Prostho Lec 6

7/28/2019 Prostho Lec 6

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References lec 6

Chapter 3, Stewarts Clinical

Removable Partial Prosthodontics

(pages 51-83)

Chapter 6, Removable Partial

(pages 49-59)

We are going to talk about direct retainers they are thenext component of the metal frame work of the removable

partial denture design in which we are discussing

Retention in metal frame work of the removable partial

denture is different from other prosthesis for example for

the complete denture we get our retention from posterior

palatal seal how ever in metal frame work partial denture

we get our retention mechanically usually by engaging the

clasp under the maximum bulge of the tooth

There are two basic types of retention :

1-Intra coronal retentionWithin the contour of the tooth crown

Like the rests ,

precision attachments ( key and key lock

combination ), semi-precision

** we won't talk about these this semester they are

more advanced

2-Extra coronal retentionOutside the contour of the tooth crown

Cast*


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* wrought wires essentially prefabricated stainless

steal wires which we bend to form the tooth shape

clasps

Clasps above or below the bulge of the tooth aboveor below the survey line are

Supra bulge and Infra bulge

Infra bulge clasps give slight more retention than

supra bulge ones the reason is due what is called

push and pull theory

It's harder to push a table than to pull it so clasps

start from below the survey line when we try toremove it it will provide more resistance and retention

than the clasps which start from above the survey

line which is pulled so have less resistance


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The precision attachments provide support , retention

and reciprocation

Direct retainers usually form an assembly part of theassembly provide retention and the other parts of the

assembly give kind of a balance to the retention

which we need

so we have retention from this arm you

see this arm is on the survey line ( themaximum bulge ) on the facial surface

we have another arm on the lingual it's a mistake to

think that both of these arms provide retention

actually one of these goes to the undercut and the

other will not go it's function is different

the clasp assembly is composed of : the rest thatgives support , the retentive arm that goes above or

below the survey line to the undercut and that what

provide retention and the reciprocation arm which

balances what the retentive arm does or it's like an

abutment

( removable partial denture provides retention

because the clasps goes to the undercut )We have two types of clasps in dentistry the

prosthodontics and the orthodontics the ortho is

designed to move the teeth but in prost once it's fully

seated in the patient mouth it should not apply any

force to the patient teeth

However it's impossible to avoid some force on theteeth


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As the clasp is going down to be fully seated it's

going to go over the maximum convexity as the

clasps is going inside and outside the mouth it will hit

the teeth in very short seconds this short time as itgoes from above the survey line to below the survey

line can cause force on the tooth and this force can

cause movement even though it's only for short

period of time because it is repeated and that's why

we need something in the opposite side of the tooth

to hold it in place and that's what we call

reciprocation .

Here you can see the rest above and the

reciprocating arm is entirely above the

survey line where the retentive arm the tip is

below the survey line( the only part of the removable partial denture that

goes to the undercut is the tip of the retentive arm )

So Direct Retainers

Passive at rest

Activated only during dislodgement

basic requirements clasp assembly1- retention from the retentive arm2- support from the rest and keeps all the other

components in place when it's fully seated in the restplace

3- stability that resist horizontal movements from thebracing arm ,the rest and the minor connecter

4- reciprocation


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receprocating arm rest retentive arm

L B

If you take a look at this diagram if this is the tooth withthe lingual side ( L ) and the buccal side (b ) you will see

as the clasp is seated the reciprocating arm touches thetooth even before the rest and the clasp engage thatmeans as this retentive arm goes over the undercut startto press on the tooth to move it this reciprocating arm (balancing arm ) prevent movement of the tooth awayfrom the retentive arm it keeps the tooth in it's specificplace and here when it at rest it should be completelypassive no any force on the tooth

5- Encriclement which provide stabilization directlywhich prevent tooth migration same idea as thereciprocation the design )the rest ,the retentive arm, thereciprocating arm and sometimes the minor connector)should go around the tooth for at least 180 degreespreventing both the denture and the tooth from migration

6- Passivity once the partial denture is fully seated thereshould be no active force on the tooth

Passivity word is used in two concepts


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1- Preventing partial denture from moving the teeth2- Passivity of the alloyIn Co-Cr alloy the main component is cobaltCr is added to prevent corrosion we say that Cr providepassivity to the alloy

