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Lec.5 Dr. Sanaa Rasheed

Support and impression techniques for RPD

All partial dentures have two things in common; they must be supported by oral structures and

they must be retained against reasonable dislodging forces.

Support: It's the resistance of the denture against movement toward the tissue. According to the

manner of support; there are three main types of RPD (Cradock classification):

1. Mucosa-borne RPD (Mucosa supported RPD): the occlusal forces are transmitted through

the saddles or denture bases of the PD and is ultimately resisted by underlying mucoperiostium

and bone. It allows varying degrees of displacement.

The amount of displacement (tissueward movement of the denture) depends on:

A. The amount of pressure applied.

B. The nature of the mucosa (thickness).

C. Area covered by the denture (the wider the area coverage, the less the displacement).

D. Fit of the denture base.

E. Type of impression technique used (anatomical, functional or selective pressure).

2. Tooth-borne RPD (tooth supported RPD): if the denture is supported on adjacent teeth by

components such as occlusal rests, the forces is transmitted to the bone via teeth and periodontal

ligaments.

3. Tooth-mucosa borne RPD (Tooth - tissue support (distal extension RPD)): when the

saddle has an abutment tooth at one end only, i.e. free-end saddle or class I and class II Kennedy

classification.

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Support for the distal extension denture base:

Distal extension RPD must depend on the residual ridge for some support, stability and

retention. Indirect retention is essential to prevent the denture from lifting away from the

residual ridge and this is why it is essential in the design in this case. The patient must be

informed of the movements to be expected with distal extension RPD.

The main problems which might occur in tooth-tissue support are:

1. Mucosa is resilient and displaceable and can lead to unstable prostheses.

2. Difficult to record mucosa at rest and function at the same time

3. In distal extension RPD under function compresses the mucosa and act as class I

lever thus it causes damaging to the abutment teeth, the solution is to record tissue

in the functional form so that the denture doesn't exert additional stress to the

abutment tooth.

Factors Influencing the Support of a Distal Extension Base:

1. Contour and quality of the residual ridge: The ideal residual ridge to support a

denture base would consist of cortical bone that covers relatively dense cancellous

bone, with a broad rounded crest with high vertical slopes, and is covered by firm,

dense, fibrous connective tissue. Such a residual ridge would optimally support

vertical and horizontal stresses placed on it by denture bases. Unfortunately this ideal

is seldom encountered. Easily displaceable tissue will not adequately support a

Indirect retention RPD distal extension under functional forces

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denture base, and tissues that are interposed between a sharp, bony residual ridge and

a denture base will not remain in a healthy state.

2. Extent of residual ridge coverage by the denture base: The broader the residual

ridge coverage, the greater is the distribution of the load, which results in less load per

unit area.

3. Type and accuracy of the impression registration: The residual ridge may be said

to have two forms: the anatomic form and the functional form. The anatomic form is

the surface contour of the ridge when it is not supporting an occlusal load. The

functional form of the residual ridge is the surface contour of the ridge when it is

supporting a functional load.

4. Accuracy of the fit of the denture base: Distal extension base is enhanced by

intimacy of contact of the tissue surface of the denture base and tissue that covers the

residual ridge. The tissue surface of the denture base must have intimate contact with

the denture bearing areas in patient’s mouth and this is gained by making good

impression.

5. Design of the RPD framework: For distal extension bases:

a. Use of more anteriorly or mesial occlusal rest is suggested as it: (1) Allow vertical

ridge loading. (2) Permits greater ridge area for support. (3) Transfers stresses to

anterior abutment.

b. Incorporation of indirect retainer to control movement of the prosthesis along the

fulcrum access.

c. Incorporation of stress release type of clasp like RPI or combination clasp.

6. Total occlusal load applied: This is influenced by reduction in the number of

artificial teeth; reduction in the size of occlusal table; and occlusal efficiency. The

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reduction in size and number of teeth will reduce the vertical and horizontal forces

acting on the partial denture and lessens the stress on the abutment teeth and

supporting tissue.

Impression techniques for RPD:

Primary impression: It is an impression made for the purpose of diagnosis or for constructing

a special tray.

Secondary or definitive impression: An impression that record the entire functional denture

bearing area to ensure maximum support, retention and stability for denture during use.

Secondary impression technique is classified into:

Anatomical impression (conventional, mucostatic).

Functional impression technique (pressure).

Selective pressure impression technique (altered cast technique).

Anatomical impression technique:

Anatomical impression is impression of the ridge when it is not under any occlusal

load (Fig. A). It is recorded with soft impression materials like alginate. When the denture is at

rest, it will adapt well to the tissue surface but under occlusal load, the rest in the direct retainer

will prevent the tissueward movement of that part of the denture near the abutment tooth, but

the distal end of the denture will show tissueward movement and compress the tissues under

occlusal load and produce lever action that will cause weakening of the abutment (Fig. B and

C). Generally anatomic impressions are preferred for tooth supported partially edentulous

arches (Kennedy’s class III and class IV cases). Anatomical impressions are contraindicated for

distal extension cases due to the reason explained above.

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Generally Kennedy’s class III and class IV arches are considered as tooth supported

partial denture. Since maximum support is obtained from the abutment teeth, functional

impression is not needed, Conventional anatomic impressions are made. The material of

choice to make a final impression for these partial dentures is irreversible hydrocolloid

(alginate).

Functional impression technique:

The objective of any functional impression technique is to provide maximum support for

the RPD bases. This allows for the maintenance of occlusal contact between natural and

artificial dentition and, at the same time, minimal movement of the base, which would create

leverage on the abutment teeth. The support form of the soft tissues underlying the denture base

of the partial dentures should be recorded so that firm areas are used as primary stress bearing

areas and readily displaceable tissues are not over loaded, only in this way can maximum

support of the denture base be obtained.

