IMPRESSION TRAY A RECEPTACLE OR DEVICE USED TO CARRY THE IMPRESSION MATERIAL TO THE MOUTH, CONFINE THE MATERIAL IN APPOSITION TO THE SURFACES TO BE RECORDED, AND CONTROL THE IMPRESSION MATERIAL WHILE IT SETS TO FORM THE IMPRESSION.
THERE ARE TWO TYPES OF IMPRESSION TRAYS:
STOCK IMPRESSION TRAY
“CUSTOM” IMPRESSION TRAY
“CUSTOM” IMPRESSION TRAYS MAY ALSO BE CALLED “SPECIAL” OR “INDIVIDUAL” TRAYS.
STOCK IMPRESSION TRAYS STOCK TRAYS COME IN A VARIETY OF DESIGNS AND SIZES. THESE TRAYS FALL INTO THREE MAIN TYPES:
FULL DENTATE TRAYS
PARTIALLY DENTATE TRAYS
THERE ARE A VARIETY OF SIZES TO SUIT MOST PATIENTS. STOCK TRAYS CAN BE MADE OF METAL (NOT DISPOSABLE) OR PLASTIC. THE PLASTIC TRAYS ARE INTENDED TO BE DISPOSABLE.
ADVANTAGES (METAL STOCK TRAYS)
DESIGNED TO BE USED MANY TIMES
MUST BE ABLE TO BE SRERILISED
USED TO TAKE FIRST IMPRESSION FOR THE PRODUCTION OF THE PRIMARY CAST
THE FLOW OF THE IMPRESSION MATERIAL CANNOT BE GUARANTEED TO COVER ALL THE AREAS OF THE TEETH OR TISSUES REQUIRED
PRESSURE POINTS CAN OCCUR WHEN USING A STOCK TRAY. THE PATIENT CAN EXPERIENCE DISCOMFORT DURING IMPRESSION TAKING DUE TO THE LARGE AMOUNT OF IMPRESSION MATERIAL USED IN THE TRAY. IN ORDER TO OVERCOME THESE PROBLEMS, SPECIAL TRAY SHOULD BE CONSTRUCTED.
CUSTOM IMPRESSION TRAYS MADE BY A TECHNICIAN FOR AN INDIVIDUAL PATIENT AND ONLY USED FOR THAT PATIENT.
CUSTOM TRAYS ARE CONSTRUCTED BY THE TECHNICIAN ON A PRELIMINARY MODEL. THE MATERIALS WHICH CAN BE USED FOR THE CONSTRUCTION OF A CUSTOM TRAY ARE AS FOLLOWS:
1. THERMOPLASTIC IMPRESSION COMPOUNDS
2. ACRYLIC RESIN – ‘SELF CURE’ OR ‘HEAT CURE’
3. PRESSURE OR VACUUM FORMED THEMOPLASTIC COMPOUNDS
4. LIGHT CURED ACRYLIC
WE WILL BE CONSTRUCTING OUR SPECIAL TRAYS WITH LIGHT CURED ACRYLIC. THIS IS BECAUSE IT IS RELATIVELY CHEAP, SAFE (NO MIXING REQUIRED) AND EASY TO ADAPT TO A MODEL. IT ALSO HAS A RELATIVELY LONG WORKING TIME.
ADVANTAGES OF CUSTOM TRAYS
Even thickness of impression material.
Controlled distortion of tissues.
Muco-static: pertaining to the normal, relaxed condition of mucosal tissues covering alveolar ridges and denture related surfaces.
Muco-compressive: pertaining to pressure on mucosal tissues covering alveolar ridges and denture related surfaces.
Even support of the impression material.
Control over the flow of the impression material.
Reduced discomfort for the patient.
Economic use of the impression material.
Reduced distortion during pouring of model due to support of impression material.
PRIMARY IMPRESSION Is an impression which is taken with a stock tray, it is not an accurate representation of the patient. Once the impression is poured a primary model is produced.
PRIMARY MODEL Is an inaccurate copy of a patient’s mouth which is used to assess and decide on a treatment plan. A primary models purpose is also to construct a custom tray on it.
SECONDARY IMPRESSION Is an impression which is taken with a custom tray, it is an accurate representation of the patient. Once the impression is poured a secondary model is produced.
SECONDARY OR MASTER MODEL This model is an accurate copy of the patient’s mouth, it is this model which is used to begin the steps in constructing an appliance or denture.
The handle must be placed so that it will not interfere with any movements of the patients’ lips during the impression taking, it should be placed where the teeth once were. The handle should be shaped so that it is:
It is easy to hold
Strong enough so not to fracture during removal
Smooth and round with no sharp edges
Positioned approximately in the centre or midline region
OUTCOME: You should now be able to know the following: