The document discusses the concept of height of contour and how it relates to areas available for retention versus support on teeth. It then discusses survey lines and classifications proposed by Kennedy, Cummer, DeVan, Blatterfein and Ney for determining ideal clasp design based on the survey line. The document outlines the basic parts of a clasp assembly and factors that determine the amount of retention such as undercut size and depth, flexibility of the clasp arm based on its length, diameter, cross-sectional form and material. Ideal clasp design and flexibility varies depending on these factors and the classification of survey line.

1. DIRECT RETAINERS(2)
DR LEENA TOMER

2. This line, height of contour (specific to the surveyor-
defined path), is the boundary between
• (1) an occlusal or incisal region of the tooth that is
freely accessible to a prosthesis, and
• (2) a gingival region of the tooth that can only be
accessed if a portion of the prosthesis elastically
deforms and recovers to contact the tooth.
This surveyor-defined path and the subsequent tooth height of contour wilI indicate the
areas available for retention and those available for support,interefence to path of insertion

3. • This line, which Kennedy called the height of contour, is its greatest convexity.
•Cummer spoke of it as the guideline because it is used as a guide in the placement of retentive
and nonretentive clasps
•To this, DeVan added the terms suprabulge, denoting the suprabulge, infrabulge

4. • The location and depth of a tooth undercut
available for retention are therefore only relative
to the path of placement and removal
• The most suitable path of placement is generally
considered to be the path of placement that will
require the least amount of mouth preparation
necessary to place the components of the partial
denture in their ideal position on the tooth
surfaces and in relation to the soft tissue.
• Then mouth preparations are planned with a
definite path of placement

5. SURVEY LINE
• Blatterfein in 1951 put forth a simple and
comprehensive classification of surveyline with
suggestion on clasp selection.

6. He described four kinds of surveyline:
1. Typical surveyline or medium
2. Atypical A or Diagonal
3. Atypical B or High
4. Atypical C or Low

7. • 1. Typical or medium:
Clasps suggested are occlusally approaching and
gingivally approaching clasps.

8. • 2. Atypical A or diagonal:
-A reverse action or hairpin clasp is recommended.
- Gingivally approaching clasp may also be used.

9. • 3.Atypical B or High Surveyline:
A wrought occlusally approaching clasp arm may be
used.

10. • 4. Atypical C or Low Surveyline:
.
An extended type of clasp arm is recommended.
Undercut may be developed by recontouring the
tooth.

11. • Ney : Recommended 3 basic surveyline with an
appropriate clasp form.
• Class I :
Surveyline runs diagonally across the tooth surface
from a low position on the side of the rest to a
high position on the other proximal side.
A cast occlusally approaching arm are
recommended.

12. • Class II:
Similar to Blatterfein atypical A or Diagonal
surveyline. Here gingivally approaching is
recommended .
• Class III:
It is the same as the Blatterfein Atypical B or High
surveyline. The wrought wire arm is used.

13. Basic parts of a clasp assembly :
• Rest : It is the part of the
clasp that lies on the
occlusal, lingual or incisal
surface of a tooth and
resist tissue ward
movement of the clasp.
• Body of the clasp : It is
the part of the clasp that
connects the rest and
shoulder of the clasp to
the minor connector.

14.  Shoulder : It is the part of
the clasp that connects the
body to the clasp terminals.
It must lie above the height
of contour and provide
some stabilization against
horizontal displacement of
the prosthesis.
 Reciprocal arm : A rigid
clasp arm placed above the
height of contour on the
side of the tooth, opposing
the retentive clasp arm.

15.  Retentive arm : It is the part
of the clasp comprising the
shoulder which is not
flexible and is located above
the height of the contour.
 Retentive terminal : It is the
terminal end of the
retentive clasp arm. It is the
only component of the
removable partial denture
that lies on the tooth
surface cervical to the
height of the contour. It
possesses a certain degree
of flexibility and offers the
property of direct retention.

16. • Minor connector : It is
the part of the clasp
that joins the body of
the clasp to the
remainder of the
framework and must
be rigid.

17. Clasp retention is based on the resistance to deformation of the metal
clasp to be retentive, it must be placed in an undercut area of the tooth where it is forced
to deform upon application of a vertical
dislodging force

18. It is this resistance to deformation along an
appropriately selected path that generates retention
 Such resistance to deformation is dependent on
several factors and is also proportionate to the flexibility
of the clasp arm
Factors determining the amount of retention
of a clasp

19. 1. Size of angle of cervical convergence [depth of
undercut]
2. How far into the undercut clasp terminal is placed
3. Flexibility of clasp arm which is product of
a. length—origin to terminal end
b. relative diameter
c. cross sectional form
d. material– cast / wrought
Factors determining the amount of
retention of a clasp

20. Size of the angle of cervical convergence
• When the surveyor blade contacts a tooth on the cast at its greatest convexity,
a triangle is formed, the apical angle of this triangle is called the angle of
cervical convergence.
• To be retentive a tooth must have an angle of convergence cervical to the
height of contour.
Factors affecting retention of a
clasp

22. • Tooth surface can be modified /
recontoured by selective grinding or
• Placement of various restorations to
achieve a more suitable path of placement
or removal
• Each abutment tooth is considered as
separate entity when designing clasp
components

