Drs. Ricardo Caicedo and Stephen J. Clark describe the use of HyFlex® CM controlled memory NiTi files, presenting the technique and four case reports



Abstract
New Nitinol files with controlled memory (CM), a metal alloy of nickel and titanium, have been introduced and manufactured by Coltene/Endo. The company reports that this file is more resistant to cyclical fatigue compared to other NiTi files, which reduces the incidence of file fracture. Some clinicians have multiple file systems in their offices to meet the demands of different clinical situations. The purpose of this new rotary file is to simplify root canal treatment and to optimize cleaning and shaping of the canal. The cutting profile of each HyFlex® CM file facilitates penetration in the canal, and presents a root canal shape corresponding with the original anatomy. This system also offers precise apical finishing, leaving the structural integrity of the root intact after endodontic therapy and allowing the adaptation of the tapered gutta-percha point to the canal anatomy.

Introduction
One of the primary goals of endodontic therapy is the complete debridement of pulp tissue from the canal, coupled with shaping of the root canal system.1,2 The ability to enlarge a canal without deviation from the original canal curvature is a primary objective in endodontic instrumentation.3,4 In the past decade, rotary nickel-titanium (NiTi) instrumentation, use of the microscope, and ultrasonic irrigation have all helped to improve the efficiency and accuracy of endodontic procedures.  
NiTi is a shape-memory alloy also commonly referred to by the name Nitinol, derived from its place of discovery, the Nickel Titanium Naval Ordinance Laboratory. William Buehler, along with Frederick Wang, discovered its properties during research at the Naval Ordinance Laboratory in 1962.5 The discovery of the shape-memory effect dates to 1932, when Swedish researcher Arne Olander first observed the property in gold-cadmium alloys. The material, if deformed while cool, returns to its undeformed shape when warmed.6,7

How does Nitinol work?
Nitinol shape-memory alloys undergo a phase transformation in their crystal structure when cooled from the stronger, high temperature form (austenite) to the weaker, low temperature form (martensite). This inherent phase transformation is the basis for the unique properties of these alloys—in particular, shape memory and superelasticity.

As depicted in Figure 1, the reversible, solid phase transformation known as the martensitic transformation is the force behind shape-memory alloys. The alloy material forms a crystal structure, which is capable of undergoing a change from one form of crystal structure to another. Temperature change and/or loading initiate this transformation. At high temperatures, Nitinol assumes an interpenetrating simple cubic crystal structure referred to as austenite (also known as the parent phase). At low temperatures, Nitinol spontaneously transforms to a more complicated “monoclinic” crystal structure known as martensite. The temperature at which austenite transforms to martensite is generally referred to as the transformation temperature. More specifically, there are four transition temperatures. When the alloy is fully austenite, martensitic begins to form as the alloy cools at the so-called martensitic start, or Ms temperature, and the temperature at which the transformation is complete is called the martensitic inish, or Mf temperature. When the alloy is fully martensitic and is subjected to heating, austenite starts to form at the As temperature, and finishes at the Af temperature.

Above its transformation temperature, Nitinol is superelastic, able to withstand deformation when a load is applied and return to its original shape when the load is removed. Below its transformation temperature, it displays the shape-memory effect. When it is deformed, it will remain in that shape until heated above its transformation temperature, at which time it will return to its original shape.

Nitinol is typically composed of approximately 54.5% to 57% nickel by weight. Making small changes in the composition can change the transition temperature of the alloy significantly. For this reason, Nitinol may or may not be superelastic at room temperature. These unique properties and adaptation of Nitinol can be used in a wide range of temperatures, making it suitable for rotary endodontic instruments.

With the advent of cone beam computed tomography (CBCT) and the wide adoption of the surgical operating microscope, some of the problems of larger NiTi tapered rotary instruments have been detected, such as the lack of flexibility and adaptation to the canal space anatomy not detected in two-dimensional radiographs. Larger NiTi instruments may transport the canal space, causing strip perforations because of their rigidity.

It is clear that prolonged reuse of NiTi rotary instruments strongly affects instrument fatigue, and recent data suggest the hypothesis that other factors (primarily errors and misuse) may be more accountable for intracanal instrument separation. Further studies are assessing the cyclic fatigue of each instrument at different levels of the shaft by altering the radius of curvature.14 The most recent research results testing the fatigue behavior of conventional NiTi instruments and CM NiTi wire concluded that the material property had a substantial impact on fatigue lifetime, and the rotary files made from CM wire had a significantly higher resistance to fracture (Nf) with lower surface strain amplitude than the conventional NiTi wire with identical design.

