The development of dentures, however the properties of acrylic

The utilization of acrylic resin as a denture base material since 1937 has reformed the dentistry hugely. The resin has fine esthetic properties, is incredible in shading and is chemically steady. It can be utilized with a straightforward strategy for the development of dentures, however the properties of acrylic resin are not perfect in all angles.
Resin polymers have been presented as denture base materials and these denture bases are made utilizing polymethyl methacrylate (PMMA). These materials have ideal physical properties and fantastic esthetics with moderately low toxic quality contrasted with other denture base materials.

 Dimensional changes which take place in heat cure acrylic resins are shrinkage and expansion which influences the fit and occlusal relationship of the denture with the underlying mucosa1. Despite the fact that acrylic resin polymer is known for its phenomenal properties, it has its own particular drawbacks like distortion. Exact duplication of trial denture into the final prosthesis is the coveted point amid handling in the lab. In any case, certain properties like dimensional errors of the materials trade off the achievement of this objective ideally. 

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Consequently a satisfactory medium ought to be chosen for storage of denture with a specific end goal to limit the distortion for dependable utilization. So it is vital to find what sort of storage medium can be prescribed keeping in mind the end goal to avert dimensional changes of the denture. Considering the importance of dimensional changes occurring during storage, the present study was undertaken to determine linear dimensional changes of commercially available heat cure acrylic resins in four liquid mediums.

Materials and method-

This in-vitro experimental study was conducted on four types of liquid medium (water,  saline, artificial saliva, vinegar) with heat cure acrylic resins. Based on the number of mediums, five dentures were determined for each group (N=20).

The fabricated denture bases were stored into following storage environments: water, saline, artificial saliva vinegar. Saline and water mediums were kept at room temperature whereas artificial saliva and vinegar mediums were stored at refrigerator temperature. The dentures were allowed to be stored in the mediums for 4 weeks where every 2 days the dentures were placed in the cast were measured using digital vernier to check for any distortion. The denture bases were measured in the posterior-palatal region, the interface between the cast and the denture. Finally after four weeks, all the measurements that were taken, were compared with the base measurements and analyzed by Paired T-testusing SPSS software version 20 at significant P-value of 0.05.

Results:

Table-1 represents the means and standard deviations of measured dentures  in four different storage mediums after every two days for four weeks. 

To determine the difference in each sample before and after storage in the liquid mediums, paired t -test was used which is shown in table 1. The data showed the dentures stored in vinegar and saline contracted 0.022mm and 0.018mm respectively and the dentures which were stored in water and artificial saliva contracted 0.010mm, 0.07mm respectively.

Discussion-

The studies done in the past years indicate that the acrylic denture base resins tend to absorb water, in this manner expansion can make up when minimal polymerization shrinkage takes place. This can clarify the minimal measure of dimensional changes seen in acrylic heat cure denture resins which were put in water which is as per the findings of this study. (2-4)
Wong et al (2), reported that tendency to absorb water in  acrylic resins can  exhibit shrinkage during setting. Expansion following water absorption can compensate for a part or all of the polymerization shrinkage or even expansion can occur.
A study performed by Goodkind 6 showed that immersion in water had no significant effect on denture base dimensions. Consani 5 reported that 90 days of storage of denture bases in water did not result in significant changes in distances
between the teeth in comparison to deflasking period. Miessi 7 reported that 180 days of immersion in water caused major dimensional changes and adaptation problems in denture bases. Some authors have reported that water storage of acrylic denture bases results in expansion due to water sorption. Water sorption forces the macromolecules apart and results in acrylic expansion 8. This expansion compensates the polymerization shrinkage of acrylic resin and improves the adaptation of denture bases with underlying tissues 7. The present study also confirmed this finding and showed that in both experimental groups, 12 days of storage in water resulted in a significant decrease in dimensional changes and compensated the polymerization shrinkage
Within the limitations of this study, water and artificial saliva were the best mediums to be used.

Reference-1. Woelfel JB, Paffenbarger GC, Sweeney WT. Dimensional changes occurring in dentures during processing. J Am Dent Assoc. 1960;61:413–30.2. Wong DM, Cheng LY, Chow T, Clark RK. Effect of processing method on the dimensional accuracy and water sorption of acrylic resin dentures. The   Journal of prosthetic dentistry. 1999;81(3):300-4.3. Koumjian JH, Holmes JB. Marginal accuracy of provisional restorative materials. The Journal of prosthetic dentistry 1990; 63(6): 639-42.4. Orsi IA, Junior AG, Villabona CA, Fernandes FHCN, Ito IY. Evaluation of the efficacy of chemical disinfectants for disinfection of heat?polymerised acrylic resin. Gerodontology 2011; 28(4): 253-7.
5. Consani RL, Domitti SS, Rizzatti Barbosa CM, Consani S. Effect of commercial acrylic resins on dimensional accuracy of the maxillary denture base. Braz Dent J. 2002;13:57–60.

6. Goodkind RJ, Schulte RC. Dimensional accuracy of pour acrylic resin and conventional processing of cold-curing acrylic resin bases. J Prosthet Dent. 1970;24:662–8. 

7.  Miessi AC, Goiato MC, Santos DM, Dekon SF, Okida RC. Influence of storage period and effect of different brands of acrylic resin on the dimensional accuracy of the maxillary denture base. Braz Dent J. 2008;19:204–8. 

8.  Duymus ZY, Yanikoglu ND. The investigation of solubility values, water sorption and bond strength of auto-polymerising and heat polymerising acrylic resin materials. Eur J Prosthodont Restor Dent. 2006;14:116–20. 

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