ARTÍCULOS ORIGINALES
Linear dimensional stability of irreversible hydrocolloid materials over time
Analía B. Garrofé, Beatriz A. Ferrari, Mariana Picca, Andrea E. Kaplan
Department of Dental Materials, School of Dentistry, University of Buenos Aires, Argentina.
CORRESPONDENCE Dr. Analia Garrofe Catedra de Materiales Dentales, Facultad de Odontologia, UBA. M.T. de Alvear 2142 (1122) CABA, Argentina agarrofe@odon.uba.ar
ABSTRACT
The aim of this study was to evaluate the linear dimensional stability of different irreversible hydrocolloid materials over time. A metal mold was designed with custom trays made of thermoplastic sheets (Sabilex, sheets 0.125 mm thick). Perforations were made in order to improve retention of the material. Five impressions were taken with each of the following: Kromopan 100 (LASCOD) [AlKr], which has dimensional stability of 100 hours, and Phase Plus (ZHERMACK) [AlPh], which has dimensional stability of 48 hours. Standardized digital photographs were taken at different time intervals (0, 15, 30, 45, 60, 120 minutes; 12, 24 and 96 hours), using an “ad-hoc” device. The images were analyzed with software (UTHSCSA Image Tool) by measuring the distance between intersection of the lines previously made at the top of the mold. The results were analyzed by ANOVA for repeated measures. Initial and final values were (mean and standard deviation): AlKr: 16.44 (0.22) and 16.34 (0.11), AlPh: 16.40 (0.06) and 16.18 (0.06). Statistical evaluation showed significant effect of material and time factors. Under the conditions in this study, time significantly affects the linear dimensional stability of irreversible hydrocolloid materials.
Key words: Dimensional measurement accuracy; Alginates.
RESUMEN
Determinación de la estabilidad dimensional lineal de hidrocoloides irreversibles
El objetivo de este estudio fue evaluar la estabilidad dimen - sional lineal de diferentes hidrocoloides irreversibles en funcion del tiempo. Se confecciono una matriz metalica con sus correspondientes cubetas individuales realizadas con laminas termoplasticas (Sabilex, de 0.125 mm de espesor). Se le realizaron perforaciones para la retencion del material. Se tomaron cinco impresiones con cada material a esta matriz, utilizando Kromopan 100 (LASCOD) [AlKr], que presenta una estabilidad dimensional de 100 horas, y Phase Plus (ZHERMACK) [AlPh], que tiene una estabilidad dimensional de 48 horas. Luego se tomaron fotografias estandarizadas a diferentes intervalos de tiempo (0, 15, 30, 45, 60, 120 minutos; 12, 24 y 96 horas), usando un dispositivo ad-hoc. Las imagenes se analizaron con software de procesamiento de imagenes (UTHSCSA Image Tool) realizando la medicion de la distancia entre las intersecciones de surcos previamente realizados en la porcion superior de la matriz. Los resultados obtenidos fueron analizados mediante Analisis de Varianza para mediciones repetidas. Se detallan los datos iniciales y finales obtenidos (media y DS): AlKr: 16,44 (0,22) y 16,34 (0,11), AlPh: 16,40 (0,06) y 16,18 (0,06). El analisis estadistico ha mostrado el efecto significativo para las variables material y tiempo. Bajo las condiciones de este estudio podemos concluir que tiempo afectaria significativamente la estabilidad dimensional lineal de los hidrocoloides irreversibles.
Palabras clave: Exactitud de medida de dimensiones; Alginatos.
INTRODUCTION
Impressions of oral structures to be used for making
casts are needed for various purposes, including
diagnoses, rigid restorations and dental prostheses.
They should be made from a material which is easy
to manipulate and provides accurate replication,
adequate dimensional stability and biological
compatibility.
The choice of material for an impression is one of
the most important steps in the process. Among the
various materials available, alginate is undoubtedly
one of the most frequently used, thanks to its
elasticity, simple manipulation technique and low
cost, among other properties.
