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ePaper created 2017-09-12, 09:26:18 | version 1.38.0
| CE article instrumentation and obturation There are several concerns about reusing NiTi in- struments. 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 un- derstanding that the long-term success of root ca- nal-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 ten 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 specialised 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 ca- nal orifice level. Glide path files, for example Path- Files and ProGlider (Dentsply Sirona), are delicate instruments and may fracture when used incor- rectly. 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 sys- tems are abundant, clinical studies on these instru- ments are sparse. Comparing NiTi and stainless steel K-Files, Pettiette et al.7 found less canal transporta- tion and fewer gross preparation errors such as strip perforations. Subsequently, using radiographic evaluation of the same patient group, they demon- strated better healing in the NiTi group.22 An earlier outcome study with three rotary preparation para- digms did not show any difference between the three systems with an overall favourable outcome rate of about 87 %.23 The most consistent clinical results are obtained when the manufacturer’s directions are followed. Fig. 2 Fig. 2: Behaviour 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 °C for standard NiTi and at about 25 °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.) 18 roots 3 2017 Instrument handling has been shown to be associ- ated with file fracture. For example, a lower rotational speed (~250 rpm) results in delayed build-up of fa- tigue12 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 manufacturing 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 recom- mendation is to move the instrument into the canal gently in an in-and-out motion for three to four cy- cles, directed away from the furcation, then withdraw to clean the flutes. It is difficult to determine exactly the apically ex- erted 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 dis- cussed 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 dentine; however, such lubricants have not been shown to be beneficial and actually did increase torque for radial- landed ProFile instruments.17 Therefore, it is recom- mended to flood the canal system with sodium hy- pochlorite (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).