problems associated with distal extension partial dentures

2. CONTENTS• TERMINOLOGY
• INTRODUCTION
• LEVER SYSTEMS
• PLANES AND AXES
• POSSIBLE ROTATIONAL MOVEMENTS IN RPD
• APPLICATIONS OF LEVER SYSTEMS IN RPD
• BIOMECHANICAL PROBLEMS WITH DISTAL EXTENSION RPD
• MANAGEMENT OF STRESS IN DISTAL EXTENSION PARTIAL DENTURE
• STRESS BREAKERS IN DISTAL EXTENSIONS RPD
• DESIGN PHILOSOPHIES IN DISTAL EXTENSION PARTIAL DENTURE
• LITERATURE REVIEW
• CONCLUSION
• REFERENCES
2

3. TERMINOLOGY:
• BIOMECHANICS: (GPT 9)
1. The application of mechanical laws to living structures, specifically the locomotor systems
of the body.
2. The study of biology from the functional viewpoint.
3. An application of the principles of engineering design as implemented in living organisms
• DISTAL EXTENSION PARTIAL DENTURE: A removable partial denture replacing the distal
most tooth or teeth on each side of one arch of the mouth.
• SADDLE: Denture base.
3

4. • LEVER: A bar or handle that moves around a fixed point, so that one end of it can be
pushed or pulled in order to control the operation of a machine or move a heavy or stiff
object.
• FULCRUM: The point at which a bar or something that is balancing is supported or
balanced.
4

5. • FULCRUM LINE:
1. An imaginary line, connecting occlusal rests, around which a removable partial denture
tends to rotate under masticatory forces; the determinants for the fulcrum line are
usually the cross-arch occlusal rests on the most distally located abutments. – GPT 9.
2. An imaginary line, connecting occlusal rests or any other rigid portion of a direct
retainer assembly located occlusally or incisally to the height of contour of the primary
abutments around which a partial removable dental prosthesis tends to rotate under
masticatory forces. - MC CRACKEN.
5

6. Introduction
• A removable partial denture that is supported by healthy natural teeth possesses
adequate stability and retention to resist functional displacement. However, a removable
partial denture that is not entirely bounded by natural teeth will move when a load is
applied.
• Such a free end saddle denture base will tend to rotate toward or away from the residual
ridge when subjected to occlusal forces or the pull of sticky foods.
• Hence, a distal extension partial denture should have some additional requirements for it
to be biomechanically stable.
6

7. LEVER SYSTEMS
7

8. 8
Class I lever
Mechanical advantage = effort arm
resistance arm

9. 9
Class II Lever

10. 10
Class III Lever

11. 11

12. KINESIOLOGY
12

13. • PURPOSEFUL MOVEMENT BY A HUMAN BEING IS AN INDEX TO THE STATE OF
HEALTH OF HIS BODY.
• THE STUDY OF SUCH MOTION IN HUMAN BODY – KINESIOLOGY
• IN ESSENCE, IT IS THE APPLICATION OF MECHANICAL PRINCIPLES TO THE BIOLOGIC
ACTIVITY AS REPRESENTED BY MUSCLE ACTIVITY.
Nagle R.J, sears V.H. Dental prosthetics.Complete dentures 1958,
the C.V mosby company, USA. P.111-122
13

14. 14
•A typical example of the first class lever
in the human body is the head as it rests
upon the vertebral column.
•Fulcrum – cervical spine axis
•Power/load – powerful postcervicsl
musculature to maintain erect posture
•Effort / work – energy required to
overcome the force of gravity which
could tend to drop the head and face
structures forward.

15. 15
The fulcrum – toes
The power arm – the heels where
the gastrocnemius muscle is
inserted
Work performed – elevation of
body weight which rests upon
bones of legs.
The forrce arm is longer than the
work producing arm.
Thus, the work produced is the
result of greater efficiency.

16. 16
Flexion of elbow in picking up
a small weight.
Fulcrum – elbow
Force – the biceps muscle
attached close to the fulcrum in
the forearm
Work done – elevation of ball
in the palm of the hand

17. 17
Lever system in mastication – 3rd
class
Fulcrum – TMJ
Force – center of action of masseter
muscle
Work – differenr positions of
opposing occlusal surfaces
This arrangement makes
mastication a very inefficient system

18. 18
When the bolus is placed more distally,
the efficiency increases as the work
arm decreases.
Third molar mastication is supposed to
be the most efficient.
Anatomically, however, the first molar is
apparently best designed to be more
efficient and clinically it is observed that
patients generate most work in first and
second molar regions rather than third
molar region

19. 19
Second class lever system in
mastication.
Fulcrum – contralateral TMJ
Force and work – same side

