Impression Trays

An impression tray is defined as “A device used to carry, confine and control the impression material while making an impression” GPT.

The choice of impression material and technique influence a tray selection.

Time, expense and accuracy must all be considered for making a tray selection.

Classification of Trays

Impression trays are mainly of two types, namely, prefabricated stock trays or custom made special trays.

Impression trays may also be classified as perforated and non-perforated trays. Perforated trays are used to carry low viscosity impression materials. The perforations help to retain the impression material on the tray by mechanical interlocking .

Stock Trays

These are prefabricated trays made in metal or plastic.

Metal trays are preferred as they are rigid and not susceptible to distortion.

Prefabricated stock trays are used for diagnostic and preliminary impressions.

Retention for the material is provided by perforations, rim locks and/or tray adhesives.

Advantages

Stock trays eliminate the time and expense of making custom trays.

Can be reused.

Disadvantages

As their fit in the mouth is only an approximation, it is not possible to obtain a uniform thickness of impression material which could cause more distortion than when custom trays are used.

More material is also required.

Custom Trays

These are fabricated individually for each patient.

Advantages

Less material is used.

A uniform thickness of impression material minimizes distortion resulting from polymerization shrinkage.

Disadvantages

It requires more time for construction.

It is more expensive.

Triple Trays

They record the prepared and adjacent teeth, opposing teeth and the maximum intercuspal occlusion (bite), hence the name triple tray.

These are indicated for single unit, less extensive restorations in each arch.

When a triple tray is used the technique is called closed-mouth impression as the patient is required to occlude while the material polymerizes.

Advantages

They eliminate the need for an articulator.

Utilize less material.

Minimize deformation of the mandible during opening.

Disadvantages

Chances of distorted impression are high if the patient is not trained properly to occlude in maximum intercuspation prior to impression making.

Distribution of impression material is not uniform.

Eccentric occlusion needs to be evaluated and adjusted during restoration delivery.

Tray Selection

An impression tray is defined as, “a receptacle into which a suitable material is placed to make an impression”.

Trays used for primary impression making are called stock trays. They are factory prepared and are available in standard sizes. They are made of metal or plastic, can be perforated or non-perforated. Disposable stock trays are also available.

As standardized trays will not adapt closely to the patients arch, an approximate tray (size and material) should be selected for each patient.

Points to be considered during Tray Selection

For Complete Denture

1)There should be at least 2-3 mm clearance between the stock tray and the ridge. It should have 5-6 mm clearance for impression compound.

2)With the stock tray in position in the patient’s mouth, the handle of the tray is tilted downwards and the posterior border of the tray is observed.

3)The tray should extend over the tuberosity and the hamular notch .

4)If the tray is too large, it will distort the tissues in the borders of the impression and will push the tissues (cheeks) away from the bone.

5)If the tray is small then modelling wax should be added along the posterior border of the tray. Soft, boxing wax can be used to create a rim that helps to adapt the borders of the tray. It also protects the fragile border tissues.

6)If it is too small, the border tissues will collapse inward to the residual ridge.

7)The tray material should not react with the impression material and it should not distort.

8)For making alginate impressions, perforated trays are used.

9)All primary impressions generally have overextended borders.

For Removable Partial Denture

1)A custom tray is essential for recording maximum labial and buccal vestibular depths.

2)Secondary impression procedures should be carried out.

3)About 5 to 6 mm spacing should be present between the teeth and the tray when placed in the mouth.

4)Holes should be made in the tray to help retain the alginate.

Tray Adhesives

These are applied to impression trays to enhance the adhesion of impression materials to intraoral impression trays.

They ensure that the completed impression remains firmly attached to the tray upon removal from the mouth.

Generally, it is composed of silicone adhesive, alcohol and a colourant. It is supplied in dark bottles which should be immediately closed after use and kept out of sunlight. Every impression material manufacturer supplies the adhesive compatible with the material.

Wax Spacer

The wax spacer of specific thickness in special tray is provided to accommodate the impression material being used to record the soft tissues state.

Use of Spacers

Certain amount of space is always necessary for the impression material as the tissue would be compressed to a certain extent during the primary impression. The use of wax spacer is more reliable because of the accuracy with which we can achieve variable thickness of impression material and thereby achieve variable compression of tissue at different areas.

Spacer Designs

1.ROY MAC GREGOR DESIGN: He recommends placement of a sheet of metal foil in the region of incisive papilla and mid palatine raphe. He also says that the other areas that may require relief are maxillary rugae, areas of mucosal damage, buccal surface of the prominent tuberosities.Finally he concludes that the relief should not be used routinely in the denture.

2.NEILL DESIGN: He recommends adaptation of 0.9mm casting wax all over except PPS area.

3.BOUCHER DESIGN: He recommends placement of 1 mm base plate wax on the cast except posterior palatal seal area. He mentions that this PPS area will act as guiding stop to position tray properly during that impression procedure. He also mentions about the placement of escape holes with number 6 round bur in the palate.

4. MORROW AND RUDD DESIGN:

He recommends to block out undercut areas with wax and than adapt a full wax spacer 2 mm short of the resin special tray all over.

Then he recommends placement of 3 tissue stops(4-4 mm) equidistant from each other.

5.SHARRY DESIGN:

He recommends adaptation of layer of baseplate wax over the whole area outlined for tray(even in PPS area).

He recommends placement of 4 tissue stops(2 mm in width located in molar and cuspid region which should extend from palatal aspect of the ridge to the mucobuccal fold) and 1 vent hole in the incisive papilla region before making the final impression with the metallic oxide impression material.

6.BERNARD DESIGN: He recommends a layer of pink baseplate wax(about 2 mm thick) attached to the area of cast that usually have the areas of the softer tissues.He recommends placement of wax spacer all around, except the posterior part of palate which according to him are at right angles to the occlusal forces.

7. HEARTWELL DESIGN: He mentions two techniques for achieving selective pressure for maxillary impressions.

In the first technique he makes the primary impression with the impression compound with non perforated stock tray, the borders are refined, later space is provided in selected areas by scraping of the impression compound.

In the second technique, he recommends fabrication of custom tray (but did not mention about the wax spacer). Border molding is done with low fusing compound.

He recommends placement 5 relief holes on the palatal region, (3 in rugae area and 2 in glandular region) before making the secondary impression with ZOE paste.

8. SHELDON DESIGN: He describe 2 technique.

In the first technique the primary impression is made with low fusing modeling compound.

The borders are refined with green stick compound. Once the operator is satisfied with retention, selective relief is accomplished by scraping in the region of incisive papilla, rugae and mid palatal areas.

In the 2nd technique he describe of making a alginate primary impression. A primary cast is poured. After analysis of cast contour, under cuts are blocked out.

Later he recommends placement of spacer or pressure control (but did not describe about the wax spacer design).

Border molding is done with green stick compound before making the secondary impression with ZOE paste.

9.HALPERIN DESIGN: He recommends the `philosophy of the custom impression tray with peripheral relief’.

According to his philosophy the slopes of the ridges are considered to be the primary stress bearing areas and therefore these areas are functionally loaded with compound during the making of the final impression.

Moreover he says that there is no need to make awash secondary impression as he considers the tray surface and the border-moulded areas as the final impression surface.