This document discusses different types of direct retainers used in removable dental prostheses. It describes primary retainers as mechanical components that directly engage abutment teeth, such as clasp assemblies. Clasp assemblies provide retention through encirclement of teeth and contacting in three areas - the occlusal rest, retentive clasp terminal, and reciprocal clasp terminal. The document also discusses factors that influence clasp retention such as flexibility, taper, length and diameter.

1. Dr SHOAIB RAHIM
BDS, FCPS
Assistant Professor
Department Of Prosthodontics

2. A direct retainer is any unit of a removable dental prosthesis
that engages an abutment tooth to resist displacement of the
prosthesis away from basal seat tissue.

3.  Primary
 Mechanically (direct retainers)
 Secondary
 Intimate contact of minor connector with guiding planes and
denture base with tissue
 Dental implants

4.  Principle of encirclement
 Ensures tooth stability
 Ensures stability of clasp assembly
 Contact in atleast three areas encircling the tooth:
Occlusal rest area
Retentive clasp terminal area
Reciprocal clasp terminal area

5.  Support
 Reciprocity
 Clasp retainers on abutments adjacent to distal extension
bases should be designed so that they avoid direct
transmission of tipping and rotational forces to abutment
 Retentive clasps should be bilaterally opposed
 The path of escapement for each retentive clasp terminal must
be other than parallel to the path of removal for the prosthesis
to require clasp engagement with the resistance to
deformation that is retention

6.  The amount of retention should always be the minimum
necessary to resist reasonable dislodging forces.
 Reciprocal elements of the clasp assembly should be located at
the junction of the gingival and middle thirds of the crowns of
abutment teeth. The terminal end of the retentive arm is
optimally placed in the gingival third of the crown

7.  Reciprocal arms are intended to resist tooth movements in
response to the retainer arm deforming as it engages a tooth
height of contour
 Reciprocal clasp arm should be located so that the denture is
stabilized against horizontal movement
 May act to a minor degree as an indirect retainer

8. Direct
Retainers
Intra-coronal
Precision
attachments
Semi-Precision
attachments
Extra-coronal
Clasp
assemblies
Supra-bulge
Infra-bulge
Attachments

10.  Elimination of visible retentive and support
components
 Provides horizontal stabilization
 Better vertical support through a rest sea located
more favorably in relation to the horizontal axis of
the abutment
 Stimulation to the underlying tissue is greater
when internal attachments are used because of
intermittent vertical massage

11.  Require prepared abutments and castings
 Require complicated clinical and laboratory
procedures
 Wear with progressive loss of frictional resistance
to denture removal
 Difficult to repair and replace

12.  They are effective in proportion to their length and
are therefore least effective on short teeth
 Difficult to place completely within the
circumference of an abutment tooth because of the
size of the pulp
 Costly

13.  All horizontal, tipping, and rotational movements of
the prosthesis are transmitted directly to the
abutment tooth
 Internal attachment should not be used in
conjunction with extensive tissue-supported distal
extension denture bases

15.  Three principal forms:
1. The clasp-type retainer which is the most
common form used, retains through a flexible
clasp arm
2. A manufactured attachment, which uses
flexible clips or rings that engage a rigid
component that is cast or attached to the
external surface of an abutment crown
3. Use of a spring-loaded device that engages a
tooth contour to resist occlusal displacement

16. A clasp assembly should consist of four component parts:
 First, there should be one or more minor connectors from
which the clasp components originate
 Second, there should be a principal rest designed to direct
stress along the long axis of the tooth
 Third, there should be a retentive arm engaging a tooth
undercut
 Fourth, there should be a non-retentive arm on the opposite
side of the tooth for stabilization and reciprocation against
horizontal movement of the prosthesis

17. Component Parts Function Location
Rest Support Occlusal, Lingual, Incisal
Minor Connector Stabilization Proximal surfaces extending
from a prepared marginal ridge
to the junction of the middle and
gingival one third of abutment
crown
Clasp arms Stabilization
(reciprocation)
Middle one third of crown
Retention Gingival one third of crown in
measured undercut

19. RPI
RPA
Bar Clasp
Combination Clasp

23. .

24.  When a small degree of undercut (0. 01 inch) exists in the
cervical third of the abutment tooth, which may be approached
from a gingival direction
 Tooth-supported partial dentures or tooth-supported
modification areas
 Distal extension base situations
 Esthetic considerations

25.  Severe buccal or lingual tilts of abutment teeth
 Severe tissue undercuts
 Shallow buccal or labial vestibules

26.  Esthetics
 Increased retention without tipping action on the abutment
 Less chance of accidental distortion resulting from its
proximity to the denture border

28.  Consists of a wrought-wire retentive clasp arm and a cast
reciprocal clasp arm

29.  When maximum flexibility is desirable, such as on abutment
tooth adjacent to a distal extension base or on a weak
abutment
 Adjustability when precise retentive requirements are
unpredictable and later adjustment to increase or decrease
retention may be necessary
 Esthetic advantage over cast clasps

30.  Flexibility
 Adjustability
 Esthetics
 Minimum of tooth surface covered because of its line contact
with the tooth
 Less likely occurrence of fatigue failures in service with the
tapered wrought-wire retentive arm versus the cast, half-
round retentive arm

31.  Extra steps in fabrication, particularly when high-fusing
chromium alloys are used
 Distorted by careless handling on the part of the patient
 Less accurately adapted to the tooth and therefore provide less
stabilization in the suprabulge portion
 It may distort with function and not engage the tooth

32. Circumferential clasp
Ring clasp
Embrasure clasp
Others

34.  More tooth surface is covered than with a bar clasp
 On the buccal surface of mandibular teeth and the lingual
surfaces of maxillary teeth, its occlusal approach increases
width of occlusal surface of tooth
 In the mandibular arch, more metal may be displayed
 Its half-round form prevents true adjustment to increase or
decrease retention

38. Used on abutments of tooth-supported dentures when proximal
undercut lies below point of origin of clasp

42.  Critical areas of an abutment that provide for retention and
stabilization (reciprocation) can only be identified with the use
of a dental cast surveyor
 This relationship of the vertical arm of the surveyor to the cast
represents the path of placement that the partial denture will
ultimately take when inserted or removed from the mouth

43.  The surveyor-defined path and the subsequent tooth height of
contour will indicate the areas available for retention and
those available for support, and the existence of tooth and
other tissue interference to the path of placement
 The apical angle is called the angle of cervical convergence

44.  Clasp retention is based on the resistance to deformation
 Such resistance to deformation is dependent on several factors
 Retention is provided primarily by flexible portion of clasp
assembly
 Retentive terminals are ideally located in measured undercuts
in gingival third of abutment crowns

45.  Size of the angle of cervical convergence
 How far the clasp terminal is placed into the angle of cervical
convergence

46.  Tooth surfaces can be recontoured by selective grinding or the
placement of restorations (mouth preparations) to achieve a
more suitable path of placement
 The path of placement also must take into consideration:
1. Presence of tissue undercuts that would interfere with the
placement of major connectors
2. The location of vertical minor connectors
3. The origin of bar clasp arms
4. The denture bases

47.  A positive path of placement and removal is made possible by
the contact of rigid parts of the denture framework with
parallel tooth surfaces which act as guiding planes
 Guiding planes can also provide additional retention for the
partial denture by limiting the possibilities that exist for its
dislodgment

48.  Flexibility of the clasp arm
1. Clasp length (measured from its point of origin to its terminal
end)
2. Clasp relative diameter (regardless of its cross-sectional form)
3. Clasp cross-sectional form or shape (whether it is round, half
round, or some other form)
4. The material used in making the clasp

49. Retentive cast clasp arm should be tapered uniformly from its
point of attachment at clasp body to its tip. Dimensions at tip
are about half those at point of attachment. Clasp arm so
tapered is approximately twice as flexible as one without any
taper

50.  The retention on all principal abutments should be as equal as
possible
 Retentive clasp arms must be located so that they lie in the
same approximate degree of undercut on each abutment tooth

51.  Must be in contact during entire period of retentive clasp
deformation
 It should be rigid
 Its average diameter must be greater than the average
diameter of the opposing retentive arm

52. THANK YOU