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 Table of Contents  
ORIGINAL RESEARCH
Year : 2020  |  Volume : 11  |  Issue : 4  |  Page : 103-107

Effect of Acrylic Polymerization on Cytotoxicity, Residual Monomer Content and Mechanical Properties


Department R&D, Spofa Dental as Markova 238, 506-01 Jicin, Czech Republic

Date of Submission02-Jul-2020
Date of Decision12-Jul-2020
Date of Acceptance17-Jul-2020
Date of Web Publication18-Nov-2020

Correspondence Address:
Zbigniew Raszewski
Swierkowa 10, Magdalenka
Czech Republic
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/denthyp.denthyp_85_20

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  Abstract 


Introduction: The aim of this study was to test and find connection between three different parameters of acrylic resins: cytotoxicity, residual monomer content in the material and flexural strength. Materials and Methods: Superacryl Plus (SpofaDental, Czech Rep.) acrylic material has been polymerized in three different ways, short-time polymerization (45 minutes), normal time (90 minutes) and long-time (7 hours 30 minutes). Flexural strength was tested in compressive instrument Shimadzu for 65 × 10 × 3.3 mm samples. Then after 24 hours the samples were broken. In 50 mm diameter and 1 mm thick samples, residual monomer was determined by gas chromatography (according to ISO standard), and the last series of the sample was used to perform cytotoxicity tests (5 mm diameter). Statistical analysis: Data were analyzed by two-way ANOVA (in GraphPad Software Inc., San Diego, CA, USA), with p-value < 0.05 as statistically significant. Results: The results from the tests show that the greatest inhibition in the development of cell cultures (VERO CCL-81) by MTT assay), was observed for the sample polymerized in the first short-time method (73.49±10%). The material also had the largest content of residual monomer 2.02±0.08% (p-value < 0.01) and lowest flexural strength 71.53±2.26 MPa. Hardened acrylic resins over 60 minutes do not adversely affect cell cultures (undisturbed growth 83.18±10.72%). The residual monomer content was below 1% (p-value < 0.01) and the mechanical resistance to fracture was over 80 MPa (p-value < 0.01). Conclusion: The use of a short polymerization method of acrylic materials can adversely affect both the mechanical properties of the prosthesis itself and its biocompatibility. From a clinical point of view, it is important to take care about the polymerization times of acrylic. Dentures for allergic patients should be carried out in long-term polymerization when the content of residual monomer is as low as possible.

Keywords: Acrylic resins, cytotoxicity, flexural strength, residual monomer


How to cite this article:
Raszewski Z. Effect of Acrylic Polymerization on Cytotoxicity, Residual Monomer Content and Mechanical Properties. Dent Hypotheses 2020;11:103-7

How to cite this URL:
Raszewski Z. Effect of Acrylic Polymerization on Cytotoxicity, Residual Monomer Content and Mechanical Properties. Dent Hypotheses [serial online] 2020 [cited 2020 Dec 4];11:103-7. Available from: http://www.dentalhypotheses.com/text.asp?2020/11/4/103/300869




  Introduction Top


Oral hygiene has a very large impact on the general health of patients. An interesting and helpful method connected with the patient’s complete health is the determination of different types of markers in the blood.[1],[2],[3]

In the case of missing teeth, one of the treatment method is to make the right prosthetic restorations, e.g. removable dentures (RD). The most commonly used materials for RD are acrylic materials. Among acrylic materials there are many materials from which denture plates are made: thermally polymerized materials, pressure cured and crosslinking at low temperatures (self-curing).[4]

When performing these acrylic dentures, there are many steps that can affect the quality of the final prosthesis. The amount of mixed monomer with the polymer, placement of the acrylic dough in the plaster form and the method of polymerization.[5],[6],[7] Some manufacturers of acrylic materials in the instructions for use show several ways of polymerization of the material: rapid polymerization, normal or long-term. Different hardening methods affect the mechanical properties of a given material, which has been widely documented in the literature.[8],[9]

