the impression process includes careful transfer of the patient 's soft and hard tissues to laboratory and is a major part of fixed prosthetic treatments . since the patient 's soft and hard tissues are transferred , having anatomic knowledge about periodontal tissues , making an accurate impression especially in the finish line , and using proper impression materials and an appropriate impression technique are important in making a suitable and accurate impression .
moreover , the significance of margin in the longevity of restoration and the effect of impression technique on marginal adaptation of restoration indicate the necessity of applying an accurate impression technique .
the accuracy of impression techniques is revealed when restoration with suitable marginal adaptation and minimum gap is obtained .
the mechanical and bonding characteristics   are also significantly influenced by the marginal fit [ 412 ] .
subsequently , the risk of carious lesions , periodontal disease , and endodontic inflammation is increased , and adverse consequences that affect the underlying health of abutments may occur [ 410 ] .
mclean evaluated more than 1000 crowns in 5 years and concluded that a marginal gap of less than 120  m is clinically acceptable ; however , in vitro studies have reported values of 100  m [ 1520 ] .
in addition to impression techniques , various factors , including preparation finish line , preparation angle , type of die , type of sprue , dye spacer , type of alloy and casting method , and type of impression material , influence the marginal adaptation .
nowadays , there are various impression materials for casting restorations that are accurate enough . in general , the common impression materials include hydrocolloid and elastomeric impression materials .
they have special properties of their own and their selection depends on the existing factors and conditions . in cases where there is not enough time for pouring the impression ,
polyether and additional silicone are used because these materials have high dimensional stability and their impression can be kept for a long time .
additional - polymerization silicone , also known as polyvinyl siloxane , was first introduced as an impression material in the 1970s .
additional silicones are very much similar to condensation silicone , except that additional silicone has higher dimensional stability ( its dimensional stability is the same as polyether ) .
the working time of additional silicone is greatly influenced by the ambient temperature and the hardened material has lower rigidity than polyether .
additional silicone is one of the most accurate and stable impression materials , which is used as single - paste , double - paste , and putty - wash systems [ 23 , 24 ] .
currently , the putty - wash type is widely used in fixed prosthodontics and removable overdentures .
this technique does not require a special tray and the putty - wash impression plays the role of special tray and saves time . moreover , there are two techniques for double - stage impression , impression with spacer and impression without spacer , and the latter was used in the present study .
based on the abovementioned discussion and diverse ideas about impression techniques , the current research was conducted to evaluate and compare the accuracies of one- and two - stage impression techniques .
a standard preparation was performed on the mandibular molar model with the following characteristics ( w&h turbine , allegra ) : 1.5  mm occlusal reduction and 1  mm axial reduction with 68 convergence ( 3 - 4 for each side ) .
round shoulder finishing line with at least 1  mm width was prepared on the lingual and facial surfaces , which were reduced in two planes [ 5 , 20 ] , and all of the line angles were rounded to reduce stress concentrations .
then , impression making was performed 20 times with one - stage impression technique and 20 times with two - stage impression with a proper tray using panacil additional silicone ( ketten bach , germany ) .
using adhesive plastic strips , stops were created in the tray to prevent sitting tray on the teeth and to provide sufficient space for impression making .
the stops were placed at the posterior and anterior parts of the tray [ 24 , 26 ] . in one - stage impression technique , putty and wash were mixed simultaneously , putty was placed in the tray , wash was put on the tooth surface , and impression was made with the same applied pressure on the tray in the mouth during impression making . in two - stage impression technique , however , an impression was made with putty from the prepared tooth and interdental papillary regions were removed , afterwards .
uniform thickness of material ( putty or wash ) must be used by applying uniform pressure for all the impression making .
if the impression material does not escape , it exerts pressure on the putty , and , after taking the impression out of the mouth , the putty returns to its original place and the impression becomes smaller consequently .
next , the impression was relined again with a layer of wash ( 3 - 4  mm )   by using uniform thickness of material ( putty or wash ) by applying uniform pressure for all impressions .
the casts of the intended impression were poured with plaster type iv over 30 minutes according to the manufacturer 's instructions ( first plaster ) .
then , the second part of plaster was added half an hour later using stone type iii . to prepare the plaster ,
the plaster powder was slowly added to the water container during a period of 10 seconds and was mixed by vacuum mixer for 30 seconds   to minimize the bubbles .
the prepared gypsum was poured in the impression on the vibrator over 3 minutes , and , one hour later , the gypsum model was extracted from the cast and randomly coded by another person
. the plaster dies with bubble at the die site and preparation finish line were excluded from the study , so a total of 40 plaster models were obtained .
all of the dies were covered with two layers of spacer with resultant thickness of 30  m .
care must be taken that the varnish is 0.5  mm short of finish line . to make the metal copings ,
after casting the metal coping , the sprue was cut and coping was seated on the dies by using fit checker and eliminating the premature points on the intaglio surface of coping . to measure the marginal gap ,
the distance between the copings margin and preparation finish line of plaster dies ( master cast ) was determined vertically in mid mesial , distal , buccal , and lingual regions by a stereomicroscope .
before running t - test with two independent variables , kolmogorov - smirnov test was used to analyze the normality of variables and the results confirmed that the data were normally distributed ( p > 0.05 ) .
having ensured the normality of variables , independent t - test was applied and the findings showed no significant difference in marginal gap between one- and two - stage impression techniques in mid buccal region ( p > 0.05 ) .
however , a significant difference was reported between the two impression techniques in mid mesial , distal , and lingual regions and average of all surfaces ( p < 0.05 ) .
