Endodontic Reboot: Part One by Drs. Gilberto Debelian, Kenneth S. Serota and Martin Trope

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Dentaltown Magazine
by Drs. Gilberto Debelian, Kenneth S. Serota and Martin Trope

“The historical sense involves a perception, not only of the pastness of the past, but of its presence.” — T.S. Eliot, “Four Quartets”

Studies assessing the diametric dimensions of apical anatomy repeatedly demonstrate that the bucco-lingual diameter is greater than the mesio-distal diameter—canals are predominantly ovoid throughout, not round1–4 (Figs. 1a and 1b).

The technical flaw most inherent—the use of a round file of any design to clean an ovoid canal configuration­—manifests as the failure to debride a substantial amount of the canal contents. A recent study showed that the mean (± standard deviation) of untreated areas ranged from 59.6 percent (±14.9, group PT/2) to 79.9 percent (±10.3, PT/1) for the total canal length and 65.2 percent to 74.7 percent for the apical canal portion, respectively5 (Fig. 2).

  • Fig. 1a. This axial view of a mandibular molar demonstrates the ovoid eccentricity of the canals and existence of an isthmus connection between the MB and ML canals consistent with the findings of numerous studies.8,9

  • Fig. 1b. The root canal space is an arborizational, anastomotic, labyrinthine complexity, morphologically comparable to the pathways of a maze. While primary canals exist, the tributaries, accessory branches and lumina of the dentinal tubuli harbor extensive tissue and microflora. The existence of these vast passages has been demonstrated throughout the past century, beginning with the work of Hess, and continues to this day with the use of microcomputed tomography. (Image courtesy of Versiani, Pécora, Sousa Nato.)

  • Fig. 2. The axial view of the obturation (microstructural replication) demonstrates the flaw in flat field film interpretation. Significant areas of the bucco-lingual dimensions of the root canal space remain uncleaned despite the illusory appearance in the radiograph.

In the 1960s, Dr. Herbert B. Schilder introduced two legacy concepts to the science of endodontics: The constricted envelope of motion for instrumentation (Fig. 3) and the use of hydraulics to enhance the rheology of the obturation material used to seal the root canal space and optimize its gravitometrics.

Notes on nickel titanium
The evolution of nickel titanium (NiTi) instrumentation manufacture has persisted with a round core blank, whether it was ground, twisted, nano-coated, heated or metallurgically reformulated. NiTi files are superelastic and able to self-center, avoid apical ellipticization and, with appropriate taper selection, prevent thinning of the coronal and middle thirds of the root. Thinning otherwise results in weakening or strip perforation.

  • Fig. 3. Dr. Herbert B. Schilder’s principles included a continuously tapering shape, maintenance of the original anatomy, an apex as small as practical, and conservation of tooth structure. A continuously tapering space was acquired using precurved hand instruments, which imposed discontinuous contact with the canal walls and created an “envelope of motion.” Transactionally, Schilder created a virtual core.

  • Fig 4. The ideal file would produce an apical size that three-dimensionally cleaned the minor apical foramen. The SAF from RedentNova is a hollow file designed as an elastically compressible, thin-walled pointed cylinder that is composed of a nickel titanium lattice. Its hollow shape allows for the continuous flow of irrigant through its lumen. It was a beginning in the paradigm shift toward minimally invasive 3-D debridement and disinfection.

NiTi files, however, are unable to effectively cleanse most of the intracanal space. Moreover, regardless of variable tip or variable taper or multiple tapers on a single file design configuration, they are unable to adequately cleanse the isthmus confluence of many canals.6 A new design in file configuration, the SAF system, was introduced to correct this deficiency by expanding in one direction when compressed in the other and by including a virtual core (Fig. 4).

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Fig. 5a. The revolution in endodontic instrumentation imparted by the first generation of NiTi instruments related to their shape memory and super elasticity. Despite the advantages, these files were susceptible to fracture due to fatigue and torsional failure.

SAF showed significant promise in terms of the degree of debris removal in complicated intracanal anatomy such as the isthmus, when compared to a competing system. However, it failed to take hold as a true replacement for traditional “round” rotary instrumentation systems.7–9 There were technical issues with the motor/pumping system, the amount of time to perform the RCT, and more. It has gained a significant global footprint once the issues were obviated, however.

