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ePaper created 2017-04-26, 11:33:32 | version 1.38.0
I C.E. article_ instrumentation and obturation There are several concerns about reusing NiTi instruments. The effectiveness of disinfection proce- dures is not clear. It has been shown that protein par- ticles cannot completely be removed from machined nickel-titanium surfaces.18 Moreover, it is clear that with additional usage, the chance for instrument fracture increases. Current recommendations advise that clinicians are judicious when reusing rotary instruments as there is no conclusive evidence of disease transmission occurring. Recently, the term minimally invasive endodon- tics has been used to describe smaller-than-usual apical sizes and, perhaps more importantly, an understanding that the long-term success of root canal-treated teeth will improve by retaining as much dentin structure as feasible.3 The thought process for this was the finding that most root-canal treated teeth survive 10 years and longer.19 In studies, the reasons cited for the extraction vary but in many cases teeth are either fractured or non-restorable for other reasons.20,21 In consequence, a smaller coronal dimension of rotaries is considered while maintaining apical sizes to support antimicrobial efficacy. There currently is no direct clinical evidence to support this strategy but it is clear that root fractures pose problems in the long-term outcomes of our patients. Another recent development is the emergence of certain specialized rotaries, such as dedicated orifice shap- ers and so-called glide path files. The orifice shapers have larger tapers, such as .08, which means that they are not flexible and can overprepare at the canal orifice level. Glide path files, for example PathFiles® and ProGlider® (Dentsply Sirona), are delicate instruments and may fracture when used incorrectly. It is recommended to use a small K-file (size #10) before any rotary instrumentation and to use a delicate touch. _Clinical results While results from in vitro studies on rotary systems are abundant, clinical studies on these instruments are sparse. Comparing NiTi and stain- less steel K-files, Pettiette et al.7 found less canal transportation and fewer gross preparation errors such as strip perforations. Subsequently, using radio- graphic evaluation of the same patient group, they demonstrated better healing in the NiTi group.22 An earlier outcome study with three rotary preparation paradigms did not show any difference between the three systems with an overall favorable outcome rate of about 87 percent.23 The most consistent clinical results are obtained when the manufacturer’s directions are followed. While these vary by instrument, a set of com- mon rules applies to root canal preparation. Root Fig. 2 Fig. 2_Behavior of controlled- memory nickel-titanium rotaries compared with standard instruments. Shown are data from Typhoon Differential scanning calorimetry, which document the transition between austenite and martensite at about 5 degrees C for standard NiTi and at about 25 degrees C for controlled-memory (CM) alloy (A). At room temperature, this results in a drastically increased fatigue lifespan (B).Image A modified and reprinted with permission from Shen et al.J Endod 2011; 37:1566-1571. 08 I roots 1_ 2017 moreover, a canal curvature that is more coronal is more vulnerable to file fracture. Instrument handling has been shown to be as- sociated with file fracture. For example, a lower rota- tional speed (~250 rpm) results in delayed build-up of fatigue12 and reduced incidence of taper lock.13 Material imperfections such as microfractures and milling marks are believed to act as fracture initiation sites.14 Such surface imperfections after manufac- turing can be removed by electropolishing but it is unclear if this process extends fatigue life.15 Manufacturers’ recommendations stress that ro- taries should be advanced with very light pressure; however, the recommendations differ with regard to the way the instruments are moved. A typical recommendation is to move the instrument into the canal gently in an in-and-out motion for three to four cycles, directed away from the furcation, then withdraw to clean the flutes. It is difficult to determine exactly the apically exerted force in the clinical setting; experiments have suggested that forces start at about 1 Newton (N) and range up to 5 N.16 Precise torque limits have been discussed as a means to reduce failure. Most clinicians use torque-controlled motors, which are based on presetting a maximum current for a DC electric motor. To reduce friction, manufacturers often recom- mend the use of gel-based lubricants in dentin; however, such lubricants have not been shown to be beneficial and actually did increase torque for radial-landed ProFile® instruments.17 Therefore, it is recommended to flood the canal system with sodium hypochlorite (NaOCl) during the use of rotaries. The best way to do this is to create an access cavity that can act as a reservoir (Fig. 3).