1. Retainers, Clasp Assemblies and
Indirect Retainers
Ting Ling Chang,
Takahiro Ogawa
and
John Beumer III
Division of Advanced Prosthodontics, Biomaterials
and Hospital Dentistry
This program of instruction is protected by copyright ©. No portion of
this program of instruction may be reproduced, recorded or transferred
by any means electronic, digital, photographic, mechanical etc., or by
any information storage or retrieval system, without prior permission.

2. Retainers, Clasp Assemblies and
Indirect Retainers

3. Retainers, Clasp Assemblies and
Indirect Retainers
Direct retainers
 That component of a removable
partial denture used to retain and
prevent dislodgment of a clasp
assembly or attachment.

4. Retainers, Clasp Assemblies and
Indirect Retainers
Clasp assembly
 The part of a removable partial denture that acts as a
direct retainer (clasp) and/or stabilizer (minor
connectors, proximal plates etc) for a prosthesis by
partially encompassing an abutment tooth

5. Types of Retainers (Clasps)
Infra-bulge
 “I”-Bar
 The approach of the
direct retainer is from
below the height of Height
contour of contour
Supra-bulge clasp
 Circumferential
clasp
 The approach of the
direct retainer is from
above the height of
contour.
Height of contour

6. “I” Bar Retainers:
 Low retentive value
compared to other types
 This is due to minimal
tooth engagement and
partly to purposeful design
since support, stability and
control of tooth position is
established with positive
rests, minor connectors
and proximal plates

7. “I” Bar Retainers:
 Effectiveness of retention
is dependent upon the
stability of the RPD which
in turn is dependent on
the presence of properly
designed minor
connectors, guide-planes
and proximal plates
 In particular, properly
contoured guide planes-
proximal plates are
integral to successful use
of “I” bar retainers

8. “I” Bar Retainers:
Benefits
 Minimal tooth contact
 Allows for easier
adjustment of the
retainer
 Enablesexact
placement of retention
contact
 Thisvery important in
extention base RPD’s
 Minimal Interference
with natural tooth
contour

9. “I” Bar Retainers:
Benefits
 Maximum natural
cleansing action
 Passive functional
movement of an extension
prosthesis (when the rest
is properly positioned)

10. Better Esthetics
“I” bars
Only the tips are visible in most patients

11. “I” Bar Retainers
Design Principles
 “I” bar should cross tooth tissue
junction at right angles and parallel to
long axis of tooth.
 Horizontal portion of the “I” bar is
placed on attached tissue whenever
possible
 Connection of “I” bar to RPD casting
should be in placed in the
interproximal area between denture
teeth (arrow).

12. “I” Bar Retainers
Design Principles
 A space is created between
“I” bar and mucosa (it is
crossing over) to prevent
tissue hypertrophy (arrows).
 Ideally “I” bar terminates in
gingival 1/3 of the tooth.
 An active “I” Bar requires
only 0.25 mm undercut.

13. “I” Bar Retainers
Design Principles
Height of
 When properly designed the “I” contour
bar engages the undercut down
into the 0.25 mm region but the
occlusal-incisal tip terminates at ..25 mm
the height of contour
 When designed in this way the
RPD will slide in out of place
with a smooth frictional
resistance

14. “I” Bar Retainers
Design Principles
 When properly designed the “I” Height of contour
bar engages the undercut down
into the 0.25 mm region but the
occlusal-incisal tip terminates at
the height of contour
 When designed in this way the
RPD will slide in out of place
with a smooth frictional
resistance

15. “I” Bar Retainers – Design Principles
The tip should be
positioned at the point
of greatest mesial distal
curvature on the tooth

16. “I” Bar Retainers: Reciprocation
An active “I” bar retainer
can be reciprocated by
one of the following:
 Opposing reciprocating
clasp at 0.00 (“I” bar or
“C” clasp)
 A combination of rest,
minor connector and
proximal plate.
 A lingual or palatal plate
 A combination of the
above

17. “I” Bar Retainers
Contraindications
 Teeth with short clinical crowns
 Guide planes-proximal plates will be short and in this
situation the “I” bar may not provide acceptable
retention
 High frenum attachments will preclude proper
positioning of the horizontal component
 Severely tilted abutments
 The “I” bar will project into the cheek. This is
particularly problem with regard to 2nd molars in the
maxilla
 Lack of attached gingiva
 Buccal of mandibular 2nd molars

18. Circumferential Clasp
Suprabulge clasp
 The flexible retentive arm originates from a minor connector or a
proximal plate and its terminal 1/3 crosses the height of contour into
the undercut (.25mm or 0.1”)
 The rigid reciprocating arm stays at or above the height of contour
Height of contour
Retentive Reciprocating
arm arm

19. Circumferential Clasp
Applications
 Posterior teeth
Note: The portion the retentive
 Tipped teeth
 Maxillary molars clasp above the height of
contour is relieved with rouge
 Better bracing
 Unilateral defects and chloroform

20. Circumferential Clasp
Applications
 Teeth with short clinical crowns in patients with long
edentulous spans requiring additional retention
 Tipped teeth (maxillary molars)
 High frenum attachments
 Minimal levels of attached gingiva (mandibular 2nd molars)

21. Circumferential Clasp
 Maxillary 2nd molars
are frequently tilted to
the buccal
 If you use an “I” bar to
engage the buccal
surface of the tooth it Courtesy Dr.GE King
will project into the
cheek and cause
irritation
 A “C” clasp is
therefore preferred in
this situation Courtesy Dr.GE King

22. Circumferential Clasp
 Maxillary 2nd molars
are frequently tilted to
the buccal
 If you use an “I” bar to
engage the buccal
surface of the tooth it Courtesy Dr.GE King
will project into the
cheek and cause
irritation
 A “C” clasp is
therefore preferred in
this situation Courtesy Dr.GE King

23. Circumferential Clasp
 Maxillary 2nd molars are
frequently tilted to the
buccal
 If you use an “I” bar to
engage the buccal
surface of the tooth it
will project into the
cheek and cause
irritation
 A “C” clasp is therefore
preferred in this
situation
 Relieve clasp except at
its terminius.

24. Circumferential Clasp
 Maxillary 2nd molars are
frequently tilted to the
buccal and the height of
contour is high on the
tooth surface
 If you use an “I” bar to
engage the buccal
surface of the tooth it will
project into the cheek and
cause irritation
 A “C” clasp is therefore
preferred in this situation

25. Circumferential Clasp
 In some patients the zone of
attached gingival around the
buccal surface of the
mandibular 2nd molar is
minimal
 If so, if an “I” bar is used to Courtesy Dr.GE King
engage this undercut
surface, it will project into the
cheek when the masseter
muscle is contracted causing
irritation to the mucosa.
 A “C” clasp is therefore
preferred in this situation

26. Circumferential Clasp
 In some patients the zone
of attached gingival around
the buccal surface of the
mandibular 2nd molar is
minimal
 If so, if an “I” bar is used to
engage this undercut
surface, it will project into
the cheek when the
masseter muscle is
contracted causing
irritation to the mucosa.
 A “C” clasp is therefore
preferred in this situation

27. Embrasure clasps
Used when restoring a patient with a
unilateral extension area
 Biomechanically this is a disadvantage.
However, when the teeth are sound and
retentive areas are available, retention
can made to be very effective with this
design
 Usually employed in association with
cast restorations
 Use in unprotected abutments is based
on caries index, oral hygiene, opposing
occlusion and tooth contours.
Disadvantage:
Insufficient space provided will
predispose to a high rate of fracture

28. Embrasure clasps
Advantage
 Easy to approach the buccal undercut in the
maxillary molar region
Risks
 Fracture – It is difficult to obtain a nonporous casting
throughout the clasp assembly

