This very unique and useful alloy has shape memory, that is, it can return to its previous shape, it also has corrosion resistance, great biocompatibility and a reasonable cost. It belongs to the group of alloys with special properties characterized by the effect of Shape memory and Super elasticity It was discovered by the metallurgical engineer William Buehlerin 1963, he named it NiTiNOL because of the elements of the alloy: Nickel (Ni), Titanium (Ti) and the name of the laboratory where it was produced (NOL) Navy Ordnance Laboratory in Maryland, United States. That is why, after the discovery of the NiTi alloy, it was introduced in dentistry. This occurs because the instrument, being rigid causes great wear on the curved wall (considered danger zones). The stainless steel, which is still used today, presents little flexibility which affects the instrumentation in curved canals, producing iatrogenic errors such as root canal deviation, foramen dislocation, and root perforation, among others. The characteristics of manual instruments can be summarized as follows: Made up of stainless steel, colored plastic handle and the increase of the standard taper of 0.02 mm per mm in the active part. The total lengths of the instruments are 21, 25, 28 and 31 mm, the color coding on the handles facilitates their identification. The diameter D2 in the active part must measure 0.32 more than the diameter D1, since the taper increase from D1 to D2 is 0.02 mm per mm of the active part and its extension must be at least 16 mm. The active part of an instrument starts at the tip and is called D1, and ends at part D2. Industry was the first to build these instruments that were known as K-type instruments, also being the most copied in the world. 28 were disclosed, and the current international standardization of the instruments was then defined. In 1981, after 26 years of studies the final norms of Specification No. 28 which presents the standards for the manufacture of files and reamers. In 1976 it was the American Standardization Association who approved Specification No. accepted the proposal of Ingle and Levine by altering its lightly and resulting to what is now known as International Standard Organization (ISO). They published the first work on standardized instruments, as well as the gutta-percha and silver cones accordingly. In 1955 John Ingle, a professor at the University of Washington USA, introduced the standardization of instruments together with Levine in 1961. The replacement of carbon steel for stainless steel instruments used today solved the corrosion problem. Among its advantages, resistance to fracture and high cutting capacity could be highlighted, but diminished by being highly corrosive due to the joint action of chemical substances such as sodium hypochlorite, negatively affecting the resistance of the file. They were manufactured without scientific criteria. The first endodontic files were made of carbon steel, whose hardness greater than dentin, prevent its use in curved canals and could cause iatrogenies such as: deformation of the apical foramen, anatomical deviation of the canal and perforations, that is due to the greater filing on the curved wall. In 1838 Edward Maynard created the first endodontic instrument starting with the spring of a clock with the objective of cleaning and widening the root canal. This entire search in order to allow the clinician the best choice in terms of evolution of instruments in endodontic to achieve a better preparation based on scientific evidence. The Exclusion Criteria were: clinical cases and reports. The inclusion criteria were: languages: Spanish, English and Portuguese any geographic area or country preferably published in the last five years and the manual search of books and magazines was also considered. The qualitative method of direct type content analysis was used the information review was based on the search information in journals indexed to Medline, Scielo, PubMed, Embase, Lilacs with the use of descriptors such as: “endodontics”, “root canal preparation”, “evolution of instruments in endodontics”, “NiTi alloys”. The purpose of this literature review article is to emphasize concepts and highlight positive and vulnerable aspects in the evolution of endodontic instruments. In the last two decades, the evolution of manual and mechanized instruments has contributed to improve the success and predictability of the endodontic treatment. The importance of chemomechanical preparation is extreme, that is why researchers have focused on the fulfillment of technical and biological objectives of pulp therapy.
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