This document discusses factors that determine the retention and flexibility of clasps used in partial dentures. It outlines several key principles of clasp design including encirclement of the tooth, providing adequate retention, support, reciprocation, bracing and stabilization. Factors like depth of undercut, angle of cervical convergence, angle of approach, length and diameter of the clasp arm, its taper, cross-sectional form and material all impact the flexibility and resulting retentive force of the clasp. Wrought alloys are more flexible than cast alloys of the same composition.

3. Retention of
Prof. Amal Kaddah
Dr. Mohamed Farouk
Dr. Amro Abdullah
Prosthodontic Department,
Faculty of Dentistry,
Cairo University

5. Is the resistance
of the partial
denture to vertical
displacement
away from the
tissues
Retention

6. 1. Have less surface area.
2. Are bathed in saliva.
3. Lower major connectors are
relieved.
The effect of physical forces is
less applicable to lower dentures
than upper because:
Contrary to maxillary m. c. that are well adapted and their
borders are beaded against the underlying tissues.
4. Strong movements of the tongue

7. 1-Adhesion
2-Cohesion
3-Interfacial s.t.
4-At. pressure
6-Gravity
Mechanical
Retention
PhysiologicalPhysical
-The physiologic
molding of the
tissues around
the polished
surfaces
- Neuromuscular
control
• Direct retainers
• Indirect R.
• Frictional fit
• Parts of the
denture
engaging tooth
and tissue
undercuts.

8. Mechanical
means of
retention
Intracoronal Extracoronal
Clasp retainer
Attachment
Attachment

9. Intracoronal attachments

10. Extracoronal attachments

12. • Frictional fit
Indirect R.
Direct
retainers
Parts of the denture
engaging tooth and
tissue undercuts.
• Clasps
• Attachments
Mechanical means of
Retention

13. Components of a clasp
Clasp Retainers
A minor connector
A rest
A Reciprocal arm
A retentive clasp arm

14. Clasp Retainer
Retentive arm
Bracing arm
Occlusal
rest
Minor connector
Occlusal view

15. Occlusally
Approaching
clasp
Gingivally
Approaching
clasp

16. Basic Principles of a Properly
Designed Clasp
1- Encirclement
Each clasp assembly must encircle more than 180 degrees
of abutment tooth

17. 180 ENCIRCLING TOOTH
continuous intermittent
Tooth can't move horizontally away from the clasp

18. Basic Principles of a Properly Designed Clasp
For a clasp to be
retentive its arm must
flex as it passes over
the height of contour
of tooth and engage
undercut in infrabulge
area of the teeth
2- Retention

19. DESIRABLE UNDERCUT
UNDESIRABLE UNDERCUT

20. 3- Support
1- Occlusal rest support prevents clasp from
being displaced in gingival direction.
2- lack of support
Basic Principles of a Properly Designed Clasp

21. 4- Reciprocation
Each retentive terminal should be opposed by a
reciprocal arm to resist any orthodontic pressure exerted
by the retentive arm during placement and removal as it
flexes about the height of contour
Basic Principles of a Properly Designed Clasp

22. 5- Bracing 6- Stabilization
All rigid parts of
clasps contribute to
this property and
resist displacement
of clasp in horizontal
direction
Basic Principles of a Properly Designed Clasp

23. The clasp should not interfere with normal
gingival stimulation and its terminal should be
away from the gingival margin
7- The clasp should be designed on biologic
as well as mechanical bases.
X
3-4mm

24. 8- Undercut is better be found within the GINGIVAL1/3 for
better esthetics & mechanics
Post Is More Readily Removed by Application of Force Near Its Top
Than by Applying Same Force Nearer Ground Level
Bracing arm better located In the apical portion
of the Middle 1/3

25. 9- There should be a minimum of 5-mm space between
any two neighboring minor connectors

26. 10- Minor connector (or proximal plate)
must contact a definite guiding plane to
dictate path of insertion

27. What are the functions of guiding planes
??????
1….
2….
3……
4…..
5…..
6….

28. The retentive clasp arm should
be passive and should not
exert any pressure against
the tooth until a dislodging
force is applied.
11- Passivity:

29. When you lose, don't
lose the lesson

30. Factors Determining the
Retentive Force of a Clasp
1- Amount of undercut used
2- Angle of cervical convergence
3-Angle of approach
4- Flexibility of clasp arm

31. Amount of retention depends on
The greater the distance we go into the undercut the greater will
be the retention considering the same rigidity.
1-Depth of undercut

32. Undercut depth depends on cervical convergence of
the tooth rather than on distance below the survey line.
2- Angle of cervical convergence

33. 3-Angle of approach
According to the way of approaching the undercut
Gingivally approaching clasp Occlusally approaching clasp

34. 3-Angle of approach
Occ. approaching clasps are pulled up to
move occlusally. Ging. approaching
clasps are pushed up to move occlusally
(Trip action of the clasp)

35. 4- Flexibility of clasp arm
The more flexible the clasp
arm, the less will be the
retention (but don’t forget the trip
action of the clasp !!!!!)
More rigid clasps can be used
in tooth supported partial
dentures

36. a. The length of the clasp arm
b. The diameter of the retentive arm
c. Tapering
d. The cross sectional form
e. The material of alloy
f. The type of alloy
The degree of Flexibility of the clasp
arm depends on the following factors

37. Increasing arm length increases the
flexibility, thus decreasing the retention
Factors affecting the Flexibility of clasp arm
a- The length of the clasp arm

38. The greater the length and tapering and
the less the diameter, the greater will
be the flexibility of the clasp arm
b. The diameter of the retentive arm
Flexibility of clasp arm:

39. Degree of taper , the retentive arm should be
uniformly tapered in such a way that the diameter at its
origin is twice that at its tip.
C- Tapering
Flexibility of clasp arm:

40. Round clasps are the
most flexible as they flex
in all direction thus
lowers the retention than
½ round and flat clasps
d- Cross sectional form

41. Flexibility of clasp arm:
f-The type of alloy
e- The material of alloy:
The wrought form is more resilient than
the same alloy of identical diameter in
cast form, because of its internal structure
Gold alloys are more flexible than
cobalt chrome alloys.

42. The wrought form is more
resilient than the same alloy of
identical diameter in cast form,
because of its internal structure

43. Every moment,
every situation,
every issue and
every concern has a
positive side. Find it
and bring it to life....