Design features of rotary root canal instruments

1. Prepared by:
Hashim M. Hussein
M.SC. of Conservative Dentistry
Alhakam A. Ibrahim
B.D.S
Design features of rotary
root canal instruments

2. Introduction
 Endodontic instruments were originally
made from carbon steel. Later on S.S
employed and since 1988 endodontic
instruments fabricated from NiTi, but
because of extreme flexibility of NiTi, they
are not designed for initial negotiation of
the root canal.
 When using the S.S files, occurrence of
procedural errors cannot be avoided
especially in case of curved canals.

3.  The super elasticity and shape memory of
NiTi alloy allows these instruments to flex
more than the S.S instruments before
exceeding their elastic limit, thereby
allowing canal preparation with minimal
procedural errors.
 Other advantages associated with NiTi
include its biocompatibility and high
resistance to corrosion.

4.  Originally, NiTi endodontic instruments
were designed for manual instrumentation.
 Then, the use of NiTi instruments coupled
to electric motors has allowed the
combination of speed and quality during
endodontic treatments, thus reducing
working time and consequently increasing
productivity and comfort for both the
operator and the patient.

5. Advanced NiTi alloy
 Possible strategies to increase efficiency and safety
of NiTi rotary include an improvement in the
manufacturing process, or the use of new alloys that
provide superior mechanical properties.
 A new NiTi alloy, termed the M-wire (Memory
shape wire) (Dentsply Tulsa Dental Specialties, OK,
USA) was developed in 2007.
 Before the grinding process, the alloy is thermally
treated to improve its properties.
 The final goal is to produce instruments with
greater flexibility and increased resistance to cyclic
fatigue, compared to those constructed from
traditional NiTi alloy.

6.  From the practical point of view, NiTi “shape
memory metal alloy” can have three different
forms:
1. Martensite: it is soft and ductile, and it can be
easily deformed.
2. Stress-induced martensite (superelastic): it is
highly elastic (rubber-like).
3. Austenite: it is quite strong and hard.
The transition from the austenitic phase to
martensitic phase can occur as result of temperature
and stress, e.g. during root canal preparation, and
after the release of stresses, the metal returns to the
austenitic phase, and the file reveal to its original
shape.

7.  This phenomenon is called stress-induced
thermoelastic transformation. Due to its
pseudoelastic properties, this material can
survive deformation without reaching the
elastic limit, thus returning to its original shape.
 Studies have shown that M-wire technology
significantly improves the resistance to cyclic
fatigue by almost 400% compared to
commercially available 25/04 NiTi files.
 Recently, NiTi rotary instruments made from a
NiTi controlled memory (CM) wire have been
introduced. The manufacturer claims that these
instruments have flexibility and fatigue
resistance superior to conventional NiTi rotary
instruments made from superelastic wire.

8. Design features of NiTi rotary endodontic
instruments
• Design features of any root canal instrument are
determined primarily by the aim and the method of its
use.
• There are a vast number of different root canal
instruments and systems. From time to time, new
instruments or modifications of the existing systems
appear. It is not always easy for dental practitioners to
be familiar with the whole variety of options.
• Nevertheless, understanding the basic features of root
canal instruments and their operation principles can
help the clinician use these instruments more
effectively, and significantly reduce the probability of
mistakes.

10. • Tip is the element of the working part that performs
the guiding function. The cutting part is the prime
element of the working section, which has cutting
blades that perform the enlargement of the root canal.
• The tip might have a sharp or rounded configuration,
depending on whether it appears:
 Active.
 Passive.
Tip

11.  An active tip has cutting edges on its surface, which
are made for the removal of dentine or obturation
material from the root canal.
 Most instruments with an active tip allow for removal
of obturation material during retreatment.
 One of the prominent disadvantages of NiTi is the lack
of tactile feedback. Therefore instruments with an
active tip require special care when operated because
of significant risk of perforation when deviation from
canal axis occur, due to insufficient instrument
flexibility or the presence of obstacles in the root
canal (hard obturation material, separated instrument
or ledge).

