Prosto Lec 6

7/28/2019 Prosto 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 the next 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 retention

Within 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 retention

Outside the contour of the tooth crownCast *

* 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 above or below the survey line are

Supra bulge and Infra bulge

Infra bulge clasps give slight more retention than supra bulge ones the reason is duewhat 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 to remove 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 reciprocationDirect retainers usually form an assembly part of the assembly 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 (the maximum

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 that gives 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

appliances; the ortho appliance is designed to move the teeth but the prosthetic

appliance once it's fully seated in the patient mouth it should not apply any force to thepatient teethHowever it's impossible to avoid some force on the teethAs the clasp goes 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 it goes 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 )


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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 rest place3- stability that resist horizontal movements from the bracing arm ,the rest and the minor

connecter4- reciprocation receprocating arm rest retentive arm

Lb

If you take a look at this diagram if this is the tooth with the lingual side ( L ) and the buccal

side (b ) you will see as the clasp is seated the reciprocating arm touches the tooth evenbefore the rest and the clasp engage that means as this retentive arm goes over theundercut start to press on the tooth to move it this reciprocating arm ( balancing arm )prevent movement of the tooth away from the retentive arm it keeps the tooth in its specificplace and here when it at rest it should be completely passive no any force on the tooth

5- Encirclement which provide stabilization directly which prevent tooth migration same

idea as the reciprocation the design)the rest, the retentive arm, the reciprocating arm andsometimes 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 there should be no active force on the tooth

Passivity word is used in two concepts1- 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 provide passivity to the alloy


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Magnitude of RetentionIMPORTANT: YOU MUST READ EXPLANATION FORCLASP 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, a thickness 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

By looking at the equation by increasing the length you increase the resistance of

the claspThere 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


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Clasp flexibility1- the length, the longer it is the more flexible it will be and it's increased by the curvature

so molar teeth have more retention2- Diameter thickness and width when increased more rigid and when decreased more

flexible .The actual material of wrought wires is stainless steel is more flexible than cast

metal which is more rigid

3- metal material, cobalt chromium is very rigid alloy while nickel chromium is softera. if its circular it bends easily with flexibility in every dimension (wrought wire) , if its

square or semi circular( cast ) part of it will be flexible and the other part won't beit would be harder and the geometry will prevent the rotation you need to read itfrom the text book .

Direct Retainer SelectionPrincipal

Pick a retainer to suit theexisting teeth rather thanprepare the tooth to fit aparticular direct retainerdesign

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 a specific 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 otherside the stress will be on the last tooth

Direct Retainers

Designed forTooth-Tissue

Borne.

Stress releasing.

Designed forTooth borne.

Non-stressreleasing.


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Types of claspsWe have something that reduces the stress of the abutment and some which

does not. Some which is very tight and one which release when there is

excessive forceNon-stress releasing retainers

1- cast circumferential claspRing clasp which goes all the way around the tooth

2- embrasure clasp but with two clasp next to each other4-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 to adjust and vise versa .

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 whichengages the undercut and we have the middle section. here in this design we start near

Edentulous space

mesial to the molar.Typical Cast

circumferential retainer.

Undercut on the

istobuccal of themolar (preferable

location).


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the edentulous area with the Reverse Action Circumferential clasp we do exactly the

same thing but 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

The thing

that is

significant about this kind of clasps is that the bracing arm is both bracing and retentive.

sometimes because 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 ring clasp

Disadvantages of a ring clasp: they are very difficult to adjust; they have poor hygiene

(especially with the presence of the strut).


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They are contraindicated when there's excessive tissue undercut: sometimes the lower piece

of metal has to go near the bone, so if there is excessive undercut they are difficult to extend.

4- 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 of the 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

5- 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 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

happens when 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 .


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the retention is needed when the patient opens his mouth and when there is a sticky material

in between his teeth so I 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 theclasp 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

bar2- 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


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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

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 clear

4- 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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Mais Tahseen Maloul

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Prosto Lec 6