Hybrid Reciprocation and Rotary Instrumentation: A Streamlined Technique

The shaping and cleaning process is a crucial procedure in root canal treatment. The initial debridement of root canals is commonly performed by instrumentation, which removes a large bulk of the pulp tissue, necrotic debris, bacterial biofilm, or previous root canal fillings. Such removal should occur with mechanical instrumentation protocols.

Mechanical instrumentation is performed using either stainless steel hand files or engine-driven nickel-titanium (NiTi) rotary and/or reciprocation files, neither of which is expected to completely clean the canal when used alone; instead, they are used to create a space that will be filled by irrigants that will provide the cleaning effect. It seems paradoxical that the shaping process generates debris and adds more noxious volume of intracanal content that will need to be removed, but this is the most common procedure performed.

While maintaining the main goal of endodontics—to clean the root canal space thoroughly until the foramen exit—a streamlined, evidence-based instrumentation technique will be discussed and described herein.

Preflaring
Preflaring the canal is a procedure that fundamentally aims to remove middle and coronal third interferences before accomplishing apical instrumentation. Preflaring has been known to prevent file fractures in the process of canal shaping by not only decreasing the occurring torsional stress, but also by shifting the area on which the stress is exerted from the tip of the file to the body. This prevents the incidence of torsional stress at the thinnest area of the instrument, which is more susceptible to fracture.

Figure 1. Comparison of the final taper preparation with diameters between the Genius Orifice Shaper 30 .08 (Ultradent Products) and a 1–4 Gates Glidden sequence.	Figure 2. Instrument fragment at the V zone in a mesial root (MB canal) of a lower molar.

Figure 3. Left: Asymmetric reciprocation movement in a 90° right cutting action and a 30° stress-releasing action (Genius pre-set [Ultradent Products]). Right: Rotary movement, 360° right cutting.

Another benefit of preflaring is that shaping the two thirds of the canal alters which size file will fit in the apical third. Clinicians typically begin shaping by directly placing a file to the apical third and trying to determine the apical diameter. From this procedure, they make judgments that limit the extent of apical shaping and how much the canal space must be enlarged. The sensation of file fit does not necessarily occur from contact to the apical third dentin walls, as is often thought, as it may be a result of interference in the coronal and middle thirds of the canal.

One of the more commonly used rotary instruments to preflare the canal is a Gates Glidden (GG) drill; however, NiTi rotary instruments are also available for this purpose. For example, the Genius Orifice Shaper (Ultradent Products), which is a NiTi rotary instrument, uses a .08 tapered double positive cutting edge with a non-cutting tip design. The file is designed to create a controlled canal enlargement and prevent lateral strip perforations—a complication commonly related to the use of GG drills. Among other disadvantages, the GG drill design doesn’t allow for a tapered preparation and keeps the debris inside the canal (Figure 1). On the other hand, the Genius Orifice Shaper design not only allows for a tapered preparation, but also concomitantly removes debris from the canal.

Figure 4. The Genius rotary and reciprocation motor.	Figure 5. The Genius reciprocation and rotary hybrid file 25 .04.

Is a Glidepath Mandatory?
A glidepath is the result of creating a smooth patency from the canal orifice opening to the foramen exit, in which a manual size 10, 15, or 20 K-file should be used with watch-winding and small axial in and out movements to find the canal path and establish patency. Usually, this procedure is done prior to the NiTi rotary and/or reciprocation instrumentation of a taper greater than .02. It has been demonstrated that the creation of a glidepath reduces the frictional forces applied to the NiTi files because the canal diameter becomes at least equal to the file tip used for shaping.

Another interesting role of the glidepath preparation concerns the anatomy of the curved section of the root canal. In this trajectory, the canal cross section is flattened, and the instrument tends to touch the outermost part of the internal walls, given the curved path that the file will be forced to perform. The flat borders of the internal canal geometry can be called the “V zone” and play a fundamental role, along with torsional and cyclic fatigue, in the tendency of the instrument to fracture and separate (Figure 2). In this case, the preparation of the glidepath helps to level these flat areas (the V zone), reducing the interference of the V zone and providing a smooth path at the canal curvature area.

