prostho sheet #2 - Dyala Al-Armouti

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prostho sheet #2 - Dyala Al-Armouti

Post by Dyala Al-Armouti on 26/2/2011, 6:59 am

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the pictures are not available here... so you have to refer to the link and download it to study the sheet well...
بسم الله الرحمن الرحيم
Prosthodontics lecture #2
Removable partial denture components




Removable partial dentures may be either cobalt chromium, or acrylic...



We will focus mainly on metal RPD (cobalt chromium)…


You can see in this picture an example of an RPD in its place

AS A REVIEW:

RPD should fulfill certain requirements:

1- Support: Resistance to occlusal or vertical seating forces towards teeth or tissues. ( so any component that resist further movements from full seating towards the teeth and tissues, provides support )

2- Stability: Resistance to horizontal forces.



3- Retention: Resistance to vertical dislodging forces away from teeth or tissues.

· The difference between retention and support is very important…

- Support: resistance against forces that are exerted TOWARDS the tissues.

- Retention: resistance against forces that are exerted AWAY from teeth and tissues…

· Since the denture has to fulfill these requirements, it should have certain components that will achieve these functions…



RPD components:

1- Major connector:

Connecting between the different components on one side to the other side of the denture, and it also provides support. (Because it's a horizontal plane and it resists vertical movement towards the tissues).

2- Minor connector:

It connects the different small components with the major connector, and it also provides stability (which is resistance to movements with the horizontal forces).



·

In this picture, you can see the major connector which is called "the lingual bar", and the minor connector that connects between the clasp (=direct retainer which provides retention)


and the major connector.



3- Indirect retainer.

4- Denture base: the part that holds the artificial teeth

and overlies the soft tissues of the residual alveolar

ridge. It provides support.

5- Reciprocal or stabilizing components that provides stability.



· Note: you have to bring a book, and look at the pictures of the components … in order to know the types of major connectors, minor connectors, and clasps and to get the whole idea.



J Don't read the text!! Just look at the pictures to know how the components look…




· Now, in this picture you can see:



1- The major connector: called anterio-posterior palatal strip.

2- Minor connector: connects the major connector with the indirect retainers (=which are rest seats).

3- Occlusal rest.

4- Guiding plates.

· Our lecture will be mainly about guiding planes and rest seats.

· In the lab you are going to prepare guiding planes on posterior teeth, and also rest seats (so you will understand it then more).



· Guiding planes:

Two or more parallel axial surfaces on abutment teeth which can be used to limit the path of insertion and improve the stability of a removable prosthesis. May occur naturally on teeth but more commonly need to be prepared.

We have said that all abutment teeth must have guiding planes on their proximal surfaces, and also on their lingual surfaces (in cases of extended major connector on top of the lingual surfaces).

· There must be planes that are parallel to each other, and all of them are parallel to the chosen path of insertion and removal.

So keep in mind that the guiding planes are parallel surfaces, most commonly prepared on abutment teeth, and might occur naturally to limit the path of insertion and removal.

(so we'll have single path of insertion and removal= the path which the denture will take from the first contact with the abutment teeth until it's fully seated, and also removed !)

· We perform surveying and identify the path of insertion and removal, and then we prepare the guiding planes according to that path.

One of the components of the RPD is the guiding plate, which will be in intimate contact with the guiding plane (to provide stability and limit the path of insertion and removal); it slides the denture in its place until it's fully seated.

- Occluso-gingival height of the plane is 2-3mm parallel to the path of insertion.



· Types of guiding planes:

1- Guiding planes on abutment teeth supporting tooth supported RPD.

2- Guiding planes on abutment teeth supporting distal extension base.

3- Guiding planes prepared on the lingual surface of abutment teeth.

4- Guiding planes prepared on anterior abutments.

· There is a slight difference between these types.





In this picture, it's class III "bounded saddle partial denture", we have two abutments with prepared guiding planes (2-3 mm in height occluso-gingivally), their preparation should follow the path of insertion and removal in all cases. And here on the denture, at the proximal surface of the artificial tooth there will be metal guiding plates which slides down the


guiding plane until it's fully seated.

