Fluoride release from orthodontic bonding agents: a comparison of three in vitro models

The aim of this in vitro study was to compare fluoride release from two cement disc models (partially varnished and unvarnished) of three orthodontic bonding materials with fluoride release from the same materials when used to bond an orthodontic bracket onto a tooth surface. A resin-modified glass ionomer cement (Vitremer) and a compomer material (Dyract Ortho) were compared with a conventional resin adhesive (Right-On). Ten discs (3.0 mm diameter and 1.5 mm thick) of each material were manufactured in a silicone mould and ten premolar teeth were bonded with orthodontic brackets using each material. All the premolar teeth were covered with nail varnish up to the bracket periphery and five of the discs for each material were coated with nail varnish on both upper and lower surfaces which halved the surface area. The fluoride released into 2 ml of deionised water, from each tooth or disc, was measured at regular intervals over 60 days. At 60 days, cumulative fluoride release was highest from the unvarnished discs, less for the varnished discs and least from the tooth-bracket model for both test materials. Despite having twice the surface area, the unvarnished disc model released only 1.2 to 1.5 times more fluoride than the varnished disc model. Compared with the tooth-bracket model, the unvarnished discs released 3.0 to 4.5 times more fluoride, whereas the varnished discs released only 2.2 to 3.7 times more fluoride. For the materials tested in this study, halving the surface of the discs did not reduce the cumulative fluoride release by half. The tooth/bracket model with minimal bonding material demonstrated substantial cumulative fluoride release. The pattern of fluoride release was similar for all models tested.