3-D Endodontic Instrumentation: Revision of a Historical Protocol

INTRODUCTION
The goal of the instrumentation phase of root canal therapy is to debride, disinfect, and shape the root canal space prior to root filling while retaining an optimal amount of tooth structure. This is of paramount importance in the regions of peri-cervical dentin (PCD) and isthmus/furcal anatomy.¹ Historically, the significant flaws of stainless steel files and reamers were their cutting geometry and rigidity. The technical protocol for these instruments, and even Dr. Schilder’s innovative envelope of motion,² failed to correct debridement inadequacies. The root canal does not natively present in the round; Dr. Schilder’s approach, although it represented an improvement, failed to address the instrument design and technique changes required to optimize shaping and cleaning of the canal space (Figure 1).

The root shape mimics the canal shape.³ Therefore, it is impossible to adequately “sculpt” the interfacial dentin of the canal unless the file chosen corresponds to the largest diameter of the nonround canal (Figure 2), which can lead to weakening or perforation of the root structure. Studies assessing the planes of geometry of the root canal repeatedly demonstrate that the buccolingual diameter is greater than the mesiodistal diameter; canals are predominantly ovoid throughout the dentition, not round.⁴ Until recently, our reliance upon flat-film radiography to assess the spatial dimensions of root filling furthered the lack of appreciation for file taper sizes and flexibility fundamentals. The z-axis was hidden from view in flat-film periapical radiographs, and only the narrower mesial-distal dimensions of the root canal space were evidenced (Figure 3a). Faux 3-D imagery could be produced, in theory, by a combination of angled mesial, distal, and central ray radiographic projections. In 2-D, cleaning to the narrowest diameter appears adequate in post-treatment radiographs. The introduction of micro-computed and cone beam computed tomography (CBCT) has changed our understanding of the planes of geometry produced by our current treatment protocols. Mapping of the root canal space by µm-CT after instrumentation demonstrates that barely 50% of the canal is cleaned (Figure 3b).^5,6 The idiom “You can’t put a square peg into a round hole” suggests an endodontic idiom: “You can’t put a round file into an ovoid canal and achieve the desired result.”

The most underappreciated sequela of round files is the creation of significant amounts of dentinal debris. Traditionally, the focus has been on the debris pushed through the apex during instrumentation to avoid postoperative pain caused by periapical inflammation. The assumption that residual debris moves coronally and is flushed from the canal by irrigants is a questionable one. In fact, debris is pushed into the nonround parts of the canal, blocking these areas from further cleaning and disinfection by irrigation solutions and adjunctive technologies.⁷

Figure 1a. As described by Schilder, the envelope of motion is generated by precurving a reamer, then rotating and withdrawing the instrument during the working cycle. All the work is done on the outstroke, obviating the potential for ledge creation.	Figure 1b. The axial view (cross section) of the mesial root of a mandibular molar demonstrates that the geometry of the canal space is irregular and elliptoid/ovoid, but not round.

Figure 2. The root shape mimics the canal shape. As such, making a round shape using the largest diameter file is clinically impractical. Using a preset taper greater than .04 jeopardizes the integrity of the root structure.

Figure 3a. Cone beam computed tomography (CBCT) provides a z-axis image that demonstrates the number of canals present. As evident in the clinical case pictures, the thinness of the dental isthmus housing the MB2 canal could readily have been compromised with a round file of the predetermined taper—a serious concern if only the flat film was relied upon (image courtesy of Dr. Martin Trope).	Figure 3b. A µCT scan shows green (untreated) and red, the latter of which is the treated portion of the canal after the use of a round file of minimum diameter. Less than 50% of the interfacial dentin is touched and debrided (image courtesy of Dr. Frank Paqué).

Figure 4. An irregular canal space is shown after instrumentation with a file (round core). Note the existing debris accumulation in the canal irregularities resultant from instrumentation (image courtesy of Dr. Gustavo De-Deus).

Figure 5a. The majority of the root canal space is ovoid. As demonstrated by the canal shape at successive levels from the apex, round files, in spite of self-centering, can weaken the root structure with a typical .06 tapered instrument and will not debride the canal in its entirety.	Figure 5b. There are approximately 157 files systems available globally, and most are made from round blanks; root canals are not created in the round.

Additionally, when irregularities are compacted with debris, increased pressure is exerted within the canal space with the attendant possibility of microfractures (Figure 4). This is of critical concern with the new generation of NiTi files, but it is not a factor with the use of the XP-3D Shaper (Brasseler USA).⁸ The trend to use fewer files and larger tapers exacerbates this potential fracture problem.

