Nickel titanium instruments for RCT Flashcards
What are traditional files made out of? What shape are they?
Stainless steel
Standard shape - ISO (International standards organisation)
What is the taper on an instrument?
The amount by which the diameter of an instrument increases from tip to handle
Taper on traditional files
Set taper of 2% (commonly referred to as ‘02 taper’
This means that for each mm from tip the diameter of file increases by 0.02mm
Sizes of traditional files
Traditional hand files are colour-coded and have a tip
size which increases in set increments
Smallest tip size is 0.06mm (an 06 file) through
0.08, 0.10, 0.15 etc, etc
The largest tip size is 1.40mm (a 140 file)
All traditional files have cutting flutes 16mm long
So due to the 2% taper, the final cutting part of the
instrument is 0.32mm wider than the tip (ie 16x0.02)
Three main configurations of traditional files
K-files
Flexible K-files
Hedstrom files
K-files shape
Created by twisting a wire to produce cutting flutes
Can be square or triangular in cross-section
Flexible K-files shape and material
Similar to K-files but their cross-sectional design
enables them to be more flexible
Can be made from stainless steel or nickel-titanium
(NiTi)
Hedstrom files shape
Made by grinding a tapered blank
Round in cross-section with a series of cones with
cutting edges
Very aggressive
(break quite easily, don’t use for apical prep)
2 main ways in which traditional files are used
Watchwind-pull
Balanced force
(& circumferential technique)
Watchwind-pull
30 degree ‘watchwinding’ each way followed by a pulling
action
Useful for both negotiating the canal and preparing the apical
third
Balanced force method
60 degree clockwise followed by 120 degree anti-clockwise
with apical pressure
Fractures off dentine which has become lodged in the flutes
Useful for preparing the apical third
Disadvantages of traditional stainless steel files
They have a tendency to produce canal
shapes which are narrow
They become increasingly inflexible in the
larger sizes
Root canals are rarely an 02 taper
The traditional preparation techniques use a lot of instruments
Must be used in a ‘reciprocating fashion – if
they are continually rotated they will fracture
The push-pull action often used with traditional files has
a tendency to create ledges
This action can also push debris into the canal causing blockages
It takes a long time to prepare a canal with traditional files/ techniques
Benefits of traditional stainless steel files
Good at negotiating canals and producing a glide-path
Nickel titanium (NiTi)
NiTi is a super-elastic metal alloy
This property provides enhanced flexibility and shapememory
This reduces the potential for canal-straightening
It also allows files to be produced with greater-taper,
whilst still retaining elasticity
Elastic deformation
Plastic deformation
Elastic modulus
Elastic: file can bend quite significantly and then will return to normal shape
Plastic: do not want to exceed file’s flexibility so it won’t go back to original shape
Modulus: NiTi is much higher
Why is NiTi so flexible?
NiTi exists in 2 forms with different
properties
Martensite
Austenite
The application of outer stress causes martensite to form
When the stress is released the martensite transforms back into austenite and the material returns to its original shape
As a result, super-elastic NiTi can be strained several times more than ordinary metal alloys without plastic
deformation
NiTi file design
Due to elasticity of NiTi and connection between diameter & stiffness, NiTi files with 2 - 6x taper are possible
NiTi files are designed to be used in continuous motion
To produce NiTi files they cannot be twisted to shape, so therefore have to be machined – this increases the cost
Most systems flatten, modify or shorten the cutting edges and vary the depth of groove, helical angle, pitch or taper to prevent the instrument from screwing and binding in the
canal wall
The tips of NiTi files still conform to the ISO tip size
standard
The tips are usually non-cutting, which allows the files to remain centred within the canal
Main difference between different NiTI files
Presence or not of ‘radial lands’
- flat area which prevents file from locking into dentine - cutting occurs through planing (acting passively) action
- some systems do not having radial lands and are more aggressive, with sharp cutting edges (acting actively)
The rake angle
The rake angle is the angle between
the leading edge of the cutting tool and the surface being cut
A rake angle can be negative, neutral or positive
Most traditional endodontic instruments have a slightly
negative rake angle
Most NiTi files have a slightly negative or neutral rake angle
NiTi grooves
The presence of grooves allows efficient removal of debris from the root canal
Most studies show that NiTi (rotary) systems are more efficient at removing superficial debris
They do however produce a thicker smear layer, particularly in the apical third
Using NiTi files in practice
Proper straight line access should initially be achieved
The files are for canal enlargement, not canal negotiation
Use a ‘crown-down’ technique
If the canal is large enough and there is plenty of space e.g. a palatal root of an upper first molar, a
glide path is already present and working length
can be established straight away without coronal
two-thirds opening
‘crown-down’ technique
Hand files should therefore be used to create a glide path up to a minimum of size 15, but ideally size 20 K-file at 2/3
working length
‘Shapers’ are then used to open the canal up to this length
Establish working length
Establish glide path to full working length
‘Shapers’ then ‘finishers’ used to prepare canal to full working length
Apex is gauged to determine size of final file
What shape do NiTi files produce?
-fixed tapers
Initially, NiTi systems used a variety of files with different taper (fixed taper of 04, 06,08,10,12)
After coronal access and creation of a glide path the different files (with different taper) were used to achieve a gradually tapering preparation, with a wider taper coronally with decreasing taper towards the apex
What shape do NiTi files produce?
-variable taper
Following fixed tapers, the concept of ‘variable taper’ was
introduced
Files no longer had one set taper, but varied the
amount of taper, starting small at the tip of the file and gradually increasing towards the shaft
This file design is sometimes referred to as the ‘upside down Eiffel Tower’
Produce a canal preparation with varying amounts of taper, but using fewer files
