prostho final lec 2 part 2

7/30/2019 Prostho Final Lec 2 Part 2

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*Quick review: after block out and relief on the master cast, we duplicate

it with the block out and the relief, now we have the refractory cast,

which is made out of phosphate bonded investment. We do our wax up on

this cast, we invest it, make our mold, sprue it, and cast the metal, then

we break the refractory cast (this is why we cant go back to the

refractory cast for fitting the framework), we cut away the sprues and we

end up with a nice metal framework.

-We take this framework back to the master cast. Our reference isalways the master cast, that's why we don't want to damage this cast,

Before we fit the framework on the master cast, we'll wash away the

block out and relief wax (remember the wax is still on the master cast

because we duplicated it to the refractory cast, and the wax wasnt lost or

molten away during duplication).

If the framework fits and everything is ok, we may proceed to processing

the acrylic which is also done on this cast (master cast).

*The master cast is used for:

1. Block out and relief.2. Fitting the metal framework.3. Processing: waxing up, setting the acrylic teeth, and

processing the acrylic.

(Question on the exam )

-Where should there be contact? Where do I want interferences?

Retentive arms, reciprocating arms, the proximal plates, superior portion

of the lingual plate, all maxillary major connectors and the meshwork in

one spot which is the tissue stop if present.

-Where are the most common interference areas?

1. Under rests2. Under the rigid portions of direct retainers (the retentive clasps)3. Inter-proximal portions of lingual plates (where the mandibular

major connector enters between teeth in the embrasures).


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4. Inter-proximal minor connectors (where the minor connector entersbetween two adjacent teeth to reach the rest).

5. Shoulder areas of embrasure clasps.

-As long as the rests are fully seated, even if the guide plate shows a

burn through, its ok because in this area the contact is normal.

-The reason the DR. mentioned this, is that youre supposed to have

contact in these areas. There's a disclosing agent, we paint it inside the

denture to see where it touches or interferes.

-Its a common mistake by many dentists, they paint the disclosing agent

on the partial denture then put it in the patient's mouth, theyll find areasin contact with the tooth (like guide plates, and rests etc...) and theyll

start to grind these areas. The point is that u needs to know where and

when to stop.

-If you keep grinding the areas that are supposed to touch, which give the

support, stability and retention, you'll destroy them and end up with

poorer support, stability and retention.

-You need to make sure that the clasps are shaped correctly; one of themost important concepts is that the end of the clasp is always tapered, it

starts broad and then as it goes toward the tip it becomes narrower. This

tapering provides the tip of the clasp with flexibility and the base with

rigidity

-With metal there's an oxide layer produced by the casting process, we

need to remove this layer and we can't remove it mechanically because

we'll destroy the surface.

-We need to remove it in a way so that we only remove one or two

molecular layers of the metal alloy, and we do this using an electro-

polisher; which is a special machine with an electro galvanic current

passing through a bath of concentrated acid (sulfuric acid). The galvanic

current is used to remove a layer of metal from the prosthesis. So we end

up with a nice, smooth, shiny surface, without changing the fit of the

prosthesis.


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-But first we have to do the mechanical polish, so we need to make sure

that we have the right equipment.

*The burs that we use for gross reduction are:

1. Heatless stones (we use a variety of stones, some are aluminumoxide, some are silicon carbide...)

2. Tungsten carbide.3. Diamond burs (the problem with diamond is that they leave a

lot of scratch marks).

4. Coarse stones: shofu coral stones, carborundum disks (carborundum is silicon carbide, and it's very abrasive, used to

cut metal because it's harder than the cobalt chromium, we use

them for polishing and trimming)


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-Usually we start with something which is coarse and rough to do the

gross reduction and then we smooth it off to make the final polish.

*A few questions to help us diagnose whose fault the defect is:dentist/technician

1. If I have a metal framework that fits perfectly on the master cast,but it has no relationship to the patient's mouth (it doesn't fit the

patient's mouth), the problem here would be due to a bad

impression which is done by the dentist.

2. If the framework doesn't fit neither the master cast nor thepatient's mouth, the problem here would be either the dentist (badimpression taking) or the technician (bad fabrication).

*Note: if the framework fits the master cast, and the master cast isn't

scratched or damaged, the problem is most likely the dentist's fault.

*Physiologic relief:

-This is specifically for tooth-tissue borne prosthesis (Class 1, 2, long

span class 4 and very rare cases of long span class 3).


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-We talked about different ways to relieve the primary abutments, likechanging the rests' position (Mesial/Distal), or choosing different types ofclasps, in addition to these things we need to relieve under the guide

plates either before the framework fabrication using wax, or after the

fabrication by trimming.

