Preparing larger apical diameters: Clinical rationale and methods

The ideal MAD is a blend of three different considerations:

1. the recommendations of the endodontic literature;
2. the avoidance of iatrogenic events in the efficient creation of larger MADs; and
3. having the technical means to prepare larger MADs predictably and safely.

These considerations are addressed in sequential order.

The recommendations of the endodontic literature

A properly created and enhanced MAD implies that dentine is being removed circumferentially at the minor construction (MC) of the apical foramen. The creation of enhanced MADs relative to smaller ones is consistent with greater volumes of irrigation, better debris removal and better master cone fit. It is virtually unanimous in the endodontic literature that larger MADs create cleaner canals than their smaller counterparts. There is no literature that we are aware of that argues that smaller MADs provide cleaner canals.

Avoidance of iatrogenic events in the efficient creation of larger MADs

Before presenting specific methods of MAD preparation, several practical considerations, principles, and concepts are elaborated upon to give context to the efficient creation of larger MADs.

1. The final canal shape should resemble a tapered funnel. The MC should be kept at its original position and size, i.e. it should be unaltered. The MC acts as a natural barrier to the extrusion of bacteria, pulp tissue, irrigants, sealer, and obturation material. The canal should remain in its original position and only be enlarged as described here. The taper and MAD should be appropriate, in order to achieve optimal irrigation and obturation hydraulics. The above goals should be achieved with a view towards avoidance of iatrogenic events. It is important to mention that the mutual goals of enhancing the MAD and leaving the MC at its original position and size are both technically possible and highly desirable. Enhancing the MAD means enlarging the canal preparation apically up to the MC (leaving the MC at its original position and size), but this enlargement does not extend beyond the narrowest diameter of the canal as it exits the root.

2. Mechanical canal enlargement should be recognised as one means of cleansing. Bactericidal irrigating solutions are required along with the mechanical means to provide the optimal antimicrobial control. The endodontic literature fully supports the concept that irrigation is required to reduce bacterial counts beyond that which is possible by instrumentation alone. Optimal irrigation methods include the use of a heated solution; removal of the smear layer; alternating liquid EDTA with the primary antibacterial solution; ultrasonic activation; and use of an adequate volume, concentration and frequency of refreshment of the solutions.

3. Access should be straight line and all files, rotary nickel titanium (RNT) and hand K files (HKF) should be able to enter the canal without deflecting off the access walls. Leaving access walls that restrict insertion of RNT files predisposes the enlargement to blockage and iatrogenic events of all types. The cervical dentinal triangle should always be removed prior to canal shaping below the orifice level.

4. Patency must be achieved and maintained throughout the entire process of canal preparation. Loss of patency is the harbinger of iatrogenic events and less than optimal cleaning and shaping. Patency can be assured through the copious use of small hand files that are utilised as productively and efficiently as possible. HKFs should be pre-curved and inserted into the canal in the same direction of the expected root curvature. HKF curvature is optimally performed with EndoBender pliers (SybronEndo).

HKFs can be trimmed to a length that is clinically relevant for the indicated purpose; for example, if the HKF is too long for the given indication, the file will buckle, kink, bend and need to be replaced. HKFs that are not appropriate for the given canal do not allow the correct amount of pressure needed to break through blockages and traverse ledges. Pre-curved and trimmed HKFs create efficiency in canal negotiation compared with their longer and un-curved counterparts and can be reciprocated as described below.

5. We use the M4 Safety Handpiece (SybronEndo) attachment once the HKF reaches the estimated or true working length (Fig. 4). Reciprocation with the M4 is safe, efficient, and saves both time and hand fatigue. It is difficult to break files using this method. If the tip of the HKF is never more than 1 to 2 mm beyond the MC and the HKF size is #10 or less, transportation is virtually unheard of. The M4 reciprocates the HKF 30 degrees clockwise and 30 degrees counter-clockwise. The M4 is not used to drive the HKF to length to negotiate the canal and it is not used to break through a calcification. The placement of the HKF to the MC is done by hand after the HKF is pre-curved and cut to the appropriate length.

