Comparison of Adaptation of Acrylic Resin Cured by Injection-molded Technique or Microwave Energy: An In Vitro Study

INTRODUCTION

Compression molding with warm triggering is a conventional method of processing dentures. Poly-methyl-methacrylate (PMMA) has superlative cosmetics and very less noxiousness in comparison with an acrylic resin.¹ Polymerization shrinkage and adaptation inaccuracies of acrylic resin are the major flaws for the regular dentures. Babu et al. stated that acrylic bases cured by New Press method and long curing cycle have more accurate denture bases in comparison with samples cured by a regular method of PMMA resin processing.² Gharechahi et al. reported that adaptation accuracies of acrylic resin specimens are affected by the compression molding technique, and the SR-Ivocap injection procedure showed a better adaptation accuracy in comparison with the conventional molding.³ Nejatian et al. reported that the specimens polymerized by a conventional water bath showed better adaptation of dentures to underlying tissue than the samples cured by an injection molding method.⁴ Babosa et al. reported that the flexural strength of dentures bases is not affected by curing microwave base resin samples by dissimilar microwave cycle.⁵ Arafa reported that the samples cured by a conventional water bath showed a better adaptation accuracy of dentures to underlying tissue than the samples cured by an injection molding method.⁶ There are very few studies regarding dimensional changes with injection molding technique or the microwaveable acrylic cured by microwave energy. Therefore this study was conducted to compare the adaptation of acrylic resin cured by the injection-molded technique or microwave energy.

AIMS AND OBJECTIVES

• To check the adaptation accuracy in the SR-Ivocap high-impact resin cured using the injection molding technique.
• To check the adaptation accuracy in the microwave acrylic resin (Onda-Cryl; Artigos Odontológicos Clássico Ltd, Brazil) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes).
• To comparatively evaluate the adaptation accuracy in the SR-Ivocap high-impact resin cured using the injection molding technique and microwave acrylic resin (Onda-Cryl) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes).

MATERIALS AND METHODS

A total of 40 maxillary acrylic bases (2 mm thick) were constituted on cast. A silicone rubber-based impression of maxillary arch was prepared and flooded with type IV dental stone to procure a cast model.⁷ An SR-Ivocap high-impact resin was used for the injection molding technique and microwaveable resin (Onda-Cryl; Artigos Odontológicos Clássico Ltd) was cured using microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes). A total of 40 test specimens were selected as stated in earlier research carried out by Babu et al.² The specimen size was 20 in each group keeping a confidence interval of 95% and a power of at least 80%. These specimens were divided into two groups.

Group I: 20 maxillary record bases were fabricated using the injection molding technique.

Group II: 20 maxillary record bases were fabricated using microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes).

RESULTS

The comparison of groups I and II mean was carried out using the unpaired t test at the right marginal limit. There was significantly a higher mean value among group I (Table 1, Fig. 3). The comparison of groups I and II mean was carried out using the unpaired t test at the left marginal limit. There was significantly a higher mean value among group I (Table 2, Fig. 4). The comparison of groups I and II mean was carried out using the unpaired t test at the right ridge crest. There was significantly a higher mean value among group I (Table 3, Fig. 5). The comparison of groups I and II mean was carried out using the unpaired t test at the left ridge crest. There was significantly a higher mean value among group I (Table 4, Fig. 6). The comparison of groups I and II mean was carried out using the unpaired t test at the palatal midline. There was significantly a higher mean value among group I (Table 5, Fig. 7).

DISCUSSION

Poly-methyl-methacrylate is the material of choice in present-day dentistry as it has excellent esthetic properties, strength, less water sorption, and little solubilty.² Poly-methyl-methacrylate is in demand and most recommended for removable prosthesis. The compression molding procedure decreases the adaptation of dentures by enhancing recess of oral mucosa and denture area. The polymerization shrinkage and distortion develop the thermal stress within the prosthesis. The microgap is the result of thermal stresses affecting the adaptation of denture bases to underlying tissue.⁹ Therefore, acrylic resin and polymerizing methods have been modified to enhance the physical and chemical properties of denture bases. The microwave curing at 690 W for 6 minutes decreases the accuracy of denture bases to underlying tissue, whereas the same resin polymerized by conventional heat polymerization has more adaptation accuracy of dentures to underlying tissue.¹⁰ The studies have shown mixed and controversial reports. Poly-methyl-methacrylate specimens cured by conventional heat polymerization have prolonged curing time; therefore, polymerization by microwave energy is suggested for better accuracy of denture bases to underlying tissue. Badr et al. reported that the test specimens prepared by microwave energy have more porousness than test samples cured by the conventional water bath. The specimens polymerized by the conventional water bath showed better adaptation of dentures to underlying tissue.¹¹ Badr et al. reported that the conventional heat polymerization technique had desirable results in fabricating water bath polymerized resins than the microwave polymerization. They reported that the little detected residual monomer in the microwave-polymerized denture is due to the elevated temperature created during the curing cycle (550 W for 3 minutes).¹¹ Firtell et al. stated that the gap between the palatal portion of the cast and denture base is the result of linear shrinkage on the posterior palatal seal area of maxillary denture.¹² Moussa et al. stated that injection-molded PMMA resin showed higher adaptation accuracies compared with the conventional pressure-packed PMMA resin.¹³ They also reported that the accuracy of denture bases to underlying tissue is affected by dissimilar positions of the base and palatal geometry. Arafa reported that the samples cured by the conventional water bath showed a better adaptation accuracy of dentures to underlying tissue than the samples cured by the injection molding method.⁶ There are very few studies regarding the dimensional changes with the injection molding technique or the microwaveable acrylic cured by microwave energy. The conflicting results were reported by the researchers. Therefore, this study was conducted to compare the adaptation of the acrylic resin cured by the injection-molded technique or microwave energy.

