Several parameters should be considered in shaping root canal spaces, these include choosing the correct:
• working length
• master apical diameter
• taper.
With regard to working length, while there is some divergence of opinion among clinicians, the natural termination point for endodontic cleanings, shaping and obturation is the minor constriction (MC) of the apical foramen. We instrument, irrigate and obturate to the MC in both vital and necrotic cases. For instrumentation, this means that the MC is left at its original position and size and that the tip of shaping instrument (rotary nickel titanium (RNT) file tip) goes up to but not beyond the MC.
Breaking down the stages of instrumentation even further, the achievement of straight line access and removal of the cervical dentinal triangle are absolutely essential to gain tactile and visual control over the orifice and hence all shaping procedures that occur beneath the orifice to the position of the MC. Canals, in the most general terms, fall into four categories, negotiable, non-negotiable, transported and non-transported. While a comprehensive discussion of each of these canal types is beyond the scope of this article, suffice it to say that if a canal is non negotiable, the clinician should work with small hand files for as long as it takes to make the canal negotiable and ultimately patent at the MC. Efforts to shape the canal should not cease until the canal is made negotiable and patent or the canal cannot be made negotiable. To shape a canal which is non-negotiable risks perforation, instrument fracture, further blockage and a host of other iatrogenic events. In addition, transported canals should be viewed in the context of whether the completion of the root canal can be achieved with a coronal and apical seal and/or whether the completed long term result risks vertical root fracture or perforation, usually at the midroot.
In our hands, master cones and obturators are taken to the MC and not placed short for “safety” to minimize the extrusion of sealer and obturating material. If the quantity of sealer and/or placement of the master cone are ideal, it is unnecessary to back the master cone or obturator away from the MC. It is simple to achieve cone and/or obturator fit at the MC in a canal shaped to an ideal taper and master apical diameter.
While RNT shaping files can remove pulp and bacteria, the true measure of canal disinfection is the penetration of irrigants into the apical regions of the roots. Conceptually, a correctly prepared taper provides a means for the irrigating solutions to reach the apical regions of the tooth that would otherwise remain untouched. The validity of this concept is found in two places in the endodontic literature. One is the fact that larger apical diameters result in cleaner canals. Studies have found that canals prepared to a #50 ISO diameter for example are cleaner than canals prepared to a #30 ISO diameter. In essence, bigger is better with regard to canal preparation as long as the preparation does not risk or cause iatrogenic events. The second evidence for this concept in the endodontic literature is the fact that non-vital teeth with apical pathology do not heal at the same rate as vital cases. This second finding is interrelated to the first in that ideally shaping and cleaning is a combination of both preparing the correct taper and master apical diameter. To maximize success then, the taper and the master apical diameter must be optimized. Especially so in non-vital cases as asepsis in vital cases is simpler to achieve (keeping bacteria out of the tooth) than disinfection (removing bacteria from the canal system).
Preparation of an ideal taper in any given root is a function of a number of different parameters. These include:
Pre-operatively assessing the risk of strip perforation and excessive dentin removal given the anatomy at hand using GG drills, Peezo Reamers and RNT orifice openers. Three dimensionally narrow roots are especially at risk as are highly fluted ones.
The types of instruments used to create the taper of the canal can influence (for better or worse) the final ideal prepared taper. For example, using GG drills the clinician must blend the various insertions to create a continuously narrowing taper in the coronal third. With the tendency of orifice openers and GG drills to “screw in” the coronal taper can easily exceed that required to adequately shape the root. For example, using GG drills, the clinician might use the #5 GG at the orifice, the #4 GG approximately a 1-2 mm below the orifice, #3 GG 2-3 mm below this, etc depending on the root being enlarged. It is more challenging to maintain a continuous taper having to blend the shapes created by these instruments, especially from the orifice to the apex, with this method compared to using a RNT files with a continuous taper along the length of the root. For example, using the Twisted File* the .08/25 TF can, once a glide path is prepared from the orifice to the apex, reach the MC in approximately 3-4 insertions. This basic taper provides the rough shaping upon which irrigation and obturation is later optimized. Said differently, .08 taper prepared throughout the root is generally more desirable relative to a .06 or .04 taper with regard to irrigation efficiency and cone fit. Obtaining a .08 taper especially in the middle root third to apical third provides what is known as “deep body shape” and while more of a clinical concept than scientific principle, obtaining deep body shape provides ideal control of irrigation, control over determination of the position of the MC and preparation of the master apical diameter as well as cone fit in the apical third. Alternatively, creating a less than optimal taper throughout the root length leads to difficulties in all of these areas manifest for example as a RealSeal One Obturator that may not go to length where it would and should otherwise easily reach true working length if the preparation were ideal. Fig. 1.