Cleaning the Last Apical Millimeter: The Apical Patency Concept
Apical patency can be defined as performing a passive debridement with small diameter files to prevent debris or collagen remnant blockage. Without removal, debris will continue to contribute to symptomatic inflammation postoperatively, even when the completed root canal appears to be of good quality radiographically. The effect of keeping this area free of debris is related to maintaining the last millimeter open to irrigation action, knowing that, by capillarity, the irrigant will passively penetrate and clean/disinfect the area.¹

• Use a K-File No. 15 or No. 20 (SS White Dental) until the cleaning working length (WL) (foramen position) in small curvature canals.
• In very curved canals or with 90° foramen exit, use an SS flexible file or a NiTi hand file.
• Replace the file with a new file every time any deformation is noticed.
• Use your patency file every time during and after the use of the NiTi instrument; To accomplish cleaning of the last millimeter (debridement effect) with the patency file, place the file at the cleaning (or patency) WL, perform 2 or 3 back-and-forth movements with small amplitude, and remove the file.

Figure 6. The Genius Orifice Shaper 30 .08 before preflaring.	Figure 7. The Genius Orifice Shaper 30 .08 after preflaring and dentin triangle removal.
Figure 8. A glidepath with a hand K-File.	Figure 9. The Genius 25 .04 in reciprocation GEN mode until the shaping working length (WL).

Engine-Driven Instrumentation Kinematics: Reciprocation, Rotation, or Both?
There is no doubt that the greatest contribution of engine-driven NiTi instruments is the final tapered preparation, facilitating the concomitant irrigation/cleaning procedure and the succeeding obturation. While hand instruments continue to be used, NiTi tapered instruments in different engine-driven kinematics offer new perspectives for root instrumentation that have the potential to avoid some of the major drawbacks of traditional hand preparation.

Reciprocation Kinematics
The NiTi file driven by reciprocation motion is a recent innovation in engine-driven instrumentation systems that shows better results in torsional and cyclic fatigue resistance and shortens the learning curve for NiTi file systems. Reciprocation had been extensively used with stainless steel files in the development of endodontic mechanical instrumentation. However, there are many differences between reciprocation applied to NiTi files and reciprocation done with stainless steel.

Symmetric vs Asymmetric Reciprocation
Symmetric reciprocating movement defines complete oscillating reciprocation as having the same reciprocating angles in both directions (clockwise [CW] and counterclockwise [CCW]). It resembles the classic watch-winding movement used with hand files, and the end result is similar to a classic manual approach. To achieve the symmetric reciprocation movement, dedicated contra-angles are available and, depending on the model, the angles of oscillation and the file type to be used are variable. With the new techniques, the tendency is to limit the application of this motion with small No. 10 and No. 15 stainless steel files to obtain an engine-driven-made glidepath.

Figure 10. To execute a non-apical pressure brushing movement with the Genius files in asymmetric reciprocation, do the following: (Image 1) Insert the file until resistance is reached (Inset A: Detail of where the file binds, usually at the V zone at the curvature); (Image 2) pull back the file until it is free (Inset B: Using back-and-forth motions against the external wall, use the tip of the file to remove interferences at the curvature); (Image 3) the file will be placed automatically at the apical third with no apical pressure.

Figure 11. A Genius 35 .04, used in reciprocation GEN mode until the shaping working length. After getting space, turn the Genius motor to Rotating P1 by pressing the Mode button and retrieve debris using the same file in rotation.	Figure 12. Proceed with the continuous ultrasonic passive irrigation every time that the file is removed to be cleaned between instruments.

