Oral Health Group

Prevention of Iatrogenic Errors in Canal Preparation: Avoiding and Managing Instrument Separation

July 1, 2010
by Richard E. Mounce, DDS and Gary Glassman, DDS

Recently, one of the authors (RM) received this e-mail asking a clinically relevant question:

“Could you tell me the percentage of a root canal therapy failing if a file was broken midway down the mesial canal of a molar? Fortunately, it wasn’t me but I need to tell the patient the prognosis. It’s asymptomatic and he was advised by the general dentist to now place a crown over the tooth. I am recommending retreatment with a specialist. It was done March, 2010.” (There was no image attached to this e-mail that was received in April, 2010).


The clinician is asking several clinically relevant questions within the larger context of the e-mail. These questions include:

How can fracture of rotary nickel titanium (RNT) files be prevented?

What is the best means of handling fracture once it happens?

What is the prognosis of various fracture locations?

Does it matter what RNT system one is using?

Are all systems the same or are there significant differences between them clinically with regard to fracture and iatrogenic potential?

This article was written to address these questions from the clinical perspective of two endodontists.

How can fracture of rotary nickel titanium files be prevented?

Prevention is the key to avoidance of RNT file separation. Several strategies are needed preventing this unfortunate occurrence. The strategies include:

The use of magnification and lighting. The value of visualization and magnification during all phases of treatment cannot be overstated, be this visualization in the form of the surgical operating microscope (SOM) (Global Surgical, St. Louis, MO, USA) and/or loupes such as the 4.8x Hi Res Class IV (Orascoptic, Middleton, WI, USA).

Adequate treatment planning pre-operatively. Every clinical case should be carefully evaluated for its complexity before starting with an eye toward anticipating avoidance of iatrogenic events. Canal anatomy where file separation and other iatrogenic possibilities exist should be anticipated. The curvature, calcification and three-dimension nature of the canal anatomy should always be assessed pre-operatively, taking into account that radiographs (digital or paper) only display a two dimensional image of a three dimensional object. In essence, the risk factors inherent in canal preparation should be anticipated to provide the clinician strategies to manage whatever the particular challenge is present. For example, if the mesial root of a lower molar is highly fluted, the risk of strip perforation is extreme if excessive force and instrument taper are used. As a result, a decision needs to be made with regard to the preparation’s final taper (.10, .08, .06, etc) and special attention must be paid to the both the means with which it is prepared but the sequence, force and rotational speed used.

In this clinical example, using Gates Glidden drills inappropriately in a highly fluted root is more likely cause strip perforation should they excessively engage dentin and “screw into” the root. As an alternative to Gates Glidden drills, the .08/25 Twisted File* (TF) can prepare the final taper in a highly fluted root and do so without the use of RNT orifice openers or Gates Glidden drills. Using TF for this indication saves time, insertions, and reduces the number of instruments needed. It eliminates the need to blend the insertion depths of the various Gates Glidden drills that are otherwise necessary to prepare a continuous taper.

Selecting the final taper before canal enlargement. The final taper of the canal system should be chosen before access to preserve tooth structure. Only removing the dentin that is absolutely necessary during access is vital because excessive dentin removal is consistent with a higher risk of subsequent vertical root fracture and strip perforation. In general, larger roots will safely allow the preparation of a larger taper and smaller and more complex roots will safely allow preparation of smaller tapers. Using TF, the palatal canal of an upper molar will usually safely allow a .10 taper, the mesial root of a lower molar will usually safely allow the preparation of a .08 taper and a severely complex root will usually safely allow preparation of a .06 taper. Because TF is manufactured by twisting nickel titanium while in the rhombohedral crystalline phase configuration instead of by grinding, it possesses the needed flexibility and cutting efficiency to safely prepare these tapers and do so with relatively few insertions. Alternatively, it is generally not possible to prepare these tapers using RNT instruments that have been manufactured by a grinding process.

Determining a preoperative estimate of the master apical diameter. Especially in non-vital teeth, preparing larger apical diameters with optimize irrigation and debris removal. Typical master apical diameters in non-vital cases are at least a #40 ISO tip size or larger. The above notwithstanding, no contraindication exists to preparing these same master apical diameters in both vital and necrotic cases.