Magnitude of Retention

IMPORTANT: YOU MUST READ EXPLANATION

FOR

CLASP FLEXIBILITY/CLASP RETENTION FROM

THETEXTBOOK REFERENCE

The slide and the lecture are not enough

The equation is called law of beam and in our case it

is the law of the retentive clasp arm which can

handle an applied load and the clasp will be deflected

away from the tooth

There are dimensions of this beam it has a width , athickness and a length

It is important to understand these because the

design of the retainer will affect how many retention

and support

** If I want to make the clasp retainer longer it will

be more flexible and the shorter it is the more

rigid

** if I want to make the thickness thin or the width

narrow it will be more flexible

And this is what the equation stands for :

D = 4PL^3 / Ewt^3

D : deflection

P : applied force

L : length

E : elastic modulus w : beam width t : thickness


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by looking at the equation by increasing the

length you increase the resistance of the clasp

there are other factors other than the material

quality of the clasp which can affect the retention:

1- The shape of the tooth or the angle of convergent

If the angle of convergent is flat the surface is smooth that

means the amount of retention is less so the depth o f the

angle of convergence is important . the more convexity of

the tooth the more retention we have

Clasp flexibility1-the length, the longer it is the more flexible it will be

and it's increased by the curvature so molar teeth havemore retention2- diameter thickness and width when increased morerigid and when decreased more flexible .The actualmaterial of wrought wires is stainless steal is more

flexible than cast metal which is more rigid3-metal material, cobalt chromium is very rigid alloy whilenickel chromium is softer4-the shape the cross sectional geometry ,if its circular it

bends easily with flexibility in every dimension (wroughtwire) , if its square or semi circular( cast ) part of it will beflexible and the other part won't be it would be harderand the geometry will prevent the rotation you need to

read it from the text book .


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Direct Retainer Selection

Principal

Pick a retainer to suit the

existing teeth rather thanprepare the tooth to fit a

particular direct retainer

design

We talked about toothtissue and tooth tooth borne

partial denture. we said that one of the major concept

of a partial denture design is the fabrication of partial

denture rotation that is going to rotate around aspecific fulcrum and this fulcrum is a tooth this tooth

is going to take too much load and in our design

when we make the clasp we do not apply too much

force of the tooth which stand right at the end of the

fulcrum.

If we have a class one partial denture with a tooth in

one side and a tissue on the other side the stress willbe on the last tooth

Direct Retainers

Designed forTooth-Tissue

Borne.

Stress releasing.

Designed forTooth borne.

Non-stressreleasing.


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Types of clasps

We have something that reduces the stress of the

abutment and some which does not. some whichis very tight and one which release when there is

excessive force

non-stress releasing retainers1-cast circumferential clasp2- ring clasp which goes all the way around the tooth3-embrasure clasp but with two clasp next to each other

4-reverse action C clasp if It's adjacent to theedentulous place

1- cast circumferential clasp

Our first choice usually of a posterior tooth is actually

called-cast circumferential clasp which starts from

above the survey line which we called supra bulge

also known as Akers clasp ( name of the dentist who

designed it ) Simple to make, hygienic. And they

have excellent stabilization because we have good

encirclement around the facial and lingual with the

rest.And because they are supra bulge they are not

considered esthetic as they appear when the patient

smile they tend to be less hygienic from below and

it's more difficult to adjust the longer it is the easier toadjust and vise versa .


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The rest adjacent to the edentulous area and the

clasp is supra bulge we have something called the

shoulder which is above the survey line and the

terminus which engages the undercut and we have

the middle section. here in this design we start near

the edentulous area with the Reverse Action

Circumferential clasp we do exactly the same thingbut the rest is on the opposite side of the edentulous

area

2 Ring clasp we have another design called a Ring clasp

it engages the tooth not complete a 360 degree but maybe

300 degrees starting from one point and we remain above

the survey line we go around the tooth and we engage the

undercut in the lingual surface

Edentulous space

mesial to the molar.Typical Cast

circumferential retainer.

Undercut on the

distobuccal of the

molar (preferable

location).


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The thing that is significant about this kind of clasps is that

the bracing arm is both bracing and retentive. sometimesbecause it is too long ( flexible ) we can add an extension

(minor connector )to reduce this flexibility or usually

mandibular molars which are mesialy and lingually tilted

we add it to them as the pic on the left above.

Which tooth is one of the most extracted teeth ?

