Minimally invasive endodontics using a new single-file rotary system

Drs. Peet J. van der Vyver, Martin Vorster, and Ove A. Peters discuss the design features of the TruNatomy^™ instruments and present case reports to illustrate the clinical application and benefits of these instruments

Introduction

The long-term retention of root canal-treated teeth depends on many factors, but it has become evident that the most common reasons for extraction of these teeth are “large carious lesion” or “unrestorable tooth,” followed by “tooth fracture,” “periodontal disease” and last of all, “endodontically related disease” (Ng, Mann, and Gulabivala, 2010).

Moreover, it is apparent that remaining structural integrity and the preservation of especially peri-cervical dentin are key factors that determine the long-term prognosis (relating to fracture resistance) in these teeth (Tang, Wu, and Smales, 2010). The term peri-cervical dentin was first described by Clark and Khademi (2010) and refers to an area roughly 4 mm coronal to the crestal bone and 6 mm apical to the crestal bone (Figure 1). According to Herbranson (2014), it appears to be critical dentin for tooth strength, and that should be conserved as much as possible to ensure long-term retention of the tooth. It is also the area of the tooth that is often destructed with access burs, Gates Glidden burs, and orifice shapers used for coronal enlargement of root canal systems.

The fact that endodontically treated teeth are more prone to fracture is largely due to the structural loss during the shaping phase of endodontic treatment and not to dehydration. Studies show minimal dehydration effects from pulpal removal with similar strength test results between vital and nonvital dentin (Sedgley and Messer, 1992, Papa, Cain, and Messer, 1994). Structural loss alone is, however, not the only cause for increased fracture incidence in teeth. The impact of irrigants, medicaments, as well as restorative procedures, and even physiological age changes should also be taken in account. Root canal therapy requires effective shaping in order to facilitate irrigation and disinfection of the canals. This should be done in such a conservative manner that the structural integrity of the tooth is respected, and dentin is preserved where possible.

Peri-cervical dentin preservation has been reported as critical in the long-term survival, especially in molars with

optimum function (Clark and Khademi, 2010). More advanced treatment options in endodontics (for example, magnification and more flexible nickel-titanium instrumentation) have therefore also shifted paradigms to a minimally invasive approach in both access cavity preparation as well as shaping of root canals in order to preserve dentin (Gluskin, Peters, and Peters, 2014).

Recently, TruNatomy™ (Dentsply Sirona), a new generation of rotary files, was launched. TruNatomy files are prepackaged, presterilized rotary instruments designed to shape root canal systems to a continuously tapering preparation with maximum preservation of peri-cervical dentin. This new file system offers the clinician more simplicity, safety, improved cutting efficiency, and mechanical properties compared to previous generations of rotating instruments. In this article, the authors will discuss the design features of the TruNatomy instruments and present case reports to illustrate the clinical application and benefits of these instruments.

Metallurgy and design features

The manufacturing of the instruments begins with a smaller initial wire blank (0.8 mm diameter) compared to the 1.1 mm diameter of other conventional rotary instruments. The post-grind thermal treatments have been further refined to produce more flexibility. This level of flexibility has been selected to complement the inherent flexibility of the fluting design and smaller maximum flute diameters.

TruNatomy instruments are manufactured using a post-manufacturing thermal process that produces a file with superelastic nickel-titanium metal properties. Due to the superelastic properties of the new wire, the files might appear slightly curved when removed from a curved root canal because the metal demonstrates less memory compared to conventional nickel-titanium or M-Wire. The file can be straightened out; if it is placed back in the root canal, it will follow the natural shape of the canal. Another advantage of this reduced memory of the files is that in cases with difficult straight-line access, it is possible to slightly precurve the files to allow easy placement into the canal orifices. All the instruments in the system have a shortened handle of 9.5 mm to further improve the straight-line access and placement of the instruments into the root canal systems.

In the case of the TruNatomy Glider and TruNatomy preparation instruments, the largest taper is at the apical extent of the shaping instruments. The instruments are designed to provide approximately the same apical sizing as the most commonly used instruments. However, they have a reduction or regressive taper as the instrument progresses coronally, allowing each instrument to maintain the 0.8 mm maximum flute diameter.

The TruNatomy system is comprised of an Orifice Modifier, a Glider, and three shaping files for different clinical applications. Regardless of the motor selected, all of the TruNatomy instruments are designed to run in continuous rotation at 500 rpm with a torque setting of 1.5 Ncm.

