ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

during the root canal preparation. Torsional failure occurs when the tip of the instrument
is locked in the canal while the shaft continues to rotate (2). Failure by torsional
overload was reported as the most common cause of separation of rotary NiTi
instruments (6). Flexural fatigue failure takes place when the instrument rotates inside a
curved canal and is subjected to an excessive number of tensile-compressive strain
cycles in the region of maximum canal curvature (3, 4).The mechanical behavior of the
NiTi files is affected by a variety of factors, such as size, taper, design, alloy chemical
composition and thermomechanical processing (5, 6). It has also been suggested that the
cross-sectional diameter and shape of the instruments affect the stress distribution
pattern and thus their torsional and banding behavior (7).
Studying the mechanical behavior of the NiTi instruments by finite element analysis is
an alternative to assess some aspects which are difficult to obtain in laboratory tests, as
the stress distribution. The major focus on NiTi instruments has been the experimental
evaluation of their behavior (8) and the reduced number of studies (8, 9) using a
numerical approach to evaluate the performance of these instruments does not compare
directly numerical and experimental results to confirm the reliability of their obtained
results. The aim of this study was to evaluate how the geometric and dimensional
differences of two commercially available NiTi instruments, ProTaper F1 and Mtwo,
affect their flexibility and torsional strength and compare the numerical results obtained
to the experimental ones to validate the models.
3. MATERIALS AND METHODS
Two commercially available NiTi rotary instruments with different cross-sectional
shapes were selected for this study: ProTaper finisher file F1 (Dentsply Maillefer
Instruments, Ballaigues, Switzerland) and Mtwo (VDM, Munich, Germany) of size 25,
.06 taper.
3.1 Experimental tests
One group (n = 12) of each type, F1 and Mtwo, were tested for bending resistance
according to specification ISO 3630-1 (10). The apparatus and test conditions were
similar to that described in the specification, with the instrument fixed at 3 mm from de
tip and then bent by 45° with respect to its long axis while the bending moment was
recorded. Another group (n = 12) of each type were tested in torsion until fracture,
based on ISO 3630-1 (10), using a torsion machine (AN8050, Analogica, Belo
Horizonte, MG, Brazil). For this test, the instruments were also fixed 3 mm from the tip
and the shaft’s end was clamped and connected to a reversible geared motor.
Continuous recording of torque and angular deflection was provided by a specifically
designed computer program (MicroTorque, Analogica).
3.2 Computational analyses
The computational analyses were performed using the commercial code ABAQUS 6.9-
1/Standard (SIMULIA, Providence, RI, USA).The geometries of the instruments were
obtained using a micro-CT scanner and de images were further processed using Mimics
to obtain the 3D models (Fig. 1). The superelastic behavior of the NiTi alloys is
described by means of a user-defined subroutine implemented in ABAQUS. The