This document discusses extracoronal direct retainers used in removable dental prosthetics. It defines extracoronal as being outside the crown of the tooth. It describes the components of clasp assemblies including the clasp arm, clasp assembly, and different types of survey lines that determine what type of clasp can be used. It outlines the basic principles of clasp design including providing support through an occlusal rest and having reciprocal arms to resist orthodontic pressure. Finally, it discusses factors that determine the amount of retention a clasp can provide such as the amount of tooth undercut, flexibility of the clasp arm, and material used.

1. Extra coronal direct retainers

2. Extracoronal is defined as: that which is outside or
external to, the crown portion of a natural tooth
Clasp (clasp arm): the component of the clasp
assembly that engages a portion of the tooth and
either enters an undercut for retention or remain
entirely above the height of contour to act as a
reciprocating element..
Clasp assembly: the part of a removable dental
prosthesis that acts as direct retainer and/or
stabilizer for prosthesis by partially encompassing
or contacting an abutment tooth.

3. Types of Survey lines
The position of the survey line is an important
factor in determination the types of
clasp that can be used

4. Four types of survey line:
1- Medium. 2- Diagonal 3- High 4- low
1) Medium survey line
Appear in the buccal or lingual surfaces of the tooth,
approximately equidistant from the occlusal surface and
gingival margin. This survey line indicates the
use of an occlusally approaching clasp, also bar type clasp
may be used.
2) Diagonal survey line
This survey line lies nearer to the occlusal surface than
the gingival margin,This survey line indicates the use of
reverse action clasp, ring clasp, back action clasp,
gingival approaching clasp with L and T bar clasp.

6. 3- High survey line
This type of survey line appears much nearer to
the occlusal than the gingival end of the tooth.
Occlusally approaching clasp is indicated
(wrought wire), back action and reverse back
action.
4- Low survey line
It occurs as a result of marked inclination of
tooth the retention obtained from extended arm
clasp or by crowning of the tooth to create
suitable undercut.

8. Components of the clasp assembly, their function and location

9. Component of clasp design
1- Rests: Yellow
2- Minor connector: Stripy (Blue& white)
3- Retentive clasp arm: red
4- Bracing arm (Blue)

10. Basic principles of clasp design
1- More than 180 degrees of the circumference of the tooth
crown should be included in the design, either through
continuous or interrupted contact.
2- Must provide support through the occlusal rest.
3- Each retentive terminal should be opposed by a reciprocal
arm to resist any orthodontic pressure exerted by the retentive
arm.
4- Retentive clasp should be bilaterally opposed e.g. buccal
retention on one side of the arch should be opposed by buccal
retention on the other side of the arch.
5- Amount of retention should be the minimum necessary to
resist reasonable dislodging forces.
6- Clasp retainers indicated in cases of free-end saddles must
possess a stress breaking action to minimize excessive torque on
abutment

11. Basic principles of clasp design
7- Ideally, the reciprocal elements should be located
at the lower part of the middle third of the crown
above the survey line, while the terminal ends of
the retentive arms at the gingival third of the crown
below the survey line.
8- The retentive clasp arm should be passive until a
dislodging force is applied.
9- The clasp assembly must be rigid except the
terminal retentive end.
10- The clasp assembly must be smooth and
polished

13. Factors determining amount of retention a
clasp is capable of generating
1- Amount of tooth undercut: The greater the amount of tooth undercut the
greater the possibility of retention.
2- How far the clasp terminal is placed in the undercut. The greater the
distance, the retentive clasp terminal is placed in an undercut, the greater
will be its retentive action.
3- Flexibility of the clasp arm which is affected by:
a- Its length from the point of origin to its terminal end.
b- Its relative diameter regardless its cross-sectional form , Increase in
diameter, results in decreased flexibility.
c- Its cross-sectional form: the flat, half round clasp arm is less flexible than
round clasp arms.
d- The material of which the clasp is constructed: Chrome-cobalt alloys are
less flexible than cast gold alloys which are also less flexible than wrought
gold clasps.
e- The taper of the clasp: Increase in taper, results in increased flexibility.
N.B. Retentive arms are tapered in 2 dimensions (lengthwise and widthwise),
while the reciprocal arms are tapered in one dimension only (lengthwise) i.e.
Reciprocal arms are less flexible than retentive arms.
f- Proper heat treatment increase flexibility