|Year : 2018 | Volume
| Issue : 6 | Page : 272-277
Dimensional accuracy of double poured casts obtained from extended pour alginate impressions: An in vitro study
Suchismita Choudhary1, Indumathi Sivakumar1, Muaiyed Mahmoud Buzayan2, Prashant Choudhary1
1 Faculty of Dentistry, SEGi University, Selangor, Malaysia
2 Department of Restorative Dentistry, Faculty of Dentistry, University of Malaya, Malaysia
|Date of Web Publication||24-Dec-2018|
Dr. Suchismita Choudhary
Faculty of Dentistry, SEGi University, Kota Damansara, Petaling Jaya, Selangor
Source of Support: None, Conflict of Interest: None
Background: A new generation of smart alginates have been developed claiming to have good dimensional stability and deformation resistance. However, there is a lack of sufficient scientific evidence in this regard. Aim: This study aims to evaluate the dimensional stability of casts generated from first and second pour of two different types alginate purported to allow multiple pour and remain dimensionally stable after storage. Methodology: Forty impressions were made of a maxillary dentulous model (standard die), using two types of alginate (Hydrogum 5.0, Cavex Colour change). Two sets of casts were generated from 10 impressions of each type of alginate immediately. Ten impressions of each type of alginate were stored wrapped in damp paper towel and stored in zip-locked bags, and two sets of cast were generated after 24 h. Casts were scanned with a three dimensional model scanner and compared with standard die. Results: No significant difference was found between standard die and second poured casts of both alginates after immediate pouring. A significant difference was found between standard die and second poured casts of both alginates after 24 h. However, percentage dimensional change was within clinically acceptable limits. Conclusion: Clinically acceptable casts can be generated by the double pouring of new generation alginates. However, for better results casts should be poured immediately.
Keywords: Deformation resistance, dimensional stability, extended pour alginate
|How to cite this article:|
Choudhary S, Sivakumar I, Buzayan MM, Choudhary P. Dimensional accuracy of double poured casts obtained from extended pour alginate impressions: An in vitro study. J Int Oral Health 2018;10:272-7
|How to cite this URL:|
Choudhary S, Sivakumar I, Buzayan MM, Choudhary P. Dimensional accuracy of double poured casts obtained from extended pour alginate impressions: An in vitro study. J Int Oral Health [serial online] 2018 [cited 2019 Jun 24];10:272-7. Available from: http://www.jioh.org/text.asp?2018/10/6/272/248433
| Introduction|| |
Alginate is one of the most frequently used dental materials since it is cost-effective. It forms a major bulk of our clinical practice even today. Alginate impression material consists of a powder that when mixed with water forms a fast-setting gel. A set alginate impression contains approximately 70% of water. Due to its high water content, moisture will be lost from the alginate causing undesirable macroscopic shrinkage and distortion of the impression if it is left exposed to air at room temperature. Despite the advantages of conventional alginate being cost-efficient and easy to manipulate, it has its own limitations namely its low dimensional stability and low tear strength. Hence, alginate impressions are good for only one pour per impression., Researchers, in the past, have recommended immediate pouring of a gypsum product into the impression because there was no adequate storage method for any hydrocolloid impression material. According to Morrow et al., the most common error made in using alginate impression materials was not pouring the gypsum product into the impression immediately. Cohen et al. measured the dimensional accuracy of three different alginate impression materials under different storage conditions and found that the immediate pouring method produced more accurate casts.,
However, due to practical circumstances alginate impressions may be subjected to a certain waiting period before being poured. As for instance, impressions require transportation to distant laboratories for pouring; limiting its potential for maximum dimensional stability. In addition, esthetic and diagnostic treatments may often require duplicate casts of one arch, one for preservation for diagnostic purpose and one for alterations (diagnostic wax-up, bleaching tray, and occlusal analysis)., Hence, the dimensional accuracy and stability of impressions when the pouring needs to be delayed or repeated later is of interest to the clinician and the laboratory technician. It not only saves clinical time but also reduces patient inconvenience and extra material cost.
Taking these factors into consideration, a new generation of smart alginates has been developed by the manufacturers. These are known as alginates with “virtual intelligence.” Manufacturers claim that they have 5–9 days of dimensional stability and good tear and deformation resistance. Imbery et al. compared a traditional alginate and with these new generation alginates and concluded that the newer generation alginates produced accurate casts at 5 days (120 h) when stored properly. Nehring and Imbrey in another similar study quoted that casts obtained after double pouring of a new generation extended pour alginate were accurate., Another study done by Haywood and Powe suggested that when alginate impressions are kept moist by completely wrapping in a damp paper towel during stone setting and poured within 45 min, two diagnostic casts could be generated from one impression with the same degree of accuracy.