*How do we know where exactly to relieve?

-As you remember we already relived under the guide plate with

parallel block out, but in most cases it's not enough, we have todouble checkwe do this by the following procedures:

-We talked about two types

of motions:

1. away from the tissue so we

need indirect retention

2. Towards the tissue, in this

case we need support (To

distribute forces between the

tooth and the tissue)


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1) We paint a disclosing agent on the fitting surface of the

prosthesis where we think it will cause problems, particularly

guide plates and sometimes lingual plates. Then place it in the

patient's mouth and rotate it, this rotation will result in pressure on

the gingival third of the tooth, (we designed our prosthesis so that it

has parallel block out, but with rotational forces there still may be


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some contact between the guide plate and the gingiva, or the guide

plate and the gingival third of the tooth).

-After we seated the prosthesis we take it out of the patient's

mouth, the disclosing agent will reveal the areas of contact.

-Normal contact areas like the upper part of the guide plate

shouldnt be relieved, but abnormal contact areas such as the

lower third (gingival third) of the guide plate should be relieved

(trimmed).

-Remember this unwanted contact (gingival contact) resulted from

rotational forces applied on the prosthesis. However when passively

seated (meaning that we only inserted and removed it), the prosthesis

should show us contact in the upper part of the guide plate (which isnormal).

-After trimming, we go back to the patient's mouth and check that the

longer any forces being appliednorelief was enough, and that there are

on the abutments


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of special impression proceduresThe aim

take impression with slight pressureis to

pressurethe same as the normal functional

. We dont face this problemin the mouth

with the short span bonded area bcz the

pressure will directly transfer to the teeth,

so, you can use conventional impression

procedure BUT in the free saddle area we

have a problem bcz the soft tissue move

and compress so we need to use thus

special procedure. And its considering a

stress area so, we need to release this

stress.

*So, our Requirement is:

record the tissues under uniform loading distribute load over as large an area delineate the peripheral extent of the denture

Factors Influencing Support of the Distal Extension Base:*

1. Quality of soft tissue covering edentulous ridgeA firm, tightly attached mucosa displaying moderate thickness (2 to 3

mm) will offer the greatest support (more thickness, more need for

functional force).

2. Type of bone in the denture-bearing area-Cancellous bone, as compared with cortical bone, is less able to resistvertical forces.bcz its sharp so excessive pressure on it end with inflamed

tissue)


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-The ideal ridge would consist of:

Cortical bone that covers dense

cancellous bone with broad rounded

crest and high vertical slops.

*Cortical bone can resist vertical forces

better than cancellous bone.

3. Design of the prosthesis.

-Knowledge of basic principles of designs guides the management of

functional forces.The use of indirect retainer will control rotationalmovement of distal extension RPD.

4. Amount of tissue coverage of denture base

-The broader the coverage of the edentulous ridge, the greater the

distribution of the load & the smaller the force per unit area

-we try to cover tissue as we can BUT we should avoid over extension.


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5. Anatomy of the denture-bearing area.-Each denture base must be made to fit the areas

that can serve as primary stress-bearing regions.

We use it to get the primary support. BUT in the

maxilla these area are cover by a thin layer of

mucosa

-In the maxillary arch we dont need to take

functional impression bcz the tissue is favorable

-In the lower arch, the crest of the ridge are

not favorable to be a denture bearing area ouraim from using the functional impression is to

transform the pressure from the crest of the

ridge to the buccal shelf area.

*conclusion:To distribute the forces of mastication to the ridge mostprimary stressefficiently, the majority of force must be directed to the

forcethat are capable of withstanding thatbearing areas

6. Fit of denture base:

-Support is enhanced by intimate contact between the mucosa andthe fitting surface of the partial denture;

7. Type and accuracy of the impression registration: -the majority of the force must be directed to portions of the ridgethat are capable of withstanding the force

*Indications for special impression procedure:1.mandibular distal extension application2.A long-span anterior edentulous base (normally including

at least the six anterior teeth).


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*Impression Methods:

1. McLean physiologic impression2. Functional reline method3. Corrected cast procedure (Selected pressure techniques)

**McLean physiologic impression:

-A dual impression technique (We call it a

dual impression technique bcz its use 2

impression materials:

1- Functional impression materialusing functional pressure

2- static impressing material (alginate)which is taken during statics (pic-upimpression)

Constructed a custom tray on a diagnostic cast, its only coveringthe edentulous area.

A functional impression was made using this tray and a suitableimpression material hydrocolloid is also taken "over-impression

Could not produce sameFunctional displacement

Generated by occlusal

forces.(actually its a problem)

-We used ZOE or polysulfide or

polyether or any rubber impression

material.

