Shaping Root Canal Systems: A “Real World” Case

This paper was written to discuss basic concepts in endodontic cleaning and shaping that can be applied irrespective of the instrumentation system utilized. Emphasis will be placed on straight-line access, achievement and maintenance of apical patency, providing optimal taper and master apical diameter, and insertion of an early coronal seal. A clinical case will be presented to add context to this discussion.

CONCEPT OF SHAPING AND SHAPING GOALS
In general concept, shaping root canal systems is quite simple. A small space within the root of the tooth is enlarged to a desired dimension in order to place a biocompatible filling that, combined with a coronal seal, will allow retention of the natural tooth. This process is designed to duplicate the net effect of extracting the tooth. Extraction resolves pulpal symptoms because the entire pulp is removed from the patient. If the clinician can remove as much pulp and bacteria from the canal space as possible, while leaving the roots and remaining tooth structure, healing can occur and the tooth remain functional.
      More specifically though, in attainment of these goals, the clinician seeks to prepare a tapering funnel throughout the length of the root from the orifice to the apex, maintain the minor constriction (MC) of the apical foramen at its original position and size, prepare a taper that allows optimal irrigation and obturation hydraulics, maintain the canal in its original position and minimize any risk of iatrogenic events, including file separation, strip perforation or the unnecessary removal of tooth structure that might later lead to vertical fracture.
      Application of these stated goals in clinical practice are obviously too complex to carry out in a number of clinical cases, hence the wide variation in endodontic case risk and difficulty. Bedeviling endodontic treatment, among other challenges, are roots of atypical length, curvature, calcification, resorption as well as challenges with anesthesia, patient cooperation, and tooth position.

CLINICAL STRATEGIES
Certain clinical strategies are productive in pursuit of these stated treatment goals irrespective of the instruments used by the clinician. These strategies, in addition to those above, include copious irrigation, straight-line access, achievement and maintenance of apical patency, removal of the cervical dentinal triangle, crown down instrumentation for the preparation of the basic taper, step back enhancement of the master apical diameter, and placement of a coronal seal at the time of treatment under the rubber dam. It has value to provide optimal magnification and visualization to the clinical setting in the form of the surgical operating microscope (Global Surgical) and loupes 4.8x HiRes, Class IV (Orascoptic). These concepts are detailed below and discussed in the context of a “real world” clinical case.

REAL WORLD CLINICAL CASE
It is both beneficial and essential to determine the case difficulty and to assess the clinical challenges that will be faced during endodontic treatment. This allows the clinician to best assess the strategies that will be required to manage the case in its entirety. In essence, to know that once started, the case can be finished. Wisdom, in a clinical context, is the knowledge of which cases can be managed well within the time, equipment, and training of the specific clinician, and which cases should be referred.
      The value of case assessment cannot be overstated. Knowing when to refer and when to treat the patient is essential. While indications for referral are myriad, they ultimately involve seeing the potential iatrogenic event before it occurs and also understanding all of the clinical challenges that may result from treatment before they happen. For example, evaluating the case pictured in Figures 1a and 1b, the clinician has not appreciated the relatively mild tipping of the tooth to the mesial and its impact on management of the apical third of the root with the bayonet curvature apically. This tooth was referred because the clinician could not negotiate the mesial root to the apex. The cervical dentinal triangle of this tooth has not been removed and while a great deal of tooth structure has been removed during access, a straight-line access has not been prepared into the coronal third of the mesial root. It would be impossible to manage the apical third of the mesial root of this tooth given these defects in access preparation.

