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ePaper created 2015-03-10, 11:16:47 | version 1.33.01
issue 1/2015 — 59EndodonticsArticle cumferential filing for approximately 20 seconds was performed in each canal to increase coronal flaring. The brushing ac- tion was performed using the TFA motion and by disengaging the instrument 1mm from working length to allow lateral cut- ting with minimal stress. Afterthe shaping procedures had been completed, the used TFA instruments (SM1 and SM2) were first inspected under the microscope at 20× magnification to detect any sign of torsional failure (flute elongation ordeformation). All deformed instrumentswerediscarded,whiletherest weresubjectedtoacyclicfatiguetest.The fatigue test was performed using a device validated in a number of previous studies conducted by the author.1¯5 The instru- mentswererotatedintoacurvedcanalus- ing TFA motion until fracture occurred. The time to failure was recorded, and the mean values and standard deviation were thencalculated.Inordertodetermineany statistical difference among the sub- groups, the data were subjected to a one- way analysis of variance, with significance set at the 95% confidence level. Results The results of the cyclic fatigue tests of used TFA instruments showed no statisti- cally significant differences between the SM1 instruments of both groups. The brushingactionperformedfor60seconds did not negatively affect the instruments’ resistance. Group A SM1 instruments had a mean time to failure of 207 seconds, while Group B SM1 instruments had a mean time of 198 seconds. Incontrast,astatisticallysignificantdif- ference was found between the SM2 in- struments of the two groups. The Group B SM2 instruments showed a significantly higherresistancetocyclicfatigue(amean time of 139 seconds) compared with the GroupASM2instruments(ameantimeof 111 seconds).The reason forthis is that the coronal flaring previously provided bythe brushingactionoftheSM1filereducedthe instrumentation stress on the SM2 file. Deformation of flutes was recorded only in three Group A SM2 files and in no Group B SM2 files. A possible explanation is that the coronal flaring previously pro- vided by the brushing action of the SM1 file reduced the torsional stress of the SM2 file. Discussion The present study shows that the clinical performance of rotary and reciprocating NiTi instruments can be related to oper- ative techniques. It has been suggested thatanyNiTiinstrumentmaybeusedboth Fig.1 Fig.2Fig.3 Fig. 1 The Elements Motor. Fig. 2 The sequence of TFA small files. Fig. 3 The mandibular molar treated. as a reamer while working towards the apex and with a brushing action as a Hed- strom file while exiting the canal, to im- provecoronalflaringand/orcircumferen- tial filing. The benefit of the brushing action is not only to enhance the extent of the canal walls touched by the file and tapered preparations, but also to provide greater coronal enlargement for safer progression of the instruments in a se- quence. The suggested brushing/circumferen- tialfilingtechniqueisthereforeneededto perform better preparation of an oval canal, as shown in the clinical case. Instru- ments are indeed designed to shape a round canal while progressing to working length, but a significant amount of cut- ting/touching of canal walls on the lateral side can be performed while instruments are exiting the canal, especially in oval canals. The author demonstrated in a previous studythatthe brushing action can be per- formedforupto30–60secondswithmin- imalornoriskofincreasinginstrumentfa- tigue. The findings of the present study, whichconcernsTFAinstrumentsusedwith TFAmotion,areconsistentwithpreviously published results.6 Moreover, in the present study, it has been demonstrated that the brushing ac- tionwithanSM1fileallowedsaferprogres- siontothefinalworkinglengthofthenext (SM2) NiTi instrument in the sequence. Group A SM2 instruments (no brushing) showed a significant reduction in resist- ance to cyclic fatigue, and this can be at- tributedtogreaterinstrumentationstress comparedwithGroupB(inwhichbrushing had been performed with the previous in- strument). Deformation of flutes was recorded only in Group A SM2 files. The reason for this might be that the coronal flaring due to the brushing action of the SM1filereducedthetorsionalstressofthe SM2 file. Conclusion Thecoronalflaringprovidedbythebrush- ing action of the first instrument (SM1) reducedtheinstrumentationstressonthe second instrument (SM2) in the TFA sequence, both in terms of torsional and flexural stress. The brushing action (cir- cumferentialfiling)performedinthepres- ent study (prolonged use for 60 seconds) did not adversely affect the mechanical resistance of the instruments. Editorial note: A complete list of references is available from the publisher.