Tripoding the Primary Cast

8/16/2019 Tripoding the Primary Cast

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Tripoding the Primary CastThe angle of path of insertion is maintained by maintaining the tilt determined for the

primary cast. To achieve this degree of tilt for the master cast, tripoding the primary

cast is done. If the path of insertion of the primary cast is not used for the master cast,

all the prosthetic mouth preparation procedures (rest seat preparation, guide plane

preparation dimpling, etc discussed later) done in relation to the path of insertion of the primary cast will become useless. Hence, it is very important for us to preserve the

tilt of the primary cast. The orientation of the cast is recorded during surveying.

Recording the spatial orientation of the cast is done by a procedure called tripoding.

Tripoding is a very simple procedure, wherein three different widely spaced out points

of a single plane are marked on the cast (ig. !".#$). These tripod points are reference

points and should not be altered till the end of treatment. The uses of tripoding include

positioning the master cast and remounting the diagnostic casts (if needed later) on

the surveying table.

%rocedure

• Tripoding is done after surveying the primary cast

• The primary cast is mounted according to the determined tilt on the surveying

table.

• & carbon marker trimmed to an angle of ' is fi*ed to the mandrel of the

surveying arm.

• The height of the hori+ontal arm is adusted such that the carbon marker

touches the tissue lingual to the teeth on the cast.


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• The hori+ontal arm is locked in that position.

• The surveying arm is moved freely. -ince the hori+ontal arm is locked to the

vertical arm, the tip of the carbon marker will lie in a single plane irrespective

of the position to which the surveying arm is moved (ig. !".#').

• &s the surveying arm is moved, two additional points in the cast that come in

contact with the carbon marker, are marked. -ince carbon marker is in the

same hori+ontal plane, all the three points marked using it will also lie in the

same plane (ig. !".#).

• ne technical consideration to be remembered is that the side and not the tip

of the carbon marker should be used to mark the tripoding points. This is

because if the tip is used, it may abrade and provide a faulty reading.

• &s an alternative to tripoding, the orientation of the cast can also be recorded

by scribing a vertical line on the base of the cast (ig. !".#b).


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Transferring the Tripod Marks

This procedure is done to orient the master cast using the same angulation of the

primary cast. &fter tripoding the primary cast, three additional reference points are

marked on the cast using the same procedure described to mark the first three tripod

marks. The commonly used additional reference points are/• 0istal marginal ridge of the first premolar.

• Incisal edge of lateral incisor.

• 1ingual cusp tip of the first premolar on the opposite side (opposite to the side

where the other two points were marked) (ig. !".#2).


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&fter marking the reference points, the primary cast is removed and the master cast is

placed on the surveying table. -ince the additional reference points are located on

distinctive anatomical landmarks, it is easy to locate them on the master cast. The

master cast is adusted in the surveying table such that the carbon marker in the

surveying arm contacts the additional reference points in the same manner as it did

with the primary cast (ig. !".#3).

Types of clasps The types of clasps are:

• 4ircumferential or &ker5s clasps

• 6ertical proection or 7ar or Roach clasps


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• 4ontinuous clasp

4ast circumferential clasp/ 8& clasp that encircles a tooth by more than !"9

degrees, including opposite angles, and which usually has total contact with the tooth

(throughout the e*tent of the clasp), with atleast one terminal being in the infrabulge

(gingival convergence) area: ; the

arms pass adacent to the soft tissues and approach the point or area of contact on the

tooth in a gingivo;occlusal direction: ;


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Types of 4ast 4ircumferential

4lasps 4ast circumferential clasps can be of !! different types. They are/

!. -imple circlet clasp (ig. !".!2")

@ Aost versatile and widely used.

@ 7est for tooth supported %artial dentures.

@ It approaches the undercut from the edentulous space.@ It engages the undercut, located away from the edentulous space.

@ 4lasp can be adusted only in one direction (i.e. buccolingually but not

occlusogingivally).

@ They cannot be used for distal e*tension cases as they engage a mesio buccal

undercut.

#. Reverse, circlet or reverse approach clasp (ig. !".!2B)

• This clasp is used when the retentive undercut on the abutment tooth is

located adacent to the edentulous space.

• 4onsider a distal edentulous condition. Csually the clasp will arise

from the distal surface of the abutment to reach the mesial undercut.

7ut this clasp is designed in such a way that the clasp arises from the

mesial side and ends on the distal undercut.• Csually 7ar clasps are preferred for distal e*tension cases. These

clasps are used when a bar clasp is contraindicated. ?.g.

• If there is an undercut area in the ridge @ %resence of a soft tissue

undercut caused by buccoversion of the abutment tooth.

• These clasps are used in distal e*tension denture base to control the

stresses acting on the terminal abutment teeth on the edentulous side.


