1. Retention in removable partial dentures is achieved through the use of direct and indirect retainers. Direct retainers make contact with the abutment tooth and include intracoronal and extracoronal attachments as well as retentive clasp assemblies. 2. Key factors in clasp design include providing adequate retention, support, stability, reciprocation, encirclement, and passivity. The flexibility, length, diameter, and material of the clasp arm all impact its retentiveness. Proper design of retentive terminals, rests, and reciprocal arms is also important. 3. Circumferential and bar-type clasps are two common extracoronal retainer designs. Circumferential

1. 1
BY:
DR. KUMARI KALPANA
PG 2ND YEAR
RETENTION AND
SUPPORT IN
REMOVABLE PARTIAL
DENTURE

2. CONTENTS
• Introduction.
• Definitions.
• Direct retainers
• Intracoronal
• Extra coronal
• Indirect retainers
• Aesthetic clasps
• Support in removable partial dentures
• References
2

3. INTRODUCTION
• Retention of a removable prosthesis is a unique concern
when compared with other prosthesis. Forces acting to
displace the prosthesis from the tissue can consist of gravity
acting against a maxillary prosthesis, the action of adherent
foods acting to displace the prosthesis on opening of the
mouth in chewing, or functional forces acting across a
fulcrum to unseat the prosthesis.
3

4. DEFINTIONS
4

5. • RETENTION that quality inherent in the dental prosthesis acting to resist the
forces of dislodgment along the path of placement.
• RETAINER any type of device used for the stabilization or retention of a
prosthesis.
• SUPPORT the foundation area on which a dental prosthesis rests; with
respect to dental prostheses, the resistance to forces directed toward the basal
tissue or underlying structures.
1. The Glossary of Prosthodontic Terms. J Prosthet Dent 2017 ;117(5S):e1-e105.

6. DIRECT RETAINERS
6

7. • A DIRECT RETAINER is any unit of a removable dental prosthesis that
engages an abutment tooth or implant to resist displacement of the
prosthesis away from basal seat tissue.

8. • DIRECT RETAINERS- That component of the partial removable
dental prosthesis used to retain and prevent the dislodgement,
consisting of clasp assembly or precision attachment. (GPT-9)
8

9. DIRECT
RETAINERS
INTRACORONAL
PRECISON
ATTACHMENTS
SEMIPRECISION
ATTACHMENTS
EXTRACORONAL
RETENTIVE
CLASP
ASSEMBLIES
SUPRABULGE INFRABULGE
ATTACHMENTS
9
2. Stewart, Rudd and Kurbker: Clinical Removable Partial Prosthodontics; 2nd ed., Euro America Inc, Publishers Tokyo,1997

10. • Intracoronal retainers: Attachments which lie within the anatomical
contour of the abutment tooth are called intracoronal attachments.
10
Keyways are contained within abutment crowns and keys are attached to
removable partial denture framework.

11. Intracoronal retainers
• Proposed by Dr. Herman Chayes in 1906.
• Attachments which lie within the anatomical contour of
the abutment tooth.
• composed of a prefabricated machined key and keyway,
with opposing vertical parallel walls.
• Retention is achieved by: frictional resistance
11
Precision attachment: manufactured by high precision techniques
and instruments.
Semi-Precision attachment: less intimate contact between matrix
and matrix component.
5/16/2021

12. Extra coronal Retainers
• Consist of components that reside entirely outside the normal
clinical contours of abutment teeth.
• They serve to retain and stabilize removable partial dentures
when dislodging forces are encountered.
• Retention form: Mechanical resistance
12
5/16/2021

13. Prothero in 1916, gave “ cone theory” of clinical crown anatomy
and provided a Conceptual basis for mechanical retention.
13
5/16/2021

14. Extracoronal Retainers
Extracoronal direct retainers consist of components that reside
entirely outside the normal clinical contours of abutment teeth.
They serve to retain and stabilize removable partial dentures when
dislodging forces are encountered. Extracoronal direct retainers
may be divided into two distinct subcategories: extracoronal
attachments and retentive clasp assemblies.
14

15. Clasp Assembly
The part of a removable dental prosthesis that acts as a
direct retainer and/or stabilizer for a prosthesis by
partially encompassing or contacting an abutment tooth.
Components of the clasp assembly include the clasp, the
reciprocal element, the cingulum, incisal or occlusal rest,
and the minor connector.
15

16. • The extra coronal retainer operates on the principle of the resistance
of the metal to deformation.
• It is of the following principal forms:-
• Clasp type retainer:
• There are two basic categories of clasps:
• Circumferential, or Akers clasps and
• Vertical projection, or bar or Roach clasps.
16

17. • Clasps mainly divided 2 types
• Occlusally approaching which approach the undercut from
the occlusal area and gingivally approaching which enter the
undercut crossing the gingival margin.
17

18. Circumferential, or Akers, clasp/retainer
18

19. Circumferential, or Akers, clasp/retainer
• It was introduced by Dr. N. B. Nesbitt in 1916.
• Design of choice for tooth- supported
removable partial denture because of its
excellent support, bracing and retentive
properties.
5/16/2021
19

20. Extracoronal bar type direct retainer / Vertical projection, or
bar or Roach, clasps:
• Assembly consists of
• A buccal retentive arm engaging measured undercut (with slight occlusal
extension for stabilization);
• Stabilizing (reciprocal) elements; proximal plate minor connector on distal;
• Lingually placed mesial minor connector for occlusal rest, which also
serves as a stabilizing (reciprocal) component;
• Mesially placed supporting occlusal rest.
• Assembly remains passive until activated.
20

21. 21

22. Parts of clasp assembly
Circumferential
Clasp
(Retentive Arm)
Reciprocating
(Bracing) Arm
Distal
Occlusal
Rest Seat Proximal
Plate
22

