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 Table of Contents  
Year : 2014  |  Volume : 1  |  Issue : 3  |  Page : 118-122

Dimensional changes in plaster cast models due to the position of the impression tray during setting

1 Department of Orthodontics, Universityof São Paulo State UNESP, Rua Humáita, Araraquara, São Paulo, Brazil
2 Department of Orthodontics, Federal Universityof Santa Maria, UFSM, Rua Marechal Floriano Peixoto, Santa Maria, Rio Grande do Sul, Brazil
3 Department of Restorative Dentistry, Universityof São Paulo State UNESP, Rua Humáita, Araraquara; Department of Restorative Dentistry, Camilo Castelo Branco University UNICASTELO, Av. Hilário da Silva Passos, Descalvado, São Paulo, Brazil
4 Department of Post Graduate Program in Dentistry, CEUMA University, Sao Luis Maranhão, Brazil
5 Department of Post Graduate Program in Integrated Dental Sciences, University of Cuiabá, MatoGrosso, Cuiabá, Brazil
6 Department of Odontopediatrics, Universityof São Paulo State UNESP, Rua Humáita, Araraquara, São Paulo, Brazil

Date of Web Publication8-Dec-2014

Correspondence Address:
Mateus Rodrigues Tonetto
Department of Post Graduate Program in Integrated Dental Sciences, University of Cuiabá, MatoGrosso, Cuiabá
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2348-2915.146488

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Introduction: The objective of this study was to assess whether the positioning of the impression tray could cause distortion to plaster casts during gypsum setting time.Materials and Methods: Fifteen pairs of master models were cast with alginate impression material and immediately filled with gypsum. Impressions were allowed to set with the tray in the noninverted position (Group A) or in the inverted position (Group B). The plaster models were digitized using a laser scanner (3Shape R-700, 3Shape A/S, Copenhagen, Denmark). Measurements of tooth size and distance were obtained using O3d software (Widialabs, Brazil) measurement tools. Data were analyzed by paired t-test and linear regression with 5% significance.Results and Conclusion: Most of the measurements from both groups were similar, except forthe lower intermolar distance. It was not possible to corroborate the presence of distortions due to the position of the impression tray during gypsum setting time.

Keywords: Digital models, impression material, leakage of plaster

How to cite this article:
Porto BG, Pinto AS, Grehs RA, Porto TS, Silva MB, Tonetto MR, Bandeca MC, Dos Santos Pinto LM. Dimensional changes in plaster cast models due to the position of the impression tray during setting. J Dent Res Rev 2014;1:118-22

How to cite this URL:
Porto BG, Pinto AS, Grehs RA, Porto TS, Silva MB, Tonetto MR, Bandeca MC, Dos Santos Pinto LM. Dimensional changes in plaster cast models due to the position of the impression tray during setting. J Dent Res Rev [serial online] 2014 [cited 2022 Nov 27];1:118-22. Available from: https://www.jdrr.org/text.asp?2014/1/3/118/146488

  Introduction Top

Study models are key tools for analysis, diagnosis, treatment plan development as well as for evaluation and final documentation of patients treated with orthodontic appliances. [1],[2],[3],[4]

Despite the importance of this tool, the plaster casts obtained in dental practices are not made in compliance with strict criteria such as: The plaster/water proportions, vacuum spatulation, leakage under controlled vibration, [5],[6],[7]] and little is known about the stability properties of the casting material. [6],[7]

Dimensional stability was defined by Nicholls [8] (1977) as the ability of the material to retain its exact dimensions over time. Thus, loss of exactitude was considered distortion with the relative movement of a single point or group of points, away from the original position, making the deformation apparent.

Alginates and hydrocolloids in general tend to distort over time, losing or gaining water, and thereby contracting or expanding. [6],[7],[9] The best results however are obtained when thealginateimpressionsare filled with plaster after 10 min, to avoid distortionof the initial expansionandelastic deformation, and not later than 1h, to avoid distortions such ascontraction and expansiondue to the movement of water and syneresis. [3],[10]

The distortionof the moldduring thesetting period, depending on the positionof the tray, is barely reported in the literatureand isa long-standing area of doubt. It was suggested by Rudd et al., [5] (1969) that the molds should wait for theplaster to hardenwith thetraynot inverted, since the inversioncould influence the accuracyand precisionof the future plaster model.

The objective of this study was to evaluate alterations in the measurements of tooth size and intercanine and intermolar distancesin dental models depending on the position of the tray during the gypsum setting time.

  Materials and Methods Top

For the present study, we selected 30 pairs of plaster models from the initial orthodontic documentation of patients who attended the Postgraduate Orthodontics Clinic at the UNESP School of Dentistry, Araraquara/Sγo Paulo.

