stress breakers in prosthodontics

Stress breakers in prosthodontics

Presented by

Sirisha.G

II year PG student

Lenora Institute of Dental sciences

Contents

• Introduction

• Aims of stress breaking

• Applications

• Distal extension RPD

• Philosophies of stress distribution

• Stress breakers in FPD

• Tooth-Implant supported FPD

• Review of literature

• Conclusion

• References

2

Introduction

Stress: (GPT 8)

Force per unit area.(perpendicular cross sectional area over which the force is applied.)

The deformation caused in a body by such a force.

An internal force that resists an externally applied load or force.

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Stress breakers: stress directors: (GPT 8)

• A device or system that relieves specific dental structures of part or all of the occlusal forces and redirects those forces to other bearing structures or regions.

• A stress breaker is something like a hinge joint placed within the denture framework, which allows the two parts of the framework on either side of the joint to move freely. (Mc Cracken)

• Nonrigid or resilient attachment

• Intracoronal/extracoronal

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Aims of stress breaking

1. To direct occlusal forces in the long axis of the abutment teeth.

2. To prevent harmful loads being applied to the remaining natural teeth.

3. To share load as early as possible between the natural teeth and saddle areas according to the ability of these different tissues to accept the loads.

4. To ensure that part of the load applied to the saddle is distributed as evenly as possible over the whole mucosal surface.

5. To provide greater comfort to the patient.

5

Applications

• Distal extension removable partial

dentures

• FPD with pier abutment

• FPD with tilted abutments

• Tooth – Implant supported prosthesis

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Distal extension RPD

• Removable partial dentures are not rigidly

connected to the teeth or tissues -

movement

• Movements Stress Damage

Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial Prosthodontics.

11th ed, Elsevier publications, Mosby Company, Delhi. P.25

7

Movement …. ???

• Teeth efficient support limite

prosthesis movement.

• The reaction of the ridge tissue to

functional forces can be highly variable.

• This disparity leads to variable amounts of

prosthesis movement.

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1. Alveolar support

2. Crown and root morphology

3. Rigidity of frame work

4. Design of occlusal rest

1. Quality of ridge

2. Denture base area

3. Accuracy of impression and denture base

4. Amount of occlusal load

Tooth support Tissue support

Problem

• As the tissues are more compressible, the amount of stress acting on the abutments is increased.

• In order to protect the abutment from such conditions, stress breakers are added to the denture.

• A stress breaker is something like a hinge joint placed within the denture framework, which allows the two parts of the framework on either side of the joint to move freely.

Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial

Prosthodontics. 11th ed, Elsevier publications, Mosby Company, Delhi. P.145

10

• Some dentists strongly believe that a stress-

breaker is the best means of preventing

leverage from being transmitted to the

abutment teeth.

• Others believe just as strongly that a

wrought-wire or bar-type retentive arm more

effectively accomplishes this purpose with

greater simplicity and ease of application.

Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial

Prosthodontics. 11th ed, Elsevier publications, Mosby Company, Delhi. P.149

11

Guidelines:

• Rule1: if the teeth are strong and the ridge

is poor, flat, knife edged or narrow –

RIGID

• Rule 2: if the teeth are weak with + or

more mobility and the ridge is strong –

STRESS BREAKER

Burns D.R,Ward J.E. A review of attachments for removable partial denture design: part 2. Treatment planning and attachment selection. Int J Prosthodont 1990;3:169-74.

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Types of stress breakers

TYPE 1

• Hinge

• Sleeve

• Cylinder

• Ball and socket

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TYPE 2: flexible conncection between direct

retainer and denture base

• wrought wire connectors

• split major conncetors

• Hidden lock partial dentures

• Disjunct partial dentures

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Wrought wire connector:

• The 12 guaze chrome wire stress breaker:

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1. The 12 gauge wire is adapted

to the refractory cast. The wire

is coated with die lubricant

and the wax up is completed.

2. The wax must not go beyond

the maximum convexity of the

wire.

3. The wire is removed and the

casting is completed.

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4. After recovering the

casting, the wire is

welded or soldered.

5. Then the connection

between the denture

base and the main

major connector is

separated to activate 12

gauge chrome wire.

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Advantages:

1. The rigidity of the 12 gauge wire avoids overloading

the mucosa.

2. The mucosa is also more evenly loaded

3. It is easy to splint teeth with this design.

4. The fabrication is relatively simple.

5. Repairs are rarely needed and are also simple.

19

Split bar major connector:• Split is provided between the denture base area and

the major connector .

