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ePaper created 2017-09-12, 09:26:18 | version 1.38.0
instrumentation and obturation CE article | abrading instruments are smaller with more elastic instruments, hence less preparation errors are likely to occur. outside of the curvature,9 current NiTi instruments, including reciprocating files, can enlarge the canal path safely while minimising procedural errors. Second, rotation in curved canals will bend in- struments once per rotation, which ultimately will lead to work hardening and brittle fracture, also known as cyclic fatigue. Steel can withstand up to 20 complete bending cycles, while NiTi can endure up to 1,000 cycles.4 Recently manufacturers have learned to produce NiTi instruments that are in the martensitic state and even more flexible than previous files. Figure 2 shows how instrument conditions (austenite vs. marten- site) are determined in the testing laboratory, using prescribed heating and cooling cycles.5 Heat-treated files with high martensite content typically do not have a silver metallic shade but are coloured due to an oxide layer, such as gold or blue. It is important to note that CM files frequently deform; however, with a delicate touch, cutting is adequate and often even superior to conventional NiTi instruments.6 It is imperative for clinicians to retrain themselves prior to using these new instru- ments to avoid excessive deformation and subse- quent instrument fracture. Preparation strategies Experimental and clinical evidence suggests that the use of NiTi instruments combined with rotary movement results in improved preparation quality. Specifically, the incidence of gross preparation errors is greatly reduced.7 Canals with wide oval or ribbon-shaped cross-sections present difficulties for rotary instruments and strategies such as cir- cumferential filing and ultrasonics should be used in those canals. Studies found that oscillating instruments recom- mended for these canal types did not perform as well,8 particularly in curved canals. Specific instru- ments developed to address these challenges include the Self-Adjusting File (SAF) System (ReDent NOVA), TRUShape (Dentsply Sirona) and XP Endo (Brasseler). However, there is no direct clinical evidence that these instruments lead to better outcomes. Canal transportation with contemporary NiTi rota- ries, measured as undesirable changes of the canal centre seen in cross-sections of natural teeth, is usu- ally very small. This indicates that canal walls are not excessively thinned and apical canal paths are only minimally straightened (Fig. 1), even when preparing curved root canals. While preparation usually re- moves dentine somewhat preferentially toward the Almost all current rotaries are non-landed, mean- ing they have sharp cutting edges, and they can be used in lateral action toward a specific point on the perimeter. This ‘brushing’ action allows the clinician to actively change canal paths away from the furca- tion in the coronal and middle thirds of the root ca- nal10 but may create apical canal straightening when taken beyond the apical constriction. Circumferen- tial engagement of canal walls by active instruments may lead to a threading-in effect, but contemporary rotaries are designed with variable pitch and helical angle to counteract this tendency. An important design element for all contemporary rotaries is a passive, non-cutting tip that guides the cutting planes to allow for more evenly distributed dentine removal. Rotaries with cutting, active tips such as dedicated retreatment files should be used with caution to avoid preparation errors. NiTi instrument usage As a general rule, flexible instruments are not very resistant to torsional load but are resistant to cyclic fatigue. Conversely, more rigid files can withstand more torque but are susceptible to fatigue. The greater the amount and the more peripheral the distribution of metal in the cross section, the stiffer the file.11 There- fore, a file with greater taper and larger diameter is more susceptible to fatigue failure; moreover, a canal curvature that is more coronal is more vulnerable to file fracture. Fig. 1 Fig. 1: Root canal treatment of tooth #3 diagnosed with pulp necrosis and acute apical periodontitis. The mesiobuccal root has a significant curve and two canals with separate apical foramina. (Case courtesy of Dr Jeffrey Kawilarang) roots 3 2017 17