Endo Tribune Middle East & Africa No. 2, 2020
Maximum curve control
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A2
ENDO TRIBUNE
Dental Tribune Middle East & Africa Edition | 2/2020
◊Page A1
was observed after multiple canal instrumentations. 20 The authors also
found surprisingly high values of
cyclic fatigue resistance and safe in
vitro use in severely curved canals. In
agreement with previous researchers, Pedulla et al. reported higher
values of fatigue resistance for HyFlex EDM files (COLTENE) even when
compared with reciprocating files
made from M-wire.21 Unfortunately,
most of the available literature on
bending stiffness and cyclic fatigue
fracture resistance of NiTi rotary or
reciprocating instruments concerns
studies performed at room temperature. However, room temperature is
not a clinically relevant temperature.
Current instruments are used at
body temperature rather than room
temperature.
Fig. 3: HyFlex EDM Max Curve sequence and size breakdown.
about 6 %, and after 100,000 deformations, it is about 4 %. Within this
range, the memory effect can be observed.16
Besides stress-induced martensitic
transformation, the lattice organisation of NiTi alloys can be altered by
altering the temperature. When a
conventional NiTi austenitic microstructure is cooled, it begins to
change into martensite. The temperature at which this phenomenon begins is called the martensite
start temperature. The temperature
at which martensite is again completely reverted is the martensite
finish When martensite is heated, it
begins to change into austenite. The
temperature at which this phenomenon begins is called the austenite
start temperature. At and above the
austenite finish temperature (Af),
the material will have completed its
shape memory transformation and
will display its super-elastic characteristics.18
Before 2011, the Af temperature for
the majority of available NiTi instruments was at or below room temperature. As a result, conventional
NiTi files were in the austenitic phase
during clinical use, showing shape
memory and super-elasticity. In
2011, controlled memory (CM) files
were introduced by international
dental specialist COLTENE. These
files are manufactured utilising a
unique thermomechanical process
that controls the material’s memory,
making the files extremely flexible
and fatigue-resistant without the
shape memory and restoring force
of other NiTi files. The Af transformation temperature of CM files
has been found to be clearly above
body temperature. As a result, these
files are mainly in the martensitic
phase at body temperature.18 When
the material is in its martensitic
form, it is soft, ductile and without
shape memory, and can easily be
deformed, but will recover its shape
and super-elastic properties upon
heating over the Af temperature.
Figs. 4a–h: Tactile-controlled activation technique explaine
Moreover, a hybrid martensitic microstructure, like that used in the
HyFlex CM files (COLTENE), is more
likely to have a better fatigue resistance than an austenitic microstructure is. At the same stress intensity,
the fatigue crack propagation speed
of austenitic structures is much
faster than that of martensitic ones.
A quantitative analysis based on the
model of the fracture process zone
showed that the martensite transformation in the shape memory NiTi
alloy caused a 47 % increase in the
apparent fracture toughness.19
Very recently, CM thermomechanical processing was combined with
an innovative machining procedure
for the manufacture of rotary NiTi
files. Electrical discharge machining (EDM) results in instruments
of increased surface hardness, cutting efficiency and extreme fatigue
resistance. In the first published
paper evaluating these files, a typical spark-machined peculiar surface
was reported and low degradation
This makes most of the previous
studies obsolete and their conclusions cannot be applied in the clinical practice. It seems that the transformation temperature (Af) of rotary
or reciprocating NiTi files might alter their clinical behaviour at body
temperature. Hulsmann et al. (2019)
reported that environmental temperature has a 500 % impact on the
lifetime of instruments.22 A transformation temperature near body temperature can result in instruments
that appear to be flexible and fatigue-resistant at room temperature;
however, at clinically relevant temperatures, the instruments become
stiffer and less fatigue-resistant.
The Af of HyFlex EDM was found to
be close to 52 °C, far above body temperature. Af temperature analysis of
EDM files revealed the presence of
monoclinic martensite B19 structure
and rhombohedral R-phase.23 Therefore EDM instruments are always
in a rhombohedral R-phase and
martensitic crystallographic state at
clinically relevant temperatures. A
martensitic structure at body temperature, like HyFlex EDM, will exert superior flexibility and fatigue
fracture resistance. The extreme
flexibility and fatigue resistance of
these files, combined with the lack of
restoring force, render them ideal for
use in the instrumentation of highly
curved and complicated canals.
HyFlex EDM Max Curve
sequence
EDM made feasible the use of a single-file enlargement approach with
rotational movement. Most cases
can be shaped quite quickly, effectively and safely by using a single
25/~ HyFlex EDM OneFile with shortstroke pecking movements, frequent flute cleaning and irrigation
between the strokes. The OneFile has
a tip size of 25 with a .08 taper. The
taper is a constant .08 in the apical 4
mm of the instruments, but reduces
progressively up to .04 in the coronal
portion of the instrument. The file
has three different cross-sectional
zones over the entire length of the
working part (rectangular in the apical part and two different trapezoidal cross sections in the middle and
coronal parts of the instrument) to
increase its fracture resistance and
cutting efficiency.21 Whenever larger
apical preparations are required,
three finishing HyFlex EDM files of
constant taper can be used (40/.04,
50/.03 and 60/.02).
For constricted and obliterated canals, thin and long roots, curved canals of more than 27° and S-shaped
canals with a curvature of smaller
than 5 mm in radius, single-file EDM
shaping is not feasible. For these
challenging cases, the HyFlex EDM
Max Curve sequence was introduced
for use with the TCA technique. With
this combination, all those cases can
be handled effectively and predictably. The new HyFlex EDM Max
Curve set includes 15/.03, 10/.05 and
20/.05 files. Under the new concepts
ÿPage A3
[3] =>
Dental Tribune Middle East & Africa Edition | 2/2020
A3
ENDO TRIBUNE
◊Page A2
Figs. 5a–g: S-shaped mesiobuccal root in a maxillary second molar managed with the HyFlex EDM Max Curve
sequence and TCA technique. Pre-op radiograph (a). Post-op radiograph (b). Access cavity preparation (c). The
15/.03 HyFlex EDM file before activation (d). The 20/.05 HyFlex EDM file finishing stroke before activation (e).
