Correlation between Shore-A-Hardness of impression materials and removal-forces - A questionable assumption

Objectives: For fixed prosthodontics and implants, polyvinyl-siloxane-materials (PSM) and polyether-materials (PM) are mainly used. Manufacturers provide a wide range of PSM and PM with different degrees of Shore-A-hardness (SH). It is supposed that the force needed to remove impressions from both teeth and casts correlate highly with the stiffness of the impression-materials (IM). Currently there is no data on the removal forces of modern IM in scientific literature. The aim of the present study was to examine the correlation between SH and removal-forces (RF), as well as separation-forces (SF) of dental IM. Methods: Two PM, nine PSM and three vinylsiloxanether-materials (VSM) were investigated. Impressions were made of a model made from polyurethane resin (Polyurock, Metalor Dental AG) based on a master model with full dentition (UK T 16 KaVo). Forces and SH were recorded during removal (Zwick 005, Zwick GmbH) of the IM from the polyurethane-model of the teeth and separation from each poured cast. At the same time SH was measured at the sample which was set in the Petri dish. Results: FS and SF ranged from 290.8(SD 21.1)-497.8(SD 28.1)N and 111.4(SD 13.5)-200.2(SD 11.7)N, respectively, and differed significantly between materials (p0.001). No values could be determined for Impregum soft (3M Espe) because after a strain of more than 1500N, the holding device broke out of the polyurethane model. SH ranged from 44.8(SD 0.2)-69.6(SD 0.4)N at the time of removal and 50.9(SD 1.1)-70.3 (SD 0.3)N at the time of separation. No correlation was found between RF and SH. Only a slight correlation was found between SF and SH. Conclusion: In-vitro-results show that SH of clinically used IM does not correlate with RF nor with SF from the cast. Keywords: impression material, Shore-A-Hardness, removal forces, in-vitro