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Impressions were made on a stainless steel master model containing a simulated two complete crown preparation with an edentulous space interposed, adding up to a total of 88 samples. The impression materials were Spidex® and Rapid®. 44 with each material were made, of which 16 were disinfected by spray for ten minutes with sodium hypochlorite 5.25%, 16 were disinfected by spray with Iodophor 10% and 12 were not disinfected . Three dimensional measurements of working casts, including, interpreparation distance (ID), height (H), and diameter (D) dimensions, were calculated using an measuring microscope graduated at 0.001 mm. Dimensional changes (mm) between the disinfected and non-disinfected working casts were assessed. A one-way analysis of variance statistical approach (ANOVA) was used to analyze the data (α=0.05).
Impression materials are used to register or reproduce the form and relation of the teeth and the surrounding oral tissues.1 Elastomeric impression materials are set through polymerization reactions, which can be categorized into two types based on the by-products formed: condensation polymerization and addition polymerization.2
In prosthodontics, impression material and prosthesis that have been exposed to infected saliva and blood provide main source of cross- contamination and additional problems in controlling cross-infection between dental office and laboratories .3,4 In view of the infectious carrier state of a significant proportion of the population and current trends in cross-infection control, thus, disinfection of the impressions is seriously recommended by the American Dental Association (ADA) and the Centers for Disease Control to prevent possible transmission of infectious diseases .5, 6 So, nowadays, additional control procedures and disinfection of impression should be a necessity and have to be followed during making and handling of dental impressions immediately after removal; however, such procedures should not alter dental impressions. To issue guidelines regarding impression disinfection, the ADA determined the antimicrobial agents to be used for different impression materials and the time, dilution, and temperature needed for each agent's optimal performance.6 The disinfecting process should be proper, but should not have adversely effect on the dimensional stability or the surface detail of the impression .7 The effects of disinfection methods on the accuracy of different impression materials have also been investigated .8-18 Apart from methodology, some studies have shown that the immersion disinfectant has no clinically relevant effect even on hydrophilic materials ; 12, 19-21 however, other studies have indicated that the dimensional stability of hydrophilic materials was adversely affected by immersion .9, 19 Some researchers have also evaluated the possible damage to the quality of the elastomeric materials impression according to the disinfectant methods, products, and time used .1, 8, 22-25
However, there is very little reported study about the effect of disinfection by spraying on the dimensional accuracy of the condensation silicon impression materials. While condensation reaction silicon impression materials are inherently unstable chemical structure because of its suppression which resulted of evaporation of volatile by-products from condensation reaction, but have more usage in Iran dental offices. Information on their behavior following spraying disinfection is, however, still lacking.
In view of the need to control infection in dental offices and dental laboratories, the purpose of this study was to evaluate the effect of the disinfection by spray atomization on dimensional accuracy of two common brands of condensation reaction silicon impression material that are currently available on the Iran dental markets. Although not of clinical importance, non-disinfected controls were also evaluated to note the effect of spraying disinfection on the impression materials. The null hypothesis was that there would be no differences in the accuracy of working casts of these 2 impression material, for the disinfected condition by spraying.
Materials and Methods
A standard, stainless steel master cast which was used to provide a dental replica of two teeth prepared for complete crowns with an edentulous space interposed, represented the clinical situation of two abutment teeth prepared to receive crown retainers splinted to two pontic forms. The effect of the two different disinfectants, hypochlorite 5.25%, and Iodophor 10% , deposited on the surface of some type of the condensation reaction silicon impressions by spray atomization was evaluated by measuring three clinically dimensions on dental stone casts recovered from the impressions of the standard master casts . A schematic representation of the standard master cast is illustrated in fig.1.
