A critical review on the conversion degree of resin monomers by direct analyses

Objectives: The aim of this study was to carry out a review on the methods of direct detection of the conversion degree of composite resins. Materials & methods: The authors searched the Cochrane Library, Embase, PubMed, and the Web of Science (ISI) for papers dated from January 1991 to November 2011. The search was complemented by manual searches of the reference lists from each identified relevant paper. The main search terms were: “degree of conversion”, “monomers”, “polymers”, “composite”, “pre-gel”, . ” d n o b e l b u o d “ d n a ” s r e m o n o m l a u d i s e r “ , ” l e g t s o p “ The inclusion criteria were studies that evaluated consolidated methods in the literature to directly evaluate the degree of conversion. Excluded studies were those considered to be unrelated to the question addressed, such as mechanical and physical properties and also monomers from adhesive systems and compomers. The selected papers were assigned to a score (A, B or C), according to the predetermined criteria. Results: A total of 45 papers were selected and 15 papers were included in the critical appraisal. Two articles were labeled as grade A, nine articles were labeled as grade B, and four articles were labeled as grade C. Conclusions: According to the definitions of evidence levels, the current article study showed a strong evidence of review. Clinical significance: The degree of conversion predicts the quality of polymers because the higher degree of conversion, the higher strength of the resin matrix. Ana Flávia Sanches BORGES1, Melody Analia CHASE2, Ana Liesel GUGGIAR2, María José GONZÁLEZ2, Andreia Rodrigues de Sousa Ribeiro ANDRADE3, Fernanda Miori PASCON4, Antonio Ricardo ZANATTA5 1 Dental Materials at Bauru Dental School University of São Paulo – USP Bauru – SP Brazil 2 Dental School of the National University of Asunción – UNA – Asuncion Paraguay 3 Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo HRAC/USP. Bauru – SP Brazil 4 Pediatric Dentistry at Piracicaba Dental School. State University of Campinas UNICAMP Campinas – SP Brazil 5 Physics at the Institute of Physics of São Carlos University of São Paulo – USP Sao Carlos – SP Brazil

papers dated from January 1991 to November 2011.The search was complemented by manual searches of the reference lists from each identified relevant paper.The main search terms were: "degree of conversion", "monomers", "polymers", "composite", "pre-gel", .The inclusion criteria were studies that evaluated consolidated methods in the literature to directly evaluate the degree of conversion.Excluded studies were those considered to be unrelated to the question addressed, such as mechanical and physical properties and also monomers from adhesive systems and compomers.The selected papers were assigned to a score (A, B or C), according to the predetermined criteria.Results: A total of 45 papers were selected and 15 papers were included in the critical appraisal.Two articles were labeled as grade A, nine articles were labeled as grade B, and four articles were labeled as grade C. Conclusions: According to the definitions of evidence levels, the current article study showed a strong evidence of review.Clinical significance: The degree of conversion predicts the quality of polymers because the higher degree of conversion, the higher strength of the resin matrix.
Borges AFS et al.

IntRoDuctIon
R esidual monomers are defined as the unreacted monomers in resin composites [1].Ideally, it is desirable for dental composites that all monomers would be converted into polymers during the polymerization reaction [1], but this does not occur.Residual monomers can be detected by the degree of conversion (DC), the determination of amount of carbon double bonds (C = C) that are converted into carbon single bonds (C -C) [1,2].
The efficiency of polymerization is influenced by the degree of conversion, (the greater number of double bonds converted into single bonds), resulting in a more resistant polymer network [3,2].Studies have shown that direct correlations with mechanical properties such as hardness, tensile strength and compressive strength are almost a consensus when the degree of conversion is evaluated [2][3][4][5].Insufficient degree of conversion results in hydrolytic degradation of monomers present in between fillers and the organic matrix [6].
The methods to measure the degree of conversion vary greatly from report to report [7].Some studies indirectly evaluate the DC, independent of leaching out tests, which consisted of an extraction of unreacted monomers in order to detect the quantity of reacted monomers [4,8].Methods that directly evaluate DC by means of spectroscopy have been successfully established [9,10].These studies verify the absorption peak of C = C bonds before and after polymerization, showing the amount the remaining C = C not converted into C -C [7].The baseline is the aromatic absorption peaks at 1609 or 1590 cm - 1 , being the internal C = C reference peak at 1638 cm -1 [11,12].The most commonly used DC calculations are based on the proportion of cured and uncured materials [1,3-5,7,13,14,15,].Since a variety of method parameters is used, it would be useful to know how quick and reliable the method of determining the degree of conversion of dental resins is.The purpose of this article is to present a systematic review about the methods of direct detection of degree of conversion of composite resins.