There are numerous publications investigating the dependence of polymerization time as a function of residual monomer content. The content of unpolymerized methyl methacrylate (MMA) is one of the factors that can affect the biocompatibility of prosthesis.[10] Residual monomer content may also be different depending on the type of material and polymerization methods. According to ISO 20795-1:2013 (Dentistry − Base polymers) standard for thermally polymerized materials it cannot be higher than 2.2% and for self-curing this value is 4.5%.[10]

One of the simplest tests that indicate whether a given material can be safe is to test its cytotoxicity to selected cell cultures. In the literature on the subject it is possible to find the work of many scientists about cytotoxicity of acrylic materials. The results obtained are not entirely consistent. Some researchers describe that acrylic materials are cytotoxic, and the second group of authors describes, that they do not inhibit cell growth.[4],[11],[12],[13],[14],[15] But there is lack of studies in the literature that combine the mechanical properties of acrylic materials (flexural strength) with residual monomer concentration and their cytotoxicity. In order for a new prosthesis to be fully functional, it must be not only mechanically resistant to breaking but also have no irritating properties and thus be biocompatible. The purpose of this article was to compare these three important parameters for Superacryl Plus (SpofaDental) − fracture resistance, residual monomer content and cytotoxicity depending on polymerization conditions. The hypothesis of this research is whether there is a relationship between the length of polymerization time and the content of residual monomer and the effect on cell cultures.


  Materials and Methods Top


Thermally polymerized acrylic resin Superacryl Plus (SpofaDental, Jicin Czech Rep. Batch number 7567891) was used for the tests. The material was mixed in a proportion of 2.2 g of powder per 1g of liquid and until the consistency of the dough was achieved (the material did not stick to the wall of the mixing container or instruments), it was stored in a glass container under cover to prevent the monomer from evaporating. The dough was then placed in a metal form and polymerized in various thermal cycles [Table 1]. The first method was fast curing (45 minutes contact with boiling water). The second series of samples were polymerized over 60 minutes (normal polymerization) and the third group has undergone long-term polymerization (7 hours at 70°C and 30 minutes at 100°C).
Table 1 Polymerization cycles for Superacryl Plus used for testing

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The acrylic samples after being removed from the mold were polished using Saint Gobain (Poland) 60 grit wet sanding paper for 2 minutes to obtain a smooth surfaces. Discs 5 mm diameter and 1 mm thickness were used for the cytotoxicity test[11] in vitro model by direct contact with (VERO CCL-81, England) by MTT assay. For this experiment 15 samples were prepared for testing, 5 for each polymerization method. The fracture resistance was tested on samples required by the ISO standard (65 × 10 × 3.3) mm,[16] using Shimadzu (Shimadzu Japan) compressive strength instrument 5 kN, breaking speed 5 mm/min.

For residual monomer content detection, five samples (15 in total) were prepared for each test. Molds 50 mm in diameter and 1 mm thick described in ISO 20795-1:2013 standard were used to study the residual monomer content. Five samples for each tested variant were prepared for testing the residual monomer content by gas chromatography methods (Shimadzu GC-7 gas chromatograph (Shimadzu, Japan).

A detailed description of the methodology of sample preparation and cytotoxicity (direct contact with Vero CCL-81 (ATCC®, UK) testing, detection by MTT methodology was presented in the work Raszewski.[11] Prior to testing, samples were stored from laboratory conditions at 23°C in the dark place.

All experiments were performed in triplicate for each parameter, which gives 9 repetitions for each parameter. Data were represented as mean ± standard error of the mean. Data were analyzed by two-way ANOVA (in GraphPad Software Inc., San Diego, CA, USA), with P<0.05 as a statistically significant.


  Results Top


Material Superacryl Plus polymerized in different curing methods has different flexural strength, which was presented in Figure 1. The highest values are obtained with long time polymerization, according to method 3 (88.55±4.67 MPa) compared to short polymerization technique (71.53±2.26 MPa) (P<0.01).
Figure 1 Flexural strength of Superacryl Plus polymerized in different curing methods.