20. `
20
Second and third class lever
systems in action during
mastication

21. PLANES AND AXES
21

22. Possible rotational movements
F. H. Balkwill clearly described the rotational movement occurring within a prosthesis
supported by natural teeth and soft tissues.
1. Rotation around the transverse axis – sagittal plane
2. Rotation around a longitudinal axis formed by the crest of the ridge – frontal plane
3. Rotation around a vertical axis located near the center of the arch – horizontal plane
The movements are dependent on the quality of supporting structures, accuracy and
denture base and the magnitude, direction, duration and frequency of the functional
Carr A.B, Brown D.T. Mc cracken’s removable partial
prosthodontics 2012, 12th ed, Elsevier publications, Mosby
company, India. P.25
Phoenix R.D, Cagna D. R, Defreest ch.F. Stewart’s clinical
removable partial prosthodontics.3rd ed. Quintessence books.
22

23. ROTATION IN SAGITTAL PLANE
• On the application of functional load, the rotation of the distal extension partial denture occurs
the line joining the occlusal rests which is called the axis of rotation or the fulcrum line.
• The amount of rotation of the denture base depends on the resiliency of the mucosa covering
residual alveolar ridge and the accuracy of the adaptation of the denture base.
• Movement of the denture base in opposite direction is resisted by the action of the retentive
of the terminal abutment and the indirect retainer.
23

24. ROTATION IN FRONTAL PLANE
• Fulcrum line extends posteriorly distal to the terminal abutment.
• It passes along the crest of the ridge to its posterior extent on the same side.
• Mediolateral rotational movement of the denture occurs relative to the edentulous alveolar process.
• In class I situation, there are two fulcrum lines around which lateral movement of the partial denture can occur.
• The anatomy of the residual ridge will play a significant role in resisting lateral movement of the denture base.
• Flat ridge with movable submucosa will offer less resistance to lateral movement as compared to well-formed
firmly bound mucosa.
• Lateral loads are also exerted on the denture by the adjacent facial and lingual musculature during
24

25. ROTATION IN HORIZONTAL PLANE
• A vertical fulcrum line is located near the midline lingual to the anterior teeth. Rotational movement
of the denture can occur around the fulcrum line in the horizontal plane. Direct mediolateral
movement of the denture occurs relative to the edentulous ridge
• This rotation is resisted by the stabilizing component of the partial denture such as the reciprocal
the minor connectors.
• Stability component on one side of the arch acts to stabilize the partial denture against the
applied on the opposite side.
• To minimize the movement, the arms of the three-arm clasp should brace the tooth completely on
and lingual surfaces.
• The minor connector should be made rigid.
25

26. APPLICATION OF LEVER
SYSTEMS IN RPD
26

27. • Class I lever
When an occlusal load is applied to a distal extension removable partial denture, the prosthesis
rotates around a fulcrum line that passes through the most posterior rests—one on each side of
the dental arch.
Displacement of the prosthesis is limited by the hard and soft tissues of the residual ridge.
Therefore, optimal resistance to displacement may be provided by broad and accurate adaptation
of the denture base(s) to the supporting tissues.
27

28. • The fulcrum (F) established at the retentive clasp terminus when a dislodging force (L) acts on
the extension base.
Phoenix R.D, cagna D. R, defreest ch.F. Stewart’s clinical removable
partial prosthodontics.3rd ed. Quintessence book 28
Power may be applied to a first-class lever in both inferior and superior directions. (a)
Circumstances tending to unseat the extension base provide superiorly directed power, or load
(L), to the lever. Functioning through a fulcrum (F) near the terminus of the direct retainer,
aspects of the denture anterior to the fulcrum move inferiorly (arrow). (b) To resist this
potentially damaging movement, appropriate design principles require an indirect retainer to be
positioned anterior to the direct retainer. In this configuration, the indirect retainer serves as a
fulcrum for the lever system, limiting inferior displacement of the lingual bar.

29. • CLASS II LEVER
29

30. • CLASS III LEVER
30

31. • DISTAL EXTENSION RPD – KENNEDY’S CLASS I AND II – CLASS I LEVER OR CLASS II
LEVER
31
R
F
L
R
F
L
L R F

32. • DEVAN determined that the mucoperiosteum of the residual ridge offers only 0.4% of
the support provided by a periodontal ligament.' In other words, soft tissues are 250
times more displaceable than are the adjacent teeth. This is particularly important
when one considers the masticatory forces placed on an extension base during
function.
• Disadvanatge of distal extension retention is that a lifting force acting at the distal end
of the base can only be resisted by the clasp on the same side.
• This cannot completely prevent any lift occuring, as it will have to move up a little on its
tooth until its retentive arm starts to act
• If the lifting force is more distal, greater rotation occurs.
• A well planned self reciprocating clasp
Phoenix R.D, Cagna D. R, Defreest ch.F. Stewart’s clinical
removable partial prosthodontics.3rd ed. Quintessence
books. P.105
32
Biomechanical problems in
distal extension partial
dentures

