Extended-pour and conventional alginates: effect of storage time on dimensional accuracy and maintenance of details

ABSTRACT Objective: This study aimed to evaluate the dimensional stability and maintenance of details of conventional and high stability alginates up to 5-day storage. Methods: Two types of alginates were selected (n=10) for this study, conventional (Hydrogum) and high stability alginates (Hydrogum 5), which were produced with the aid of a cylindrical metal block and a ring-shaped metal mold (Specifications 18, 19, and 25, ANSI/ADA). Ten images were obtained from the molds for the dimensional stability test, which were taken immediately after their production and at each different storage periods (15 min, 24 h, 48 h, 72 h, 96 h, and 120 h) by a digital camera. The specimens were kept hermetically sealed in plastic bags (23°C) and then used to obtain 140 (n=70) dental stone models, used in the detail reproduction test, in which the angular accuracy of three grooves (20 µm, 50 µm, and 75 µm) was observed at each period. The details reproduction accuracy was classified using a predetermined score classification. Measurements of dimensional changes were made in the Corel DRAW X6 program. The data were submitted to the Student’s t-test (α?#8197;= 0.05). Results: A statistically significant difference concerning the size of the matrix was observed after 24h for both alginates, and a statistically significant negative linear dimensional change (contraction) was verified after 24 h of storage (1.52% for the high stability alginate, and 1.32% for the conventional alginate). The high stability alginate kept the full details for 72 hours, while the conventional alginate, for 24 h. Both alginates reproduced the 75 µm groove at all storage periods. Conclusion: Impressions made with both alginates presented satisfactory clinical results when the alginates were immediately poured.


INTRODUCTION
Optimal dental impression material should have dimensional stable performance over time and allow the pouring of the dental stone according to the convenience of the operator. 1 Hydrocolloids and synthetic elastomeric polymers are the most frequently used materials to obtain dental impressions. When higher precision is required, some nonaqueous elastomeric impression materials can be used, like polysulfides, polyethers, and condensation or addition silicones. 2 However, alginate is one of the most used impression materials, especially due to its satisfactory technical properties and low cost.
Their details reproduction allows creating stone cast models with good details of accuracy when correctly used. However, it is a very sensitive material, requiring a strict protocol of use. Certainly, one of the main causes of clinical failure when using alginate is its tendency to experience syneresis and imbibition, often as a result of mold storage issues, long time between molding and casting, and insufficient disinfection procedures. [3][4][5][6] Alginate has poor detail reproduction and dimensional instability when stored for long periods. 5,6 Therefore, its immediate pouring or right after disinfection is recommended. New products have been launched in the dental market to overcome some of these disadvantages inherent to the material composition, claiming to be dimensionally stable for up to 5 days (extended-pour alginates), being known as new generation of alginates due to their high stability. 3,7-10 Bitencourt SB, Catanoze IA, Silva EVF, Turcio KHL, Santos DM, Brandini DA, Goiato MC, Guiotti AM Extended-pour and conventional alginates: effect of storage time on dimensional accuracy and maintenance of details 5 Dimensional stability is one of the most important properties of impression materials to obtain accurate dental stone models. 11 The main limitation of alginate is its dimensional change after being removed from the mouth and stored for periods over 15 min. 12 Thus, the analysis of dimensional stability and maintenance of details of these new high stability products is required, since the immediate pouring of the impressions may not always be possible, especially if the impression needs to be sent to a dental laboratory. 7 Although this material is generally used to obtain diagnostic casts, it is frequently used during the manufacturing of the partially removable dental prostheses. 6,11 Therefore, the knowledge about the dimensional behavior of impression materials is of utmost importance, and this study aimed to provide new information to the clinician about the different products available in the market, providing scientific support to the selection of the best materials.
This study aimed to evaluate the influence of 5-days storage on the dimensional stability and maintenance of details of molds obtained from two types of alginate (conventional and high stability). Two null hypotheses were tested, in which there was no statistically significant difference between both alginates, regarding their dimensional stability; and that the dimensional stability and maintenance of details would not be affected by the storage time.

