機能印象材のレオロジカルな性質に関する研究

Abstract

The details of basal surface of complete or partial denture have intimate relationship with the retention, stability and support of the denture in oral cavity, as well as the preservation of oral tissue under the denture. The ordinal impression can not always result in a satisfactory fit of the denture to the oral tissue in functional movement, and hence it would be desirable to take impression on functioning especially in the case of constructing the complete denture. Recently several impression materials have preferably been used for this purpose in the clinical practice. The clinical performance of these materials may be closely concerned with their rheological properties. Therefore, some of the properties were examined in association with the clinical evaluation using several commercial materials. The functional impression materials employed were Ivoseal, F.I.T.T., Hi-soft, Hydro-cast, Softone and Visco-gel. Torsional shear strain was applied to the specimen at 90 minutes after mixing, as all of the materials were found to finish their setting by this time. Most of the materials showed high rate of elastic recovery for the torsional strain at the beginning. The elastic recovery decreased quickly as the duration time of the strain increased, and the stress relaxation became much easier than that of the ordinal impression material. Such characteristic tendencies could be favorable in taking functional impression. Only Hi-soft exhibited markedly low elastic recovery and fast stress relaxation even at 90 minutes from start of mix. It means that the impression of this material may be likely to be greatly deformed on removal from the oral cavity. When the specimen was subjected to repeated compressive stresses, each for 0.3 seconds with a 0.3 second interval from the end of the previous loading, most of the compressive deformation occurred in earlier period of the loading. As the ageing time before loading increased, the deformation decreased gradually and it became extremely small in 3 hours after mixing. From the clinical experiments, the morphological change of the impression was found to be affected by the flow of the material. It might be due to the interaction of the material with the mucous membrane under compression, and the material having poor flow left large compression evidences on the membrane. It was also found that the impression lost the details of its surface appearance if the impression was held in the oral cavity for more than 3 hours. It can be assumed that the material which is elastic against the momentary strain and viscous against the long-term strain is desirable for taking functional impression. On the basis of this assumption, F.I.T.T., Hydro-cast, Softone and Visco-gel seemed to be well available for functional impression among the materials employed in this study. In clinical use, it is considered from the present investigation that the reasonable duration time of the impression in the oral cavity should be in the vicinity of 3 hours in the case of taking the final functional impression.