· Now, in distal extension RPD (for purposes

that you'll learn about later on !) the

guiding planes should be little bit shorter than those in the bounded saddle RPD. (it will be 1.5-2 mm)… the only difference will be in

the height, while it also should be prepared according to the path of insertion and removal. (The difference in the height will allow stress breaking effect for the denture, you'll learn why later on!!!)

Mainly, the rest seats on anterior teeth will be on the lingual surfaces (on the cingulum, just above the cingulum, or on the incisal edges). We'll see later on.



Now in this picture, you can see major connector, minor connector, and rest on the lingual surface of the tooth.


Here, the guiding plane is the minor connector itself, they must



be in intimate contact, and inserted upon the selected path of


insertion and removal.



For anterior abutments, keep in mind that prepared guiding planes will provide better esthetics, (if they are not prepared, there will be gaps between the denture and the abutments… while preparing them will close these gaps)










· Preparation of the guiding planes:

This will be done mainly using diamond burs (straight fissure, or little bit tapered, with rounded or flat tip, it doesn't matter).

We care about having straight diamond part (2-3 mm).

You hold the bur parallel to the selected path of insertion and removal and prepare them as you can see in the picture below.

This is a practical step that you will understand more in the lab.








Keep in mind !! You don't cut the tooth straight

bucco-lingually, but you have to go with you bur with the

curvature of the tooth. Otherwise you will be cutting too

much of the tooth surface.



That means, when you look at the tooth from the occlusal aspect, its proximal surface should be curved bucco-lingually but when you look from the buccal aspect, it should be straight occluso-gingivally, 2-3 mm in height, (this idea is the same as a cylinder!)


Look at the pictures below to understand it more…














When the preparation is done well, (to the correct line that you can see in the 1st picture from the left), very small amount of the tooth is cut, and the purpose of preparation is achieved without exposing dentine.

While if you try to make it flat bucco-lingually, then you'll be cutting lots of the tooth structure… ending up in the dentine… and this is absolutely wrong.

Remember… if you have two abutments, it's essential to look at them to make sure that the guiding planes are parallel to each other as well as to all guiding planes on the rest of the abutments.

· Functions of guiding planes:

1- Make insertion and removal easier. (By creation of single path of insertion and removal)

2- Stabilize the prosthesis against horizontal stresses.

3- Reduce the block out area and improve appearance

4- Contribute to indirect retention and frictional retention.

5- Prevent clasp deformation

6- Provide reciprocation.

· We'll talk about these functions in details:

- It increases stability, when it's prepared well, the denture can be inserted in a single path of insertion and removal, and it will have the ability to move only in that direction, thus improving resistance against horizontal forces.














- Reciprocation:

The clasp tip (which is the only component that is placed below the surveying line) is flexed to move from the non-undercut to the undercut area, it exerts some sort of force on the tooth. So, there must be another component on the other side of the retentive tip of the clasp that provides resistance against movement in that direction.

J Whenever you have a retentive tip, there must be a reciprocal component at the other side.

-another explanation: passing through the maximum bulge of the tooth, the clasp tip will cause flexion of the tooth (pushing it to the other side)… and the reciprocal component is essential to resist this flexion, and it may be a guiding plate, or reciprocal arm of the clasp… (We'll talk about it later on, in the lecture about direct retainers).

So, practically you will prepare guiding surfaces, with which the reciprocal component will be in contact. (Opposing the retentive tip of the denture).

-

Without a reciprocal component, each time the clasp enters the undercut area or exit; it will push the tooth to the other side, causing movement of that tooth from its place away from the clasp.








· Prevention of clasp deformation:

As you know the clasp tip has to reach the undercut. So, if it can be inserted in different paths, then it will be flexed to different degrees and it will lose its proper flexibility, and fatigue will occur with time, and it might break as well.

The solution was to make the clasp according to the measured undercuts (by choosing certain type of clasp, certain dimensions, and certain flexibility that suits each case in particular).

- By limiting the path of insertion and removal clasp will be flexed only the amount that is intended, and will maintain its proper flexibility.

