Prosthodontics lec #9

Last lecture we talked about class 3 and 4, this lecture is going to be about the Design of a RPD

We will go in the same order we learned before which starts with The Outline

the outline includes:

the classification determination of primary abutments guiding planes tilt

Slide 3

1- First we outline the edentulous area

in the above example it’s > Classification: class 1 (bilateral free end saddle)

2- then we determine the primary abutments which are the second premolars on both sides .

Primary abutments are the teeth adjacent to the edentulous area.

3- Determine the guiding planes

here they are located on the distal surfaces (the green lines in the above picture)

4- Tilt

the best tilt for the above case is an anterior tilt

explanation for the advantages of this tilt:

first advantage: the undercuts that are distal to the premolars will be reduced, resulting in minimum preparation for the path of insertion (Exposure of surface) .

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second advantage : the lingual bulge underneath the teeth will be reduced ; by tilting the cast we are exposing this area and so avoiding trauma and interference to the tissues .

Note that these advantages apply to both sides of the cast

recall that :

-the alternative tilt is related to the path of insertion and removal -the path of displacement is fixed and vertical - if we didn’t use a tilt both paths would be the same and that requires more preparations and application of clasps.

- The tilt is related to surveying that we did and studied before.

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The second step in Design is providing Support

remember we treat class 1 and 2 in a category different than the category of class 3 and 4 and there are cases that come in between . In the last slide the doctor summarized these different categories.

The main difference is the source of support; in class 1 and 2 it’s a dual support (both tooth and tissue ) . these differences indicate that we will have differences in the design .

The disadvantage of having dual support :

the most important point is that we have differences in compressibility ; as tissues are more compressible than teeth.

-this point is considered in the design , impression and all stages mentioned later on

-the denture will sink in areas that are more compressible and to avoid this problem in uniformity we perform certain procedures.

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This is the MAIN difference between classes 1 ,2 and 3, 4

factors influencing support

1- the contour of ridges :

more developed ridges provide more surface area and so better support .

2-the coverage:

we aim for maximum coverage, since more coverage means the load per unit area is less.

*Think about the feet of the camel as an example; they are wide and therefore will not sink in the sand.

3- Type and accuracy of impression registration

the more accurate the impression is, the better the fit will be.

The fit of the denture reflects the accuracy of the impression already taken

4- design of the partial denture framework

5- Total occlusal load

there are certain factors we can control to reduce the occlusal load and accordingly match the support we have provided. we will talk about certain factors we will control to reduce the occlusal loads and consequently reduce the loads on the ridges .

note in the above picture

- on the left the well extended areas will have better support .

- the well extended flanges will also be more supportive since they are providing maximum coverage.

* maximum coverage is within physiological limits .

- these provide better support , bracing and all mentioned properties required. but here we are mainly talking about support .

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- we didn’t talk about these factors in class 3 and 4 since they have uniform support

-these factors are under your control , except for the anatomy that the patient has ( his ridge for example ) .

- these factors reduce the difference of compressibility between teeth and the tissues and improve support.

Type and accuracy of impression registration

- we have two types of impression:

1- Anatomic 2- Functional / Mucofunctional

in the Anatomic:

- We are taking the impression for the tissues as they are. - We control this in the secondary impression by using flowy (low viscosity material) as we did with the complete dentures.- Materials with low viscosity only reflect fine details of the tissues without any compression.

- For this type of impression we also use spaced trays

on the other hand in the functional impression :

- tissues should be displaced within physiological limit.

-In the same procedure we perform close fitted tray and higher viscosity of the material

this is the second difference between the category of class 1, 2 and class 3, 4 . in classes 3 and 4 we use an anatomic impression because these classes are uniform ; no displacement or support by the soft tissues ( only coverage above the soft tissues ) .

* recall we treat class 4 as a dual case : if it was a long span we will treat it as a free end saddle ( tooth-tissue supported) , while if it was a short span we treat it as a bounded saddle .

