3. INTRODUCTION
■ Removable partial denture is an appliance that
allows controlled movement in function, under
load to avoid impingement of tissues and injury
to abutments.
■ The function of the indirect retainer is to
prevent the distal-extension base from moving
away from its seat because of cheek and tongue
forces, sticky foods, or gravity on a maxillary
prosthesis.
4. DEFINITIONS
INDIRECT RETAINER (GPT 9) - The component of a removable partial denture
that assists the direct retainer(s) in preventing displacement of the distal-
extension denture base by functioning through lever action on the opposite side
of the fulcrum line when the denture base attempts to move away from the
tissues in pure rotation around the fulcrum line.
6. ■ The lever system represented by a distal extension removable partial
denture could magnify the applied force of occlusion to the terminal
abutments, which would be undesirable.
7. Concept of Levers
■ In its simplest form, a lever is a rigid bar supported somewhere along its
length. It may rest on the support or may be supported from above. The
support point of the lever is called the fulcrum, and the lever can move
around the fulcrum.
■ There are three types of levers: first, second, and third order levers. Each
lever modifies the intensity of the force acting on the denture to a
different degree.
8. First Order Levers
■ In this lever the fulcrum is in the center, resistance is at one end and effort
(force) is at the other end.
9. Second Order Levers
■ In this lever, the fulcrum is at one end, effort is at the opposite end and
resistance or load is the center. This type of lever action occurs in indirect
retention of a removable partial denture, When a displacing force tends to
lift a denture from one end (effort), the anterior most point of the major
connector will act as the axis of rotation (fulcrum), the intermediate zone
of the denture, which is lifted by the force will form the resistance (load) of
the lever
10. Third Order Levers
■ In this lever, the fulcrum is at one end, resistance is at the opposite end and
effort is at the centre.
11. Forces Acting on the Denture
Movements of the denture can occur in three planes:
Movement towards the tissue : this movement is prevented by the presence of
rests. Rests are a part of the clasp assembly located on the occlusal surface of
the teeth. The rests act as a vertical stop when the occlusal forces push the
denture down (towards the tissue).
Movement towards the teeth (away from the tissue) : These forces affect the
retention of the denture. This movement is prevented by the activation of the
direct retainer, which is passive in normal conditions.
Movement in buccolingual direction (horizontal) : This movement affects the
stability of the denture. The movement is prevented by the action of the
stabilizing components of the denture. For example, major connector, denture
flange and so on.
12. Rotational Movement
■ Presuming that direct retainers are
functioning to minimize vertical
displacement, rotational movement will
occur about some axis as the distal
extension base or bases move either
toward, away, or horizontally across the
underlying tissue.
■ Rotational movement is possible around
various axes in the mouth :
– Horizontal axis
– Longitudinal axis
– Vertical axis
13. Horizontal Axis
■ This axis known as the fulcrum line, is the center of rotation as the distal
extension base moves toward the supporting tissue when an occlusal load
is applied.
■ The axis of rotation may shift toward more anteriorly placed components,
occlusal or incisal to the height of contour of the abutment,
■ as the base moves away from the supporting tissue when vertical
dislodging forces act on the partial denture.
■ These dislodging forces result from the vertical pull of food between
opposing tooth surfaces, the effect of moving border tissue, and the forces
of gravity against a maxillary partial denture.
14. ■ Vertical tissue ward movement of the
denture base is resisted by the tissue of
the residual ridge in proportion to the
supporting quality of that tissue, the
accuracy of the fit of the denture base,
and the total amount of occlusal load
applied.
■ Indirect retainers should be placed as
far as possible from the distal extension
base, affording the best possible
leverage against the lifting of the distal
extension base.
15. Longitudinal Axis
■ A second movement is rotation about a
longitudinal axis as the distal extension
base moves in a rotary direction about
the residual ridge. This movement is
resisted primarily by the rigidity of the
major and minor connectors and their
ability to resist torque.
■ If the connectors are not rigid or if a
stress-breaker exists between the distal
extension base and the major
connector, this rotation about a
longitudinal axis either applies undue
stress to the sides of the supporting
ridge or causes horizontal shifting of
the denture base.