Gutta-percha cones (20/.05) (f). Clinical image of the pulp floor after obturation (g).
of dentinal preservation, flaring can
be avoided in order to reduce unnecessary tissue removal from the
peri-cervical area. The HyFlex EDM
Max Curve sequence can be used
with a single-stroke TCA technique.
After canal identification and negation, a minimum glide path of 10/.02
should be achieved with stainlesssteel hand files before moving to the
Max Curve rotary sequence. After
making the 10/.02 hand file super
loose, the 15/.03 HyFlex EDM file is
used to shift the manually achieved
glide path to a smooth glide path
that all subsequent rotary files can
follow.
After the 15/.03 file has reached the
predetermined length, the 10/.05
HyFlex EDM follows in order to enlarge the middle part of the canal
safely without binding the delicate
tip. The apical 3 mm of the 10/.05 file
functions as a guiding tip (without
engaging the canal walls, Fig. 3).
The 20/.05 HyFlex EDM file follows
as a finishing file to give the final
smooth shape. Once a 20/.05 enlargement has been achieved, the canal can be filled with a 20/.05 guttapercha cone and GuttaFlow bioseal
About the author
Dr Antonis Chaniotis, Greece
Graduated from the University of Athens’s School of Dentistry in Greece in
1998. In 2003, he completed a three-year
postgraduate programme in endodontics at the same school. Since 2003, he
has owned a private practice limited
to microscopic endodontics in Athens.
For the last ten years, he has served
as a clinical instructor affiliated with
the undergraduate and postgraduate
programmes at the Department of Endodontics of the University of Athens’s
School of Dentistry. From 2012 to 2014,
he was a clinical fellow teacher at the
University of Warwick in the UK. He
lectures extensively nationally and internationally, and he has published articles
in local and international journals. He
currently serves as an active member of
the Hellenic Society of Endo dontology,
a certified member of the European Society of Endodontology and an international member of the American Association of Endodontists.
bioceramic sealer (COLTENE). The
sequence is easy to remember and
works effectively and safely even in
tricky situations.
Tactile-controlled activation
To minimise file engagement, TCA
was developed (Fig. 4a). This instrumentation technique can be defined
as the activation of a motionless engine-driven file only after it becomes
fully engaged inside a patent canal.24
TCA utilises file activation only after maximum engagement of the
flutes is reached and tactile
feedback of the anatomy is
felt. Inserting files passively
(non-activated) inside the
root canals and using CM files
that can be pre-bent before
file insertion is useful, especially when complicated canal systems are encountered
and limited mouth opening
hinders canal negotiation
and visualisation. TCA can
be divided into in-stroke and
out-stroke movements.
After accessing the pulp
chamber and locating the
canal orifices, technical patency to length is confirmed
and the canal is enlarged up
to 10/.02. The first file of the
Max Curve sequence to be
used, the 15/.03 file, is mounted on to the handpiece of
an endodontic motor and
inserted passively inside the
canal to the point of maximum frictional resistance
(point A, Fig. 4b). The file is
activated and pushed apically
(in-stroke) until the activated
file resists further advancement (point B, Fig. 4c) and
withdrawn from the canal.
After file withdrawal, the file
is inactivated and the flutes
are cleaned and checked for
any possible deformations.
Irrigation and patency confirmation follow. The second
time that the same file is inserted passively inside the canal it will bind deeper inside
the anatomy (point B, Fig. 4d).
Activating the file again the
same way will guide the file
even closer apically to length
(point C, Figs. 4e–g). The work
to be done by this file is completed when the file can reach
working length (point D, Fig.
4h) without having to activate it. After reaching working length, the second file
of the Max Curve set is used
the same way. The delicate
apical 2 mm of the 10/.05
file will always remain loose
inside the canal, guiding the
file through the anatomy
without risking engagement
and breakage. The 20/.05 that
Figs. 6a–g: S-shaped mesial rootof a manibular second molar managed with HyFlex EDM Max Curve sequence.
Pre-opradiograph revealing a deep distal carious lesion (a). Periapical radiograph with 15/.03 HyFiex EDM file
taken to working length, reaviling the anatomy after the deep distal margin elevation (b). Post-op radiograph
after completion of the root canal obturation (c). Post-restoration radiograph (d).
follows will provide the final canal
shape to disinfect and obturate the
canal.
Instrumentation to larger apical
preparations can be achieved the
same way to the desired apical instrumentation width. For challenging cases, as seen in Figures 5 and 6,
a 20/.05 enlargement might be ideal
in order to balance the clinical disinfection procedures with the risks of
damaging the challenging anatomy
or separating the instruments. The
TCA technique aims at minimising
the time of engagement with an
activated file by using file activation only when needed for advancement. With this instrumentation
technique and the HyFlex EDM Max
Curve sequence, most anatomical
root canal variations can be enlarged
safely.24
guided only by anatomy itself. The
TCA technique minimises the time
files are under engagement. This procedure maintains continuous tactile
feedback during instrumentation.
For challenging anatomies, special
sequences like the HyFlex EDM Max
Curve set help clinicians to keep on
track.
NiTi files with CM effect are extremely flexible and fatigue-resistant. They
can be activated inside the canal and
move passively around the curves
Editorial note
Conclusion
This article was originally published
in roots-international magazine of
endodontics, Issue 4/2019.
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