Care of Impressions and Making Casts PDF 2020

Care of impressions and Making casts  Removable Partial dentures  2020  Georges SAYEGH  Content Definitions Impression making Cast fabrication Procedures Requisites Types Trouble shooting guide Impression Makin...

Care of impressions and Making casts  Removable Partial dentures  2020  Georges SAYEGH  Content Definitions Impression making Cast fabrication Procedures Requisites Types Trouble shooting guide Impression Making  Impression making skill is important when diagnostic casts are required as part of planned therapy for the patient.  An irreversible hydrocolloid impression material, also known as “alginate” is used for making impression for diagnostic casts. CAST FABRICATION  Cast Fabrication by the Modified Two Stage Technique.  Step 1) Measure the stone by weighing 100 g of dental stone in the rubber bowl on the lab scale.  Be sure to "zero out" the weight of the bowl prior to measuring stone.  Step 2) Place the premeasured artificial dental stone and water into the vacuum, power mixer bowl.  Completely, incorporate the water and powder with the large spatula to wet all the powder.  The consistency of the stone should be thick enough so that you can turn the bowl upside down and the stone will not run out.(creamy consistency).  Step 3: First Pour With gentle vibration from the dental vibrator, use the small metal spatula or laboratory brush to add small increments of mixed dental stone to the impression.  Begin at one distal end and allow the stone to fill each tooth individually.  The stone will be completely set after 45 min. but you can pour a base after about 20 min.  Step 4: Second pour After the required time, soak the cast (First pour) in water before beginning the second pour. Once the cast has been soaked for several minutes, measure stone and water as before and vacuum mix.  Place the mixed stone on the plastic plate and build a "patty" to a thickness of approximately 25 mm  Gently, invert the poured impression into the patty.  With gentle hand vibration slightly push the impression into the patty. Diagnostic Casts  A life size reproduction of a part or parts of the oral cavity and/or facial structures for the purpose of study and treatment planning.  Diagnostic casts are artificial stone replicas of the patient.  these will include the soft and hard tissues of the maxillary and mandibular jaws.  These are usually made from Type III artificial dental stone but can be made from all types except Type II, impression plaster. Practically, they are not made from Type II stone, model plaster, due to its poor physical properties. Uses of Diagnostic Casts  Diagnostic or study casts are used to plan treatment for patients. individual uses for this purpose including: occlusal analysis, soft tissue assessment for prosthodontics, periodontics, oral surgery, and orthodontics, hard tissue assessment for occlusion, and fabrication of splints, custom trays, provisional restorations, and other Requisites for casts  All casts for removable partial dentures should exhibit the following qualities.  1-All surfaces to be contacted by the prosthesis should be accurate and free of voids and nodules. Removal of nodules resulting from voids or bubbles in the impression is essential, but hand carving in critical areas is not acceptable.  2-The surface of the cast should be hard ,dense and free of any grinding sludge left by the cast trimmer.  3-The cast extensions should include all of the area available for denture support, for example,3 to 4 mm beyond the hamular notches on maxillary cast ,and 3 to 4 mm beyond the retromolar pad on the mandibular cast.  4-The peripheral roll should be complete and approximately 3 to 4 mm deep.  5-Side walls of the cast should be vertical and may be tapered slightly outward but should not be undercut.  6-The base of the cast should be not less than 15 mm thick at the thinnest place and the lingual region of mandibular cast should be trimmed flat and smooth.The lingual peripheral roll,however,should not be removed.The cast should show no indications of having been wet, washed ,or brushed in tap water. THANK YOU