Studies on the dimensional stability of alginate1-5,
have given special attention to syneresis and
imbibition3,6,7. Syneresis is the loss of water by
evaporation from the surface or exudation, which
makes the gel to contract. Imbibition, in contrast, is
the absorption of water during immersion. Both
these processes alter the original dimensions of the
gel and thereby its dimensional stability6,7. It is
important to take syneresis and imbibition into
account because any dimensional change that occurs after removing the impression from the
mouth will produce inaccurate pours and casts.
Manufacturers add fillers and small amounts of
certain components in order to control consistency,
setting time, elasticity, resistance and dimensional
stability. Some alginates which have recently
appeared on the market promise dimensional
accuracy and stability for up to 5 days.
The literature describes different storage methods for
alginate impressions during the time until the cast is
made, including waiting times and standardized
temperature and moisture conditions2-5.
The aim of this study was to determine and compare
linear dimensional stability of two irreversible
hydrocolloids which are sold as materials with
improved dimensional stability over time. We did not
standardize the temperature and storage conditions
of the impression until the cast was made, in order to
replicate the conditions to which the materials are
normally exposed during everyday use.
MATERIALS AND METHODS
Individual trays were made from 0.125 mm thick thermoplastic sheets (Sabilex) on a metal mold, following a standardized insertion path when the impression was taken (Figs.1 and 2). Perforations were made to improve retention of the material. The impressions were made with two irreversible hydrocolloid commercial brands: Kromopan 100 (Lascod, batch 161.301.119.201, expiry date 03/2014) [AlKr], for which the instructions specify that the maximum time to making the cast is 100 hours; and Phase Plus, (Zhermack batch 93131, expiry date 09/2014) [AlPh], for which the instructions specify that maximum time is 48 hours. We defined n=5 for both groups. In order to ensure precise quantities, the powders were weighed on a digital scale (Ohaus Analytical Standard) with a precision of 4 decimals, and the water was measured with a measuring cylinder. Both materials were prepared according to the manufacturers’ instructions for mixing, work ing and gelling times. The impressions were washed for 30 seconds and stored in a hermetically sealed rigid container with cotton soaked in water apart from the samples to prevent any direct contact with the material.
Fig. 1: Individual tray made of thermoplastic sheet.
Fig. 2: Lateral view of the metal mold used as a model for taking impressions.
A piece of metal 5 mm long was placed in each impression to calibrate the software used to measure the distances between marks. Photographs were taken of the impressions with a SonyR digital camera (model DSC-W1), using an ad-hoc device to standardize the impression-camera position (Figs. 3 y 4). Photographs were taken at the following time intervals: 0, 15, 30, 45, 60, 120 minutes; 12, 24 and 96 hours. For each picture, the impression was removed from the container, placed in the device and photographed within 45 seconds. The pictures were analyzed with image analysis software (UTHSCSA ImageTool Version 3.0) to measure the distance between the intersections of the grooves formed by the upper part of the mold (Fig.5). Data were analyzed statistically by ANOVA (Analysis of Variance for repeated measures for the time factor) using SPSS software version 9.0. Significance was established as α = 5%.
Fig. 3: Ad-hoc device for taking photographs.
Fig. 4: Lateral view of the impression positioner in the ad-hoc device.
Fig. 5: Measuring with Image Tool image processing software.
RESULTS
Table 1 shows arithmetic means, standard deviations and percentage of dimensional variation for the different times compared to the first measurement taken (0 minutes). For Phase Plus, change over time was more constant (Fig. 6). Its greatest stability was observed at 15 minutes from time 0 and lowest at 96 hours. At 24 hours there was already a difference of 0.91% compared to the original measurement. However, the difference was never statistically significant (p>0.05). For Kromopan 100, variation was not constant. It was greatest at 24 hours (0.04%) and lowest at 12 hours. At 96 hours dimensional change was 0.61% with respect to the original measurement. The difference was not statistically significant (p>0.05).
Table 1: Variation in size (mean and standard deviation) expressed as difference between initial and final value at each time evaluated.
Fig. 6: Dimensional changes (in millimeters compared to
baseline) recorded for each impression material (Kromopan
and Phase Plus).