· Improved appearance:

We said that especially in anterior saddles, when you prepare guiding planes, the denture will extend filling the gaps between the abutment tooth and the artificial tooth… thus improving appearance.











RESTS AND REST SEATS


· The rest: A rigid extension of a removable partial denture which contacts a remaining tooth or teeth to dissipate vertical or horizontal forces.

· Rest seat: That portion of a natural tooth (or restoration) prepared to receive an Occlusal, Incisal, Lingual, Internal or semiprecision rest.

· So, the rest is a component of the RPD, while the rest seat is a preparation of the tooth to receive that component.



- There are different classification for the rests:

1- based on the relation of the rest to the direct or indirect retainers:

A- Primary Rest (placed along with the clasp assembly.)

B- Secondary or Auxiliary Rest (the one placed for indirect retainer.)

(These types were not explained, but they are mentioned in the slides).

2- Based on the position of the rest on the abutment (according to the surface on which you prepare the rest seat, where the rest comes in contact with the tooth).

They can be:

- Occlusal Rest.

- Cingulum Rest.

- Incisal Rest.

- Interproximal occlusal rest.

Functions of Rests:

The main function that you have to understand is providing support; we said that support is the resistance against the movements toward the tissues and teeth, so these rests prevent further sinking of the denture on the tissues and teeth (this is very important)…

- Vertical forces will be transmitted with the long axis of the teeth to the underlying bone that will dissipate these forces.

· The main difference between the cobalt-chromium partial dentures and the acrylic partial dentures is having the rests and rest seats and thus depending on teeth as well as tissues to provide support.

-you are supposed to know the classification of partial dentures, according to being tooth supported, tissue supported, or tooth-tissue supported. (As you took with dr.mahmood)



Functions of the occlusal rests:

1. Transmit stress along the long axis of the tooth.

2. Secure the clasp in a proper position.

3. Assist in distribution of occlusal load.

4. Prevent extrusion of abutment.

5. Prevent food trapping between abutment and clasp.

6. Provide resistance to lateral forces.

7. Sometimes contribute to indirect retention.

8. Used to close small spaces where a tooth cannot be placed.

9.

Help to build up occlusal plane of a tilted tooth.




Now, we'll discuss these functions one by one:

1- Transmit stress along the long axis of the tooth.

You prepare the seat to have certain dimensions and slopes and certain angles so that ultimately forces will be transmitted to the center of the tooth. (With the long axis-through the root-to the underlying bone)



2- Secure the clasp in a proper position



In the picture to below, we have a rest, a retentive tip, and a reciprocal arm at the other side. The rest in its position prevents further sinking of the denture toward the tooth, and it makes the clasp stay at its position during function.










3- Assist in distribution of occlusal load.



Part of the occlusal forces is transmitted to the tissues, and part to the teeth. If you distribute the occlusal, incisal, and cingulum rest seats on the remaining teeth and plan the distribution of support on teeth and tissues well, then you can achieve proper transmission of forces to them.


4- Provide resistance to lateral forces.

Mainly the rest seats will be round shaped, and the rests

themselves are saucer shaped.

This way they can move within the rest seat dissipating the lateral



forces, that may destruct the tooth if not dissipated…




5- Contribute to indirect retention…



In this stage we won't talk about this principle in details, but you have to know that there will be a rotational movement around the axis seen in the picture below, and extension of the rests on teeth will prevent such movements of the denture.






- Just keep in mind that the rests provide

indirect retention, which is very important

in distal extension prosthesis

6- Close small spaces where a tooth cannot be placed.



In the picture below, between the 1st molar and the 2nd molar there is a big space, but not that big for an artificial tooth to be placed in, so you can put rests on both teeth and close the space with metal plate. This way it's hygienic and at the same time the space is closed.




7- Build up Occlusal Plane of a tilted tooth:

If we have a tilted tooth, or a tooth where the

occlusal surface is out of contact, you can extend



the rest on the occlusal surface to have proper occlusal contact of the tooth. (As you can see in this picture !!)