23. Components of clasp assembly
• Rest: Part of the clasp that lies
on the occlusal, lingual, or
incisal edge or surfaces of a
tooth and resists tissue ward
movement of the clasp by
ensuring that the retentive
terminal of the clasp remains
fixed in the desired, or
planned depth, of undercut.
23

24. • Body: Part of the clasp that
connects the rest and shoulders
of the clasp to the minor
connectors. It, like all
components, except the
retentive terminal, must be rigid
and must lie above the height of
contour.
24

25. • Shoulder: Part of the clasp that connects the body to the
clasp terminals. The shoulder must lie above the height of
contour and provide some stabilization against horizontal
displacement of the prosthesis.
25

26. • Reciprocal Arm: A rigid clasp arm
placed above the height of contour
on the side of the tooth opposing
the retentive clasp arm.
• Reciprocal arm also helps stabilize
the partial denture against lateral
movement.
26

27. • As the retentive terminal passes over the greatest bulge of the tooth, the
metal must deform.
• This deformation generates a positive lateral force against the tooth.
• If the tooth were not supported against this destructive lateral force,
damage to the supporting periodontal ligament and bone could occur.
• The position of the reciprocal arm in relation to the retentive arm is
critical.
• It must be designed to contact the tooth before the retentive clasp does,
and to remain in contact while the retentive terminal passes the height of
contour.
27

28. • Retentive Clasp Arm: Part of the
clasp comprising the shoulder,
which is not flexible, is located
above the height of contour, and
the retentive terminal.
28

29. 29

30. • Retentive Terminal: The distal third
of the clasp arm. It is the only
component of the removable partial
denture to lie on the tooth surface
apical to the height of contour. It is
this position of the flexible terminal
in the undercut that provides the
direct retention.
30

31. 31
Recontouring of enamel
surfaces can effectively
change height of contour. In
this case height of contour is
being lowered to allow
placement of a reciprocal
clasp arm in a more
favorable position.

32. •Minor Connector: Part of the
clasp that joins the body of the
clasp to the remainder of the
framework.
•It must always be rigid.
32

33. • Approach Arm: Component of bar,
or vertical projection clasps
• The approach arm is a minor
connector that projects from the
framework, runs along the mucosa,
and turns to cross the gingival
margin of the abutment tooth.
33

34. • Retentive terminal is portion
of vertical projection clasp
positioned below survey line.
34

35. REQUIREMENTS OF
CLASP ASSEMBLY
35

36. • All clasps must be designed so that they satisfy the following six
basic requirements:
• 1. Retention
• 2. Support
• 3. Stability
• 4. Reciprocation
• 5. Encirclement
• 6. Passivity
36

37. Retention of the prosthesis
• Clasp retention is based on the resistance to deformation of the
metal. For a clasp to be retentive, it must be placed in an undercut
area of the tooth where it is forced to deform upon application of a
vertical dislodging force. It is this resistance to deformation along an
appropriately selected path that generates retention.
37

38. These include:
•Tooth (planned and executed by the
clinician) and
•Prosthesis (to be planned by the dentist
and executed by the laboratory technician)
factors.
38

39. •The angle of cervical
convergence (depth of
undercut) and
•How far the clasp terminal
is placed into the angle of
cervical convergence.
Tooth
factors
include:
39

40. • Size and distance into the angle of cervical convergence
• Angle of cervical convergence: The angle formed by the tooth
surface below the height of contour with vertical plane, when the
occlusal surface is oriented parallel to the horizontal plane.
40

41. • Relationship of height of contour, suprabulge, and infrabulge:
41

42. 42

43. 43

44. How far the clasp terminal is placed into the angle of cervical
convergence??
44

45. • Prosthesis factors:
Flexibility of the clasp arm
• Whatever type of clasp is used a denture will be retained successfully
only as long as the force required to flex the clasps over the
maximum bulbosities of the teeth is greater than the force which is
attempting to dislodge the denture.
• The retentive force is dictated by tooth shape and by clasp design.
45

46. • Flexibility of the clasp arm:- Clasp flexibility is the product of clasp
length (measured from its point of origin to its terminal end),
• Flexibility is directly proportional to the cube of its length
46

47. • Cross section: round > half round
• Modulus of elasticity: more the modulus - less flexibility
• Diameter of clasp: flexure inversely proportional to the
diameter.
• Alloy: wrought > cast
47

48. 48

49. 49

50. The flexibility of a clasp is dependent on its design:
• The longer the clasp arm the more flexible it will be, all other
factors being equal. The length of a circumferential clasp arm is
measured from the point at which a uniform taper begins.
• Flexture is directly proportional to the cube of length.
50

51. 51

52. 52

53. Permissible Flexibilities of Retentive Cast circumferential and bar-type arms
of type IV gold alloys
Circumferential Bar-type
Length(inches) Flexibility(inches) Length(inches) Flexibility(inches)
0 to 0.3 0.01 0 to 0.7 0.01
0.3 to 0.6 0.02 0.7 to 0.9 0.02
0.6 to 0.8 0.03 0.9 to 1.0 0.03
53

54. Permissible Flexibilities of Retentive Cast circumferential
and bar-type arms for cobalt-chromium alloys
Circumferential Bar-type
Length(inches) Flexibility(inches) Length(inches) Flexibility(inches)
0 to 0.3 0.004 0 to 0.7 0.004
0.3 to 0.6 0.008 0.7 to 0.9 0.008
0.6 to 0.8 0.012 0.9 to 1.0 0.012
54

55. • The greater the average diameter of a clasp arm the less flexible
it will be, all other factors being equal.
• A clasp should be half as thick at the tip as at the origin.
55

56. 56
Cross- section

57. 57

58. 58

59. • Flexibility is also dependent upon the alloy used. The most commonly used
alloy is cobalt chromium.
59

