The Effect of Residence Time on the Tensile Properties of Superelastic and Thermal Activated Ni-Ti Orthodontic Wires

Since the 1980s, different devices based on superelastic alloys have been developed to fulfill orthodontic applications. Particularly in the last decades several researches have been carried out to evaluate the mechanical behavior of Ni-Ti alloys, including their tensile, torsion and fatigue properties. However, studies regarding the dependence of elastic properties on residence time of Ni-Ti wires in the oral cavity are scarce. Such approach is essential since metallic alloys are submitted to mechanical stresses during orthodontic treatment as well as pH and temperature fluctuations. The goal of the present contribution is to provide elastic stress-strain results to guide the orthodontic choice between martensitic thermal activated and austenitic superelastic Ni-Ti alloys. From the point of view of an orthodontist, the selection of appropriate materials and the correct maintenance of the orthodontic apparatus are essential needs during clinical treatment. The present work evaluated the elastic behavior of Ni-Ti alloy wires with diameters varying from 0.014 to 0.020 inches, submitted to hysteresis tensile tests with 8% strain. Tensile tests were performed after periods of use of 1, 2 and 3 months in the oral cavity of patients submitted to orthodontic treatment. The results from the hysteresis tests allowed to exam the strain range covered by isostress lines upon loading and unloading, as well as the residual strain after unloading for both superelastic and thermal activated Ni-Ti wires. Superelastic Ni-Ti wires exhibited higher load isostress values compared to thermal activated wires. It was found that such differences in the load isostress values can increase with increasing residence time.

Revitalização urbana de centros históricos: uma revisão de contextos e propostas: a Ribeira como estudo de caso

This dissertation focuses on the theme of urban revitalization in historical centers. It starts from a survey of international and national concepts and experiences about the subject, in special, the historical center of Ribeira, in Natal, Brazil. It gives more attention to local management and it is dedicated to the study of objectives and actions developed, basically, by two segments that manage works in the district. Companhia Docas do Rio Grande do Norte aims at the economical development of the state and it implies the enlargement of capacity of loading and unloading of the harbor of Natal and the Natal city counsil aims at urban preservation and revitalization. It was analysed speeches ans projects related to Ribeira and it was discovered that Natal has done very little to revitalize its historical center in the last two dacades. It analyses what seemed, or at least, should be a conflict of interests between incompatible projects, revealing both conciliatory proceedings and connivance between the participants in decision making. The conclusion directs attentions to the threat to historical building in the city and the importance of enhancing discussion about the future of our historical sites