The impression materials evaluated in this study were the condensation reaction silicon impression material (Spidex® ; Coltene AG, Altstatten, Switzerland) and (Rapid®; Coltene AG, Altstatten, Switzerland) which used with the putty-wash technique. Each impression was contained within a perforated metal stock tray with retentive rims (Omnident GmbH, Rodgau, Germany) to make all impressions. All impressions were made by a single investigator. All materials were mixed at room temperature (25°C) and placed within the working time recommended by the manufacturer. The impressions were allowed to polymerize approximately 3 times longer (15 minutes) than the time recommended by the manufacturer to ensure adequate polymerization occurred at room temperature .13
In each of the 2 disinfected groups, 32 impressions were made( 16 impression for Spidex® and 16 impression for Rapid®) . A solution containing a 10% iodophor (Biotrol ,inc., North Salt lake ,Utah) or a 0.525% sodium hypochlorite solutions (Lacroix,Colgate-Polmolive,France) for application by spray atomization, were used for disinfection. After the impressions were removed from the master model, they were rinsed for 10 seconds under running water, and then air dried. The impressions were sprayed by disinfectant for 10 minutes, then rinsed again for 10 seconds under running water, air dried, and left for an additional 110 minutes before impression pouring. Additional 24 impressions (12 impressions with Spidex® and 12 impressions with Rapid®) were made as non-disinfected group by the same manner. The non-disinfected impressions were left for 120 minutes before forming gypsum casts. Type IV gypsum (Towerock ; Kettenbach GmbH, Germany) was used to form the working casts. The recommended ratio of 20 ml of distilled water to 100 g of powder was used. The powder and water was first mixed by hand for 10 seconds, then vacuum mixed (Multivac 4; Degussa GmbH, Hanau, Germany) for an additional 30 seconds. The gypsum was vibrated into the impressions and allowed to set for 60 minutes. The casts were left at room temperature for 24 hours to dry after being removed from the impressions.
Measurements of the three clinical dimensions ,interpreparation distance (ID), height (H), and diameter (D) were a made by one calibrated examiners using a measuring microscope (Measurescope MM- 400; Nikon GmbH, Düsseldorf, Germany) capable of measuring to 1 μm. Each measurement was repeated three times on each stone cast, and the mean of these measurements was recorded. The measurements were made blinded to the type of impression material and to the disinfection condition. The degree of dimensional distortion observed in the stone replicas was expressed as a percentage of change from the measurement values for the standard.
For an alpha level of 0.05 a sample size for each group to achieve a power of 95% and efficient size 0.5% was calculated.
The data collected from the investigation of two impression materials treated with two disinfectants, were analyzed for each clinical dimension with an analysis of variance (ANOVA) by using statistical software (SPSS 16.0; SPSS, Inc, Chicago, Ill).All hypothesis testing was conducted at α=0.05.
Considering the non-disinfected group as the baseline for evaluation of the dimensional changes, we can confirm the influence of the disinfectants. Table 1 shows the mean values and standard deviations of the dimensional stability of each impression material and disinfectant agents .The comparative results between each disinfected impression materials with similar non-disinfected materials have been shown in Table 2.As can be seen in table 2 there were no statistically significant differences in H and D dimensions between disinfected and non-disinfected Rapid® impression material, although ID dimension changed significantly. In Spidex® group disinfection of impressions resulted in significant changes in ID and D dimensions. There was also a significant difference in H dimension between non-disinfected and iodophor disinfected Spidex® groups.
In spite of significant differences based on disinfection protocol, it is important to note that all the dimensional accuracy measurements were lower than 0.4% and were within ADA specification 19 requirements (≤0.5% dimensional change).
The null hypothesis was that there would be no differences in the accuracy of working casts of these 2 impression material, for the disinfected condition by spraying. This hypothesis was rejected for the Spidex® impressions since there were statistically significant differences among disinfected dental stone casts and non-disinfected group, but was partially accepted for the Rapid® impressions. In most situations, the detected differences were small in magnitude and of minor clinical significance, in light of other factors such as tooth mobility, 26 mandibular deformation during opening, 27 potential inaccuracies during laboratory processes, 28, 29 and the clinically accepted values for marginal gaps of crowns (150-100 µm). 30, 31
In this study spray disinfection technique was used in order to disinfection of impressions. The spray technique has been investigated 17 and shown similar antimicrobial activity compared to the immersion method, however unlike the immersion method will not cause dimensional changes. 32 Following the ADA specification for elastomeric impression materials, in the present study condensational silicon impressions were made of stainless steel dies and the protocol does not include impression trays with tray adhesive and because of the type of impression technique (putty/wash system) it is the same as making a clinical impression with stock tray. When making an impression in a stock tray, impression shrinkage results in oversized dies, which is advantageous for compensating of wax pattern and casting alloy shrinkage.9 The oversized die could be helpful in full seating of a casting crown. On the other, hand following disinfection of impressions dimensional stability may be changed in the result of potential impression expansion. Thus, it is critical to maintain the balance between impression shrinkage and expansion and to know how much the dimensional accuracy of the impression material might be affected by disinfection process. Al-Omari et al.20 reported that changes of impressions produced by certain disinfectants were compensated by the setting expansion of the stone used to make the casts. This means that, provided they occur in the right direction, the changes of impressions and casts can balance each other, producing stone casts that are dimensionally closer to the original object than the impressions .16 For this to happen, the impression material should expand during disinfection to a degree analogous to the expansion that the stone mix would have on setting .21
There are many origins for the dimensional changes in dental impression materials. All the elastomers exhibit a light contraction during polymerization as a result of the volume reduction due to the cross link and alcohol evaporation. This is true just for the condensation silicones. The incomplete elastic recovery may also give rise to impression with the different dimensions when compared to the original .1
In Spidex® samples, differences between the disinfected and non-disinfected conditions were significant . However, there were no significant differences, for any dimensions except (ID), when comparing the control and 2 disinfectant conditions for Rapid ® samples. Such differences between two impression materials may be related to the presence of certain ingredients such as surfactant in ample quantity in Rapid® impression material .The Rapid® condensation-cured reacted well to the disinfection showing a slight differences but not significant .
