stress breakers in prosthodontics

1. Stress breakers in
prosthodontics
Presented by
Sirisha.G
II year PG student
Lenora Institute of Dental

2. 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

3. 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.
3

4. 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
4

5. 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

6. Applications
• Distal extension removable partial
dentures
• FPD with pier abutment
• FPD with tilted abutments
• Tooth – Implant supported prosthesis
6

7. 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

8. 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.
8

9. 9
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

10. 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

11. • 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

12. 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.
12

13. Types of stress breakers
TYPE 1
• Hinge
• Sleeve
• Cylinder
• Ball and socket
13

14. 14

15. TYPE 2: flexible conncection between direct
retainer and denture base
• wrought wire connectors
• split major conncetors
• Hidden lock partial dentures
• Disjunct partial dentures
15

16. Wrought wire connector:
• The 12 guaze chrome wire stress breaker:
16

17. 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.
17

18. 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.
18

19. 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

20. 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.
20

21. 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

22. • 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

23. 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
23

24. 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.
24

25. Advantages:
o Independent movement between the tooth supported
and tissue supported parts decreases the forces on
periodontally weakened remaining teeth.
Disadvantages :
o It is technically difficult to fabricate
o Patient may complains of rattling of the framework
during mastication.
25

26. 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
26

27. 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

28. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial
Prosthodontics, 3rd ed. Quintessence books, India. Pp. 512
28

29. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial
Prosthodontics, 3rd ed. Quintessence books, India. pp.512
29

30. 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
Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial
Prosthodontics, 3rd ed. Quintessence books, India. pp.233-240
30

31. 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.
Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial
Prosthodontics, 3rd ed. Quintessence books, India. pp.233-240
31

32. • 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 -
32

33. 33

34. Advantages Disadvantages
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.
34

35. 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
Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial
Prosthodontics, 3rd ed. Quintessence books, India. pp.233-240.
35

36. Advantages Disadvantages
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 inexpensive
36

37. Stress breakers in FPD
• FPD with pier and malaligned abutments
• The connection between the pontic and retainer / within
the pontic
• Non rigid connectors
37

38. Pier abutments
• An edentulous space on both sides of a
tooth, creates a lone freestanding pier
abutment
38

39. • 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
39

40. Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J
AM Dent Ass 1973;87:1195-99
40

41. 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
41

42. 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
42

43. • Size of the connectors
• Shape of the connectors
• Position of the connectors
43

44. • Key and keyway/ Dovetails
• Split pontics / tapered pins
• Cross pin and wing
44

45. 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

46. 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.
46

47. 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.
47

48. • 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.
48

49. • 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.
49

50. 50

51. 51

52. 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
52

53. 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 pontic
pattern.
• 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.
53

54. 54

55. 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.
55

56. • 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
56

57. • 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.
57

58. • 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.
58

59. 59

60. Tooth – Implant supported
FPD
• Implant supported fixed dental prosthesis has been
proven as an efficient modality of treatment.
However, tooth and implant…?
60

61. 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

62. 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,mechanical
failure, caries and loss of occlusal contacts have been
reported in the literature associated with this treatment
approach.
62

63. 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
63

64. • 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

65. • 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

66. • 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-rigidly
connected 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

67. • 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

68. • 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

69. • Becker et al., suggested to splint implant to two teeth
when non-rigid connectors are considered.
69

70. Complications associated with tooth implant
supported 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

71. 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

72. 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 direction
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
72

73. • 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;3:143-150
73

75. • 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

76. • 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

77. 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

78. 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

79. 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

80. 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