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Background  The aim of this study is to calculate and compare the play and torque expression of 0.018 and 0.022 bracket slots when engaged with archwires of different size, cross section and material.    Methods  Eight orthodontic brackets, two of slot height 0.018 and six of slot height 0.022, from different manufacturers, were measured and fixed to a vertical support. Twenty-four archwires of differing size, cross section and material were selected, measured and tested in each bracket of compatible slot width. Compression testing by Instron dynamometer and geometric calculations enabled us to determine the play angle of each bracket/archwire combination, and the angle at which a clinically efficacious force couple, sufficient for dental movement, is exerted.    Results  All bracket/archwire combinations considered were found to have play angles far above the ideal. This is ascribable to the slots being oversized with respect to the manufacturers' claims. Likewise, some archwires were found to be oversized, while others undersized.  When the same archwire was tested with brackets from different manufacturers, the play and torque expression differed, despite the same nominal dimensions of the slots. When the same bracket was tested with the same size archwires, their construction material was found to influence the torque expression, due to the difference in elastic modulus, but not the wire/slot play.    Conclusions  The dimensional precision of orthodontic brackets and archwires and the rigidity of the latter have a profound influence on the torque expression of pre-angled appliances.

Torque expression capacity of 0.018 and 0.022 bracket slots by changing archwire material and cross section