(a) Increased critical surface tension is seen with which orthodontic bracket ? Ceramic Polycarbonate Stainless steel Ni Ti Ans. (C). Stainless steel Orthodontics Materials: scientific and clinical aspect: Brantley First edition Page 414. Bracket Material : Critical surface angle ( Dyne/cm) Stainless Steel :40.8 Polycarbonate:32.8 Alumina : 29.8 The higher the surface tension of orthodontic bracket suggest high plaque retaining capacity. So stainless steel have high plaque retaining capacity Q.57. (b) Which contemporary orthodontic wire has maximum surface friction? Stainless steel wire AJW wire TMA wire Niti Wire Ans. (C). TMA wire The surface roughness results and the studies on frictional forces between archwires of various types and brackets suggest complex interactions. The frictional force is higher for TMA wire than for SS and NiTi wires [17-20], which can be attributed to a “cold-welding” feature of TMA wire leading to a repeated stick-slip movement of the bracket relative to the archwire . However, the present study found that the surface roughness of TMA wire was only around one-third those of NiTi wire and LH wire, which indicates that the frictional force between the bracket and archwire cannot directly be predicted from the surface roughness of the archwire. J. R. Bednar, G. W. Gruendeman and J. L. Sandrik, “A comparative study of frictional forces between orthodontic brackets and arch wires,” Am. J. Orthod. Dentofacial Orthop., 100: 513-522, 1991 Which of the following metal shows the property of Twinning: Cobalt chromium alloy Ni-Ti alloy Stainless steel alloy All Ans. (B). Ni-Ti alloy An alternative mode of permanent deformation in metals is twinning, in which there are small atomic movements on either side of a twinning plane that result in the atoms having a mirror relationship, In metals that have relatively few slip systems, twinning is favored over dislocation movement; this mechanism is also favored at high strain rates and at low temperatures. “Twinning has significance for deformation of a-titanium alloys, which are highly important for some dental implants and also of interest for cast restorations. Ref:- PHILLIPS In a-titanium the (cia) ratio involving the lattice parameter (a) in the basal plane and the lattice parameter (c) in the perpendicular direction to the basal plane (see 2-14) is slightly less than the ideal value of 1.633 for the hcp structure. This deviation results in additional slip planes and the tendency to readily undergo twinning. Twinning is also the mechanism for reversible transformation between the austenitic and martensitic stnictures in nickel-titanium orthodontic wires, which has considerable clinical significance
Posted on: Wed, 16 Oct 2013 09:57:19 +0000
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