Clinically, a small HKF is inserted to the estimated or true working length and the M4 placed upon it under the rubber dam. The tip of the HKF should be taken to or slightly beyond the MC. This patency file is the only instrument that is taken beyond the MC. Deciding which HKF to reciprocate with the M4 is based on the degree of resistance that the canal gives to the advancement of the HKF; for example, the #6 HKF will easily reach the true working length, while the #8 will reach this depth but only with resistance. The #8 is placed to the MC and left at this level. The M4 is then placed onto the #8. With a vertical amplitude of 1 to 3 mm, the M4 is used in the canal for 15 to 30 seconds. As the M4 reciprocates the #8 (30 degrees clockwise and 30 degrees counter-clockwise), the canal will become minimally enlarged from the action of the file. The file will have progressively less resistance to vertical movement and reciprocate freely.

The M4 requires lubrication at all times, ideally a liquid solution of 2% chlorhexidine or 5.25% sodium hypochlorite. The use of a viscous EDTA gel in reciprocation is neither required nor advisable as removal of all the gel (particularly in the apical third) may not be possible using even the most strident irrigation protocol.

The M4 fits onto any electric motor with an E-type attachment and is used at 900 RPM on the 18:1 setting. After the #8 in this example has been used, the canal will become the diameter of a #10 HKF, and after irrigation and recapitulation a #10 HKF is inserted into the canal and reciprocated. Reciprocation of the M4 in the manner described will prepare the canal to the diameter of a #15 HKF, after which the canal is ready for RNT enlargement.

If the HKF is a size #15 or larger and particularly if the file is reciprocated short of the MC, transportation of the canal is a risk. Copious irrigation and recapitulation after the use of the M4, as well as working at the MC (i.e. at the true working length), will go far towards avoiding iatrogenic events of all types. It is noteworthy that the recommended use of reciprocation in this article is for the early enlargement of canals to make the glide path and not for the final canal preparation. Final canal preparation is made with RNT files in the manner that is suggested below. The use of reciprocation for the final canal preparation above a size #30 is consistent with apical transportation.

6. The correct working taper must be prepared for the given root anatomy. Roots that are more complex should be prepared to less taper than their simpler counterparts. Specifically, highly curved, calcified, and atypical anatomies are generally prepared to less taper than a root that is straight and appears easily negotiable radiographically. While a comprehensive discussion of what taper could and should be prepared in any given root system will be addressed later in this article, at this stage it should be remembered that different RNT systems have varying abilities to prepare larger tapers throughout the canal. Files that are manufactured by grinding have less flexibility and fracture resistance than those that are manufactured by twisting, such as the Twisted File (TF, SybronEndo; Figs. 1–3). Knowledge of the relative degree of taper that can be prepared with a given system is a prerequisite for choosing the desired taper, RNT file sequence and determining whether orifice openers might be used or are necessary.

Figs. 1–3_The Twisted File (SybronEndo, Fig. 1). The large and small pack configuration of the enhanced Twisted File sizes (Figs. 2 & 3).

The preparation of larger tapers in a complex root requires that the clinician always be aware of perforation risk. Removal of more dentine than necessary will weaken the root and place the tooth at excessive risk of subsequent vertical root fracture, even if the root is not perforated in the initial treatment.

7. The rationale for the final termination point of root canal irrigation, instrumentation and obturation varies greatly amongst clinicians. I use the MC for the final termination point in both vital and necrotic cases, with patency through the MC a paramount goal. Determining the position of the MC occurs as a function of correlating all of the different sources of information as to its position. For example, the clinician can estimate the initial working length from the preoperative radiographs. This initial length can be correlated to the place within the canal where the clinician feels a tactile ‘pop’ with the HKF as it passes through the MC. This length should be very close if not identical to the electronic length determined by using an apex locator. In addition, once the canal is prepared, if patency has been achieved and maintained, a paper point should be able to mark the exact location of the MC with a reproducible and small spot of moisture or haemorrhage. Correlating all of this evidence and taking electronic lengths at various junctures in the enlargement process can confirm the exact position of the MC. When enlarging canals, the working length gets shorter depending on the length of the canal and degree of curvature. It is important to check the working length frequently to avoid over instrumentation apically from this shortening.

