The impression materials used for the study were condensation silicone putty and light body consistencies (ZETAPLUS), addition silicone putty and light bodied consistencies (AQUASIL) and polyether heavy and light bodied consistencies (IMPREGNUM TM SOFT).These three impression materials were named as group 1, group 2 and group 3 respectively.

METHOD - Standardized stainless steel master die as per ADA specification number 19 was fabricated. The master die consists of a ruled block and a mold ring with dimensions of 31mm height and 38mm width (ruled block). A 3mm height and 29.97mm diameter step has been made on the sides of the die to which the metal mold ring fits. The die consists of three parallel lines inscribed on the surface of the die named as X, Y and Z. The dimensions of the mold ring are 38mm (outer ring), 30mm (inner ring) and 6mm height which fits around the borders as a mold for the impression material [3] [Table/Fig-1,2]. The distance between the three parallel lines is measured in the microscope at four specific points referred to as C, D and C’, D’ which was appx 5mm. A total of 45 samples of the stainless steel master die were made (n=45) which were divided into three groups of 15 for each material. A stainless steel ring to be used as a mould to make an impression was placed on the die. Now, double mix single impression method was used to make the impressions. Impression material was mixed according to the manufacturer’s instructions and was loaded into the mold to make an impression of the die. After loading the mold, impression material was immediately covered by a thin sheet of polyethylene followed by application of sufficient force on a rigid flat metal plate to seat it firmly against the mold. To compensate for polymerization of the material at room temperature rather than at mouth temperature, the impressions were allowed to set for twice the manufacturer’s recommended setting time [4]. The impression was then recovered from the mold and numeric coding system (1-15) was used to identify the samples [Table/Fig-3].

[Table/Fig-1]:

Schematic representation of the master die.,

[Table/Fig-2]:

Master die,

[Table/Fig-3]:

Condensation silicone samples.

Samples were measured using stereomicroscope (MAGNUS MSZ-Bi) of 0.65x magnification with the help of image analysis software (IMACE PRO-INSIGHT VERSION 8) [Table/Fig-4].The distance between the cross lines CD and C’D’ were measured in the die and the measurement was recorded as reading A. Distance between the cross lines CD and C’D’ reproduced in the impression were measured before autoclaving, immediately after autoclaving and 24 h after autoclaving and these measurements were recorded as B1, B2 & B3 respectively. The measurements were made at the intersection of the vertical and horizontal lines in the stereomicroscopic view [Table/Fig-5]. Dimensional change percentage was calculated using the following formula: Dimensional change% = A – B_1/2/3 /A x 100.

[Table/Fig-4]:

Stereomicroscope

[Table/Fig-5]:

Stereomicroscopic view of addition silicone sample

The dimensional stability was compared at three different time intervals for three different materials using one-way-ANOVA [Table/Fig-6]. A student t-test was used to ascertain the statistical difference between the groups [Table/Fig-7].

The present study evaluated and compared the effect of autoclaving on 3 different elastomeric impression materials. Within the group, significant difference in the dimensional change was observed before and immediately after autoclaving. Statistically no much dimensional change was observed between before autoclaving and 24 h after autoclaving in condensation and addition silicone. Significant difference in the dimensional change was observed in polyether material in all the three time intervals.

Elastomeric impression materials may exhibit dimensional instability due to polymerization shrinkage, release of bye products due to chemical reactions, thermal changes or incomplete elastic recovery from deformation [5]. Saliva and blood contaminated impressions are often the source of contamination between the clinic and dental lab personnel. As part of infection control protocol, proper handling of dental impressions must exist among the office staff as well as between office and dental laboratories. Sterilization results in destruction of all forms of microbial life where as disinfection results in destruction of specific pathogenic microorganisms [6].

In the current study, impressions were made from stainless steel dies following the ADA specification for elastomeric impression materials. Although this provides a protocol that can be easily replicated by others, it is not the same as making a clinical impression. For example, the protocol does not include impression trays with tray adhesive [7]. When making an impression in a tray, impression shrinkage translates into oversized dies, which is advantageous for the fabrication of a cast restoration [8]. The oversized die can help compensate for wax pattern and casting alloy shrinkage producing a crown more likely to seat. Thus, it is important that impression shrinkage is consistent, serving as a reliable factor within the expansion and shrinkage equation associated with cast restorations. This clarification is valuable in order to appreciate the clinical ramifications of dimensional accuracy changes associated with potential impression expansion following disinfection [9-12].

The mean dimensional change and standard deviation values for three different elastomeric materials before autoclaving, immediately after autoclaving and 24 h after autoclaving were tabulated [Table/Fig-8], for addition silicone and polyether the difference between the mean dimensional change before autoclaving, immediately after autoclaving and 24 h after autoclaving was statistically significant (p<0.05) but for condensation silicone it was not statistically significant (p>0.05).

The study revealed that it is necessary to delay the casting of autoclavable elastomers by 24 h and it is advised not to autoclave the polyether material as they show slight sticky (matte) surface and higher mean dimensional change because of their hydrophilic nature. Tullner et al., reported that soaking the polyether for 15 min with a solution of sodium hypochlorite, iodophor or glutaraldehyde did not produce clinically relevant changes in the impressions. So, polyether can be sterilized by following cold sterilization [13,14].

Among the various sterilization procedures available for disinfecting the elastomeric impression materials autoclaving is considered to be most effective procedure. Various studies have revealed that dimensional change exhibited by elastomers after autoclaving is significantly low. Within the limitations of this study it is considered that autoclaving is the most effective procedure for condensation and addition silicone but for polyether autoclaving results in altering the surface details (matte surface) so following disinfecting procedure recommended by the manufacturer or following cold disinfecting procedure is indicated.