29. Embrasure clasps
 Disadvantage when used in virgin teeth
 Frequently, too little tooth reduction is made in the
proximal area
 As a result these clasps are susceptible to fracture
because of lack of metal bulk and it is difficult to obtain
a nonporous casting throughout the clasp assembly

30. Embrasure clasps
Disadvantage when used in virgin teeth
 This
problem can be avoided by placing full
veneer crowns on the abutments

31. Indirect Retainers – Myth or Reality
Definition:
The component of a removable partial denture that assists the direct
retainers in preventing displacement of the distal extension denture bases
by functioning through lever action on the opposite side of the fulcrum line
when the denture base moves away from the tissues in pure rotation around
the fulcrum line.

32. Indirect Retainers – Myth or Reality
Frank RP & Nicholls JI. An investigation of the effectiveness of
indirect retainers. JPD 1976;38:494-506.
 Methods: Tested the following combinations: 1) D rest 2nd PM/canine
rest, 2) D rest 2nd PM/M rest 1st PM, 3) D rest 2nd PM/no indirect
retainer, 4) M rest 2nd PM/no indirect retainer, 5) M rest 2nd PM/M rest 1st
PM, 6) M rest 2nd PM/canine rest. Applied dislodging forces with Instron
machine unilaterally, bilaterally, anteriorly and posteriorly. A wrought-wire
clasp was substituted for the bar clasp on both 2nd premolars. Also rested
with and without guide planes. Attached rests to framework with
autopolymerizing acrylic resin.
 Conclusions:
 The type of clasp arm used has a much greater influence on the amount of
denture base displacement than does the presence or location of an indirect
retainer
 Usefulness of an indirect retainer in preventing occlusal displacement of a
denture base appears to be very limited. It is probably more effective in
distributing forces to teeth other than the direct abutments than in preventing
denture base lifting
 Guiding planes are important in preventing denture base lifting
 Mesial rest placement decreases the distance from the fulcrum line to the indirect
retainer, but this does not seem to increase denture base dislodgement.

33. Indirect Retainers – Myth or Reality
 In this example the
mesial rest on the right
maxillary premolar acts
as an indirect retainer.
 The forces of gravity
tend to displace the
obturator portion of the
prosthesis, down and out
of the defect. The
indirect retainers resist
this displacement

34. Indirect Retainers – Myth or Reality
 The musculature of the lip tends to lift the prosthesis away from
the tissue bearing surfaces in the anterior region. The indirect
retainers resist this rotation.
 Indirect retention provided by:
 The rests on the 2nd molars act as indirect retainers
 The longer the rests the more effective the indirect retention
Axis of
rotation
(fulcrum
line)

35. Clasp Assemblies and Reciprocation and
Encirclement

36. Reciprocation
 Resistance to horizontal forces (primarily in the
buccal and lingual direction) exerted on a tooth by
an active retentive element
 Provided by the rigid reciprocating clasp arm
opposite the retentive arm, minor connectors,
proximal plates, lingual plates etc.
 Prevents tooth movement that may result from
over adjustment of a retentive clasp arm (i.e.
making the I-bar, or the retentive arm too tight)

37. Clasp assembly
Components
 Rest
 Clasp
 Directretainer
 Reciprocating clasp
 Minor connector
 Proximal plate
 Major connector

38. Retainer (Clasp) Assembly
Minor connectors Proximal plates
Direct retainer
Rests Circumferential type
Direct retainer,”I”-Bar
A Clasp (retainer) assembly is a combination of several RPD
components that engage an abutment tooth extra-coronally for
support, stability and retention of the partial denture.
•Direct retainers
•Rests
•Minor connectors – proximal plates
•Reciprocating elements

39. Encirclement
The clasp assembly must engage more than 180
degrees of the circumference of the tooth
 Encirclement is achieved with a combination of retainers
proximal plates, minor connectors and extended rests
Otherwise the abutment tooth may away from the
clasp assembly
Examples of the methods used to accomplish this
task are shown