Determining a preoperative estimate of the true working length. Before the true working length is confirmed (later in the instrumentation sequence), the clinician should provide an estimated working length against which the resistance of the RNT file to apical advancement should be referenced tactilely. The true working length should be determined using hand K files and an electronic apex locator prior to taking a RNT file into the apical third of a canal after which a glide path is prepared. Determining the true working length accurately and only taking the RNT files to that anatomic canal location prevents over instrumentation and transportation of this vital clinical landmark. Such an action will also help prevent iatrogenic outcomes including extrusion of irrigants and obturation materials.

Preparing straight-line access. Straight-line access will allow the clinician to see all of the canals with one mirror view. Such a view can help the clinician assess whether all canals have been located and assure that the pulp chamber has been unroofed before commencing canal enlargement.

Removing the cervical dentinal triangle. One method to simply and efficiently shape the orifice (and remove the cervical dentinal triangle) is to use the .08/25 TF in a brushing motion, up and away from the pulp chamber floor. This is highly efficient and optimal relative to using Gates Glidden drills which can otherwise push debris apically, block small canals with debris and possibly transport the canal.

Using the correct pressure on the RNT file, TF or otherwise. The correct pressure on a RNT file during insertion is the equivalent of that needed for a #2 sharp lead pencil. Using excessive pressure is not only unproductive with regard to making apical progress, but also the primary harbinger of file separation due to the excessive torsional force being placed on the instrument.

The clinician should achieve and maintain apical patency prior to placing an RNT file into any portion of the canal anatomy beyond the orifice. Once the orifice is shaped and the cervical dentinal triangle is removed, hand K files can be placed into the canal below the orifice. Starting with #6 hand K files even in a canal that appears to be easily negotiable is advised. Unexpected multiplanar curvatures can easily provide a different tactile experience in the canal than that expected radiographically.

Precurving hand K files and trimming them as needed prior to apical third negotiation can often allow apical progression where a straight hand K file will not advance. The insertion of pre curved hand K files is gentle, passive and done with the intent to assure patency, not enlarge the canal per say. Driving a hand K file to length forcefully can carry debris apically and also (if the hand K file is large enough) potentially cause apical transportation. Once the canal is negotiable and patent through the minor constriction of the apical foramen, the glide path described in #10 b
elow can be prepared. It is noteworthy that when a hand K file passes through the minor constriction a tangible tactile “pop” can often be experienced. A mental note of the location of this sensation should be correlated to the estimated working length, the electronic apex locator reading, and ideally a bleeding point working length determination taken later in the treatment (a bleeding point or moisture point should be visible at the tip of a paper point if the canal has been kept open, patent and negotiable through the minor constriction).

Preparing a glide path prior to placing a RNT file into any portion of the canal. A glide path is prepared to give the RNT file a reproducible track within the canal to progress apically. It is the minimal enlargement of the canal to a #15 hand K file equivalent.

Clinically, once the first hand K file reaches the estimated working length, an electronic apex locator is placed on the file and the true working length determined if a reliable and reproducible reading can be obtained. For example, if the first hand K file to reach the minor constriction is a #6 hand K file, an electronic length should be attempted. If a reliable reading is not possible, the canal is enlarged with a #8 hand K file, and a reading attempted. Once the true working length (position of the minor constriction) is determined, the canal is enlarged by hand to the equivalent of #15 hand K file.

One method to rapidly and efficiently increase the size of the canal from a #6 to a #15 hand K file equivalent is the M4 Safety Handpiece. The M4 is a reciprocating handpiece that moves a hand K file 30 degrees clockwise and 30 degrees counterclockwise. In essence, the M4 replicates the manual use of hand K files in early canal enlargement. While some preparation methods advocate that the entire canal be enlarged using reciprocation, this is not the method advocated by the authors.