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0isadvantages/

• If sufficient occlusal clearance is not present, the thickness of the clasp

has to be reduced. This will affect the strength of the clasp.

• The occlusal rest away from the edentulous space does not protect the

marginal ridge of the abutment tooth adacent to the edentulous space.

Hence, an additional rest must be placed to provide the necessary

protection.

•

%oor aesthetics as the clasp runs from the mesial to the distal end of thefacial surface.

• Dedging may occur between the abutment and its adacent tooth if the

occlusal rest is not well prepared.

$. Aultiple circlet clasp (ig. !".!39)

• It is a combination of two simple circlet clasps oined at the terminal

end of the reciprocal arms.

• It is used for sharing the retention with additional teeth on the same

side of the arch when the principal abutment tooth has poor periodontal

support.

• It is a mode of splinting weakened teeth.

• It5s disadvantages are similar to that of simple and reverse circlet

clasps.

'. ?mbrasure clasp or modified crib clasp (ig. !".!3!a)• It is a combination of two simple circlet clasps oined at the body.

• It is used on the side of the arch where there is no edentulous space.

• The clasp crosses the marginal ridges of two teeth to form the double

occlusal rest. The clasp emerges on the facial surface and splits into

two retentive arms. ?ach retentive arm engages the undercut located on

the opposite side of the tooth.

• Interpro*imal tooth structure should be removed to provide sufficient

thickness of the metal. The clasp may break if the metal is too thin.

• Indications/ It is used in Eennedy5s class II and class III cases without

any modifications.


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• ccasionally, a very small edentulous space can be closed by a

modified embrasure clasp called pontic clasp (ig. !".!3!b).

. Ring clasp (ig. !".!3#)

• 4onsider a distal edentulous condition with a distolingual undercut

where a reverse circlet clasp cannot be placed (no buccal undercut). In

such cases, the retentive arm is e*tended all around the tooth from the

distobuccal end to termiante in the distolingual undercut across the

mesial side of the tooth.

• It is used in cases with lingually tipped molar abutments. Aandibular

molars usually tip mesiolingually and the ma*illary molars tip

mesiobuccally. Hence, the retentive undercut will be on the

mesiolingual side for the lower molar and mesiobuccal side for the

upper molar.

• &s the clasp is long, additional support should be provided by adding

an au*iliary bracing arm from the denture base minor connector to the

center of the ring clasp on the buccal surface. (ig. !".!3$).

0isadvantages/

• &lteration in the food flow pattern.

• It cannot retain its physical Fualities.

• 0ifficult to adust or repair.

• Increased tooth surface coverage.

4ontraindications/

• If the buccinator5s attachment lies close to the lower molar.

• If the bracing arm will have to cross a soft tissue undercut.


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2. ishhook or hairpin clasp or reverse action clasp (ig. !".!3')

• It is a type of simple circlet clasp, which after crossing the

facial surface of the tooth loops back to engage the pro*imal

undercut beneath its point of origin. It is used in conditionswhere the undercut is near the edentulous space.

• Cpper arm is rigid and the lower arm is fle*ible. The upper arm

should be positioned above the height of contour in such a way

that it does not interfere with occlusion.

Indications/

• The undercut is adacent to edentulous area.

• %resence of a soft tissue undercut.

0isadvantages/

• It has poor aesthetics.

•It tends to trap and accumulate food debris.

3. nlay clasp (ig. !".!3)

•

It is an e*tension of a metal crown or onlay with buccal and lingualclasp arms.


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• It is used in the occlusal surfaces of submerged abutment teeth (that are

below the occlusal plane) so that the normal occlusal plane can be

restored with an onlay.

• If the onlay clasp is made of chrome alloy, the opposing tooth should

be protected with a gold crown. 7ecause the chrome alloy can produce

massive attrition of enamel.

• &s this clasp covers large amount of tooth structure, it may lead to

breakdown of enamel surfaces. Hence, it should be used only in a

caries resistant mouth.

". 4ombination clasp (ig. !".!32)

• & cast circumferential clasp cannot be used when an undercut is

adacent to the edentulous space, as it will produce destructive rotatory

forces on the distal abutment.

• In such cases, a fle*ible wrought wire retentive arm is used to replace

the rigid cast alloy retentive arm. These clasps are called combination

clasps as they combine the two.

• &s it has a greater fle*ibility it can be placed in a deeper undercutwithout any ha+ard to the abutment.

• It is used in ma*illary canines and premolars due to its superior

aesthetics.

&dvantages/

The round configuration of the wrought wire gives two advantages

• It has a thin line contact, which collects less debris and is easy to

maintain.

• It can fle* in all planes.

0isadvantages/

• Tedious lab procedures.

• ?asily breaks or distorts.

• %oor stability.


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B. Half and half clasp (ig. !".!33)

• It has a retentive arm arising from one direction and a reciprocal arm

arising from another.

• Two minor connectors are needed for this design. The first minor

connector attaches the occlusal rest and the retentive arm to the maor

connector. The second minor connector connects the reciprocal arm,which is similar to the bar clasp with or without an au*iliary rest.

• This design produces large tooth coverage, which can be reduced by

converting the reciprocal arm into a short bar with an au*iliary

occlusal rest.

• This design is intended to provide dual retention.

!9. 7ack;action clasp (ig. !".!3"a)

• It is a modification of the ring clasp.

•

Here the minor connector is connected to the end of the clasp arm andthe occlusal rest is left unsupported. 0isadvantages/

• 1ack of support to the occlusal rest reduces its function.

• It has both biological and mechanical unsound principles.


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!!.


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It is a minor connector that connects the retentive tip to the denture base minor

connector. It is semi circular in cross section and should cross the gingival margin at a

right angle. The approach should closely adapt over the soft tissues and cannot be

fabricated over soft tissue undercuts. This is the only fle*ible minor connector

designed in a R%0.

Retentive terminal (ig. !".!"9)

It varies for each type of bar clasp. It should end on the surface of the tooth below the

undercut. It can be paired or singular. The terminal, which enters the undercut, is

called retentive finger and the other terminal is called the non;retentive finger. It helps

to give a push type retentive force.

&dvantages of bar clasps

•It is easy to insert and difficult to remove.

• It is more aesthetic, as it covers less tooth structure.

0isadvantages of bar clasps

• It tends to collect food debris.

• It has increased fle*ibility but reduced bracing and stabili+ation.

• &dditional stabili+ing units are needed.

0esign considerations

• &pproach arm should not impinge as it crosses the soft tissue.

• Go relief is given so the tissue surface of the approach arm should be smooth

and polished.

• &pproach arm should cross the gingival margin at a B9 angle (ig. !".!"!).• It is used if the favourable undercut is present near the edentulous area.


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• The approach arm is a minor connector arising from the denture base. It arises

from the edentulous area near the undercut. It runs vertically upwards to the

height of contour of the abutment where it splits into its terminal ends.

• The tip of the retentive terminal should always point to the occlusal surface.

• The bar clasp should be placed as low as possible on the tooth.

Types of 7ar 4lasps

7ar clasps have been classified based on the shape of the retentive terminal. ?ach type

is described in detail below.

T clasp (ig. !".!"#)

• Csed in a distal e*tension denture base with a distobuccal undercut

• 4an also be used for a tooth supported partial denture with natural undercuts.

-ince the clasp is designed to use the e*isting undercuts without creating new

ones, it is referred to as clasping for convenience.

• It should not be used on a terminal abutment tooth if undercut is located awayfrom edentulous space.

• -hould not be used over a soft tissue undercut

• The clasp has good aesthetics but due to the fle*ibility of approach arm it

lacks the bracing Fuality.

Aodified T clasp (ig. !".!"$)

• It is similar to T clasp but the non;retentive finger (usually the mesial

terminal) of the T5 terminal is removed.

• It is used in canines and premolar for better aesthetics.


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0isadvantages

• It does not have !"9 encirclement.

clasp (ig. !".!"')

clasp is basically a T clasp modified to suit certain abutments where height

of contour is high at faciomesial and faciodistal line angles but low at the

center of the facial surface.


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I clasp (ig. !".!")

• Csed on distobuccal surface of canines for aesthetics.

• nly the tip of the clasp (#;$ mm) contacts the tooth. Hence, stability and

encirclement is decreased.

Infrabulge clasp or mirror view clasp/ by AA 0e6an (ig. !".!"2)

• The approach arm for the retentive terminal arises from the border of the

denture base, either as an e*tension of a cast base or attached to the border of aresin base.


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• There are two occlusal rests on each abutment tooth.

• The lingual aspect of the abutment may be plated (supported) or left open.

• It is more fle*ible because the portion of the metal base that gives rise to the

approach arm has an incomplete cut. The cut is prepared either by machining

or placing a thin matri* band during casting (ig. !".!"3).

• The retentive arm can also be made of wrought wire, which has higher

fle*ibility. The wire may be soldered to the metal base or embedded in the

resin base.

&dvantages/

• Aore aesthetic as it is placed more interpro*imally.

• Increased retention without any tipping action on the abutment.

• Resists distortion during handling.

I;bar

It is a modified I type roach clasp introduced by Eratochvil. It has a mesial rest

arising from a maor connector, an I;bar retentive arm and a long pro*imal plate.

Tripoding the Primary Cast

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