Despite the manufacturers' claims of increased dimensional stability and improved strength properties, the studies on the extended-pour alginates have provided mixed results. There is also the absence of sufficient scientific publications or studies on the accuracy of the multiple poured casts obtained from these newer generation alginates. Considering the lack of evidence-based data to use these type of alginates, the present study was designed to evaluate the dimensional changes of an extended pour alginate after delayed and double pouring of the impression. The null hypothesis for the study was that dimensional accuracy would not differ significantly after double pouring of the two types of extended pour alginates regardless of the cast pouring time.
| Methodology|| |
To evaluate the effects of delayed and multiple pouring on the dimensional stability of extended pour alginate, an acrylic maxillary arch was duplicated from a Frasaco typodont dentulous model (AG-3 Standard Restorative Typodont) and was used as the standard die of our study. On the standard die, three metal posts were inserted accordingly onto the 1st molars (16, 26) and left central incisor (21). A surveyor was used to attach the metal posts into the respective teeth to ensure parallelism between each pole. These poles act as specific reference points for cast measurements of arch length and arch width. The arch length and arch width measured for the standard die was 40.41 mm and 47.13 mm, respectively.
Our present study utilized a modified articulator similar to the one used in a study conducted by Wandrekar et al. using a hinge type articulator whereby the maxillary acrylic standard die was mounted onto the upper rim of the articulator. The lower portion of the articulator consisted of an acrylic housing supported by metal pins and attached to the lower member of the articulator. This housing was used to position the metal stock tray during the impression procedures. This modified articulator ensured accurate positioning of the tray and uniform pressure during impression making [Figure 1] and [Figure 2].
Two extended pour alginates were used for the study. These were Hydrogum 5 (Zhermack, LOT no.-216271) and Cavex Colour change (Cavex, LOT no. 161013). The alginates were manipulated following manufacturer's instructions. A volume of 45 ml of distilled water was added to 42 g of alginate powder; the alginate mixture was agitated for 2 s and was placed in an alginate mixer (Blendex Fully automatic alginate mixer) for 12 s. All impressions were mixed for 12 s consistently. The alginate mix was placed in the metal stock trays which was then placed on the acrylic housing of the modified articulator for the impression procedure [Figure 3]. The maxillary standard die was wetted with water before each impression to simulate the moist intra-oral environment. A total of 20 impressions were be made for each type of alginate. 10 impressions were poured immediately. Casts were poured in vacuum mixed Microstone (140 grams/40 ml water) immediately after removal. Casts were separated from trays after 45 min and a second cast was immediately generated. Ten impressions each of both the type of alginate were not poured immediately and were wrapped in damp paper towel and stored in zip-locked bags at room temperature for 24 h [Figure 4]. Two sets of casts were generated from the impression at the interval of 45 min [Figure 5].
The standard die and the casts were scanned with three-dimensional (3D) model scanner (Ceramill map300). The measurements were done using 3D computer-aided design Tool viewer. All the measurements obtained were compared to that of the standard die and statistical analysis (ANOVA and posthoc Turkey's test) was performed [Figure 6] and [Figure 7].
| Results|| |
After cast pouring the recorded measurements for both types of alginates showed dimensional changes when compared to the standard die measurements.
ANOVA (One-way ANOVA) followed by a post hoc Tukey's test was performed on the obtained data. P < 0.05 was considered statistically significant.
[Table 1] and [Graph 1] show the mean arch length measurements, P values and percentage dimensional change of both types of alginate after first and second time pouring of the impressions immediately.
|Table 1: Arch length comparison of immediate poured casts with the standard die|
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[Table 2] and [Graph 1] show the mean arch length measurements, P values and percentage dimensional change of both types of alginate after first and second time pouring of the impressions after storage for 24 h.
|Table 2: Arch length Comparison of delayed poured casts with the Standard die|
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[Table 3] and [Graph 2] show the mean arch width measurements, P values and percentage dimensional change of both types of alginate after first and second pouring of the impressions immediately.
|Table 3: Arch width Comparison of immediate poured casts with the Standard die|
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[Table 4] and [Graph 2] show the mean arch width measurements, P values and percentage dimensional change of both types of alginate after first and second pouring of the impressions after a delay of 24 h [Table 4] and Graph 2].
|Table 4: Arch width comparison of delayed poured casts with the standard die|
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No significant difference was found in arch length and width between the standard die (40.41, 47.13 mm), first and second poured casts of both Hydrogum 5 and cavex color change after immediate pouring [Table 1], [Table 2], [Table 3], [Table 4]. The immediate second poured casts had measurements closer to the standard die. However, there was statistically significant difference between the standard die measurements and the cast obtained from first and second pour of both types of alginate after a storage period of 24 h.
