Neutrons and X-rays show how to prepare durable tooth cement

The neutron images (left row) detect the distribution of liquids in this filled tooth, whereas the X-ray-CT shows the microstructure and pores in the material. A comparision of both images allows to see which pores are filled with liquids. Credit: HZB

Heloisa Bordallo und Ana Benetti, University of Copenhagen, collaborate in the development of a strong material for tooth fillings made out of glass ionomer cement. Credit: University Copenhagen

There are many ways to mix cements for tooth fillings, but it can be difficult to tell which way works best. Now, a team of scientists from Copenhagen, Denmark, has come up with an answer: They used neutron imaging and x-ray-microtomography at HZB to analyze fillings with glass ionomere cements, prepared by different methods. Their results, now published in  Scientific Reports, demonstrate how much the order of mixing steps matters to obtain a nearly homogenous filling without large liquid-filled pores which reduce stability.

Tooth decay is a serious health problem and since ancient times, cavities have been filled with a variety of materials which have to resist huge mechanical forces as well bacteria and chemicals.  One of the classics, silver amalgam filling, has the disadvantage of containing mercury, which can poison the environment. And composite materials based on acrylate do normally not last for a lifetime under the harsh conditions in the mouth.

An interdisciplinary team of dentists and materials scientists from Niels Bohr Institute at the University of Copenhagen, Denmark, decided therefore to study an alternative mercury-free material. Their choice fell on glass ionomer cement. “Glass ionomer cement has the advantage that it does not need an intermediate layer of adhesive to bond to the tooth and it also has the interesting property in that it releases fluoride, which helps to prevent cavities (caries). The material also has good biological properties, while it is almost as strong. Our research therefore focuses on understanding the connection between the microstructure of the material and its strength in order to improve its properties,” explains Ana Benetti, dentist and researcher at the Faculty of Health and Medical Sciences at the University of Copenhagen.

Glass ionomer cement also has the property that when pulverised, it can be mixed with a liquid by hand without the use of special equipment and the material does not need to be illuminated with a lamp to harden (this is necessary for composite materials). This is a great advantage in remote areas without electricity like in Africa, China or South America.

Different mixing methods

The researchers studied two kinds of glass ionomer cement. The cement itself is the same, but a mix of acids was blended into one of them. Because of that two different kinds of liquids were used to mix the cement powder, either ordinary water (for the cement with acids) or water with an acidic mixture (for the cement without acids). The question now was whether it was best to mix the acid up with the cement powder or with the water? “It is ok for the material to be porous, however if the pores contain liquids, it can be a problem, since that can make it easier for the fillings to break,” explains Heloisa Bordallo, materials researcher at University of Copenhagen. To find out, they contacted Nikolay Kardjilov and Ingo Manke at the neutron source BER II at Helmholtz-Zentrum Berlin, who are experts in 3D imaging with neutrons and X-rays. 

Neutron- and X-ray-tomography with high resolution

“Our neutron tomography instrument CONRAD II provides the highest spatial resolution worldwide, comparable to the resolution of the micro-CT with X-rays, which we can do in the lab next door”, Kardjilov explains. First they took X-rays of different teeth with the cement fillings to get an accurate image in 3D of the microstructure, showing location and size of the pores. The neutron tomography allowed then to detect the presence of hydrogen atoms and liquids inside the pores and in the material.

Strongest materials with "acidic" water

The results show that the “easiest way” of preparation with ordinary water is not best: When the cement is already mixed up with acids and just ordinary water remains to be added, the material is weakest. “We get the strongest material by having cement powder mixed with water that has had acid added to it. So it is better to have the acid in the water – it helps to bind the liquid faster and stronger to the cement and there is less water in the pores,” explains Heloisa Bordallo.

There is still too much loose liquid in the pores, so now the research is continuing with new mixtures where they will try adding natural minerals to the cement.

The scientists published their results in the Open Access Journal Scientific Reports, doi:10.1038/srep08972

You can find a nice press release by Niels Bohr Institute of University of  Copenhagen here.

arö/Uni Kopenhagen