Chapter 2 Preliminary Impressions PDF

Chapter II PRELIMINARY IMPRESSIONS CARE AND POURING Preliminary impression : An impression made for the purpose of diagnosis or construction of a tray. https://www.ua.edu.lb/images/logo.png Preliminary cast : A positive reduction of the upper or lower jaw tissues that is made in an impression a...

Chapter II PRELIMINARY IMPRESSIONS CARE AND POURING Preliminary impression : An impression made for the purpose of diagnosis or construction of a tray. https://www.ua.edu.lb/images/logo.png Preliminary cast : A positive reduction of the upper or lower jaw tissues that is made in an impression and over which impression trays may be fabricated. Requirements : 1. All surfaces to be contacted by the tray or denture should be accurate and free of voids or nodules. 2. The surfaces of a cast should be hard , dense and free of any grinding sludge left by the cast trimmer. 3. A cast should extend sufficiently to include all of the area available for denture support. For example , a mandibular cast should extend 3 to 4 mm beyond the retromolar pads. 4. The peripheral roll should be complete and no deeper than 3 to 4 mm , and the edge of a cast extending out from this roll should be approximately 3 to 4 mm wide. 5. The side walls of a cast should be vertical or slightliy tapered outward , but not undercut. 6. The base of a cast should not be less than 15 to 16 mm at the thinnest point. 7. The tongue space on a mandibular cast should be flat and smooth when trimmed , but the lingual pripheral roll should remain intact. 8. A cast should not show signs of having been wet or washed with tap water. The two most frequently used materials for complete denture preliminary impressions are alginate irreversible hydrocolloid and modeling plastic ( impression compound ). Alginate Preliminary impressions Alginate is used to make impressions for diagnostic casts ,removable partial dentures and preliminary impressions for complete dentures. It is an irreversible material that transforms from sol (soluble salt of alginic acid ) to gel (insoluble salt ). The impression should be poured as soon as possible because the alginate dehydrate rapidly which leads to distortion. Pouring procedure 1. Examine the impression carefully for voids in critical areas and pulling away of the alginate. It is better to reject any bad impression than remaking the restoration. 2. Wash the impression with a gentle stream of cold water. Sprinkle stone on the impression to disclose any saliva remaining then scrub it with a camel hair brush and flush with running water. 3. Remove moister with a gentle stream of air. 4. Remove the excess of alginate that extends beyond the back of the tray to prevent any contact between the alginate and the bench. 5. Weight the stone and water according to the manufacturer recommendations. Be careful to store stone in dry and closed containers. 6. Mix the stone and water in a vacuum mixing machine to prevent air bubles in the mixture. 7. Gently pour the stone into the impression without letting the alginate touch the vibrator. 8. We only add plaster in one corner of the impression. We cover the entire anatomic portion of the impression during the first pour. 9. Apply the remaining stone as droplets to create undercut areas which will be used in forming the base of the cast. 10. Put the impression on the bench or in a tray holder and wait while it sets. 11. After the initial set of the stone place the impression in slurry water for 3 to 5 minutes to moister the stone for a better grip with the base. 12. Remove the impression from the slurry water an check the height of the stone before pouring a base with the same type of stone. 13. Mix stone with the same ratio onto a glass or plastic surface and form it into a base. 14. Add a small amount of stone on the undercuts of the first pour. 15. Invert the poured impression onto the patty and draw the stone up onto the sides of the first pour. 16. Remove the excess carefully especially in the tongue area in the mandibular ridge. 17. Separate the cast from the impression approximately 45 min to 1 hour. 18. Examine the case carefully and remove all nodules. 19. Soak the cast in slurry water for a few minutes then trim it on a cast trimmer. The soak will prevent sludge from trimming to stick on it. 20. Permit the cast to dry. 21. Identify the cast by labeling it.  PROBLEM AREAS 1. Failure to identify a faulty impression. 2. Too long a delay in pouring the impression. 3. Poor pouring technique. 4. Incorrect trimming on the cast trimmer.  MODELING PLASCTIC IMPRESSIONS It is also called impression compound. It is composed of gum dammar , prepared chalk and other materials. It is used especially for complete denture impressions.