16. ■ A third movement is rotation about an
imaginary vertical axis located near the center
of the dental arch.
■ This movement occurs under function because
diagonal and horizontal occlusal forces are
brought to bear on the partial denture.
■ It is resisted by stabilizing components, such as
reciprocal clasp arms and minor connectors
that are in contact with vertical tooth surfaces.
■ Stabilizing components on one side of the arch
act to stabilize the partial denture against
horizontal forces applied from the opposite
side. It is obvious that rigid connectors must be
used to make this effect possible.
Vertical Axis
17. FULCRUM LINE
FULCRUM LINE (GPT 9) - 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.
There are two types of fulcrum lines:
■ Retentive Fulcrum Line – It is an imaginary
line, connecting the retentive points of
clasp arms around which the denture
tends to rotate when subjected to forces
such as the pull of sticky foods. Once the
denture begins to rotate the retentive
arm becomes active and provides
retention by engaging the undercut.
18. ■ Stabilizing Fulcrum Line – It is an imaginary line, connecting occlusal rests,
around which the denture tends to rotate under masticatory forces.
Moving the stabilizing fulcrum line away from the point of action of
occlusal forces provides indirect retention.
19. Secondary Fulcrum Line
■ A second fulcrum line is produced by
indirect retention. The prosthesis rotates
about the primary fulcrum line if the
denture base is forced tissue ward , and
about the secondary fulcrum line if the
denture base is lifted off the tissue .
■ However, if a second fulcrum line is not
produced by indirect retention, the
prosthesis rotates about the primary
fulcrum line whether forces move the
denture base toward or away from the
tissue.
20. CONTRAINDICATIONS OF
INDIRECT RETAINERS
■ Unfavorable alignment of the anterior tooth
■ Lingually tipped tooth.
■ Adjustability of the patient.
■ Space: lack of horizontal overlap as well as vertical space between the
lower incisors and the cingulum of the upper teeth
■ Speech alteration
22. ■ The indirect retainer components should be placed as far as possible from
the distal extension base, which provides the best leverage advantage
against dislodgment.
23. ■ Distal extension bases generally act as a class I lever.
■ This is the most hazardous system and causes damage to both
the tooth and the supporting tissues
■ But when an indirect retainer is incorporated the fulcrum is
converted into a Class II lever
■ Indirect retainer becomes the fulcrum, the retentive clasps
the resistance and the power is the force that tends to move
the denture base away from the tissue
24. ■ Although the most effective location of an indirect retainer is commonly in the
vicinity of an incisor tooth, that tooth may not be strong enough to support an
indirect retainer
■ In such a situation, the nearest canine tooth or the mesio-occlusal surface of
the first premolar may be the best location for the indirect retention, despite
the fact that it is not as far removed from the fulcrum line.
■ Whenever possible, two indirect retainers closer to the fulcrum line are then
used to compensate for the compromise in distance.
INDIRECT RETENTION IS THE
APPLICATION OF CLASS 2 LEVERS
25. Mesial Rest vs Distal Rest
■ As the rest is moved anteriorly , it moves
the point of rotation anteriorly as compared
to the conventionally placed disto-occlusal
rest. Mesial rest reduces the need for
indirect retention.
■ It also transmits forces to the edentulous
ridge in a more nearly vertical direction.
Reduces tilting potential of distal abutment.
■ Retainer will have a tendency to disengage
under load and engage retentive area under
dislodging force which leads to equal load
distribution .
26. Advantages of Mesial Rests
■ Minor connector placed in mesial aspect tend to resist distal displacement
of RPD.
■ In distal rest, when distal extension move towards the tissue the distal rest
pulls over the tooth thus applying stress on the abutment, whereas in
■ mesial rest case , it allows the distal extension base to move towards the
tissue and clasp to move more towards the retentive area of the tooth ,
thus applying less stress to the abutment tooth .
27. ■ Retentive clasp engages the retentive area thus resisting movement.
■ Use of a mesial rest instead of a distal rest on the terminal abutment tooth
does not decrease indirect retention.
29. 1. 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.
