tray definition and classification

Impression trays

Definition and classification

Impression tray• It is a device used to carry ,confine & control

impression material while making an impression

Objectives

• Impression try must be rigid because Flexible trays cause distortion of impression

• It should be dimensionally stable• It should provide uniform space for impression

material.• It should be smooth to avoid injury to oral structure• It should not distort the tissue in the vestibular areas• It should support set impression materials when

removed from mouth so that a cast can poured

Parts of the tray

handle

flanges

floor

1- Body: which consist of : a) floor. b) flanges.2- Handel.

• 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.

• The handle is an extension from the union of the floor & labial flange in the middle region (midline), it's (L) in shape so that it will not interfere with lip during impression procedure.

Classification of traysBased on whether they are prefabricated or individualized• 1- prefabricated Stock tray: used for primary

impression procedure.• Perforated trays• Non perforated trays• Rim lock trays• 2- individualized Special tray/ custom try which is

used for final impression procedure.• Close fitting special try• Loss fitting or spacer special try

• Based on shape 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.

• 3-Tray with combination flat and oval floor are suitable for partial denture work.

Based on material used for fabrication

• a) metallic.• b) Non- metallic.Material used for fabrication stock Tray 1- metal Tin-lead alloyStainless steel2- plastic

• Material used for fabrication of custom tray• Tray compound• Tray acrylic • Shellac• Vacuum formed thermoplastic resin sheet• Wax• Old denture

advantagesI-stock tray1-Rigid and support the set impression material.2- Dimensionally stable3- Tray is smooth, so injury to oral tissues is avoided. 4- Multiple set in several patients.II- Custom trays1- Dimensionally accurate impression can be made2- Uniform space available for impression materials3- Over and under extension of tray flanges can be avoided4- Non stress bearing area can be relived with the use of spacer

advantages

III- Tray acrylic special tray1- easy to fabricate.2- require no special equipment.3- sufficient dimensional stable4- can be made thin but reasonably rigid.5- modified easily, by grinding with acrylic bur.IV- Shellac special trays:Impression tray can be fabricated very rapidly using thermoplastic shellac plate.V-Thermoplastic resin special trays:Very minimal amount of time is used in constructing the tray.

disadvantages

I- Stock trays:1- dimensionally accurate impression is difficult to make with stock tray2- uniform space is not available for impression material.3- flanges of stock trays may be over extended or under extendedII- Custom trays:1- can be used only for individual patient.2- fabrication of custom trays is time consuming based on the material selected3- Rigidity is less when compared to metallic stock tray.4- Dimensional stability is less when compared to metallic stock tray

disadvantages

III- Tray acrylic special traysDimensional stability is less due to polymerization sshrinkage.IV- Shellac special trays:1- lack of dimensional stability, especially during application of heat when border moulding the tray with impression compound.2- Brittle material, so chances of breakage while impression making and fabrication of cast.V- Thermoplastic resin special trays:1- specialized equipment requirement required.2- Expensive due to the requirement of costly equipment

Factors effect in selection of trayI- based on the type of impression1- Diagnostic impression = stock trays.2- Primary impression= stock tray 3- Secondary impression= custom traysII- Based on complete or partial absence of teeth:1- Edentulous impression = non perforated trays2- dentulous impression• Perforated trays• Non perforated trays with special adhesive.III- Based on impression material used:1- Alginate= perforated trays2- Impression compound= non perforated trays.