A : 6 molars because of bad oral hygiene with the

lack of knowledge that it is not a deciduous tooth so

extracted early in life because of caries then when the 7

erupts there is nothing in the place of 6 so the 7 tilts

mesially and lingually with such a tooth it is very difficult

to put a clasp in the right position so we use the ringclasp

Disadvantages of a ring clasp: they are very difficult to

adjust; they have poor hygiene (especially with the

presence of the strut).

They are contraindicated when there's excessive tissue

undercut: sometimes the lower piece of metal has to gonear the bone, so if there is excessive undercut they are

difficult to extend.


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3- Double Embrasure Clasp

the rests are back to back . some people like it a lot

some hates it a lot because they require a preparation ofthe tooth so the clasp will go to the embrasure.

however if I don't have an edentulous space how can I

put a rest for the minor connector ?? as u can see that

the embrasure clasp is

going to the facial

surfaces of both of the

teeth

4- C clasp ( Hair pin clasp )

It is the least desirable design

When we survey a tooth we mark the maximum convexity

of the tooth . some teeth are a symmetrical when the under

cut of the same tooth is law in one side and high on the

other . so sometimes we are forced to start the clasp from a

point and goes to the undercut to the rest distal corner . the

question is how to start from above from the distal and

finish here ?? ( look at the pic )


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So we have to go and then come back and it is very difficult

to adjust and not hygienic and technically difficult to make

But there are other solutions is putting the rest on the

mesial and bring the clasp down, there is different choices.

There is an unusual type of clasps where there is a

concavity in the occlusal plane we can solve it by adjusting

the occlusal plane using metal on top of the tooth ( not

important )

Now we finished the non stress bearing design we will

start talking about three Stress bearing design

when the denture goes away from the tissue or goes down

to the tissue we will end up with excess amount of force

on this tooth . the clasp is designed not to over load this

tooth

There is something called mesial rest concept when I

have an edentulous area the best place to put the rest

mechanically to put it in the point as close to the

edentulous area except in tooth tissue borne prostheses

we usually put the rest away from the edentulous area. if u

place it on the distal you will over load the tooth as the


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force vector is almost horizontal and the posterior teeth

and the tissues are not designed to get lateral forces. But

on mesial rest concept because the arm of rotation is

larger we have more vertical forces and this what happenswhen we change the position of the rest .

What happens with the clasps when we put the rest on the

distal side when the patient bite on the partial denture?

the clasp will extract the tooth .

the retention is needed when the patient opens his mouth

and when there is a sticky material in between his teeth soI don't need retention now ( we get retention from the

clasp ) .you can see that the clasp is working while the

patient is biting and we don't won't this to happen so as he

bite he extract his own tooth . but if we put the rest

mesially and when the patient bite the clasp will go away

from the tooth ( stress relief ) it works when it must and it

disengage when we don't won't it to work .

1- and this is what is called RPI system

"R" Rest (always mesial)

"P" Proximal Plate (distal)

"I" I - Bar (buccal)

We essentially have a bar or an infra bulge which comes

from below it provides aesthetics

bar


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2- RPA Clasps

in RPA we have the R ( mesial ) the P but the A here is

from wrought wires where in PRI was cast metal from I bar

now the A here is coming from above so it is supra bulge

and what they did actually they brought wrought wires and

bend it instead of metal clasps

Used where infrabulge approach not possible

"R" Rest (always mesial)

"P" Proximal Plate (distal)

"A" Aker's retentive arm (always

wrought wire)

3- Combination Clasp

Circumferential clasp with wrought wire clasp

Bracing and retentive arms originate from distal rest

Guide plane must not run entire occluso-gingival

height

Used when : 1- Tooth-borne cases with poor

prognosis for posterior abutments

2- Mesial rest and infrabulge approach not possible


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Notes about the lecture :

1- The dr emphasized to read the lecture from the

book as he was talking briefly

2- The dr changed the syllabus so this lecture is not

as 2009 one he said that everything in this lec will

be easier when we take the biomechanics lecture

which is on this Sunday

3- He was talking in Arabic all the time so I tried my

best to make everything clear4- Forgive me as there are not enough pictures in the

script but I don't have the slides

Finally I would like to say hi to my lovely adorable

friends Ala2 , hashim , sawsan and Areej love u

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Prostho Lec 6