Rheological properties of elastomeric impression materials for selective pressure impression technique

Purpose: The selective impression technique is recommended for removable prosthodontics. However, the viscoelastic rheological properties of impression materials, particularly the time-dependent viscoelastic changes in impression material immediately after tray seating, are unknown. The objective of this study is to clarify the viscoelastic rheological properties of impression materials by focusing on tray seating. Methods: In this study, eight types of polyvinylsiloxane (PVS), two types of polyether, and two types of alginate impression materials were used. The storage modulus (G'; degree of stiffness) and loss tangent (tan δ; degree of hardening) were determined as functions of time from 0 to 360 s, commencing immediately after the completion of mixing, using a stress control-type rheometer. Thus, G' and tan δ at 0 s and 20 s were compared. Results: Stiffness was found to be widely distributed (4.49-0.26×10⁴Pa) among PVS-types, even immediately after mixing the impression material. There was also variation among polyethers (1.55-0.5×10⁴Pa) and among alginates (0.64-0.21×10⁴Pa). The hardening of all impression materials progresses beyond 20s after the completion of mixing. Conclusions: The G' values varied with each impression material, even immediately after mixing, and the accurate impression-taking time was determined from the results of tan δ. These results provide unique insight into the selective impression technique.