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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 9  |  Issue : 2  |  Page : 83-89

A comparative study of dimensional stability of two popular commercially used denture base resins


Department of Dental, RD Gardi Medical College, Ujjain, Madhya Pradesh, India

Date of Submission25-Sep-2019
Date of Acceptance14-Nov-2019
Date of Web Publication3-Feb-2020

Correspondence Address:
Dr. Nidhi Dinesh Sinha
Department of Dental, RD Gardi Medical College, Agar Road, Surasa, Ujjain - 456 006, Madhya Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijmd.ijmd_46_19

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  Abstract 


Background: A good dimensional stability is a necessary prerequisite for successful complete denture retention and its functional efficacy. This study was conducted keeping in mind the cost factor and the need of a common man. Out of many available products, conventional heat cure resin (Dental Products of India [DPI] and Lucitone 199 [LUC]) are the standard of choice. For suitable understanding of these two products in accordance to the above-stated statement, the current study was conducted.
Aim: This in vitro study was carried out to compare the dimensional stability of commercially available polymethylmethacrylate (PMMA) DPI and high-impact LUC denture base materials processed using a long curing cycle, with and without terminal boiling.
Methodology: By using a chrome cobalt metal base plate, a total of forty specimens of acrylic base plates were fabricated. They consisted of two major groups of twenty specimens each, fabricated from conventional heat cure DPI and LUC heat cure resins. Each group so fabricated was further subdivided into two equal subgroups of ten specimens each, which were subjected to long curing cycle, with and without terminal boiling. The results for each group were compared by paired sample t-test for the studied objective, and data were analyzed by SPSS software version 20 (IBM Corporation, Armonk, NY USA). The level of statistical significance was fixed at the customary level ≤0.05.
Results: The dimensional stability values of LUC were found superior (P < 0.05) for both curing processes as compared to DPI.
Conclusion: PMMA reinforced with butadiene styrene fabricated using long curing cycle with terminal boiling showed the highest value of dimensional stability and the least was seen with conventional PMMA processed with long curing cycle without terminal boiling.

Keywords: Curing cycle's dimensional stability; Dental Products of India; Lucitone 199


How to cite this article:
Sinha ND. A comparative study of dimensional stability of two popular commercially used denture base resins. Indian J Multidiscip Dent 2019;9:83-9

How to cite this URL:
Sinha ND. A comparative study of dimensional stability of two popular commercially used denture base resins. Indian J Multidiscip Dent [serial online] 2019 [cited 2020 Feb 24];9:83-9. Available from: http://www.ijmdent.com/text.asp?2019/9/2/83/277451




  Introduction Top


The main objective while planning a complete denture and a distal extension partial denture fabrication is to obtain a denture base that conforms to the underlying tissues with utmost precision and retention. Both polymerization shrinkage of the denture base material and tissue changes underneath the denture base hamper this accuracy.[1],[2] Dimensional stability of the polymethylmethacrylate (PMMA) denture base is influenced by several factors such as the technique of polymerization, denture base thickness, and internal stresses induced by different coefficients of thermal expansion of gypsum and acrylic resin.[3],[4],[5],[6],[7]

The processing technique influences the rate of residual monomer conversion.[8] Decreased degree of conversion may affect the physical properties as well as the biocompatibility. Harrison and Huggett in 1992 evaluated the effect of polymerization cycle on residual monomer levels and concluded that terminal boiling leads to markedly reduced residual monomer content. They stressed that the rate of heat application was an important influencing factor.[9] The search for an absolute ideal denture base material, does not seem to make a final destination; although poly methyl methacrylate is the most popular among prosthodontists for almost over eight decades, still it has limitations such as shrinkage, water sorption, low impact, and flexural strength, to name a few. No doubt modifiers and fillers have unendingly brought research in its forefronts[10],[11],[12],[13],[14],[15],[16],[17] to recreate a new dimension in the properties of poly methyl methacrylate, but the applications of conventional heat cure resin in dentistry have not yet ceased, and researchers will continue to look at the original material PMMA and strive for its betterment in many ways.

