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Endo tips    Better Endo    Endo abstracts    Endo discussions

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for the individual posts where they are

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  
they 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 don't 
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
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