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Impression trays

Definition and classification

Dr. Shanai M.

In complete denture prosthesis we make 2 impressions for patient: a primary impression and final or secondary impression. To make an impression we should have impression tray.

Impression tray

It is a device used to carry ,confine & control the impression material from patients mouth while making an impression. During impression making, the tray facilitates insertion and removal of impression material from the patients mouth

Parts of the tray

handle
flanges
floor
1- Body: which consist of : a) floor. b) flanges.
2- Handel.
The handle is an extension from the union of the floor and labial flange in the middle region (midline), its (L)in shape so that it will not interfere with lip during impression procedure

There are upper tray to make impression of upper arch & lower tray to make impression of lower arch.
The difference between them is that in the upper tray, there is the palatal portion we called (vault) & in the lower tray there is the lingual flanges.

Types of trays

There are 2 main types:
1-stock trays: these are used for primary impression procedure.
Special trays or individual trays : these are used for final impression procedure.

1.stock trays :impression trays serve to carry the impression to the mouth and support it in the correct position while its hardening. This type of the trays can be used for several patients and used for making primary impression.

Types of stock trays

1- Stock tray for dentulous patient (Square).
2- Stock tray for edentulous patient (oval).
We can distinguish between them by: stock trays for dentulous patient have long flanges, wide and flat floor, while the other have short flanges, oval and narrow floor.
Tray with combination flat and oval floor are suitable for partial denture work.

Stock trays can be classified according to impression material in to two types

1.perforated stock tray.
A. perforated stock tray without rim lock
B. perforated stock tray with rim lock
These types used with alginate impression material.

2. non-perforated stock tray.

• non-perforated stock tray without rim lock used with impression compound.
• Non-perforated stock tray with rim lock used with alginate impression

Based on material used for fabrication of stock tray

a) metallic (plastic)
b) Non- metallic(metal Tin-lead alloy or Stainless steel)

Factors effect in selection of stock tray:

1- The type of material used in the primary impression procedure. e.g. impression compound we used non-perforated tray, because it will be stick on the tray. And if we use alginate material we should use perforated stock tray.
2- Size of the arch.
3- Form of the arch. (round , square , taper ).
4- The stock tray must covered all the anatomical landmarks needed in complete denture & this is a most important point.
5- Stock tray should give a sufficient space to impression material in all direction.

Special tray (individual or Custom Trays)

An individualized impression tray made from a cast recovered from primary impression. It is used in making a final impression.
Special tray is constructed on primary cast. As edentulous ridge show variations of shape and size (some have flattened ridges and other have bulky ridge), for this reason stock tray can fit the ridge only in an arbitrary manner, so special tray is constructed

Advantages of special trays:

1- Economy in impression material ( used less impression material required in special tray).
2- More accurate impression.
3- Special tray provides even thickness of impression material. This minimize tissue displacement & dimensional changes of impression material.
4- The work with special tray is more easier & quicker than modifying stock tray to provide accurate impression.
5- Special tray is more accurately adapted to the oral vestibules, this helps in better retention of denture.
6- Special tray are less bulky than stock tray which is more comfortable for the patient.

Materials used for construction of special tray

Special tray can be constructed by the use of different materials; this is depending on the type or technique of impression tacking. It can be constructed from:
1.cold cure acrylic resin or self cured acrylic resin or auto polymerizing acrylic resin (more common)
Visible light cured acrylic resin (VLC)
Shellac base plate.
Impression compound
Heat cure acrylic resin (rarely)

Types of special tray

We have two types of special tray:
1.spaced special tray (with stoppers or with out)
Closed fitted special tray


1.spaced special tray
In special tray with stoppers we should have 4 stoppers, 2 at anterior area (canine area) and two at posterior area (first molar area) in both sides. A baseplate wax sheet 1mm in thickness is adapted the on cast (after heating the wax) and a window open on the wax sheet in area of stoppers by removing the wax to make the stoppers and then put a uniform layer of self cure acrylic resin upon it. When we remove the wax, there is a space with 4 stoppers which will stop the special tray in the mouth of the patient and stop the pressure on the material during make the impression .



While for spaced one without stopper, a baseplate wax 1mm in thickness is adapted on the cast (after heating the wax) then put the acrylic resin on it, when the wax is removed there is space without stoppers.






In close fit special tray we used only separating medium on study cast and a self curing acrylic resin tray is mixed and uniformly adapted over the cast, so that the tray will be about 2-3 mm in thickness.
Acrylic resin handle is attached in the anterior region of the tray to facilitate removal of the final impression.

Techniques or methods of construction of special trays

1. Finger adapting dough method
2. Sprinkle-on acrylic method


1. Finger adapting dough method
• Make the outline, blocking of undercuts and apply separating medium
• Proportion the acrylic tray material (powder/liquid) according to manufacture recommendations and mix in a suitable container.
• Once it reaches dough stage, roll the resin to uniform and desired thickness to make the tray uniform thickness
• Hand adapt the material to the cast to avoid over thinning the resin on the convex portions of the cast
• Remove the excess material from the cast and make handle of the tray from these excess.
• Cure the tray under inverted plaster bowel.
• Remove the set tray from the cast














Sprinkle-on acrylic technique
This technique used for construction of individualized impression tray
Eliminate undercuts on the cast with a thin coat of wax
Paint cast with separating medium (cold mold seal)
Place acrylic resin powder (polymer) in a container with a perforated top (like salt shaker). Place the (liquid) monomer in a dappen dish.
Shake the polymer on the bored area with a glass medicine dropper, add monomer to the saturation point. Continue to build this over the entire denture- bearing area to thickness that will yield a rigid tray (a minimum of 2.5 mm)
Just before the final polymerization, remove the tray, reseat on the cast, and allow complete polymerization.
Reduce the borders to coincide with out line on the cast (2mm under extended).
Roughen the ridge area on the top of the tray anteriorly at midline, Then make a handle from resin and attached to the tray at this area

Criteria for special tray construction

The impression tray must not impinge upon movable structures.
The borders must be under extended (2mm)
The posterior limits of the impression tray should be slightly over-extended to ensure inclination of the posterior detail for development of the post-dam area in appear tray.
The tray should be rigid and of sufficient thickness that it will not fracture during its use
The tray must have a handle for manipulation and the handle must not interfere with functional movement of the oral structures.
The tray must be smooth on its exposed surfaces, and should have no sharp corner or edge which would injury the patient