Functional impressions cause displacement of tissues so is only needed in classes 1 and 2 .

in classes 1 and 2 the functional impression will reduce the difference in compressibility explained by the following :

when I take the impression which causes displacement of tissues , the reflected prosthesis will also be causing displacement and the fitting surface will be on a displaced tissue and any further load will not cause any more displacement and sinking .

The altered cast technique is another type of the functional impressions used in class 1 and 2 , which we will discuss. _____________________________________________________________________________________________

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The occlusal load

the less the occlusal load , the less the forces are on the tissues and a better support will be provided.

factors under our control :

- the number of teeth we can drop of few teeth and increase the support .

- Smaller occlusal table transmits less load.

Think of the occlusal table as an entrance for forces to the ridge. The smaller the entrance, the less the load and the better support we will have.

In the above image the ridges are the same . The smaller occlusal surface on the right causes less forces which are more manageable and better tolerated.

Wide coverage

maximum coverage results in fewer loads per unit area

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reducing the saddle movement under occlusal loads:

Methods:

-mucofunctional concept that we discussed earlier.

-sufficient support (sufficient retention is a more accurate description)

to explain this point we will think of them step by step :

- in class 1 and 2 we have a terminal abutment tooth .- this terminal abutment is adjacent to the free end saddle

- if the clasp that was holding the abutment tooth is short or not engaging the undercut - not following the criteria that I need - any load will cause its’ displacement and disengagement .

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-While if we had a proper design and proper material of the clasp engaging the undercut, the load will be resisted. take the example of the stand ( jumper plank ) in the swimming pool , it will sink with the force of a standing person , bit if it was firmly attached posteriorly it will not sink .

- all these methods under our control reduce the differences in compressibility and compensates for the difference.

Now we will talk about support

- support is directly provided by the rests - these rests could be either mesial or distal

- in the bounded saddle we don’t use the concept of ‘’mesial and distal’’ since the rest is mesial on one tooth and distal on to the other tooth in the bounded saddle. So we use the concept ‘’ adjacent to the edentulous area ‘’ .

in class 1 and 2 we have two options :

the rest is either adjacent to the edentulous area ‘’ Distal ‘’ on the primary abutment, or away from the edentulous area ‘’ Mesial ‘’ on the primary abutment .

We will go over the advantages and disadvantages of each and then decide which one we should use.

*the disto-occlusal rest

the only advantage to the disto-occlusal rest -which is adjacent to the edentulous area- is that :

- in the disto-occlusal rest the guiding plate / proximal plate holds the rest so we don’t need an additional minor connector to hold the mesial rest ( less components )

the disadvantages are many :

- it creates a class one lever system !_________________________________________________________________________________________

what are these systems ?

-Lever systems have three classifications -in levers always think of a rigid fulcrum, a resistance, and a force.

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the first classification is a see-saw

in which we have a fulcrum in the middle , a force on one side and resistance on the other side

in classification 2

we have the same components (fulcrum , force and resistance ) , but the fulcrum would be on the side , the resistance is in the middle and the force on the other side .

* if the resistance and force were flipped we would have a class 3 lever system (below is an image from the internet the effort represents the force and the load represents the resistance )

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Now we will reflect these cases on dental cases

in the disto-occlusal rest

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- the fulcrum is the rest- the resistance is the clasp (resisting the denture to come out )-the force is the occlusal load on the distal extension

The disadvantages in this case : ( you can’t just say the disadvantage is being a class 1 lever )

1- the disadvantage is the activation of this class on the teeth ( not the fact whether it’s class 1 or 2 , the disadvantage is the ACTIVATION on the tooth and transmission of forces on the tooth! )

- in the above picture the load on one side causes a force on the opposing side like a see-saw around the fulcrum. Now the clasp on the opposing side got activated

2- the second disadvantage : the entire force is directed on the terminal end of the tooth so it’s like you are causing a tipping force on the tooth away from the axis and opening the contact between the abutment tooth and the tooth adjacent to it mesially. This results in tooth impaction, periodontal problems and caries in addition to tipping and tilting of the tooth and with time this ends in losing the tooth.