40. Reciprocation and Encirclement
 The clasp assembly must engage and encircle the tooth
by more than 180 degrees of its circumference
 Otherwise, the tooth may move out from under the clasp
assembly during function

41. Reciprocation and Encirclement
 The clasp assembly must engage and encircle the
tooth by more than 180 degrees of its circumference
This is an example
of insufficient
encirclement

42. Reciprocation
 In the classical RPI
system (UCLA-
Kratochvil)
reciprocation is
provided by the
proximal plate and
the minor connector

43. Reciprocation
 Reciprocation of the “I” bar on the buccal surface
of the molar is accomplished with the proximal
plates

44. Reciprocation
 Reciprocation of the “C” clasp on the lingual surface of the
molar is accomplished with the proximal plate and the
circumferential (“C”) clasp on the buccal side
Requires rouge relief to make
this design function properly

45. Reciprocation
 Reciprocation of the “I” b bars on both premolars is
accomplished with the proximal plates and the lingual plate

46. Reciprocation
 In this example,
reciprocation provided
by the buccal “I” bar
(the lingual “I” in this
patient is the retentive
retainer) and the
proximal plate

47. Reciprocation
 Reciprocation for the retainers on the cuspid and
the molar in this unilateral case is accomplished
with the proximal plates and lingual plate

48. Bracing
Courtesy Dr. T Berg

49. Bracing (Stability)
Definition:
 Resistance to horizontal lateral or torsional
components of force generated during
mastication or eccentric movements of the
mandible
 It is provided primarily by the rigid portions of a
clasp assembly such as the rigid reciprocating
clasp elements, minor connectors, proximal
plates, lingual plates and extended occlusal
rests.

50. Bracing (Stability)
 Guide planes of teeth
engaged by proximal
a
plates provide stress
reduction in several
areas including
buttressing (a)

51. Bracing
 Inthis bilateral extension case, bracing is
enhanced by the proximal plate, the minor
connector and the “I” bar retainer

52. Types of Bracing
 In this patient most of the bracing is provided by
the distal proximal plates. However the lingual
plate supplements bracing to significant degree

53. Bracing
 During mastication significant lateral forces will be generated. In
patients presenting with unilateral defects such as this, additional
bracing is necessary in order distribute these forces equitably among
the remaining teeth
 Here, in addition to the bracing effect of the proximal plates on the 2nd
molar and the 1st premolar, additional bracing is provided by plating
the lingual surfaces of the remaining dentition.

54. Bracing (Resistance to lateral forces)
 Patients with unilateral dentition and large edentulous spaces
such as in this case, require additional bracing. Here, in
addition to the bracing effect of the proximal plates on the !st
and 3rd molars molar additional bracing is provided by plating
the lingual surfaces of the remaining dentition.

55. Bracing (Resistance to lateral forces)
More bracing is required in maxillary resection defects because
of the large edentulous extension area and long lever arms.
Lingual plating is frequently employed In these types of defects.

56. Bracing
 As the defect
becomes larger and
with the remaining
dentition in a linear
fashion, bracing
becomes more
aggressive. In this
patient the lingual
plate embraces all of
the remaining teeth.

57. Types of Bracing
 Reciprocation and encirclement of the molar in
this rotational path RPD is accomplished with
the proximal plate and the extended rest
engaging its buccal slopes

58. Passivity
 There should be no active force on any abutment teeth
when the framework is seated and the retainers are
engaged. Its retainers should only by activated when a
dislodging force is applied

59.  Visit ffofr.org for hundreds of additional lectures
on Complete Dentures, Implant Dentistry,
Removable Partial Dentures, Esthetic Dentistry
and Maxillofacial Prosthetics.
 The lectures are free.
 Our objective is to create the best and most
comprehensive online programs of instruction in
Prosthodontics