Resistance to sliding of orthodontic appliances in the dry and wet states: influence of archwire alloy, interbracket distance, and bracket engagement.

Abstract

Having established dimensional and mechanical characteristics, the resistances to sliding (RS) were measured in vitro for various archwires against stainless steel brackets. Using stainless steel ligatures, a constant normal force (300g) was maintained while second-order angulation (straight theta) was varied from -12 degrees to +12 degrees. Using miniature bearings to simulate contiguous teeth, five experiments each were run in the dry or wet states with human saliva at 34 degrees C as a function of four archwire alloys, five interbracket distances, and two bracket engagements. Outcomes were objectively analyzed to establish when theta=0, and the relative contact angles ( theta(r)) were replotted. Critical contact angles (theta(c)) that were determined via experimentation were in good agreement with theory. Slopes and y-intercepts were tabulated from linear regression equations of RS against theta plots in both the passive (theta < or = theta (c)) and active ( theta > or = theta(c)) configurations, for which theta = theta(c) identified the boundary between classical friction and binding phenomena. Stiffer archwires and shorter interbracket distances exacerbated binding, whereas, once corrected for differing bracket engagement, RS was independent of slot dimension. Unlike earlier results in the passive configuration, in the active configuration couples comprised of titanium alloys (NiTi and (beta-Ti) had higher RS values in the wet versus the dry state. For those archwire alloys evaluated, two empirical expressions were adduced that comprise the binding component, the yield strength or elastic limit, and the beam length, which implicitly represent the stiffness, flexibility, and interbracket distance.