Aim and objectives

The select acrylic resin brands Dental Products of India (DPI) and Lucitone 199 (LUC) were evaluated and compared for the linear dimensional changes, which were compared with respect to two different polymerization cycles.


  Methodology Top


Fabrication of metal template – A cast metal plate of even thickness of 2 mm was made in chrome cobalt alloy (S-U-Duranium, Chrome cobalt alloy for model castings, Schuler-Dental-ULM, Lucitone 199, Germany). Four sheets of 0.5-mm-thick S-U-Flexible Wax (Veined) (Schuler-Dental ULM) were used to form the wax pattern. The casting procedure was done using an Induction casting machine (Modular 3N, Asec Galoni; S Colombano L. (Milano) Italy, preheating furnace Asec Galoni; Model 210 (Italy) E600, and a vacuum mixer (Whip-Mix). Three vertical stops were designed on the wax pattern: one (A) in the region of the incisive papilla and one each (B and C) in the region just anterior to the hamular notch. These vertical stops were replicated in the cast metal plate [Figure 1], and they formed the baseline for the measurements of dimensional stability in the study. Thus, the dimensions A-B and A-C were the measurements in the anteroposterior direction, i.e., transarch and the dimension B-C was in the crossarch direction.
Figure 1: Cast base plate made in cobalt–chromium of even thickness of 2 mm with reference points

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Method for fabrication of test samples – A maxillary edentulous white rubber mold (Size 50 – MP Sai) was chosen to obtain the desired master casts. After careful manipulation of water–powder ratio and vacuum mixing of Type III gypsum product (Goldstone Asian Chemicals Rajkot, India), forty master casts were duplicated from the mold, using a vibrator/Unident (Unident Instruments (India) private limited, New Delhi)/regulator [Figure 2].
Figure 2: Edentulous white rubber mold used to duplicate forty master casts

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Commercially available heat cure acrylic denture base materials of two different manufacturers, namely, high-impact heat cure acrylic resin (LUC – Dentsply Pvt. Ltd., Rajkot, India) and conventional DPI heat cure resin (Dpi-Heat Cure Resin – light veined) DPI, a division of The Bombay Burmah Trading Corporation Ltd., were the selected materials for the study [Figure 3].
Figure 3: Materials and equipment used in the study

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The denture base samples were distributed into four groups with ten samples in each. Group 1 and Group 2 were DPI – conventional heat cure resin. Group 1 was processed with long curing cycle with terminal boiling for 1 h at 100°C and Group 2 was processed with long curing cycle without terminal boiling. Groups 3 and 4 comprised of samples of LUC high-impact resin, of which Group 3 was processed with long curing cycle with terminal boiling at 100°C for 1 h and Group 4 was processed with long curing cycle without terminal boiling.

A standard brass dental flask size – medium (Jabbar and Co Products, Delhi, India) was taken, and the master cast with the custom metal plate positioned on it was invested in Type IV gypsum product (Kalrock – Kalabhai Karlson Pvt Ltd). After application of two layers of the separating media (DPI – heat cure cold mold seal), the counter pour was done. The investment material was allowed to set. Subsequently, the two halves of the flask were separated [Figure 4]. The custom metal plate was carefully lifted with a plaster knife so as not to chip off the underlying mold space in the investment material.
Figure 4: Investment and separation of cast metal base plate in Type IV gypsum

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The conventional DPI heat cure resin was proportioned according to the manufactures' instructions with a polymer-to-monomer ratio of 3:1 by volume and packed in the dough stage. A polyethylene sheet (separating sheets – DPI) was placed over the resin for the trial closure [Figure 5], and the flask was closed.
Figure 5: Trial closure while packing heat cure resin

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The flask assembly was placed into a hydraulic press (OMEC CE Muggio-MI-Italy, Type P188/3), and pressure was applied incrementally. Gradual application of pressure permits the resin to flow evenly throughout the mold space. The application of pressure was continued till the denture flask was firmly closed without any gap [Figure 6].
Figure 6: Hydraulic press demonstrating the application of incremental pressure and dental acrylizer used in heat polymerization

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The two halves of the flask were separated, and the polyethylene sheet was removed with a rapid continuous tug. The flash material was carefully teased away with a sharp knife. Subsequently, another trial closure was made. The final flasking was done after the second trial closure. The flask was allowed to bench cure for 15–20 min.