The original models were duplicated, in view of the risk of damage to the permanent record of the patient. The moldings were made using the Morellin 7 tray with alginate (Jeltrate, Dentsply, York, PA, United States), undertaken by the same professional. The plaster used was of the special rock type (Durone V, Dentsply, York, PA, United States), spatulated under vacuum (proportion powder/water: 19 ml/100 g) and immediately poured onto a vibrator, in order to decrease the presence of bubbles in the cast. Two moldings were made: In the first, the plaster was poured in and left to set [Group 1 - [Figure 1]]; in the second molding, once filled with plaster, the entire piece was placed on a work surface, with the tray and mold on top of the plaster [Group 2 - [Figure 2]].

The 26 reference points were identified on the models [Figure 3] and [Figure 4] with a number 3 pencil, to help in the positioning of the measuring instruments. The images of the plaster casts were obtained using the non destructive laser scanning technique, with the reading being carried out by means of noncontact laser surface sweep (3Shape R-700, 3Shape A/S, Copenhagen, Denmark), with the model with a precision of 0.005" and 400 points per inch. The analyses wereperformed by the sameexaminerusing the toolsof the O3d software application (Widialabs, Brazil) [Figure 5] and yielded the following measurements:
Figure 1: Upright tray and mold filled with plaster awaiting setting (Group 1)

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Figure 2: Tray and mold filled with plaster and inverted on the base of the work surface awaiting setting (Group 2)

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Figure 3: Points marked out on the upper model. Red points = points of buccalcusp, black points = mesiodistal points

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Figure 4: Points marked out on the lower model. Red points = points of buccal cusp, black points = mesiodistal points

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Figure 5: Example of the measurement of the upper intercanine distance

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  • Tooth size : Mesiodistal diameter of the first molars, premolars, canines, and incisors of both arches;
  • Intercanine distance : Distance between the tipsof the cuspsof the permanent canines;
  • Intermolar distance : Distance betweenthe tips of themesiobuccalcuspsof the firstpermanent molars.

The evaluation of possible changes in tooth size and the distances obtained by the measuring instrument was performed usingthe paired t-test and regression analysis, both at a significance level of 5%.

  Results Top

For the calibration of themeasurement method, 10 pairs of models, randomly selected by casual sampling, were measured twice by the same examiner using digital images from the O3d software tools and at 1 week's interval. Reliability of the measurement process of the variables was evaluated by employing intraclass correlation coefficient (ICC) whichwas 0.99.

In [Table 1] we see that the difference between the values obtained in the two groups evaluated was small, ranging from 0.01 to 0.21 mm, most of the measurements being statistically similar exceptfor thel ower intermolar distance which presented statistically highervalues in models where the plaster remained on top of the mold [Group 1].
Table 1: Mean and standard deviation of the variables of Groups 1 and 2; and of the difference between groups and Student's t-test

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The mean differences in intercanine and inter molar distances, as well as in tooth sizeare shown in [Figure 6] and [Figure 7].

In order to show the similarity among the results obtained by both groups, regression analysis was performed for the measurement of distances [Figure 8] and of tooth size[Figure 9].
Figure 6: Mean of the differences in intercanine and intermolar distances

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Figure 7: Mean of the differences in mesiodistal tooth size

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Figure 8: Linear regression for the mesiodistal diameter of teeth of Group 1(a) andGroup 2(b)

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Figure 9: Linear regression for the intercanine and intermolar distances in Group 1 (a) and Group 2 (b) obtained from the digital image

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  Discussion Top

The influence of waiting time for  filling the mold with plaster on the dimensional stability and deformation of models has been the subject of many investigations. [1],[2],[4],[11],[12] However, little is known about the influence of positioning of the tray during the setting time of gypsumand obtaining the model, a fact that motivated this study.

The instrument selected for the measurements of tooth size and distance was the O3d systemdue to the excellent results obtained by the authors inunpublishedstudies, in which the measurements performed by these instruments have proved accurate and reproducible when compared to the caliper and MicroScribe.

The differences between the measurements obtained in this study were small, ranging from 0.01 to 0.21 mm [Table 1] and were statistically similar between the two groups except for the lower intermolar distance which was statistically higher for Group 1. Which may beattributed to a probable error in the measurement method (O3d) due to the difficulty in visualizing the lower molarcuspindigital imaging models since the buccal and occlusal wallsappeared to be onthe same plane. Tarawneh et al., [12] (2008) found statistically significant differences when evaluating the reproduction of the occlusal surface of molarsand premolarsin theirdigital image susingalginate and elastomer, attributing this differenceto the anatomy of these teeth, to the qualityof the moldsand to the typeof impression material used.

