Effect of Shore-A-Hardness and Viscosity on Impression Accuracy

Methods: A steel model with different cones (convergence-angles: 6° and 10°) was prepared and ten two-step-putty-wash-impressions with six material combinations were taken. Three putty-materials (SHA 45, 65, 75) and two wash-materials (disc-diameter ADA No. 19 (DD): 37mm, 44mm) were used. Impressions were poured with type-IV-plaster (Fuji-Rock, GC, Japan). After 48 hour storage time diameter, roundness (5 planes with 24 measuring points each) and die height were assessed with a 3-D coordinate measuring machine (Thome, Germany). For statistical analysis parametric procedures (Welch, Games-Howell) (p<0.05) were used (normally distributed data).

Results: Deviation in diameter ranged from (Mean±StD) -0.85%±0.29% (75 SHA/DD 44mm) to -2.99%±0.27% (SHA 45/DD 37mm), 6° cone and from -1.23%±0.31% (SHA 65/DD 44mm) to -1.82%±0.58% (SHA 45/DD 37mm), 10° cone. Roundness-deviations between 14µm (SHA 75/DD 44mm) and 48µm (SHA 45/DD 37mm) were observed for the 6° cone and between 16µm (SHA 65 putty/DD 44mm) and 60µm (SHA 75/DD 44mm) for the 10° cone. The combination SHA 45/DD 37mm showed highest differences (-1.74%±0,62%/ 6°; -2,29%±1,29%/ 10°) in die-height for both convergence-angles, while the lowest changes where shown for combinations SHA 65/DD 44mm (-0,48%±0,16%/ 6° cone) and SHA 75/DD 44mm (-0,65%±0,45%/ 10° cone). Overall there was a significant correlation in between SHA, viscosity and the three target variables.

Conclusion: Within the limitations of this study it can be concluded, that the accuracy of a two-step-putty-wash-impression is related to putty SHA and wash viscosity. Thus adequate materials should be preferred in clinical practice.