Everything old is new again is an intriguing euphemism as demonstrated by the fact that rotary instrumentation based upon shape memory alloys is not a modern era concept; it was introduced in the late 19th century.1 Everything works, everything fails, another euphemism, reflects an interesting facet of endodontic therapy; however, as Figure 1 suggests, the concept needs to be elucidated further. There are biologic mandates for all successful treatment procedures and it is a synthesis of the clinician’s knowledge and technical acumen, in addition to the tools used that lead to positive treatment outcomes and predictable clinical success.
Figure 1: Rube Goldbergian endodontics – simplicity based on sound foundational precepts defines sophistication – in this vainglorious attempt, a complex approach is hallmarked by convolution and ineptitude.
Figure 2: Legacy shaping is based on the concept of a continuously tapering funnel. While this may be relevant in the apical third to ensure an apical stop, the removal of peri-cervical dentin and root structure in furcal areas and bifidities can lead to iatrogenic fracture. As evidenced by the cbCT image, residual structure and its supportive capacity are analogous to the third law of motion. You can’t exceed tolerance with the application of force unless the containment is suitable.
An endless array of files have been brought to market for negotiating and shaping canals over the past decades. Today, there are in excess of 70 different rotary or reciprocating shape memory instrument systems in the global marketplace. Regardless of the system configuration, the mechanical objectives of canal preparation remain consistent with those delineated by Dr. Herb Schilder some forty years ago2; however, increasingly, the caveat, “within reason” has become of extreme importance as the shaping of the root canal system is being linked with an uncomfortably increased incidence in root fracture (Fig. 2).3
The purpose of this article is to showcase a new file system, The Zone™ (ZendoDirect AG, Switzerland), that iterates design features from the past and introduces a myriad of technical innovations for its clinical application in the preparation of shaped root canal systems (Figs. 3a, b).
Figure 3(A) & 3(B) (below): Preparation technique and instruments and final preparation size have to be defined individually for each root canal system. Legacy shaping is truly a thing of the past. Its acceptance was based on technical limitations of the armamentarium available and at this juncture, it is no longer relevant.
Figure 3(C). The Zone has a .25mm tip diameter, a 0.06 constant taper, is designated for single use only, is packaged in a sterile blister pack and can be used with any motor at a rotation speed of 400 rpm and 4n/cm.
Our understanding of how design impacts on the physical properties of files such as stress, stress concentration point, strain, elastic limit, elastic deformation and plastic deformation, along with metallurgy augmentation, have engendered a radical shift in design parameters as well as the type of motion used in the instrumentation process. Versiani and Zapata, among others, have reinforced our appreciation of the exotic, labyrinthine complexity of the root canal system with the exquisite detail evidenced by the use of micro-computed tomography (Fig. 4).4,5 The concern about single-file shaping has been eliminated by the ability of this technology to demonstrate the degree to which a single file effects debridement and creates optimal shaping.
Figure 4: tudies conducted by the principals of the Root Canal Anatomy Project (http://rootcanalanatomy.blogspot.ca) showed that all systems perform similarly in terms of the amount of touched dentin walls. None of the systems were capable of completely preparing the oval-shaped root canals.
The Zone rotary system was created to take into account the anatomical variations evident in the coronal, transition and apical regions of the root canal systems of all tooth types. The instrument incorporates variable cross sections along the length of the blade: the apical zone presents with a variable three cutting edge design, the second, prior to the transition section, has a cross-section that progressively changes from three to two cutting edges, and the coronal is designed with two cutting edges (Fig. 5).
Figure 5: The advantages of this unique design ensures that one single instrument with three variable cross-section zones does the job of three files. Guided down the glide path by three cutting edges. The ZONE’s flexibility assures the maintenance of the original canal path and curvature and the variation of cross-sections offers an optimal cutting action in three zones of the canal. The two-cutting-edge zone on the instrument coronal portion offers optimal cutting and coronal debris removal while the variable pitch of The Zone reduces the possibility of a screw-in effect. One of the most unique features is the ABC (Anti Breakage Control) for additional safety; the instrument will unwind to avoid separation.
STEP ONE – Access Cavity Preparation
Radiographs should be taken to facilitate the assessment of canal difficulty, approximate the working length of the root canal and calibrate the degree of compensation for dystrophic or other calcification that has occurred in the pulp chamber,in order to create a lesser-angled journey to the canal orifices taking into account optimal preservation of peri-cervical dentin. We have left the Gates-Glidden era behind as the implications of root weakening and perforation so elegantly described by Kuttler, McLean et al.6 leave no doubt as the risk potential these instruments present. The Z-FLARE™ is used to remove orifice obstructions–no more than 3mm below the lip of the canal orifice creating a four to six degree divergent flare thus preservi
ng peri-cervical dentin. All coronal constrictions can be removed with the Z-FLARE (Figs. 7, 8). In procedural descriptions in this article, the reader is to axiomatically include the routine use of copious irrigation with sodium hypochlorite throughout each step.
Figure 6: The tapering funnel need only exist in the continuum between the apical region and its continuum with the transition zone. The risk factors associated with undetectable furcal concavities in flat film representation are simply too high to ignore.
Figure 7:The Z-FLARE is used with a rotation speed of 300 to 600 rpm in a brushing motion.
