Dental Hypotheses

: 2019  |  Volume : 10  |  Issue : 1  |  Page : 20--24

A Comparative Analysis on the Accuracy of Special and Prefabricated Impression Trays Using Additional Silicone in a Laboratory Model

Ladan Jamshidy, Mohammad Macvandi 
 Department of Prosthodontics, School of Dentistry, Kermanshah University of Medical Sciences, Kermanshah, Iran

Correspondence Address:
Ladan Jamshidy
Department of Prosthodontics, Dentistry Faculty, School of Dentistry, Kermanshah University of Medical Sciences, Shariati Street, Kermanshah, Kemanshah Province


Introduction: Impression technique plays a pivotal role in fixed prosthodontic treatments; one of the main steps of impression is selection and preparation of an appropriate tray. Impression technique with insufficient accuracy causes distortions and complications that consequently result in treatment failure. The present study compares the impression accuracy of special and prefabricated trays in a laboratory model. This article compares the accuracy of special and prefabricated impression trays for achieving better impression and finally better crowns with good adaptation. Materials and Methods: In this study, a laboratory-fabricated resin model, resembling a mandibular first molar, was used on a dental arch and prepared by standard method for full crown restorations with a finish line of 1-mm depth and 3° to 4° convergence angle. Impression was made from a laboratory-fabricated resin model, 40 times with monophase medium-body polyvinyl siloxane in special trays and 40 times with putty and light-body polyvinyl siloxane using double-mix impression technique without spacer in prefabricated trays. To measure marginal gap, distance between the restoration margin and preparation finish line was vertically determined on each die in the mesial, distal, buccal, and lingual regions. Statistical analysis was performed and data were analyzed by SPSS 20 software using independent samples t-test. The data were significantly different at P < 0.05.Results: The mean value of special tray impression (72.72) was lower than that of prefabricated tray impression (91.27). No significant difference was observed between two techniques in the mesial and buccal regions (P > 0.05). However, a significant difference was reported between the techniques in the lingual and distal regions.Conclusion: The present study indicated a higher accuracy for the special tray impression than prefabricated tray impression.

How to cite this article:
Jamshidy L, Macvandi M. A Comparative Analysis on the Accuracy of Special and Prefabricated Impression Trays Using Additional Silicone in a Laboratory Model.Dent Hypotheses 2019;10:20-24

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Jamshidy L, Macvandi M. A Comparative Analysis on the Accuracy of Special and Prefabricated Impression Trays Using Additional Silicone in a Laboratory Model. Dent Hypotheses [serial online] 2019 [cited 2019 Jul 23 ];10:20-24
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Impression making for dental restoration is an inseparable part of prosthodontics, and impression technique is an important stage of the treatment procedure, which greatly affects the ultimate outcome. It is also important for dentists to be sufficiently aware of the problems associated with impression making and the solutions to come up with.[1] Because the clinical success of the fixed prosthodontics depends on the precise reconstruction of the prepared tooth through impression, and the impression technique determines the accurate reproduction of preparation finish line, numerous discussions have been directed toward the effect of impression techniques on the accuracy of the reconstruction cast of the prepared tooth. Accordingly, a frame with good adaptation can reduce the difficulties of carries and periodontal diseases.[2] The mechanical and bonding characteristics[3] are also significantly influenced by marginal fit.[4],[5],[6],[7],[8],[9],[10],[11],[12] Inaccurate marginal fit through plaque accumulation, microleakage, and cement breakdown cause to increase the risk of decayed lesions, periodontal disease, and endodontic inflammation, and subsequently leads to consequences affecting the underlying health of abutments.[4],[5],[6],[7],[8],[9],[10]

Several studies have evaluated the values of maximal marginal gap.[13],[14],[15],[16],[17],[18],[19],[20] McLean et al.[13],[14] evaluated more than 1000 crowns in 5 years and concluded that a marginal gap of less than 120 μm would be clinically acceptable; however, in vitro studies have reported values of 100 μm.[15],[16],[17],[18],[19],[20] In addition to the impression techniques, other factors such as preparation finish line, preparation angle, die type, die spacer, number of spacers and investments,[2] alloy type and casting technique,[21] and impression materials[22] affect marginal adaptation.

