the science,principles and requirements of direct and indirect retainers in removable partial dentures.

1. Direct and indirect
retainers
Vinay Pavan Kumar .K
2nd year P G student
Dept of Prosthodontics
AECS Maaruti College of Dental Sciences

2. Direct
retainers
Indirect
retainers
Definition
Classification
Principles of
design
Extracoronal
retainers
Types
Definition
Factors
affecting
types of indirect
Requirements retainers

3. Direct retainer: It is that component of a
removable partial denture that is used to retain
and prevent dislodgment, consisting of a clasp
assembly or a precision attachment (GPT 8)

4. Direct retainers
Intracoronal Extracoronal
Precision Semi-precision Extracoronal Clasps
Attachment

5. Intracoronal retainers
In 1906 the principle of the internal attachment was first
formulated by Dr. Herman E.S. Chayes

6. Extracoronal Retainers
 Prothero provided a Conceptual Basis for
mechanical retention

7.  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.

9. 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.

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

11. Principles of Clasp Design
1. Encirclement- more than 180 degrees in greatest
circumference if the tooth engaged by the clasp assembly
2. Occlusal rest - to prevent the movement of the clasp arms
cervically.
3. Each retentive terminal should be opposed by a reciprocal
component

12. 4. Clasp retainers on abutment teeth adjacent to distal
extension bases should be designed to avoid direct
transmission of forces to the abutment
5.The amount of retention should always be the minimum
necessary to resist reasonable dislodging forces.
6. Reciprocal elements – junction of gingival and middle
third
Terminal retentive arm – gingival third

13. Functional requirements of the clasp
Retention
Support
Stability
Reciprocation
Encirclement
Passivity

14. Retention
 Is obtained by the incorporation of a flexible element of
the clasp into the undercut.
 Sufficient undercut to be engaged to obtain desired
retention.
 Force from the clasp arm on flexing must be within the
tolerance of the PDL and must be less to prevent
deformation of the clasp arm itself.

15. Factors affecting retention
◦ Tooth factors: Size of the angle of cervical
convergence
◦ How far the clasp terminal is placed into the angle of
cervical convergence

16. Clasp arm flexibility
 Material used: cast chrome (0.010”)
cast gold (0.015”)
wrought alloy (0.020”)

17.  Length of the clasp
•The longer the clasp arm the more flexible.
• Flexibility is directly proportional to the cube of its length.
•By increasing the length, the horizontal stresses imparted
to the abutment during placing and removal is reduced

18.  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

19. Support
 Support is the quality of the clasp assembly to resist
displacement of the prosthesis in the apical direction.
 a rest must contact the surface of the abutment tooth at
a properly prepared surface- rest seat

20.  A properly prepared rest will prevent the
tissueward movement of the prosthesis.
 maintains the position of the clasp assembly in
relation to the abutment.
 Transmits forces along the long axis of the
abutments

21. Stability
 Resistance to horizontal displacement

22. Reciprocation
 Counteracts lateral displacement of an abutment when
retentive clasp terminus passes over the height of
contour

23. Encirclement
 Prevent movement of abutment away from associated
clasp assembly
 More than 180 degrees

24. Passivity
 Prevent the transmission of the adverse forces
to the associated abutment
 Be passive until a dislodging force is applied

25. Classification of extra-coronal retainers
 Supra bulge clasps (occlusally approaching,
circumferential clasps)
 Infrabulge clasps (gingivally approaching,
projection or bar clasps)
 Combination clasps

26. Circumferential clasps
 The cast circumferential clasp design was introduced by
Dr N B Nesbitt in 1916.
 Simple, easy to construct- excellent support, bracing,
retentive properties.
 Close adaptation to tooth therefore minimises food
entrapment
 Disadvantage- covers large amount of tooth surface

27.  Circlet clasp.
 Reverse circlet
 Multiple circlet clasp
 Embrasure clasp.
 Reverse action or hair pin clasp
 Ring clasp.
 Back action and reverse back action clasp

28. Simple Circlet clasp
 Tooth support RPD
 Undercut remote from
edentulous area
 Half round
 Disadvantages
- Increase tooth coverage
- compromised esthetics

29. Variations of circlet clasp
 Back action clasp
 Reverse back action
 Ring clasp
 C clasp or hair-pin clasp

30. Reverse circlet clasp
 Undercut located adjacent to edentulous area
 Kennedy class I ,II
 Disadvantages
- Lack of rest adjacent to
edentulous area
- Poor esthetics

31. Multiple circlet design
 2 simple circlet clasp joined at the terminal aspect of
their reciprocal elements
 Principle abutment is periodontal compromised and the
forces are distributed between multiple abutment teeth