• When occlusal forces are applied they are transferred

more towards the tissue supported base and then

they are transmitted to the abutment teeth.

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Hidden lock partial denture:• This is a two piece casting, the top half, which is the

major connector supporting the direct retainers and other

rigid components, is cast first.

• The bottom half, which is the connector between the

denture bases, is cast to the major connector next.

Cecconi B.T, Kaiser G, Rahe A.L. Stress breakers and the removable partial denture. J

Prosthet Dent 1975;34:147-51

21

• The hidden lock is created by mechanical means, and the

split between the two connectors is made possible by the thin

oxide shell that forms during the making of the two sections.

• What appears to be a conventional lingual bar or linguoplate

actually is two bars connected by a movable joint at the

midline

Disadvantages

• More prone to collect debris and become un hygienic.

• And also there may be chances of tissue trap at the junction

between the two parts.

22

Disjunct Removable partial denture:o Tooth borne & mucosa-borne parts of denture are

disjoined.

o Tooth borne part providing splinting of remaining teeth &

only retention for mucosa borne part.

Geissler P. R, and Watt D. M. Disjunct dentures for patients with teeth of poor prognosis.

Dent Pratt 1965;15:421-23

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Structural details:

• The tooth borne part is a lingual plate and thus provides

stabilization for the remaining teeth.

• The tissue borne part is a lingual bar which consists of

denture bases along with the teeth at its terminals.

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Advantages:

o Independent movement between the tooth supportedand tissue supported parts decreases the forces onperiodontally weakened remaining teeth.

Disadvantages :

o It is technically difficult to fabricate

o Patient may complains of rattling of the frameworkduring mastication.

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Philosophies of design

• These philosophies are based upon three approaches to

force distribution.

1. Stress equalization / broken stress philosophy

2. Physiologic basing

3. Broad stress distribution

Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics, 3rd

ed. Quintessence books, India.pp. 234

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Stress equalization

• Proponents believe that rigid connections between

denture bases and direct retainers are damaging, and

that stress directors are essential to protect the

abutments

• Articulated prosthesis

• Hinge – most common

• Ball and socket

Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics,

3rd ed. Quintessence books, India. pp.233-240

27

Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial

Prosthodontics, 3rd ed. Quintessence books, India. Pp. 512

28


Prosthodontics, 3rd ed. Quintessence books, India. pp.512

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Advantages Disadvantages

Minimize the tipping forces on

abutment teeth, thereby limiting bone

resorption.

comparatively fragile

Minimal direct retention because the

denture bases operate more

independently than do those used in

conventional removable partial

denture applications

Costly

Constant maintenance

Difficult / impossible to repair


Prosthodontics, 3rd ed. Quintessence books, India. pp.233-240

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Physiologic basing

• Proponents - Equalization can best be accomplished by

recording the anatomy of the edentulous ridge in its

functional form and ensuring that the associated denture

base accurately reflects this anatomy.

• Depressing the mucosa during impression

• Relining the denture base after it has been constructed.


Prosthodontics, 3rd ed. Quintessence books, India. pp.233-240

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• Denture bases formed over compressed tissues will

show an increased ability to withstand vertical forces.

• The prosthetic teeth and occlusal rests will be positioned

above the existing occlusal plane when the prosthesis is

not in function -

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physiologically stimulating effect on the

tissues of the residual ridges

promotes tissue health and reduces the

necessity for frequent relining or

rebasing procedures.

Premature contacts between the

opposing teeth and the prosthesis

during closure.

The minimal retention requirements

lightweight prostheses minimal

maintenance and repair.

Difficult to produce effective indirect

retention because of the vertical

movement of the denture and the

minimal retention provided by the direct

retainers.

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Broad stress distribution

• Advocates - distributing forces over as many teeth and

as much of the soft tissue area as possible – prevents

trauma

• Additional rests and clasp assemblies and broad

coverage of denture bases


Prosthodontics, 3rd ed. Quintessence books, India. pp.233-240.

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Wider force distribution Increased coverage – patient

acceptance

Minimised lateral forces Oral hygiene maintenance

Multiple clasp assemblies added

horizontal stability as like

removable splinting retaining

periodontally weak abutments for

longer time.

