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Stone dental impression8/24/2023 Although some studies showed that the surface hardness was not affected by impression disinfection 22, others disagree with these results 13. Cyanoacrylate increases the surface hardness 150% and abrasion resistance 48% 6,9. Hardeners are added to stone to improve the hardness of the cast. These details can be affected by the type of impression material used and disinfection method 1,3,4,10. 25 of ADA revealed that details reproduction is satisfactory when a copy line of 50μm in diameter reproduced continuously across the stone cast. Working cast model and die of gypsum used in fabrication of dental restoration must provide dimensional stability, strength, and resistance to abrasion as well as they must reproduce surface details 5. It is of a good practice to separate the impression from the cast before the hydrocolloid dehydrates because that the dried alginate can cause abrasion on the cast during its removal 7. The ADA specification no 18 accept an error of 75 μm in the alginate impression as a maximum dimension change 2. These changes in dimension can be explained by two main phenomenons syneresis which results in impression contraction and imbibition that makes the impression expands 12,14. Unfortunately, alginate is not dimensionally stable thus it is necessary to pour the impression immediately after molding has been accomplished 12,15 or within up to 60 minutes if the impression is kept in 100% humidity 23. 18-1992 for alginate impression materials 2. This product is regulated by ISO 1563:1990 and its properties are described in ANSI/ADA specification no. Commercial alginate has some additives to calcium salt which improves its properties such as diatomaceous earthfiller to increase rigidity and facilitate mixing, tetra sodium pyrophosphate (retarder), magnesium oxide (pH modifier) and setting aids such as sodium fluorosilicate 2,14. It consists of a powder containing calcium or potassium sulfate and fillers as primary components. The alginate is a hydrocolloid material used in dentistry since 1940 because of its easy mixing, low cost and well accepted by patient 8,15,17. The most extensively used materials for this purpose are alginate and gypsum products 8. These replicas are made from impressions that poured to make what called cast model or die 14,19,21. In dentistry, there is a need for a replica of oral-dental tissues for diagnosis and treatment of oral diseases 11,19. Keywords: alginate impression dental stone contact time cast model properties. Conclusion: According to the results of this study, pouring of impression up to 9 hours can negatively affect the cast model properties in the term of details richness. In the meanwhile, hardness was improved with increasing contact time. Results: The dimension stability of cast models was not affected by increasing contact time between type III dental stone and alginate impression while surface details decreased. The samples were evaluated under light microscope for surface details, measured by digital caliper for dimension stability and hardness was tested by making indentation on the cast then measuring the depth using digital caliper. Thirty-seven cast models were separated after one hour (control group) and 30 cast models were separated from impressions after 9 hours. Material and methods: Sixty-seven cast models were obtained from stainless steel cylinder using alginate impression material and type III dental stone. Objective: To measure the effect of different contact time between the alginate impression and type III dental stone on cast model properties in the terms of dimension stability, hardness and surface details reproduction. III Department of Prosthodontics, Faculty of Dentistry, Thamar University – Thamar – Yemen II Department of Prosthodontics, Faculty of Dentistry, Al-Gazira University – Wad Medani – Sudan I Department of Oral Rehabilitation, Faculty of Dentistry, Khartoum University – Khartoum – Sudan Effect of contact time between alginate impression and type III dental stone on cast model propertiesĪmna Adam Ibrahim I,II Mohammed Nasser Alhajj I,III Magdi Wadie Gilada I
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