Korean J Orthod.  2021 Jul;51(4):270-281. 10.4041/kjod.2021.51.4.270.

Effectiveness of medical coating materials in decreasing friction between orthodontic brackets and archwires

• Arici N
• Akdeniz BS
• Oz AA
• Gencer Y
• Tarakci M
• Arici S

• KMID: 2518520
• DOI: http://doi.org/10.4041/kjod.2021.51.4.270

Abstract

Objective
The aim of this in vitro study was to evaluate the changes in friction between orthodontic brackets and archwires coated with aluminum oxide (Al₂O₃), titanium nitride (TiN), or chromium nitride (CrN). In addition, the resistance of the coatings to intraoral conditions was evaluated.
Methods
Stainless steel canine brackets, 0.016-inch round nickel–titanium archwires, and 0.019 × 0.025-inch stainless steel archwires were coated with Al₂O₃ , TiN, and CrN using radio frequency magnetron sputtering. The coated materials were examined using scanning electron microscopy, an X-ray diffractometer, atomic force microscopy, and surface profilometry. In addition, the samples were subjected to thermal cycling and in vitro brushing tests, and the effects of the simulated intraoral conditions on the coating structure were evaluated.
Results
Coating of the metal bracket as well as nickel–titanium archwire with Al₂O₃ reduced the coefficients of friction (CoFs) for the bracket–archwire combination (p < 0.01). When the bracket and stainless steel archwire were coated with Al₂O₃ and TiN, the CoFs were significantly lower (0.207 and 0.372, respectively) than that recorded when this bracket–archwire combination was left uncoated (0.552; p < 0.01). The friction, thermal, and brushing tests did not deteriorate the overall quality of the Al₂O₃ coatings; however, some small areas of peeling were evident for the TiN coatings, whereas comparatively larger areas of peeling were observed for the CrN coatings.
Conclusions
Our findings suggest that the CoFs for metal bracket–archwire combinations used in orthodontic treatment can be decreased by coating with Al₂O₃ and TiN thin films.

Keyword

Thin film coating; Coefficient of friction; Bracket; Archwire

Figure

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  Figure 1 Illustration of the brushing device (A), tribometer (B), and the bracket and archwire holder of the modified tribometer (C) used for evaluating changes in friction between orthodontic brackets and archwires coated with different coating materials. In the brushing test setup (A), the toothbrush moves back and forth linearly on the brackets ligated to a flat archwire. In tribometer, the bracket is attached to a 1 cm diameter metal ball and rests on the top without moving in a fixed position (B). The bottom plate to which the archwire connected (C) is controlled by a motor that can move at the micron level and a special software. After the archwire is placed in the bracket slot, it moves linearly.

• 

  Figure 2 Scanning electron microscopy (SEM) images and atomic force microscopy (AFM) of nickel-titanium (NiTi) and stainless steel (SS) archwires. The SEM images of the uncoated NiTi (A), titanium nitride-coated NiTi (B), uncoated SS (C), and chromium nitride (CrN)-coated SS (D) clearly show that there is an increase in smoothness on their surfaces after the coating process. The AFM images show that the surface roughness of the uncoated SS archwire surface (E) decreased with the CrN coating (F).

• 

  Figure 3 Scanning electron microscopy (SEM) images and surface profilometry results for alumina (Al2O3)-coated (A, B), titanium nitride (TiN)-coated (C, D), and chromium nitride (CrN)-coated (E, F) bracket slots. The pits and irregularities on the bracket edges were filled with coating materials as shown in the SEM images (A, C, E) and profilometer images (B, D, F) of coated btacket slots with Al2O3 (A, B), TiN (C, D), and CrN (E, F).

• 

  Figure 4 Graphical representation of the coefficients of friction (CoFs) for uncoated (CON), alumina (Al2O3)-coated (AlO), titanium nitride (TiN)-coated, and chromium nitride (CrN)-coated bracket and nickel–titanium (NiTi) and stainless steel (SS) archwire combinations. Among all groups, the lowest mean CoF value was observed in the AlO coated bracket / AlO coated SS archwire group, whereas the highest mean CoF value was observed in the uncoated bracket / TiN coated SS archwire group.

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  Figure 5 Scanning electron microscopy images show the frictional wear patterns for uncoated bracket and nickel–titanium (NiTi) archwire (A, B), alumina (Al2O3)-coated bracket and uncoated stainless steel archwire (C, D), titanium nitride (TiN)-coated bracket and uncoated stainless steel archwire (E, F), and chromium nitride (CrN)-coated bracket and CrN-coated NiTi archwire (G, H) pairs. The arrows indicate areas of heavy wear that occurred on the coatings and underlying materials during friction tests.

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  Figure 6 Scanning electron microscopy images of uncoated (A), alumina (Al2O3)-coated (B), titanium nitride (TiN)-coated (C), and chromium nitride (CrN)-coated (D) bracket samples after brushing and thermal cycling tests. In areas where toothbrush contact is very high, such as the outer surfaces of the bracket wings, it appears that the Al2O3 coating (B) was not affected much by brushing and thermal cycling tests, but the TiN coating (C) was slightly affected and the CrN coating (D) was very much affected.

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