Cognitive Dissonance
The introduction of NiTi files fostered a transition to instruments that would potentially obviate the flaws inherent in the use of carbon and stainless-steel files. NiTi files are super-elastic and self-centering, and they avoid ellipticization of the apical terminus. With appropriate taper selection, NiTi instruments should prevent thinning of the coronal and middle thirds of the root, thus preventing wall weakening or strip perforations. However, each generation of NiTi files (whether ground, twisted, or heat treated) shaped and cleaned far less debris than expected from the root canal space. Unfortunately, while a few systems include .04 tapers, the vast majority of single or multitapered files use .06, .07, and .08 tapers. Some of the latest systems use asymmetrical rotary motion, conforming S shaping, and reciprocal motion. Unfortunately, separation of a NiTi instrument due to taper lock, cyclic fatigue, and torsional resistance remains an omnipresent concern. The advantages of super elasticity and self centering were incalculable; however, the improvements were compromised by the persistence of round core manufacturing (Figure 5). The flaw in every iteration of NiTi files remains the same: The cutting geometry produces a round shape.

Figure 6. The Booster Tip (BT) (Brasseler USA) has no cutting flutes on the first 0.25 mm. The next 0.25-mm section has 6 cutting flutes that alter the apical extent of the canal to a #30/.02 (size/taper) instrument. The tip design of traditional NiTi instruments enables the BT to follow the glide path rather than actively cutting and risking ledging or torsional failure if the tip inadvertently catches in an irregularity in the canal wall (image courtesy of Sebastian Ortolani Seltenerich).

Figure 7a. The file dimensions of the XP-3D Shaper (Brasseler USA) are shown in its martensite phase and in its austenite phase.	Figure 7b. The tooth shapes the canal, not the file, as evidenced in the cross-sectional images at 1, 3, 5, and 7 mm from the apex.

Inevitability of Bio-Minimal Adaptive Shaping
Representing a new generation of adaptive/virtual core files, the XP-3D Shaper has dramatically changed the landscape of endodontic instrumentation. The XP-3D Shaper was designed to adapt to the anatomical shape of the canal while respecting the native framework of the root canal space without packing debris into untouched areas. The XP-3D Finisher (Brasseler USA) has a “reach” of at least 3.0 mm, so it has the ability to touch even the widest canal diameters without making any changes in the original shape of the canal.⁹

Booster Tip
The Booster Tip (BT) (Brasseler USA) lead section fits into the pre-established glide path, ensuring precise guidance and centering of the instrument. A traditional glide path instrument produces a .15/.02 or .10/.04 (size/taper). There are no cutting flutes on the lead section of the BT, ensuring precise guidance and centering of the instrument. The XP-3D Shaper has a BT that enables the instrument to follow the glide path into the apical component to a depth of 0.25 mm. The next 0.25-mm section of the BT is configured with 6 cutting flutes. Rotation of these flutes sizes the next 0.25 mm of the canal space from a .15/.02 to a .30/.02 (size/taper) instrument; thus, the apical size chosen for the XP-3D Shaper is #30 (Figure 6).

Figure 8a. In small (mesial) canals, the Shaper file will first reach a 0.30 diameter and, in time, increase the canal taper subject to the resistance of the dentin. The virtual/adaptive core prevents the packing of debris in irregularities.	Figure 8b. The µCT image on the left shows the packing of the debris into the isthmus by a reciprocating file. The image on the right shows the canal after preparation with the XP-3D Shaper. Increased resistance due to the packing of debris is a common flaw in round NiTi files that can result in fracture.
Figure 9. This image shows the comparison of the mechanism of cutting by a file made from a round blank and the XP-3D Shaper. No matter how much relief is provided by reducing the taper along a file with an apical third taper of .06, .07, or .08, enhanced resistance is created and irrigation turbulence is not enhanced. The opposite is true of the Shaper.	Figure 10. A photoelastic stress analysis using a monochromatic light source and plastic models is shown here, demonstrating that (a) a reciprocating file creates high stress in the apical third, (b) a rotational file shows strong stress in the apical third, and (c) the XP-3D Shaper file shows no stress in the apical third.

XP-3D Shaper
To better explain the unique properties of the file, the physical characteristics of the XP-3D Shaper’s MaxWire NiTi technology must be understood. At room temperature, the XP-3D Shaper is in the martensite phase, enabling it to be bent and more readily placed in the canal. No more than 3 to 5 easy up-and-down strokes (swaths) of the serpentine XP-3D Shaper with the BT should result in an apical terminus shaped to a #30 file and a canal taper of .02 (Figure 7). The choice of a 0.3-mm diameter enables a #31-gauge irrigating needle to approximate the working length, preventing vapor lock. Maximal irrigation efficiency is ensured. Additionally, a shelf for seating the GP point prior to root filling is created. With an increasing number of strokes, the file has the capacity to expand from tapers of .01 to .02, .04, .06, or .08 while maintaining the flexibility of the original .01 taper. At body temperatures, the file attains its austenite characteristics and attempts to achieve its potential of a .08 taper: a maximum that is needed in only the most unique cases.

