Lec.10 Types of Clasp Assemblies
Lec.10 Types of Clasp Assemblies
Lec.10 Types of Clasp Assemblies
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3rd
year / College of Dentistry/University of Baghdad (2017-2018) Page 1
Indications:
a. It is most logical clasp to use with all tooth-supported partial dentures because
of its retentive and stabilizing ability.
b. On free end extension when minimal undercut is utilized.
Contra indication:
a. When the retentive undercut may be approached better with a bar clasp arm.
b. When esthetics will be enhanced by using bar clasp arm.
Advantages
a. Excellent bracing qualities.
b. Easy to design and construct.
c. Less potential for food accumulation below the clasp compared to bar clasps.
Disadvantages:
a. More tooth surface is covered than with a bar clasp arm because of its occlusal
origin.
b. On some tooth surfaces, particularly the buccal surface of mandibular teeth and
the lingual surfaces of maxillary teeth, its occlusal approach may increase the
width of the occlusal surface of the tooth.
c. In the mandibular arch, more metal may be displayed than with the bar clasp
arm.
d. Its half-round form prevents adjustment to increase or decrease retention. True
adjustment is impossible with most cast clasps.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 2
The circumferential type of clasp may be used in several forms:
Indications:
a. It is used when a proximal undercut cannot be approached by other means. For
example, when a mesiolingual undercut on a lower molar abutment (isolated
lower molar such as in Class II modification one) cannot be approached directly
because of its proximity to the occlusal rest area and cannot be approached with a
bar clasp arm because of lingual inclination of the tooth.
Disadvantages
a. Covers a large area of tooth surface, therefore requiring meticulous hygiene.
b. Very difficult to adjust due to the extreme rigidity of the reciprocal arms.
c. The lower bracing arm should be at least 1mm from the free gingival margin
and relieved to prevent impingement of the gingival tissues.
Example of use of embrasure clasp for a Class II partially edentulous arch: Embrasure clasp
on two left molar abutments was used in the absence of posterior modification space.
Indications:
a. In an unmodified Class II or Class III partial denture, where there are no
edentulous spaces on the opposite side of the arch to aid in clasping.
Occlusal and proximal surfaces of adjacent molar and premolar prepared for embrasure
clasp. Note that rest seat preparations are extended both buccally and lingually to
accommodate retentive and reciprocal clasp arms.
Disadvantages:
a. Extensive interproximal reduction is usually required.
b. Covers large area of tooth surface – hygiene considerations.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 5
Other less commonly used modifications of the cast circumferential
clasp are:
1. Back action clasp
a. The back-action clasp is a modification of the ring clasp.
b. It is used on premolar abutment anterior to edentulous space.
c. The undercut can usually be approached just as well using a conventional
circumferential clasp, with less tooth coverage and less display of metal.
2. Multiple clasp
The multiple clasp is simply two opposing circumferential clasps joined at the
terminal end of the two reciprocal arms.
Indications:
a. It is used when additional retention and stabilization are needed, usually on
tooth-supported partial dentures.
b. It may be used for multiple clasping in instances in which the partial denture
replaces an entire half of the dental arch.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 6
c. It may be used rather than an embrasure clasp when the only available retentive
areas are adjacent to each other.
Disadvantage:
a. Its disadvantage is that two embrasure approaches are necessary rather than a
single common embrasure for both clasps.
3. Half-and-half Clasp
a. It is consists of a circumferential retentive arm arising from one direction and a
reciprocal arm arising from another.
b. The second arm must arise from a second minor connector, and this arm is used
with or without an auxiliary occlusal rest.
c. Reciprocation arising from a second minor connector usually can be
accomplished with a short bar or with an auxiliary occlusal rest, thereby
avoiding so much tooth coverage.
d. There is little justification for the use of the half-and-half clasp in bilateral
extension base partial dentures.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 8
prevent more denture base movement. This is concept include RPI and RPA
clasps.
RPI clasp
RPI clasps are referring to the: R = Rest, P = Proximal plate, and I = I-bar.
These are component parts of the clasp assembly. Basically, this clasp assembly
consists of:
a. A mesioocclusal rest with the minor connector placed into the mesiolingual
embrasure, but not contacting the adjacent tooth.
Occlusal view
A distal guiding plane, extending from the marginal ridge to the junction of the
middle and gingival thirds of the abutment tooth, is prepared to receive a proximal
plate .The buccolingual width of the guiding plane is determined by the proximal
contour of the tooth.
b. The proximal plate, in conjunction with the minor connector supporting the
rest, provides the stabilizing and reciprocal aspects of the clasp assembly.
c. I-bar should be located in the gingival third of the buccal or labial surface of
the abutment in a 0.01-inch (0.25mm) undercut. The whole arm of the I-bar
should be tapered to its terminus, with no more than 2 mm of its tip contacting
the abutment. The retentive tip contacts the tooth from the undercut to the
height of contour. This area of contact along with the rest and proximal plate
contact provides stabilization through encirclement.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 9
The horizontal portion of the approach arm must be located at least 4 mm from
the gingival margin and even farther if possible.
The bar clasp arm arises from the denture framework or a metal base and
approaches the retentive undercut from a gingival direction. The bar clasp arm has
been classified by the shape of the retentive terminal. Thus it has been identified
as T, Y, L, I, U and S. I shape bar is prefer than other shapes because this shape
being biologically and mechanically sound.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 11
There are several other types of bar clasps; example:
Infrabulge clasp:
It is designed so that the bar arm arises from the border of the denture
base, either as an extension of a cast base or attached to the border of a
resin base. It is made more flexible than the usual bar clasp arm.
Advantages:
a. Its interproximal location, which may be used to esthetic advantage.
b. Increased retention without tipping action on the abutment.
c. Less chance of accidental distortion resulting from its proximity to
the denture border.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 12
Advantages:
a. The flexibility.
b. The adjustability.
c. The esthetic appearance of the wrought-wire retentive arm over other retentive
circumferential clasp arms (It may be used in smaller diameters than a cast
clasp, with less danger of fracture).
d. Minimum of tooth surface covered because of its line contact with the tooth,
rather than having the surface contact of a cast clasp arm.
e. A less likely occurrence of fatigue failures.
Disadvantages:
a. It involves extra steps in fabrication, particularly when high-fusing chromium
alloys are used.
b. It may be distorted by careless handling on the part of the patient.
c. Because it is bent by hand, it may be less accurately adapted to the tooth and
therefore provide less stabilization in the suprabulge portion.
d. It may distort with function and not engage the tooth.
Indications:
a. When maximum flexibility is desirable, such as on an abutment tooth adjacent
to a distal extension base where only a mesial undercut exists on the abutment
or on a weak abutment or where a large tissue undercut, contraindicates a bar-
type direct retainer.
b. It may be used for its adjustability when precise retentive requirements are
unpredictable and later adjustment to increase or decrease retention may be
necessary.
c. When esthetic required over cast clasps, because wrought -wire is round, light
is reflected in such a manner that the display of metal is less noticeable than
with the broader surfaces of a cast clasp.
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year / College of Dentistry/University of Baghdad (2017-2018) Page 13