Special Tray Materials and Types

Special Tray: a device that is used to carry, confine and control impression material while making a final impression.

Fabrication: a. Tray outline To make a tray outline, one must know how to interpret the anatomic land marks on the primary cast obtained from preliminary impression. Maxillary Cast * With an indelible pencil, draw a line transversely across the posterior border connecting the two hamular notches, 2 mm distal to fovea palatine. * Draw a line outlining the muco-buccal fold at the point where the buccal reflection leaves the lateral wall of the alveolar ridge.

Mandibular cast * With an indelible pencil, draw a line distal to the retromolar pad continue this line buccaly in an inferior and anterior direction following the masseter groove to the beginning of the external oblique ridge. * Progressing posteriorly, follow the oblique ridge to the buccal frenum attachment carry the outline above the frenum attachment and end the buccal frenum. * Allow space for the labial frenum attachment. * On the lingual side, make a line inferiorly from distal aspect of retromolar pad to lingual tuberosity. * From lingual tuberosity, extend a line anteriorly and inferiorly to mylohyoid ridge but 2 or 3 mm opposite the cuspid eminence. * Mark the outline similarly on the other side, join the two lines anteriorly by following the sublingual mucus membrane.

b. Methods Autopolymerizing resin impression trays. a. Sprinkle on methods 1. After making the outline on the cast, block out the severe undercuts with wax and adapt a layer of base plate wax to cast for relief. 2. Apply separating medium on the stone cast and over the relief wax. 3. Sprinkle polymer powder on the cast and relief wax and saturate with liquid monomer from an eye dropper, until uniform layer of approximately 2 mm thickness is obtained. 4. Avoid excess resin built up in the palatal region of maxillary casts or in the mucobuccal fold areas of mandibular casts. 5. Cure the tray under an inverted plaster bowl to reduce the porosity. 6. Mix more resin, when it is in the dough stage, form handles and adapt them to the impression tray. 7. After setting of resin, remove the trays from cast and trim it with bur and do necessary corrections.

b. Finger adapted dough method * Make the outline, blocking of undercuts, as described for sprinkle on method. * Proportion the acrylic tray material according to manufacturer 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 handles of the tray from these excess. * Cure the tray under inverted plaster bowel. * Remove the set tray from cast and follow the other steps as mentioned in the previous method, store the tray on the cast until needed.

2. Vacuum-adapted method *After making the tray outline and block the undercuts, place the relief on the cast, such as a wet sheet of non asbestos casting ring lining material, that will not melt during heating of the thermoplastic resin sheet. *Center the cast on the vacuum adapted plate and a sheet of appropriate size thickness in the heating frame, rotate the heating unit into position and continue heating until the specified sag in the material occurs. *Lower the frame and resin sheet onto the cast and start vacuum adaptation. Once adaptation is completed, allow it to cool and then remove the cast from the unit. Remove the tray from the cast and trim the borders. *Add handles from cold cure resin or from pre formed metal handles and store the tray on cast until needed.

3. Shellac Method * Once the tray outline is made on cast, block out undercuts on the cast with plaster. * Adapt a layer of wet non asbestos casting ring lining material to the cast for relief. * Make the shellac of proper thickness by adapting it and to be in intimate contact with cast and relief material. * Make a handle from a shellac material and adapt it to the tray. Remove the tray from cast and trim the excess. Store the tray on cast until needed.

Tray Handle * Only one handle is placed in the tray, whether upper or lower tray. (some authors suggest three handles for mandibular tray). * Handles should approximate the position of the teeth on the finished denture and should not interfere with the lip movement. * The dimensions of the tray, approximately about: Thickness= 3-4 mm. Length= 8 mm. Height= 8 mm. * Place horizontal grooves across the facial and lingual surfaces of the handles to improve the grip.

Evaluation and correction The maxillary special tray * Buccal and lingual flanges should be 2 mm shorter than tissue reflections * Move cheeks an lips to observe clearance for frenum. * Tray should contain hamular notches and extend 2 mm posterior to the vibrating line. * If the tray is displaced when the patient open his mouth widely, then there is over extension in the hamular notches region.

The mandibular special tray * Adjust the buccal flange to a line parallel to the ridge crest and 2 mm short of the external oblique ridge. Also, it should be no interfere with buccal frenum is present * The tray should be 2 mm shorter than tissue reflection point. * Reduce the tray 2-3 mm to allow a space for buccinator muscle. * Ask the patient to place their tongue in the right buccal vestibule. Dislodgment of the tray indicate over extension of lingual flange on left side in the posterior one third region. * Over extension of anterior two thirds of lingual flange is checked by asking patients to place their tongue well back on the palate. * To check the tray at lingual frenal attachments, ask the patient to open their mouth wide and place the tip of the tongue on margin of upper lip.

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