As much healthy tooth tissue as possible must be maintained; therefore, it is recommended that when the working length has been achieved in the first 3 to 5 strokes, an additional 10 long strokes will achieve a .04 taper that is sufficient to adequately debride the root canal space in very tight canals. In larger canals, the file will easily create larger tapers as lesser dentinal resistance is met. In these larger, noncomplex canals, light brushing and up to 30 long strokes will result in over 90% of the walls being touched due to its serpentine shape. (Figure 8).

Figure 11. The dimensions of the XP-3D Finisher (Brasseler USA) are shown in the martensite and austenite phases. At body temperature, the last 10.0 mm of the instrument during rotation achieves a sickle shape with a diameter of 3.0 mm. Pressure on the bulb can further enhance the tip diameter.	Figure 12. The anatomy of the canal will cause the XP-3D Finisher to expand or contract and enter small irregularities in the canal walls with an up-and-down motion. No other file can reach these indentations.
Figure 13a. The natural expansion and contraction of the XP-3D Finisher contacts the irregularities on the canal walls. It is insufficiently sturdy so as to alter the original shape created by the XP-3D Shaper.	Figure 13b. The Finisher creates a robust turbulence within the irrigating solutions. Studies have shown it to remove microflora to a depth of 40 μm.

Figure 14. A study by Alves et al demonstrated that the reduction of residual debris in the canal space using the XP-3D retreatment Finisher was 69% greater by comparison to standard round files.

To summarize, the file is adaptive to the original shape of the canal; thus, the tooth shapes the canal space in contrast to round NiTi files where the file shapes the tooth. As shown in Figure 7a, the file has a sinusoidal/serpentine shape. The space available for this shape in motion enables a light brushing technique to adapt and debride 90% or more of the walls in larger, noncomplex canals that contrasts dramatically with the debris removal done by round NiTi files. As previously discussed, round files will pack debris into canal irregularities—a major drawback in sufficiently cleaning a canal. The serpentine shape, “virtual core,” and .01 taper of the XP-3D Shaper enables it to adapt to the canals, ensuring that debris remains in turbulent solution, permitting its optimal removal from the canal (Figure 9). This enables the irrigants to work maximally as the canal is shaped. Tests using photoelastic models have shown that apical pressure is not built up using the XP-3D Shaper, obviating concerns regarding microcracks. Round core files should produce significant generation of apical pressure (Figure 10). Recently, innovative irrigation devices have been introduced in the endodontic armamentarium as adjuncts to the traditional side-vented needle and passive ultrasonic irrigation (PUI), such as the EndoActivator (Dentsply Sirona), the Endosafe Plus (Vista Dental Products), and the Endovac Pure (Kerr) (apical negative pressure irrigation).¹⁰

XP-3D Finisher
The XP-3D Finisher (Figure 11) is used in addition to the XP-3D Shaper. The Finisher’s design allows it to access and scrape untouched components of the canal walls without altering the canal shape created by the XP-3D Shaper. The file has a tip diameter of 0.25 mm with a .00 taper. It is extremely flexible and, thus, has tremendous resistance to cyclic fatigue. The spoon-shaped design of this file is created in a mold in the austenite phase. At room temperature, the martensite phase can be manipulated to any shape. Upon insertion into the canal, as the file is heated to body temperature (35°C), it tries to revert to the austenite phase (Figure 12). In the austenite phase, it forms a uniquely shaped cleaning instrument. At body temperature, the apical 10.0 mm of the file transforms into a bulb/sickle shape, while retaining a depth of 1.5 mm. Without squeezing the bulb, rotation of the file produces a tip size of 3.0 mm. However, if the bulb is “squeezed,” the tip will expand to a maximum of 6.0 mm. The instrument can’t cut; thus, the only impact is scraping, which removes microbes up to a depth of 40 μm in the tubules (which is commensurate with root planing in periodontic therapy).^11,12 As the file is moved up and down in the canal, a vigorous agitation of the irrigants (NaOCl and EDTA) occurs, which adds to an enhanced inhibition or eradication of microflora presence from the root canal space (Figure 13).

Retreatment
The XP-3D Finisher file has also been modified for retreatment. The core is 0.03 in diameter with a 0.0 taper. This provides a more robust adaptation to the interfacial dentin, thus enhancing the removal of residual gutta-percha and debris from the irregularities (Figure 14). A study by Alves et al13 demonstrated that the reduction of residual debris in the canal space using the XP-3D retreatment Finisher was 69% greater in comparison to standard round files.