32. Embrasure clasp
 2 simple circlet joined at bodies
 Used on side of the arch where there is no edentulous
space
 Can be used only when adequate tooth preparation is
possible

33. C-clasp design
 Fish hook” or “Hairpin” clasp
 Simple circlet clasp with loop back retentive arm
 Sufficient crown height
 Disadvantages
- Insufficient flexibility
- Tooth coverage
- Esthetics compromised

34. Combination clasp
 Cast metal reciprocal arm and wrought wire retentive
arm
 abutment adjacent to Kennedy class I and II area
Advantage
• kinder to the tooth
can engage greater undercut
Disadvantage
• more prone to breakage than cast
• minimal stabilizing

35. Gingivally approaching clasps /Bar/Roach
type
Approach the undercut gingivally and have a push type of
retention.

36. Approach arm
• It is a minor connector that connect the retentive tip to
the denture base.
• It crosses the gingival margin at right angle and it is the
only flexible minor connector.
• Flexibility of the clasp is controlled by the taper and
length of the approach arm
• More esthetic

37. Retentive terminal
• It should end on the surface of the tooth below the
undercut.

38. T-clasp
 Kennedy class I and II
 Undercut locate adjacent edentulous area
 Contraindication
- Severe soft tissue undercut
- Height of contour locate near
occlusal surface

39. Modified T-clasp
 No retentive horizontal projection
 Kennedy class I and II
 Undercut locate near adjacent edentulous area
 Canine and premolar
 Advantage - esthetics

40. Y-clasp
 Equivalent to T-clasp
 Approach arm terminates in the cervical third
 Mesial and distal projection terminate near occlusal
surface

41. I bar
 Kennedy class I and II
 RPI
- Mesial rest
- Proximal plate
- I bar

42. Flexible clasps

43. • Acetal resin clasps are esthetic and are available in
sixteen different shades.
• To evaluate the effect of cast Co-Cr and acetal resin
clasp on the surface of tooth.
• The retentive force of cast Co-Cr clasp showed a
decrease from 12.4 N to 8.1 N.
• The retentive force of acetal resin clasp, reduced
from 5.2 N to 4.03 N at the completion of experiment.
• Acetal resin clasps do not abrade the surface of tooth
and maintain retention
A comparative study on Co-Cr and Acetal resin clasps; Pal .H etal
TPDI • January 2014, Vol. 5, No. 1 pg 9- 13

44. Implants as direct retainers
• Eliminates a visible clasp
• placement of an implant within a modification space to
the advantage of retentive needs requires
consideration of anterior, mid, or distal placement
• retainers utilizing teeth have always been restricted to
tooth locations at either end of a span

45. Indirect retainer
 Resists rotational displacement of an extension base
from the supporting tissue
 Kennedy class I, II and IV

47. Factors determining indirect
retainer
 Occlusal rests must be held in rest seats by direct
retainer
 Distance from fulcrum line
 Placed on definite rest seat to prevent slippage
 Rigidity

48. Auxiliary functions
 Reduce A-P tilting of abutments
 Stabilization – auxiliary guide planes
 Anterior teeth stabilized
 Auxiliary rest – stress distribution
 Visual indication for reline

49. Forms of indirect retainer
 Auxiliary occlusal rest
 Canine extension from occlusal rest
 Continuous bar retainer & lingual plate
 Rugae Support

50. Auxiliary Occlusal Rest

51. Canine rest

52. Rugae Support

53. Major connectors - cingulum bars and
linguo plates

54. References
 Carr AB, Mc Givney GP, Brown DT, McCracken’s
Removable Partial Prosthodontics, 12th edition ,
Canada, Elsevier Publishers, 2005 , pp:68-102
 Stewart, Phoenix, Cagna, De Freest, Clinical
Removable Partial Prosthodontics, 3rd edition,
2001, USA, Quintessence publishers, pp:53-96
 Grant AA, Johnson W, An Introduction to
Removable Denture Prosthetics, 1st edition,
USA, Churchill livingstone, 1983, pp: 96-101

55.  Davenport JC , Basker RM, Heath JR, Ralph JP,
Glantz PO, Retention ,Brit Dent J 2000;189(12):646-
657
 Davenport JC , Basker RM, Heath JR, Ralph JP,
Glantz PO, Hammond P, Bracing and reciprocation
,Brit Dent J 2001;190(1):10-14
 Davenport JC , Basker RM, Heath JR, Ralph JP,
Glantz PO, Hammond P, Clasp design , Brit Dent J
2001;190(2):71-81
 Davenport JC , Basker RM, Heath JR, Ralph JP,
Glantz PO, Hammond P, Indirect Retention ,Brit
DentJ 2001;190(2):128-132