Both of the impression materials are condensation silicones but they show differing responses to the test disinfection materials. This indicates that extrapolation across different brands within a single generic group is not wise .This result is in agreement with the results of Martin et al. 18
The dimensional changes in the present study which represent expansion of tested impression materials following disinfection are in accordance with Thouati et al study.25 Such an expansion can offset the polymerization shrinkage and therefore improve the precision of the resulting cast. Other investigator also have been confirmed the presence of an improvement in the precision of impressions in condensation silicone immersed in the disinfectant .24
Both spray disinfectants resulted in dimensional changes on the condensational silicone impressions. The alterations vary according to the disinfectant employed because the vaporization of alcohol as a by-product of polymerization is inhibited. With all of the measurements it was observed that Iodophor disinfectant did not affect each impression material similarly For the Rapid material, the Iodophor spraying resulted in an increase in the interpreparation dimension, more than the hypochlorite spraying and non disinfectant stone casts. It may be due to expansion of the impression after disinfection and reduction of (D) dimension of stone dies.
Both of disinfectant materials caused a statistically significant increase in (D) and (H) dimensions in Spidex impressions. Others have reported a decrease in the (D)dimension of improved stone dies when disinfecting impressions with immersion disinfectants.33.It seems that the polymerization shrinkage of the Spidex impression material do not completely negated by the use of a spray disinfectant, as demonstrated by the increase in percent change of the measurements with the cast sprayed.
The largest dimensional changes that had been occurred during the disinfection process was %0.4 in the (ID) dimension of (Rapid®-Iodophor), (H) dimension of (Spidex®-Iodophor) and (D) dimension of (Spidex®-Iodophor). This finding is in accordance with the study of Johansen and Stackhause that showed the condensation silicone had shrunk 0.44%.34
Thouati et al (1996) observed that the elastomer immersion in 5.25% sodium hypochlorite solution for 30 minutes caused expansion of the impressions which are in agreement with the result of present study . 25
There are also some studies in the literature that are not in accordance with the finding of present study. Adabo et al (1999) investigated the effect of disinfecting methods on the dimensional stability of six elastomeric materials. They concluded that there were a significant difference among the elastomers used and that the interaction between the material and the treatment was not significant 8.Matyas et al investigation also showed that there were no significant dimensional changes when condensation silicone impressions were sprayed or immersed in the iodophore and chlorine compound 35. This different results may be related to using different brands within single generic group.
All of dimensional changes that had been occurred during the disinfection process ranged from 0.1% to 0.4%. According to ADA specification 19 criteria, elastomeric impression material should not exhibit more than 0.5% dimensional change within the first 24 hours 36. Thus it seems that changes in the mean dimensions resulting from disinfection with two disinfectants by a spray were significant but not great enough to cause critical positional distortion of teeth when fixed partial denture restorations are made.
Limitations of this study include the following. The impressions were made from a simulated crown preparation was made of stainless steel and clinical consideration were not stimulated, then these conditions differ from those of teeth in the natural oral environment, since not only soft tissue, saliva and sulcular fluid were not present but also ,the intraoral temperature would be different.
Also, this study used stock, sterilizable, full-arch metal impression trays, whereas, in some regions, disposable full-arch and dual-arch plastic trays are used.
Based on the results of this study, the following conclusions were drawn:
1- There was a significant difference between the condensation silicone impressions materials used. Rapid® has higher dimensional stability than the Spidex® material after disinfection with two disinfectant materials.
2- Both of the impression samples showed significant difference in (ID) dimension.
3-Disinfection by Idophor resulted in more dimensional changes in contrast to sodium hypochlorite.
4- The dimensional changes for each specimen of condensation silicone impression materials were lower than the maximum linear dimensional changes (%5) recommended by ISO 4823 and spray atomization disinfection technique with sodium hypochlorite and iodophor can be a safe method for preservation of dimensional stability of the material.
The authors are grateful to Tabriz University of Medical Science for funds to enable this research to be carried out.