CLOSING COMMENTS
Preliminary studies of XP-3D files have shown remarkable removal of soft tissues, less residual dentinal chips in an isthmus, and bio-minimalistic shapes of the root canal space (with an optimal taper of .04), resulting in lower dentinal stress (ie, fewer microcracks). An efficient debridement and disinfection of the apical third area is achieved by the Booster Tip and the serpentine design of the Shaper. Have we achieved the ideal fusion of technology and biology for long-term, positive, patient-centered treatment outcomes? Perhaps. What we do know has been achieved is a redress of a design flaw that has persisted for far too long.
A second follow-up article will address the use of bioactive materials for cold hydraulic obturation.This should bring endodontics much closer to the ultimate objective of an impenetrable root canal space.

References

1. Clark D, Khademi J. Modern molar endodontic access and directed dentin conservation. Dent Clin North Am. 2010;54:249-273.
2. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am. 1974;18:269-296.
3. Wolf TG, Paqué F, Zeller M, et al. Root canal morphology and configuration of 118 mandibular first molars by means of micro-computed tomography: an ex vivo study. J Endod. 2016;42:610-614.
4. Peters OA, Laib A, Rüegsegger P, et al. Three-dimensional analysis of root canal geometry by high-resolution computed tomography. J Dent Res. 2000;79:1405-1409.
5. Paqué F, Balmer M, Attin T, et al. Preparation of oval-shaped root canals in mandibular molars using nickel-titanium rotary instruments: a micro-computed tomography study. J Endod. 2010;36:703-707.
6. De-Deus G, Belladonna FG, Silva EJ, et al. Micro-CT evaluation of non-instrumented canal areas with different enlargements performed by NiTi systems. Braz Dent J. 2015;26:624-629.
7. Paqué F, Al-Jadaa A, Kfir A. Hard-tissue debris accumulation created by conventional rotary versus self-adjusting file instrumentation in mesial root canal systems of mandibular molars. Int Endod J. 2012;45:413-418.
8. Bayram HM, Bayram E, Ocak M, et al. Effect of ProTaper Gold, Self-Adjusting File, and XP-endo Shaper instruments on dentinal microcrack formation: a micro-computed tomographic study. J Endod. 2017;43:1166-1169.
9. Metzger Z. From files to SAF: 3D endodontic treatment is possible at last. Alpha Omegan. 2011;104:36-44.
10. Miller TA, Baumgartner JC. Comparison of the antimicrobial efficacy of irrigation using the EndoVac to endodontic needle delivery. J Endod. 2010;36:509-511.
11. Azim AA, Aksel H, Zhuang T, et al. Efficacy of 4 irrigation protocols in killing bacteria colonized in dentinal tubules examined by a novel confocal laser scanning microscope analysis. J Endod. 2016;42:928-934.
12. Bao P, Shen Y, Lin J, et al. In vitro efficacy of XP-endo Finisher with 2 different protocols on biofilm removal from apical root canals. J Endod. 2017;43:321-325.
13. Alves FR, Marceliano-Alves MF, Sousa JC, et al. Removal of root canal fillings in curved canals using either reciprocating single- or rotary multi-instrument systems and a supplementary step with the XP-endo Finisher. J Endod. 2016;42:1114-1119.

Dr. Serota received his DDS degree from the University of Toronto Faculty of Dentistry in 1973 and his endodontics certificate and MMSc degree from the Forsyth Institute affiliated with the Harvard School of Dental Medicine in 1981. He is an instructor in the graduate endodontic program of the University of Toronto Faculty of Dentistry. He is the founder of ROOTS and NEXUS, online educational forums that have provided clinicians from around the world the ability to mentor one another in many evolving dental disciplines. He can be reached via email at kendo@endosolns.com.

Disclosure: Dr. Serota reports no disclosures.
 
Dr. Trope was born in Johannesburg, South Africa, where he earned his BDS degree in dentistry in 1976. He received his endodontics degree from the University of Pennsylvania in 1980. He received the JB Freedland Professorship for his contribution to endodontics as well as the Louis I. Grossman award for cumulative publication of significant research by the American Association of Endodontists. Currently, Dr. Trope serves as clinical professor in the department of endodontics at the University of Pennsylvania and maintains a private practice in Philadelphia. He can be reached via email at martintrope@gmail.com.

Disclosure: Dr. Trope is a consultant for Brasseler USA and FKG Dentaire.
 
Dr. Debelian received his DMD degree from the University of Sao Paulo, Brazil in 1987 and completed his specialization in endodontics from the University of Pennsylvania in 1991. In 1997, he received his PhD from the University of Oslo, Norway. Dr. Debelian is an adjunct professor in the department of endodontics at the University of Pennsylvania and the University of North Carolina and maintains a private practice limited to endodontics in Oslo. He can be reached via email at gildeb@me.com.
 
Disclosure: Dr. Debalian is a consultant for FKG Dentaire.

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