Asymmetric reciprocation can be defined as the sequence of repetitive and imbalanced back-and-forth rotational movements, which mimics Roanne’s balanced force hand filing motion.² Limiting the angle of rotation in the cutting verse to higher than the angle of rotation in the opposite noncutting verse prevents the taper lock phenomenon. Thus, torsional stress is reduced by preventing binding of the file, and safety is improved. So, practically each time that a file is cutting dentin in a CW motion, there is a certain degree of torsional deformation that develops on the axis. If this deformation is maintained under the limits of the plastic deformation, there will be no structural changes. Furthermore, a very important feature of asymmetric reciprocating-based instrumentation is its efficiency in negotiating root canals. Depending on the design of the file, the instrument can more smoothly advance toward the apical third compared with continuous rotation.

Another advantage of using an asymmetric reciprocation motion is the use of more lateral movements (also called brushing movements), which make use of the side cutting ability of the files, increasing passive flare and getting a tapered preparation without the use of high taper files. The operator can repeat the movement more times during instrumentation without risking high levels of mechanical stress to the instrument (torsional and cyclic fatigue).

The efficiency of asymmetric reciprocation motion has been compared with rotary movement (Figure 3) in terms of the time required to prepare a curved canal, cutting efficiency, and cyclic fatigue resistance. To date, the findings of several studies suggest that asymmetric reciprocation motion improves cyclic fatigue resistance³ and extends the life span of NiTi files. The amplitude of reciprocation has a significant influence on the cyclic fatigue life of the files, and the cleaning effectiveness of reciprocation files are comparable with rotary file instrumentation.⁴

Asymmetric Reciprocation Movement vs the Single-File Technique
Root canal preparation using the single-file technique has been recommended to simplify the endodontic procedure, showing a very attractive approach in terms of saving both time and cost. To be able to open the space with just one file, the design of the single file has to have a tip size of ISO 25 with an apical taper of .08 (in the last 4 mm) that reduces toward the coronal end. This configuration allows a high tapered preparation at the apical third, but does not ensure the enlargement necessary to shape the space for ideal irrigation cleaning.⁵ Using more than one instrument during instrumentation, the clinician can irrigate more frequently, and debris may have less opportunity to accumulate in a tooth that is more frequently irrigated.⁶ Another interesting point (for commercial reasons) is that the cutting direction of the single file is CCW (left cutting or reverse design).⁷ There is no evidence that left cutting is more efficient8 or adds any advantage to instrumentation safety.

Asymmetric reciprocation is a movement in which the cutting angle is bigger than the releasing forces angle, resembling Roanne’s balanced forces technique. The direction of the cutting angle can be right (or CW), as with all endodontic files, or left (or CCW), as with the single-file technique files category.

A Hybrid Asymmetric Reciprocation and Rotary System
It seems to be clinically reasonable to start the instrumentation with the safety of asymmetric reciprocating movement, opening the root canal space securely based on the benefits of stress reduction of the asymmetric reciprocation movement until the apical limit, and then to finish with rotation to carry out debris.

Until 2015, if the clinician chose to use the 2 movements of left cutting asymmetric reciprocation and rotation, the clinician had to use 2 different file systems. They would start with left cutting reciprocation from the single-file system, change the file for a rotary file system, and then try to match size and taper to the proper rotary file in sequence. It is very difficult to perfectly match the single-file technique instrument with a rotary file. The unique apical .08 taper design of the single file is not duplicated in any other rotary file, forcing the operator to have to choose a larger instrument to finalize the canal shaping. Therefore, the safety benefits generated by the reciprocation movement end up being left aside when the operator is required to use the rotary movement with a larger-sized instrument that would generate some degree of enlargement.