     The case in Figure 2a is another case in point. The canals of both roots are calcified. This said, the abrupt curvature present in the distal root at the junction of the middle and apical third would be a significant risk of instrument fracture. In addition, the calcified pulp chamber also could cause access difficulties that might include leaving pulp chamber unroofed and add the possibility of canal blockage because the pulp chamber tissue and debris might be carried into the roots (if the full extent of the pulp chamber is not uncovered and all calcified remnants in the pulp chamber are not removed appropriately). When evaluating the final treatment radiographs, it is evident that straight-line access has been prepared and the cervical dentinal triangle has been removed. Both of these actions can only lead to greater tactile control in the apical third of both of these roots as well as optimal irrigation as greater volumes of irrigation are able to reach the apical third when the final taper and master apical diameter are prepared in addition to heating and activating the irrigation for these particular cases.
      In these clinical cases, the irrigation was heated, performed with a side-venting needle, activated ultrasonically using the MiniEndo (SybronEndo) and an ultrasonic file adapter and stainless steel K file. The final prepared taper and optimal apical diameter were prepared using the Twisted File (TF) system (SybronEndo). All of the roots were prepared with .08 taper except the distal root of the tooth shown in Figure 2b (.10 TF taper). All master apical diameters were enhanced to at least a No. 40 or No. 50 ISO preparation size using a .06/30, .06/35, .04/40 and .04/50 TFs sequence after either the .08 and .010 TF master apical taper was prepared. The case was obturated with RealSeal bonded obturation material (Figures 3 and 4).

Figure 3. TF system (.12/25, .10/25, .08/25, .06/25 and .04/25).	Figure 4. Large Apical Assorted (30/.06, 35/.06, 40/.04)

     The authors use TF due to its fracture resistance, flexibility, and cutting prowess. Created by twisting nickel titanium in the rhombohedral crystalline phase configuration, an intermediate crystalline phase configuration between austenite (nickel titanium at rest) and martensite (nickel titanium under stress), the file does not have the scratches and machining marks of nickel titanium files resulting from manufacturing that uses a grinding process. This is noteworthy because these machining marks become the focus of cyclic fatigue and torsional stresses that might otherwise fracture the file. As a result of its manufacturing process, TF can be used in many roots (approximately 75%) to shape the basic preparation to a .08 taper in 2 to 4 insertions without the use of rotary nickel titanium (RNT) orifice openers or other instruments such as Gates Glidden drills. Production of a larger master apical diameter is simple and efficient (the sequence of which is outlined above) due to the larger tapers created in the basic preparation and the strength and flexibility of the larger apical sizes. A larger taper such as .08 throughout the length of the canal allows a smaller taper such as a .06 and/or .04 to easily reach the apical third to shape the larger apical diameter.
      RealSeal (SybronEndo) is the material suggested and used for obturation in these cases because in vitro and in vivo there is persuasive evidence in the endodontic literature that while the bond is not a mono block and is relatively weak relative to direct composites, that the bond within root canal spaces created is enough to diminish coronal microleakage in a statistically significant way when measured across the totality of the canal. The material is utilized exactly as gutta-percha and is available both in a master cone-based form and in obturator-based form. The exact instrumentation sequence employed in these 2 cases is provided in the following text.

EVALUATION OF A “REAL WORLD” CLINICAL CASE
The case pictured in Figures 5a to 5e was necrotic. Radiographic evaluation of the tooth revealed mild curvature and internal resorption in both the mesial and the distal roots (Figures 5a, 5b, and 5e). Access was made through a crown. Access through a crown virtually always makes the treatment more clinically difficult in that the usual landmarks that direct the clinician to the pulp chamber are often removed relative to access into a pulp chamber that is otherwise unobstructed. The case was instrumented with TFs used in a crown down manner using .08/25 TF and .06/30 TF. Irrigation was carried out with 5.25% sodium hypochlorite and liquid EDTA 17%. After the master cone trial film was taken in Figure 5c, the clinician decided to refine the apical preparation for better conefit. The .06/30 was then inserted to the true working length (TWL). During this insertion the .06/30 TF separated and was found radiographically beyond the apex (Figure 5d).
      This case is interesting and worthy of discussion from a variety of perspectives. First off, accurate determination of TWL is an essential component in providing excellent treatment. Preservation of the MC at its original position and size, as mentioned earlier, cannot be overstated in its vital importance. Determination and verification of TWL repeatedly throughout the process of cleaning and shaping is essential. Ideally, the TWL should be estimated from the initial radiographs before the case is ever started. This estimated working length is correlated with the resistance that the clinician receives as the canal is negotiated. In many teeth, the clinician will feel a tactile pop at the MC as the first hand K file passes through the narrowest diameter of the canal. Once the first hand K file reaches the TWL in negotiation, the clinician should attempt to obtain an electronic working length if possible. When the first RNT file, in this case TF, reaches the TWL, it should be taken again with an electronic apex locator. When the last TF reaches the TWL, the TWL should be taken again electronically. Conefit films have value and can allow the clinician to adequately visualize the final anticipated obturation.