60. • SUPPORT –
• Support is the property of a clasp that resists
displacement of the clasp in a gingival
direction.
• The prime support units of a clasp are
occlusal, lingual, or incisal rests.
• Rests provide only vertical support.
• Other elements of the partial denture must
resist horizontal displacement.
60

61. 61

62. • RECIPROCATION –
• Each retentive clasp terminal must be
opposed by a reciprocal clasp arm or
another element of the partial denture
capable of resisting horizontal forces
exerted on the tooth by the retentive
arm.
• The reciprocal arm of the clasp is
positioned on the opposite side of the
tooth from the retentive arm.
• Because the reciprocal clasp must be
rigid, it must be positioned above the
height of contour. It should be placed as
close to the height of contour as possible,
no higher than the middle third of the
tooth and preferably at the junction of
the gingival and middle thirds. 62

63. • ENCIRCLEMENT –
• Each clasp must be designed to encircle more than 1800
(more than half the circumference) of the abutment tooth.
• Encirclement may be in the form of continuous contact,
(circumferential clasp) or broken contact (bar clasps).
• If broken encirclement is planned, the clasp assembly must
contact at least three different tooth areas (normally the
occlusal rest, the retentive terminal, and the reciprocal
terminal) that embrace more than half the tooth’s
circumference.

64. 64

65. 65

66. • PASSIVITY –
• A clasp in place should be completely passive.
• The retentive function is activated only when dislodging forces
are applied to the partial denture.
• If the clasp is not seated, the retentive terminal cannot reach
the depth of undercut it was planned to reach and therefore
always applies force to the tooth, producing pain.
66

67. Cast
circumferential
clasp:

68. 68

69. 69
Maximum retention occurs when a retentive clasp
tip is lifted to near the survey line.7
If contact between the proximal plate and the
guiding plane is lost before the retentive tip reaches
the survey line, retention decreases rapidly.8

70. Disadvantages of the cast circumferential clasp:
• More tooth surface is covered than with the bar clasp, which
may lead to decalcification of the enamel surface or caries.
• The circumferential clasp also changes the morphology of
the abutment crown. The normal buccolingual contour of
the tooth is altered, which may interfere with the normal
food flow pattern. This could lead to damage of the gingival
tissue because of a lack of physiologic stimulation of this
tissue.
70

71. 71
If these clasps are positioned
high on the tooth, they can
increase the width of the
food table, which in turn
causes greater occlusal force
to be exerted on the tooth.
Because of the half-round
configuration of the cast
circumferential clasp, it is not
possible to truly adjust the
clasp with pliers.

72. Rules for use:
• The retentive clasp arm should originate above
(occlusal to), and its terminal third should be positioned
below (gingival to) the height of contour.
• The retentive terminal should always point toward the
occlusal surface, never toward the gingiva. This helps
produce a curved clasp and results in greater flexibility.
72

73. •The retentive tip should terminate at the mesial or distal
line angle of the abutment tooth, never in the center of
the facial or lingual surface.
•The clasp arm should be kept as low on the tooth as
possible without violating its relation with the height of
contour.
• In this position it will have greater mechanical advantage
and esthetic result.
73

74. • Types of circumferential clasps:
• Simple Circlet Clasp –
• The simple circlet is the most versatile and widely used clasp.
• It is most often the clasp of choice on tooth-supported
removable partial dentures where the available retentive
undercut permits its use.
74

75. • This clasp usually
approaches the
undercut on the
abutment tooth from
the edentulous area
and engages the
undercut remote
from the edentulous
space.
• The limitations to the
use of the simple
circlet clasp are the
same as for all cast
circumferential
clasps.
75

76. • Reverse, or Reverse Approach Circlet Clasp –
• The reverse circlet clasp is used when the retentive undercut is
located on the surface of the abutment tooth adjacent to the
edentulous space.
• In a distal extension edentulous ridge partial denture the reverse
approach circlet clasp helps control stresses transmitted to the
terminal abutment tooth on the edentulous side.
76

77. 77

78. Disadvantages:
• Sufficient clearance with
opposing occlusion so that clasp
can have enough thickness to
maintain strength.
78

79. • An occlusal rest on the surface of the tooth away from
the edentulous space does not protect the marginal
gingiva adjacent to the abutment tooth.
• This marginal gingiva may, be traumatized if food packs
between the denture and the proximal surface of the
tooth.
79

80. • Reverse circlet clasp often
gives a poor esthetic result
with excessive display of
metal, particularly on
premolar.
80

81. 81
Multiple
circlet
clasp –
The multiple circlet clasp is essentially two
opposing simple circlet clasps joined at the
terminal end of the two reciprocal arms.
Its use is primarily in sharing the retention
responsibilities among several abutment
teeth on one side of the arch when a
principal abutment tooth has lost some of
its periodontal support.

82. 82
This may be considered a form of splinting of weakened teeth by
a removable partial denture.

83. • Embrasure Clasp, or Modified Crib Clasp –
• The embrasure clasp is essentially two
simple circlet clasps joined at the body.
• It is most frequently used on the side of the arch where there is no
edentulous space.
• The clasp must cross the marginal ridges of two teeth, emerge to cross
the facial surfaces of both the end, and engage undercuts on the
opposing line angles of these teeth.
83

84. • Occlusal rest
preparations must be
made on both teeth, and
tooth structure must be
removed from the buccal
inclines of both teeth to
provide space for
adequate thickness of
metal.
• Breakage of these
clasps in function is not
uncommon because of
insufficient tooth
preparation.
84

85. • Ring Clasp –
• The ring clasp is most often
indicated on tipped molars.
• Most unsupported mandibular
molars tend to drift and tip in a
mesiolingual direction, whereas
maxillary molars tip in a
mesiobuccal direction.
85

86. 86
• On a mandibular molar the
clasp encircles the tooth
beginning on the mesiobuccal
surface and terminating in an
infrabulge area on the
mesiolingual surface.
• On a maxillary molar the
direction of the clasps
reversed; it begins at the
mesiolingual surface and
terminates in an undercut on
the mesiobuccal surface.