There are various kinds of impression materials for casting restorations that are adequately accurate. The selection of these materials depends on personal taste, ease, and to some extent economic issues. The accuracy of these materials is not a basis for their selection, because they are not very distinct clinically.[23] However, alginate is currently one of the most widely used materials in initial impressions to obtain a primary model for diagnostic purposes, treatment plan, and making temporary prostheses or special tray.[24] The choice of tray also depends on the type of impression material. For impression making with reversible hydrocolloid (agar), special trays are required that are cooled by water, but for simple prosthetic treatments with irreversible hydrocolloid (alginate) and elastomer materials, prefabricated trays can be suitable. Using the trays will undoubtedly lead to impression distortion, but the distortion can be minimized by taking a series of measures.

In special tray impression, as the name suggests, a special tray is made for each patient using diagnostic casts for tray construction. The special trays are more advantageous than the prefabricated ones[25] and are used for elastomeric impression materials, because these materials are very accurate in 2 to 3-mm constant layers. Many authors do not approve of prefabricated trays, because the nonuniform size of the impression material is vulnerable to distortion.[26],[27] Nevertheless, it has been reported that the mean difference between the thickness of impression materials in special and prefabricated trays is less than 1 mm; hence, both trays lack a uniform thickness.[28] The special tray should be hard and have stops on occlusal surface for proper positioning in the mouth.[29] The special tray should involve 3 to 5 mm of gingival margin and should also be positioned up to 3-mm behind the last tooth in the dental arch.[25] The prefabricated trays are probably accurate enough for single-unit restorations, especially when polyvinyl siloxane or polyether impression materials are used.[24] The purpose of our study is to compare the accuracy of two impression techniques, the special tray, and prefabricated tray, using marginal gap measurement.

 Materials and Methods

A standard preparation was performed on the mandibular molar model with the following characteristics (Alegra dental turbine, W&H company, Bürmoos, Austria): 1.5-mm occlusal reduction and 1-mm axial reduction with 6° to 8° convergence (3°–4° for each side).[7] A 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 the line angles were rounded to reduce stress concentrations. For impression making, the die was fixed in the dentic. Through the study (approved and supported by Kermanshah University of Medical Sciences with permission-code 93473), 40 impressions constructed by metal prefabricated trays were created using putty and light body polyvinyl siloxane (PVS; Kettenbach, Germany) through double-mix impression technique without spacer. Forty special tray impressions were constructed by means of monophasic medium body PVS; the first impression was constructed with irreversible hydrocolloid using an appropriate prefabricated plastic tray. Then the special tray was made on the primary cast with self-curing acrylic (Dentium, Cypress, California, USA) according to the manufacturer’s instructions. All the procedures applied for cast pouring followed by Alhavaz and Jamshidy.[45] After 30 min of cast pouring, the plaster cast was extracted and randomly coded. The unqualified plaster samples (i.e., those with bubbles at the die site and preparation finish line) were excluded. All the dies were covered with two layers (collectively 30-μm thickness) of spacer. After varnishing (0.5-mm above finish line),[11] the metal copings were waxed on the dies. All the samples were invested and casted in same laboratory conditions. After cutting sprue from the casted metal copings, they were sitted on the dies using Fit Checker (Fit Checker™ Advanced Blue; GC America, Inc., Alsip, Illinois, USA). The distance between the margin of copings and preparation finish line of plaster dies (master cast) was measured vertically in the mesial, distal, buccal, and lingual regions via a stereomicroscope (Model: SZM-SMD; Optika, Ponteranica, Italy) with standard method.[30] For statistical analysis, the data were subjected to “Kolmogorov–Smirnov” test for determining the normality, and to “independent samples t-test” for determining the difference between the trays using SPSS-20 software (SPSS Inc., Chicago, Illinois, USA). Data were considered statistically different at P-value <0.05.


The variables were normally distributed in the present study (P > 0.05). Subjecting the normal data to independent samples t-test approved a significant difference between two impression techniques in two levels of comparison [Table 1];] as it might be concluded from Figure 1, in the distal and lingual regions, the special tray is more accurate than the prefabricated one (P < 0.05); the difference between the trays in the distal region is more conspicuous than the lingual region. Two other regions, the buccal and the mesial, are not appropriate indicators statistically (P > 0.05) to demonstrate the advantages of the special tray compared with the prefabricated one. In the total region, however, a significant difference was registered between the trays (P < 0.05).{Table 1}

In the special trays, marginal gap in the lingual region is more minimized than other regions, whereas the gap is in its maximal condition for the buccal region. In the special trays, four regions ordered by marginal gap from the least to the most are lingual (61.813 μm), distal (63.475 μm), mesial (79.488 μm), and buccal (86.1 μm). Same observations would be distinguishable for the prefabricated trays; the lingual region has the least marginal gap in the series, whereas the buccal region possesses the greatest. In the prefabricated trays, four regions ordered by marginal gap from the least to the most are lingual (85.038 μm), distal (89.638 μm), mesial (91.825 μm), and buccal (98.563 μm).