8. Instrumentation sequences vary greatly depending on the clinician, the anatomy and the system being used. With RNT files, there are two primary canal preparation objectives, the ‘basic preparation’ (getting a .06 or .08 tapered #25 RNT to length) and then the ‘enhanced preparation’ (preparing a MAD larger than a size #25 in this example) that ideally should follow. It is noteworthy that the initial diameter of the MC as reported in the literature is approximately .28 mm on average across all root anatomy. Any MAD below this size is likely to leave portions of the apical canal untouched, resulting in a compromise in the ultimate canal cleanliness. Said differently, if the clinician stops apical preparation at the MC following achievement of the basic preparation and fills at this MAD, they are arbitrarily imposing onto the root a diameter that may be clinically irrelevant.

Crown-down instrumentation implies that the canal is prepared with larger tapers and tip sizes and decreasing progressively to smaller tapers and tip sizes. With the sequence, each successive file is inserted further apically than its predecessor. For example, after shaping the orifice, crown-down instrumentation is demonstrated by a sequence of .06 files that are used from #40 to 35 to 30 to 25 to 20 and finally #15 tip sizes. This sequence can be repeated until the desired taper and tip size is taken to the MC, be that a size #20 or 25.

Alternatively, the clinician could use a variably tapered sequence of files—using a pack of K3 VTVT (SybronEndo) RNT instruments, for example—moving apically. Such a sequence might be .10/25, .08/25, .06/35, .04/30, .06/25 and .04/20. Using this technique, the taper and the tip size diminish sequentially.

After this initial basic preparation, the clinician can subsequently prepare an enhanced MAD. The preparation of enhanced MADs is generally done with a step-back sequence. Step-back instrumentation requires that the clinician use the files from smaller to larger tip sizes; for example, if the basic preparation took the canal to a .06/25, then the #30, 35 and 40 tip sizes would be used. Instruments of a smaller taper can be used to prepare the enhanced MAD. Smaller tapers are required because much like a small cup fits into a large cup, smaller tapered RNT files fit into larger tapered preparations. Clinically, if the basic preparation was made with .06 instruments, .04 RNT instruments will fit inside the .06 preparations. As a result, the .04 instruments only cut on their tips in the creation of larger apical diameters. If the clinician is using TF and the final taper is a .08, the .06 and .04 TF tapers (which can prepare enhanced apical diameters) can fit inside the .08 taper easily.

The creation of larger MADs through enhanced apical shaping is rapid and efficient. This added step, of obvious benefit as described in the endodontic literature, requires only that the clinician insert the increasing tip sizes to length, which is followed by irrigation and recapitulation. Then the next larger tip size can be entered until the clinician has prepared the desired MAD. The sequence for this enhanced shaping is detailed below.

9. Various RNT systems have inherent limitations and advantages relative to other systems in preparing enhanced MADs and larger tapers. Some RNT files cut more efficiently, some are more flexible and some allow the preparation of larger MADs, some do not. Some systems may not have tip sizes that allow enhanced MAD preparation; for example, some systems do not have tip sizes greater than a size #40, or if they do, the files are so stiff that preparing larger diameters is only possible in a relatively straight canal. In order to appreciate fully the ability of some RNT systems to shape canals to larger MADs, the clinician will optimally practise in extracted teeth. Such practice allows the clinician to determine the true functionality of various RNT systems with regard to the creation of enhanced MADs.

There are several viable clinical methods for creating larger MADs. TF is a viable and advisable choice for both the basic preparation and creating enhanced apical diameters. TF is available in .12/25, .10/25, .08/25, .06/25/30/35, and .04/25/40/50 tapers and tip sizes. With this variety of file sizes and tapers, virtually any tooth can be prepared irrespective of the degree of difficulty.

TF is fundamentally different from other RNT instruments manufactured by grinding. TF is never ground against the grain structure of the metal. Twisting nickel titanium creates cutting flutes. In order to twist nickel titanium, it must be converted from the austenite crystalline phase configuration (CPC) to the rhombohedral (R) crystalline phase configuration. R phase is an intermediate CPC between austenite and martensite. Austenite is the CPC of NT at rest; martensite is the CPC of NT when a stress has been placed upon it. The ability of NT to flex when a stress is placed upon it is a result of the martensitic transformation, a transformation that realigns the atoms of the CPC to allow the metal to exhibit super-elasticity. Up to the point that the NT has reached its elastic limit, the metal will return to its original shape. After reaching the elastic limit, if the metal is stressed further, it deforms and is in the plastic range of function. It will not return to its original shape. For RNT files that have been manufactured by grinding, deformation is a clear indication for disposal. TF has a much wider plastic range of function than ground NT files. As a result, if there is some deformation (unwinding) of the file flutes, TF can still be utilised. Unwinding of TF is not an immediate indication for disposal as it would be with RNT instruments manufactured by grinding.