| Discussion|| |
Over the years, dental materials have been developed to improve their dimensional stability, reproducibility, and handling properties. The accuracy and functional efficiency of prosthesis depends on an accurate impression which is reproduced on a gypsum cast. The accuracy of impression with repeat pours is of paramount importance as duplicate casts are usually required for various laboratory procedures. These duplicate casts can be used as working or refractory casts so that the master cast remains unaltered. Keeping this in mind, a new generation of alginates known as extended pour alginate has been introduced by manufacturers claiming to exhibit better dimensional stability and tear resistance compared to conventional alginates. A study conducted by Walker et al. showed extended pour alginates (Kromopan) were more accurate after 100 h of storage. The study explained that extended pour alginates could have a different chemical constituents which compensates for the initial shrinkage of the material, henceforth rendering extended-pour alginates more dimensionally stable over a period. Fellow and Thomas in there study suggested that extended-pour alginates exhibit better dimensional stability due to the different chemical composition compared to conventional alginates. Extended-pour alginates have a higher filler: Alginate ratio and Ca: Na ratio. There is a decreased level of soluble alginate which leads to a lower alteration in stability as a lower weight percentage of gel is invariably subject to fewer changes in dimension.
In the present study, the dimensional stability of casts generated from double pouring of alginate impressions was measured and compared with the standard die. Forty-five minutes was chosen as the maximum time to ensure complete setting of the stone to simulate the realistic conditions of dental office. It was possible to avoid tearing the alginate impression material while removing the stone cast by ensuring that the while pouring the cast the stone does not to extend beyond the depth of the vestibule.
Impressions imbibed water resulting in smaller first poured-casts. During gypsum setting, impressions lost water resulting in second-poured casts larger than first-poured casts. Hence the dimensions obtained for the second poured casts were closer to the standard die and more accurate. This can attributed to the centrifugal tensile forces on the impression material as described by Imbery et al. Impression material tightly adhered to the stock tray at the periphery subjects the material to centrifugal tensile forces. Due to shrinkage of the material by syneresis and the centrifugal tensile forces, the material will shrink outwards resulting in an increase in cast diameter.,
No significant differences were noted between the first and the second poured casts and the measurements of the standard die for both types of alginate impressions after immediate pouring. Clinically, also it was difficult to distinguish the two casts.
However, a significant difference was found between the first and second poured casts after 24 h of storage for both types of impressions. There could have been some dimensional changes of the impression material in our study which could have been due to evaporation and syneresis due to loss of water from the damp towel and the impression, which resulted in an arch width increase and difference in measurements compared to the standard die.,
ADA specification number 18 does not specify a specific allowable threshold value for the dimensional change of alginate impression materials. However, according to Alcan et al., the author has considered the percentage of dimensional change ranging from 0.48% to 0.90% to be clinically acceptable. This was because as the changes within this range was very small in terms of millimeters and had no adverse effects on digital modeling. Imbery et al. had chosen a 0.50% difference to be their maximum allowable dimension change for their study. By utilizing the similar threshold change as chosen by Alcan et al., the values of dimensional change in our study fell within this range hence rendering our impressions clinically acceptable for diagnostic purpose despite the statistical differences after 24 h of storage [Table 5].
Therefore, cast obtained by pouring the impression twice can be used for diagnostic purpose, fabrication of bleaching trays, mouthguards or for diagnostic wax-up. It has been suggested that by metal grinding, an incorrectness of up to 150 μm at the metal framework try-in stage could be easily adjusted, so this discrepancy might be considered acceptable. However, further investigations will be required to evaluate whether these casts can be used as master models for fabrication of clinically acceptable prosthesis.
The limitations of this study include that the acrylic master model used to make the impressions were prepared with the least number of undercuts to prevent the distortion of the material during removal. In clinical situations, removal of impressions from the patient's oral cavity could cause distortion of the material on removal. Furthermore, in vivo conditions such as the effects of blood, saliva, and oral cavity temperature were not considered. In addition, impressions in our study were not subjected to the use of disinfectants such as that in a clinical setting.
| Conclusion|| |
Dimensional accuracy and stability of impressions when the pouring needs to be delayed or repeated later is utmost importance to both the clinician and the laboratory technician. It not only saves clinical time but also reduces patient inconvenience and extra material cost. From the present study, we can conclude that two diagnostic casts can be generated from one impression with the same degree of accuracy as two casts made from taking two separate impressions, providing the alginate does not tear on removal. However, best results are obtained when the impressions are poured immediately.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]