Orthodontic archwires

Another application of dental composites is orthodontic archwires. One example is a unidirectional pultruded S2-glass-reinforced dimethacrylate thermoset resin. Depending on the yam of glass fiber used, the fiber volume fraction varied from 32 to 74 percent The strength and modulus were comparable with those of titanium wires. Orthodontic brackets were also made from composites with a polyethylene matrix reinforced with ceramic hydroxyapatite particles, resulting in isotropic properties and good adhesion to enamel. 

Kobayashi S, Ohgoe Y, Ozeki K, Hirakuri K, Aoki H (2007) Dissolution Effect and C3dotoxicity of Diamond-Like Carbon Coatings on Orthodontic Archwires. J. mater, sd. mater, med. 18 2263-2268. 

Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic archwires, and cochlea implants. 

FIGURE 38.9 Schematic illustration of the stainless steel wire and TiNi SMA wire springs for orthodontic archwire behavior. (Modified from Wayman, C.M. and Duerig, T.W. (1990). London Butterworth-Heinemann, pp. 3-20.)... 

Orthodontics is concerned with tooth movement to optimal positions, using metallic archwires ligated to brackets bonded to enamel or dental restorations by adhesive resins, as well as using suitable other metallic appliances, to provide appropriate forces and bending moments in vivo. The force generated by a bent orthodontic wire is proportional to its elastic modulus, and relatively light and continuous forces are considered to be optimum. There is considerable interest in nickel-titanium orthodontic wires, which have the lowest elastic modulus of the major wire alloys [7]. 

The nickel-titanium orthodontic wire (Nitinol, 3M Unitek, Monrovia, CA, USA) that was first marketed [6] had a heavily cold-worked stable martensite structure [25], but superelastic wires [21] were subsequently introduced and followed [34] by wires having shape memory in the oral environment [7]. For the most efficient treatment, the archwire that is bent by the clinician for treating malpositioned teeth should return completely to the initial undeformed state during the process of tooth movement. Complete recovery will not occur for superelastic wires if there is permanent deformation [33], whereas full recovery will take place for nickel-titanium wires with in vivo shape memory. 

Three nickel-titanium wires studied by TMDSC had been previously investigated by conventional DSC [25] superelastic Nitinol SE (3M Unitek) Neo Sentalloy, which has in vivo shape memory (GAC International, Bohemia, NY, USA) and nonsuperelastic Nitinol. The Nitinol SE and Neo Sentalloy archwires had 0.016 inch x 0.022 inch cross-sections, and the Nitinol archwires were 0.019 x 0.025 inch, also a clinically popular size. In addition, the effect of permanent deformation on transformations in Nitinol SE and Neo Sentalloy was studied after bending the archwires to 135° with orthodontic pliers. 

Orthodontics is an area in which polymers are desirable for both their esthetic appeal and shape-memory effect. In 2007, Eliades published an opinion paper on projected future materials for orthodontics and discussed research into polymer-based archwires [67]. In the following year, Jung and Cho demonstrated the use of shape-memory polyurethanes for arch wires [68]. An in vitro dental model was used to test the correction of misaligned teeth and can be seen in Fig. 10. The melt spun polymer, synthesized from 4,4 -methylene bis(phenylisocyanate) and PCL-diol, was stretched to the length required to realign the teeth and attached to stainless... 

Some possible apphcations of shape memory alloys are orthodontic dental archwire, intracranial aneurysm clip, vena cava filter, contractile artificial muscles for an artificial heart, vascular stent, catheter guide wire, and orthopedic staple [Duerig et al., 1990]. 

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