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Complete denture impressions

Dr. Shanai M.

DENTAL IMPRESSION

A negative imprint of an oral structure used to produce a positive replica of the structure to be used as a permanent record or in the production of a dental restoration or prosthesis..
COMPLETE DENTURE IMPRESSION
is a negative registration of the entire denture bearing, stabilizing and border seal areas present in the edentulous mouth

OBJECTIVES OF IMPRESSION MAKING

• RETENTION
• STABILITY
• SUPPORT
• ESTHETICS
• PRESERVATION OF REMAINING STRUCTURES

RETENTION

the ability of denture to resist the displacement against vertical forces( along the bath of of placement)
Retention resists the adhesiveness of food, the force of gravity, & the forces associated with the opening of jaws.
STABILITY
The quality of a denture to be firm, steady, or constant, to resist displacement by functional horizontal forces or rotational movement.


Retention is the constant relation of the denture base to underlying soft tissues, while stability is the relation of the denture base to underlying bone. These relations may be constantly changing.
Retention must hold the denture in its position when its set at rest.
Stability must resist displacement by rocking when a force is applied to teeth over a limited area.

SUPPORT

It is the resistance to vertical forces applied in a direction toward the basal seat, therefor, the greater the amount of area covered the greater the support.

When the natural teeth are missing ,the alveolar ridge & their covering of mucosal tissue become the supporting elements.

The best support for denture is the compact bone covered with fibrous connective tissue

retention= denture base + soft tissue
Stability= denture base + bone
Support= denture base + bone+ soft tissue

ESTHETICS

The border thickness of the denture flanges should be varied with the need of each patient in accordance with extent of residual ridge loss. The vestibule fornix should be filled, but not overfilled, to restore contour.
Thicker denture flanges are preferred in long-term edentulous patients to give required labial fullness.


PRESERVATION OF REMAINING STRUCTURES (residual alveolar ridge and soft tissues)

Preservation of the remaining residual ridge is physiologically accepted that with the loss of stimulation of natural teeth, the alveolar ridge will atrophy or resorb. Prosthodontist should keep in mind the effect of impression material and technique on the denture base and the effect of the denture base on the continued health of both the soft and hard tissues of the jaws.

PRIMARY IMPRESSION

The negative likeness of the oral tissues made for the prepare of diagnosis, treatment planning, or the fabrication of tray. It’s the first impression made for the patient and from which the study cast was produced. This impression is obtained by stock tray.
For upper stock tray, the posterior border of the tray should cover the maxillary tuberosity and hamular notch, anteriorly should include the antero-alveolar ridge
For lower stock tray posteriorly should cover the whole area of retromolar pad area and anteriorly include the alveolar ridge

Materials used for making primary impression

1- impression compound
2- alginate impression material.
3- Rubber base impression material (heavy body).

STEPS IN MAKING AN IMPRESSION

• Preliminary examination of the patient
• Selection of the tray
• Selection of the material
• Making impression


Preliminary examination of the patient
A complete case history and thorough clinical examination is done.

Factors that can complicate impression making are identified.

Patient education.

Seating of the patient

Position of the operator for maxillary impression

Position of the operator for mandibular impression

Selection of tray:
The beginning of good impression starts with the selection of the correct stock tray.

The space available in the mouth for upper impression is studied carefully by observation of the width and height of the vestibular spaces with mouth partly open.


And in the lower the general form and size of basal seat is studied.

PRIMARY IMPRESSION PROCEDURES IN ALGINATE

The objective is to obtain a preliminary impression that is slightly overextended around the borders.
Border molding:-
The shaping of impression material by manipulation or action of muscles adjacent to it.
1.Selection of stock tray, , 3-5 mm space for impression material.
2. load the tray with impression material
3. The loaded tray is positioned in the mouth. Tray to be rotated into the mouth and seated first at the back of the mouth.
4. Position borders at hamular notches posteriorly.
5. Lift the tray anteriorly.
6. Tray is held in the mouth in while Labial and buccal borders to be molded.

(Mandibular impression with alginate)

Primary impression using impression compound

.Softened in water bath and kneaded.

.Compound placed in the tray.

Molded with fingers to ridge form.

Gently warmed over a flame

Before insertion, tempering in warm water bath.

Tray should be gently seated.

Patient instructed for tongue movements and to purse lips.

Impression should cover all denture bearing area.

Primary cast (study model or diagnostic cast)
A cast formed from a preliminary impression for use in diagnosis or the fabrication of an final impression tray
Production of study cast (primary cast)
The primary impression is poured or casted in plaster (after beading and boxing) to get the primary cast or study model which is the positive reproduction of the oral tissues.
The plaster mixed with water by the saturation method in the ruber bowel. when the plaster became hard, the cast is separated from the impression by the use of hot water (55 C˚- 60C˚). When using very hot water, the impression compound will sticky and its will be difficult to remove from cast. The special tray will be constructed on primary or study cast which is used to make final impression.

FINAL OR SECONDARY IMPRESSION

The impression that represents the completion of registration of the surface or object.
Its negative likeness or registration of the entire denture bearing, stabilizing area and border seal of mandible and maxilla for the purpose of fabricating a prosthesis.
The final impression made from special tray and is used for making master cast which must be poured with stone material.

Materials used for final impression:

1. Zinc-Oxide Eugenol impression material (ZOE)
2. Alginate impression material
3. impression plaster.
4. elastomers impression materials
A- polysulphide (rubber base)
B- poly ether
C- silicon (light body)
5- waxes


The techniques used for making final impression
• Mucostatic impression technique (non-pressure technique).
• Muco-compression or functional impression technique (pressure or closed mouth technique).
• Selective pressure impression technique
Master cast or final cast
A replica of the tooth surfaces, residual ridge areas and or other parts of the dental arch and or facial structures used to fabricate a dental restoration prosthesis.

Boxing an impression and maxing the castes

Boxing : is the enclosure of an impression to produce the desired size and form of the base of the cast and preserve desired details.
Boxing impression can be used for primary and final impression for complete denture. This procedure cannot usually be used on impression made from hydrocolloid materials (e.g. alginate) because the boxing wax will not adhere to the impression material and the alginate will be distorted.

Advantages of boxing

• To facilitate pouring the impression with plaster or stone
• Produce the desired size and form of the base of the cast
• Provide adequate thickness of cast
• Preserve desired details and borders of the impression
• In the lower impression, boxing makes the reproduction of the lingual borders and tongue space easier

Materials used for boxing impression

1. beading wax: a strip of wax is attached all the way around the outside of the impression approximately (2-3mm) below the border and sealed to it with hot wax knife.

2. boxing wax:

a sheet of wax is used to made the vertical walls of the box and its attached around the outside of the beading wax strip so that it does not alter the borders of the impression, the width of the boxing wax is about 10-15 mm above the impression

3. base plate wax:

a sheet of wax can be used to fill the tongue space in the lower impression that is sealed to lingual border of the impression and should be located just below the lingual border of the impression.

4. Artificial stone

is mixed according to manufacturer direction and sufficient stone is poured into the final impression so that the base of the cast be formed from 10-15 mm in thickness. The cast is called master cast.

Completed Master Cast

Base (15-20 mm )thick

Common faults in impression making
1.Poor selection of the tray.
2. in sufficient material loaded in the tray.
3. Excessive material loaded in the tray
4. Failure to press the tray completely to position (insufficient seating pressure)
5. Excessive seating pressure.
6. Incorrect position of the tray before final seating it (un-centralization).
7. Obstruction of proper flow of the material by lips, cheek or tongue.