Resistance to sliding of orthodontic brackets with bumps in the slot floors and walls: effects of second-order angulation.

OBJECTIVE: Two stainless steel (SS) bracket designs with two bumps across the slot floors and rounded slot walls and one polycrystalline alumina (PCA) bracket design with one bump along the slot floor were compared to conventional SS and PCA brackets. METHODS: For each design, the resistances to sliding of four archwire-bracket couples, two in the dry state and two in the wet (saliva) state, were measured at 32 second-order angles between -12 and +12 degrees. RESULTS: When clearances existed, the kinetic coefficients of friction of the SS brackets ranged from 0.13 to 0.15 in the dry state and from 0.17 to 0.18 in the wet state. For the PCA brackets, the kinetic coefficients of friction were 0.18 for both designs in the dry state and were 0.18 for the brackets with bumps and 0.24 for the brackets without bumps in the wet state. SIGNIFICANCE: Overall, the bumps and rounded slot walls did not reduce classical friction. When the brackets with rounded slots were modeled, the critical contact angles for binding were larger than the values based on the apparent dimensions, but were less than those based on the true dimensions. When clearances did not exist, the rates of binding were greater for the SS brackets with bumps and rounded walls than for the conventional brackets. The rates of binding increased because the rounded walls forced the archwires into complex shapes. Both PCA bracket designs had similar rates of binding; but unlike the conventional design, the brackets with the bumps did not permanently deform the archwires at large angles.