Incorporation of Lucrin-TPO into Resin Based Composites
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The use of resin based composites (RBCs) continues to increase at different rates internationally, partly due to the demise of amalgam. They provide a versatile and robust restoration material  , however, in recent years the biocompatibility of dental composite restorations has come into question . Degree of conversion and degree of crosslinking are key concepts within RBC formulation as they directly impact the biocompatibility of dimethacrylate resins  - the reason for which being the potential leaching effect of individual components from RBC formulations, e.g. monomers, free radicals and photoinitiators . Evidence has shown that improvement of mechanical properties and degree of conversion of conventional photoinitiator systems, i.e. camphorquinone/amine systems (CQ) can be seen with an increase in photoinitiator concentration. However, such increase in concentration is limited by a threshold, after which no further advantages are seen. Alongside this, such an increase often warrants unwanted side-effects i.e. a yellowing effect  and potential CQ toxicity . As a result, CQ is now looked at being replaced by alternative photoinitiators such as Lucrin-TPO (TPO) due to their apparent improved degree of conversion (DOC) and degree of crosslinking (DCR)  without the need for a large increase in photoinitiator concentration whilst subsequently holding the potential to reduce the issue of component leaching from RBCs. The aim of this current study is to investigate the effects of photoinitiator chemistry on the mechanical properties of RBCs.
A total of four RBC formulations were used within this project. Of the four RBC’s two different photoinitiators were used: CQ and TPO. RBC 1 and RBC 3 were based on CQ whilst RBC 2 and RBC 4 were based on TPO. Within each photoinitiator group, alternative filler quantities were used. Table 1 provides the detailed compositions of RBCs 1-4.
Curing of composite specimens
Each sample was cured using the light engine (Lumencor, inc®) at an irradiance of 1000 Mw/cm2. The power output to attain such irradiance was calculated using a calibration curve which was obtained using a fibre based spectrometer (USB 4000, Ocean Optics, Dunedin, USA). CQ based RBCs were cured using cyan light at a power output of 70. TPO based RBCs were cured using blue light at a power output of 29. As such, the absorption spectrums of both TPO and CQ could be attained.
Degree of conversion (DC)
DC was measured using static samples via attenuated total reflectance (ATR). The DC for each RBC was calculated three times, with each sample having the following dimensions: 6mm in diameter and 2mm in thickness.
Vickers hardness testing
A silicone cylindrical hardness testing was carried out using the Vickers Hardness testing method (Duramin tester, Struers). Three samples for each RBC formulation (1-4) and two different irradiance times (9 and 20 seconds) were tested after up to 48 hours on both upper (cured) and lower (non-cured) surfaces. Each sample side was tested three times and an average acquired for each irradiance group. Every sample was tested using a load of 1.961N for a period of 10 seconds.
Depth of cure (DoC)
A silicone cylindrical mould with dimensions of: 6mm in diameter and 12mm in depth were filled with experimental composite. Three samples for each RBC and two different irradiance times (9 and 20 seconds) were obtained and then extracted from the mould. The sample was then tested using the ISO4049 method. Such method involves a scrape test whereby all ‘non-cured’ material is scraped off, and the length of the remainder of the sample is measured using a digital calliper. The obtained value was then divided by two.
For each data population the standard deviation was calculated. Alongside this, when two sets of data samples were compared against each other, a one-way analysis of variance (ANOVA) was calculated.
Photoactive resins which are cured using a light source utilise photoinitiator systems which absorb light of a certain wavelength, creating excited states which initiate the process of polymerization. It is known that several factors affect the efficacy of RBC curing, e.g. light source and irradiance time; which can be optimized to improve the mechanical properties of dental composites .
The incorporation of alternative photoinitiator systems such as TPO into RBC formulation has been proposed to produce a more optimal light cure, and thereby improving the overall mechanical properties of the RBC when compared to the more traditional photoinitiator system of CQ . According to the kinetic parameters of the present investigation, TPO has been shown to produce a higher DC (see Figure 1 and 2) and a higher DoC (see Figure 3 and 4) when compared to CQ. Alongside this, TPO has shown to produce higher hardness values (Figures 5 and 6).
The mannerism by which CQ and TPO absorb light occurs via two different pathways. CQ reacts with its co-initiator (DMAEMA) in its excited triplet state to generate one active free radical, whereas TPO cleaves directly onto the molecule in question, generating two free radicals (6). It is for this reason that despite the same irradiance time, due to the presence of a larger number of free radicals, TPO is shown to generate a higher DC compared to CQ (Figure 1 and 2). Due to the nature of the obtained DC values (via static samples), the rate of polymerization was not attained. However, looking at the difference in DC for TPO between the 1, 9 and 20 second values, the difference is fairly constant compared to those presented by CQ. This would indicate that TPO produces a more constant rate of polymerization in comparison to CQ.
In reference to the ISO 4049 method which was used to assess the DoC for RBCs 1-4, it is assumed that the leftover hardened specimen is not ideally cured, and hence this is accounted for by the division factor of 2 . It can be clearly seen (Figure 5 and 6) that TPO is shown to produce better DoCs in comparison to CQ. This could be indicative of the issue of opacity; CQ is known to have a yellow tinge in colour , and thus altering the opacity of the composite compared to TPO. Opacity is a factor which affects light transmission through the material, and such difference in colour could result in CQ producing the visibly lower DoC.
The assessment of RBC hardness post-curing is a common mechanical test used to study the curing efficacy. The obtained results are related to DC  and for this reason, it is not surprising that in this study, TPO based RBCs generated higher hardness values compared to CQ based resins (Figure 3 and 4). Alongside this, light transmission through RBCs is affected by surface reflection, absorption and scattering as a result of filler particles, thus a reason for the variation of hardness between both the upper and lower surfaces of the specimens.
The incorporation of TPO into RBCs has shown to be a promising alternative to CQ, producing higher degrees of conversion, hardness and depth of cure. It is important to understand the importance of the different pathways for free-radical formation in dimethacrylate systems. When considering the clinical applications of TPO it is imperative to consider the wavelength outputs of light curing units, which would require a shift towards the UV range.
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