The M4 is used at 900 rpm on the 18:1 setting of an endodontic motor with an E-type coupling. The vertical amplitude of movement during reciprocation is approximately 1-3 mm and takes place for 15-30 seconds. Clinically, the first hand K file that reaches the true working length is reciprocated. If for example, the first hand K file to reach the true working length is a #6, it is reciprocated with the time, amplitude and settings above. Reciprocating a #6 hand K file prepares the canal to the size of a #8 hand K file equivalent. Reciprocating the #8 hand K file will create the size of a #10 hand K file equivalent. Reciprocating the #10 hand K file creates the size of a #15 hand K file equivalent.

Once the canal is prepared to the equivalent of a #15 hand K file equivalent, the canal is ready for RNT enlargement. Lubrication should always be present during reciprocation. After each reciprocation, the canal is irrigated and a hand K file is placed into the canal to assure that the canal remains open and negotiable.

The M4 is placed on to the head of the hand K file after the file has been placed to the MC. The hand K file is not placed into the M4 inserted into the canal and driven apically. Trying to drive a hand K file apically with a reciprocating handpiece will predictably ledge, transport and block canals, especially with hand K files larger than a #8.

Inserting RNT files for the correct length of time. Each insertion of a RNT file should optimally be 2-3 seconds. RNT files should not be used like a toilet plunger. The ideal motion of insertion is a continuous and controlled apical motion with the insertion of the RNT, withdrawal, and use of the next file in the sequence. Repeatedly driving a RNT file into a canal risks removal of excessive dentin, locking of the instrument tip and apical debris propulsion.

Using the RNT files in the correct sequence. Improper instrument sequencing blended with forceful insertion increase the chances of instrument fracture, canal blockage, and will otherwise diminish tactile control. Ideally, a crown down sequence is employed as the clinician moves towards the apex, meaning that RNT files are used from larger tapers and tip sizes to smaller to prepare the basic taper of the canal before the master apical diameter is finalized. As a RNT file is moving apically, because the canal is narrowing, an increased resistance to advancement should be expected. If a file will not advance passively, it should be withdrawn and the next smaller file of an appropriate taper and/or tip size utilized.

When the clinician is preparing the master apical diameter, in distinction from the crown down preparation of the basic taper of the canals, RNT files should be used from smaller tip sizes to larger tip sizes, in essence in a step back method. A recommended sequence of TF instrumentation using these concepts is presented in detail later in this paper.

Irrigating frequently. Ideally, the clinician should irrigate after every RNT insertion.

Recapitulating frequently. Ideally, the clinician should recapitulate after every RNT insertion and aforementioned irrigation.

Once the clinician has determined true working length and reaches this length with RNT, every effort should be made to assure that the given RNT file does not progress beyond the minor constriction (MC) of the apical foramen. Taking RNT files beyond the MC can cause apical perforation, canal transportation and increase extrusion of irrigants and obturation materials and leave bacteria at the apex in untreated canal space.

What is the best means of handling fracture once it happens? What is the prognosis of various fracture locations?

If a RNT instrument fractures, the case should be referred to a specialist unless there are significant extenuating circumstances. Once in the hands of a microsurgical specialist, fragments in the coronal third and those above the point of first curvature can generally be removed safely and efficiently, even in upper and lower second molars. Files below the point of first curvature are much more challenging to remove and yet it is often possible to remove these files if a portion of the file can be reached above the point of curvature.

The pulpal status of the tooth at the time of access matters significantly. Vital inflamed cases are more forgiving of fractured instruments with regard to success and healing relative to non-vital cases. Non-vital cases in which instruments fracture and which contain uncleaned and unfilled space around and below the fragment have a slightly less favorable prognosis. Biofilm remaining in non-vital canals around and below the canal after fracture does not favor healing. Vital inflamed cases in which an instrument fractures will have a somewhat greater chance for clinical success relative to non-vital ones.

Where in the treatment sequence fracture occurs is significant. Cases where the fracture occurs late in the cleaning and shaping of a root canal will have a better chance of healing relative to those where fracture occurs early in the sequence. Late sequence fractures will occur in slightly cleaner canals than early fractures and as a result are often better tolerated.