HyFlex® CM
Based on this research, a new Nitinol rotary instrument (HyFlex® CM) has recently been marketed that is machined from a wire (termed CM-wire) previously subjected to a proprietary, novel, thermomechanical processing procedure. The manufacturer has reported that this new CM-wire instrument has considerably improved flexibility, resistance to cyclic fatigue, and good adaptation to the canal space anatomy, compared to conventional rotary instruments that are machined from superelastic austenitic NiTi wire. Clinical use seems to indicate that these new HyFlex® CM rotary instruments have outstanding clinical fatigue resistance (Graph 1 and Table 1).



HyFlex® CM NiTi files have been manufactured utilizing a unique process that controls the material’s memory, making the files extremely flexible but without the shape memory of other NiTi files. This gives the file the ability to follow the anatomy of the canal very closely, reducing the risk of ledging, transportation, or perforation compared to other instruments with rebound effect (Figure 2).

The manufacturer reports that the shape and strength of files with straightened spirals can be restored during autoclaving. This means that the file appears to regain its shape after sterilization and reuse. Files not returning to original shape should be discarded. In vitro research analyzing the cutting capacity of HyFlex® CM instruments after multiple uses is in process.

The HyFlex® CM Rotary NiTi File technique
For best results, a new file should be used for each procedure. The number of times the instruments can be re-used depends on the treatment and procedure. Instruments must be inspected before and after use. Hyflex® CM files may respond to pressure, torque, and resistance with a lengthening of the spirals, which allows the file to avoid binding to the walls and therefore increases fracture resistance. During autoclaving, the instruments can regain their shape (Figures 3A-3D). However, care must be taken if the instruments start to rewind in the opposite direction during use because the files will not regain their shape and should be discarded. If after autoclaving, a file has multiple spirals that appear lengthened, or the file appears to not be functional, the file should be discarded.

The following are general recommendations for the instrumentation process:
•    Sterilize files prior to use. See sterilization instructions below
•    Use of a slow-speed handpiece is required. Operate the handpiece at 500 rpm. Recommended torque setting is up to 2.5 N·cm (25 mN·m)
•    Irrigate/lubricate the canal frequently when using the files
•    Clean file flutes after each insertion into the root canal
•    Recapitulate between each step with the patency file

An advantage of HyFlex® CM files is that they can be used with multiple techniques (crown-down, step-back) or with the single-length technique proposed by the manufacturer (Figure 4). Figures 5 and 6 show the handle characteristics, ISO color sequence, taper, flute design, and cross sections of the rotary instrument.

Sterilization instructions
•    Remove files from the packaging prior to sterilization. Note that due to the unique features of the alloy, the files may be slightly bent as a result of the packaging process. This can be gently corrected by hand.
•    Place the devices in a file block, support, or container to avoid any contact between instruments.
•    If the disinfecting solution contains a corrosion inhibitor, it is recommended to rinse the instruments just before autoclaving.
•    Put the file kit in a sterilizable wrap or container. Steam sterilization at 134°C/273°F for 6 minutes; for prion inactivation, 18 minutes is recommended.
•    Keep devices in the sterilization packaging stored in a dry, clean environment.

Step-by-step single-length technique instructions
Figures 7-9 show the disposition of the instruments from left to right and a method of placing them together with the K-files in a sponge during clinical use.



Coronal patency
After gaining straight-line coronal access, assess working length (WL) with an electronic apex locator and create an apical glide path to a K file 02/20 hand file size (Figure 10).





After use
•    Immediately after use, soak all instruments in a detergent and disinfecting solution combined with a proteolytic enzyme, if possible
•    If debris is left on the file, use a soft toothbrush for cleaning
•    Note the Sterilization Instructions

Clinical cases
Case 1 (Figures 18-24)
This case involved a mandibular molar (tooth No. 30) with pulp necrosis treated with HyFlex® CM files. Figure 18 shows the occlusal access and the final access preparation of each canal with straight-line glide paths developed with ultrasonic tips and an Endo-Z bur (Dentsply). This is important so that the files do not accumulate cyclic fatigue stresses in the coronal third of the canal. Figure 19 shows the canals after instrumentation with the last rotary file, .04/40, and after medication with calcium hydroxide.
Postoperative radiographs show the shaping objective has been reached with the rotary file. Figures 20 and 21 show the obturation process with an apically accurate tapered point fit with tapered gutta-percha, GuttaFlow® (Coltene Whaledent) filling material/sealer, and an ideal access cavity with the mesial and distal access walls parallel to the mesial surface of the tooth. Postoperative radiograph and photography show the final coronal restoration with a secondary monoblock using ParaCore® dual cure core and resin cement (Coltene Whaledent) with integrated bond system before definitive restoration with a full coverage crown (Figures 22 and 23).



