Suzan Hussein

Dr. Sandra

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8/4/2015

Prosthodontics 2

Lecture No.

Date:

Doctor:

Done by:

Sheet Slides

Hand Out

Designed by: Hind Alabbadi

University of JordanFaculty of Dentistry

Fourth year – 2nd semester 2014-2015

Secondary Impressions: Anatomic and Physiologic Impressions

Last lecture we talked about tooth preparation and how to prepare rest seats and guiding planes and tooth modifications, in this lecture we are going to talk about how to obtain an impression for a RPD.

We are going to talk about the impression for a bounded saddle design (tooth supported RPD) and the impression for the distal extension design (tooth and tissue supported RPD).

Impressions for tooth-supported RPDs

In tooth-supported removable partial denture forces are directed through the rests and transmitted to the abutments through the long axis of the teeth to the underlying PDL.

Teeth absorb these forces before the forces can be transmitted to the tissues of the residual ridge.

In tooth and tissue supported, forces are distributed between the teeth and the mucosa. This difference in support make impression making not as simple and straight forward as tooth supported RPD’s.

Tooth-supported removable partial denture can be constructed from a single impression (single step). Stock trays or custom trays can be used, depending on the case. For example if there was only one missing tooth, it would be simpler to use a stock tray.

So stock trays can be used in bounded saddle that is simple and the edentulous span is small. There are plastic and metal trays, each have their advantages and disadvantages. Plastic trays are disposable. The metal trays can be sterilized and reused.

We already know the impression technique: The adhesive is sprayed on the tray and is left for 5 minutes. The alginate is loaded onto the tray and we take the impression. Before taking the impression, an important step is to take a small amount of alginate on our fingers and wipe the surfaces of the teeth and the area of the rest seats and guiding planes to make sure that these areas have no air bubbles. We inspect the impression

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taken and make sure that all the prepared areas are visible. The extension is very important, especially if we want to use major connectors such as the lingual bar we need to make sure there is enough space for it. Then we pour the impression.

Custom trays are used when the edentulous span is longer or when there is a distal extension .In this case we have to do border molding for that area.

Impression material options:

In general we can use any elastomeric material: alginate, polyether, polysulfide or silicone. The only difference is the cost. Alginate is the cheapest and is the most commonly used but needs to be poured immediately.

Impressions for tooth and tissue supported RPDs

General Considerations:1) Impression procedures for tooth-mucosa borne partial dentures should be designed to record teeth and mucosal tissues in a manner that maximizes support. (This is achieved by using a special technique, in which we want to compensate for the difference in resiliency between the teeth and the mucosa).

2) muco-osseous tissues have been described as being approximately 25 times more displaceable than the dento-alveolar tissues, so we want to account for this difference in displacement by our impression technique that we are going to use.

Before we talk about the impression technique for tooth and tissue supported rpd’s we need to know where it gets its support from.

Teeth –tissue borne support is affected by the following:

1) Contour and quality of the residual ridge. A patient with a residual ridge that is broad and has a good height is different than a patient with a thin ridge, knife edge or fragile mucosa. The first will surely have much better support.

2) Extent of residual ridge coverage by the denture base. That’s why when we have a distal extension; we do border molding the same as we do for a complete denture, to get the

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maximum extension of our denture base to maximize support. As we increase the area coverage, support and retention are increased.

3) Type and accuracy of the impression registration

4) Accuracy of the fit of the denture base. This depends on the impression taken and on the handling of the impression by the technician in the lab.

5) Design of the RPD framework. If the design has any faults for example instead of using RPI, the rest was placed distally on the abutment tooth, no matter how well our impression was we won’t have good support.

6) Total occlusal load applied. A patient who has an opposing complete denture is different than a patient who has opposing natural dentition or a patient with bruxism for example.

Regarding the design, we already mentioned an example if we didn’t apply an RPI. Another example is if we didn’t pay attention to the indirect retainer. In distal extensions we have to use an indirect retainer, which is by definition: a rest seat that is placed anteriorly and its purpose is to assist in preventing the dislodgment of the distal extension of the denture. Upon displacement or upon function, the posterior part of the denture that will prevent it from coming out of place is the retentive tip of the clasp arm. Anteriorly if there is no rest seat to prevent the sinking toward the tissues, the denture is going to fall into the tissues and we totally lose the support. In this case regardless of the impression technique that we used, the design is incorrect and there won’t be proper support.

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The ideal residual ridge to support a denture base would consist of:

1) Cortical bone that covers relatively dense cancellous bone 2) Broad rounded crest and high vertical slopes 3) Covered by firm, dense, fibrous C.T.

This would provide optimal vertical and horizontal support against the stresses

Impression registration

1) Anatomic form:

- We register the surface contour without occlusal loads applied

2) Functional form:-we obtain our impression under function so we register the surface contour with occlusal loads applied.

•Distortion and tissue displacement can occur due to:

a) Viscosity of the material. For example if we took the impression for the distal extension using impression compound which has high viscosity.

b) Insufficient thickness of impression material. We use wax as a spacer and we cut windows were we make tissue stoppers. The tissue stoppers act as a reference point every time we insert and remove the tray and allow for sufficient thickness of material by preventing the application of excessive pressure. Alginate requires about 3mm of material to prevent tearing and to have the appropriate physical properties.

Anatomic form impression: One stage impression method using an elastic impression material Represent the hard and soft tissue at rest without occlusal loads.

