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Instrument asymmetry: from behavior of rotary triple-­‐helix to Revo-­‐S.
F. Diemer *, J-P. Mallet*
*Département d’Odontologie Conservative Conservatrice et Endodontie, Faculté d’Odontologie, Université Paul Sabatier, Toulouse, France.
INTRODUCTION During cleaning and shaping a root canal, endodon=c files produce important stress. Instruments characteris=cs such as sec=on (1), blades as well as pitch length (2) influence repar==on and intensity of stresses the instrument is subjected to. The aim of this study is to compare the stress and torque of three asymmetric triple helix Nickel-­‐Titanium (Ni-­‐Ti) files used in con=nuous rotary mo=on.
METHODS Three endodon=c triple helix Nickel-­‐Titanium files (HeroShaper®, Micro-­‐
Mega) 21 mm long, ISO size #30 for =p diameter, 0,06 taper and 1.18-­‐2mm pitch length were tested. Each instrument differs by the machining brought toward the center (0%, 4% or 6%) of one face of the triple helix. The instruments were mounted on a machining center (CV 800,GSP) that simulates the root canal prepara=on in polyurethane resin block (Cibatool BM 5450, Astorit). A dynamometer records the axial stresses (Fz) and the torque (Mz) transmi`ed by the instrument. Data of each third of the progression was measured (at 5, 10 and 15mm). An analysis of the variance and PLSD Fischer’s tests for a 5% α risk with the null hypothesis that the three instruments generate the same stress levels were performed. R3
R2
R3
R3
R1
R3
Fig 1 : Centering of machining the third blade provides an asymmetrical cross secBon
RESULTS The dynamometer recorded the stresses transmi`ed by the instruments. The axial component of the stress (Fz) decreases with the centering of the third face of the triple helix, even though the torque remains constant. Fig 2: Example of Mz torque curb (N.cm) and Fz axial constraint curb (N) registered by the dynamometer. Variance analysis shows no differences with torque registered but significant differences with axial stresses since 5mm (p<0,0001). The PLSD Fischer’s test confirms these differences (p<0,0001). Fig 3: Average value of torque Mz (N.cm) and axial constraint Fz (N) at three levels of penetraBon (5, 10 and 15mm). DISCUSSION Our experimental model allows to record two value generated by asymmetrical triple helix Ni-­‐Ti instruments. The first one concerns Fz: it is about measuring the ac=on of the instrument’s blades in the block’s resin. The second one assesses the torsionnal stress which is mechanically represented by the moment (Mz) or the torque generated by the endodon=c instrument on the simulated root canal. Condi=ons of use, speed and movement are iden=cal for all samples. This protocol is similar to the one adopted by Tepel et al (3) one. The maximal penetra=on in the simulator only under the weight of the sample (5,0N) allows comparing the cudng efficiency of the instrument tested. Though these values came out in an other protocol, they could be compared to the measure of the axial component of the stress (Fz) of our model. This protocol had been validated for the study of the effect of pitch length on the behavior of rotary triple helix root canal instrument (2). The Fz value is always oriented toward the shank of the instrument and is maximal on retreat. Its registra=on is the expression of the file ac=on during the picking movement of the machine. The file tears out chips of polyurethane resin during the rotary movement and the ascent. The resin block is then lifed up. CONCLUSION Finally, the asymmetric triple helix nickel-­‐=tanium files used in con=nuous rotary mo=on allow decreasing the stresses generated. On the bases of this experimenta=on, an instrument sequence was proposed by Micro-­‐Mega with this innova=ng characteris=c. The improvement of those instruments named Revo-­‐S, based on an asymmetry of the blades, has allowed performing a simplified instrument sequence in order to answer both biological (efficient shaping and cleaning) and ergonomic (simplifica=on and safety) impera=ves which are crucial to perform ini=al endodon=c treatments in general prac=ce. REFERENCES : 1 Turpin YL, Chagneau F, Vulcain JM. (2000) Impact of two theorical cross-­‐sec=on on torsional and bending stresses of nickel-­‐=tanium root canal instrument models. J Endod. 26:414-­‐7. 2 Diemer F, Calas P. (2004) Effect of pitch length on the behavior of rotary triple helix root canal instruments. 30:716-­‐8. 3 Tepel J, Schäfer E, Hoppe W. (1995) Proper=es of endodon=c hand instruments used in rotary mo=on. Part I. Cudng efficiency. J Endod. 21:418-­‐21. .