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64 — issue 2016 Endodontics Article the same reason, some instruments have a nonuniform taper, which usually is smallerinthecoronalpart,inordertogain more torsional strength in the apical part and more flexibility in the coronal part. Nevertheless, instruments with shorter working parts or nonuniform tapers need GPconeswiththesamedesignanddimen- sions in order to allow a good match be- tween the prepared canals and the obtu- rating materials. Matching instruments with nonuniform tapers with GP tapered cones The same differences in dimensions pre- viously described 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 arootcanalpreparedwithnonuniform-ta- perinstruments,theGPconewillprobably not go to working length, because of the greaterdimensionsoftheconeinthemid- dle or coronal part. This could be consid- ered GP taper lock. Thisisadifferentproblemtothatexpe- rienced by dentists in the past, which was mainly related to cone fitting in the apical part, and consequently requires a differ- ent approach. Choosing a cone with a smallertipsizemaynotsolvetheproblem, while choosing a smaller-taper cone may significantlyincreasetheriskofiatrogenic errors such as underfilling and overexten- sion of the cone through the apical for- amen,becausethetug-backinthecoronal part does not allow for correct fitting of the apical part of the cone. Therefore,thebestandeasiestsolution is to choose brand-associated GP cones that precisely fit the root canal prepara- tion achieved by the specific NiTi instru- ments and allow for ideal 3-D filling and good apical tug-back. However, with the K3XF system, clinicians could use both types of cones (i.e., the 0.04–0.06 cones or TF/TFA cones) because they will both fit the root canal preparation in the apical andmiddlethirdswell,wheretug-backand 3-D matching are more critical. More clinical hints Thus far, dimensions and sizes have been discussedtohelpcliniciansunderstandthe difficulties in matching instruments and cones. However, there are also clinical ways to seek to solve problems encoun- tered during these procedures. The fol- lowing advice may be useful for both in- struments with nonuniform tapers and many instrumentation techniques. — Create greater coronal flaring If a GP cone does not perfectlymatch the root canal preparation and thus does not reachtheworkinglength,onepossibleso- lution is to increase the coronal flaring by brushing withthe last instrument used. By doing so,the NiTi instrument will increase the dimensions of the prepared canal in the coronal part, solving the problem of GP taper lock. — Ensure correct apical fit Clinicians may experience two different clinical problems inthe apical fit:the need forabetterapicaltug-back,whichmayre- quire slightly cutting the tip of the master cone, and the fit related to the amount of canal transportation. The first situation may occur when, owing to the different dimensional tolerance, a GP cone is slightly smaller than the nominal size, in- creasing the risk of overfilling during ob- turation. In such a case, the advice is to in- crease the dimensions of the mastercone slightlybycutting 0.5–1 mm offthetip, or ideallytorecalibratethemasterconepre- cisely using a tip-snip device. This can also occur if a canal is slightly overinstru- mented (e.g., owing to an error in deter- mination of the working length or in the position of the rubber stop on the file). In such a situation, the apical constriction would have been modified and the cone fitwouldhavetocompensatefortheerror byincreasing the tip size of the GP master cone. Some NiTi instruments (HyFlex; TFA; TRUShape,DENTSPLY;NEONITI,NEOLIX; etc.) are significantly more flexible than the majority of competing NiTi rotary in- struments. As a consequence, they tend to follow and maintain the original trajec- tory of the root canals more precisely, minimizing canal transportation. Canal transportation frequently occurs when a rigid file is inserted into a curvature and tends to straighten it by cutting more in the inner part of the curvature coronally and in the outer part apically. However, this error, which can affect the quality of debridement, makes insertion of master GPconeseasier,especiallywhencomplex, double or triple curvatures are present. For this reason, clinicians using such flex- ible NiTi instruments may experience slightlymoredifficultinsertionofthemas- ter GP cone to the working length. If this problem occurs, once again slightly in- creasing flaring by circumferential filing can help. Conclusion Itmaybeconcludedthatclinicianswhouse instruments with nonuniform tapers or withreducedworkingpartsshouldprefer- ably use brand-associated GP cones that perfectlymatch withthe prepared canals. By doing so, fitting the master GP cone becomes much easier and more pre- dictable. In the very few cases in which problems still arise, the clinical hints pro- vided may help practitioners to under- stand the problem and find a proper solution. Article_Gambarini_00-00.qxp_Layout 1 02.03.16 20:48 Seite 3 Article_Gambarini_00-00.qxp_Layout 102.03.1620:48 Seite 3