2. Distance from the fulcrum line. The following three areas must be
considered:
a. Length of the distal extension base
b. Location of the fulcrum line
c. How far beyond the fulcrum line the indirect retainer is placed
3. Rigidity of the connectors supporting the indirect retainer. All connectors
must be rigid if the indirect retainer is to function as intended.
4. Effectiveness of the supporting tooth surface. 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
30. Mechanical Disadvantage of Denture Design
■ The clasp is always nearer to the indirect retainer (fulcrum) than is the displacing
force. The clasp is therefore working at a mechanical disadvantage relative to the
displacing force.
■ The RPD design should strive to reduce the mechanical advantage of the
displacing force by placing the clasp axis as close as possible to the saddle and by
placing the indirect retainers as far as possible from the saddle.
31. The mechanical advantage of a lever is expressed as the quotient of the
power arm length divided by the resistance arm length. It can be reduced
in the following ways:
■ Resistance arm is made longer by moving the indirect retention
mesially to prepared rest seat on cingulum of canine tooth.
■ Resistance arm is lengthened again by moving retentive clasp tip from
mesial toward distal of primary abutment tooth.
33. 1. Act as auxiliary guiding planes.
2. Anterior teeth supporting indirect retainers are stabilized against lingual
movement.
3. May act as auxiliary rest to support major connector especially in
mandible
4. It counter acts the horizontal forces.
5. It may provide the first visual indications for the need to reline an
extension base partial denture
6. Indirect retention prevents the retentive clasp tips from becoming a
fulcrum about which the prosthesis could rotate when forces move the
denture base away from the tissue
35. Fulcrum lines found in various types of partially edentulous arches, around
which denture may rotate when bases are subjected to forces directed toward
or away from residual ridge. The most advantageous position of indirect
retainer differs based on the type of arch :
■ 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.
36. ■ In Class II arch, fulcrum line is diagonal, passing through abutment on
distal extension side and the most posterior abutment on opposite side.
37. ■ If abutment tooth anterior to modification space lies far enough removed
from fulcrum line, it may be used effectively for support of indirect retainer.
38. ■ In Class IV arch, fulcrum line passes through two abutments adjacent to
single edentulous space
39. ■ In Class III arch with non-supporting 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.
40. ■ 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.
43. ■ Auxiliary Occlusal Rest
■ Canine Rests
■ Canine Extensions From
Occlusal Rests
■ Cingulum Bars (Continuous
Bars) and Linguoplates
■ Modification Areas
■ Rugae Support
■ Incisor Rests
■ Lingual Rests
■ Cummer Arms
■ Embrasure Hook
■ Denture Base
■ Direct – Indirect Retention
The indirect retainer may be in one of several forms. All are effective
in proportion to their support and the distance from the fulcrum line.
44. 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 same principle applies to any maxillary
Class I partial denture when indirect retainers
are used.
45. ■ Indirect retainers for Class II partial dentures are usually placed on the
marginal ridge of the first premolar tooth on the opposite side of the arch
from the distal extension base.
■ Bilateral rests are seldom indicated except when an auxiliary occlusal rest
is needed for support of the major connector or when the prognosis of the
distal abutment is poor and provision is being considered for later
conversion to a Class I partial denture.
46. 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. Lingual or incisal rests,
may be used.
47. 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 extension is used to effect indirect retention by increasing the
distance of a resisting element from the fulcrum line. This method is
particularly applicable when a first premolar must serve as a primary
abutment.
■
48. ■ Even when they are not used as indirect retainers, canine 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.
49. Cingulum Bars (Continuous Bars)
and Linguoplates
■ In Class I and Class II partial dentures, a cingulum bar or linguoplate may
extend the effectiveness of the indirect retainer if it is used with a terminal
rest at each end.
■ In tooth-supported partial dentures, a cingulum bar or linguoplate is placed
for other reasons but always with terminal rests.
■ In Class I and Class II partial dentures especially, a continuous bar retainer
or the superior border of the linguoplate should never be placed above the
middle third of the teeth so that orthodontic movement is prevented
during the rotation of a distal extension denture.
50. ■ This guideline is not as important when the six anterior teeth are in nearly
a straight line, but when the arch is narrow and tapering, a cingulum bar or
linguoplate on anterior teeth extends well beyond the terminal rests—and
orthodontic movement of those teeth is more likely.
