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How many times should an instrument be used
by Barry Musikant on Mon Apr 11, 2011 10:35 pm
Source:  www.endomailmessageboard.com

I was reading a post on the other message board that concerned the number of times an endodontic
instrument should be used before replacing it. The general consensus was 1 time. The reasons all
made good common sense and included the following:

1. After the first usage, the instruments get progressively duller and take more rotations to cut
   the same amount of dentin progressively increasing the risk of breakage.

2. Even though a rotary NiTi instrument may look perfectly intact, it doesn’t mean that if examined
   with SEM it would not show the propagation of cracks in its body.

3. Since we don’t know how many times we can use the instruments before they do fracture, the better
   part of caution is to dispose of them after one use.

4. You can’t really sterilize them well because of the debris that gets caught up in the microfractures
   that occur with usage.

I don’t argue that such reasoning makes sense when the subject is confined to NiTi used in rotation.
However, I see no reason why this cautionary approach is necessary when using stainless steel reamers
with either a tight watch winding stroke or in the 30º reciprocating handpiece. 30º arcs of motion
pretty much eliminate torsional stress and cyclic fatigue as issues to be concerned with. Stainless
steel is much tougher than NiTi and maintains its cutting edge much longer.

Since stainless steel will demonstrate all sorts of abuse prior to breakage such as unwinding or
overwinding there is ample visible evidence to make a determination when to throw away the instrument.
Stainless steel reamers used in a reciprocating handpiece will become dull way before they ever break
and that is easily discernible with usage. I typically use the reamers about 6-8 times before replacing
them unless we are talking about 08  or 10 reamers that can be turned into pretzels when first
attempting to negotiate a highly calcified canal and, therefore, must be replaced.

We should not make generalizations based on the vulnerabilities of NiTi. They are simply one design
that is used a specific way that increases its vulnerabilities to fracture and their weaknesses
should not be ascribed to other systems that are designed and used in a different manner. Rotary NiTi
is not the end of the road, merely one step along a road that is forever being built.

Always interested to hear other comments.

Regards, Barry Musikant

hi, i think its not practical to through after one use. it will  cost me  much?
what about 3 times cuz many dentisit claim that 3 times  is fair enough?
and i use the protaper from dentsply  thay claim that if it going to break it breaks from the upper end
(which then can be withdrawn easily) is that right? - ketty

Ketty, When it comes to rotary NiTi, the risk of fracture goes up as the tip size and taper of the
instruments increase and as the abruptness of the canal curvature becomes greater. An instrument will
work many times without separation if it is used in a straight canal. As the canal curvature increases,
the tip size and taper of the instruments make it increasingly vulnerable to breakage. So, to pick an
arbitary number of uses will not predictably determine whether or not an instrument will remain intact.
Where a rotary NiTi instrument will break is determined more by the canal being shaped than anything
inherent to the instrument. A sharp apical curve will more likely result in apical breakage while a
highly curved coronal entry point will lead to fracture more coronally.

Frankly, using relieved reamers with 30º reciprocation allows me to use the instruments 6-7 times
effectvively. In fact, even if overused the downside is dullness not separation, something that is a
lot less stressfull to my stomach lining. I believe reciprocation will eventually replace rotary as the
safest method of motion using stainless steel designed as relieved reamers that can record the curvature
while negotating thru the canals with the least resistance.

Regards, Barry Musikant

by Hal on Fri May 06, 2011 9:24 pm

DO you have any advice for the best cleaning of the reamers?  I have my assistant wiping them clean with
Orange Solvent before the ultrasonic bath because I noticed some remaining debris after ultrasonic and
autoclaving alone. - Hal

Hal, It sounds like you are doing the right steps. In fact, you go thru the extra step of orange solvent.
I've never heard my assistant talk about that. I have not noticed debris on the instruments we are using.
I know she wire brushes all the instruments prior to the ultrasonic bath. Perhaps that is a step that will
more predictably remove debris. The instruments implicated in the retention of debris are the rotary NiTi's.
Even before use they may have cracks on their surface which tend to widen and propagate with use.
For those using NiTi a number of times before replacement, this is an additional concern and it is
accentuated by rotation. 30º reciprocation is a lot kinder to the instruments.

Regards, Barry Musikant

thank you dr Barry for your great answers
i think that you dont use k file but insteade you use reamers only ?but all books and even what we use to
study in dental school they said k file is better? what do you think?  sorry for asking too much - ketty

Ketty, The best response I have come up with in determining whether or not the design of a reamer
(more vertically oriented flutes) or a file (more horizontally oriented flutes) is better is to use the
analogy of shaving ones face with a blade. The only reason the blade works well is because its shaving edge
is at right angles to the plane of motion. That principle holds true whether shaving ones face or shaving
dentin from the walls of a canal. Since our predominant motion is watch winding which is  horizontal,
it stands to reason that the blades will shave more effectively if they are at right angles to that plane
of motion. The reamers are. The files are not. This is such a basic concept that I believe schools are
embarassed to admit the obvious. Yet if you look at all the rotary NiTi systems that are used with a
horizontal motion they all have vertically oriented flutes. I could go on, but I think yhou get the point.

Regards, Barry