87. •C, Fishhook or
Hairpin Clasp:
• The C clasp is essentially a
simple circlet clasp in which
the retentive arm, after
crossing the facial surface
of the tooth from its point
of origin, loops back in a
hairpin turns to engage a
proximal undercut below its
point of origin.
87

88. 88
The upper part of the retentive arm must be considered to be a
minor connector and should be rigid. The lower part of the clasp
arm should be tapered; it is the only flexible part of the clasp
arm.

89. • The crown of the abutment tooth must have sufficient occlusogingival
height to accommodate this double width of the clasp arm.
• The upper and lower arms of the retentive clasp must also be shaped
such that food debris will not be retained between them.
• There must be sufficient space between the arms so that the metal
may be adequately finished and polished.
89

90. Indication:
• C clasp is indicated for use when the retentive clasp must engage an
undercut adjacent to the occlusal rest or edentulous space and a soft
tissue undercut precludes the use of a bar clasp. (The approach arm of a
bar clasp must never cross a soft tissue undercut.)
• The C clasp is also indicated when the reverse circlet clasp cannot be
used because of lack of occlusal space.
90

91. Disadvantage:
• The clasp covers a considerable amount of tooth structure, which
may trap food debris. It is not a good choice for a young patient or
one who is prone to caries, and it is often unacceptable esthetically,
particularly on premolar teeth.
91

92. 92
• Onlay clasp - The onlay clasp is
an extended occlusal rest with
buccal and lingual clasp arms.
The clasp may originate from
any point on the onlay that will
not create occlusal
interferences.

93. 93
Onlay clasp may be indicated where occlusal surface of
one or more teeth is below occlusal plane.

94. 94
Onlay clasp covers a large amount of tooth
structure and may lead to breakdown of
enamel surfaces. Opposing enamel occlusal
surfaces may also be subjected to rapid wear.

95. • Combination Clasp:
• The combination clasp consists of a wrought round wire retentive
clasp arm and a cast reciprocal clasp arm.
• The cast reciprocal arm is normally a circumferential clasp, but a
bar clasp may be used.
• The wrought wire retentive arm is a circumferential clasp arm.
95

96. Indication:
• On an abutment tooth adjacent to a distal extension space when
the usable undercut on the tooth is on the mesiobuccal surface.
Note - The greater flexibility of the wrought wire acts as a stress
equalizer, preventing the undesirable forces created by the lever
action of the retentive clasp tip from lifting or torquing the
abutment tooth as downward forces occur on the denture base.
• Because of greater flexibility of wrought wire, a combination clasp
can be placed in a deeper undercut area. Usually clasp can be
positioned lower on crown of a tooth, in gingival third, resulting in
a more esthetic appearance.
96

97. 97
Disadavantage
• The main disadvantage of the combination clasp is that it does
require extra steps in laboratory fabrication.
• It is also more prone to breakage or damage than a cast clasp.
• It can be easily distorted by careless handling by patients

98. Bar, or Vertical Projection, Clasps
• The bar clasps approach the undercut or retentive area on the tooth
from a gingival direction, resulting in a “push” type of retention.
• This push retention of bar clasps is more effective than the “pull”
retention characteristic circumferential clasp.
98

99. 99
Vertical projection clasps approach retentive undercut from a
gingival direction. Entire approach arm must contact mucosa.

100. 100
Because of gingival approach to the retentive
undercut, bar clasp is said to have ‘push” type
retention.

101. Rules For Use:
• The approach arm of the bar clasp must not impinge on the soft
tissue it crosses. It is not desirable to provide an area of relief under
the arm, but the tissue side of the approach arm should be
smoothed and polished.
• The minor connector that attaches the occlusal rest to the
framework should be strong and rigid to provide some bracing.
• The approach arm must always be tapered uniformly from its
attachment at the framework to the clasp terminal.
101

102. • The approach arm must never be designed to bridge a soft tissue
undercut. Otherwise, food will be trapped, and the cheeks or lips will
be irritated.
• The approach arm should cross the gingival margin at a 90-degree
angle. This sensitive and critical area must be protected from
irritation by as little interference with normal function as possible.
102

103. 103
Retentive terminal of bar clasp, as tip of
circumferential clasp, should point toward
occlusal surface. Clasp must be kept as low on
tooth as possible.

104. Types of Bar Clasps:
• T Clasp:
• The T clasp is used most often in combination with a cast circumferential
reciprocal arm.
• The retentive terminal and its opposing encircling finger project laterally
from the approach arm to form a “T”.
• The approach arm should taper gradually and uniformly from its origin to
the retentive terminal.
• The approach arm contacts the tooth only at the height of contour.
104

105. 105

106. Indications:
• T Clasp, the most frequently used vertical projection group
clasp is most often used on terminal abutment tooth on distal
extension edentulous ridge.
• The T clasp can also be used for a tooth-supported partial
denture when the retentive undercut is located on the
abutment tooth adjacent to the edentulous space.
106

107. 107
Contraindications:
The T clasp should not be used on a terminal
abutment adjacent to a distal extension base if
the usable undercut is located on the side of
the tooth away from the edentulous space.
The T clasp can never be used if the approach
arm must bridge a soft tissue undercut. This
produces a food retention area as well as
potential irritation to the lips and cheeks.

108. 108
Clasp is indicated for premolars and canines for esthetic reasons.
Care must be taken to ensure 180-degree encirclement of
abutment.
Modified T Clasp:
The modified T clasp is essentially a T clasp with the nonretentive
(usually mesial) finger .