The TruNatomy Orifice Modifier is characterized with a modified triangular cross section, 7.5 mm of active cutting flutes on the 16-mm shank, and an  ISO tip size of 20 with a fixed 0.08 taper. The Orifice Modifier’s main function is to modify the canal orifice conservatively while still retaining coronal anatomy and to create an ideal receptacle for the introduction, scouting, and canal preparation instruments. Compared to the ProTaper SX™ (Dentsply Sirona) or ProTaper Next™ XA Opener (Dentsply Sirona) instrument, the TruNatomy Orifice Modifier has several advantages:

1. Smaller coronal maximum flute diameter of 0.8 mm compared to the 1.2 mm of the SX instrument
2. Shorter active cutting flutes distance of 7 mm compared to 14.5 mm of the SX instrument
3. Shorter handle of 9.5 mm compared to the 11 mm handle for the SX

Another difference is in that the Orifice Modifier is used with two-to-three gentle, smooth, apical movements (amplitudes) of 2 mm-5 mm into the root canal instead of backstroke brushing motions that are used for the SX instrument. This protocol results in a more conservative orifice relocation compared to the SX instruments, which are more aggressive with an increased risk of over-preparation of the coronal aspect.

TruNatomy Glider is available in 21 mm, 25 mm, and 31 mm lengths. The cross section is a cantered parallelogram. The instrument has a tip size of ISO 17 with an average taper of 0.02 and 14 mm of active cutting flutes. The glider is designed with a regressive variable taper ensuring that the shank ends up again with a maximum flute diameter of 0.8 mm.

For the preparation instruments, there are three different tip-size instruments for development of a final root canal shape. The Prime instrument (red stopper and color band) is the workhorse of the group and is called for in nearly every case. It has a tip size of ISO 26 with an overall decreasing taper that averages at 0.04.

The Medium (green stopper and color band) is used for larger cases and situations where more apical shape is desired. It has a tip size of ISO 36 and a similarly decreasing taper of about 0.03. Each of these shaping instruments have 16 mm of cutting blades and are available in 21 mm, 25 mm, and 31 mm lengths with off-centered parallelogram cross sections. The Small (yellow stopper and color band) is used for extremely curved canals where the Prime is not able to reach working length with ease or in cases where glide path preparation was very difficult. This instrument has a tip size of ISO 20 and a taper of 0.04.

Clinical guidelines for the use of TruNatomy instruments

1. Create adequate access.

It is always important to prepare a cavity that will ensure adequate access into each root canal system after removal of all the pulp chamber contents. Because TruNatomy files have less memory compared to conventional nickel-titanium or M-Wire instruments, it is possible to slightly prebend the tip of the file to allow easy insertion into a secured canal orifice that fails to have complete straight-line access or in cases where patients present with limited mouth opening. Their high-fatigue resistance, specifically high on the shaft, allows their use in a restricted access. The TruNatomy Orifice Modifier is used to create and refine the coronal opening as it transitions into the root canals. The Orifice Modifier is used with two-to-three gentle, smooth apical movements (amplitudes) of  2 mm-5 mm into the root canal. Regardless of the preparation, the TruNatomy Orifice Modifier is recommended to be used in all canal systems.

2. Negotiate canals to patency, create a reproducible micro glide path (RMG), and enlarge glide path.

With an estimated working length obtained from a preoperative radiograph, a size 08 or 10 K-File is negotiated to patency in the presence of a viscous chelator. After the establishment of patency (Figure 9), a working length is determined (Figure 10), and the canal is ready for the preparation of an RMG, using manual stainless steel instruments (Van der Vyver, 2011). It is recommended to use size 08 or 10 K-Files in a vertical in-and-out motion with an amplitude of 1 mm from working length and gradually increasing the amplitude to approximately 4 mm, as the irregularities are removed from the dentin wall (Van der Vyver, 2011). A “super loose” size 10 K-File is the minimum requirement (Bürklein, et al., 2012; Van der Vyver, 2011). To confirm that a reproducible glide path is present, a size 10 K-File is taken to full working length. The file is withdrawn 1 mm and should be able to slide back to working length by using only light finger pressure. Thereafter, the file is withdrawn 2 mm and should be able to progress to working length, using the same protocol. When the file can be withdrawn 4 mm-5 mm and slid back to working length, an RMG is confirmed. After establishment of an RMG, it is recommended to enlarge the glide path further to create as macro glide path. The micro glide path can be expanded by using the TruNatomy Glider using three easy amplitudes in a pass. If the Glider instrument does not reach length, remove and clean the instrument, re-irrigate the canal space, and then re-insert the instrument for another three-amplitude pass. Repeat this until working length is reached.

