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ePaper created 2016-05-18, 11:28:18 | version 1.38.0
canal preparation and filling technique | 27 roots 2 2016 tary instruments have a nonuniform taper (e.g., Pro- Taper; and HyFlex EDM, Coltène/Whaledent) or a working part smaller than 16 mm (e.g., Twisted Files [TF], Kerr; and TFA). Tip sizes and tapers of NiTi instruments While some instruments have a nonuniform taper, the majority of endodontic NiTi rotary instruments have a uniform taper, and the associated techniques areintendedtocreateatleasta0.04-or0.06-tapered preparation. For this reason, GP cones of greater ta- pers are usually sold in 0.04 and 0.06 tapers. However,NiTiinstrumentswiththesamenominal size and taper may not have the same dimensions and consequently not create an identical root canal preparation, since the length of the working part maybedifferent(Fig. 1).Forexample,ina25.06K3XF instrument (Kerr; or other instruments, including RevoS, MICROMEGA; ProFile, DENTSPLY; and Race, FKG Dentaire), the working part is 16 mm, while in a 25.06 TF instrument, it is 10 mm. Even if the taper and tip sizes are the same, a 25.06 K3XF instrument will enlarge the root canal to 1.21 mm. This calcula- tion can be made as follows: 0.06 mm increase for each millimeter, multiplied for 16 mm = 0.96 mm + 0.25 mm tip size = 1.21 mm. In contrast, a 25.06 TF instrument (a file with a reduced working part) will enlarge the canal to a lesser extent: 0.85 mm (0.06 mm × 10 mm = 0.60 mm + 0.25 mm tip size = 0.85 mm). SimilardifferencescanbefoundbetweenanyNiTi instrumentwithaconventional16 mmworkingpart comparedwithanyotherinstrumentwithareduced working part. NiTi instruments with a shorter work- ing part are widely used because a shorter working part creates less stress during instrumentation by reducing taper lock and torsional stress in the coro- nal part, the largest section of the instrument. With a lower operative torque, efficiency and safety are moreeasilyimproved.Forthesamereason,somein- strumentshaveanonuniformtaper,whichusuallyis smallerinthecoronalpart,inordertogainmoretor- sionalstrengthintheapicalpartandmoreflexibility in the coronal part. Nevertheless, instruments with shorterworkingpartsornonuniformtapersneedGP coneswiththesamedesignanddimensionsinorder to allow a good match between the prepared canals and the obturating materials. Matching instruments with nonuniform tapers with GP tapered cones Thesamedifferencesindimensionspreviouslyde- scribed between instruments (e.g., K3XF compared with TF) can be found between 0.04-/0.06-tapered GP cones and cones with nonuniform tapers (e.g., ProTaper and TFA cones). The first few millimeters are usually similar, but in the middle or coronal part, the GP cones might be much wider. Therefore, if a 0.04-/0.06-tapered GP cone is used in a root canal preparedwithnonuniformtaperinstruments,theGP conewillprobablynotgotoworkinglength,because Fig.1: Comparison of instruments and cones with uniform and nonuniform tapers. 16 mm long; 0.06 taper SM2 TFA GP cone SM2 25.06 TFA 25.06 GP cone 25.06 K3 16 mm working part 10 mm long; 0.06 taper 10 mm 22016