The crown-down approach for root canal space preparation has been optimized through the past decade by advances in rotary nickel-titanium (NiTi) instrumentation.1 The improved canal architecture produced by these instruments in conjunction with enhanced thermosoftened obturation techniques has demonstrably increased the predictability of endodontic success. However; regardless of the iterations in file design, the myriad complexity of root canal system morphology makes it is increasingly apparent that the “one system fits all” paradigm is no longer applicable.
In addition, as the concepts of deep shaping, apical patency and apical gauging2-4 have become more and more mainstream, a greater appreciation of the need to fashion an apical control zone has taken hold.5,6 The ‘Apical Control Zone’ is a matrix-like region created in the apical third of the root canal space. This zone demonstrates an exaggerated taper from the clinician defined apical preparation terminus regardless of whether this is spatially a linear or point determination. The enhanced taper in the apical control zone provides resistance form against the condensation pressures of obturation and acts to prevent the extrusion of the filling material during obturation.7 Its formation is a function of the geometries incorporated into the design of NiTi instruments as well as the stainless steel (SS) hand file sequence used (Fig. 1).
This article addresses the synergy potential possible by combining the disparate design features of two rotary NiTi instrumentation systems currently available; Protaper files (Fig. 2a) and ProSystem GT files (Figs. 2b & c). The geometries of each produce an excellent shaping definition to the root canal space. Used in concert, with appropriate irrigation protocols,8 the shaping achieved enables debridement, disinfection and three-dimensional obturation in an unprecedented manner.
GLIDE PATH
Straight-line access to the apical region is essential in order to retain the natural flow of the root canal space during preparation in x, y and z axes. An ideal access entails the creation of a Class I inlay preparation to deroof the pulp chamber and align the axial walls of the preparation in a divergent orientation without undermining the peripheral coronal tooth structure. In the coronal segment, this orientation initiates the Glide Path (Fig. 3). Spatial alignment of the cusp tip, the pulp horn, the canal orifice and the interface of the middle and apical one-third intersection enables retention of the apical foramen in its original anatomical position during shaping.
The Glide Path is then further extended to the apical terminus by blueprinting a straight-line access pathway with smaller sized hand files; with this surveyed pathway, rotary NiTi instruments with their self-centering ability and their super-elastic capacity can readily negotiate even the most complex system. However, if the irregularities in the glide path cannot be recontoured, then the apical architecture is best shaped with SS hand files. The elasticity of NiTi files does not compensate for their propensity to fracture in aberrant canal anatomy.
Furthermore, the creation of the Class I inlay style preparation defines the intra-chamber preparation for the coronal restorative component that will be placed after root canal treatment has been completed. The seamless integration of root canal space and restoration minimizes interface steps capable of producing stress vectors in the tooth that could lead to fracture during function (Fig. 4).
INITIAL PREPARATION
The chamber is accessed, pulp tissue and dystrophic calcification removal completed; warmed 5.25% sodium hypochlorite is left to soak in the chamber for 10 minutes. The chamber is then flushed and a caries detection agent used to landmark all orifices.
Preliminary debridement and passive exploration of the canal confines is made with ISO standard .02 taper SS files, #’s 08 through #20 in sequence (Fig. 5). Negotiation of the root canal system with hand files orients the operator as to the x, y, and z direction of the root canal space and identifies anatomic anomalies that may prevent the NiTi instruments from reaching the apical terminus. If the SS files can initially traverse the root canal space and negotiate to the EAL determined working length at this juncture, all the better, however; in the more typically calcified root canals this is neither expected, nor is it the intention of this initial phase of the shaping procedure. The “apex last approach” will eventually enable determination of an accurate working length without forcing instruments to place regardless of the degree of anatomic challenge presented by the canal.9
All hand files are used in a “watchwinding” motion; quarter turn clockwise followed by a quarter turn counterclockwise. This feeds the file into the canal until it first binds. At the initiation of treatment, instruments bind in the coronal aspect of an unflared canal, not in the apical region. Once engaged, the file is retrieved coronally away from the furcal wall along a linear path of no more than 2mm to ensure that the path of penetration is retained.
The depth of insertion of the #20 file is measured and this length transferred to the Protaper Shaping File No. 1 (purple ring) / S1 and auxiliary Shaping file/SX (no ring). The Sx relocates the coronal aspect of canals away from external concavities and will produce more shape, as desired, within any canal. Without pressure, and in one or more passes, the S1 and Sx are allowed to passively cut into the canal until their apical travel slows. They may be used like a brush to laterally cut dentin on the outstroke until the coronal to middle 1/3 of the canal are optimally prepared (Fig. 6). Applicable axioms; never push, exert only light pressure, avoid in and out pecking motion, repeatedly clean flutes in order to minimize compaction stress creation.
At this point, except in the most extremely calcified cases, it is reasonable to assume that a #08 or #10 file can be negotiated to the apical foramen and an electrometric length determination (EAL – ROOT ZX, J. Morita, Irvine CA) reading taken. Retaining the distance between reproducible occlusal and apical reference points is a function of apical patency.10 A combination of the EAL determined working length (WL) and paper points, in distinct contrast to the traditional use of the radiographic terminus is the most accurate means by which to determine the true canal preparation termination point11 (Fig. 7).The EAL will be used throughout the procedure to reconfirm WL as a more direct path to the terminus is established during shaping procedures which invariably shortens the WL to a degree.
