What Zinc concentration in teeth reveals

Teeth are composites of mineral and protein, with a bulk of bony dentin that is highly porous. This structure is allows teeth to be both strong and sensitive. Besides calcium and phosphate, teeth contain trace elements such as zinc. Using complementary microscopy imaging techniques, a team from Charité Berlin, TU Berlin and HZB has quantified the distribution of natural zinc along and across teeth in 3 dimensions. The team found that, as porosity in dentine increases towards the pulp, zinc concentration increases 5~10 fold. These results help to understand the influence of widely-used zinc-containing biomaterials (e.g. filling) and could inspire improvements in dental medicine.

Teeth have a complex structure: The dental pulp with the nerves is surrounded by dentine, a porous bony material, covered externally by enamel in the mouth and cementum in the roots. Although dentine is criss-crossed by countless micrometre-sized dentine tubules, teeth can withstand decades of cyclic, repeated forces. The density of the dentinal tubules increases towards the pulp, meaning that the dentine becomes increasingly porous towards the inside. Dentine tubules conduct stimuli to the nerves and enable cold and hot sensations, via connection to cells located in the pulp and root canals. Similar to bone, dentine contains mostly calcium and phosphate (in nanocrystals) as well as organics and a range of trace elements that are normally homogeneously. But the trace element zinc is almost non-existent in some regions, yet quite prevalent near the pulp Up to this study, it was not known what the concentration of natural Zinc is, and how it is distributed in healthy teeth.

Ideal teeth

The team, led by Prof. Dr. Paul Zaslansky, Charité Berlin, and Dr. Ioanna Mantouvalou, HZB set out to solve this question, only to realise that discarded human teeth are usually contaminated with zinc from treatment or toothpaste. They therefore made use of cattle teeth, widely discarded from slaughterhouses. These show e.g. through infrared studies performed with the IRIS team of BESSY II a high similarity to human teeth, being only much younger and without histories of dental treatment or toothbrushing.

Where is zinc?

‘First, we made fine cuts and examined them under a scanning electron microscope. These images revealed that the material between the individual tubules is almost perfectly homogeneous,’ says HZB SyncLab responsible Mantouvalou. The team then examined the teeth using different dental and industrial 3D X-ray tomography systems to map their three-dimensional microstructure, in particular density. As expected, tooth density decreases near the pulp, corresponding to an increase in tubules. These findings made it possible to model the material correctly and evaluate the experimental data from maps of micro-X-ray fluorescence spectroscopy. ‘We recorded the signals from many elements, in particular calcium, phosphorus and zinc. While calcium and phosphorus, both of which originate from the nanocrystals of dentine, are distributed evenly, we observed and quantified a very sharp increase in the zinc concentration from the outside to the inside, i.e. towards the pulp,’ says Mantouvalou.

Useful insights

‘These results are very helpful for the further improvement of dental care, for example whether the dentist should recommend low or high zinc containing materials during treatment,’ explains Zaslansky. In healthy teeth, zinc is enclosed in the dentine. However, contact with acids, whether through caries or through root canal treatment with zinc-containing pastes, could potentially chemically activate enzymes, with possible negative effects. ‘Studies on human teeth are needed to confirm our bovine-tooth based hypothesis’.  

Zinc as an indicator

Another important point: The study shows that Zinc could serve as a good proxy to determining bony-material mineral density. ’Bone density is a huge concern for many patients: everyone knows that we want calcium and more mineral for bones to be strong. But actually, maybe what we want is a good balance of micro porosity?’ suggests Zaslansky and concludes : ‘We find unexpectedly that Zinc can likely be used as a very sensitive measure of gradients in material density, which may change over the lifetime. Density is linked to mechanical competence of bony tissues, and should neither be too high or too low, to serve in the human body. With high sensitivity methods such as X-ray fluorescence, we may be able to take samples and monitor density changes with ageing, for example due to use of well-chosen dental fillings or oral pastes.’

Note:

The collaboration took place as part of the DFG-funded programme ‘FOR 2804: Materials Science of Teeth in Function’.