The clinical success of fixed prosthodontics depends on precisely reproducing the preparation finish line and marginal adaptation of the restorations.[22] An accurate impression is one of the most important stages of the treatment. The marginal adaptation of a frame as an effective factor in periodontal health and longevity of restoration[31] is influenced by various factors, including preparation angle,[32] preparation finish line,[31] impression material,[33] impression technique,[34] die construction materials (plaster or epoxy),[35] sprue pattern and number,[36] investment type,[37] and casting technique.[38] Marginal adaptation is a key factor in the success or failure of metal-ceramic frames.[31]

If the gap between the prepared tooth and frame margins is more than acceptable standards, the dental cement dissolves rapidly,[5] microorganisms accumulate in gaps and under-frame carries,[39],[40] and pulp stimulation, subgingival floor alterations, gingival inflammation and discoloration in gingival margin, increased pocket depth, and gingival loss occur consequently.[41],[42],[43],[44]

In most in vitro studies, similar to the present study, prepared and standardized dies have been used to evaluate the copings’ accuracy.[45] In the present study, preparation of 80 plaster dies from a dental model yields a precise control on the parameters such as the convergence rate of axial walls and preparation finish line. So it was an attempt to evaluate and compare the impression techniques, whereas the rest of variables were kept constant and under control. Moreover, the sectioning technique as a plausible cause of marginal distortions and accuracy reduction in the measurements was not applied.

Our findings showed that the marginal gap made by special and prefabricated trays was less than 120 μ that is clinically satisfying.[46] Therefore, the impression copings are suggested to be made intraorally by special and prefabricated trays. Furthermore, better marginal adaptation can increase the strength of copings, and reduce microbial plaques,[47] thereby minimizing periodontal and esthetic complications. The marginal gap means in both the techniques were clinically acceptable, but the reduced mean of special tray makes the special tray superior than the prefabricated one. Millstein et al.[46] indicated that the casts made by special tray are more accurate than those made by prefabricated one. Moreover, our findings are in contrast with Rueda et al.,[48] in which they emphasized on lack of conspicuous difference between the two types of techniques. However, the difference can be attributed to the different applied impression techniques.Sazgara and Nahidi[49] reported that the metal trays and PVS putty-wash impression in closed tray reveal a similar accuracy as the special tray, but the accuracy was significantly lower in prefabricated plastic tray than special and prefabricated metal trays; the difference can be related to different techniques and measurements. In addition, Carrotte et al.[50] determined the minimum level of distortion (about 50 μ) for prefabricated metal trays and hard plastic trays, and the maximum level of distortion for prefabricated soft plastic trays (180–210 μ). Bomberg et al.[28] declared that special trays are required for making impression with elastomeric impression materials.

Less marginal gap in copings fabricated by the special tray compared with the other tray may be due to good adaptation of special tray with arch led to reduce the thickness of PVS, so this low thickness is necessary for accurate impression making. No significant difference between the two techniques was observed in the mesial and buccal regions. This may be associated with mesial and buccal direction of thumb finger pressure above the tray during impression making that may dislodge impression tray from other sites (distal, lingual); so this dislodgement is less in the special tray than the prefabricated one in the distal and lingual sites, because of good adaptation of special tray in these regions.

Because of the handmade nature of the special trays, differences in the thickness of the trays resulted from human errors cause to affect the trays’ stability, and ultimately the accuracy of the impressions. Accordingly, it is suggested that in next studies special trays would be made of prepared resin sheets which have same thickness.


The findings of this study indicated no significant difference between the two impression techniques, special and prefabricated trays, in the mid-mesial and buccal regions (P > 0.05), but a significant difference was reported between the two impression techniques in the mid-lingual, distal regions and in total (P < 0.05). Thus, the accuracy of special tray impression technique was higher than that of prefabricated tray. However, because the mean marginal gap in both methods was clinically acceptable, both techniques are suggested to be used.

Financial support and sponsorship

The current research was financially supported by the research council of Kermanshah University of Medical Sciences.

Conflicts of interest

There are no conflicts of interest.


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