Review
• Question addressed by this review: What is the most reliable methodology to directly evaluate the degree of conversion of resin composites?

• Literature search
The authors searched for papers on Cochrane Library, Embase, PubMed, and Web of Science (ISI) databases from January 1991 to November 2011.The search was supplemented by manual searches of the reference lists of each relevant paper identified.The search terms were "degree of conversion", "monomers", "polymers", "composite", "pre-gel", "post-gel", "residual monomers", and "double bond".The search was limited to in vitro studies.Only original papers written in English were included.Interim reports, abstracts, letters, short communications, reviews, and chapters in textbooks were excluded.First, the papers identified in databases were printed as abstracts, or full-text articles, if the abstract was missing.In a second step, three authors independently selected relevant papers, and those considered of interest for this review were ordered in fulltext versions.During the evaluation process, reference lists were manually searched.

• Inclusion and exclusion criteria
After the appraisal, papers were included in this review when they showed consolidate methods in the literature to directly evaluate the degree of conversion.Excluded studies were those considered to be unrelated to the question addressed, such as mechanical and physical properties and also monomers from adhesive systems and compomers.

• Evaluation of scientific papers and levels of evidence
The papers that met the inclusion criteria were subjected to critical appraisals, which were carried out independently by at least two reviewers.Data was extracted using a pilottested form, and each paper was given a score of A, B or C, according to predetermined criteria for methodology and performance, as presented in Table 1.Based on the literature evaluated and A Critical Review on the Conversion Degree of Resin Monomers by Direct Analyses on the scores, the final level of evidence was judged according to the protocol of the Swedish Council on Technology Assessment in Health Care [16] and modified by Kantovitz et al. [17] as described in Table 3.

