Operative dentistry aje qualtrough, jd satterthwaite la morrow, pa brunton

62


Chapter 3
Fig. 3.5
Coronal shaping – ledge removal at (a) gives straight line access to (b).
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Principles of endodontics


63
the instrument is based on the diameter of its tip (the diameter at D
1
)
expressed in hundredths of a millimetre. Thus an ISO size 50 file will
have a tip diameter of 0.50 mm.
ISO standardisation also uses a colour for each size, as shown
below:
Colour
Nominal Size
Pink
06
Grey
08
Purple
10
White
15
45
90
Yellow
20
50
100
Red
25
55
110
Blue
30
60
120
Green
35
70
130
Black
40
80
140
Specialised half sizes are colour-coded in the following way:
Purple/Gold
12
White/Gold
17
Yellow/Gold
22
Red/Gold
27
Blue/Gold
32
Green/Gold
37
The length of the instrument is measured in millimetres and is 
indicated on the packaging. The normal lengths available are 21, 25,
28 and 31 mm.
Endodontic instruments vary according to metal alloy, tip design,
mode of manufacture and shape of cutting flutes. There are also a
large number of files available that are non-ISO and these vary in
taper and length of cutting blade.
Alloys
The properties of root canal instruments are related to the alloy, taper
and cross-sectional configuration. Most instruments are constructed
either from stainless steel or nickel titanium.
Nickel titanium is composed of approximately 55% nickel and 45%
titanium by weight, and instruments constructed from this material
are about three times as flexible as stainless steel instruments of the
same dimensions. This flexibility facilitates the shaping of very curved
canals. Nickel titanium has shape memory; that is, when deformed, 
it will return to its original shape. This is in contrast to many other
metal alloys (for example stainless steel) that, if bent or twisted, may
remain permanently deformed. This difference in mechanical properties
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influences the method of construction. Files made from stainless steel
may be made by taking a blank of material, with a rectangular or
square cross-section, and twisting it to form cutting flutes. This is not
possible with nickel titanium due to its shape memory, so in order to
create cutting flutes, the shape must be machined from a blank rather
than being twisted.
A major advantage of nickel titanium files is that they are flexible 
at greater tapers than stainless steel and they may therefore be used 
in rotary handpieces. A disadvantage is that they are prone to fracture
and removal of separated fragments may be difficult.