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8/4/2015 Prosthodontics lecture #23 Suzan HusseinWhen taking an impression using this form the rest seat will prevent the denture from sinking any further during function when the upper teeth come in contact with the lower teeth and there is contact with the rest seats, the denture wont sink any further in the area of the abutment, but what about the distal extension? There is nothing to prevent it from sinking, and as we said that there is a difference in the resiliency between the tooth and the mucosa it will sink further and eventually this will cause torqueing of the abutment and loosens the abutment, even if we used rpi but because I obtained an anatomic impression I put the tooth and the mucosa under the same amount of pressure (alginate everywhere and finger pressure everywhere), so what happens is upon function there will be this space and it because of the difference in the support between the teeth and the tissue.So we should consider the need of mechanical stress breaker but this is not sufficient on its own.

As a summary, when taking the impression using the anatomic form, when the denture is upon occlusal loading: 1) Rest will act as a definite stop 2) Limited movement near the abutment 3) Distal end able to move freely – receive the most occlusal load 4) Torque to the abutment teeth

A removable partial denture fabricated from an onestage impression, which only records the anatomic form of basal seat tissue, places more of the masticatory load on the abutment teeth and that part of the bone that underlies the distal end of the extension base. The balance of the bony ridge will not function in carrying its share of the load. The result will be a traumatic load to the bone underlying the distal end of the base and to the abutment tooth, which in turn can result in bone loss and loosening of the abutment tooth.

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8/4/2015 Prosthodontics lecture #23 Suzan HusseinDisadvantages of anatomic form: 1) Bone loss on the distal end of the ridge due to the continuous compression2) Cantilever action of the distal extension base against the abutment teeth - Loosening of the abutmentsSo we should take an impression in two steps:

Resilient soft tissues of the primary supporting areas must be mildly displaced to promote their contribution to support.Secondary and non-supporting areas should be recorded at rest to minimize the forces directed to these areas which are more susceptible to pressure-induced resorption.

• This is accomplished by a dual impression technique.–Teeth in their anatomic positions.–Residual ridge which must record the soft tissues in their functional form.This technique allows the occlusal load to be evenly distributed.

Functional impression:- Need to record the tissue that supports the distal extension RPD in its functional form& relating them to the remainder of the arch-The more the mucosa displaces under function, the more rebound there is likely to be.

McLean, 1936 was the first one to introduce the concept of functional impression. The idea was to produce a tray for the distal extension (a sectional tray that only covers the distal extension). In this picture we can see green stick material acting as a bite block. Then we place the impression material in the sectional tray and then ask the patient to bite down so we are obtaining the impression during function under occlusal load. Now we took an impression for the distal extension, with the impression held in place, a stock tray is used to carry the impression material and is placed in the patient’s mouth without applying any load, in this way an impression is obtained for the teeth in the anatomic form and for the distal extension in a functional form.

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8/4/2015 Prosthodontics lecture #23 Suzan Hussein-Functional impression of the distal extension ridges under closing pressure with a custom tray

-Overall hydrocolloid impression with the first impression held in position.

This impression technique has a disadvantage that it will cause bone loss. As long as the patient is wearing the denture the tissues are under compression which will cause loss of blood supply and bone loss, so this technique didn’t solve the problem it just delayed it.

From here came the modification from Hindels, 1952:

He thought that we wanted to apply some pressure on the distal extension but not during occlusal load as to not have continuous pressure. He used a similar idea with a custom tray only for the edentulous areas; we do border molding and then take an impression for the edentulous area only under finger pressure, until now we are talking about an anatomic impression. How are we going to make it functional? We do modification in the stock tray, we make openings in the area where we have edentulous span and when we take the overall impression instead of applying occlusal load we apply differential pressure, in the area that is edentulous we apply extra pressure, so the mucosa is slightly compressed under our finger pressure ,while in the teeth area we apply no pressure (anatomic).In this way the compression is less than mclean technique and it is functional in sense but it has a disadvantage that we can’t really control finger pressure, we may apply too much or too little force so this technique is difficult to standardize.

-Dual-impression technique

-Anatomic impression of the ridges

-Specially made custom tray for the second irreversible hydrocolloid impression

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8/4/2015 Prosthodontics lecture #23 Suzan Hussein-Finger pressure applied through holes in the tray to the underlying anatomic impression of the ridges

•Disadvantages of both techniques:

After we construct the denture according to our impression technique, the forces will depend on two factors:

–If the clasp action is sufficient to maintain the denture base in its intended position, this may result in compromised blood flow with adverse soft tissue reaction and bone resorption.

–If clasp action is not sufficient to maintain that functional relationship of the denture base to the soft tissue, this will result in floating denture(displace slightly) and rebound of the tissues, the denture will always be high in occlusion with premature contact and patient dissatisfaction.

Dentist’s aren’t aware that there is problem in compression and when that patient comes to the clinic complaining that the denture feels high they solve this problem by grinding the teeth. In this case they solve the problem temporarily but it becomes as if we took an anatomic impression and after a while the patient will have severe bone loss in the distal extension and loose abutments that need extraction.

To overcome all these problems the best method is to use the Altered Cast Technique.

Which is instead of taking the functional impression in the impression stage we take aanatomic impression using which ever material then we do the frame work and after constructing the framework we take the impression for the distal extension.

Applegate, 1955 Introduced altered cast technique.

-Trays attached to the metal framework in the distal extension and then Fluid-impression wax (Korecta Wax IV) is applied ( this was before we started using zinc oxide) and take the functional impression and then the ridges of the master cast cut away according to the new impression.

GOOD LUCK

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