Preventive dental programs

No added retentive components

Rigid – excellent horizontal stability

No need for relining and rebasing

Easy and inexpensive36

Stress breakers in FPD

• FPD with pier and malaligned abutments

• The connection between the pontic and retainer / within

the pontic

• Non rigid connectors

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Pier abutments

• An edentulous space on both sides of a

tooth, creates a lone freestanding pier

abutment

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• Physiologic tooth movement, arch position of the

abutments, and a disparity in the retentive capacity of

the retainers

• Studies in periodontometry have shown that the

faciolingual movement ranges from 56 to 108 microns

and intrusion is 28 microns.

• stresses in a long-span prosthesis

• Standlee and Caputo suggest that tension between the

terminal retainers and their respective abutments, rather

than a pier fulcrum, is the mechanism of failure.

Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J AM

Dent Ass 1973;87:1195-99

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Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J

AM Dent Ass 1973;87:1195-99

40

Non rigid connectors

GPT 8

• A connector that permits limited movement between otherwise independent parts of a Fixed partial denture. The nonrigid connector is a broken-stress mechanical union of retainer and pontic

• Internal connector: a non rigid connector of varying geometric design using a matrix to unite the members of an FPD

• Subocclusal connector: an interproximal non rigid connector positioned apical to and not in communication with the occlusal plane

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Indications:

1. Malaligned abutments

2. Pier abutments

3. Longspan FPD

4. Distal abutments with questionable prognosis

Contraindications:

1. Teeth with latge pulp chambers

2. Abutment with reduced clinical occlusogingival height

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• Size of the connectors

• Shape of the connectors

• Position of the connectors

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• Key and keyway/ Dovetails

• Split pontics / tapered pins

• Cross pin and wing

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Keyway position..?

• Nearly 98% of posterior teeth tilt mesially when

subjected to occlusal forces.

• If the keyway of the connector is placed on the distal

side of the pier abutment, mesial movement seats the

key into the keyway more solidly.

• Placement of the keyway on the mesial side, however,

causes the key to be unseated during its mesial

movements.

Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J

AM Dent Ass 1973;87:1195-99

45

Dovetail

• It is necessary to align the path of insertion of the

keyway with that of the distal abutment.

• This technique is best suited for relieving stress at

midspan on long pontics.

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Procedure:

• The wax pattern for the retainer on the pier abutment is fabricated on the working cast.

• A deep box form is carved into the distal surface of the wax pattern to create space for the placement of the plastic keyway pattern.

• Place the working cast, with the wax pattern seated, on the table of a surveyor.

• Assemble the key and keyway portions of the connector, and lock the mandrel that extends from the top of the key portion of the pattern into the vertical spindle of the surveying instrument.

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• Manipulate the surveyor table until the mandrel and attachments are parallel with the path of insertion of the distal preparation.

• Then lower the plastic pattern to the middle retainer wax pattern and lute it in place with sticky wax

• Remove the key portion and complete the middle retainer wax pattern by blending the distal surlace with the keyway.

• The pattern is then invested, burned out, and cast.

• After the casting has been cleaned and air abraded, carefully cut off any part of the keyway portion of the attachment that protrudes above the occlusal surface.

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• Place the casting on the working cast, and place the prefabricated plastic pattern for the key into the keyway.

• At this point the pontic wax pattern is attached to the plastic key.

• The pontic pattern is completed, removed from the working cast, invested, burned out, and cast.

• After the casting is recovered from the investment, the mandrel and any excess on the top portion of the key are carefully reduced so the key and keyway are flush.

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50

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Split Pontic

• It is particularly useful in tilted abutment cases

• The wax pattern for the anterior three-unit segment

(mesial retainer-pontic-pier retainer) is fabricated first,

with a distal arm attached to the pier retainer. The

underside of the arm is shaped like the tissue-contacting

area of a pontic.

• A surveyor is used to position either the key or the

keyway segment

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Procedure:

• Invest, burn out, and cast the mesial three-and-a-halfunit segment.

• After preliminary finishing, seat the cast segment on the working cast. Place the plastic pattern down into it (if the keyway is in the casting), or down onto it (if the key was left facing upward on the pontic base).

• Wax the distal retainer and the disto-occlusal two-thirds of the ponticpattern.

• The pontic can be metal-ceramic, but there should be a thin collar of metal around the periphery of the ceramic section.

• Try it on the prepared teeth in the mouth, making adjustments as necessary.

• Cement the mesial segment first, followed immediately by the distal segment.

• No cement should be placed between the two segments of the pontic.

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54

Cross-pin and Wing

• The cross-pin and wing are the working elements of a

two-piece pontic system that allows the two segments to

be rigidly fixed after the retainers have been cemented

on their respective abutment preparations.