23. • Clasp retention is based on resistance of metal to
deformation
• Without guiding plane the clasp retention will be either
detrimental or practically non existitng
• Relative uniformity of retention will depend on the
location of retentive part not in relation to HOC but to
angle of cervical convergence
• Retention on principle abutments should be nearly equal
as possible…retentive arm should be in the same
approximate degree of undercut on each abutment

24. • Retentive clasp arms must be located so that they lie in
the same approximate degree of undercut on each
abutment tooth, despite the variation in the distance
below the height of contour.
• The measurement of the degree of undercut by
mechanical means is achieved by the help of an
undercut gauge attached to a dental surveyor.
DEGREE OF UNDERCUT

25. Retentive areas are not sufficient to resist
reasonable dislodging forces when cast is
surveyed at its most advantageous position
(occlusal plane parallel to surveyor table) even
though guide planes could be established with
minor tooth modification
Tilting cast creates functionally ineffective
tooth contours, which are present only in
relation to surveying rod and do not exist
when compared with most advantageous
position
(position in which restoration will be subject to
dislodging forces in an occlusal direction)

26. Clasps designed at tilt are ineffective without development of corresponding guide planes to
resist displacement when restoration is subject to dislodging forces in occlusal direction

27. The more vertical walls (guiding planes) that
are prepared parallel, the fewer the
possibilities that exist for dislodgment
 If some degree of parallelism does not exist
during placement and removal, trauma to the
teeth and supporting structures and strain on
the denture parts are inevitable
clasp retention will either be detrimental or practically
nonexistent

28. The retentive force is dictated by tooth shape and by clasp
design. Though clasps 1 and 2 are in an undercut of
0.25mm, 1 offers more retention than 2.

33. Flexibility of clasp arms
Length of the clasp arm :
The longer the clasp arm, the
more flexible it is, all other factors
being equal.
The length of a clasp arm is
measured from the point at which
a uniform taper begins
clasp arm should be tapered
uniformly from its point of
origin through the full length of
the clasp

36. • The diameter of a clasp arm is inversely proportional
to its flexibility.
• The average diameter to be considered is at a point
midway between its origin and its terminal end.
• The thickness of the clasp arm in the buccolingual
direction is to be considered rather than the width in
the occluso-gingival direction.
Diameter of clasp arm

37. Cross-sectional form
• Round cross-sectional form enables the clasp to be
flexible in all directions whereas the half-round form
limits the flexibility to only one direction. Indication
=class I ,II
• Cast clasps are half round in form and they flex away from
the tooth, but edgewise flexing is limited.

38. • If the cross-sectional area of clasp is doubled,
the stiffness will be increased 4 times and the
flexibility reduced 4 times.

39.  Type IV gold alloys and cobalt-chrome alloys
which have different modulus of elasticity.
 The modulus of elasticity of cobalt-chrome alloys
is greater than that of cast gold, which have a
higher modulus than wrought gold wires.
Material used for clasp arm :

40.  The retentive terminal has to be flexible and therefore
have low modulus of elasticity. The reciprocal elements
have to be stiff and unyielding and have high modulus of
elasticity.
 Therefore a clasp of the same cross-section is stiffer in
cobalt-chrome than in cast gold.
This can be overcome by using longer clasps of thinner
section and by doing contour modification so as to
reduce the degree of undercut.

41. • The alloy may be cast or wrought in nature. Wrought
wires have greater flexibility than a cast structure due
to its grain structure being fibrous.
Structure of the alloy

42. The tensile strength of a wrought structure is at
least 25% greater than that of the cast alloy from
which it was made.
Wrought forms can be used in smaller diameters
to enhance flexibility and they offer minimum
friction and can have a stress breaking effect.

43. • Flexibility may exist in any form
• but it is limited to only one direction in the case of
the half-round form.
• The only universally flexible form is the round
form,which is practically impossible to obtain by
casting and polishing.
• Because most cast clasps are essentially half round in form, they may
flex away from the tooth, but edgewise flexing (and edgewise
adjustment) is limited
• For this reason, cast retentive clasp arms are more acceptable in
tooth-supported partial dentures in which they are called on to flex
only during placement and removal of the prosthesis.

44. • SHAPED DIFFERENTLY THAN RETENTIVE ARM
• AVERAGE DIAMETER IS GREATER THAN THE
RETENTIVE ARM TO INCREASE THE DESIRED
RIGIDITY
• CROSS SECTION IS HALF ROUND
STABILIZING ARM

48.  Choice must be biologically & mechanically sound based on the
diagnosis & t/t planning
Exracoronal retainer should be considered as
combination of different components of RPD
framework designed to work together &
located to perform specific functions of
support , stabilization , reciprocation &
retention .It is not necessary that all the
components are physically attached to each
other or originate from major/ minor
connector of framework
SELECTION OF CLASP

49. 1. Is it flexible enough to satisfy the purpose
esp. for distal extension base?
2. Adequate stabilization is there to resist
horizontal & rotational movements?
3. Will rigidity be provided where it is needed?
4. Is clasp design applicable to malposed
/rotated abutment teeth
5. Can it be used despite the presence of tissue
undercut
Affirmative answer

50. 6. can clasp terminal be adjusted to increase
/decrease retention ?
7. Does clasp arm cover minimum of tooth
structure
8. Will clasp be as inconspicuous as possible
9. Will the width of occlusal table not be
increased?
10.Is clasp arm likely to be distorted or
broken? If so ,can it be replaced
.