detailed diagram of the marginal gap in mesial , lingual , distal , and buccal side in the one- and two - stage impression techniques is demonstrated in figure 1 .
as mentioned , making impression for oral restoration and dental morphology is an integral part of prosthodontics , and accurate impression is undoubtedly one of the most important stages of fixed prosthetic treatments .
ignoring this treatment stage will result in inaccurate impression and consequently a prosthesis with improper adaptation .
lack of accuracy in impression making leads to repeated impression making , which is costly and time - consuming for the patient .
thus , selecting the best and most accurate impression technique seems to be necessary for a successful treatment [ 1 , 2 ] .
one- and two - stage impression techniques are acceptable for many clinicians , and no significant difference has been reported in most of the studies .
therefore , given the diversity of ideas about the impression technique , the effect of various factors on the treatment outcome , and the notion that application of the most accurate impression technique requires repeated impression in special cases due to inaccurate preparation finish line , the current research was conducted to evaluate and compare the accuracies of one- and two - stage impression techniques . in most in vitro studies similar to the present study , the prepared and standardized dies have been desirably used to evaluate the accuracy of copings , since 40 gypsum dies were made from one dental model , and precise control was exerted on preparation parameters like convergence rate of axial walls and preparation finish line .
hence , the impression technique variable was evaluated and compared and the rest of the variables were kept fixed and under control .
also , sectioning technique was not used in the current study , which might have caused marginal distortion and reduced accuracy of measurement due to heat production .
the results of this study revealed that the marginal gap in the fabricated copings by one- and two - stage impression techniques was less than 120  m , which is clinically acceptable according to some studies [ 1420 ] .
better marginal adaptation at the margins can decrease the rate of fracture by increasing the consistency and can decrease microbial plaque and periodontal disease and complications consequently .
therefore , the copings made by one- and two - stage impression techniques are recommended to be utilized .  moreover the mean marginal gap is lower in the copings of two - stage impression .
therefore , this technique is preferred to one - stage impression technique , which is in line with the results of the studies conducted by mahshid et al . , caputi and varvara , nissan et al . , and dugal et al .
franco et al . reported no significant difference between one - stage impression technique and impression technique with a spacer .
the result of this study was similar to the result of franco 's study , but in this study we did not use a spacer .
this subject probably was due to impression making with the spacer which is induced impression material remaining in the tray , with more accurate impression making than without a spacer .
evaluated the dimensional accuracy of stone casts based on the impression material and three impression techniques .
they found that stone casts had high dimensional accuracy , and one - stage and two - stage putty - wash impression techniques and monophase light - body impression technique were not significantly different for marginal gap .
's study and those of the current study can be due to the type of impression material .
used two types of materials : silicone and gypsum ; furthermore , we use only gypsum material and this may make difference in results .
however , better marginal adaptation may be associated with proper thickness of wash in this study with proper pressure by applying 10 newtons during impression making , which may have made up for the putty condensation and reduced the pressure on putty impression in the second stage , thereby minimizing the intensity of condensation resulting from the return to the initial state .
no significant difference between one- and two - stage impression techniques in mid buccal region may be associated with buccal direction of thumb finger pressure above tray during impression making , which may dislodge impression tray from other sites ( mesial , distal , and lingual ) , which is compensative in two - stage technique with wash , but there is no fortunate compensation in the one - stage technique .
buccal direction during impression making induces tray dislodgement in other surfaces such as mesial , distal , and lingual regions , which forms a greater marginal gap . in two - stage impression making ,
the marginal gap is compensated by wash material in the second stage , whereas , in one - stage , we do not have the second stage for compensating marginal gap in such surfaces .
the findings of this study indicated no significant difference between one- and two - stage impression techniques in mid buccal region ( p > 0.05 ) , but a significant difference was observed between the two impression techniques in mid mesial , distal , and lingual regions and in average four surfaces ( p < 0.05 ) .
therefore , it can be argued that the accuracy of two - stage impression technique was higher than that of one - stage impression technique .
however , marginal gaps in both techniques were clinically acceptable ( marginal gap is less than 120  m ) and suggested .
since the present study was performed in a laboratory , it was not possible to analyze the effect of such factors as blood , saliva , oral temperature , and special clinical conditions on the accuracy of impression techniques , which might have significant impacts on the obtained results ; for example , high temperature in the oral cavity which is due to some hot foods may produce dimensional changes in coping special margins , because coping in this region is so thin , and this change may influence vertical gap .
hence , future studies are recommended to examine this issue in clinical conditions and with different impression materials .
 
 introduction . 
 one of the main steps of impression is the selection and preparation of an appropriate tray . 
 hence , the present study aimed to analyze and compare the accuracy of one- and two - stage impression techniques . materials and methods . a resin laboratory - made model , as the first molar , was prepared by standard method for full crowns with processed preparation finish line of 1  mm depth and convergence angle of 3 - 4. impression was made 20 times with one - stage technique and 20 times with two - stage technique using an appropriate tray . to measure the marginal gap , 
 the distance between the restoration margin and preparation finish line of plaster dies was vertically determined in mid mesial , distal , buccal , and lingual ( mdbl ) regions by a stereomicroscope using a standard method . 
 results . 
 the results of independent test showed that the mean value of the marginal gap obtained by one - stage impression technique was higher than that of two - stage impression technique . 
 further , there was no significant difference between one- and two - stage impression techniques in mid buccal region , but a significant difference was reported between the two impression techniques in mdl regions and in general . conclusion . 
 the findings of the present study indicated higher accuracy for two - stage impression technique than for the one - stage impression technique . 
1. Introduction
2. Materials and Methods
3. Results 
4. Discussion
5. Conclusion
Limitations and Suggestions