* صناعة نظري / ثاني اسنان كركوكد.حسن29/11/2017 135
Synonyms: Primary impressions. Preliminary impressions. Diagnostic impressions.*
Objectives:
Preliminary impression is as important as final impression. An accurate preliminary impression helps to control the accuracy of the remaining impression procedures. Preliminary impression should record all areas to be covered by the impression surface of the denture and the adjacent landmarks. The peripheral borders of preliminary impression should be accurate in both height and width for the special tray to be properly extended. The maxillary impression should include the hamular notches, fovea palatine, entire buccal vestibule, frenum attachments, palate and entire labial vestibule. The mandibular impression should include the retromolar pad, the buccal shelf areas, the external oblique ridge, frenum attachment, sublingual space, retromylohyoid space, the posterior mucus membrane floor of the mouth. The completed preliminary impression must closely resemble the eventual denture base.*
Instruments and Materials: I- For preliminary impression made with alginate:
*Instruments and materials: II- For preliminary impression made with impression compound.
Stock impression trays (non perforated). Thermostatically controlled water bath, containing compound. Rubber bowel. Alcohol torch. Bunsen burner and matches. Mouth wash. Napkin and mirror. Trimming knife. Wax knife. Large pair of scissors.*
Preparation:
Choose the correct impression material and impression technique. Arrange all the instruments and materials required for the impression procedure. (sterilization should followed at every step). Patient should be seated up right in a comfortable position with the occiput resting firmly in the head rest. Dentist should instruct the patient about the procedure before the impression taking. It is advisable to practice placing the tray in the patient mouth before taking the impression.*
Selection of impression trays.
The beginning of a good impression starts with the selection of correct impression tray. Always remember, with alginate use perforated tray, and with impression compound use non perforated tray. The tray selected should cover the entire denture bearing area and extended to the tissue reflecting point. The tray should approximately 5 mm larger than outside surface of the residual ridge. In maxilla, tray must include both hamular notchs and vibrating line posteriorly. In mandible, tray must cover the retromolar pads posteriorly. For making alginate impression, Under extension of tray= add impression compound or wax. Over extension of tray= trimmed with bur.
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Making impression. I- Using alginate:
Mandibular arch: taking first as it is easier to the patient.Select the appropriate impression tray.Instruct the patient to rinse his mouth vigorously with cold water. Mix the alginate and load the tray.Insert the tray with operator sitting or standing in front of the patient, with the patient mouth slightly above the operator’s elbow.Instruct the patient to lift his tongue up and out then seat the tray posteriorly into retromylohyoid area. Then ask him to relax the tongue and lift the cheeks and lip.Hold the impression trays lightly in place for two minutes with no movement.Remove the impression with one quick sharp controlled movement, then inspect it for its acceptability. **
Making impression. II- Using alginate:
Maxillary arch: Ask the patient to open his mouth in order to reach the material to the retrozygomatic areas opposite to the tubersoities. The operator is standing behind the patient. The tray handle acts as a guide for the midline. Exert bilateral pressure to cause the alginate to flow and record the shape of the tissues. Hold the impression tray in place for 2 minutes without any movement, then remove the tray with one sharp controlled movement. Inspect the impression for its acceptability.*
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Making impression. Using impression compound.
Mandibular arch:Soften the impression compound in water bath at temperate of 55°C to 70°C. knead the compound with fingers by folding the materials inwards from the periphery to the center.The material is moulded with fingers to the approximate shape of the ridge and it can be lightly flamed to improve its flow.The loaded tray is then seated firmly in patients mouth and the procedures carried out as described for alginate preliminary impression.When the impression has hardened, remove from the mouth and check it for any deficiencies or over extension. Any correction can be done by adding or removing from the materials the re-soften the surface of impression before reseating in patient’s mouth.Once a satisfactory impression has been made, it must be hardened by placing in cold water. **
Making impression. Using impression compound.
Maxillary arch: Soften the impression compound, knead it and obtain the size and shape of a golf ball. Place the material in the centre of selected tray and mould it with finger to obtain the shape of the ridge and palate. The remaining procedure are as described for the lower impression compound.*
Impression evaluation:
The impression should include the entire denture bearing area. The surface of impression should be smooth. The border areas should be rounded and include the impressions of frenal attachments. If any errors are recorded (over extension, under extension, incomplete impression, large voids, separation of impression from tray during removal, etc..), the impression should be repeated or modified.
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