• Accommodating abutment teeth with disparate long

axes. The path of insertion of each tooth preparation is

made to parallel the long axis of that tooth.

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• Attach a vertical wing, cut out of a piece of baseplate wax, to the mesial surface of the distal retainer wax pattern.

• The wing should parallel the path of insertion of the mesial abutment preparation, extend out 3.0 mm mesially from the distal retainer, have a 1.0-mm thickness faciolingually, be 1 0 mm short of the occlusal surface, and have an undersurface that follows the intended contour of the underside of the pontic.

• Invest, burn out, and cast the distal retainer, with wing.

• Seat the retainer on the cast, and drill a 0.7-mm hole through the wing with a twist drill in a handpiece.

• Place a 0 7-mm-diameter pencil lead through the hole and build the wax pattern around the lead and the wing. Remove the lead, withdraw the retainer-pontic wax pattern, and replace the 0.7-mm lead in the hole in the pontic pattern to maintain the patency of the hole during investing and casting

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• Assemble the two parts of the fixed partial denture on the working cast. Use a tapered 8/0 machinist reamer to ream a smooth, tapered hole through the pontic and wing, following the pilot hole produced by the 0.7-mm pencil lead.

• Fabricate a pin of the same alloy used for the fixed partial denture casting. A mold can be made by drilling a hole in a piece of aluminum with the machinist reamer and filling the hole with autopolymerizing resin.

• An impression of the reamer can be made with polyvinyl siloxane impression material and filled with resin or molten wax. Invest, burn out, and cast it It must be long enough to extend all the way through the pontic-wing assembly. Try the pin for fit in the components on the cast.

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• Cement the retainer with the wing first, followed by the retainer-pontic segment . Seat the pin in the hole with a punch and mallet.

• Remove excess length from the pin both facially and lingually.

• If it is ever necessary to remove part of this fixed partial denture, the pin can be tapped out and the parts dealt with separately.

• This technique requires no special patterns and does allow for a completely rigid prosthesis when completed.

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59

Tooth – Implant supported

FPD

• Implant supported fixed dental prosthesis has been

proven as an efficient modality of treatment.

However, tooth and implant…?

60

Advantages:

• splinting of a natural tooth to an implant

• Increased mechanoreception

• Additional support for the total load on the dentition.

• Connecting teeth with implant broadens treatment possibilities for the restorative dentist Reduces the cost for teeth replacement and Avoids the use of cantilevers.sadvantages:

• Higher need for maintenance and repair with such conncections.

61

Problem:

• The natural tooth and the osseointegrated implants have dissimilar mobility patterns and this may subject the implant to excessive stresses.

• Numerous studies have reported pronounced marginal bone loss or failure of implant to osseointegrate . This led to the controversy of whether connecting implant to the natural teeth is a viable option.

• Various complications like, intrusion of the teeth,mechanicalfailure, caries and loss of occlusal contacts have been reported in the literature associated with this treatment approach.

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Mismatch in tooth and implant movement:

• The natural teeth are attached to the alveolar bone by means of periodontal ligament fibers; whereas osseointegrated implant is rigidly anchored

• The tooth exhibits normal physiological movement in vertical, horizontal and rotational direction to the bone

• Osseointegrated implants exhibit only linear movement during the entire loading cycle in proportion to the applied load without initial rapid movement due to lack of periodontal ligament – viscoelastic nature

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• A healthy natural tooth can move 200 μ in response

to a 0.1 N force while an implant can be displaced 10

μ or less.

• The ratio of the amount of movement of the tooth in a

healthy periodontium to that of an implant has been

estimated to be 10:1 and 100:1

64

• Physiologic movement of the natural tooth causes the

prosthesis to act as a cantilever generating maximum

resultant load up to two times the applied load on the

implant.

• Implant would receive higher amount of loads in function

and could lead to potential complications.

65

• Types of connection

1. Rigid connection: The tooth is rigidly connected to the implant with a fixed dental prosthesis.

2. Non rigid connection: The tooth is non-rigidlyconnected to the implant by means of precision attachments, non-precision attachments and telescopic restorations. It acts as a stress breaking element.

3. Resilient connection: It incorporates a flexible component that simulates the periodontal ligament. It acts as a stress absorbing element.

66

• Different types of non rigid connectors are described

with most common being key and key way.

• The placement of the key way on the natural teeth

seems to be beneficial as it would allow for

physiological tooth movement under function.

67

• Biomechanical studies demonstrate that a shift of force

distribution from the superstructure to the supporting

teeth occurs

• when non-rigid connectors are used and tooth intrusion

was considered as potential complication of non-rigid

connection with frequent emergency appointments.

• Non-rigid connectors should be used with caution as it

increases unfavorable stresses on the abutment.

68

• Becker et al., suggested to splint implant to two teeth

when non-rigid connectors are considered.

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Complications associated with tooth implantsupported prosthesis

Biologic complications:

• Gradual bone resorption around the implant neck

• Bone fracture

• Loss of osseointegration

• Peri-implantitis

• Endodontic problems - caries after cement dissolution

• Root fracture.

70

Technical complications:

• Mechanical damage to the teeth or implant and includes fatigue

• Induced implant fracture

• Fracture of abutment screw

• Loosening of abutment screw

• Loss of prosthesis cement bond to tooth or abutment

• Abutment fracture

• Teeth or root fracture

• Tooth intrusion

• Fatigue induced prosthesis fracture.

71

Guidelines

Guideline 1: Splint implants to natural teeth only when

the teeth need support: Teeth do not stabilize

implants.

Guideline 2: Do not end the fixed prosthesis on the

weakest splinted abutment.

Guideline 3: Regardless of the connection teeth must

be cemented using definitive cement

Guideline 4: For a natural pier abutment between two

implants a stress breaker is not indicated

Guideline 5: Design of the prosthesis should allow

minimal movement in a buccolingual directionShenoy V.K, Rodrigue S.J, Prashanti . E, saldanha S.J.R. Tooth Implant supported

Prosthesis: A Literature review. J Inter Discip Dent 2013;3:143-150

72

• Connecting implant to natural teeth is accompanied by

various adverse sequelae.

• It is paramount to formulate a treatment plan for

predictable treatment outcome.

• A risk benefit analysis and anticipated complications

should be presented to the patient and appropriate

consent obtained before the treatment plan is finalized.

• The main focus should be to reduce the risk of intrusion

of the tooth and of overloading the implant.

Shenoy V.K, Rodrigue S.J, Prashanti . E, saldanha S.J.R. Tooth Implant

supported Prosthesis: A Literature review. J Inter Discip Dent 2013;3:143-15073

• Mc Leod N.S (1977) made a theoretical analysis of the

mechanics of the Thompson dowel semiprecision

intracoronal retainer. The analysis locates the center of

rotation during function and identifies the factors that

affect its position. The degree to which the dowel should

be relieved to permit unrestricted rotation has been

established

75

• Arunkumar G et al (2011) Three-dimensional finite

element analysis of the stress distribution around the

implant and tooth in tooth implant-supported fixed

prosthesis designs in order to suggest a design, which

transmits less stress to the bone.

• From the study, it could be suggested that if natural teeth

and implants are used as support for fixed prosthesis,

the NRC should be placed on the implantsupported site

to reduce the load on the implant and natural teeth.

76

Conclusion

• Stress breakers may not be in regular use. However,

mandatory usage is needed in specific conditions.

• The patient should be educated about the maintenance

• Regardless of design, most stress breakers effectively

dissipate vertical forces which is the purpose for which

they are used

77

References

1. Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s

Removable Partial Prosthodontics. 11th ed, Elsevier

publications, Mosby Company, Delhi. P.25

2. Shenoy V.K, Rodrigue S.J, Prashanti . E, saldanha

S.J.R. Tooth Implant supported Prosthesis: A Literature

review. J Inter Discip Dent 2013;3:143-150

3. Shillingberg H.t. Fundamentals of fixed Prosthodontics ,

3rd ed. Quintessence Books, India.

4. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s

Clinical Removable Partial Prosthodontics, 3rd ed.

Quintessence books, India. 78

5. Akulwar R. F, Kodgi A. Non rigid connector in

managing pier abutment in FPD. J Cli Diag Res

2014;8:12-14

6. Bevilacqua M et al. The influence of cantilever length

and implant inclination on stress distribution in maxillary

implantsupported fixed dentures. J Prosthet Dent

2010;105: 5-13

7. Steffel V.L, Columbus, Ohio. Fundamental Principles

involved in partial denture design. J Am Dent Ass

1951;42:534-545.

79

8. Cecconi B.T, Kaiser G, Rahe A.L. Stress breakers and

the removable partial denture. J Prosthet Dent

1975;34:147-51

9. Burns D.R,Ward J.E. A review of attachments for

removable partial denture design: part 2. Treatment

planning and attachment selection. Int J Prosthodont

1990;3:169-74.

80

Education

stress breakers in prosthodontics