STEP TWO – Establishing the Glide Path
The glide path is prepared using a pre-curved (to match the canal curvature obtained from the pre-operative radiograph) #10 Stainless Steel K-type hand file (Precizen™ #10). A #15 K file is used to determine working length (WL). The glide path is a reproducible smooth tunnel from the endodontic access orifice to the apical foramen. The size of the tunneled shaft is validated by slowly increasing a #10 file’s vertical movement down and up the radicular glide path. In this way, the operator can determine that a loose #10 endodontic manual file will effortlessly follow an unrestricted path along the preliminarily opened root canal space (Fig. 8).
Figure 8:The lack of glide path establishment and glide path enlargement is often the cause of ledge formation, transportation, and blockage of root canals followed by obturation short of the minor apical diameter. Each step in the instrumentation protocol is a chain of events and any chain is only as strong as it’s weakest link. protocol is a chain of events and any chain is only as strong as it’s weakest link.
STEP TWO – Establishing the Glide Path (complex)
If canal constrictions prevent the #10 K file from easily reaching the EWL, use a #08 K file along with copious irrigation until the #10 K file reaches the EWL. The glide path preparation is then completed using Zendo® Pathfinder Files™ in continuous rotation (250–400 rpm at 1.2 N.cm). The Pathfinder Files have superior flexibility due to their small diameters (#12 & #17) and their minimal taper (.03) and with their non-cutting tip can work their way down canals where the use of hand files may be impossible. Use the Pathfinder 1 to the WL followed by the Pathfinder 2 to the WL according to their recommended protocol. Copiously irrigate after each instrument passage (Figs. 9a, 9b). Finally, use a #15 K file to determine working length (WL), and use The ZONE as described in Step Three.
Figure 9(A): The Pathfinder files are used with a rotation speed of 250 – 400 rpm with a maximum torque of 1.2 N/cm. Their operative dynamics is a slow and unique downward movement in a free progression without pressure. In the case of too much resistance, the file is withdrawn and the movement apically restarted.
Figure 9(B): The P1 has a 0.13mm apical tip and the taper along the length is 0.03mm. The P2 has a 0.17mm apical tip and a 0.03mm taper as well. Both files come in 21, 25 and 29mm lengths.
STEP THREE – Shaping the Root Canal with The ZONE
Irrigate thoroughly with sodium hypochlorite. A chelating gel can also be used during the root canal shaping. Work the ZONE down to two-thirds of the WL using an “in and out” amplitude movement without pressure. Withdraw the instrument with an upward brush stroke movement in order to enlarge the canal. Remove The ZONE from the root canal and clean it. Irrigate the canal and check patency with a #10 K file.Reintroduce the ZONE into the canal and work it to 3 mm from WL using an “in and out” amplitude movement without pressure. Remove the ZONE from the root canal and clean it. Irrigate and re-check the canal patency with a #10 K file. Reintroduce the ZONE into the root canal and work it to the WL by performing the recommended “in and out” amplitude movement. The WL can be reached in one or more passages (file withdrawal, cleaning of the file, irrigation and patency check) depending on the complexity of the canal anatomy. A coronal brush stroke filing movement can then be performed if necessary, based on the root canal anatomy. Remove and clean the instrument and irrigate the canal when apical resistance or a slight apical pull is encountered. The speed of rotation is 350-450 rpm at a maximum torque of 4 N.cm. The steps are repeated until the WL is reached (Fig. 10).
Figure 10. The apical terminus in many cases will not be a .25mm diameter. The Zone file has created a safe tunneled path for apical gauging with larger hand files to ensure that the apical stop at the minor apical diameter is an accurate representation of the native anatomic state.
Shaping of the canal space with this technology is intentionally minimalistic. No doubt new file systems will continue to come to the marketplace; however, as evidenced by the latest reciprocating systems, the taper needs
to be reduced and the tendency for apical fracturing eradicated.7 The latest iteration of the Protaper series has recognized that .08 and .09 simply exceed allowable tolerances.
As clinicians, we must rely on the elegant simplicity produced by design engineers who have iterated the various generations of nickel-titanium file shapes and configurations to ensure that breakage is non-existent and the use of less and less files in conjunction with enhanced irrigation protocols is the standard. The ZONE file system is another road less taken on the journey to endodontic excellence. OH
Dr. Serota is on the advisory board of The Endo Academy (TheEndoAcademy.com). He maintains a private practice limited to endodontics in Mississauga and visits Timmins, Ontario twice a month to service the underserviced endodontic needs of North Ontario’s dentists. Oral Health welcomes this original article.
1. Hülsman M, Peters OA, Dummer PMH. Mechanical preparation of root canals: shaping goals, techniques and means. Endodontic Topics 2005;(10);30–76
2. Schilder H. Cleaning and shaping the root canal. Dental Clinics of North America 1974;18(2)
3. Peters OA. Current challenges and concepts in the preparation of root canal systems: A review. J Endo August 2004;30(8):559-67
4. The root canal anatomy project: A micro-CT study guide. http://rootcanalanatomy.blogspot.ca
6. Kuttler S, McLean et al. The impact of post space preparation with Gates-Glidden drills on residual dentin thickness in distal roots of mandibular molars. JADA July 2004;135:903-09
7. Bürklein S, Schäfer E. Incidence of dentinal defects after root canal preparation: Reciprocating versus rotary instrumentation. J Endo April 2013;39(4):501-04