3- third disadvantage : Less alveolar bone for supportwe start to calculate the alveolar support from the rest backwards as shown by the line in the image below(and this is less than the amount of alveolar support in the mesial rest which will be covering a longer area)

Mesio-occlusal rest :

Disadvantage: Requires an additional minor connector needed to hold the mesial rest (remember this is the advantage for the distal-occlusal rest)shown in the image below

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it’s a class 2 lever system

the advantages :

1- when a force is applied the clasp will disengage from the undercut as shown below

try to imagine the forces moving around the fulcrum line . the guiding plate moves downwards disengaging the tooth which is good since it’s not transmitting forces ,also the retentive tip disengages .This causes DEACTIVATION OF class 2 lever . note that there could be cases where it’s activated and causing a negative effect, but according to the forces and arrangement here it is deactivated

2-when the force is mesial , it will enhance the contact between the tooth and the adjacent tooth . while remember in the disto-occlusal rest the force was terminal causing the tipping of the tooth distally and opening the contact between the two teeth . - Here any force is transmitted to the adjacent tooth and therefore is not destructive

Follow the arrows below for better understanding

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3-the third advantage is having more alveolar bone utilization for support

the distance between the distal part and the line shows that the socket of the tooth is also included here for support. While in the disto-occlusal rest this distance was shorter.

4- Stress breaking effect

-relates to what we mentioned about deactivation of class 2. -Once the forces are no longer transmitted this is called stress breaking effect.

in other words , when the proximal plate disengaged and the tip disengaged , no more stresses are being transmitted to the tooth

* notes : since the retentive tip is flexible enough to flex around the bulbous part of the tooth it means it’s flexible enough to move out of its’ undercut - we use the I-bar since it is more flexible than the C-clasp in addition to its’ aesthetic advantage.

-stress breaking effect is a result of the disengagement of the components.

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now we will talk about the guiding planes:

- recall in the lab we prepared only 2-3 mm on the proximal areas.

- if we prepared the entire length extending occluso- gingival , it will not allow disengagement so the advantages of having a short guiding plane are :

1-a stress breaking effect 2- being more conservative in preparing the tooth 3- staying away from the gum and not irritating it ; as if it was long it will extend all the way to the gum .

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4- since teeth are moving in their sockets under the loads ( even though it’s as minimal as 0.2 ) , If the plate was extending for a long distance it will cause locking of the tooth and preventing the tooth from distributing the load on the periodontal ligaments and the physiological movements that should happen .

*these are disadvantages for a long guiding plane and advantages for a short guiding plane.

*this concept is called* the Crolls’ concept .

*It’s short with a range of 2 mm, but the long guiding plane it’s extended from the occlusal surface to the gingival.*guiding planes are always short.

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So now we finished support and said retention will be achieved by an I-bar on premolars wherever the undercut is “buccally , mesiobuccal, distobuccal”, depending on the undercut and surveying.

We talked about clasp axis: it’s a line extending between the tips of the clasps.

*If we had a vertical load on the denture :1- what’s the component that will prevent it from sinking towards the tissue ? rests and coverage.2- what’s the component that will prevent it from moving away from tissues? clasps.

If we consider the fulcrum in the picture, the movement will be rotational, if there was a force anteriorly what will happen ?

the major connector will sink anteriorly causing trauma to the tissues and sub-gingival area.

So we need components on the teeth anteriorly that will prevent the denture from moving away from tissues , these components are rests two rests , one on each side ( the red rests not the pink one “direct rests”) and they are called indirect retainers.

## a student asked the dr. what if we used a lingual plate as a major connector would that substitute the rests anteriorly ?

the answer is even if we placed a lingual plate we will have rests under it (why?) because under lingual plates we have a proclined teeth surface and without any preparations or rest seats, a strong force may cause the lingual plate to slip and damage tissues.