The manipulation of the metallurgical properties of nickel titanium by thermomechanical processing and alloying treatments has led to the most significant iteration in the manufacture of endodontic files. The transition from the martensite phase to the austenite phase is dependent on temperature and stress—not time—because there is no diffusion involved. It is the reversible diffusionless transition between these two phases that results in the special properties. Unfortunately, fracture still occurs because of cyclic fatigue and torsional failure when the elastic limit is exceeded (Fig. 5a).

The new generation of NiTi alloys have transformation temperatures much higher than those of conventional austenitic materials used in previous generations of rotary instruments, and will transform at close to body temperature.

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Fig. 5b. Heat treatment (thermal processing) is one of the most fundamental approaches in adjusting the transition temperatures of nickel titanium alloys and affecting the fatigue resistance of NiTi endodontic files. Newer alloys such as MaxWire transforming close to body temperature have demonstrated superior resistance to cyclic fatigue and torsional failure.

A recent study of commonly used NiTi endodontic files showed that a temperature increase to 37 degrees Celsius, simulating body temperature, substantially decreased the fracture resistance of all instruments tested—the temperature effect on the latest generation of nickel-titanium files10 (Fig. 5b).

Unique endo instruments
Brasseler’s XP-3D instruments feature an MaxWire alloy that allows the martensitic instrument to transform to a predetermined austenitic shape at body temperature.

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Fig. 6. An overview of the unique features of the XPendo Shaper are demonstrated. The discontinuous adaptive debridement motion kinesis mimics Schilder’s envelope of motion exactly.

In the absence of a solid core, this system allows the tooth to dictate the canal configuration achievable and allows for thorough cleaning. (Fig. 6 details various features of the XP-3D Shaper.)

The booster tip lead section fits into the pre-established glide path, ensuring precise guidance and centering of the instrument. A traditional glide path instrument is used consistent with a .15/.02 (size/taper) instrument. There are no cutting flutes on the lead section of the booster tip, and the XP-3D Shaper instrument slips into the prepared apical component of the glide path to a depth of 0.25 millimeters. The next 0.25mm section of the booster tip is configured with six cutting flutes.

Rotation of these flutes sizes the next 0.25mm of canal space anywhere from a #25 to #60 tip with the rest of taper variable, dependent on the desires of the manufacturer or clinician. In the XP-3D Shaper the apical size chosen is #30. Thus, the booster tip enables achieving a #30 tip size within five swaths after the creation of the #15/.02 glide path.

The core taper of the XP-3D Shaper file is .01 and at room temperature in the martensite form, the file appears and acts in terms of flexibility and cyclic fatigue like a traditional #30/.01 solid core file; however, at body temperature the austenite shape is snakelike and has a taper of .08 when no resistance is met. The file now maintains its properties in terms of flexibility and resistance to cyclic fatigue of the #30/.01 tapered file, but can potentially create a shape of #30/.08. The final taper of the file is time-dependent. When the file has reached the working length, it is “squeezed” to a #30/.02 shape. At body temperature, it will expand to its austenite shape. It is the resistance of the dentin that limits its achieving its full potential, dependent on time.

If after reaching working length the file is used in with approximately 10 additional swaths, it will reach a #30/.04 size; 10 more swaths, a #30/.06, etc. To maintain critical coronal dentin, dentinal girth, the authors believe that a #30/.04 shape is ideal. Of paramount importance: The time of use of the file is extremely safe in terms of cyclic fatigue resistance. The snakelike shape of the file and its virtual/adaptive core and mechanism of use allow the file to adapt to the true or native shape of the canal.

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Fig. 7a. A traditional NiTi file from round blank is represented by the color red, the XPendo Shaper by the color blue. The sinusoidal motion of the XPendo Shaper—in contrast to the round file which augurs— demonstrates the benefit of adaptive debridement. In conjunction with the Finisher, unprecedented levels of debris removal and disinfection are possible. (Image courtesy of Dr. Gustavo de Deus.)

Fig. 7a demonstrates the difference between the ability of a standard round NiTi file to clear a less-than-ideal volume of intracanal debris, in contrast to the debridement achieved by the XP-3D Shaper’s adaptive discontinuous contact of the canal walls. The desired minimally invasive shape achieved with this instrument is shown in Fig. 7b (p. 80).