The achievement of an ideal taper is part of the larger process of canal enlargement, a process that should be determined by adherence to a rigid set of principles that are independent of the means used to achieve these preparation goals. These principles are to leave the canal in its original position, achieve narrowing cross sectional diameters, leave the MC in its original position and at its original size, and achieving a taper and master apical diameter that are appropriate for the given root dimensions. These root dimensions to be considered are the root length, the root width (mesial to distal and buccal to lingual) and the presence of fluting, possible resorption (internally and/or externally) amongst other considerations. Achieving the correct taper depends, in part on optimizing all these variables correctly.
Creation of ideal tapers clinically has its roots in the treatment planning that should occur taking into account the principles and concepts described above. With these concepts and principles held in mind during their application, the means that are used to prepare the canal are secondary to the correct adherence to the principles required. With this as a background, we use the Twisted File for the creation of taper prior to accentuation of the master apical diameter. Specifically, if the root has significant bulk, the .10-tapered TF (.10/25 TF) is used throughout the length of the canal. Sufficient bulk to allow this taper is commonly found in the palatal canals of upper molars and the distal roots of lower molars. More intermediate roots, such as the mesial root of a lower molars and bicuspid canals can easily accept a .08 TF taper prepared with the .08/25 TF. In extremely complex curvatures and calcified roots, the .06 taper can easily be created with TF (.06/25 TF) prior to the accentuation
of the master apical diameter. Interestingly, the preparation of a .06 taper can often be followed up with a .08 taper in such complex canals.
Clinically, using the mesial root of a lower molar as an example, after achievement of a straight-line access and removal of the cervical dentinal triangle the orifice of the canal should be shaped to allow insertion of subsequent hand K files as well as TF. Shaping the orifice is done with the TF that the clinician expects will become the final taper along the entire length of the root. For example, if the clinician is shaping the mesial root of a lower molar, the .08 TF is placed into the coronal 3-4 mm to create space for subsequent instruments as described and this is the final expected taper to be prepared along the length of the root. With irrigant in the access, a #6 or #8 hand K file is placed into the canal to the MC which can often be felt as a tangible ‘pop’ If no pop is felt, the clinician should stop at the position of the estimated working length (the estimate of the true working length that is determined from the initial radiographs taken before starting the case).
Once the first hand file reaches the estimated working length the clinician can take an electronic length to confirm the position of the true working length. Once this vital information is gathered, in this clinical example, the clinician can take the .08/25 TF in successive insertions (alternating irrigations and recapitulations) down the canal to the true working length. Once the .08/25 TF reaches the MC, the true working length is taken again electronically and verified. Using the TF system, the master apical diameter is enhanced from the initial diameter of a #25 to a larger size by using the .06/30, .06/35 and .04/40 and .04/50 as desired. Because each of these subsequent TF instruments (.06/30, .06/35, .04/40, .04/50) are cutting only on their tips, they only need to be inserted once to prepare the portion of the canal in which they are functioning.
If, in the clinical scenario above, the .08/25 TF will not move apically without undue pressure, the next smaller TF should be used. In this case, the .06/25 would be inserted if the .08 were not moving with the ease expected. In complex anatomy, it is not uncommon to use the .08/25 and .06/25 in tandem to create the desired final apical taper.
The clinical considerations in the preparation of an idea taper has been described as well as relevant clinical means to create this taper. Emphasis has been placed on a need to avoid iatrogenic events as well as optimization of irrigation and obturation hydraulics. We welcome your feedback. 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 is in private practice in Endodontics in Vancouver, WA, USA.
Dr. Gary Glassman graduated from the University of Toronto, Faculty of Dentistry in 1984 and is a graduate of the Endodontology Program at Temple University in1987. He has been on staff at the University of Toronto, Faculty of Dentistry in the graduate department of endodontics. Gary is a fellow of the Royal College of Dentists of Canada, and the endodontic editor of Oral Health dental journal. He maintains a private practice, Endodontic Specialists in Toronto, Ontario, Canada. He can be reached through his website www.rootcanals.ca
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