     This level of diligence in determination of the TWL can go far toward avoidance of the file separation that occurred. Specifically, the clinician should constantly and carefully measure the position of the RNT file tip in the canal. This is more involved in actual practice than it might sound. For example, the clinician should consider using the laser markings for length reference present on many RNT file systems. Using a rubber stopper is inherently inaccurate as often the stopper moves on the file if touched leading to inherent inaccuracy.

     In clinical practice, it is important to see the reference point in relation to the laser marks or rubber stopper to measure the depth of RNT file insertion. Tactile control over the RNT during insertion is critical. Some RNT files have a tendency to “screw in.” Some don’t cut particularly well and some are more or less flexible than others. The variations in RNT file systems with regard to tactile control based on the various design features and manufacturers’ recommended use and anatomy encountered are infinite. This said, having the right tactile control over the file to insert the file only to the desired length using the correct rotational speed and tactile insertion pressure and speed couldn’t be overstated. Using TF as an example, the file should be inserted in a smooth continuous and controlled motion that takes approximately 2 to 3 seconds. While SybronEndo recommends 500 rpm for TF, TF can be rotated at higher speeds. In any event, the file is never plunged back into the canal repeatedly with the intention of trying to gain additional length.

Figure 6. Clinical case shaped with TFs and obturated with RealSeal One Bonded Obturators (SybronEndo).

     The separation in this clinical case likely occurred because the file was inserted past the MC and into the apical bone. It is very likely the file was taken into an acute angle as it exited from the root even though this might not be evident from the fragment or radiographic appearance of the root. This is avoidable if the clinician remains cognizant of the optimal depth of insertion (TWL) and blends this knowledge with the correct pressure and rotational and insertion speed during RNT file introduction into the canal (Figure 6).

SUMMARY
This paper was written to discuss basic concepts in endodontic cleaning and shaping from a clinical perspective for general practitioners. Emphasis was placed on straight-line access, achievement and maintenance of apical patency, providing optimal taper and master apical diameter and placement of an early coronal seal. Several cases were evaluated which detailed the needed preoperative assessment of clinical and technical challenges, while also discussing strategies to overcome these particular issues; in particular, to consider using the M4 Safety reciprocating handpiece attachment in creation of the glide path prior to RNT file use and crown down instrumentation to achieve the ideal taper and step back techniques to prepare the enhanced master apical diameter. We welcome your feedback.

Dr. Mounce is in private practice in endodontics in Vancouver, Wash. He lectures globally and is widely published. He recently authored the nonfiction book, Dead Stuck (DeadStuck.com; Pacific Sky Publishing), about “one man’s stories of adventure, parenting, and marriage told without heaping platitudes of political correctness.” He can be reached via e-mail at the address lineker@comcast.net.

Disclosure: Dr. Mounce is on the advisory council of SybronEndo. He receives honoraria for certain parts of this work, such as lecturing.

Dr. Friedman is a general dentist with a keen interest in endodontics. He is in private practice in Rapid City, SD. He can be reached via e-mail at the address jfriedmandds@gmail.com.

Disclosure: Dr. Friedman reports no conflicts of interest.