12.  A passive tip does not have cutting edges and
does not possess cutting properties.
 A passive tip reduces the risk of instrument
deviation from the canal axis, and as a
consequence the risk of transportation or
ledge formation.
 The majority of NiTi root canal instruments
have passive tips.

13.  Taper is described as the amount of file diameter
increase per millimeter along the working surface
from the tip toward the file handle.
 In the past, as an ISO, a hand file was fluted and
tapered at a constant 2% for 16mm. New rotary
files incorporate a wide variation of constant or
variable tapers at different lengths of working
surface.
 Most instrument tapers are fixed (meaning they
increase at a standardized, consistent rate from
the tip of the file up to the end of flutes). These
fixed taper file systems range from 0.02mm taper
to 0.12mm.
Taper

14.  Some of the newer instruments have a
variable taper built into the instrument.
Thus within a single instrument the taper
varies, sometimes starting at a specific
taper and then altering the tapers as it
travels the shank so as to improve
efficiency in its cutting.
 It has been reported that instruments with
progressive taper can shape canals more
quickly than constant taper instruments.

16.  The fluting is a specific surface with a
certain configuration, which is created on
the working part to impart the cutting
ability to the instrument.
 In general, the fluting is formed by
grinding out a groove of a specific profile
onto the cylindrical or conical NiTi blank
with appropriate diameter.
 As a result of the grinding process,
adjoining flutes form the cutting blade.
Flute

17. • The fluting is characterised by the following parameters:
1. Helical angle.
2. Pitch.
3. Depth of fluting.
4. Configuration of fluting.

19.  It is the angle formed between the blade
and the long axes of the instrument.
 Variable helical angles (HA) are an
important aid to moving debris up and
out of the canal.
 Additionally, a constant HA file is more
prone to debris accumulation. This
debris accumulation can lead to the
need for increased torque, which can
lead to potential separation.
Helical angle

21.  Pitch is the distance between a point on
the leading edge and the corresponding
point on the adjacent leading edge.
Pitch
 It is very important
because a constant pitch
will work much like a
wood screw and pull you
into the tooth.

22.  A variable pitch will significantly decrease
the tendency of the file to get sucked down
into the tooth.
 In addition, a smaller pitch distance would
give more resistance to the file and less
cutting efficiency.

23.  The rake angle (RA) is the angle formed by the
leading edge and the surface to be cut. The RA can
be negative, neutral or positive.
Rake angle
 If the angle formed by the
leading edge and the
surface to be cut is acute,
the RA is said to be
negative.

24.  It is well known that the cutting efficiency of a
file depends upon the rake angle of its cutting
blades. Since dentine is a dense and resilient
material, instruments having a negative RA
are less efficient and require more energy to
cut dentine than files with a positive RA.
 Most endodontic instruments have a slightly
negative or substantially neutral RA. The result
is a scraping rather than cutting action.
 The ideal RA is slightly positive because an
overly positive RA will result in too actively
cutting of dentine and probably threading.

25.  The surface with the greatest diameter that
follows the groove (where the flute and land
intersect).
Cutting edge
 The cutting edge
forms and
deflects chips
from the wall of
the canal and
snags soft tissue.
Its effectiveness
depends on its
angle of
incidence and
sharpness.

26. • It's a flat area that is located directly
behind the cutting edge of the instrument.
Radial land

27.  The land touches the canal walls at the
periphery of the file and reduces the
tendency of the file to screw into the canal,
reduces transportation of the canal, reduces
the progression of microcraks on its
circumference, supports the cutting edge; and
limits the depth of cut.
 Radial lands on rotary files will increase
lateral resistance. Increased resistance
will results in increased torque
requirements which is not a good thing for
rotary files as this will elevates the danger
of instrument fracture.

28. Radial-landed
instrument cross section
Non-landed instrument
cross section
• So the lack of this area allows the instrument to be
sharper and consequently more efficient, in addition
this will results in a decreased thickness of metal.
• The result of less metal is a dramatic increase in
flexibility. Theoretically, the radial land improves
irrigation flow apically and the movement of debris
coronally.