TF is never ground against the grain structure of the metal in its manufacture, there are no surface imperfections of metal roll over on its surface or at its cutting edges. These surface imperfections and defects are the preferred site of instrument fracture due to cyclic fatigue and torsional stress. The lack of these defects in part explains the clinical capabilities of the TF.

The Twisted File

TF has several advantages over traditional RNT instruments because it is not constrained by the limitations of ground files. Its attributes are the following:

1. TF can achieve the basic preparation in one or no more than two files in 90 per cent of the clinical cases encountered. The most common single-file basic preparation is taken to the .08/25 and when two files are used in tandem, the .06 and .08 are most commonly used.

2. TF can prepare larger tapers than those possible with ground RNT files; for example, with ground RNT files, .06 is the most common taper prepared. When using TF, the canals can be easily and safely prepared to .08 and .10 taper, assuming that the given root is not at undue risk for perforation or that these tapers risk excessive dentine removal that predisposes subsequent root fracture. For larger canals, such as the palatal canal of an upper molar, .10 taper can often be taken to the apex. For more intermediate canals, .08 taper can usually be taken to the apex (for example, in the mesial root of a lower molar). For narrow, curved and highly calcified complex canals, the .06 taper can usually be taken to the apex.

The .12-tapered TF is generally used as an orifice opener and not to the apex. The .04 TF is used to take a small canal (that has a glide path present) and makes a minimal canal enlargement to facilitate the bulk shaping that will follow. For example, if the glide path is created and the root is particularly complex, the .04 TF can make the canal slightly larger and make the subsequent bulk shaping more efficient and reduce the risk of file separation.

3. TF requires fewer insertions to prepare the canal relative to ground nickel titanium files. Generally, only three to four insertions of the TF are needed to reach the apex and perform the basic preparation of the canal prior to the enhanced preparations.

4. With practice in extracted teeth, the number of TF instruments used and the number of insertions needed by the clinician to achieve the same task will diminish. In essence, cases that once required two TF for the basic preparation may now require one, and what may have taken four to five TF insertions will require three to four.

Canal preparation with the Twisted File and the creation of enhanced master apical diameters

For the basic preparation phase of treatment, TF crowns down (as described above) from larger to smaller tapers. For the preparation of enhanced MADs, they are used step back, from smaller to larger tip sizes. The detailed steps of this process are described here.

1. After straight-line access and removal of the cervical dentinal triangle, the orifice is shaped. Orifice shaping is performed bearing in mind that whatever the initial size of the orifice preparation, the taper of the final shaped root canal must be continuous along the length of the root. As a result, in large and simpler roots (such as the palatal canal of an upper molar), the first file into the coronal third is usually the .10 TF. In roots of more intermediate complexity (such as the mesial root of a lower molar), the .08 TF is the first file into the canal. For highly complex canals, such as the lower anterior teeth, and canals of significant curvature and calcification the .06 TF should be the first file used.

If the clinician desires to create more taper later using TF, the given initial file can be used with a brushstroke up and away from the furcation, in other words towards the root with the greatest bulk of dentine. It is also noteworthy that it is possible to create greater tapers after first creating a smaller one. However, once the root is shaped, the taper cannot be made smaller. As a result, it is always safer to be more conservative when preparing the initial taper of canals. Dentine can always be removed; it can never be put back.

2. The canal is negotiated with HKFs to the estimated working length and a glide path created, optimally using the M4 as described above.

3. When the first HKF reaches the estimated working length, the electronic apex locator is used to obtain the true working length.

