A comparison of the forces required to produce tooth movement in vitro using two self-ligating brackets and a pre-adjusted bracket employing two types of ligation.

Friction in fixed appliance systems has received considerable attention in the recent literature, although that attributable to the type of ligation used has not been fully investigated. This in vitro study of 0.022 x 0.028 inch slot Minitwin, Activa ('A' Company, San Diego, California, USA), and SPEED brackets (Strite Industries Ltd., Cambridge, Ontario, Canada), investigates friction in two forms of self-ligating brackets and in two methods of ligating Minitwin straight wire brackets with polyurethane elastomeric ligatures. The resistance to sliding of rectangular archwires through the ligated brackets was measured on a vertically mounted Instron testing machine. The stainless steel archwires used were straight lengths of 0.016" x 0.022", 0.017" x 0.025", 0.018" x 0.025", and 0.019" x 0.025" Nubryte Gold (GAC International Inc., Central Islip, New York, USA). The results showed a significant reduction (P < 0.01) in frictional resistance in the Activa brackets compared with SPEED brackets by a factor of approximately 15. When the SPEED brackets were compared to Minitwin brackets, the reduction in friction was by 50-70 per cent (P < 0.01). The placing of 'figure-of-eight' elastomeric ties increased friction by a factor of 70-220 per cent compared to conventional elastomeric ties (P < 0.01) except for 0.016" x 0.022" archwires. The results indicate that self-ligating brackets require less force to produce tooth movement because they apply less frictional contact to the archwire than conventionally tied siamese brackets.