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Fig. 7b. Minimally invasive endodontics.

The distinctions of greatest importance between the XP-3D Shaper and conventional NiTi instruments are: The Shaper does not compact debris on the flutes, resulting in increased frictional resistance, as it provides substantial space in the lumen or the virtual core; nor does it force the debris apically, as evidenced in instruments used with reciprocating motion.11

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Fig 8a. Photoelasticity is an experimental technique for stress and strain analysis, useful for conditions of complicated geometry or loading. As evidenced by the accompanying images, the XPendo Shaper demonstrates the least stress in the apical third. Internal tests performed at FKG Dentaire, Switzerland.

Because the points of contact on the dentinal walls are discontinuous, less stress is applied and thus less cyclic fatigue created than with conventional instruments, which can be readily demonstrated in photoelastic testing models (Fig. 8a, p. 80).12

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Fig. 8b. Protaper Next was the first example of an attempt to migrate away from the auguring peck and pull motion of most NiTi files. Its swaggering motion was an improvement in regard to emulating the constricted envelope of motion. However, its foundation remained a round blank with all the attendant issues related to cyclic fatigue and torsional failure.

Fig. 8b demonstrates that efforts have been made with other file systems to emulate the uniqueness of the adaptive core design of the XP-3D Shaper; however, regardless of the design alterations, a solid round core remains. The overall result is that this file can create a #30 tip size with different tapers, depending on the preference of the practitioner, while adapting to the natural anatomy of the canal.

Effective disinfection
Inhibition or eradication of microflora presence from the root canal spaces is a multifactorial conundrum. The bulk of the microbes reside in the primary canal in a planktonic/loose form. However, there is a vast network of labyrinthine irregularities acting as a microbial reservoir that communicates with the primary canal. While irrigation with disinfectants may be very effective against planktonic microbes, it is not sufficiently effective when the microbes are in biofilm form or in canal irregularities.

The ability of organisms within the residual biofilms to create an adaptive mechanism to the environmental changes resulting from the treatment protocol can result in recrudescence of the pathosis.13 The biofilm must be eliminated before the disinfectants will work. This is analogous to scaling and root planing in periodontal therapy.

As referenced previously, most files produce a final round shape on any given canal cross section, and as such the practitioner is limited in the capacity to scrape the walls of the nonround root canal space; at best, a round file can “brush” the walls to facilitate an enhanced disinfection. Alternative methods must be applied to remove toxins unreachable by the traditional files.

Unfettered access
The XP-3D Finisher was designed to be adjunctive to the XP-3D Shaper. The Finisher has many properties that allow it to gain access to and scrape untouched components of the canal walls, and the turbulence it produces in the canal irrigant enhances its antimicrobial properties. The file is a #25 tip diameter with .00 taper. It is extremely flexible and thus has tremendous resistance to cyclic fatigue. Its primary action within the root canal is to scrape the walls that it contacts, rather than debride and sculpt a shape into the wall of the canal.

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Fig. 9a. The apical 10mm of the file transforms into a bulb more coronally and a tip in the last few millimeters. When rotating at canal temperature, the Finisher exhibits a total expansion of 3mm.

When the file is cooled below 35 degrees Celsius, it’s in the martensite phase and can be bent to any other shape. When the file is heated to body temperature (37 degrees Celsius), it will change to the austenite phase. When the file is rotated in the austenite phase, it creates a uniquely shaped cleaning instrument; the apical 10mm of the file transforms into a bulb shape coronally while retaining a tip in the last few millimeters. Since the depth of the spoon is 1.5mm, the total diameter of the bulb and tip is 3mm. However, if the bulb is “squeezed,” the tip will expand to a maximum of 6mm. Relatedly, if the tip is “squeezed,” the bulb will likewise expand to a #600 file (Fig. 9a).

However, because the instrument can’t cut, the only impact on the dentin is optimized scraping. Therefore, if moved up and down in the canal, the bulb and tip will expand or contract in concert with the natural 3-D diameter of the canal. Maximum loss of length when transforming from straight to full austenite phase is 1mm.