· Now, we'll explain the types of rests

according to their position (occlusal, incisal,

cingulum, and interproximal.



A- Occlusal rest: A rigid extension of a partial denture that contacts the Occlusal surface of the tooth.



In an occlusal view, the rest should be of certain dimensions.

And keep in mind that the rest seat preparation should be maintained within enamel, otherwise you will expose dentine, causing caries and sensitivity of the tooth.

The proper dimensions of occlusal rests and rest seats in an occlusal view:

Half the bucco-lingual distance of the tooth. (the bucco-lingual distance is that distance between the maximum bulges of the tooth bucco-lingually)



And it should be one third the mesio-distal distance of the tooth… this is shown in this picture…


So you must measure the dimensions of the tooth, and

prepare the seat according to them, otherwise it might be too

big or too small !

· The mentioned dimensions are the ones for the

premolars, while for molars it's one fourth the distance mesiodistally.

(It's almost the same size. But because of the difference in sizes of teeth, the ratio is different,, molars are bigger than premolars so 1/3 premolar=1/4molar)

This type of rest might be called mesio-occlusal or disto-occlusal (according to its position)



In the next picture, it's a proximal view of the tooth in which the rest seat is rounded= saucer shaped (it should NOT box shaped, to allow certain kind of movement of the rest within the rest seat upon function and to dissipate the harmful lateral forces and don't transmit


them to the long axis of the tooth.

· Now look at the buccal or lingual

view (mesio-distal view). In the next page

You can see the shape of the rest

seat on the marginal ridge, and how it goes deeper towards the center of the tooth (the deepest portion is toward the center of the tooth)…



It's about 1.5 mm in depth, and if you measure the angle between the long axis of the tooth and the floor of the rest seat it should be less than 90 ̊…










- If it's more than 90 ̊ , forces will be transmitted in a direction other than the one that we want. But if it's less than 90 ̊, forces will be transmitted toward the long axis of the tooth.

B- Interproximal occlusal rest:



The same principles as the occlusal rest, whenever you have a certain type of direct retainers (which is the double aker), there will be two clasps adjacent to each other on an adjacent teeth. So, in this case you prepare two rest seats on these adjacent teeth, and you open the embrasure lingually for the minor connector (or the rest of the direct retainer) to be in contact with the tooth in this direction…


(This will be discussed in the lab as well !!! )



· Why do we prepare rest seats ?? is it

necessary? Yes… it's very important

to produce a favorable tooth surface for support, to prevent interference with the occlusion, and to reduce the prominence of a rest.

- If you don't prepare the rest seats, and you just put the rest on the tooth surface, it will be prominent, and the patient will feel that it's irritating (feeling it with his/her tongue)…

- It might interfere with occlusion if there is an opposing tooth, and even if there is no opposing tooth, and you didn't provide rest seats, the direction of support will not be optimum…


















In this picture…

In case #1 : you can see that without preparing a rest seat, upon function the rest will slide down, and it won't transmit forces to the long axis of the tooth.

But in case #2 : the forces will be transmitted through the rest seats to the long axis of the tooth, thus providing the proper support.



Q: why do we put the rests on areas of occlusion ?? why don't we just put them away??

Because the aim of using rests and rest seats is to transmit forces that are exerted by occlusion J

· In the next picture,, the rest and rest seats are located on the marginal ridges, (not the fossae) and are not extended toward the central fossae as well… as we said it's only one third the distance mesiodistally !!
















** now in the next occlusal view (case #1),, you can see that the occlusal rest seat is triangular in shape with the base toward the marginal ridge, and the apex toward the center of the tooth, but keep in mind that it should be rounded triangle (with no sharp angles),, it should be all rounded to facilitate movement of the rest within its seat…






In the mesio-distal view (in case #2)... you can see the floor and the wall of the rest seat, the floor should be inclined toward the center of the tooth, (so that the deepest part is also toward the center of the tooth.)

C- Anterior rests:

There are no occlusal surfaces, but there is an incisal edge, lingual surface, and a cingulum.