Case 2 (Figures 25-36)
This case involved maxillary lateral incisors (teeth Nos. 7 and 10) with pulp necrosis and asymptomatic apical periodontitis, instrumented with HyFlex® CM file sequence (Figures 25-36), and filled with .04/40 taper gutta-percha with GuttaFlow® filling material. A post-space was created, and a tertiary endodontic monoblock interface with a size 4.5 ParaPost® (PP) Taper Lux Post was bonded with ParaPost® ParaCore® dual cure core and resin cement with an integrated bonding system (ParaBond®: Non-Rinse Conditioner and Adhesive), which seals and protects against marginal microleakage as well as reinforces the abutments, producing better biomechanical performance. Figure 34 is the postoperative radiograph showing apical curvature of tooth No. 7 bypassed with precurved stainless-steel K-files and finished with a precurved HyFlex® CM rotary file .04/40. Note the conservative coronal shapes (Figure 36), the excellent apical control, and the accessory lateral canals at the apex filled by GuttaFlow® (Figures 28, 30, and 34). This result required only four HyFlex® CM files per canal.



Case 3 (Figures 37-43)
This case involved a mandibular molar (tooth No. 19) with pulp necrosis and asymptomatic apical periodontitis that was treated similar to Case 1—a mandibular molar shaped with five HyFlex® CM files in the mesial canals, and a three-file rotary sequence of .06-20, .04-30, .04-40 HyFlex® CM files. In the distal canal, WL was reached with a .06-20 file because it was a large canal, starting directly from Step 4 of the single-length technique. Note the excellent three-dimensional cleaning and filling of the entire canal (Figure 43). Also, note that the structural integrity is intact after endodontic therapy, and the prepared shapes flow together with the original curvature of the canals as demonstrated in Figure 39 with the paper points copying the canal anatomy after the canals were dried.



Case 4 (Figures 44-54)
This patient presented with a maxillary premolar (tooth No. 12) with pulp necrosis and a Class 1 crown fracture without a periodontal defect (Figure 46). Figure 45 shows the occlusal access and the result after access of each canal with straight-line glide paths developed with ultrasonic tips and an Endo-Z bur prior to rotary instrumentation with HyFlex® CM files using a single-length technique. The canals were filled with .04/40 taper gutta-percha cone with GuttaFlow® filling material, which combines two products into one: gutta-percha in powder form with a particle size of less than 30 μm and a silicone sealer (Figures 47 and 48). Warm vertical condensation was used, and a palatal post space was reinforced with a tertiary endodontic monoblock interface using a size 4.5 PP Taper Lux Post bonded with PP ParaCore® dual cure core and resin cement. Figure 52 shows the abutment preparation before the final impression. Figures 53 and 54 illustrate the final restoration in place.



Conclusion
The development of the HyFlex® CM rotary file system, a new Nitinol rotary instrument, with two types of flute blades, may be used to perform a root canal with a simplified instrument sequence. With its great flexibility, the HyFlex® CM rotary file system preserves both the root canal anatomy and minimizes file separation, imperatives which are fundamental to performing predictable endodontic treatments.

Acknowledgment
Case 4 crown restoration was carried out by Dr. Paula L. Collins, FDP, Outpatient Center, University of Louisville, School of Dentistry.

Disclaimer
The authors of this article have no commercial ties with the manufacturer or any conflicts of interest to disclose.

Bios
Ricardo Caicedo, Dr. Odon, is a Specialist in Endodontics, Specialist in Health Professions Education, and Professor Emeritus of the Colombian Dental College, UNICOC, Bogota, Colombia. Currently, he is Associate Professor, and Director of the Endodontic Pre-Clinical course, Advanced Endodontics Course for senior students, and Clinical Instructor of Post-Graduate Endodontics, Department of Oral Health and Rehabilitation, Endodontic Division, at the University of Louisville, School of Dentistry, Louisville, Kentucky. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Stephen J. Clark, DMD, is Professor of Endodontics and Director of the Post-Graduate Endodontic Program, Department of Oral Health and Rehabilitation, Endodontic Division, at the University of Louisville School of Dentistry.

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