109. 109
Body or center of the Y clasp should be kept as low on tooth as
possible, and fingers should be permitted to rise as dictated by height of
contour.
Y Clasp:
is basically a T clasp; its configuration occurs when the height of contour
on the facial surface of the abutment tooth is high on the mesial and
distal line angles but low on the center of the facial surface.

110. I Clasp and I Bar:
• The I-clasp may occasionally be used on the distobuccal surface of
maxillary canines for esthetic reasons.
• There is a definite danger involved in using this clasp.
• Because the only contact of the retentive clasp with the abutment tooth
is the tip of the clasp, an area of 2-3 mm, encirclement and horizontal
stabilization may be compromised.
110

111. 111
The I clasp may be used on the distobuccal surface of
maxillary canines for esthetic reasons. There must be a
posterior abutment tooth for this clasp to be successful.
Encirclement may be a problem

112. RPI – SYSTEM :
Fulfills the requirement of proper clasp design &
minimises the stress on abutment tooth
112

113. The RPI system is a
combination of occlusal rest
(R) distal guide plate (P) and
gingivally approaching I bar
clasp (I) used primarily with
mandibular distal extension
saddles
113

114. 114

115. 115

116. Advantage of R P I clasp :
• On occlusal force the I bar move mesio –gingivally from the
tooth & proximal plate move further into the under cut of the
tooth
• I bar & proximal plate disengage the abutment & there by
reduce torquing of the tooth
• mesial minor connector together with proximal plate provides
necessary reciprocation
• mesial rest eliminates the potential pump hand effect
• has minimal contact – used on carious prone patient.
116

117. Contraindications :
• Shallow vestibule
• lingually tilted tooth
• labially/ bucally flared tooth
• soft tissue undercut
• high floor of the mouth in which
lingual plate is used
117

118. Modification in construction of RPI :
• RPA – system
• Two tooth splinted
•3 – unit fixed prosthesis
•Isolated tooth with no rest
118

119. RPA CLASP DESIGN

121. Kratochvil, F J, and Caputo, A A: Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J PROSTHET DENT 1974; 32:52.
• According to Kratochvil and Caputo, the physiologic adjustment of
extension-type removable partial denture frameworks in the mouth
alleviates adverse tipping forces on abutment teeth and directs the
forces within the long axis of the teeth.
• Thompson and associates analyzed seven commonly used removable
partial denture designs and found the mesial rest and I-bar retainer
exhibited the most favorable distribution of vertically applied forces..
121

122. 122
Aesthetic Clasp Design
Aesthetic clasp design for Removable partial dentures: A literature review SADJ June
2005;60(5):190 - 194

123. 123
Back-action clasp:- Owen reported its use on upper premolars. The clasp
arm bends backwards at the buccal bulge of the tooth to reach the distal
undercut, increasing its length and making it less obvious.

124. 124
Equipoise clasp - Goodman developed and described the equipoise
system, the action of which is based on the principles of the backaction
clasp. The equipoise clasp was developed claiming to address all the
requirements of a successful clasp as well as aesthetics and favorable load
distribution to the abutment. Clasp tips are placed in preparations in the
enamel of the proximal surfaces of the abutment teeth.

125. 125
Modified equipoise clasp The sound enamel preparations were deemed
destructive and a modification of the equipoise clasp was proposed by De
Kock and Thomas. They showed it to be a practical and viable option for
improved aesthetics and acceptable retention for Kennedy Class IV situations.

126. 126
Hidden clasp These clasps have been advocated for the Kennedy Class IV
situations. The design achieves its aesthetic qualities by engaging the
proximal undercuts often naturally present on teeth. Disadvantages:-
(a) complex designs,
(b) permanent deformation after repeated flexure,
(c) abutment displacement as no reciprocation is provided,
(d) rotation of the clasp if a restricted path of placement is not used with
resultant loss of retention,
(e) variable retention and,
(f) difficulty in cleaning

127. 127

128. 128
Flexible lingual clasp According to a clinical report by Pardo-Mindan
and Ruiz-Villandiego, a lingual clasp is indicated when the buccal arm is
not to be seen.
In this case a rigid clasp with increased flexibility and limited length
emerges from a mesial minor connector or proximal plate. With this clasp,
however, the abutment needs to be crowned. The rest seats are prepared
within the crown.

129. 129
Disadvantages include that of cost (due to crowns) and the
fact that its use is limited to the mandible only.

130. 130
RLS-system This is the acronym for mesio-occlusal rest,
distolingual bar and distobuccal stabilizer. It has been advocated for
distal extension RPDs when the RPI system cannot be used due to
lack of a buccal undercut, or when aesthetics would be severely
compromised.

131. 131
Twin-flex clasp or spring-clasp This is a flexible clasp utilizing mesial-distal
retention. It is adjustable and can be used with the normal conventional path of
insertion, with resultant improved aesthetics. It consists of a wire clasp soldered
into a channel that is cast in the major connector.
Disadvantages include
irreparability once fractured,
the major connector being very thick over the wire,
increased cost due to extra laboratory procedures, and
toxicity because of galvanic corrosion.

132. 132
Twin-flex improved clasp
The authors claim that as this clasp is not soldered onto the
framework, toxicity associated with galvanic corrosion is
eliminated. They further claim that the major connector is
not so thick, clasps are easily adjustable and replaceable and
it can be used on all RPD designs.

133. 133
Case Report

134. Use of a Polyetheretherketone Clasp Retainer for Removable Partial
Denture
134
14. Ichikawa T, Kurahashi K, Liu L, Matsuda T, Ishida Y. Use of a Polyetheretherketone Clasp Retainer for Removable Partial Denture : A Case Report. 2019;
• The working model was scanned with
dental scanner and
• Designed the clasp retainer was with CAD
software.
• Milling machine was used to shape the
clasp from PEEK disc.

