Shear Bonding Strength and Thermal Cycling Effect of Fluoride Releasable/Rechargeable Orthodontic Adhesive Resins Containing LiAl-F Layered Double Hydroxide (LDH) Filler

Abstract

This study aims to investigate the shear bonding strength (SBS) and thermal cycling effect of orthodontic brackets bonded with fluoride release/rechargeable LiAl-F layered double hydroxide (LDH-F) contained dental orthodontic resin. 3% and 5% of LDH-F nanopowder were gently mixed to commercial resin-based adhesives Orthomite LC (LC, LC3, LC5) and Transbond XT (XT, XT3). A fluoroaluminosilicate modified resin adhesive Transbond color change (TC) was selected as a positive control. Fifteen brackets each group were bonded to bovine enamel and the SBS was tested with/without thermal cycling. The adhesive remnant index (ARI) was evaluated at 20× magnification. The fluoride-releasing/rechargeability and cytocompatibility were also evaluated. The SBS of LC, LC3, and LC5 were significantly higher than XT and TC. After thermal cycling, the SBS of LC, LC3, and LC5 did not decrease and was significantly higher than TC. The changes of ARI scores indicate that failure occurred not only cohesive but also semi-cohesive fracture. The 30 days accumulated daily fluoride release of LC3, LC5, and TC without recharge are higher than 300 μg/cm². The LDH-F contained resin adhesive possesses higher SBS compared to positive control TC. Fluoride release and the rechargeable feature can be achieved for preventing enamel demineralization without cytotoxicity.

Keywords: cytotoxicity; fluoride release; lithium aluminum (fluoride) layered double hydroxide; orthodontic adhesive resin; shear bond strength test; thermal cycling.

Figure 1

Illustration of SBS sample preparation and testing procedures. (a) Each incisor was embedded in epoxy resin and the buccal enamel was exposed by grinding on sandpaper. (b) Use etchant or primer on the bovine enamel first according to the manufacturer’s recommendations. (c) A 350 g weight was applied to each bracket perpendicular to the exposed enamel for 60 seconds. (d) The adhered bracket was de-bonded at a constant displacement rate of 1 mm per minute.

Figure 1

Illustration of SBS sample preparation and testing procedures. (a) Each incisor was embedded in epoxy resin and the buccal enamel was exposed by grinding on sandpaper. (b) Use etchant or primer on the bovine enamel first according to the manufacturer’s recommendations. (c) A 350 g weight was applied to each bracket perpendicular to the exposed enamel for 60 seconds. (d) The adhered bracket was de-bonded at a constant displacement rate of 1 mm per minute.

Figure 2

The shear bond strength (SBS) of non-fluoride releasing resin-based adhesives (XT and LC) compare to LDH-F contained adhesives (XT3, LC3, LC5), and fluoride releasing resin-based adhesive (TC) control group. Note: different letters label statistically significant between-group differences.

Figure 3

The shear bond strength (SBS) of LC, LC3, LC5, and TC before and after thermal cycling test.

Figure 4

The SEM observation of residual adhesive resin on bracket base after thermal cycling. (a) LC, (b) LC3, (c) LC5, (d) TC.

Figure 5

Daily fluoride releasing profile of (a) LC, LC3, and LC5 compared to TC. (b) Daily fluoride releasing profile of LC, LC3, and LC5 at 1–30 day (the initial release period), (c) at 31–60 day (fluoride recharge period), and (d) at 61–90 day (secondary release period).

Figure 6

The cell viability of L929 cells cultured in extract medium of LC, LC3, LC5, and TC compared to blank and positive controls (5% DMSO) at day 1 and day 3. The same lower-case and upper-case letters presents the same statistic level of cell viability at day 1 and day 3, respectively.