| A precise model is one of the key factors in making a good restoration,which is the key and the indispensable thing to success of restoration , however, many factors in clinic will affect the accuracy of the models.The operator's experience and skill level, the type and the performance of impression materials, ambient air humidity and temperature in clinical operation , disinfection of impression and so on, have different effects on accuracy and dimensional stability of the impression.Elastomeric impression materials is the most accuracy optimal in clinic, the best impression of a series of materials tody. Elastomeric impression materials includes: addition silicone rubber, condensation silicon rubber, polyether rubber, polysulfide rubber, which have the merit that good mobility, plasticity, elasticity, toughness, strength and little shrinkage in volume and so on. However, in clinical work, there are a variety of unsuccessful restorations caused by deformation of models obtained from elastomeric impression materials . Therefore, pursuing the more accurate model has been a goal and aspiration of clinicians.At present, many of the dental clinics in our country do not have their own denture processing center, and impressions of the restoration often need to be mailed to the remote processing unit, which will bring about a great changes to the impressions as the surrounding changing in the mail process.In addition, because of the need of tachnician, dentists are often required to send impressions not gypsum dies. The effect of temperature, humidity and time-varying impact on the impression accuracy rarely reported at home and abroad.Content :The content of this study is to discuss the accuracy and three-dimensional stability of impressions or models taken by three kinds of silicone rubber impression materials which can be influenced under different temperatures, humidities, and storage times, in an attempt to lay a solid theoretical foundation for the clinical work.Methods:The two common clinical vinylPolysiloxane impression materials: Silagum (Germany, DMG company, abbreviation DMG), EXAFINE (Japan, FUJI company abbreviation GC) and the condensation silicone rubber added hydrophilic group:Coltène @ Rapid Liner (Switzerland, Coltène AG Company abbreviation KT) were selected to conduct an experiment, in which after taking impression by those three kinds of silicone rubbers'light body, imprssions will be stored under different temperatures and humidities, then changes in linear dimensions on the surface of the imprssions will be measured respectively after a period of time (24 h). And stainless steel dental arch molds of lower mandible will be made to research the size rate of change between each corresponding point on the plaster model and tooting after simulating taking impression and pouring plaster model at eight different storge times in clinic by the Silagum silicone rubbers'heavy body and light body.Results:1.There were differences among humidity(P<0.05)and materials(P<0.05). No statistic significance of DMG was found between 96% and in water (P﹥0.05), showing the smallest change. Under humidity of 30%, the linearity size was significantly small compare with others (P<0.05). GC was significantly different between humidity of 30%, 50% and 75%(P<0.05), except for 96% and water (P﹥0.05). Finally KT have the smallest change at 96% and water(P﹥0.05). The largest change was found at humidity of 30%, 50% (P﹥0.05).2.There were differences among temperature (P<0.05) and materials (P<0.05). Among the three kinds of materials, KT showed the biggest change of linearity size under all kinds of extreme temperatures . two kinds of vinyl- Polysiloxane impression materials was no statistical difference(P﹥0.05); Compared different temperatures: three materials all changed smallest at 23℃; The significant difference happened compared with other different temperature(P<0.05).3.There were differences among time (P<0.05), three dimensional changes in the percentage of stone dies was no difference at 72 h,168h in most condition (P﹥0.05). Storage time of 2 h or 5h have the minimum change percentage in most cases. Conclusion:1.Higher humidity was useful for the precision of impression. The effect of the humidity on DMG material was less significant than the two others. DMG and KT have the smallest change at the humidity of 96% and in water,while GC at 75% . Under every humidity, KT showed the biggest change of linearity size.2.temperature significantly affects the precision of impressions. The comparatively large distortion generally occurred as a result of the high and low temperature extreme. We recommend that impressions should be poured in stone according to manufacturers'specifications before being mailed to a dental laboratory.3.Time also significantly affects the precision of stone dies (23℃,50%). The dimension of stone dies poured from 0,0.5h storage time changed comparatively large(-0.29%～-1.72%). Storge time between 2,5h make the most precise models(-0.01%～-0.43%).
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