135. 135
• PEEK clasp installed in denture base
14. Ichikawa T, Kurahashi K, Liu L, Matsuda T, Ishida Y. Use of a Polyetheretherketone Clasp Retainer for Removable Partial Denture : A Case Report. 2019;

136. 136
a) b)
c)
14. Ichikawa T, Kurahashi K, Liu L, Matsuda T, Ishida Y. Use of a Polyetheretherketone Clasp Retainer for Removable Partial Denture : A Case Report. 2019;

137. 137
Advantages Disadvantages
Aesthetic Difficulty of
polishing
Biocompatible Difficulty in
adjusting retention
capacity
Bacterial
Adherence is easily
overlooked
Preoperative
designing is
necessary

138. Indirect retainers
• When the distal extension denture base is
dislodged from its basal seat, it tends to rotate
around the fulcrum lines.
• Theoretically, this movement away from the
tissue can be resisted by the activation of the
direct retainer, the stabilizing components of
the clasp assembly, and the rigid components
of the partial denture framework that are
located on definite rests on the opposite side
of the fulcrum line away from the distal
extension base.
• These components are referred to as indirect
retainers.
138

139. • The indirect retainer components
should be placed as far as possible
from the distal extension base, which
provides the best leverage advantage
against dislodgment.
139

140. 140
Mandibular distal extension removable partial denture showing distal
extension base being lifted from the ridge, the clasp assembly being
activated and engaged, with the indirect retainer providing stabilization
against dislodgement. Lift of distal extension base is effectively controlled
by the indirect retainer when the direct retainer and proximal plate act to
maintain the clasp assembly in place during base movement away from
the supporting tissue.

141. 141

142. 142
Beams are supported
at various points
Lifting force will
displace entire beam in
absence of retainers

143. 143
With direct retainers (dr) at fulcrum, lifting force will depress
one end of beam and elevate other end

144. 144
With both direct and indirect retainers (ir) functioning, lifting force will
not displace beam. The farther the indirect retainer is from the
fulcrum, the more efficiently it should control movement.

145. FACTORS INFLUENCING EFFECTIVENESS OF INDIRECT RETAINERS2
The following factors influence the effectiveness of an
indirect retainer:
• The principal occlusal rests on the primary abutment teeth must be
reasonably held in their seats by the retentive arms of the direct retainers.
If rests are held in their seats, rotation about an axis should occur, which
activates the indirect retainers.
• If total displacement of the rests occurs, there would be no rotation about
the fulcrum, and the indirect retainers would not be activated.
145

146. • Distance from the fulcrum line. The following three areas must be
considered:
• Length of the distal extension base
• Location of the fulcrum line
• How far beyond the fulcrum line the indirect retainer is placed
• All connectors must be rigid if the indirect retainer is to function as
intended.
• The indirect retainer must be placed on a definite rest seat on
which slippage or tooth movement will not occur. Tooth inclines and
weak teeth should never be used to support indirect retainers.
146

147. 147
In Class I arch, fulcrum line passes through the most posterior
abutments, provided some rigid component of framework is
occlusal to abutment’s heights of contour
Fulcrum lines found in various types of partially edentulous arches,
around which denture is rotate when bases are subjected to force
directed toward or away from the residual ridge

148. 148
•In Class II arch, fulcrum line is diagonal, passing through abutment
on distal extension side and the most posterior abutment on opposite
side.
•If abutment tooth anterior to modification space lies far enough
removed from fulcrum line, it may be used effectively for support of
indirect retainer.

149. 149
•In Class III arch with posterior tooth on right side, which has a poor
prognosis and will eventually be lost, fulcrum line is considered the
same as though posterior tooth were not present. Thus its future loss
may not necessitate altering original design of the removable partial
denture framework.
•In Class Ill arch with nonsupporting anterior teeth, adjacent edentulous
area is considered to be tissue-supported end, with diagonal fulcrum
line passing through two principal abutments as in Class II arch.

150. 150
In Class IV arch, fulcrum line passes through two
abutments adjacent to single edentulous space.

151. AUXILIARY FUNCTIONS OF INDIRECT RETAINERS
An indirect retainer may serve the following auxiliary functions:
• It tends to reduce anteroposterior-tilting leverages on the principal
abutments. This is particularly important when an isolated tooth is
being used as an abutment.
• Contact of its minor connector with axial tooth surfaces aids in
stabilization against horizontal movement of the denture. Such
tooth surfaces, when made parallel to the path of placement, may
also act as auxiliary guiding planes.
151

152. • Anterior teeth supporting indirect retainers are stabilized against
lingual movement.
• It may act as an auxiliary rest to support a portion of the major
connector facilitating stress distribution.
• For example, a lingual bar may be supported against settling into the tissue by
the indirect retainer acting as an auxiliary rest.
• It may provide the first visual indications for the need to reline an
extension base partial denture. Deficiencies in basal seat support are
manifested by the dislodgment of indirect retainers from their
prepared rest seats when the denture base is depressed, and rotation
occurs around the fulcrum.
152

153. FORMS OF INDRECT RETAINERS2:
Auxiliary Occlusal Rest
• The most commonly used indirect retainer is
an auxiliary occlusal rest located on an
occlusal surface and as far away from the
distal extension base as possible.
• In a mandibular Class I arch, this location is
usually on the mesial marginal ridge of the
first premolar on each side of the arch.
• The ideal position for the indirect retainer
perpendicular to the fulcrum line would be in
the vicinity of the central incisors.
153

154. Canine Rests
• When the mesial marginal ridge of the first
premolar is too close to the fulcrum line or
when the teeth are overlapped so that the
fulcrum line is not accessible, a rest may be
used on the adjacent canine tooth.
• Such a rest may be made more effective by
placing the minor connector in the embrasure
anterior to the canine, either curving back onto
a prepared lingual rest seat or extending to a
mesio-incisal rest.
154