-I return it to the patient mouth using

a perforated tray then I will get an

over impression.

-You notice in the picture we have 2

colors the blue is the functional


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impression and the white is the alginate.

*McLean Disadvantage:

clasps is sufficient :may result in compromised blood flow withadverse soft tissue reaction and resorption of the underlying bone

clasps is not sufficient: the denture base will be occlusallypositioned (premature contact)

*Functional reline method

Done after the partial denture has been completed (done at a laterdate) but the maclean is taken during the secondary impression.

Adding a new surface to the intaglio of the denture base (functionalrelying) we add the new layer to the fitting surface.

The partial denture is constructed on a cast made from a singleimpression with a soft metal spacer underneath to ensure a

uniform space for the impression material (we use the spacer to

create a space for the relaying technique).


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The patient must keep the mouth partially open to permitappropriate tissue control and visual assessment

modeling plastic like a green stick is applied to the intaglio(fittingsurface) of the denture base

1mm of modeling plastic is removed from the intaglio surface andan impression is made by ZOE or any rubber material.

:Disadvantage*

failure to maintain the correct relationship between the frameworkand the abutment teeth during the impression

Failure to achieve accurate occlusal contact following the relineprocedure bcz its must take while the patient opening his mouth so

we cant control occlusion force.

Occlusal discrepancies must be corrected: slight: accomplishedcorrection directly in the mouth, majority of cases: remount on an

articulator.


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*Altered cast technique:

-This subject is very important, and often asked about in exams.

-The difference in the mobility and

compressibility between the tissue and

the tooth is one of the main problemswe face when dealing with partial

dentures. We can minimize this

problem using the altered cast

technique.

*Note:For tooth-tissue borne prostheses, the general term is "physiologic

impressions for tooth-tissue borne RPDs" or "special impression

procedures for tooth-tissue borne RPDs" rather than "altered cast

technique.

-There is more than one type of

physiologic impression/special impressionfor tooth-tissue borne RPDs. The Dr.

described two in the slides:

(1) corrected cast technique.

(2) Reline technique.

*Corrected cast procedure (selected pressure techniques)

bcz some alteration is done foraltered cast techniqueWe also, called it-

the master cast during this technique.

acrylic base processing.beforeThis method is done-

weand thenof the patient's mouth,static impression-mucoWe take a-

pour the cast and fabricate our metal framework. To reduce the


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movement resulting from the difference in compressibility between the

teeth and the tissue, we have to compress the tissue.

-This compression can't be done on the cast (because stone is not

compressible), so we fabricate a tray on the framework only in theedentulous areas (while the framework is on the cast), make sure the trays

are 2mm short from the depth of the sulcus, because we're going to

border mold.

-Place the prosthesis in the patient's mouth, check and see the peripheries

(the tray is 2mm short of the sulcus), now border mold. After border

molding we need to take an impression.

**Step by step:1-We adding an impression tray after the

framework have been fitted using a chemically

activated or light-activated resin. We use its

for the free saddle areaimpression only

2-Undercuts that would interfere with removal of

the tray are blocked out

-separating medium is then placed

-Tray is adapted to the master cast

-Boarders trimmed using a laboratory knife and

gently rounded

-the edge of the tray should be 2 to 3 mm from

the depth of the buccal vestibule


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-Border molding for a corrected cast is basically the same as that for acomplete denture covers

-the buccal flange to the most posterior extent also border molding for the

lingual and distolingual flanges

*Impression materials that we can use (they need to be thin and

muco-compressive) are:

with 0.5mm spacer or without spacer (usually it'sZinc oxide eugenol1)

our material of choice)

(Ex. Kerr Korecta wax or Iowa wax).Functional impression wax2)

They are very thin, we paint them on the internal surface of the tray


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(they're designed to soften at the mouth temperature), we apply them in

the patient's mouth and we press, they will compress the tissue.

(Ex. polyvinyl siloxane)/Regular body addition silicon3) Medium

-After taking the impression we go back to the cast, this impression will

not fit the cast because the cast was made with a muco-static impression

while this is a muco-compressive impression, so we have to alter our cast.

-We alter the cast by removing or

cutting away the stone in the edentulous

areas. Now put the framework in its

place and use the trays attached to it as

an impression to pour the edentulous

areas which were removed from the

original cast.

-Before pouring we do boxing. Now we pour the stone and we end up

with a cast that is mixed with two colors, one color representing the

original cast, and another representing the new stone that we poured.

-The new stone is lower in relation to the original cast because it's

compressed. It's compressed within normal physiological limits so that it

won't irritate the patient. The new edentulous areas are taken under

function = muco-compressed = muco-displaced.


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*summary:


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prostho final lec 2 part 2

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