors)seating action of posteriors)

Non-rigid connector (stress breaker)Special Problem: Pier Abutment

Where periodontal support is adequate, a simpler

approach could be a mesial cantilever pontic.

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Implant supported FPD #5-#6-#7 SPECIAL PROBLEMS: Tilted Molar Abutment

Discrepancy between long axis of molar

and premolar abutments

25o - 30o - maximum angle of tilting

SPECIAL PROBLEMS: Tilted Molar Abutment

Mesial wall must be over-reduced ( resistance)

Distal adjacent tooth may intrude on the path of insertion


Plan path of insertion / preparation design on diagnostic

cast.

Surveyor may help in determination


Occlusal reduction is not always the same as clearanceneeded.

Remove only enough to provide necessary space forthe restoration.

Allows for longer axial wall length.


Molar uprighting Places abutment in better position for preparation

Distributes forces under loading through long axis oftooth (helps eliminate mesial bony defects)

Enables replacement of optimum occlusion

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Orthodontic Movement: Molar UprightingTilted Molar Abutments: Proximal Half Crown

Proximal Half Crown does not involve distal wall

A 3/4 crown rotated 90o

Requirements:

Caries-free distal surface

Low incidence of caries

Even marginal ridge height

Proximal Half

Crown Retainer

Tilted Molar Abutments:

Telescopic Coping and Crown

Full crown preparation and coping

with path of insertion in long axis of

tooth

Full coverage crown compensates

for discrepancy in paths of insertion

Must over-reduce molar to

accommodate the thickness of

coping and crown

Tilted Molar Abutments: Non-Rigid Connector

Allows slight movement - short span

Keyway in distal of premolarto avoid intrusion ofmolar (mesial seating action)

Must prepare box in distal of premolar preparation

Canine Replacement FPD (Complex)

Pontic lies outside the inter-abutment axis

Stress is greater on maxillary arch Forces inside arch (weak - tension)

Stress more favorable in mandibular arch Forces outside arch (strong compression)

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Resin-bonded hybrid cantilever

Full crown retainer central incisor #9

Resin-bonded wing / mesial preparation #7

Favorable occlusion

Cantilever FPD:

Replacement of First Premolar

Limit pontic occlusion to distal fossa.

Use full veneer retainers on the 2nd premolar

and 1st molar.

Cantilever: First Premolar

Resin-bonded retainer on canine

(mesial rest)

When using a rest on a cantilever pontic, always

place a rest prep in a restoration on the abutment.

Caries

Missing tooth less than space

Change proximal contour / occlusion

Button pontic

Importance of resistance form: clinical crown length;

facial lingual grooves; minimal taper

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Cantilever FPD: Molar Replacement

Very Unfavorable

Extreme forces generated by posterior position(Class 2 lever)

Occlusal forces place tensile stress on 2o retainer

Cantilever FPD: Replacement of First MolarUnfavorable

Pontic size small (premolar)

Light occlusal contact; no excursive

contact

Pontic and connector

Maximum O-G height for rigidity

Good crown:root ratio of abutments

Clinical crowns - maximum preparation

length and resistance form

fpd lecture 2009-2

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