Sliding mechanics of coated composite wires and the development of an engineering model for binding.

A tribological (friction and wear) study, which was designed to simulate clinical sliding mechanics, was conducted as part of an effort to determine the suitability of poly(chloro-p-xylylene) coatings for composite orthodontic archwires. Prototype composite wires, having stiffnesses similar to those of current initial and intermediate alignment wires, were tested against stainless steel and ceramic brackets in the passive and active configurations (with and without angulation). Kinetic coefficient of friction values, which were determined to quantify sliding resistances as functions of the normal forces of ligation, had a mean that was 72% greater than uncoated wire couples at 0.43. To improve analysis of the active configuration, a mathematical model was developed that related bracket angulation, bracket width, interbracket distance, wire geometry, and wire elastic modulus to sliding resistance. From this model, kinetic coefficients of binding were determined to quantify sliding resistances as functions of the normal forces of binding. The mean binding coefficient was the same as that of uncoated wire couples at 0.42. Although penetrations through the coating were observed on many specimens, the glass-fiber reinforcement within the composite wires was undamaged for all conditions tested. This finding implies that the risk of glass fiber release during clinical use would be eliminated by the coating. In addition, the frictional and binding coefficients were still within the limits outlined by conventional orthodontic wire-bracket couples. Consequently, the coatings were regarded as an improvement to the clinical acceptability of composite orthodontic archwires.

Stress relaxation and recovery behaviour of composite orthodontic archwires in bending.

The viscoelastic behaviour of prototype composite orthodontic archwires was evaluated using a bend stress relaxation test. Archwires having 10 different volume fractions of reinforcement were subjected to constant bending radii in a water bath at 37 degrees C for time periods of up to 90 days. The wires were subsequently released and left unconstrained for the same testing conditions. Creep-induced changes in the unconstrained bending radii of the wires were measured at specific times during both phases (stress relaxation and recovery) of the test. The statistical analysis showed that stress relaxation behaviour was strongly correlated with the archwire reinforcement level. The final relaxation varied, with decreasing reinforcement, from 2 to 8 per cent. Archwire recovery was not correlated with reinforcement level, and revealed a final viscous loss of only 1 per cent. The relaxed elastic moduli in bending of the composite wires were similar to the elastic moduli in bending of several conventional orthodontic archwire materials. Losses that were associated with the viscoelastic behaviour varied with decreasing reinforcement level from 1.2 to 1.7 GPa. Because these modulus losses were minimal, each archwire retained sufficient resilience to be applicable to the early and intermediate stages of orthodontic treatment.

Frictional characteristics of composite orthodontic archwires against stainless steel and ceramic brackets in the passive and active configurations.

The frictional characteristics of prototype composite archwires were investigated. The resistance to sliding was measured in the dry state for wires with three different volume fractions of fiber reinforcement against stainless steel, polycrystalline alumina, and single crystal alumina orthodontic brackets. Each archwire and bracket combination was tested at 34 degrees C with twelve different normal forces (from 0-400 g) and six different angulations (from 0 degrees -12.5 degrees ). The kinetic coefficients of friction were determined from the slopes of linear regressions through plots of the resistance to sliding versus normal force data. The y-intercepts of these regressions were also evaluated as indicators of the binding magnitude. The tested archwire samples were examined for wear using a scanning electron microscope. A fully factorial model analysis-of-variance showed no significant differences in the frictional coefficients for changes in bracket material, reinforcement level, or angulation. Highly significant differences were observed in the y-intercepts for changes in the reinforcement level and angulation. Highly significant, positive, and linear correlations between the y-intercepts and angulations were also established. Abrasive wear of the composite surface was observed at the archwire-bracket interface, particularly at higher normal forces and angulations. Relative to other frictional studies of metallic archwire materials, the composite archwires had higher kinetic coefficients of friction than stainless steel but lower coefficients than either nickel titanium or beta-titanium archwires against all bracket materials tested.