Preventing Nickel Titanium Rotary Instrument Breakage
Nickel
Titanium (NiTi) Rotary instruments have become a popular replacement
for traditional hand files for root canal preparation. Proven
advantages of rotary instruments are an inherent ability to pull
tissue and dentin debris out of the canal spaces and maintain
a centered canal preparation. Although Nickel Titanium is a highly
flexible metal it has limitations when stresses are produced during
rotation.
Nickel Titanium instrument separation
or breakage is caused by excess torque and / or cyclic fatigue.
A torque failure is caused by a single rotational event while
a cyclic fatigue failure is caused by multiple flexing events
(over-use of the instrument).
Events
that cause increased torque or cyclic fatigue are:
(1) Canals that come together at sharp angles. (2) Curved
canals that change directions abruptly.
Torque
is increased by: insufficient lubrication,
increased pressure, increased canal curvature, increased surface
area, decreased rotational RPMs.
Cyclic Fatigue is increased by:
increased canal curvature, decreased radius of curvature,
increased diameter of the instrument, and increased taper
of the instrument.
A three year
tracking study of NiTi instrument failure indicates that 65-75%
of separation occurs in the mesial canals of lower molars.
Lower molars often have secondary
(S-Shaped) or Tertiary (Bucco-lingual) curvatures. 40-50% of
mesial roots have 2 mesial canals that unite in the apical 2-3
mm’s and have internal ledges.
MINIMIZING
FILE SEPARATION:
- Preparation
with NiTi instruments takes time, don’t rush.
- Use an
electric torque controlled handpiece specifically designed for
NiTi instrument use.
- Never
force the instrument in the canal, very light pressure only.
- Use crown-down
shaping. Enlarge the coronal 1/3 of the root first using larger
NiTi rotary instruments in a larger to smaller order.
- Then explore
the apical 1/3 of the canal using very small files such as 6,
8, and 10.
- Create
a glide-path by enlarging the apical 1/3 to at least a size
15 before using rotary files.
- Maintain
lubricants and irrigants in the canal spaces at all times. Clean
the flutes of all debris.
- After
each insertion, inspect the instrument for shiny spots indicating
wear or unwinding flutes.
- Consider
changing from.06 to. 04 taper if a larger taper fails to go
to length.
- Small
size NiTi instruments should be considered “single use” instruments.
- Make sure
the instrument is rotating when entering and exiting the canal.
- A good
rule: “Take what the canal will give you and don’t over-prepare
the space”.
If an instrument separates stop
what you are doing. Take a radiograph and assess where the instrument
is in the canal. Depending on location, separated instruments
can be removed using a surgical operating microscope and ultrasonic
tips.
Before dismissing the patient,
inform them that a procedural mishap has occurred. Explain that
a small piece of a biocompatible metal instrument has separated
in the tooth and that you are going to refer the patient to an
endodontist to have them attempt to remove it.
Jerome
CE, Hanlon RJ. Identifying multiplanar root canal curvatures using
stainless-steel instruments.
J Endodon 2003;29:356-358.
ESCONDIDO
ENDODONTICS 488 EAST VALLEY PARKWAY SUITE 307 ESCONDIDO, CA 92025
(760) 739-1400
FAX (760) 739-1100 www.escoendo.com
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