Influence of alternative photoinitiators in composite resins : A literature review

Objective: To evaluate the influence of alternative photoinitiators present in composite resins related to yellowing, color stability, photosensitivity, light-curing efficiency, degree of conversion, and microhardness. Materials and methods: The literature was reviewed by two independent reviewers at PubMed. The search strategy was carried out using the following descriptors: (Photoinitiators, dental) AND (Curing lights, dental) AND (Composite Resins) NOT (Case report). The electronic search was carried out until March 2019, without language restrictions, and inclusion and exclusion criteria were applied in the selection. Results: Thirteen articles met the previously established criteria, and all were included. Of these, most were in vitro studies. After the aging of the composite, all systems suffered yellowing, however the isolates with camphorquinone were the most affected and, although there is a greater tendency to yellowing, this does not directly affect color stability. However, the literature is still conflicting about which system has significant color stability. Regarding photosensitivity, all photoinitiators have different absorption peaks. In this sense, the photopolymerization must be chosen with caution, as the conversion of monomers and microhardness proved to be directly linked to the correct combination of photoactivator and photopolymerizing unit. Conclusion: Alternative photoinitiators have a lower yellowing rate and good color stability. They can be up to five times more sensitive than systems containing isolated camphorquinone. Only third-generation light-emitting diodes photopolymerize satisfactorily composite resins that have alternative photoinitiators in their composition, ensuring a good degree of conversion and acceptable Knoop microhardness values.


Introduction
Self-perception of dental aesthetics has become common in people seeking a harmony relation to the face. The composite resin has become one of the most widely used dental restorative materials, mainly due to its aesthetics similar to dental element. (Vervliet, et al. 2018, Shimokawa, et al., 2017 Most composites have only camphorquinone (CQ) as photoinitiator. CQ has as limitation the yellowing of body resin color due to interaction with tertiary amines into composites (Shimokawa, et al., 2017). Therefore, manufacturers have been replacing or inserting CQ alternative photoinitiators (AP), such as type I photoinitiators, which means that they do not need a coinitiator molecule to generate free radicals and are capable of generating these radicals from the cleavage of the molecule itself. Examples of this photoinitiators are 1,2phenylpropanodione (PPD) and acylphosphine oxides: mono-alkylphosphine oxide (MAPO), diphenyl-2,4,6-trimethylbenzoyl phosphine oxide (Lucirin TPO) derived from MAPO, phenylbis(acyl) phosphine oxide BAPO, phosphine oxide or also called Igarcure 819. These compounds have white coloration, perfectly suitable to be applied for restore whitening teeth (Vervliet, et al. 2018, Oliveira, et al., 2015.
Aiming AP activation, different photoactivators can be used, among them quartz halogen lamp and the light emission diode (LED). The light spectrum emitted by these devices should correspond to the absorption peak of photoinitiator present into restorative material (Shimokawa, et al., 2017, Price, 2017.
The different light absorption peaks of photoinitiators present into composite resins need to be respected to ensure complete polymerization, avoiding interference with mechanical properties (Price, 2017), such as: microhardness, which is crucial to the clinical success, which is directly related to degree of conversion (DC) of the composite. When DC is high it is provided optimum microhardness, but DC is influenced by irradiance and light Development, v. 9, n. 12, e32091211128 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i12.11128 5 spectrum emitted by photoactivator, which should be relative to alternative photoinitiator used (Santini, et al., 2013, Pahlevan, et al., 2016. Manufacturers are currently inserting more than one photoinitiator into resin composites, forming combined systems, but this specification is not always exposed on the material label. Clinicians could use inappropriate photopolymerization devices and this could interfere negatively properties of composite resins since not all APs are polymerized by blue LED light (Santini, et al., 2013).
Given the above, it was aimed to perform a literature review about the influence of alternative photoinitiators used in composites resins, assessing yellowing and color stability, photosensitivity, photopolymerization efficiency and degree of conversion and microhardness.

Search Strategy
An online search was performed by two independent reviewers (NMRA and RVFD) in the PubMed database using as a search strategy with the following descriptors: (Photoinitiators, dental) AND (Curing lights, dental) AND (Composite Resins) NOT case report. The electronic search was performed to march 2019 with no language restrictions.

Eligibility Criteria and Study Selection
In vitro studies and literature reviews were considered according to the following inclusion criteria: (a) In vitro research using resins composed of the types of photoinitiators explicit in their composition. (b) In vitro research using bovine dental elements for testing; and (c) Research evaluating physical, mechanical and/or other characteristics of composite resins that contained alternative photoinitiators present. Case reports or studies that did not meet the inclusion criteria, were not considered.
After articles selection, those not directly related to the topic were excluded. Studies that met the inclusion criteria or those with inconclusive information in the title or abstract were selected for full-text evaluation in a second round of this review (Fig. 1).
After inclusion of the studies and evaluation of the full text, the data were recorded in table1. Research, Society and Development, v. 9, n. 12, e32091211128 2020 (CC BY 4. Studies not included in this review were excluded for the following reasons: o Experiment with insertion of Lucirin TPO in composite resin aiming only to improve its concentration (Miletic & Santini, 2012).

Results
Thirteen articles met the criteria previously established by our study (Table 1). The results are described below.

Yellowing and Color Stability
After aging of the composite resin, the yellowing rate over time is higher in any light curing system. But, isolated camphorquinone (CQ) systems are the most affected, thus turning yellow when compared to type I systems containing phenylpropanoldione (PPD), Lucerin TPO, or BAPO which are alternative photoinitiators that already have a lighter color, or combined systems with PPD and CQ and TPO + CQ when cured with the same LED unit (Silami, et al. 2013, Oliveira, et al., 2015. However, color stability over time is higher in composites with camphorquinone, although it has a higher tendency to yellowing. This does not directly affect the color stability of composites which use CQ as the main alternative photoinitiator (Silva, et al., 2011).
to avoid this intrinsic yellow color, avoiding compromises clinical materials properties (Shortall, et al., 2012) and still influence light curing (Schneider, et al, 2016).
For this reason, it is important to note that inadequate polymerization may decrease mechanical and physical properties into composites with only CQ (conventional) or CQ and alternative photoinitiators (composites for bleached teeth), leading to cavity retention failures, increased solubility, presence of marginal infiltration and pulp response to unpolymerized monomers. Each photoinitiator has a particular absorption spectrum, sensitive to certain wavelengths. Therefore, for the activation of the photoinitiators, the light curing devices must emit radiation near their absorption peak. If emission and/or absorption capacity are not compatible, then little or no polymerization will occur (Shortall, et al., 2012, Schneider, et al., 2016. Camphorquinone is a photoinitiator which is photoactivated when exposed to wave length of 468nm (Brandt, et al., 2011). MAPO, BAPO and phenylpropanedione need wave length of 381, 371, and 410nm, respectively to be activated, and only a portion of their absorption ranges are within the visible light spectrum (Shortall, et al., 2012, Lee, et al., 2012 Finally, degree of conversion and microhardness are closely linked to light curing. If the photoactivator unit does not emit sufficient light at wave lengths that are absorbed by the photoinitiator, then the polymerization rate may be inefficient (Neumann, et al., 2006, Brandt, et al., 2010. Thus, the monomer-polymer conversion and material microhardness are significantly reduced causing a relatively inferior material in terms of physical and mechanical characteristics. This is an important clinical factor to be observed for the correct choice of resin type (and inherent photoinitiator type), and an appropriate photocuring unit to be used.

Conclusion
Alternative photoinitiators (AP) have a lower yellowing rate when compared to isolated systems with only camphorquinone (QC) in the composition. However, color stability