Orthodontic brackets attached directly to the tooth are retained by a:

Bonding of resins (bracket-adhesive) to etched enamel is based on micromechanical interlocking produced by the resin tags that are formed by the penetration of the resin into micro-porosities created in the surface of the enamel after acid etching. The essential components of composite resin are a resin matrix, filler particles, a coupling agent and a curing system. ​Key comment: The chemical method of bonding tried using GIC failed to withstand the compressive, shear stress in orthodontic bracket bonding.  Textbook of ORTHODONTICS Sridhar Premkumar