155. Canine Extensions From Occlusal Rests
• Occasionally a finger extension from a premolar rest is placed on the prepared
lingual slope of the adjacent canine tooth.
• Such an extensions, continuous bar retainers, and linguoplates should never
be used without terminal rests because of the resultant forces effective when
they are placed on inclined planes alone.
155

156. Cingulum Bars (Continuous Bars) and Linguoplates
• Technically, cingulum bars (continuous bars) and linguoplates are not
indirect retainers because they rest on unprepared lingual inclines of
anterior teeth.
• The indirect retainers are actually the terminal rests at either end in the
form of auxiliary occlusal rests or canine rests.
156

157. Modification Areas
• The primary abutments in a Class II, modification 1 partial denture are
the abutment adjacent to the distal extension base and the most distal
abutment on the tooth-supported side.
• The fulcrum line is a diagonal axis between the two terminal
abutments.
157

158. 158
Class II, mod 1, removable partial denture framework. Fulcrum line,
when denture base is displaced toward residual ridge, runs from left
second premolar to right second molar. When forces tend to
displace denture away from its basal seat, supportive element (distal
occlusal rest) of direct retainer assembly on right first premolar
serves as indirect retainer.

159. 159

160. What is support?
• It is the foundation area on which a dental prosthesis rests. With
respect to dental prosthesis, the resistance to displacement
towards the basal tissue or underlying structures. (GPT 9)
160

161. • Two Different type of Removable Partial
Dentures exist
• Tooth Supported
• Tooth – Tissue Supported
161

162. Difference in the Resiliency of mucosa & periodontal membrane
162
Distortion of tissues over edentulous ridge will be approximately 500
micrometer under 4 newtons of force, whereas abutment teeth will
demonstrate approximately 20 micrometer of intrusion under the same
load.

163. Length of residual ridge influences the amount of support
163
Longer the edentulous area covered greater the potential lever action
on the abutment teeth

164. Contour of residual ridge influences support & stability
164
B - flat ridge provides
good support but poor
stability
C – sharp spiny ridge
provides poor support &
poor stability
D – displacable tissue
on ridge provides poor
support & poor stability

165. TOOTH AND TOOTH
TISSUE SUPPORTED
PARTIAL DENTURE
165

166. Support for
Tooth
supported
partial
denture 1
• The potential support provided by
an abutment can be evaluated by
considering:
Periodontal health
Amount of supporting bone
Crown & root ratio
Tooth morphology
Location of tooth in arch
Relation of tooth to other
support units
166

167. REST & REST SEATS

168. Rest
• A rigid component resting in a recessed preparation on the occlusal, lingual or
incisal surface.
• Rest is a projection or attachment, usually on the side of an object.(GPT)3
• Provides vertical support.
168

169. Rest Seat
• Rest seat: the prepared recess in a tooth or
restoration created to receive the occlusal, incisal,
cingulum, or lingual rest. (GPT)3
169

170. Functions 1
• The primary purpose of the rest is to provide
vertical support for the partial denture. In doing
so, it also does the following:
1. Maintains components in their planned
positions
2. Maintains established occlusal relationships by
preventing settling of the denture
3. Prevents impingement of soft tissue
4. Directs and distributes occlusal loads to
abutment teeth
170

171. Occlusal Rest Seat
Form
• Rounded triangular shape
• Apex near center of tooth
• Spoon shaped
171

172. • Base of triangle should be one third the bucco-lingual
width of the tooth.
• Size varies from 13 to 12 of the mesio-distal diameter
and half the width from cusp tip to tip.
172

173. • Marginal ridge must be lowered and rounded 1-1.5mm
(Bulk of metal to prevent fracture)
• The width of the rest at the base should be 2.5 mm for premolars and
molars.
• The minimum thickness should be 1mm at the thinnest portion of the rest
and a minimum of 1.5 mm where it crosses the marginal ridge.
173

174. • Floor inclined towards the center
• Angle formed by rest and minor connector
should be less than 900
174
POSITIVE REST SEAT

175. 175
• Deepest portion is central
• Floor should be concave or
spoon shaped ball-&-socket
joint
• Prevents horizontal stresses
& torque.

176. Occlusal rests
on amalgam
restorations
• Placing occlusal rests on large
amalgam restorations is hazardous
at best. The primary reason for
attempting this is economics, since
an amalgam restoration costs less
than a comparable cast gold
restoration. The unfavorable flow
characteristics and poor tensile
characteristics of amalgam increase
the probability of restoration
failure.
176

177. Occlusal
rests on gold
restorations
• When a cast gold restoration is
planned for an abutment tooth,
the wax pattern should display
ideal contours. An appropriate
rest seat should be carved into
the wax pattern. Upon
completion of the casting
process, restoration contours
should be refined in preparation
for delivery.
177

178. Occlusal Rest Seat Form
• Adjacent Tooth
• Rest is not flared
to facial line
angle
• Lingual flared
more - space for
minor connector
178

179. For Mesially-tipped
molar abutment.
• Patient could not
afford crown to
improve axial
alignment or
orthodontic treatment
to upright the molar.
Occlusal rests will be
used on mesio-occlusal
and disto-occlusal
surfaces to support
restoration and direct
forces over greatest
root mass of
abutment.
179

180. Lingual Rest Seats
• Usually in the canine, due to its well-developed
cingulum
• When canine is not available, an incisor may be
used
180

181. • The outline of the lingual rest is half moon
shape.
Dimensions are:
- Mesiodistal length 2.5 – 3mm
- Labiolingual width 2mm
- Depth should be greater than 1.5mm.
181

182. A slightly rounded V is prepared on the lingual surface at
the junction of the gingival and the middle one third of the
tooth. The apex of the V is directed incisally. This
preparation may be started by using an inverted, cone
shaped diamond bur and progressing to smaller, tapered
bur with round ends to complete the preparation.
182