4. TF is used crown down from larger to smaller tapers until the basic preparation is created. Depending on the type of root being prepared, the first TF in the canal can, in many instances, be the final file in the sequence to create the basic preparation before enhancing the MAD. For example, in many canals, the .08 TF can be used crown down in repetitive insertions (irrigating and recapitulating between each insertion) to reach the apex as a single-file technique. If the single TF will not move apically without undue pressure with each successive insertion, the next smaller taper can be used in a two-file tandem to create the basic preparation. For example, the .10 and .08 can be used in a two-file technique in larger canals or .08 and .06 can be similarly used in canals that are more complex. It is noteworthy that when the .06 TF reaches the apex first, if indicated, it can generally be followed with the .08 TF in order to prepare a larger taper.

TF is used in our hands at 900 rpm with the torque control off. Insertion is passive and withdrawn after one continuous and controlled motion. Once resistance is met, the file is withdrawn and the canal irrigated and recapitulated after the TF reinserted. TF can be used in approximately two to three teeth if used with the gentle and passive insertion recommended here. It is not advised to pump TF up and down in the canal or use it with a pecking motion. The file should be turning on entry into the canal.

5. When the .08/25 or .06/25 TF reaches the true working length, the clinician can gauge the apex. Gauging is a simple means of determining an approximation of the initial diameter of the foramen to guide the final MAD to be prepared. For example, if a #25 HKF will not penetrate or move beyond the true working length when gentle pressure is applied, this is considered the initial diameter of the canal at the MC and gauges the apex. In this clinical example, if the #25 HKF binds at the apex, the canal can be prepared three to four sizes larger than the initial file to bind at the true working length. In this example, if the #25 HKF binds at the true working length, the .06/30 would be followed by the .06/30 and .04/40.

As described by the mechanism before, the smaller tapers of the .06 and .04 tapers fit inside the .08 taper and will cut only on their tips. If the clinician were to use a different instrumentation system to achieve larger MADs (K3, SybronEndo; LSX instruments, Lightspeed, Discus Dental), the principle employed would be the same with the smaller tapers fitting into the larger one.

6. After TF preparation, a cone can be fitted for obturation. While there are many ways of selecting master cones to fit a preparation, a general concept favoured by specialists is to pick a universal master cone that is then custom fitted for the particular canal. Such a universal cone is the .06/20, which is trimmed to fit the apical size. For example, the .06/20 is a size 50, 5 mm back from its tip. When a #50 MAD is prepared, the .06/20 cone is trimmed 5 mm from its tip and cone fit is attempted. The mathematics that underline this are simple: 1 mm back from the tip of a .06/20 master cone, the cone is .26 mm in diameter, 2 mm back it is a .32 mm, 3 mm back it is .38 mm, 4 mm back it is .44 mm and 5 mm back it is the aforementioned .50 mm. Other common universal master cones that can be trimmed in the same manner are the FM and M sizes.

Figs. 5 & 6: RealSeal bonded obturation material (SybronEndo, Fig. 5). RealSeal bonded obturators (SybronEndo, Fig. 6).

We use RealSeal (SybronEndo) for obturation (Figs. 5 & 6). RealSeal is a synthetic polymer of polyester that binds to the sealer and canal wall to provide a statistically significant resistant reduction in leakage across the totality of the canal space when measured in vivo and in vitro. Alternatively, if the use of master cones is not preferred, RealSeal can be used in an obturator-based form (Figs. 7a & b).

Figs. 7a & b: Clinical cases treated to enhanced apical diameters as described in the article.

In this article, a clinically relevant discussion of the rationale and preparation of enhanced MADs has been presented. Emphasis has been placed on patency, selecting the correct taper for the root anatomy and the clinical value of preparing larger MADs and doing so without undue iatrogenic risk in the process.

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Editorial note: This article was originally published in roots Vol. 5, Issue 2, 2009.

Author info

Dr Richard E. Mounce lectures globally and is widely published. He has a private practice specialised in endodontics in Vancouver, Washington. Dr Mounce offers intensive customised endodontic single-day training programmes in his office for groups of one to two doctors. For information, contact Dennis at +1 360 891 9111 or e-mail RichardMounce@MounceEndo.com.

Dr Gary D. Glassman has authored numerous publications and is on the staff in the Faculty of Dentistry in the graduate Department of Endodontics at the University of Toronto. A renowned international lecturer on endodontics, Dr Glassman has presented at major dental conferences around the world. He is an endodontic editor for Oral Health and maintains a private practice in Toronto, Ontario. He can be reached through his website: www.rootcanals.ca.