Repair enamel defects with hydroxyapatite film coating



A group of Japanese researchers recently developed a process to repair enamel defects by depositing a pulsed laser deposition (PLD) process of erbium-doped yttrium-aluminum garnet (Er-YAG) to forming a coating of hydroxyapatite (HAp) film on the teeth.

To study: Hydroxyapatite Film Coating by Er: YAG Pulsed Laser Deposition Method for Enamel Defect Repair. Image Credit: Maxx-Studio / Shutterstock.com

This process was tested on a sample of bovine enamel with tricalcium phosphate (α-TCP), precursor of HAp, coated on the demineralized surface using the Er-YAG-PLD method. The resulting coating showed improved wear resistance, cell uptake and micro-Vickers hardness recovery. This study is published in the open access journal Materials.

Importance of hydroxyapatite in dental treatment

Hydroxyapatite (HAp, Caten(purchase order4)6 (OH)2) is used in a variety of applications, such as in medical and dental devices intended for implantation in living bodies. Orthodontic treatment leads to damage to the enamel during the removal of brackets due to the high bond strength between them.

Enamel rods that are decalcified during etching are eroded to form thinner enamel, and the enamel remaining on the surface of the teeth lacks strength and luster. In addition, the enamel remaining on the surface of the tooth is cell-free and hardly self-repaired after damage from tooth eruption.

Handpiece schematic graphic consisting of straightened C400F contact tip and bulk a-TCP target.

Handpiece schematic diagram consisting of straightened C400F contact tip and α-TCP bulk target. Image Credit: Chen, L et al., Materials

Therefore, it is ideal for restoring and preserving dentin using a thin layer of non-stoichiometric HAp fluorocarbonate crystal, which is the main constituent of teeth. Nevertheless, the adhesion between two ceramics is difficult to obtain due to the difference in surface roughness.

As a solution to this, the Er-YAG laser with a wavelength of 2.94 µm and a low photon energy of 0.42 eV can be used in the PLD method. As the energy of the photons is too low to directly break bonds, ablation causes spontaneous evaporation of the hydration shell of laser targets, resulting in generation of vapor and an increase in pressure followed by micro- explosion of these vapor pockets which cause mechanical rupture. laser targets and physically contribute to the ablation process.

SEM images of (a) (10 kV) a-TCP powder, deposition of the a-TCP layer by (b) (10 kV) 1 pulse and (c) (5 kV) 25 pulses of Er: YAG- PLD.  (d) (15 kV) Plate crystallites of HAp after hydrolysis by 25 pulses of PLD.  Cross-sectional SEM of HAp-coated samples by one pulse (e) (10 kV) and 25 pulses of Er: YAG-PLD (f) (10 kV) after hydrolysis.

SEM images of (a) (10 kV) -TCP powder, deposition of the α-TCP layer by (b) (10 kV) 1 pulse and (vs) (5 kV) 25 pulses of Er: YAG-PLD. (D) (15 kV) Plate crystallites of HAp after hydrolysis by 25 pulses of PLD. Cross-sectional SEM of HAp-coated samples by pulse (e) (10 kV) and 25 pulses of Er: YAG-PLD (F) (10 kV) after hydrolysis Image credit: Chen, L et al., Materials

About the study

In this study, the raw α-TCP powder was compressed using a hydraulic press under a pressure of 0.3 MPa for 60 seconds to form compacted target specimens in the shape of a 7 mm long disc and 5 mm in diameter, followed by soaking in pure water for ablation during Er-YAG-PLD.

In addition, fresh cattle teeth were cut into rectangular shapes and polished using silicon carbide sandpaper to a roughness of 4000, followed by etching with phosphoric acid. at 35% for 30 seconds and cleaning in deionized water (DI) using an ultrasonic bath for 10 minutes.

Subsequently, α-TCP coated bovine dental discs were hydrolyzed in artificial saliva at a temperature of 37 ℃ for 2 days immediately after Er-YAG-PLD to obtain HAp coated samples.

Observations

Visual inspection revealed that the α-TCP layer deposited immediately after Er-YAG-PLD had a coarse and dull appearance and was strong enough not to be removed by an airflow or ultrasonic bath. of 10 minutes. Scanning Probe Microscopy (SPM) showed a significant reduction in surface roughness after the brushing test and the formation of a smooth surface for a new HAp film coating.

Micro-Vickers hardness results indicated a significant reduction (from 329 to 164 HV) in bovine enamel hardness after etching with 35% phosphoric acid, but the hardness recovered from 89.4% and 91, 8% after deposition of the α-TCP coating and the coatings were converted to post-PLD HAp, respectively.

Scanning electron microscopy (SEM) results reveal that α-TCP and HAp coexist in the deposited coating and attachment of living HPdLF cells to the HAp film coating and bovine enamel after 24 hours of incubation. The presence of calcium on the tooth surface played an important role in the adsorption of proteins to specimens using calcium bridges.

In addition, only the α-TCP coating, which is the precursor of HAp, has been successfully converted to HAp using Er-YAG-PLD from several potential choices, such as calcium hydrogen phosphate dihydrate, α-TCP, β -TCP and diphosphoric. acidic targets. Even direct use of HAp targets failed to achieve adequate bond strength with enamel and dentin.

The temperature change during the Er-YAG-PLD process in vitro increased by 0.1 in 5 seconds and by 0.4 in 10 seconds, which was well below the temperature rise of 5.5 which can damage or even destroy the dentin pulp.

A typical SEM image of HPdLF cells attached to the surface of (a) the hydroxyapatite coating and (b) bovine enamel.

A typical SEM image of HPdLF cells attached to the surface of (a) the hydroxyapatite coating and (b) bovine enamel. Image Credit: Chen, L et al., Materials

Conclusion

The researchers used the Er-YAG-PLD treatment to deposit a HAp film coating on demineralized tooth enamel using α-TCP as a precursor and without any unwanted pulp effects related to the high temperature. Although a hardness recovery of 100% is difficult to achieve due to the complicated hierarchical structure of natural enamel, a hardness recovery of 91.8% was quite impressive.

Therefore, the Er-YAG-PLD technique is a promising choice for enamel repair in the future.

Reference

Chen, L., Hontsu, S., Komasa, S., Yamamoto, E., Hashimoto, Y., Matsumoto, N., Hydroxyapatite Film Coating by Er: Pulsed YAG Laser Deposition Method for Defect Repair enamel. Materials 2021, 14, 7475. https://www.mdpi.com/1996-1944/14/23/7475#cite

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