■ Although these are intended primarily to stabilize weak anterior teeth, they
may have the opposite effect if not used with discretion.
51. Modification Areas
■ Occasionally the occlusal rest on a secondary abutment in a Class II partial
denture may serve as an indirect retainer. This use will depend on how far
from the fulcrum line the secondary abutment is located.
■ 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 anterior abutment on the tooth-supported side is a secondary
abutment, serving to support and retain one end of the tooth-supported
segment and adding horizontal stabilization to the denture.
52. • If the modification space were not present, as in an unmodified
Class II arch, auxiliary occlusal rests and stabilizing components in
the same position would still be essential to the design of the
denture.
• However, the presence of a modification space conveniently
provides an abutment tooth for support, stabilization, and
retention. If the occlusal rest on the secondary abutment lies far
enough from the fulcrum line, it may serve adequately as an
indirect retainer.
53. Rugae Support
■ Some clinicians consider coverage of the rugae area of the maxillary arch as
a means of indirect retention because the rugae area is firm and usually
well situated to provide indirect retention for a Class I removable partial
denture.
■ Although it is true that broad coverage over the rugae area can conceivably
provide some support, the facts remain that tissue support is less effective
than positive tooth support and that rugae coverage is undesirable if it can
be avoided.
■ The use of rugae support for indirect retention is usually part of a palatal
horseshoe design. Because posterior retention is usually inadequate in this
situation, the requirements for indirect retention are probably greater than
can be satisfied by this type of tissue support alone.
54. ■ In the mandibular arch, retention from the
distal extension base alone is usually
inadequate to prevent lifting of the base away
from the tissue. In the maxillary arch, where
only anterior teeth remain, full palatal
coverage is usually necessary.
■ In fact, with any Class I maxillary removable
partial denture extending distally from the first
premolar teeth, except when a maxillary torus
prevents its use, palatal coverage may be used
to advantage.
■ Although complete coverage may be in the
form of a resin base, the added retention and
reduced bulk of a cast metal palate make the
latter preferable.
■ However, in the absence of full palatal
coverage, an indirect retainer should be used
with other designs of major palatal connectors
for the Class I removable partial denture.
55. Incisor Rest
■ Incisor rests are placed at the incisal angles of the anterior teeth in enamel.
These are the least desirable rests for anterior teeth and are indicated only
when the canines are absent.
■ Preparation is begun with a small safe-side diamond disk held parallel to the
path of insertion. Depth - 1.5-2 mm. It should be approximately 2-3 mm inside
the proximal angle of the tooth. Mesiodistally, the rest should be concave.
■ The preparation is completed using a small flame-shaped diamond point. The
notch created by the disk should be rounded.
■ Factors to be considered in determining the site and form of rest placed on
incisor:
– Root form
– Root length
– Inclination of the tooth
– Crown / Root ratio
57. Lingual Rests
■ Lingual rests are primarily given on maxillary canines. If a canine is not
present, multiple rests are spread over several incisor teeth instead of a single
tooth.
■ Lingual rest seat preparations on enamel should never be attempted on
mandibular anterior teeth because of lack of thickness of enamel. However, a
lingual rest on cast restoration may be used on any anterior tooth, either
maxillary or mandibular.
58. ■ Lingual or cingulam rest is preferred over incisal rest due to:
– Mechanical advantage.
– Closer to center of rotation
– More esthetic
– Less prone to breakage.
■ It is prepared if the enamel of the tooth is sound, there is good oral hygiene &
caries index is low.
■ No.2 or 4 round diamond or cylindrical diamond stone with a rounded tip is
used to prepare the rest seat
■ The outline form should be half-moon shaped.
■ Should follow a smooth curve from one marginal ridge to the other, crossing
the center of the tooth incisal to the cingulum. Deepest point of the rest seat is
over the cingulum. It should follow general contour of the cingulum. The base
of the rest seat should be placed about 1mm cervical to the height of the
cingulum.
