In Vitro Assessment of Cyclic Fatigue Resistance, Cutting Efficiency and Surface Changes of Two Rotary NiTi /

Abstract

Cleaning and shaping of the root canal system is considered one of the essential steps in endodontic treatment. The biological objective of cleaning and shaping the root canal system is to clean and totally remove all pulp tissues, bacteria and its toxins in order to achieve good prognosis. Before the introduction of rotary nickel-titanium files (NiTi), cleaning and shaping was done using stainless steel hand –driven files. These files lack enough flexibility which resulted in procedural errors as ledge formation, canal blockage and perforation. Furthermore, instrument separation was another adverant sequelae. There are two types of instrument separation mechanisms: Torsional fracture also called ductile which occurs when there is a lock of the instrument tip or any part of the instrument in the canal while the shaft rotates continuously. Plastic deformation is considered a specific sign characterizing this type of fracture. The second mechanism is Flexural Fracture also called brittle which occurs due to metal fatigue, the instrument rotates freely in the curvature generating compression and tension cycles at the point level of maximum flexure into which fracture occurs. This repeated tension and compression cycles increase the cyclic fatigue of the instrument. Unlike torsional fracture there are no specific pattern characterizing this type of fracture. In order to improve the NiTi rotary endodontic files, many attempts have been made to improve the flexibility and the fracture resistance including introducing new alloys and new designs. Accordingly, Memory shaped wire (M-Wire) has been introduced which is characterized by improved mechanical properties to give the endodontic instruments more flexibility and resistance to cyclic fatigue compared with the conventional non M-wire NiTi wire. Protaper Next (PTN, Dentsply Tulsa Dental) is one of the systems introduced with the new M-Wire alloy characterize by variable tapers and off- centered rectangular crosssection. Meanwhile, the introduction of different designs to improve the cyclic fatigue resistance of rotary NiTi files have been exposed in the market. Protaper Universal (PTU) is characterized by having a triangular cross section, variable taper system to decrease the torsional loads as well as reducing the fatigue of the instrument and its potential breakage. The Protaper Gold (PTG) system differs than PTU by its metallurgical property which is characterized by having superior flexibility and cyclic fatigue resistance. These innovations in alloys, geometrical designs and surface treatment act on the behavior of the enlarging tools during the cleaning and shaping phase. Furthermore, the effects of these modifications on incidence of instrument separation needs to be delineated