MATERIAL AND METHODS
Two types of alginates were used to make the test specimens, the conventional and high stability alginates (Hydrogum ® and Hydrogum 5 ® , Zhermack Spa, Badia Polesine, Italy). The test specimens were prepared according to ANSI/ADA specifica-tions#18, 19, and 25, 13-15 using a cylindrical metal block and a ring-shaped metal mold ( Fig. 1) in order to test their dimensional stability and maintenance of details. The ring-shaped metal mold was adapted to the upper part of the cylindrical metal block, to leave a space where the material was placed.
The alginates were manipulated according to the instructions of the manufacturer at 23 ± 2°C and at relative humidity of 50 ± 10%. After manipulation, the material was placed in the mold using a spatula. After insertion, a polyethylene strip was laid, followed by a glass plate. A 1,500 g load was placed on the glass plate to extrude the excess of impression material, and kept under pressure. Subsequently, the material was immersed in distilled water at 35 ± 1°C and kept in an incubator Bitencourt SB, Catanoze IA, Silva EVF, Turcio KHL, Santos DM, Brandini DA, Goiato MC, Guiotti AM Extended-pour and conventional alginates: effect of storage time on dimensional accuracy and maintenance of details for three min longer than the minimum time recommended by the manufacturer. After this time, the set was removed from the incubator, the mold separated from the metal matrix, and the specimens were carefully removed to avoid distortions.   The maintenance of details of the dental stone models was tested in a stereomicroscope (Olympus) under a low-angle illumination at 13× magnification. In the detail reproduction test, the angular accuracy of three grooves (x =50 ± 5 µm; y =20 ± 5 µm; z =75 ± 5 µm) molded in each sample was recorded,

LINEAR DIMENSIONAL CHANGE RESULTS
The frequency distribution of each measurement was analyzed for the assumption of normality, which was confirmed for the quantitative data.     Tables 2 and 3, and Fig. 6 (A and B) show the results of the maintenance of details test for the two alginates, depending on the storage time, according to the classification of Goiato et al. 16 In this scale, score 0 is assigned to full reproduction of two of the three grooves; score 1, to full reproduction of the three grooves, without accurate angles; and score 2, to full reproduction of the three grooves, with accurate angles.

DISCUSSION
Alginates are used currently, day after day at dental offices, especially due to their several advantages. They present low cost, well acceptability by the patient, easy manipulation and execution technique, as well the possibility of achieving a detailed impression all in a single step, being outstanding for primary prosthetic, orthodontic and design imprints. However, alginates present some disadvantages like lower accurate reproduction than elastomeric impression materials, and poor dimensional stability for complex cases. 17 In this study, the linear dimensional change (% mean) for high stability alginate and conventional alginate at 24 h of storage was of -1.52% and -1.32%, respectively ( Table 1). The values were statistically different from the immediate period after mold production and at 15 min of storage. These data revealed that the alginate molds, once removed from the mouth, experience some contraction associated with syneresis and evaporation. 11 According to the manufacturer, if the gypsum can not be immediately poured, the impression should be stored in a bag sealed at room temperature (23°C), and delayed for up In the methodology used in this study, the molds were stored in sealed bags without relative humidity, following the recommendations of the manufacturer. According to literature guidelines, 11 when they need to be stored, this must be done in containers with a relative humidity of 100%. This factor might explain the unsatisfactory or less than expected results obtained.
The ADA specification #18 13 does not stipulate a maximum clinically acceptable value for the dimensional changes of alginates. 3,9,18,19 However, for the elastomeric materials, these values are well defined in the literature and may not exceed 1%, according to the ANSI/ADA specification #19. 14 The polysulfides present a shrinkage percentage in 24 h that vary from 0.4% to 0.45%; condensation silicones, from 0.38% to 0.6%; addition silicones, from 0.14% to 0.17%; and polyethers, from 0.19% to 0.24%. 11 There are controversies regarding the dimensional change values considered clinically acceptable for alginates, ranging from 0.1% to 0.8%.6 According to Imbery et al. 3 and

CONCLUSION
The findings of this study showed that impressions made with these materials should be immediately poured to have satisfactory clinical results, since dimensional alterations were found after 24 hours, even on high stability alginates. Overall responsibility: AMG.