So we need actual preparation , something to engage. So we put indirect retainers under it rests. In this case it’s a ledge on both canines ( lower teeth less prominent cingulum so we don’t use inverted V ). But we won’t be able to see the rests because the plate is hiding it.

** so as a conclusion lingual plates provide indirect retention but there must be preparations on the teeth under it (not on primary abutments teeth but on canines which are considered secondary abutments), and if we turned the plate we will see a bulge in it where engaging rests are seen. this is called +ve support.

** to complete our design and in order for it to be efficient, we need reciprocation.

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** in this system we said we use : mesial rests + I-bar + proximal plate , and reciprocal arm won’t be needed (why ?) because the proximal plate and the minor connector extending from the mesial rest provide reciprocation and keep the teeth fixed in its place.*Note: reciprocation isn’t provided by a single point , it’s should be covering the teeth (surrounding it).

**But if we used distal rest we need a reciprocal arm. (why?) in the mesial rest situation we have a rigid component which is the proximal plate and the minor connector, so from two sides the tooth is fixed , so reciprocation is achieved but in the distal rest situation minor connector is not there and we only have proximal plate which is not enough to provide reciprocation, we have to add a reciprocal arm to cover the surface and achieve reciprocation and keep the tooth fixed.

** a student asked in case if I-bar clasp do we need a reciprocal arm ? The reciprocal arm and the I-bar are two different clasps, retention is achieved by retentive arm. Ex. On a molar we use a c-clasp and we have a retentive arm on one side (which engages in the undercut,

gives us retention and its tip is the only flexible component in the RPD) and on the other side the reciprocal arm.

** a reminder: the farther the indirect retainer the more efficient it is longer resistance arm.

* if we talked practically, we should put them on centrals but aesthetically we put them on canines or premolars.

** in this picture we have class II kenndy : It’s a combination of class I and class III (how?)

1- Class I : ** the free end saddle we use mesial rest + I-bar + proximal plate. ** reciprocation provided by minor connector and proximal plate>** using clasp axis to provide all the right components.

2- Class III (unmodified) :** cross arch stabilization .

** a student asked , in this case why didn’t we put a clasp anteriorly ?

It’s not necessary because we have can take advantage of the undercuts anteriorly, and aesthetically it’s not very acceptable.

Simplicity in the design,

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** now we will talk about bracing :Lower denture , anterio-posterio and lateral movement .** bracing occur when the denture is full seated.

in the picture we can see the forces exerted on the denture ,which components will provide bracing ?

**It has to be a rigid component against a rigid surface. In this picture we have two components that achieve bracing:

1- I-bars supported by teeth.2- Retromolar pad. Both of them will brace and don’t move backward.

in this picture If the forces were anteriorly not laterally, major connector won’t provide bracing (why?) nothing is behind the major connector to stop it from moving so it will go out.

But if the forces we posteriorly major connector will provide bracing.

**note: a component can provide both bracing and reciprocation depending on the forces and its direction.

** Altered cast technique :

#Free end saddles are liable to be displaced under occlusal .#This is as a result of the displaceability of the mucosa.#The altered cast technique is employed to try and prevent this by:

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1- taking an impression of the mucosa under controlled pressure. 2- The metal framework is constructed on a cast produced by a mucostatic impression material, usually

alginate. 3- Baseplates are then constructed in self-cured acrylics on the framework in the saddle areas, these are close

fitting.4- Impression paste or a medium viscosity silicone paste is then applied to the fitting surface of the self-cured

acrylic. 5- The denture is then inserted in place, held in place by the framework only, no finger pressure is applied as

this would lead to over displacement of the mucosa.6- Border moulding is then carried out as the is impression material is setting.

In the laboratory1- the free end saddle areas on the master cast are sectioned off. 2- The denture is then positioned on the model and the new saddle areas are poured. 3- The resulting model represents the free end saddle areas under conditions, which mimic functional load. 4- Denture construction then continues as normal. The distribution of loading of the free end saddles is improved

and denture is more stable.

Lab steps:

Done by : Rula Amarin & Noor Hilal.

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