Polymerization was done by two different polymerization cycles. Polymerization cycle 1 involved heat polymerization at a temperature of 74°C for 7 h and then increasing the temperature of the water bath to 100°C for 1 h (terminal boiling) Polymerization cycle 2 was carried out by heat polymerization at a temperature of 74°C for 7 h without terminal boiling in a dental acrylizer (Dent Cure – Puneet – ISO Certified) [Figure 6]. After completion of the chosen polymerization cycle, all the test samples were bench cooled for 30 min.

Method of measurement

In the first part of the study, the metal template was measured with respect to the three predetermined points [Figure 1] A, B, and C with the help of digital Vernier calipers at a resolution of 0.01 mm (Aerospace digital Vernier caliper, Mitutoyo, Japan). This formed the baseline values.

Subsequently, the measurements were made on the processed denture bases with respect to the three points A, B, and C inscribed on them [Figure 7]. Dimensional changes were recorded. The difference between the baseline values and the measurements on the processed denture bases was calculated and expressed as a negative reading, to indicate the linear shrinkage after polymerization.
Figure 7: Measurement with Vernier caliper on custom metal plate and acrylic base plate demonstrating transarch shrinkage

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  Observations and Results Top


Observational analysis: Baseline dimensional measurements on cobalt–chromium plate were as follows: transarch dimension (AC) 43.25 mm, transarch dimension (AB) 43.26, and crossarch dimension (BC) 46.21 mm. The mean dimensional changes after polymerization of the heat cure resin samples, the corresponding standard deviation, and the mean difference from the baseline values on cobalt–chromium plate for the two studied materials are expressed in [Table 1]. Here, LUC recorded lesser mean dimensional changes.
Table 1: Distribution of mean, standard deviation, and mean difference of dimensional changes after processing of all samples of the two studied materials

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Inferential analysis

A comparison was made between the mean dimensional changes of all the study group samples in their anteroposterior dimension, i.e., transarch A-C and A-B, and the mean dimensional changes in the crossarch dimension, i.e., B-C for both the materials. [Table 2] presents the paired sample t statistic results on this regard.
Table 2: Paired samples test for comparison between anteroposterior dimension and crossarch dimension of all samples after processing

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The t results demonstrated that there is a statistically significant (P = 0.05) difference between the mean dimensional changes in the crossarch as compared to the mean dimensional changes in the anteroposterior dimensions.

As a next step, the mean dimensional changes after heat processing of the two materials LUC and DPI conventional heat cure resin were measured and compared in all the three dimensions. The mean standard deviation and the “P” value were determined and are expressed in [Table 3].
Table 3: Paired samples test of comparison between mean dimensional changes of Lucitone and Dental Products of India groups

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After applying the Student's t-test for the above table, it was observed that there is a highly significant difference between the mean dimensional changes between LUC and DPI conventional heat cure resin in all the three dimensions. LUC demonstrated highly statistically significant result (P = 0.01–0.00) in almost all comparable dimensional parameters except in AC transarch (P = 0.125). Furthermore, the mean dimensional changes between the two processing techniques used, but within the same material, were compared and analyzed. The mean values, standard deviation, and the “P” value after applying the Student's t-test are expressed in [Table 4].
Table 4: Paired samples test for comparison of mean dimensional changes between two processing techniques of the same denture base material

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No extragroup variability for dimensional changes was observed (P = 0.29–0.58) except for LUC in AC dimension (P = 0.01).

After applying the Student's t-test, it was seen that there is no significant difference between the mean dimensional changes after processing of the samples of the same material but different polymerization cycles, i.e., LUC 2 – LUC group 2 – heat polymerization with long curing cycle and with no terminal boiling and LUC 1 – LUC group 1 – heat polymerization with long curing cycle and with terminal boiling and DPI 2 – DPI conventional resin group 2 – heat polymerization with long curing cycle and with no terminal boiling and DPI 1 – DPI conventional resin group 1 – heat polymerization with long curing cycle and with terminal boiling.