In 2015, a new open endodontic motor concept and a set of NiTi files designed to be able to achieve the highest performance in both asymmetric reciprocation and rotation (Genius endodontic system [Ultradent Products]) were introduced to the market. The Genius system allows the clinician to begin with one ISO 25 .04 file in right cutting asymmetric reciprocation and finish with a larger file in reciprocation (first, until the WL), concluding with the same file in rotation to optimize debris removal. The design of the Genius file and the kinematics of the reciprocation movement allow a free tip crown-down approach with brushing movement, avoiding overstress in the file during instrumentation. The right cutting direction permits the use of asymmetric reciprocation in open space until the WL, minimizing file fatigue and optimizing the cleaning effect in rotation. Plus, the operator does not need to change the file used in reciprocation to another system or size/taper; this would result in a stress decrease in rotation once the space was already opened and the file was working freely.

Genius Open Concept Motor
The Genius motor (Figure 4) is a micro-stepper motor that enables full control in terms of either direction of movement (CW and CCW) and is classified as an open motor, which means that the operator can set any different parameters possible. Regarding the rotary movement setup provided, it can be used in different values of torque and speed, types of automatic reverse (stop, reverse, or stop and reciprocation), and direction of movement (forward or reverse). In reciprocation movement, the angles in CW and CCW directions can be set and stored in different programs. It comes pre-set for the Genius files in 300 rpm in rotation with a 0.5-Ncm torque. In asymmetric reciprocation, it is set for 90° CW and 30° CCW motion in 350 rpm.

Genius Files
Genius files (Figure 5) are made of thermal-treated austenite NiTi alloy, designed with an S “double right-cutting edge” cross section, enhanced flutes, and a non-cutting tip with a safe and anti-blocking tip design. They present a constant .04 taper, compatible with right cutting asymmetric reciprocation and rotary movement, and operate ideally in reciprocation with a 90° “cutting” CW and 30° “releasing” CCW movement provided by the preprogrammed Genius file settings built into the Genius motor. This 90° CW/30° CCW asymmetric reciprocation movement reduces the risk of file fracture due to torsional and cyclic fatigue and creates a smooth path to the apical limit. Studies reported that decreasing the reciprocation range of instruments resulted in increased cyclic fatigue resistance with less canal transportation and more centered preparations.^4,9

After reaching the apical limit in reciprocation, the Genius files can be used in a 360° rotation movement to retrieve debris more efficiently from the canal. The file will not be under the pressure of the dentin canal walls since the space was previously opened by the same file in asymmetric reciprocation.

Studies have shown that the 2-file Genius sequence showed comparable results to the 7-rotary-file sequence in terms of bacteria and endotoxin cleaning properties¹⁰ and to 5 rotary file sequence with greater tapers in terms of the final dimensions of the shaped canals.¹¹

Genius Hybrid Reciprocation and Rotary Technique
1. Scout the canal with an SS K-File No. 10 or No. 15.
2. Set the Genius motor to rotation by pressing the Mode button (Rotating P1).
3. Use the Orifice Shaper in rotation until the curvature (proceed with a measurement of the straight part of the canal at the radiographic image to determine the Orifice Shaper WL) (Figures 6 and 7).
4. Determine the cleaning and shaping WLs using an apex locator.
5. Create the glidepath, if necessary (Figure 8).
6. Set the Genius motor to reciprocation by pressing the Mode button.
7. Use the 25 .04 file in reciprocation GEN mode to the WL in a crown-down approach and perform a brushing movement without apical force (Figures 9 and 10).
8. Proceed patency with a hand K-File (the same used during the electronic measurement) until the cleaning WL (Figure 8).
9. Finish with the final file (30, 35, 40, or 50) according to the anatomy of the canal (Figure 11). Use reciprocation movement until the WL is achieved. Set the motor to rotation by pressing the Mode button. Use the same final file in rotation for 5 seconds to remove debris.
10. Achieve all instrumentation with irrigant/lubricant inside the canal. At each instrument exchange, use copious irrigation, preferably with continuous passive ultrasonic irrigation (Figure 12).