59. Cummer Arms
■ Metal arm extending from saddle or
connectors to end on unprepared palatal or
lingual surface of an anterior tooth (upper
canines)
■ Disadvantes:
– Liable to cause tooth movement as
excessive load is placed on a single
tooth
– Can be used only on canines because of
their long roots
– Liable to get distorted
■ Advantages:
– Used when insufficient space exists for
the continuous clasp
60. Embrasure Hook
■ Placed in the embrasure between the anterior teeth.
■ It covers the occlusal and incisal surfaces of the teeth and continues over to
the labial or buccal surfaces.
■ They are attached to continuous clasp or linguoplate. on unprepared tooth
and are more esthetic.
61. Denture Base
■ Indirect retention is provided by extending the denture base anteriorly
onto the rugae area.
■ Anterior part of the denture is indirectly retaining the posterior part
■ Although the tissue support is les effective than tooth support, denture
bases are used as a means of indirect retention
■ This is because of the wide palatal coverage and the palatal tissues being
firm in consistency
■ If they become flabby, the efficiency of the bases decrease
■ It provides indirect retention and also some support
■ Cannot be used as effectively in the lower arch
62. Direct – Indirect Retention
■ These are used in certain cases
■ In this example the reciprocal arm of the direct retainer provides the
needed indirect retention also
63. CONCLUSION
■ Extension-base partial denture patients are the most common
partial denture patients. It is essential for us to understand and
utilize indirect retainers in our dentures.
■ Therefore, with the use of proper biomechanics in the design
and construction of removable partial denture, the dentist can
deliver removable partial denture with high level of
predictability and acceptance from the patients.
64. REFERENCES
■ Carr AB, Brown DT. McCracken's Removable Partial Prosthodontics. Elsevier
Health Sciences; 2010 Jun 22.
■ Stewart KL, Rudd KD, Kuebker WA. Clinical removable partial
prosthodontics. Ishiyaku EuroAmerica; 1992.
■ Nallaswamy D. Textbook of prosthodontics. JP Medical Ltd; 2017 Sep 30.
■ Nallaswamy D. Textbook of prosthodontics. JP Medical Ltd; 2017 Sep 30.
■ Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P.
Prosthetics: Indirect retention. British dental journal. 2001 Feb;190(3):128.
■ Richard P. Frank, Jack I. Nicholls . An investigation of the effectiveness of
indirect retainers The Journal of Prosthetic Dentistry, Volume 38, Issue
5, November 1977, Pages 494-506.
■ William E. Avant. Indirect retention in partial denture design. J Prosthet
Dent 2003 :90;(1):1-5
Editor's Notes
#8:These levers are more efficient and easily controlled. This type of lever can occur in patients with distal extension partial dentures. The direct retainer will be the fulcrum, effort end lies on the point where the denture takes up the occlusal load (area where the artificial teeth are located) the load is the region of the anterior end of the major connector. Using an additional rest (indirect retainer) to shift the fulcrum line prevents lever action in these dentures.
#16:Such stabilizing components are essential to any partial denture design regardless of the manner of support and the type of direct retention employed.
#23:Indirect retainer principle. A, Beams are supported at various points. B, A lifting force will displace the entire beam in the absence of retainers. C, With direct retainers (DRs) at the fulcrum, the lifting force will depress one end of the beam and elevate the other end. D, With both direct and indirect retainers (IRs) functioning, the lifting force will not displace beam. The farther the indirect retainer is from the fulcrum, the more efficiently it should control movement.
#25:An indirect retainer should be placed as far from the distal extension base as possible in a prepared rest seat on a tooth capable of supporting its function.
#30:The text emphasizes that for a removable partial denture to function correctly, the occlusal rests must be securely held in place by the retainers. If they are not, the necessary rotation around a fulcrum line will not occur, and the indirect retainers won't engage, leading to potential instability and improper function of the denture.
#34:1) 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
#45:The ideal position for the indirect retainer perpendicular to the fulcrum line would be in the vicinity of the central incisors, which are too weak and have lingual surfaces that are too perpendicular to support a rest. Bilateral rests on the first premolars are quite effective, even though they are located closer to the axis of rotation
#46:Bilateral rests on the mesial marginal ridge of the first premolars are generally used in preference to rests on incisor teeth.