As a final step, the mean dimensional changes of all the study groups were then compared and their mean values, standard deviation, and “P” values are presented in [Table 5].
Table 5: Paired samples test for comparison of difference of means of dimensional changes of all four study groups

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LUC maintained better dimensional stability in both curing cycles (P = 0.04,0.05) over DPI.


  Discussion Top


In the study conducted, it is observed that LUC high impact is a better material than conventional DPI as dimensional changes observed with LUC are less. Such an inference can be attained from the P values as described in the above-mentioned tables. High-impact resins contain copolymers of low-molecular-weight butadiene styrene-b copolymer. The exact nature of this inclusion is manufacturers' trade secret. The presence of this polymer in LUC helps in reinforcing the material, reducing polymerization shrinkage, and also adding strength and hardness to the material.

Polymerization shrinkage leads to dimensional changes in the denture base, resulting in distortion of the palate of maxillary denture and therefore its inaccurate fit on the supporting tissues.[18] The other aspect is the alteration of the position of teeth on the maxillary and mandibular dentures, thus resulting in alteration in the final occlusion and also an increase in the vertical dimension.[19]

According to a study by Woelfel and Paffenbarger, the greatest distortion occurs in the crossarch region when the denture is deflasked. This has been attributed to the release of internal stresses developed during processing, and the difference in the coefficient of thermal expansion between stone cast and acrylic resin.[20] It has been observed that this shrinkage in the crossarch dimension can be up to 0.9 mm. If the shrinkage in the posterior region is maintained up to 0.5 mm, it does not cause a serious misfit, but when it is increased to 0.9 mm, the dentures did not fit properly.[21],[22] Furthermore, from the observations, we can arrive at an inference such as the long curing cycle with terminal boiling is a better processing technique as against a long curing cycle without terminal boiling. In the present study conducted, polymerization shrinkage was less with samples that were processed using long curing cycle with terminal boiling.

Lamb et al. observed that a higher proportion of polymer (5:3) resulted in low levels of residual monomer than the ratio of 4:3.[23],[24] If residual monomer is present, less monomer conversion occurs which may result in increased sorption and solubility.[25],[26] Whereas Sheridan et al. reported that the cytotoxic effect of acrylic resins was the most in the first 24 h after polymerization, and the author claims that the prosthesis should be stored in water for 24 h before placing in the patients' mouth.[27]

Dimensional stability is an important physical property to ensure that a denture can maintain its shape over a period of time.[28],[29] Shrinkage can occur during polymerization and cooling,[30] whereas expansion can occur when exposed to an increase in temperature. Water absorbed into the material acts as a plasticizer and decreases the mechanical properties such as hardness and transverse strength and also influences the dimensional stability.

The results of the above study conducted were in close proximity with the study of Vallittu,[31] and it was observed that low dimensional accuracy was found with unreinforced heat cure PMMA, whereas reinforced heat cure PMMA with glass fibers showed greater dimensional accuracy. A study conducted by Chen et al.[32] also observed that after polymerization of heat cure resins, more dimensional changes in molar-to-molar linear shrinkage were evident.


  Conclusion Top


Once the clinical try-in procedure is accomplished, the clinician expects that the occlusal scheme and the tooth arrangement are replicated in the final prosthesis. However, due to processing errors, some of it is lost and has to be re-achieved by selective grinding. The present data justify this observation. Disinfection of the dentures is also a major problem in terms of maintaining the dimensional stability.[33] As microwave disinfection is superior over various chemical disinfection methods, it is maintained that the influence of microwave disinfection has no significant changes on the dimensional stability if the temperatures are moderate.[34] Microwave irradiated resins also show a reduced amount of residual monomer than heat-polymerized resins.[35],[36] Efforts to constantly improve the conventional PMMA with a better curing cycle and reinforcements such as fillers and copolymers to achieve a more dimensionally stable and more precise replication in retention and occlusion will be in the move.

Finally, the present study has been conducted in a laboratory and the results may not be equivalent to clinical findings even though efforts were made to simulate the clinical conditions. The data obtained in the study pertain to the conditions in which they were tested in the materials and methodology used and are subject to change.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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