New advance could lead to regrowth of decayed enamel

By Dr. Liji Thomas, MDSep 3 2019

Modern dental practice is concerned with preserving teeth rather than extracting them, even if tooth decay is present. Various types of fillings have been used, including materials like composite resins, ceramics and amalgams.

However, these often become loose in a few years because they are a far cry from the native enamel, and have a clear boundary across which demarcation is marked. Now, Chinese scientists have created a new technique which can produce a 3 mm thick layer of repair enamel which cannot be distinguished from the real thing.

Enamel is 96% fluoridated carbonate apatite crystals that are packed in a tight assembly, all pointing the same way. This gives it its unique resistance to high stress. Enamel is, however, produced only during the growth phase of the body, in childhood and adolescence. The mature tooth enamel does not contain cells, making it incapable of repairing any damage. This is one primary reason for the persistence of tooth decay.

Some methods used to remineralize decayed enamel include application of mineral-rich solutions, using peptides or proteins to hasten mineral deposition, or the use of hydrogels in the tooth cavity to form a scaffold for minerals to grow on. However, enamel grows in layers with extreme complexity under precise control – which is why it’s difficult to replicate artificially. The crystal mineral growth in enamel is covered by amorphous-phase mineral, so that it grows as one continuous layer o (epitaxy) rather than as a series of separate crystals (polycrystals). Former methods have only resulted in the formation of polycrystals and not epitaxial growth.

Replication of the complicated structure of enamel, see paper for further explanation of images https://advances.sciencemag.org/content/5/8/eaaw9569

How was the study done?

The current research appears to have cracked this problem by evolving a method to induce epitaxial enamel regeneration using a special gel to build a suitable boundary structure between hydroxyapatite (HAP) (a less complex mineral used to model enamel growth) and amorphous calcium phosphate (ACP), which is the main component of enamel.

The particles in current use are either too large to form such a crystal layer (at about 20 nm, like ACP), or too small, like calcium phosphate ion clusters (CPICs), making them unstable and tending to form clumps. This led to the use of additives to stabilize CPICs, such as liquids which can be induced by polymers to crystallize. However, the addition of such organic molecules to achieve crystallization makes the enamel weak, defeating the purpose.

The researchers therefore used a removable organic additive in the form of triethylamine (TEA), a small highly volatile molecule to prevent CPIC clumping. This novel TEA-stabilized CPIC solution remained stable for 2 or more days, unlike other CPICs. However, evaporation of the alcohol removed the TEA content completely leaving pure ACP.

HAP rods (a substrate similar to human enamel) were dipped into the CPIC-TEA solution and withdrawn. This showed continuous ACP crystallization occurring on the HAP. The HAP then continued to grow epitaxially without leaving any gap between the ACP and the HAP, so that the crystalline phase is now covered with a single-crystal amorphous-phase layer even as it grows. This exactly mimics the natural process of enamel mineralization. The key to this success is achieving a continuous crystalline-amorphous interface unlike the traditional 20nm ACP used so far, which leaves a discontinuous boundary between the substrate and the particles.

The CPIC is compatible with enamel, as shown by the excellent wetting of enamel with the solution forming a continuous coating. High-resolution transmission electron microscopy (HRTEM) confirms that the enamel and the ACP exist and behave as a continuum across the boundary. The initially deposited ACP precursor layer on the enamel slowly changed to HAP until only a perfectly crystalline HAP layer is visible.  The final enamel has even better mechanical strength, frictional coefficient and wear resistance than the native enamel.

What did the study show?

The crystallization of HAP in epitaxial manner from ACP exactly replicated the natural structure and strength of native enamel. The study also showed that the characteristic fish-scale shape of natural enamel structures could be achieved using this crystallization process. Gratifyingly, the CPIC replicated this structure within 48 hours, such that scanning electron microscopy failed to detect a seam between the natural and repaired enamel. This technology is thus capable of repairing whole teeth in vivo.

TEA is is not toxic below 62.5 mg/day and is widely used in pharmaceuticals. Moreover, TEA can be volatilized completely by the evaporation of ethanol.

At present, this technique can be used to grow only a thin layer of enamel, about 3 microns thick, beyond which epitaxial growth in the ACP layers stops. To increase the thickness, other methods must be tested such as a more stable form of ACP or better directional crystallization. However, a simpler technique could be simply repeating the process to thicken the repair layer in steps. The results must now be confirmed and the technique tested for safety in humans before clinical trials are set up.

The strengths of this technique are the exact replication of the complex enamel structure with identical or better mechanical performance, by setting up a mineralization boundary along which epitaxial continuous crystallization can occur. This approach could be extended to other structurally complex materials to design a range of bioinspired materials.

Researcher Zhaoming Liu says, “We hope to realize tooth enamel regrowth without using fillings which contain totally different materials and we hope, if all goes smoothly, to start trials in people within one to two years.”

The study was published in the journal Science Advances on August 30, 2019.