The brushing movement (Figure 10) is performed as follows:

Passively introduce the file inside the canal. Once resistance is felt, withdraw the file 0.5 to 1.0 mm and, without forcing the instrument apically, create circumferential flaring with a lateral brushing action and without engaging the tip. The amplitude of the brushing movement should be minimal, and, preferably, the free tip of the file will be touching the dentin walls laterally. Do not work the instrument more than 5 to 10 seconds. Interferences will be gradually removed anatomically alongside the canal path, and the file will progress toward the apical area. Light apical pressure will be necessary only in the final millimeters of the WL.

CLOSING COMMENTS
There are significant advantages in the correct application of the reciprocation movement during the engine-driven instrumentation of the root canal system. It is important to emphasize that reciprocation is not a synonym for the single-file system. The hybrid use of right CW cutting reciprocating and rotary motions provides evidence-based safety and efficiency for the instrumentation phase, significantly reducing the chance of instrument fracture and improving canal shaping.

References

1. Lambrianidis T, Tosounidou E, Tzoanopoulou M. The effect of maintaining apical patency on periapical extrusion. J Endod. 2001;27:696-698.
2. Zuolo ML, Carvalho MC, De-Deus G. Negotiability of second mesiobuccal canals in maxillary molars using a reciprocating system. J Endod. 2015;41:1913-1917.
3. Ferreira F, Adeodato C, Barbosa I, et al. Movement kinematics and cyclic fatigue of NiTi rotary instruments: a systematic review. Int Endod J. 2017;50:143-152.
4. Saber Sel D, Abu El Sadat SM. Effect of altering the reciprocation range on the fatigue life and the shaping ability of WaveOne nickel-titanium instruments. J Endod. 2013;39:685-688.
5. Azim AA, Griggs JA, Huang GT. The Tennessee study: factors affecting treatment outcome and healing time following nonsurgical root canal treatment. Int Endod J. 2016;49:6-16.
6. Robinson JP, Lumley PJ, Cooper PR, et al. Reciprocating root canal technique induces greater debris accumulation than a continuous rotary technique as assessed by 3-dimensional micro-computed tomography. J Endod. 2013;39:1067-1070.
7. Grande NM, Ahmed HM, Cohen S, et al. Current assessment of reciprocation in endodontic preparation: a comprehensive review—Part I: historic perspectives and current applications. J Endod. 2015;41:1778-1783.
8. Plotino G, Ahmed HM, Grande NM, et al. Current assessment of reciprocation in endodontic preparation: a comprehensive review—Part II: properties and effectiveness. J Endod. 2015;41:1939-1950.
9. Shin CS, Huang YH, Chi CW, et al. Fatigue life enhancement of NiTi rotary endodontic instruments by progressive reciprocating operation. Int Endod J. 2014;47:882-888.
10. Cavalli D, Toia CC, Flores Orozco EI, et al. Effectiveness in the removal of endotoxins and microbiological profile in primary endodontic infections using 3 different instrumentation systems: a randomized clinical study. J Endod. 2017;43:1237-1245.
11. Drukteinis S, Peciuliene V, Dummer PM, et al. Shaping ability of BioRace, ProTaper NEXT and Genius nickel-titanium instruments in curved canals of mandibular molars: a MicroCT study. Int Endod J. 2018 Jun 1. [Epub ahead of print]

Dr. Ramos graduated with a degree in dentistry from the State University of Londrina (1987) in Brazil. He is a specialist and PhD in endodontics and a former endodontics sector coordinator at the State University of Londrina. He has had 3 endodontic textbooks published and has written more than a dozen chapters for various endodontic books. He accomplished all of this while working part time exclusively as an endodontic specialist at his private office from 1987 to 2012. Since 2012, he has been the Ultradent R&D and clinical affairs endodontic manager. He can be reached via email at carlos.ramos@ultradent.com.

Disclosure: Dr. Ramos is the clinical affairs endodontic manager for Ultradent Products.

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