Method: Vinyl polysiloxane impression material (express 3M) was placed between 2 approximating discs on a universal testing machine and the pressure generated at the centre of the upper disc was recorded. The lower disc was perforated at the centre and at 3, 6, 9, 12, 15, 18, 21, & 24mm from the centre with 2mm perforations. The perforations were sealed. Each perforation, in turn, was unsealed and the impression pressure as the discs were approximated was recorded for 5 approximations. Data was analyzed by ANOVA with post hoc multiple comparisons (Tukey-B) at the 0.05 level of significance.
Results: The peak pressure means (S.D.) were 76.88(8.34), 176.8(10.79), 214.83(9.13), 256.80(9.13), 289.91(7.58), 316.23(7.78), 340.71(5.03), 357.46(11.09), 378.83(11.50), and 378.02(26.57)KPas. There was a significant difference between the groups (p<0.00001). The Tukey-B showed significant difference for each of the holes up to and including the hole 15mm from the centre (alpha=0.05). The holes at 18 and 21mm and the holes at 21, 24mm & ‘no hole' formed homogenous subsets.
Conclusion: In this in vitro experiment, perforations have a decreasing effect on impression pressure as they become more distant from the sensor. At distances greater than 21mm the effect of the perforation is not statistically different from no perforations.