Does it matter what RNT system one is using? Are all systems the same?

Clinicians can create beautiful results with any one of a number of various systems. We use TF for its flexibility, cutting efficiency, reduced number of required files as well as reduced number of required insertions. TF is available in the following tapers and tip sizes: .12/25, .10/25, .08/25, .06/25/30/35 and .04/25/40/50. For the vast majority of roots encountered, the .08 taper will be the final prepared taper. This taper is made with the .08/25 TF.

In general, the sequence of TF instrumentation uses the .08/25 TF followed up with the .06/25, .06/30, and .04/40 TF (assuming that a glide path is present and the clinician wishes to prepare this master apical diameter. This sequence follows the suggested protocol above, using the instruments crown down (from bigger tapers and tip sizes to smaller) to create the shape of the basic preparation and taper (.08)
and then step back (from smaller tip sizes to larger tip sizes) to prepare the master apical diameter (in this clinical example, a #40 master apical diameter).

Blending the above strategies for prevention of RNT instrumentation and TF sequence, clinical use of TF is detailed here sequentially in outline form:

An estimated working length is taken radiographically.

Straight line access is achieved (as described above)

The cervical dentinal triangle is removed (as described above) with the .08/25 TF brushing up and away from the furcation. As a result, the orifice is defined for ease of hand K file insertion.

Small hand files (starting with a #6 or #8) are placed to the estimated working length to gain patency. These files are precurved and inserted gently and passively.

The true working length is determined with an electronic apex locator once the first hand K file reaches the estimated working length.

The glide path is prepared to the true working length. The glide path is completed when a #15 hand K file will spin freely at the true working length. Making the glide path is far simpler using the M4 Safety Handpiece.

The .08/25 is inserted and moved apically in a 2-3 second continuous and controlled motion after which it is removed. The file is not pumped forcefully to gain apical length. The flutes of the file are wiped after every insertion. The canal is irrigated and recapitulated after every insertion of the RNT.

If the .08/25 will reach the true working length passively in several insertions (3-4), it is used to length, if it will not, the .06/25 is inserted instead. If needed, a tandem of the .08/25 and .06/25 will be used to prepare the basic taper of the canal. If the .06/25 reaches the apex first, the .08/25 can then be take to the apex to prepare the final taper as desired.

The master apical diameter is prepared using the .06/30, .06/35 and .04/40 in succession as desired. Each file should reach the apex easily and in one insertion.

After the final activated bactericidal irrigation, the smear layer is removed with a liquid EDTA solution such as SmearClear*, the canal rinsed with distilled water, the canal dried and obturation commences. Activation of irrigation can take place with sonic, ultrasonic, negative pressure, rotating plastic file, and heated irrigation techniques, etc.

The preparation resulting from the above sequence has a .08 taper and a #40 ISO master apical diameter. A warm vertical condensation technique may be used to obturation the root canal system at this time.

This article was written to address several clinical relevant questions that arose out of an e-mail asking about separation of a fractured rotary nickel titanium file. Emphasis has been placed on principles that can prevent RNT fracture and iatrogenic outcomes of all types, and early referral. We welcome your feedback.

*SybronEndo (Orange, CA, USA) OH

Dr. Mounce is the author of the non-fiction book Dead Stuck, “one man’s stories of adventure, parenting, and marriage told without heaping platitudes of political correctness.” Pacific Sky Publishing. DeadStuck.com

Dr. Mounce lectures globally and is widely published. He is in private practice in Endodontics in Vancouver, WA, USA.

Dr. Gary Glassman graduated from the University of Toronto, Faculty of Dentistry in 1984 graduated from the Endodontology Program at Temple University in 1987. The author of numerous publications, Dr. Glassman lectures globally on endodontics and is on staff at the University of Toronto, Faculty of Dentistry in the graduate department of endodontics. Gary is a fellow of the the Royal College of Dentists of Canada, and the endodontic editor for Oral Health Journal. He maintains a private practice, Endodontic Specialists in Toronto, Ontario, Canada. He can be reached at: www.rootcanals.ca

Oral Health welcomes this original article.