Resistance to sliding of titanium brackets tested against stainless steel and beta-titanium archwires with second-order angulation in the dry and wet states

Introduction: With the increased awareness of Ni allergies, alternative alloys for orthodontic products must be identified. The properties of these new products must be determined. Methods: Rectangular (0.017 × 0.025 in) stainless steel (SS) and beta-titanium (β-Ti) archwires were tested against commercially pure titanium brackets (CP-Ti, 0.018-in slot) in the dry state and with whole human saliva. Resistance to sliding (RS) was measured as a function of 5 normal forces (N, 200 to 950 cN), 32 angles (θ, –12° to +12°), and 1 interbracket distance (IBD, 18 mm). Results: With clearance between the archwire and the bracket (passive region, θ ≤θc), the frictional coefficients (μ) of the SS archwire and the CP-Ti bracket couples were 0.12 and 0.13 for the dry and wet tests, respectively; for the β-Ti archwire and the CP-Ti bracket couples, the μ values were 0.29 and 0.28 for the dry and wet tests, respectively. For an θ without clearance (active region, θ ≥θc), RS increased as a function of θ and N. To examine the rates of binding (μBI) in this active region, the value of classical friction (mean of the passive region data) was subtracted from RS to yield BI, and the value of θc was subtracted from each θ to yield relative contact angles (θr). Because of the unique relationship between the frictional and mechanical properties of these SS and β-Ti archwires tested against the CP-Ti brackets at a large IBD, the μBI values for these archwire-bracket couples were nominally equivalent (24 to 30 cN per degree). Clinical outcomes would be unaffected by this 6 cN per degree (approximately 0.2 oz-force per degree) difference. When all kinetic data in the elastic region (θr ≤5°) were combined, μBI equaled 28 cN per degree. Above this region (θr ≥5°), the data for the SS archwire and CP-Ti bracket couples were less scattered than those for the β-Ti archwire and the CP-Ti bracket couples. This demarcation from linearity was designated as θz and indicated the end of the elastic region and the beginning of the plastic region, above which sliding can eventually cease. This region (θ ≥θz), the binding region (θc ≤θ ≤θz), and the classical friction region (θ ≤θc) were described in a model. Conclusions: This model explains, in part, the equivalent values of μBI for SS and β-Ti archwires tested against CP-Ti brackets.