183. All line angles must be eliminated, and the rest seat must
be prepared within the enamel and must be highly
polished. Shaped, abrasive rubber polishing points,
followed by flour of pumice, produce an adequately
smooth and polished rest seat.
183

184. Lingual rest
seats on
cast
restorations
• When a crown is to be placed on an
anterior tooth and a rest seat is
required, the rest seat should be placed
in the wax pattern. The cingulum of the
restoration should be accentuated to
allow development of a rest seat that
will direct occlusal forces along the long
axis of the tooth.
184

185. Incisal Rests
• Inferior mechanically & esthetically
• Less torquing potential
• Compared to lingual rests these are less widely used.
• Mandibular canines are mostly used for these rests.
185

186. Use of such rests may be justified by the following
factors:
• 1. They may take advantage of natural incisal faceting.
• 2. Tooth morphology does not permit other designs.
• 3. Such rests can restore defective or abraded tooth
anatomy.
• 4. Incisal rests provide stabilization.
• 5. Full incisal rests may restore or provide anterior
guidance.
186

187. • An incisal rest seat is prepared in the form of a
rounded notch at the incisal of an incisor, with the
deepest portion of the preparation apical to the
incisal edge. The notch should be beveled both
labially and lingually, and the lingual enamel
should be partly shaped to accommodate the rigid
minor connector connecting the rest to the
framework.
187

188. • An incisal rest seat should be approximately 2.5 mm wide and 1.5
mm deep so that the rest will be strong without having to exceed
the natural contour of the incisal edge angle of a canine or on the
incisal edge
188

189. Denture
Base

190. TYPES
 Tooth Supported Partial Denture Base
 Distal Extension Partial Denture Base
190

191. Tooth Supported Partial Denture Base
• In tooth supported prostheses denture base is
primarily a span between 2 abutments
supporting artificial teeth.
• Occlusal forces transferred to abutments-Rests.
• Prevent horizontal migration of all abutment
teeth in partially edentulous arch and vertical
migration of teeth in opposing arch.
191

192. Distal
extension
partial
denture base
192
Snow shoe principle-broad coverage
furnishes the best support with least
load per unit area is principle choice
for providing maximum support.
Support is the primary
consideration and is critical to
minimize functional movement and
improve prosthesis stability.

193. Support for the Distal Extension Denture
Base1
• Distal extension RPD is unique in that its support is
derived from abutment teeth , which are
comparatively unyielding & from soft tissue overlying
the bone which may be comparatively yielding under
occlusal forces.
• The tooth – tissue supported RPD exerts excessive
pressure on the abutment teeth as the soft tissue
under the denture base compresses.
193

194. 194

195. Because the tooth-supported base has an abutment tooth at each
end on which a rest has been placed, future relining or rebasing
may not be necessary to reestablish support. Relining is necessary
only when tissue changes have occurred beneath the tooth-
supported base to the point that poor esthetics or accumulation of
debris results.
195

196. For these reasons alone, tooth-supported bases
made soon after extractions should be of a
material that permits later relining. Such
materials are the denture resins, the most common
of which are copolymer and methyl methacrylate
resins.
196

197. Primary retention for the removable
partial denture is accomplished
mechanically by placing retaining
elements on the abutment teeth.
Secondary retention is provided by the
intimate relationship of denture bases and
major connectors (maxillary) with the
underlying tissues.
197

198. Retention of denture bases has been described as
the result of the following forces:
• adhesion, which is the attraction of saliva to the denture and
tissues;
• cohesion, which is the attraction of the molecules of saliva to
each other;
• atmospheric pressure, which is dependent on a border seal
and results in a partial vacuum beneath the denture base
when a dislodging force is applied;
• physiologic molding of the tissues around the polished
surfaces of the denture; and
• the effects of gravity on the mandibular denture.
198

199. conclusion
199

200. References
200

201. References
1. The Glossary of Prosthodontic Terms. J
Prosthet Dent 2017 ;117(5S):e1-e105.
2. Stewart, Rudd and Kurbker: Clinical
Removable Partial Prosthodontics; 2nd ed.,
Euro America Inc, Publishers Tokyo,1997
3. Carr AB, Mc Givney and Brown DT: Mc
Craken’s Removable Partial Prosthodontics;
11th ed.
4. J. C. Davenport, R. M. Basker, J. R. Heath,
J. P. Ralph, and P-O. Glantz; Retention;
British Dental Journal.
201

202. 5. Stratton RT and Wiebelt FJ: An atlas of Removable Partial Denture Design.
6. Eliason CM. RPA clasp designs for distal extension dentures. J Prosthet Dent
1983; 49: 25-7.
7. Krol, A. J.: Clasp design for extension-base removable partial dentures. J
PROSTHET DENT 29:408, 1973.
8. Kratochvil, F. J.: Influence of occlusal rest position and clasp design on
movement of abutment teeth. J PROSTHET DENT 13:l 14, 1963.
202

203. 9. Kratochvil F J and Caputo A A. Photoelastic analysis of pressure on teeth and bone supporting
removable partial dentures. J PROSTHET DENT 1974. 32:52,
10. William E. Avant; Classics article : INDIRECT RETENTION IN PARTIAL DENTURE DESIGN; JPD 1966.
11. J. C. Davenport, R. M. Basker, J. R. Heath, J. P. Ralph, and P-O. Glantz; indirect Retention; British
Dental Journal.
12. Aesthetic clasp design for Removable partial dentures: A literature review SADJ June 2005;60(5):190
- 194
13. J. C. Davenport, R. M. Basker, J. R. Heath, J. P. Ralph, and P-O. Glantz; Support; British Dental
Journal.
14. Ichikawa T, Kurahashi K, Liu L, Matsuda T, Ishida Y. Use of a Polyetheretherketone Clasp Retainer for
Removable Partial Denture : A Case Report. 2019;
203

204. THANK YOU
204