3. Select the correct TruNatomy file

The following guidelines can be used for TruNatomy file selection.

a. TruNatomy Prime file (26/04)(Figure 4)

Any canal where a size 08 and 10 K-File have to be negotiated to working length, followed by preparation of a glide path or where a size 15 K-File fits loose in the canal to working length. This will probably be the case in the majority of root canal systems with an average length and moderate curvatures in the midroot and apical regions.

b. TruNatomy Medium file (36/03) (Figure 5)

Any canal where a size 20 or 25 K-File fits loose in the canal and is not necessary to negotiate and prepare a glide path with smaller instruments. This will usually include larger diameter, relatively straight root canals. This file can also be used after the Prime file if more shape is desired or if it is felt that not enough infected dentin was removed from the canal.

c. TruNatomy Small file (20/04)(Figure 6)

The Small file is mainly used when the Prime 26/04 file does not passively progress apically or when the operator feels unsecure with the Prime file after the canal was negotiated to patency and a glide path prepared. When this Small file reaches working length, the clinician may accept the canal preparation or, alternatively, if more shape is required, to further enlarge the canal with the Prime file. However, in canals with severe apical curvatures and very long root canals or in canals where the glide path preparation was very challenging, the TruNatomy Small file can be used to start root canal preparation with more safety. When this file reaches working length, the clinician may again accept the canal preparation or alternatively, if more shape is required, further enlarge the canal with the Prime file.

4. Canal preparation with the TruNatomy preparation files.

The selected preparation file in the presence of an irrigation solution (typically sodium hypochlorite) is allowed to passively advance inward and to progress down the canal upon activation, using three easy amplitudes in a pass until working length is reached. If the selected preparation does not reach length, remove and clean the instrument, re-irrigate the canal space, and then reinsert the instrument for another three-amplitude pass. Repeat this until working length is reached, and remember to take enough time for the shaping instruments to expand and contract to promote conforming shaping. In addition, sonic activation of irrigating fluids are recommended to enhance cleaning efforts. The authors recommend using the EDDY^® Endo Irrigation Tip (VDW) driven by an airscaler (SONICflex™ Lux 2000L, KaVo).

Case report 1

The patient, a 25-year-old female, presented with irreversible pulpitis on her maxillary left first premolar that had a history of a previous pulp cap procedure and a large Class II composite restoration. (Figure 7A shows the preoperative periapical radiograph and orifice modification with the TruNatomy Orifice Modifier).

After glide path preparation with stainless-steel K-Files and the TruNatomy Glider (Figure 7B), both root canal systems were prepared with a single TruNatomy Prime instrument (Figure 7C). Two TruNatomy Prime Conform Fit™ gutta-percha cones were placed, and the fit confirmed radiographically (Figure 7D). Figure 7E shows the postoperative result after root canal obturation using the continuous wave of condensation technique (Calamus^® Dual Obturation Unit, Dentsply Sirona) with AH Plus^® Root Canal Sealer (Dentsply Sirona) and two TruNatomy Prime Conform Fit gutta-percha points.

Case report 2

The patient, a 58-year-old male, presented with a nonvital maxillary right second molar. A preoperative periapical radiograph revealed a very deep, previously placed composite restoration (Figure 8A). After access cavity preparation, four root canal systems were detected (two mesiobuccal, distobuccal, and palatal). After orifice modification with the TruNatomy Orifice Modifier and glide path preparation with K-Files and the TruNatomy Glider, the four root canals systems were prepared with a single TruNatomy Prime instrument. The fit of four Prime TruNatomy Conform Fit gutta-percha cones were confirmed radiographically before the root canal systems were obturated (Figure 8B). Figure 8C depicts the postoperative result after obturation of the root canal systems and placement of fiber post and composite core. The clinical procedure of this case can be viewed on the following link: https://you tu.be/OplAENoh3b8.

Case report 3

The patient, a 30-year-old, presented with a nonvital maxillary left lateral incisor previously restored with a zirconia crown (Figure 9A). After access cavity preparation, the canal was located, and a size 20 K-File was used for length determination (Figure 9B) using an electronic apex locator. The size 20 K-File was loose in the canal; it was decided to complete canal preparation with a single TruNatomy Medium instrument. A size Medium TruNatomy Conform Fit gutta-percha cone was placed, and the fit confirmed radiographically (Figure 9C). Figure 9D illustrates the postoperative radiograph after the canal was obturated using the continuous wave of condensation technique (Calamus Dual Obturation Unit) with Pulp Canal Sealer (Kerr) and a Medium TruNatomy Conform Fit gutta-percha point. The clinical procedure of this case can be viewed on the following link: https://you tu.be/sG5paE_2FN4.

Case report 4

The patient, a 65-year-old male, presented with a nonvital maxillary right second premolar with a history of a previous emergency root canal treatment and placement of calcium hydroxide as an intracanal medicament. The temporary restoration and calcium hydroxide paste were removed before length determination was done using an electronic apex locator, and the working length was confirmed radiographically. The canals were extremely difficult to negotiate, and when the size 08 K-Files that were used for length determination were removed, it was noted that the buccal root canal system presented with an S-curvature.