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The residual monomer concentration is also closely related to the polymerization method of acrylic material. Its lowest content was determined by gas chromatography for samples polymerized according to method 3 (P<0.01), as shown in Figure 2.
Figure 2 Residual monomer concentration is depended on curing method.

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The polymerized material in a short period of time according to the first method shows small inhibition of cell growth. The material cured in more than 90 minutes does not show a negative effect on cultures (P<0.01). These results are shown in [Figure 4].
Figure 4 Control cell viability.

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  Discussion Top


The conducted research indicates that the hypothesis put at the beginning of this research is true. There is a relationship between the polymerization time of the material and the content of residual monomer in the material, which in turn affects the cytotoxicity of the cell cultures. This is visible for samples polymerized in a short period of time, which possesses cytotoxic properties, while also having a high residual monomer content and have a low fracture resistance (73.49±2.26 MPa). The polymerized material has a residual monomer content more than 2.02 ± 0.08%. Methyl methacrylate as a low molecular weight substance not built into the polymer structure, and has a plasticizing effect for all material.[6] Therefore, Superacryl Plus polymerized for 45 minutes has a lower mechanical resistance to fracture. This phenomenon has already been described by the authors.[5],[6],[7],[8] Rapid polymerization process produces shorter polymeric chains, which also adversely affects the material’s resistance to fracture.[7],[8]

Too high a residual monomer concentration may also adversely affect the biocompatibility of acrylic material by causing an allergic reaction within a short period of time after a new prosthetic restoration.[9],[10],[11],[12]

Gradual polymerization of the material at 70°C causes the slow generation of free radicals as a result of the decomposition of dibenzoyl peroxide contained in the powder. It reacts gradually with a monomer that attaches to the polymer chains contained in the powder (more ordered polymer structure).[18] Superacryl Plus polymerized This method has the highest fracture resistance 88.55± 4.67 MPa.

Material polymerized with long time curing cycle does not inhibit the growth of cell cultures, which can grow undisturbed in the direct contact of acrylic plastic (these values of over 100%). In the case of short-time polymerization, the amount of monomer (above 2%) already begins to adversely affect the cells of the kidney of an African green monkey (survival rate at 73%). They were used for the previous series of tests and as a reliable model they were utilized again for Superacryl tests.[11] Thanks to this, the results obtained in this series of tests are easily comparable with those of Premacryl and Superpont C + B, whose results have already been published.[11]

Cytotoxicity results presented by other authors who tested the resins which polymerized in self-cure methods indicate a higher degree of cytotoxicity.[9],[12],[14],[15],[17] Retamoso et al. indicate, however, that the dyes contained in acrylic materials intended for orthodontic appliances are not cytotoxic.[12]

Souto-Lopes, studying various cell cultures, clearly states that cytotoxicity is the effect of unpolymerized MMA, which in some cases can be converted into even more toxic formaldehyde.[13]

Materials available in the form of CAD-CAM discs are the least cytotoxic, which clearly indicates the processing of acrylic and its impact on the biocompatibility of the material. The material cured in industrial conditions under high pressure and in the long run has a very low residual monomer content.[14]

The second variable that can affect whether the material turns out to be cytotoxic or not is the time after which the acrylic samples have been tested since polymerization. Numerous studies indicate that residual monomer is present only for the first few days after polymerization, and its quantity decreases over time. Some of the unpolymerized monomer evaporates, some is washed away by water (saline) and some minor amount is attached to the polymer chains.[4]

Residual monomer content may also depend on how the samples are handled, according to Santos RL and others. They noticed that chemical polishing with MMA baths increases the residual monomer content.[4],[19],[20],[21],[22] Therefore, in this work, the author used mechanical polishing with sandpaper.


  Conclusion Top


The use of a short polymerization method of acrylic materials can adversely affect both the mechanical properties of the prosthesis itself and its biocompatibility. In the case of short time polymerization, it seems appropriate to cure the acrylic material in warm water to reduce the residual monomer content.

Acknowledgement

The author would like to thank SpofaDental for providing acrylic materials for research.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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