Table 2 provides the result of the statistical analysis, showing that there was a significant effect for the time factor (p=0.026), and no significant effect for the brand variable (p=0.403) or for interaction between variables (p=0.264).
Table 2: Analysis of variance. Test of Within-Subjects Contrasts.
DISCUSSION
The literature we reviewed revealed two ways of
evaluating dimensional stability of impression
materials: on the impression material itself2,3 or on
the models made from an impression1,4,5,8. Our study
recorded dimensional stability directly from the
impression at different times: 0, 15, 30, 45, 60, 120
minutes; 12, 24 and 96 hours, in order to avoid any
changes which might occur as a result of using other
materials.
The papers by Chen1, Bayindir2 and Lapria Faria8 all evaluated time intervals not longer than 24
hours, probably due to the possible dimensional
changes in viscoelastic materials over long periods
of time. Bayindir et al.2 evaluated changes in temperature and pH, and variations in dimensional
stability for 4 alginate brands by measuring directly
under a microscope for a maximum 2 hours at
10-minute intervals, concluding that the linear
dimensional changes in the four irreversible
hydrocolloids differed significantly for the variable
time and for the variable material.
In contrast, our study, in agreement with Walker3,
Imbery4, and Sedda5, evaluated dimensional
accuracy over longer periods of time. Walker et al.3 evaluated dimensional changes in three alginate
brands at 30 minutes, 48 and 100 hours, showing
that all materials contracted after being stored for 30
minutes, and that conventional alginate continued to
contract over a longer time, and was thus most
accurate at 30 minutes. In contrast, alginates with
longer pouring times, Alginmax and Kromopan 100,
underwent minimal dimensional changes at all
storage times. Nevertheless, Kromopan 100 had the
best accuracy at 100 hours. Imbery et al.4 studied
the effects of extended storage time on the
dimensional accuracy of irreversible hydrocolloids
(one conventional and another with extended
pouring time). Their results indicate that when the
manufacturer’s instructions are followed, either by
pouring immediately or at 5 days, both materials
produced plaster casts with no significant difference
compared to the master model. The material with
extended pouring time was less accurate when
poured immediately and at 24 hours, whereas
conventional alginate was less accurate when
poured at 3 and 4 days. They conclude that extended
pour alginate tends to produce smaller models
compared to the mold used; and that this type of
alginate did not change dimensionally by more than
0.5% at any of the times at which measurements
were taken. Sedda5 et al. studied the dimensional
stability of five alginate brands, over different times,
on models obtained from impressions stored at
constant temperature and moisture. Their results
showed that the dimensional stability of the alginate
impressions were material- and time-dependent.
Another variable considered in our study, which is
not often found in the literature, was exposing
impressions to different temperature ranges.
Although this variable was not fully controlled, all
impressions were taken and evaluated at the same
time, so they all underwent the same conditions.
These temperature changes also contribute to
dimensional change, and it should be considered,
for example, that a decrease in temperature produces
slight contraction6. Our study did not control for the
temperature at which the impressions were stored,
because we intended to replicate usual manipulation at a dental office or laboratory. It is important to
take this into account when attempting to replicate
environmental conditions to which they are usually
exposed while being moved to the laboratory, in
order to create protocols for alginate impression
manipulation and ensure that more accurate plaster
casts are obtained.
Nevertheless, the lack of standardization in the
techniques for evaluating hydrocolloid dimensional
stability and the variability in the results found in
this and other studies9 do not enable manipulation
protocols to be established. Thus, according to
the type of cast that needs to be made, it is important
to determine the waiting time between taking
the impression and making the cast. When a
cast is needed for diagnosis, small dimensional
changes may not have great impact on its clinical
application. However, when more accurate casts
– working casts – are needed, and the storage time
and/or transportation conditions and temperatures
that the impression will undergo are unknown, it is
advisable to pour it immediately.
CONCLUSION
Under the conditions in this study, we conclude that time may significantly affect the linear dimensional stability of alginates sold as materials with greater dimensional stability.
ACKNOWLEDGMENTS
This study was supported by University of Buenos Aires, UBACyT Program. ( number20020090100178).
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