Results
A total of 115 papers were originally identified based on the main search terms.Initially, 70 papers were excluded after evaluation.A total of 45 papers were selected.Fifteen papers were included in the critical appraisal project based on the inclusion and exclusion criteria.The papers were labeled as grade A if all criteria were described.Papers were labeled as grade B if one criterion was not described.Papers were labeled as grade C if more than one criterion was not described (Table 1).The grades B and C are concerned methodologies not completely described.The criteria of this classification are described in Table 2. Two articles were labeled as grade A, nine articles were labeled as grade B, and four articles were labeled as grade C. According to the definitions of evidence level, modified by Kantovitz et al. [17], the current article was classified with strong evidence of review (Table 2).The thirty papers that indirectly verified the degree of conversion evaluated the degree of conversion of adhesive systems or evaluated the degree of compomer conversions were not submitted to grade classification (Table 4).The basic requirements for classification of the studies reviewed were the methods, tested, light curing/ intensity, specimens description, laser /wavelength, peak vibration reference, the number of specimens (N), degree of conversion (DC) calculation, and exposition of acquired spectra (Table 1).
The main methods shown in this review were: the Fourier Transform Infrared (FTIR), the Raman and the FT-Raman (Fourier Transform Raman) spectroscopy.From 15 articles reviewed, 14 evaluated the Degree of Conversion (DC) of resin composite monomers using an Infrared technique [2][3][4][5]7,13,14,[18][19][20][21][22].During the IR absorption (or reflection) experiments, the technique probes the monomer characteristics by measuring the absorbed radiation in the infrared range.The process is very efficient, since it occurs when resonance takes place between the incident radiation and the vibrational frequency characteristic of a specific arrangement of atoms or molecules [23].FTIR, or Fourier transform IR, refers to the use of an interferometer (to separate the spectral components) and a mathematical algorithm to accelerate the measurement A Critical Review on the Conversion Degree of Resin Monomers by Direct Analyses [24].While the FTIR technique is based on the absorption (or reflection) of radiation, Raman spectroscopy probes the light scattering processes [23,24].Within this context, it is important to remark that [23,24]: (a) the absorption of radiation is much more efficient (~ 109 times) than scattering, (b) IR absorption measurements are suitable for polar systems (in which there is a large transfer between atoms or molecules), as the chemical bonds formed by polymerization of resin monomers, and (c) Raman scattering is best suited for systems in which the electron cloud may be deformed.More specifically, during the Raman measurements, when laser light strikes a sample, it acts by deforming the electron cloud, which will cause scattering.Based on these aspects, the use of FTIR absorption spectroscopy is the easiest and simplest method [25].FT-Raman scattering provided an alternate method of obtaining degrees of conversion for these systems and was particularly useful for measuring spectra of materials without any sample preparation [5].
A study of DC should specify what monomer system is evaluated, even if the resin composite studied is commercially available, because it could influence the values of DC.The total replacement of TEGDMA by BisEMA decreases the DC in the Bis-GMA based resin matrix [25].Some articles that used the DC analysis did not specify the monomer system in the methodology [1,14,15,20].
The quality of polymerization is influenced by the light cure type, and by its wavelength and intensity.Some studies specified what type of light cure was used, if halogen or LED, and their wavelengths and intensities [1,4,5,13,15,18,25].Other studies only cited the type of light and the time of irradiation [2,3,7,13,14,20,22,26] .The specimens greatly varied among the studies reviewed.Some studies did films [3,7,22]; while others studies did circular specimens [7,8,10,12,16,24,20] and some studies powdered the polymers [2,22,25].The description of procedures included citing the dimensions of a mold used as a standard for specimens' fabrication, until the weight of monomers and polymers used are obtained.One study did not describe the specimens [20].However, all studies inspected the specimens before and after polymerization.This is a basic requirement in order to obtain the DC of monomers directly from IR and/or Raman spectroscopy, and identifying the wavenumbers of the aliphatic and aromatic bands [1,4,5,13,14,15].
The DC calculations from all the articles reviewed were obtained from the spectrum of the monomer absorbance of the aliphatic (C = C) band at approximately 1637 cm-1 and the absorbance of the aromatic (C -C) band at approximately 1608 cm-1 [5,14,15,18,19,20,23,25,26].The band peak variations up to ~ 5 cm-1, as obtained by different authors, is not significant because they can be associated to variations in composition and/or geometry of the structures [23,24].The ratio between C = C/C -C of polymers and C = C/C -C of monomers are taken as the fraction of unreacted double bonds in the polymer and it is important to directly compare the spectra to follow the conversion process [23,24].If they are not subject to problems of background and do not experience changes in their weights (which may suggest the absence of vibration modes), the fraction of material converted is directly proportional to the peak intensity or area.Seven reviewed articles did not show the spectra acquired [3,7,13,19,20,22,25].
Whereas the IR absorption methods are carried out under standard conditions (broad band and low-intensity IR radiation sources) [23], Raman measurements can be performed following many different experimental approaches.Considering that the conversion degree can be influenced by the laser radiation characteristics (photon energy, power, probed area), it is fundamental to specify all the experimental details involved in the Raman analyses [24].The two studies based on the Raman technique described only the photon energy or laser wavelength [13,5].

Table 1 -
Topics from each article includes in this review A Critical Review on the Conversion Degree of Resin Monomers by Direct Analyses

Table 3 -
Definitions of evidence level*

Table 4 -
Articles excluded of this review