The small core diameter of the file maintains its flexibility and cyclic fatigue resistance, causing it to scrape, not shape, the dentinal walls. This, plus the turbulence that is created in the irrigant, results in a large surface area of the canal being touched by the file and removal of biofilm that would never be removed by round files.

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Fig. 8b. Protaper Next was the first example of an attempt to migrate away from the auguring peck and pull motion of most NiTi files. Its swaggering motion was an improvement in regard to emulating the constricted envelope of motion. However, its foundation remained a round blank with all the attendant issues related to cyclic fatigue and torsional failure.

Fig. 9b shows the action of the XP-3D Finisher. In the “M-phase” the finisher is placed in the canal before it changes to full austenite phase. The middle illustration demonstrates full austenite phase at canal temperature; the file will expand to the extent that is determined by the canal anatomy. By moving the finisher up and down in a 7–8mm swath, it expands and contracts per the anatomy of the canal. A recent study demonstrated the efficacy of the XP-3D Finisher, compared with traditional modes of hard tissue debris.14 The results are reflected in Fig. 10, pg. 84.

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Fig. 10. The image reflects the distal views of 3-D reconstructions of the mesial root canal systems of four mandibular molars before (green) and after (red) canal preparation with reciprocating instruments. The figures demonstrate the effectiveness of the Finisher in the apical region.

A more recent study showed that the XP-3D Finisher had the greatest bacterial reduction, when compared with standard needle irrigation, sonic agitation with the EndoActivator, and photon- initiated photoactivated acoustic streaming.15

Another study demonstrated that the inclusion of the XP-3D Finisher as an adjunct to conventional needle irrigation and passive ultrasonic irrigation enhanced biofilm eradication more than a continuous irrigation protocol in an in vitro biofilm model.16

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Fig. 11. The preoperative periapical radiograph shows a mesiodistal resorptive defect. The CBCT images show that this is internal resorption that also extends buccolingually. The postoperative radiograph shows that at the second visit the canal is filled completely, which is an indication that the tissue and debris has been removed. The original shape of the canal has been maintained, and the tooth has not been further weakened by the cleaning procedure.

Fig. 11 (p. 84) is an example of the unique action of the finisher. The irregularity in the canal is in a mesio-distal dimension because of internal resorption. The Finisher enabled removal of debris and tissue in the irregularity while retaining the original shape of the canal and preventing further weakening of the root.

Retreatment considerations
There is a third file in the XP-3D instruments—the XP-3D Finisher R—designed for retreatment. This file is a #30/.00, making it slightly stiffer and more efficient in removing root fillings materials adhering to the canal walls, especially in the curvature or oval areas. The residual amount of filling material remnants when a tooth is retreated is difficult to calculate; however, studies using histologic evaluation of teeth with post-treatment apical periodontitis show evidence that bacterial colonization is associated with the canal remnants.

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Fig. 12. MicroCT images of representative specimens subjected to retreatment procedures. Only the apical segment of roots was reconstructed. (a) The initial microCT scan taken after root canal filling. (b) A postpreparation microCT scan taken after retreatment procedures with both systems: left canals with Reciproc; right canals with Mtwo. (c) The final microCT scan after using the XP-Endo Finisher.

A new supplementary strategy using a finishing instrument was evaluated for its ability to improve filling material removal. A study showed substantial reduction in residual contents when the “Mtwo” system and “Reciproc” system were used for re-treatment. The results using the XP-3D Finisher instrument were encouraging. The remaining filling volume showed a 69 percent reduction in volume contents. In canals with residual filling material, an adjunctive approach with the XP-3D Finisher instrument significantly enhanced removal17 (Fig. 12).

Preliminary studies on XP-3D instruments have shown remarkable removal of soft tissues, fewer dentinal chips residual in the isthmus and canal walls after instrumentation, and low dentinal stress (fewer microcracks). The minimally invasive conservative instrumentation engenders a low amount of dentin removed coronally, and efficient debridement and disinfection of the apical third area.

Have we achieved the ideal fusion of technology and biology for long-term positive treatment outcomes? Perhaps.

What has been achieved is a redress of a design flaw that has persisted for much too long.

(Editor’s note: Part 2 of this series will address how the use of bioactive materials for cold hydraulic obturation takes us yet another step closer to the goal of predictable endodontic success.)


