Principles of Impression Making
Impression Trays Impression Materials Impression Techniques
What is an Impression?
An impression is a negative record
or imprint of the tissues of the oral cavity that forms the basal seat of the denture.
What is a Cast?
A Cast is a positive reproduction of the Oral tissues of the upper jaw or the lower jaw, over which denture bases or other restorations may be fabricated.
What is the connection between Impression and Cast?
An impression is a negative record or imprint of the tissues of the oral cavity
which when poured in Dental Plaster - reproduce a positive form and shape of the same oral tissues (Cast)
What is an Impression Tray?
An impression tray is a device used to carry,
confine and control the impression material
while making an impression.
It also helps to remove the impression
from the mouth.
Parts of a Tray
Any impression tray has at least two parts:
A body, and
Classification of impression Trays
(Trays may be Perforated OR Non –Perforated)
What is the difference between Stock trays and Custom trays?
Stock trays are ready-made and comes in specific sizes. So stock trays must be selected for best fit.
Stock trays are usually meant to be used with specific impression materials.
They are re-useable after sterilization.
Custom trays are fabricated on the particular patient’s cast thereby making it unique to the patient. This is why custom trays always are a better fit than stock trays.
They are useful only for the particular patient- then discarded.
(Choosing stock trays Types of trays Choosing Impression materials for stock trays)
What type of stock tray should I choose?
Stock Trays are always selected to the best fit.
This selection is based on -
The type of mouth you want to make an impression of-
Dentulous Mouth – use Dentulous Trays.
Edentulous – use Edentulous Trays.
Partially Edentulous – use Combination Trays.
Type of impression material you want to use.
Elastic Impression Materials – generally use Perforated trays.
Inelastic materials – generally use Non-Perforated trays.
How can I identify between Dentulous and Edentulous trays?
Dentulous Trays – Square Cross-Section For patients with teeth!
Edentulous Trays – Rounded Cross-Section
For patients without teeth!
The Cross-Section of the trays are different-
What type of Impression Materials can I use with Stock Trays?
The stock trays may only be minimally adjusted to get a better fit. This is done by using wax or modelling compound.
This means that the impression material must make-up for the areas where the tray is not accurate in its fit.
So impression materials used with these trays must posses adequate bulk/ body/ viscosity.
Why use Custom Trays?
(Advantages, Materials in construction, Methods of construction)
Patients ridges are unique, and vary from patient to patient. Sometimes there are differences in the ridge resorption pattern in different parts of the same patient’s mouth.
Custom trays are meant to enable the dentist to overcome these difficulties and make accurate impressions.
To make custom trays you must make a preliminary cast using stock trays!
This means you end up making two sets of impressions!
What are the advantages of using Custom Trays?
Important advantages of using Custom Trays are –
The better fit ensures a more accurate reproduction of details.(Less bulk of impression material means less dimensional change!)
The borders can be refined to a much greater extent to obtain excellent results. (The functional depth and width of the sulcus, can be recorded!)
Areas of relief can be planned, if needed, in a custom tray.
Patient comfort and tolerance is better.
Materials used to make Custom Trays
Types of Custom Trays
What is the difference between a close-fit tray and a spaced tray?
CLOSE –FIT TRAY - As the name suggests, it is adapted directly on to the cast without any wax spacer.
Usually used with impression materials that have a light viscosity to obtain a wash impression, e.g. light bodied elastomers, ZOE impression paste.
TRAY WITH SPACER AND STOPS- these trays use a wax spacer to provide space for the impression material. This is because impression materials used here need extra space as they have higher viscosity e.g. Alginate, medium and heavy bodied elastomers.
What is a Spacer and what is a Stop?
The material used to provide space for the impression material is the SPACER, usually WAX.
The Acrylic extensions that help to stop the tray for over-seating during impression procedure is called the STOPS.
Four Stops are commonly employed in these trays, two in the Canine regions and two in the Molar regions.
A Tray with Spacer and Stops
Methods of Custom Tray Construction-
Shellac Base Plate – they are thermoplastic resins which may be heated over a dry flame and adapted to a cast. Talcum Powder or a light coat of Vaseline may be smeared to keep the material from adhering to the cast.
Impression Compound – A type of Impression compound is called ‘Tray Compound’ and may be used to construct a custom tray. It is also thermoplastic and is manipulated using a water bath.
Light – Cure Acrylic Resin – comes as soft flexible sheet which can be shaped on the cast as desired and then cured in a light curing unit.
Self – Cure Acrylic Resin – comes as a powder-liquid system which is manipulated to a dough stage, then shaped as a sheet of uniform thickness. This sheet is then adapted on the cast. It cures by chemical reaction emitting heat in a few minutes.
Heat – Cure Acrylic Resin – Here first the custom tray is constructed with base-plate wax to the desired thickness, then flasked and processed with heat-cure acrylic resin.
SELF CURE ACRYLIC CUSTOM TRAYS
Completed Special Trays – are 2mms short of the sulcus depth to allow space for Green stick (low fusing Impression Compund ) border moulding.
Trays are refined and Border Moulding is done to get the functional depth and the width of the sulcus.
After border moulding, the Final Impression is made usually with a material which has good flow – ‘Wash Impression’
Three impression techniques are followed while making an final impression-
Selective Pressure technique
Non Pressure technique
Preparing a Master Cast
(Methods of Beading, Method of Boxing, Advantages of Boxing)
Once an acceptable Final Impression has been made using the Custom Tray and disinfected, it is time to prepare the impression so as to obtain a detailed Master Cast.
This preparation of the Final impression is done by Beading and Boxing techniques.
Why do you do Beading & Boxing?
The advantages of Beading and Boxing are many –
It allows preservation of the borders of the impression.
Boxing produces a denser cast as the technique allows for vibration to get rid of air bubbles.
The thickness of the base of the cast can be controlled, and the master cast will have land area of predetermined dimensions and shape.
Good density and strength is a prerequisite for using master casts with mounting plates.
What is Beading? Types?
Beading can be done in two ways-
Beading using wax – Here a wax strip/ roll about 4mm wide is adapted 3mm below the impression border on the tray side of the impression.
Beading using Dental Plaster and Pumice –
Here the impression is seated into a thick mix of 1:1 plaster and pumice with the impression surface upward (tray side into the mix). This is then shaped so that a beading is obtained 3mm below the border of the final impression. Once set, boxing wax is used to box the beaded impression. It is then poured.
Wax beading does NOT adhere to Alginate and most Elastomeric impressions, so the Plaster & Pumice method is useful here.
What is Boxing?
Boxing is a technique to ‘box’ the impression to produce a container into which Dental Stone (Type III) can be poured.
This facilitates the use of a vibrator, and limits the stone expansion, thus obtaining a denser cast.
What is the importance of the Land Area?
The Land Area is the part of the cast that is developed by the Beading procedure.
The Land Area provides adequate thickness to protect the border from accidental damage.
If the border gets damaged then the denture may have a distorted border leading to a lack of retention, support or stability.