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ePaper created 2017-04-26, 11:33:32 | version 1.38.0
C.E. article_ instrumentation and obturation austenitic form can withstand deformations of up to 7 percent without permanent damage or plastic deformation.4 Knowing this is critical for rotary en- dodontic instruments for two reasons. First, during preparation of curved canals, forces between the canal wall and abrading instruments are smaller with more elastic instruments, hence less preparation er- rors are likely to occur. Second, rotation in curved canals will bend instru- ments 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 colored 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 ad- equate and often even superior to conventional NiTi instruments.6 It is imperative for clinicians to retrain themselves prior to using these new instruments to avoid excessive deformation and subsequent instru- ment 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 prepa- ration errors is greatly reduced.7 Canals with wide oval or ribbon-shaped cross-sections present dif- ficulties for rotary instruments and strategies such as circumferential filing and ultrasonics should be used in those canals. Studies found that oscillating instruments rec- ommended 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 (ReDentNOVA, Raana, Israel), TRUShape® (Dentsply Sirona, Tulsa, Okla.) and XP Endo® (Brasseler, Savannah, Ga.). How- ever, there is no direct clinical evidence that these instruments lead to better outcomes. Canal transportation with contemporary NiTi rotaries, measured as undesirable changes of the canal center seen in cross-sections of natural teeth, is usually very small. This indicates that canal walls are not excessively thinned and apical canal paths are only minimally straightened (Fig. 1), even when 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. (Photos/Provided by American Association of Endodontists) preparing curved root canals. While preparation usually removes dentin somewhat preferentially toward the outside of the curvature,9 current NiTi instruments, including reciprocating files, can enlarge the canal path safely while minimizing procedural errors. 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. Circumferential 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 contempo- rary rotaries is a passive, non-cutting tip that guides the cutting planes to allow for more evenly distrib- uted dentin 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 Therefore, a file with greater taper and larger diameter is more susceptible to fatigue failure; roots 1_ 2017 I 07