33. 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.
33

34. 34
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

35. 35
Factors which influence the magnitude
of stress transmitted to the abutment
tooth
The quality of the
undergirding
The form of
the residual
ridge
The type of
the mucosal
covering
The length
of the span

36. 36
Flexibility
of clasp
Design of
clasp
Material
Contact
with the
tooth
Type of
abutment
CLASP
‘Whiplash’- a fleeting , unreciprocated thrust
exerted by the retentive arm against the tooth each
time the prosthesis is placed and removed.
Clasp arm constructed in
chromium-based alloy
must be fabricated using
a smaller cross-
sectional diameter.

37. 37
Harmony
Opposing
occlusion
Area of the
base
occlusio
n

38. MEANS OF STRESS CONTROL
• RETENTION
• STRATEGIC CLASP POSITIONING
• CLASP DESIGN
• THE COMBINATION CLASP
• INDIRECT RETENTION
• IMPRESSION MAKING
• THE OCCLUSION
• THE DENTURE BASE
• THE MAJOR CONNECTOR
38

39. • RETENTION AS A MEANS OF STRESS CONTROL:
1. ADHESION AND COHESION: Although it is impossible to develop a peripheral seal around the
borders of a removable partial denture, adhesion and cohesion can still contribute to retention. To
maximize this effect, each denture base must cover the maximum area of available support, and it
must be accuratel adapted to the underlying mucosa.
2. ATMOSPHERIC PRESSURE: Not a major factor. Possess a major role in maxillary partial as there
is a potential anti retentive gravitational force. The retentive potential by atmospheric pressure is
achieved by accurate seal of the peripheries of the dneture.
3. FRICTIONAL CONTACT: Guiding planes are prepared surfaces that are parallel to each other and
parallel to the path the denture takes as it is inserted and withdrawn from the mouth. The frictional
contact of the prosthesis against these parallel surfaces can contribute significantly to the retention
of the removable partial denture.
4. NEUROMUSCULAR CONTROL: The innate ability of the patient to control the actions of the lips,
cheeks, and tongue can be a major factor in the retention of a removable prosthesis. A patient who
lacks the ability or coordination to control the movement of these structures may not be able to
retain a prosthesis.
39

40. • STRATEGIC CLASP POSITIONING AS A MEANS OF STRESS CONTROL:
1. QUADRILATERAL
2. TRIPOD
3. BILATERAL
40

41. • CLASP DESIGN AS A MEANS OF STRESS CONTROL:
41

42. 42

43. 43

44. 44

45. • COMBINATION CLASP AS A MEANS OF STRESS CONTROL:
45
The combination clasp on the second
premolar consists of a flexible facial
wroughtwire clasp, a lingual cast reciprocal
clasp, a discoocclusal rest, and a distal
guide plate.

46. • INDIRECT RETENTION AS A MEANS OF STRESS CONTROL:
An indirect retainer is a component that helps resist rotation and/or displacement of a
removable partial denture. As a result, the indirect retainer is located on the side of the
fulcrum line opposite the denture base.
46

47. PRINCIPLE OF INDIRECT RETAINER
47

48. • The indirect retainer should be placed as far as possible from the distal extension base to afford the
best possible mechanical advantage
48

49. Davenport et al. Indirect retention. British dent J
2001;190:128-132 49

50. • FACTORS EFFECTING EFFECTIVENESS OF INDIRECT RETAINER:
1. Effectiveness of direct retainer
2. Distance from the fulcrum line
3. Location of the fulcrum line
4. Rigidity of the connectors supporting the IR
5. Effectiveness of the supporting tooth surface
50

51. • FORMS OF INDIRECT RETAINER:
1. Auxiliary occlusal rest
2. Canine extension from the occlusal rest
3. Canine rest
4. Continuous bar retainers and linguo-plates
5. Rugae support
6. Indirect retention from major connectors
7. Incisal rest
8. Double lingual bar or kennedy’s bar
9. The cummer arm
Osborne J, lammie G.A. Partial dentures 1985, 4th ed, CBS
publishers, new delhi. P. 231-240 51

