This Bioactive Filler Had Better Polymerization Kinetics, Study Says

Bioactive composite materials help prevent secondary caries in a restored tooth. However, the additives that benefit the hard tissues and optimize the tooth also affect the polymerization process, not in a good way. An October 2021 Study in Scientific Reports looked at a lower sodium version of a bioactive glass that might have the benefits of a bioactive filler without inhibiting polymerization results or rates.

The study, "Polymerization kinetics of experimental resin composites functionalized with conventional (45S5) and a customized low-sodium fluoride-containing bioactive glass," investigated how the 2 bioactive glasses (BG) performed. One is the BG 45S5, and the other is a low-sodium (Na Fluoride (F)-containing BG). While the BG 45S5 did slow the polymerization rate and inhibit the final results, the Low-Na F-containing BG showed a negligible influence compared to controls.¹

The following summarizes who did the research, why, what they thought would happen, how they did it, and what they learned.

Who Did The Research?

• Matej Par, Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Zagreb, Croatia
• Katica Prskalo, Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Zagreb, Croatia
• Tobias T. Tauböck, Department of Conservative and Preventive Dentistry, Centre for Dental Medicine, University of Zurich, Zurich, Switzerland
• Hrvoje Skenderovic, Institute of Physics, Zagreb, Croatia
• Thomas Attin, Department of Conservative and Preventive Dentistry, Centre for Dental Medicine, University of Zurich, Zurich, Switzerland
• Zrinka Tarle, Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Zagreb, Croatia

Why Did They Test the 2 Bioactive Glass Fillers for Their Effect on Polymerization Kinetics?

Bioactive materials are useful in dentistry. The restorative material's ability to positively affect the surrounding dentition is a feature that piques many dentists' curiosity.

A bioactive material evokes a response from the living tissue, organisms, or a cell, including the formation of hydroxyapatite, per the International Journal of Contemporary Dental and Medical Reviews. The ideal properties of bioactive materials are that it kills bacteria as well as prevent their formation, it is sterile, it stimulates dentine formation, and it maintains the pulp of the tooth. Clinicians use bioactive materials for pulp capping, permanent restorative dentistry, dentinal tubule occlusion, regenerating bone, and tooth remineralization.²

Bioactive materials can help prevent secondary caries that cause composite resin restorative failures, sometimes in otherwise sound restorations. So, materials development teams started putting reactive additives to protect the hard tissue of a restored tooth from decay. By discouraging bacteria formation to inhibiting and promoting remineralization to improve the restorative seal, these additives provided many benefits to restorations. The bioactive glasses are one the most capable ones.

However, some bioactive glasses (BG 45S5) used have inhibited polymerization kinetics, both in the result and polymerization speed. The rate of polymerization kinetics affects the resulting polymers chain length, composition, distribution, and tacticity.³ The polymerization rate is essential to optimize because all the polymer's properties, chemical, mechanical, and physical, depending on the tacticity, a term that describes the arrangement of the groups on the hydrocarbon chain.⁴

Because of the benefits of the BG on the composites, the researchers wanted to determine if the alternative for BG 45S5, Low-Na F-Containing BG, would provide the benefits without adverse effects on the polymerization kinetics of the materials.

In other words, they wanted to see if they could keep all the pros of the initial BG material and eliminate its cons by replacing it with a new experimental version.

What Did They Think Would Happen?

The researchers hypothesized that:

1. The polymerization kinetics would not affect the composite layer's type, amount, or thickness.
2. The curing light's transmittance through 2mm would not be affected by either type or amount.

How Did They Test Them?

The researchers started by using a similar preparation and grinding for the fillers to ensure similar particle sizes of the BG filler materials and the control. Then, they mixed the BG fillers in a 60:40 mixture of bisphenol-A-glycidyl methacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA). Then, they added the photoinitiators, camphorquinone, and dimethylamino benzoate and blended them in a dark container for two days. Alongside the ones they mixed just for the experiment, they also used Charisma Classic (Kulzer) as a reference.

The researchers tested the polymerization kinetics using a degree of conversion measurements using a Fourier transform infrared Spectrometer and an attenuated total reflectance accessory. The materials were placed in 6mm diameter thicknesses of .1mm and 2mm thicknesses and cured for 20 seconds. Then, they collected the FTIR spectra continuously for 5 minutes. Five specimens of each type of composite were tested.

Light transmittance was measured during light curing. First, the team collected the light passing through the specimens with a lens directed into a charge-coupled device array fiber spectrometer. Spectra were gathered at a rate of 20-seconds, the time of light curing. Then they divided this measurement by the light intensity of the empty specimen compartment to get the light transmittance percentage.

Then, the research team did a statistical analysis of the data to determine their results.

What Did They Learn?

The researchers determined that the 2mm thick specimen reduced the light transmittance in both BG types. However, they only observed a reduction in polymerization rate and degree of conversion in the experimental composites with BG 45S5, not Low-Na F-containing BG. Moreover, that rate was affected by the amount of BG 45S5 present in the material. The more of it there was, the more the polymerization rate decreased, as did the degree of conversion.

On the contrary, Low-Na F-Containing BG showed a viable and beneficial filler for light-cured dental resin composites. Furthermore, it showed a negligible influence on polymerization. Although it did result in lower light transmittance, the presence of Low-Na F-Containing BG did not impair the polymerization kinetics.

By comparing the polymerization kinetics between the .1mm and 2mm specimens, the researchers also learned that improving light transmittance for BG 45S5 did not improve polymerization kinetics. This analysis led researchers to believe that the BG 45S5 material directly impacts the polymerization reaction.

So, does that mean that materials developers will switch to a new BG in future materials? It might; it might not. Time will tell. However, the science published here indicates that making a switch to a Low-Na F-containing BG for a filler load in a bioactive resin composite is an opportunity for materials manufacturers to move forward.

References

1. Par, M., Prskalo, K., Tauböck, T.T. et al. Polymerization kinetics of experimental resin composites functionalized with conventional (45S5) and a customized low-sodium fluoride-containing bioactive glass. Sci Rep 11, 21225 (2021). https://doi.org/10.1038/s41598-021-00774-w.

2. Snehal Sonarkar, Rucheet Purba, "Bioactive materials in conservative dentistry," Int J Contemp Dent Med Rev, vol.2015, Article ID: 340115, 2015. DOI: 10.15713/ins.ijcdmr.47

3. What is Polymerization?. Bartlby.com. Accessed February 14, 2022. https://www.bartleby.com/subject/science/chemistry/concepts/polymerization-kinetics.Jankowski, BSChE, Patricia. Tacticity of Polymers: Definition, Types & Examples. Accessed February 14, 2022. https://study.com/academy/lesson/tacticity-of-polymers-definition-types-examples.html.

4. Jankowski, BSChE, Patricia. Tacticity of Polymers: Definition, Types & Examples. Accessed February 14, 2022. https://study.com/academy/lesson/tacticity-of-polymers-definition-types-examples.html.