Effect of bench-delay after flask cooling on the posterior teeth movement in maxillary complete dentures

A classic article shows that the subsequent expansion seen after acrylic resin water storage is commonly greater than the shrinkage occurring during the polymerization. Due to this unavoidable base polymerization shrinkage, a certain lack of dimensional accuracy has been accepted as one of the disadvantages for the construction of complete dentures, which is partially compensated by water imbibition.


INTRODUCTION
A classic article shows that the subsequent expansion seen after acrylic resin water storage is commonly greater than the shrinkage occurring during the polymerization 16 . Due to this unavoidable base polymerization shrinkage, a certain lack of dimensional accuracy has been accepted as one of the disadvantages for the construction of complete dentures, which is partially compensated by water imbibition 13 .
According to the diffusion theory for assessing the quantitative aspects of the kinetics of water sorption, the diffusion coefficient governs the rate of water sorption and the time required to reach equilibrium, which is also proportional to the specimen thickness 11 . The denture fits better after water sorption than immediately after processing, considering that the shape of the oral tissues is remained unchanched 4 . Thus, good wetting characteristics of the denture base are important since materials with different surface energies have varying wettabilities 22 .
The water sorption by dentures in clinical use occurs during the first three months, and the resultant expansion partly compensates the polymerization shrinkage 10 , where liquid equilibrium and consequent dimensional stability of the denture base is often attained 15 .
A study showed that the dimensional changes of light-cured, heat-cured, and self-cured denture base resins were different from each other after storage for seven days at 37ºC in distilled water 5 . However, the linear dimensional changes occurred were similar in all resin brands after storage in water for thirty, sixty and ninety days at 37ºC, and these discrepancies were so small that they were not clinically detectable 6 .
Another article stated that the immersion in water at room temperature for 1 hour, one day and one week reduced the base dimensional changes in the continuous-injection technique, whilst no influence was observed for the trial-pack technique, and these dimensional changes were influenced by the palate shape 17 .
It has been demonstrated that the water saturation of both dry heat and wet heat-processed dentures is relatively low because of the high initial water content. The linear expansion associated with water sorption does not entirely compensate for the processing shrinkage of dry and wet heat-processed dentures, and do not demonstrate any statistically significant difference in resulting shrinkage 20 .
The aim of this investigation was to determine the effect of water sorption on the posterior teeth shift in maxillary complete dentures, after storage for one week, one month, and three months at temperature of 37ºC, when the flasks were bench stored for 3 hours after cooling in the curing water.

MATERIALS AND METHODS
Ten maxillary complete dentures were made from similar stone casts simulating an arch without irregularities in the alveolar ridge crest. The wax denture base-plates, with thickness of 2mm, were made on the respective casts. The height of the occlusion wax-rims was 20mm in the labial sulcus of the cast, and 10mm in the second molar region. The upper stone casts were mounted in a Mondial 4000 semi-adjustable articulator (Bio-Art Dental Products, São Carlos, SP, Brazil) with the wax-rim interocclusal relation in accordance with the teeth of a lower stone cast in the following references: intercondilar distance in M, Bennett angle of 15º, and condilar guide of 30º. In order to accurately mount all dentures on the articulator, similar V-shaped notches were carved in the base of the upper casts.
The arrangement of the left anterior teeth was initiated with the carved wax-rim to serve as a guide for the central and lateral incisors, and canine positions. The same procedure was employed in the right arch. The left posterior teeth were arranged starting with the first pre-molar until the second molar. The same procedure was used in the right arch. The teeth arrangement for the interocclusal relationship was anterior vertical overlap and Angle class I for the posterior teeth.
Metallic reference pins were placed in the incisal border of the labial cusp of the first pre-molars, and mesiolabial cusp of the second molars. The dentures were flasked conventionally in Herodent Soli-Rock dental stone (Vigodent, Rio de Janeiro, RJ, Brazil), using a standard metallic flask (J. Safrany Metallurgy Co., São Paulo, Brazil). The waxed dentures were softened for 10 minutes in boiling water. The two halves of the flasks were separated, the wax removed and the stone cleaned with detergent and boiling water. After bench cooling, one coat of Isolak sodium alginate (Clássico Dental Products, São Paulo, SP, Brazil) was used as mold separator.
The heat-cured acrylic resin (Clássico Dental Products), based on the polymethylmethacrylate, was prepared with a polymer : monomer ratio of 3:1 by volume, and the plastic dough was packed in the flasks under a final packing pressure of 1,250 kgf. Twelve hours after the final flask closure, the acrylic resin was polymerized in water bath cycle at 74ºC for 9 hours. After polymerizing, the flasks were cooled slowly inside the water bath, removed from the thermo-polymerizing unit (Termotron Dental Products, Piracicaba, SP, Brazil), and bench stored for 3 hours. After this period, the dentures were deflasked, polished, and the PM-PM (premolar to pre-molar), M-M (molar to molar), LPM-LM (left pre-molar to left molar) and RPM-RM (right pre-molar to right molar) distances were measured with a STM microscope (Olympus Optical Co., Tokyo, Japan) with an accuracy of 0.0005mm.
After water storage at 37ºC for periods of one week, one month, and three months, the PM-PM (pre-molar to pre-molar), M-M (molar to molar), LPM-LM (left pre-molar to left molar) and RPM-RM (right pre-molar to right molar) distances were measured in the same manner as used following the denture deflasking. Resulting data were submitted to ANOVA and Tukey´s test (5%).

RESULTS
No statistically significant difference was observed in the PM-PM, M-M, LPM-LM and RPM-RM distances among deflasking period and water storage periods, when the flask was bench stored after cooling in curing water bath (Table 1).

DISCUSSION
Classic studies have shown that the expansion promoted by water sorption of the heat-cured resin bases compensates, in part, for the shrinkage which occurs during the denture polymerization 10,13 .
The effect of the water sorption on the teeth movement is difficult to evaluate due to the divergent results showed by several studies. The literature shows that the water sorption by resin base occurs during clinical use and the resulting dimensional changes may affect the occlusion of the denture, and the complex expansion of the bases following the water sorption causes changes in the occlusion due to movement of the teeth 11 . Although the reduction of the vertical dimension is small, this may cause discomfort to patients 13 .
Based on other studies, it was expected that expansion would occur when the deflasked dentures were stored in water, since the water imbibition by acrylic resin is a well-established occurrence 10,13 . However, the results from this study showed that water storage promoted values with no statistical significant differences when compared to the deflasking period (Table 1). These results are not consistent with an other study, which showed that the monomer remaining immediately after deflasking affects the water uptake level 6 . Conversely, our results agree with the same study showing no significant differences in the linear dimension change of acrylic resin specimens stored in water for up to three months 6 . Following storage in water for thirty days, the acrylic resin bases also showed no significant change in dimensional expansion 1,3 .
Studies showed differences in the total water content at saturation according to the cooling flask method. Thus, similar linear changes were observed in the bench cooled denture bases and in those cooled slowly inside the water bath 20 . Dimensional changes by water sorption cause expansion apparently due to the entry of water between molecules of the polymethylmethacrylate 2,4 , resulting in a plasticizing effect 12 , when absorbed during polymerization or immersion in water 18 . This study suggests that the amount of water uptake by volume of mass was not sufficient to cause posterior teeth movement in all storage periods. Since the initial water content in the wet heat-processed dentures is great 20 , the subsequent saturation level by water storage decreased. In contrast, a study verified that water immersion for eight weeks compensated, in part, the polymerization shrinkage, and that dimensional changes were influenced according to the different region of the dentures 7 .