A size 08 and 10 K-File were used to secure a reproducible micro glide path before the glide path was expanded using the TruNatomy Glider. It took several passes with the glider before working length was reached in both root canals. Taking into account the fact that glide path preparation was extremely difficult and the fact that the buccal root canal presents with an S-curve (not visible on periapical radiographs), it was decided to complete canal preparation with the TruNatomy Small instrument. After canal preparation, two TruNatomy Conform Fit Small gutta-percha cones were placed and the fit confirmed radiographically. Again, upon removal of the cone from the buccal root canal, the S-curvature was imprinted on the gutta-percha cone. This confirmed that the TruNatomy file maintained the original root canal anatomy. The canals were obturated using the continuous wave of condensation technique (Calamus Dual Obturation Unit) with Pulp Canal Sealer and two Small TruNatomy Conform Fit gutta-percha points. The clinical procedure of this case can be viewed on the following link: https://youtu.be/FaK0tXqULM8

Case report 5

The patient, a 71-year-old male, presented with irreversible pulpitis on his mandibular maxillary right first molar previously restored with a zirconia crown (Figure 11A). An access cavity was prepared, and four root canal systems were located followed by length determination using and electronic apex locator. After orifice relocation using the TruNatomy Orifice Modifier, a glide path was prepared using stainless-steel K-Files and a TruNatomy Glider. Root canal preparation was done with a single TruNatomy Prime instrument. Figure 11B illustrates postoperative radiograph after the canal was obturated using the continuous wave of condensation technique (Calamus Dual Obturation Unit) with Pulp Canal Sealer and four Prime TruNatomy Conform Fit gutta-percha points. Note the conservative coronal root canal preparations in the peri-cervical area. Figure 11C depicts a mesio-angulated view showing the four obturated root canal systems.

Case report 6

The patient, a 45-year-old female, presented with irreversible pulpitis on her mandibular right first molar (Figure 12A). A preoperativCBCT scan revealed the presence of a mid-mesial root canal system in the mesial root (Figure 12B). After access cavity preparation, and removal of pulp calcifications in the pulp chamber, the three main root canal systems were located (mesiolingual, mesiobuccal, and distal). The groove between the mesiolingual and mesiobuccal  canals was roughed with a Start-X^® No. 3 tip (Dentsply Sirona) to remove an overlapping dentin ledge, exposing the internal anatomy of the groove. A micro-debrider (Dentsply Sirona) was used to locate the orifice of the mid-mesial canal. A size 08 C+ file (Dentsply Sirona) was used to negotiate the initial few millimeters of the constricted canal.

Canal orifices were relocated and opened coronally with the TruNatomy Orifice Modifier before the three mesial root canal systems were negotiated to full working length and apical patency. It was noted clinically and on CBCT that the distal root canal system was very wide in a buccal-lingual direction (Figure 12B), and it was possible to place a size 20 K-File to full working length.

Working lengths were determined by using an electronic apex locator and confirmed radiographically (Figures 12C and 12D). It was noted that the mid-mesial canal joins the mesio-lingual canal and the mesio-lingual joins within the mesio-buccal canal in the apical 2 mm of the root canal system. A reproducible micro glide path was established in all five root canal systems using a size 08 and 10 K-File before the glide paths were expanded with the TruNatomy Glider.

Taking into account that there were three root canal systems in the mesial root, the authors decided to use the TruNatomy Small file for root canal preparation and maximum preservation of root structure. Figure 12E shows a magnified view of the pulp chamber floor. Note the large amount of tooth structure that was still intact after root canal preparation with the TruNatomy Small file. The distal root canal was prepared with a TruNatomy Medium file.

Root canal irrigation was achieved by using 17% EDTA and heated 3.5 % sodium hypochlorite activated with the EDDY Endo Irrigation Tip (VDW) driven by an airscaler. Obturation of the canals was achieved by using TruNatomy Conform Fit gutta-percha cones and TotalFill^® BC Sealer Highflow (FKG) using the continuous condensation technique. Figure 12F shows a parallel view of the obturation result. Note the maximum preservation of the root structure in the peri-cervical region of the mesial and distal root canal systems. However, the mesio-angulated view (Figure 12G) shows that the full extent of the lateral anatomy that was cleaned and obturated. The clinical procedure of this case can be viewed on the following link: https://youtu.be/MxVKMc-E2VM.

Conclusion

Respecting original canal anatomy, preserving dentin and therefore maintaining the structural integrity of teeth should form an integral part of root canal shaping and preparation. In this paper, the authors illustrate the clinical guidelines, applications, and advantages of the recently launched TruNatomy system. With a renewed focus on dentin preservation and benefits like improved performance and efficacy, the TruNatomy instruments offer the clinician superior debridement while respecting original canal anatomy.

Reprinted with permission by International Dentistry African Edition.

van der Vyver PJ, Vorster M, Peters OA. Minimally invasive endodontics using a new single-file rotary system. Int Dent Afr Ed. 2019;9(4):6-20.