52. • IMPRESSION MAKING AS A MEANS OF STRESS CONTROL:
1. Physiologic impression techniques: Record the ridge portion of the cast in its
functional form by placing an occlusal load on the impression tray during the
impression procedure.
i. The Mc Lean – Hindels
ii. The functional reline
iii. The fluid wax
2. Selected pressure impression techniques: Intended to equalize the support
between the abutments and the soft tissues and to direct forces to the portions of the
ridge that are most capable of withstanding such forces. This is accomplished by
relieving the tray in some areas while allowing the impression tray to contact the ridge
in other areas.
i. The corrected cast or altered cast technique.
52

53. • OCCLUSION AS A MEANS OF STRESS CONTROL:
A smoothly functioning occlusion that is in harmony with the movements of both the
temporomandibular joints and the neuromusculature will minimize the load transferred to
the teeth and soft tissues. The contacts of the remaining natural teeth must be the same
whether the removable partial denture is in the mouth or not.
1. Harmonious intercuspation
2. Size of food table
3. Occlusal pattern of the posterior teeth
53

54. • DENTURE BASE AS A MEANS OF STRESS CONTROL:
1. BROAD COVERAGE
2. LONG FLANGES
3. TUBEROSITY AND RETROMOLAR PAD COVERAGE
4. PROPER EXTENSIONS
5. ACCURATE ADAPTATION
6. CONTOUR OF THE POLISHED SURFACE
54

55. • MAJOR CONNECTORS AS A MEANS OF STRESS CONTROL:
55

56. NEED FOR STRESS BREAKERS:
• 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.
• 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.145
56

57. 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.
57

58. Types of stress breakers
TYPE 1
• Hinge
• Sleeve
• Cylinder
• Ball and socket
58

59. 59

60. Type 2: flexible conncection between direct retainer and denture base
• Wrought wire connectors
• Split major conncetors
• Hidden lock partial dentures
• Disjunct partial dentures
• Swinglock partial dentures
60

61. WROUGHT WIRE CONNECTOR:
• The 12 guaze chrome wire stress breaker:
61

62. 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.
62

63. 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.
63

64. 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.
64

65. 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.
65

66. 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 66

67. • 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.
67

68. Disjunct removable partial denture:
oTooth borne & mucosa-borne parts of denture are disjoined.
oTooth 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 68

69. 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.
69

70. 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
oPatient may complains of rattling of the framework during mastication.
70

71. swinglock removable partial denture:
71

72. 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 72

73. 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 73

74. Phoenix R.D, cagna D.R, defreest C. F. Stewart’s clinical removable
partial prosthodontics, 3rd ed. Quintessence books, india. Pp. 512 74

75. Phoenix R.D, cagna D.R, defreest C. F. Stewart’s clinical removable partial
prosthodontics, 3rd ed. Quintessence books, india. Pp.512 75

76. 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 76

77. 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 77

78. •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 -
78

79. 79

80. 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.
80

81. 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.
81

82. 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
82

83. REVIEW OF LITERATURE
83

84. • Lee H. E et al in 2008 made a biomechanical analysis of distal extension removable
partial dentures with different retainers. They compared various retainer designs with
different design philosophies based on stress breakers and rigid support. They
Fabricated six types of retainers for the analysis: A wrought wire clasp; Rest - Proximal
plate - I-bar (RPI) system; Conical crown Telescopic retainer; Rest - Bi-proximal plate -
I-bar (RPPI) system; Ring clasp and Cast circumferential clasp. Within the limitations of
their experiment, different retainer designs had different effects on the abutment and
residual ridge. The retainers designed based on a rigid support philosophy concentrated
more stress at the abutment and generated less stress at the residual ridge than those
retainers designed with a flexible support.
84

85. CONCLUSION
85

86. REFERENCES
1. Phoenix R.D, cagna D. R, defreest ch.F. Stewart’s clinical removable partial prosthodontics.3rd ed.
Quintessence book
2. Carr A. B, mc givney G. P, brown D. T. Mc cracken’s removable partial prosthodontics. 11th ed,
elsevier publications, mosby company, delhi.
3. 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.
4. Cecconi B.T, kaiser G, rahe A.L. Stress breakers and the removable partial denture. J prosthet dent
1975;34:147-51
5. Geissler p. R, and watt d. M. Disjunct dentures for patients with teeth of poor prognosis. Dent pratt
1965;15:421-23
86

87. 6. Phoenix R.D, cagna D.R, defreest C. F. Stewart’s clinical removable partial prosthodontics, 3rd
ed. Quintessence books, india. Pp. 512.
7. Osborne j, lammie g.A. Partial dentures 1985, 4th ed, CBS publishers, new delhi. P.
231-240
8. Lee H E et al. Biomechanical analysis of distal extension removable partial dentures
with different retainers. J dent sci 2008;3:133-39
9. Nagle R.J, sears V.H. Dental prosthetics.Complete dentures 1958, the C.V mosby
company, USA. P.111-122
87