1. Wu MK, van der Sluis M, Wesselink PR. The capability of two hand instrumentation techniques to remove the inner layer of dentine in oval canals. Int Endo J 2003;36:218-224
2. Wu MK, Wesselink PR, Walton RE. Apical terminus location of root canal treatment procedures. OS, OM, OP, OR, Endo Jan 2000;89(1): 99–103
3. Khademi A, Yazdizadeh M, Feizianfard M. Determination of the minimum instrumentation size for penetration of irrigants to the apical third of root canal systems. J Endo May 2006;32(5):417–420
4. Jung IY et al. Apical anatomy in mesial and mesiobuccal roots of permanent first molars. J Endo May 2005;31(5):364–368
5. Paque F et al. Preparation of oval-shaped root canals in mandibular molars using nickel-titanium rotary instruments: A micro-computed tomography study. J Endo April 2010;36(4):703-707
6. Song M, Kim HC, et al. Analysis of the cause of failure in non-surgical endodontic treatment by microscopic inspection during endodontic microsurgery. J Endo Nov 2011;37(11):1516-1519
7. Solomonov M, Paque F et al. The challenge of C-shaped canal systems: A comparative study of the Self-Adjusting File and Protaper. J Endo Feb 2012;38(2):209-214.
8. Mannocci F et al. The isthmuses of the mesial root of mandibular molars: a micro-computed tomography study. Int Endo J July 2005;38(8):558-563
9. Villas-Boas MH, Bernardineli N et al. Micro-computed tomography study of the internal anatomy of mesial roots of mandibular molars. J Endo Dec 2011;37(12):1682-1686
10. De Vasconcelos RA et al. Evidence for reduced fatigue resistance of contemporary rotary instruments exposed to body temperature. J Endo May 2016;42(5):782-786
11. Bürklein S, Shäfer E. Apical extruded debris with reciprocating single-file and full-sequence rotary instrumentation systems. J Endo June 2012;38(6):850-852
12. Souza Bier CA, Shemesh H et al. The ability of different nickel-titanium rotary systems to induce dentinal damage during canal preparation. J Endo Feb 2009;35(2);236-238
13. Chavez de Paz F. Redefining the persistent infection in root canals: possible role of biofllm communities. J Endo June 2007;33(6):652-662
14. Leoni GB, Versiani MA et al. Ex Vivo evaluation of four final irrigation protocols on the removal of hard-tissue debris from the mesial root canal system of mandibular first molars. Int Endo J May 2016;49(5)
15. Azim AA, Aksel H et al. Efficacy of 4 irrigation protocols in killing bacteria colonized in dentinal tubules examined by a novel confocal laser scanning microscope analysis. J Endo June 2016;42(6):928-934
16. Bao P, Shen Y, Lin J, Haapasalo M. In vitro efficacy of XP-endo finisher in 2 different protocols on biofilm removal from apical root canals. J Endo article in press
17. Alves FRF, Marceliano-Alves MF 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 Endo 2016;42(7):1114-1119

Check it out! Earn CE credits— online, anytime
Next Level Endodontics’ webinar curriculum “Online Foundations of Predictable Endodontics Success” allows dentists to learn more about endodontics—and earn CE credit while doing so. To find out more, go to dentaltown.com/ce.

Author Dr. Gilberto Debelian received his DDS degree in 1987. In 1989, he completed the post-graduate program in endodontics at the University of Pennsylvania. In 1997, he received his PhD from the University of Oslo, Norway. Debelian now serves as an adjunct professor at the Department of Endodontics at UPenn and is in private practice in Oslo.
Author Dr. Kenneth S. Serota received his endodontics certificate and MMSc degree from the Forsyth Institute at Harvard. He is on the editorial boards of Endodontic Practice and Endo Tribune, and is the founder of ROOTS and NEXUS, online educational forums for those interested in cutting-edge dentistry.
Author Dr. Martin Trope received his BDS degree in dentistry in 1976. In 1980 he moved to Philadelphia to specialize in endodontics at the University of Pennsylvania. Trope now serves as a clinical professor in the Department of Endodontics at UPenn and also is in private practice

Drs. Debelian, Serota and Trope are involved with endodontic education through the training facility Next Level Endodontics in Philadelphia, which Trope owns. Information: nextlevelendodontics.com.


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