The criteria for functional impression technique: The impression must:

1. Record and relate the tissues under uniform loading.

2. Distribute the load over as large an area as possible.

3. Accurately delineate the peripheral extent of the denture base.

Methods for Obtaining Functional Support for the Distal Extension Base:

A. McLean‘s technique (closed mouth):

The technique consists of making an impression of the edentulous ridge in border-

moulded denture base tray which is provided with occlusion rims.

Fig. A Fig. B Fig. C

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High detail impression material is used to record ridge areas under biting stresses like

ZnO eugenol impression material.

After setting of ZnO eugenol; it is removed, inspected, reinserted; and then an overall

alginate impression is made with the ZnO impression seated in the mouth using a stock

tray. The teeth in the anatomical form and the tissues in the functional form will be

produce after pouring the cast into the impression.

This technique is done at the master impression stage (during reproduction of master cast

and before the construction of metal framework)

B. Hindle‘s technique (opened mouth):

The same idea of McLean‘s technique but instead of the

occlusion rims, use finger pressure through 2 circular

openings in the posterior region of the hydrocolloid

impression tray (done at the master impression stage).

These two technique also called the pick-up impression techniques.

Selective pressure impression technique (altered cast technique):

It is also called Selective Tissue Placement Impression Method or altered cast technique.

In this method one anatomical impression is made of the entire ridge and one physiological or

functional impression is made only on the edentulous portion. The main aim of altering method

is:

1. To capture the mucosa in its functional form so that the base can be related to the metal

framework in the same relationship that exists between the abutment tooth and the

supporting mucosa when occlusal loads are applied.

2. To accurately extend borders for greater area coverage, as to reduce stress per unit area

of alveolar bone and contribute maximally to the support, stability and retention of the

denture.

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In this technique, the master cast is made using the anatomical (first impression), on this

anatomical impression the framework will be made and fitted on the cast. This

anatomical master cast is altered according to functional impression, which is made later

(second impression). (Done after fitting of metallic framework).

Indication of altered cast technique:

1. Mandibular distal extension cases, both Kennedy Class I and II cases.

2. Extensive edentulous spans.

3. Any case where periphery is distorted and needs correction.

4. Less necessary in maxilla due to major connector contact with palate.

Procedure for altered cast:

1. The tentative outline of the denture base is drawn on the master cast with the framework

in place. This will permit greater accuracy in tray extension.

2. Remove the framework and place one thickness of baseplate wax spacer over the

edentulous ridge.

3. Make special tray with auto-cure acrylic on the framework over the edentulous area(s)

after applying separating medium on the master cast. Trim the acrylic to the base outline

on the cast, allow it to harden, and trim and round the flanges with acrylic burs.

4. Border mold intraorally with green stick compound.

5. The framework must be fully seated while doing cheek molding and tongue movement.

6. Remove the border-molded tray from the mouth, scrape the compound and remove the

wax spacer.

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7. Apply impression material like zinc oxide - eugenol, one of the elastomers (polysulfide,

polyvinyl siloxane) or fluid wax in the acrylic tray attached to the framework then seat

the framework inside patient mouth with pressure only over rests (DO NOT PLACE

PRESSURE ON THE DISTAL EXTENSION PORTION OF THE TRAY). When fluid

wax used some authors name the technique as fluid wax impression technique.

8. After setting of the impression material, remove it from patient mouth and check for

voids or over-extensions that might indicate improper seating.

9. The edentulous area in the master cast is cut away with a saw. The cast is sliced using

two cuts, one buccolingual and one anteroposterior.

10. The buccolingual cut is made 1mm behind the terminal abutment while the

anteroposterior cut is made 1mm lingual or medial to the lingual sulcus.

11. Vertical grooves are prepared on the cut walls of the cast.

12. The framework along with the functional impression is placed over the cut master cast.

13. The framework is sealed to the master cast using softened

modeling plastic or sticky wax.

14. The impression is beaded and boxed in continuity with the

master cast.

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15. The area of the impression is poured in different colored stone.

16. After we get the altered cast, remove the framework, then burn the tracing compound

and auto cure acrylic.

17. Return the framework on the new cast and fabricate record base.

Possible causes of an inaccurate or weak cast of a dental arch:

1. Distortion of the alginate impression due to: (a) use of non-rigid impression tray, (b)

partial dislodgement from the tray, (c) shrinkage caused by dehydration, (d) expansion

caused by imbibition, (e) by attempting to pour the cast with stone that is already begun

to set.

2. A ratio of water to powder (dental stone) that is too high which will result in weak cast.

3. Improper mixing of dental stone which results in weak cast.

4. Trapping of air because of insufficient vibration.

5. Premature separation of the cast from the impression.

6. Failure to separate the cast from the impression after complete setting of gypsum which

will result in chalky cast.

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Control of gag reflex during impression making:

1. Tell patient to relax and breathe through their nose during the procedure.

2. All the instrument must be out of the sight of the patient and he/ she must not see the

mixing of impression material as these will initiate the gag reflex.

3. Avoid touching the dorsum of the tongue with the back of the tray and seat the

impression as quickly as possible.

4. Use thicker mix of Alginate.

5. Set the patient in upright position.

6. Carry out the impression technique using as little material as possible.

7. Desensitize the surface of the mucous membrane (soft palate and posterior third of the

tongue) with application of local anesthesia on the surface.

8. Seat the tray posteriorly first.

9. The patient's head should be brought forwards and downwards.