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

Nickel titanium rotary instruments
The use of nickel titanium instruments reduces the risk of procedural
errors during cleaning and shaping, such as blocks, ledges, transporta-
tions and perforations (see later). However, their use is associated
with an increased incidence of instrument separation.
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Chapter 3
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Principles of endodontics


67
There is a wide range of instruments which vary in cross-sectional
configuration to some degree. Most have a radial land, that is, a flat
planing surface (in contrast to the sharp planing/cutting flutes of
twisted hand files), which centralises the instrument in the canal and
prevents the instrument from cutting into the canal walls. Cutting of
dentine is achieved by engagement of the flutes in the canal walls,
debris being theoretically extruded in a coronal direction. Recom-
mendations regarding techniques vary according to the system, but
the basic principles remain the same in that the canal should be 
rendered free from infected material and shaped in such a way that 
it may be adequately obturated. As is the principle for hand shaping
procedures, coronal opening should precede apical and mid-root
shaping.
Electronic apex locators
The latest generation of these devices is useful in determining the
position of the apical constriction. Many recent models measure
impedance rather than electrical resistance, and are less sensitive to
ionic solutions within the canal. They should therefore function in the
presence of NaOCl, EDTA solution, blood or pus.
Following coronal flaring, one lead from the device is attached to an
earthing clip and is positioned on the patient’s lip to complete an elec-
trical circuit. The floor of the access cavity and 2–3 mm into the canal is
dried. The other lead is clipped to the top of a file. The machine is
switched on and the file is slowly advanced into the canal. Some
devices show a digital display as the apex is reached and emit an 
audible bleep. Other devices have a visual display indicating the 
position at which the end point is reached. The figures should not 
be taken as an accurate indicator of file tip distance from the apex 
until zero is reached. If the apex locator is used through a metallic
restoration (such as amalgam or a crown), care must be taken to 
prevent contact of the file with the restoration otherwise shorting of
the reading will occur.
When a zero reading is displayed, the distance stop on the file is
moved to contact a suitable reference point and the file then removed.
The distance between the file tip and the stop is measured and
recorded together with the reference point. A diagnostic radiograph is
then exposed as described previously.
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