In this study, there was a higher mean marginal discrepancy value among group I at all the measured points (right marginal limit, left marginal limit, right ridge crest, left ridge crest, and palatal midline). The microwaveable acrylic resin (Onda-Cryl; Artigos Odontológicos Clássico Ltd) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes) showed significantly lesser adaptation inaccuracies and more accurate denture base than the injection molding techniques. The probable reason may be that the residual monomer was less due to raised temperature during a curing cycle (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes). The microwave-polymerizing system has three necessary parts: microwave power generator (magnetron), waveguide, and microwave chamber or cavity where the samples have to be processed, and additional parts include control system, tuner, and circulator. The waveguide transfers microwave energy between free space and conveys to the specimens. The microwave chamber transfers, adjusts, and returns back the microwave energy.¹⁴ The wattage (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes) of microwave energy affects the physical and mechanical properties (dimensional stability) resulting in lesser adaptation inaccuracies and more accurate denture base adaptation to underlying tissues. The time power setting of microwave energy has been proved adequate for curing the denture base resin. The SR-Ivocap high-impact resin cured using the injection molding technique showed significantly higher adaptation inaccuracies and less accurate denture base than Onda-Cryl resin polymerized by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes).

The probable reason may be that polymerization shrinkage and release of thermal stresses from PMMA results in adaptation inaccuracies, which further results in dimensional instability of the final denture base.¹⁵ The microgap between denture base and the underlying tissue results as there is curing depreciation of denture base and a chance of cooling depreciation in the direction of the middle portion of the denture and distortion due to impounding of the surface balance of the crestal area.¹⁶ The denture base resin has been cured using microwave energy. The advantages include energy-saving rapid heating rates, short processing time, and deep penetration of the microwave energy. Therefore, the microwaveable acrylic resin (Onda-Cryl) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes) can be a viable option for fabrication of dentures.

CONCLUSION

The microwaveable resin (Onda-Cryl, Artigos Odontológicos Clássico Ltd) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes) showed significantly lesser adaptation inaccuracies compared with denture base fabricated using the injection molding techniques.

• The microwaveable acrylic resin (Onda-Cryl; Artigos Odontológicos Clássico Ltd) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes) showed the lowest amount of adaptation discrepancy at the right marginal limit (0.358 mm), left marginal limit (0.821 mm), right ridge crest (0.008 mm), left ridge crest (0.007 mm), and palatal midline (0.023 mm).
• SR-Ivocap high-impact resin cured using the injection molding technique showed the highest amount of adaptation discrepancy at the right marginal limit (1.245 mm), left marginal limit (1.985 mm), right ridge crest (0.600 mm), left ridge crest (0.720 mm), and palatal midline (0.093 mm).
• The microwaveable acrylic resin (Artigos Odontológicos Clássico Ltd) cured by microwave energy (320 W for 3 minutes + 0 W for 3 minutes + 720 W for 3 minutes) showed more accurate denture base than using the injection molding techniques.

STRENGTH OF THE STUDY

The denture base resin has been cured using microwave energy. The advantages include energy-saving rapid heating rates, short processing time, and deep penetration of the microwave energy.

LIMITATIONS IN THIS STUDY

A single microwaveable acrylic resin was used in this research. Therefore, further research need to be carried out with different microwaveable acrylic resins with different polymerization cycles.

FUTURE RESEARCH DIRECTION

Future studies are required to comparatively evaluate the dimensional accuracy of the conventional compression molding method, microwave polymerization, and injection molding technique.

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