The reproducible Glide Path to the apical terminus is checked and then the Protaper sequence S1, S2 (white ring) and F1 [yellow ring] (D0 diameter/taper of .20/.07) is carried to the full established working length. Depending on the length, diameter and curvature of the canal, the F1 will generally achieve length in one pass, a function of its reverse taper design.12 The finishing criterion is to remove the F1, inspect its apical flutes and, if they are loaded with dentin, the shape is cut (Fig. 8).
APICAL GAUGING
Apical gauging is the technique where the apical diameter of the canal is measured after the shaping objective file (SOF), the file of suitable taper based on root morphology, has cut to working length. This is necessary to confirm that apical continuity of taper exists and that the tapered preparation extends all the way to the terminus of the canal. For the sake of example, if the SOF taken to length has a .20mm tip diameter, gauging is done with a #15 file passed through the canal terminus. A #20 K-file is taken to the terminus without pushing or cutting dentin. If it lightly binds at length and the #25 and #30 files bind shorter in the canal, there is apical continuity and the shape of the canal has been determined. If the #20 K-file can still pass through the end of the root canal without meeting resistance, the K-file that binds at length is
determined and the shape is adjusted by taking the same size tapered instrument into the canal with a larger tip size (Fig. 9).
HYBRID APPROACH
Deep shape is essential for ideal cone fit and enables penetration of obturation condensers to an optimum depth. The deep shape rendered by the Protaper files is now further augmented with the use of ProSystem GT files (Fig. 10). The geometries of the GT files were designed to mimic the natural anatomy of the root canal space when it first forms and therefore idealize the shape created. The 20/.10 GT file is used first regardless of root size and continues in a crown-down fashion until the first GT file gets to length (20/.10, 20/.08, 20/.06). As a general rule, small roots (mandibular incisors, 2 & 3 canal premolars, mesial roots of lower molars and buccal roots of upper molars) are shaped to a .06 or .08 taper; canals with abrupt apical bends or multi-planer curves, an.06 taper exclusively is used. Medium roots are: distal roots of lower molars and palatal roots of upper molars, and usually have a .10 taper. Large roots are: lower cuspids, upper anteriors and one-canal pre-molars.13
The goal of the root canal space preparation is to enable a gutta-percha cone of suitable size and shape (ideally a non-standardized fine-medium or medium cone) to intimately fit the preparation. After the initial sequencing with the 20/GT file series, a fine-medium or medium gutta-percha cone is inserted and the degree of shaping achieved or required assessed (Fig. 11). If the cone comes up short of the terminus, the file sequence is repeated by recapitulation (Fig. 12) until the master cone to be used (fine-medium or medium) fits intimately and exactly (Fig. 13).
The difference in the exaggerated taper of the Apical Control Zone achieved with the Protaper files initially and then augmented by the ProSystem GT files is shown in figure 14. It remains to be seen whether we can ever achieve 100% predictable endodontic success; however, one thing is patently clear. Each innovation in canal debridement and disinfection, each iteration in irrigant formulation, and each successive evolution in obturation technology brings us that much closer to the day when apical periodontitis can be successfully eliminated and the possibility of its recrudescence negated.OH
Dr. Yosef Nahmias maintains a private practice dedicated to endodontics in Oakville, ON. An international lecturer, he is the creator and chief editor of Endoweb — an Endodontic e-zine (www.endoweb.com).
Dr. Kenneth S. Serota maintains a private practice, Endodontic Solutions, dedicated to endodontics in Mississauga, ON. He is the founder of ROOTS — an online Endodontic discussion group, contributing Endodontic consultant to Oral Health, and on the editorial board of Endodontic Practice.
Neither author has a fiduciary interest in the products discussed, nor are they affiliated with the manufacturing company in any way.
Oral Health welcomes this original article.
REFERENCES
1.Yun HH, Kim SK. A comparison of the shaping abilities of 4 nickel-titanium rotary instruments in simulated root canals. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003 Feb;95(2):228-33
2.West JD, Roane JB. Cleaning and Shaping the Root Canal System. Pathways of the Pulp. 7th. Ed.,1998. Mosby, St. Louis.
3.Buchanan LS: Shaping root canals, Part 3. Large-root canals with small coronal and apical diameters. Dent Today. 1999 Nov;18(11): 76-9.
4.Buchanan LS. Shaping root canals, Part 4. GT File technique in large root canals with large apical diameters. Dent Today. 1999 Dec;18(12):64-9.
5.Buchanan LS. The predefined endodontic preparation: instrument and technique updates for the greater taper system. Dent Today. 2001 Jan;20(1):56-60, 62-5.
6.West JD. Finishing: the essence of exceptional endodontics. Dent Today. 2001 Mar;20(3):36-41.
7.Serota KS, Nahmias Y, Barnett F, Brock M, Senia S. Predictable Endodontic Success: The Apical Control Zone. Dent Today (in press)
8.Glassman GD, Serota KS. A Predictable Protocol for the Biochemical Cleansing of the Root Canal System. Oral Health, July 2001.
9.Yu DC, Schilder H. Cleaning and shaping the apical third of a root canal system. Gen Dent. 2001 May-Jun;49(3):266-70.
10.Goldberg F, Massone EJ. Patency file and apical transportation: an in vitro study. J Endod. 2002 Jul;28(7):510-1.
11.Rosenberg, DB. The paper point technique: Part I. Dent Today. 2003 Feb:22(2):80-86.
12.Peters OA, Peter CI, Schonenberger K, Barbakow F. Protaper rotary root canal preparation: effects of canal anatomy on final shape analysed by micro CT. Int Endo Journal 2003 Feb;36(2):86.
13.Buchanan LS. The standardized-taper root canal preparation — Part II. GT file selection and safe handpiece-driven file use. Int Endod J. 2001 Jan;34(1):63-71.