The similarity between the measurements obtained in both groups can be seen by comparing the mean difference for the distances and tooth size [Figure 6] and [Figure 7] and from the high coefficient of correlation obtained [Figure 8] and [Figure 9]. Brosky et al., [11] (2003) observed noinfluence of the inversionof the trayon the accuracyand reproductionof plaster casts. These authors, however, used elastomeras casting material, which is considered to be the material that best reproduces dental anatomyand suffers the least dimensional changes. [7],[12]

The type ofalginateand the storage period of alginate impressions can influence the quality andaccuracy of measurements on dental models. [1],[2],[13] Linear measurements showed changes on the fourth day of storage, butwere not clinically significant. [1],[2] The type of alginate and storage time were not interfering factorsin this study because all molds were obtained with alginate in the same manufacturing batch and castimmediately after collection.

Other factors thathave been carefully observed and implemented in this studywere: The proportions of gypsum and water recommended by the manufacturer (19ml/100g) and mixing carried out under vacuum, to decrease the like lihood of bubbles in the plaster. Several authors recommend these factors and consider the mimportant for obtaining a good plaster cast that is accurate and reproductive. [4],[5],[6],[7],[11],[13],[14],[15]

Although there are differencesin the measurements analyzed, in this study, they are lesser than 0.21 mm, thus considered to be clinically acceptable by Hirogaki et al., [16] (2001) who adopt 0.30 mm as the limit of clinically in significant difference. Thus, the inversion of the mold to obtain a cast with apredefined base, a common practice among orthodontists, can be performedwithout the occurrence ofchangesto the casts.

  Conclusion Top

Based on the method used, the position of the tray does not affect the dimensional stability of theplaster model.

  References Top

Alcan T, Ceylanoglu C, Baysal B. The relationship between digital model accuracy and time-dependent deformation of alginate impressions. Angle Orthod 2009;79:30-6.  Back to cited text no. 1
Dalstra M, Melsen B. From alginate impressions to digital virtual models: Accuracy and reproducibility. J Orthod 2009;36:36-41.  Back to cited text no. 2
Skinner EW, Pomes CE. Dimensional stability of alginate impression materials 1946;33:1253.  Back to cited text no. 3
Torassain G, Kau CH, English JD, Powers J, Bussa HI, Salas-Lopez AM, et al. Digital models vs. plaster models using alginate and alginate substitute materials. Angle Orthod 2010;80:474-81.  Back to cited text no. 4
Rudd KD, Morrow RM, Bange AA. Accurate casts. J Prosthet Dent 1969;21:545-54.  Back to cited text no. 5
Nassar U, Aziz T, Flores-Mir C. Dimensional stability of irreversible hydrocolloid impression materials as a function of pouring time: A systematic review. J Prost Dent 2011;106:126-33.  Back to cited text no. 6
Nassar U, Hussein B, Oko A, Carey JP, Flores-Mir C. Dimensional accuracy of 2 irreversible hydrocolloid alternative impression materials with immediate and delayed pouring. J Can Dent Assoc 2012;78:c2.  Back to cited text no. 7
Nicholls J. The measurement of distortion: Theoretical considerations. J Prosthet Dent 1977;37:578-86.  Back to cited text no. 8
Coleman RM, Hembree JH, Weber FN. Effects storage time on the accuracy of casts made from different irreversible hydrocolloid impression material. Am J Orthod 1979;438-46.  Back to cited text no. 9
Anseth KS, Bowman CN, Bannon-Peppas L. Mechanical properties of hydrogels and their experimental determination. Biomaterials 1996;17:1647-57.  Back to cited text no. 10
Brosky ME, Major RJ, DeLong R, Hodges JS. Evaluation of dental arch reproduction using three-dimensional optical digitalization. J Prosthet Dent 2003;90:434-40.  Back to cited text no. 11
Tarawneh FM, Panos PG, Athanasiou AE. Three-dimensional assessment of dental casts' occlusal surfaces using two impression materials. J Oral Rehabil 2008;35:821-26.  Back to cited text no. 12
Seeda M, Casarotto A, Raustia A, Borrachini A. Effects of storage time on the accuracy of casts made from different irreversible hydrocolloids. J Contemp Dent Pract. 2008;9:1-12.  Back to cited text no. 13
Chen SY, Liang WM, Chen FN. Factors affecting accuracy of elastrometric impressions materials. J Dent 2004;32:603-9.  Back to cited text no. 14
Shafa S, Zaree Z, Mosharraf R. The effects of custom tray material on the accuracy of master casts. J Contemp Dent Pract 2008;9:49-56.  Back to cited text no. 15
Hirogaki Y, Sohmura T, Satoh H, Takahashi J, Takada K. Complete 3-D reconstruction of dental cast shape using perceptual grouping. IEEE Trans Med Imaging 2001;20:1093-101.  Back to cited text no. 16


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]

  [Table 1]


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