· Types of anterior rests:

1- Lingual or cingulum rests, classified according to their shape into:

A- Inverted V cingulum rest

B- Cingulum ledge

C- Cingulum ball rest

2- Incisal rest (Hook)

Explanation for them one by one !!!!!!!

A- 1- Inverted V cingulum rest seats:

Keep in mind that there shouldn't be sharp angles.

It's dimensions: 2.5-3 mm mesio-distally…

The depth of the floor is 1.5 mm, and the depth of the wall is 2 mm…

The floor should be inclined labio-gingivally, (not straight, not inclined labio-incisally !!)

The wall should be free of undercuts. So, when you look at the tooth from an occlusal aspect, you can see the whole floor.

These mentioned properties are important to provide definite stop for the rest and transmit the forces to the long axis of the teeth…

You can prepare this kind of rest seats on prominent cingulum (mainly on upper canines,, lower canines, and upper incisors to lesser extent,, not used in lower incisors because of its too small cingulum)





This type of rest seats provides better esthetics and stress transfer than incisal rest seats…






2- Cingulum ledge:

It looks like the inverted V, but it's not complete over the cingulum, and its position here is on top of cingulum while the inverted V was a little bit higher.

Usually the ledge is located on one side of the tooth, slightly crossing the midline, but mainly on one side ! and it breaks the marginal ridges, so if you look at the proximal surface you'll see the rest seat (while it wasn't visible proximally in cases of inverted V, because that type doesn't cross the triangular ridges on both sides.

Same as inverted V, it has a floor that is inclined labio-gingivally to provide proper stress transfer of the forces, and a wall that is free of undercuts.
























3- Cingulum ball:

It's very infrequent to use, it's one of the least commonly used rest seats because it's too small, so that the amount of provided support is minimal and also there is a big chance that you may leave undercuts which will in turn interfere with the seating of the denture…

It has also a floor and a wall, located on one side of the tooth, and prepared using round bur… mainly !!

- You can see in the picture below, porcelain fused to metal crowns, on which the rest seats can be prepared… (this means that this type of rest seats can be prepared on restorations, or on porcelain fused to metal crowns,they are more preferably prepared on metal surfaces…

- You ask the technician to prepare the rest seat on these crowns…

-



It's prepared least preferably on porcelain, because of the susceptibility of the porcelain for cracking…












In a proximal view, you can see the cingulum ball rest seat that has a wall and a floor… remember that the wall is essential to be free of undercuts for proper seating of the denture…








· These lingual (cingulum) rest seats are indicated when:

- The cingulum is prominent. (Preferably on upper canines)

- The patient practices good oral hygiene. Because it's on the lingual surface and patients tends to forget cleaning that area, so the patient should have good oral hygiene provided with cingulum rest seats.

*if the patient has poor oral hygiene, the RPD is not indicated any way, but if you have to provide this patient with an RPD, then you can choose other types of rest seats such as incisal rests.

- Low caries index.

B- incisal rests:

The problem with this type is the esthetics, you can see that the rest is visible on the labial surface, so we don't use it for upper teeth, we mainly use it for lowers.



In this picture you can see the incisal hook, dimensions are around 2.5 mm mesio-distally and 1.5 mm occluso-gingivally, because they are connected with the major connector on the lingual surface through the minor connector,


then you have to create a space (as a shallow tunnel) to receive

the minor connector. It looks like a saddle, and you prepare a







bevel labially (you'll learn about it more in the lab).








· This type of seats is indicated when:

1- Tooth morphology does not permit other designs

(When the cingulum is not prominent enough to make cingulum rest seat for example).

2- When the incisal edge is completely lost, the incisal rest can restore the lost contour.

*the problem with these rest seats, is that forces are applied on the incisal edges away from the center of the tooth. While the cingulum rest seats were closer to the center of the tooth and to its long axis…

And the other problem is esthetics !! as well as the amount of provided support is not ideal!!

That's it J !!
Wish you all the best of luck J
Dyala Al-Armouti
Prosthodontics lecture #2 – dr.nadia erefij
22/2/2011
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Dyala Al-Armouti

عدد المساهمات : 639
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تاريخ التسجيل : 2009-09-06
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