The Shape of Success

In root canal therapy, shaping is a key step in creating clean canal contours. To accomplish this type of shape shifting, endodontic files are needed. We explore the world of files and how they can help your clients obtain successful outcomes.

Although root canal treatment (RCT) starts with the use of burs to create an access cavity, from there on out, endodontic files become the stars of the show. Used to remove infected tissue from root canals, these tiny corkscrew-like instruments, trace their origin back to 1838, when Washington, D.C., dentist Edwin Maynard took a file to a watch spring to treat an infected tooth.

Though today’s files feature higher-tech construction, the basic principle is not much different. Essentially bits of wire that have been twisted or machined are used to remove infected material from canals through filing, cutting and scraping, with the help of cleansing and lubricating irrigants. Files are also used to shape canals, to ensure that the natural contours of the root are retained and to allow for thorough cleaning.

The number of files available, ranging from manual to motorized, can be mind bending. But if you count endodontists among your customers, you may want to become conversant in how files are designed and what they are meant to accomplish. At the very least, you’ll want to make sure you can tell a barbed broach from a K-file.


POINT OF SALE | JUST FILE IT

  • Hand files are an important part of every endodontic armamentarium, and are often used in conjunction with motorized files.
  • Rotary files are reportedly efficient in removing debris from canals during cleaning and shaping.
  • Reciprocating files are said to be less likely to break under cyclic stress than rotary files.
  • Developments in metallurgy and design are leading to files that are flexible enough to handle curves, less subject to separation, remove less healthy dentin, clean more efficiently and offer controlled memory.

Hand Files

Hand files may seem archaic next to their motorized counterparts, but they offer greater tactility and a measure of safety not found in instruments spinning at 300 rpm. They are still widely used for part or even all of the RCT procedure. Simple and efficient, they are available in stainless steel or nickel-titanium (NiTi), and come in a range of International Organization for Standardization (ISO) color-coded lengths and diameters.

Among the most prevalent manual files are K-files and Hedstrom files (H-files), which are basic go-to instruments for RCT. In fact, K-files, typically made from twisted square, rectangular or triangular wire, were the first commercially available endodontic file. Used with a filing or watch-winding motion, they are designed for enlargement of canals. Hedstrom files, made from machined round wire, are used for canal enlargement and debridement via a push/pull motion, cutting on the pull stroke.

Also used for enlargement are reamers, which do the job through a rotate-and-pull motion. Barbed broaches, basically tiny pieces of barbed wire, are used for quick removal of pulp tissue — sometimes in one piece — from straight canals.

Hand files are often used to start the RCT process, securing patency and forming glide paths, through which irrigants and instruments can easily travel. But these days, many practitioners will eventually switch over to power tools for the remainder of the work.


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MOTORIZED FILING

Engine-driven endodontic files wouldn’t amount to much without motors and handpieces. As luck would have it, several manufacturers offer motors as part of their endodontic file systems. A few even incorporate other procedure aspects, such as obturation, into their range of operation, and many are available in cordless versions for lightweight portability.

While some motors are dedicated to rotary or reciprocation, increasing numbers utilize technology that can alter filing mode in response to load, or file pressure, so that either a rotary or reciprocating motion can be employed as needed. Others offer settings for both modes, allowing easy programming. Some even offer easy switching between high-speed and endodontic handpieces, and even have the ability to sense imminent file breakage and stop before it happens.

Handpieces range from basic rotary contra-angles to those designed for counterclockwise reciprocation. They may accommodate high- or low-speed operation and offer torque control for endodontic procedures, enhancing safety and efficiency. As a bonus, many are streamlined to permit superior access and visibility.


Rotary and Reciprocating Files

Files designed for use with low-speed rotary handpieces can really speed up the RCT cleaning and shaping process. Long made of stainless steel, rotary files are now available in nickel-titanium, which reportedly makes them more resistant to cyclic fatigue and file separation. And, unlike stainless steel files, NiTi versions have no problem staying centered in the canal and are flexible enough to follow curved anatomy without breaking. This helps limit iatrogenic errors, such as ledging, zipping and perforations inside the canal.

Reciprocating files, have become popular in some endodontic circles more recently. “Files that rotate simply spin continuously through the full 360 degrees of motion. Reciprocal instruments rotate through a smaller arc and then stop, and reverse the direction of the spin for a short time after which, they return to the forward partial rotation,” explains Robert Roda, DDS, a Scottsdale, Arizona-based endodontist and a past president of the American Association of Endodontists. “This motion keeps files centered in the canal, and also minimizes the chance of instrument separation compared to a file in a full rotational motion. The back-and-forth reciprocal motion does not let the instrument get bound in tight canals, so there is less likelihood of instrument fracture.”

Used with reciprocating handpieces, reciprocating files are designed to cut in a counterclockwise direction. They are reportedly less apt to break under cyclic stress than their continuously rotating counterparts, even when the same file is used for much of the procedure.1

But opinions are divided on this. Allen Ali Nasseh, DDS, a Boston-based endodontist, who is also president and CEO of Real World Endodontics educational seminars, says that when single-file systems are used with reciprocation, excessive torque on the dentinal walls can result as the final master file is forced into a small canal. “Reciprocation,” he says, “can be much safer for dentin if the cutting is incremental and shaped among several reciprocating files rather than a single file. Another down side of reciprocation is the excessive debris accumulation in the root canal that requires frequent irrigation.”

Likewise, Roda notes that reciprocating files appear to push more debris toward the apex and into lateral canals and other anatomical spaces. “This makes it more difficult to clean fins and webbing between canals,” he says. “It may also cause more postoperative discomfort after the instrumentation procedure.”

Peter Z. Tawil, DMD, program director of graduate endodontics in the Department of Endodontics, University of North Carolina School of Dentistry in Chapel Hill, concurs that rotary movement augers debris more efficiently coronally, but argues that reciprocating files are designed with a higher pitch and flute space to accommodate for debris removal. But while Tawil concurs that rotary movement augers debris more efficiently coronally, he argues that reciprocating files are designed with a higher pitch and flute space to accommodate for debris removal.

In any case, Nasseh believes that both rotation and reciprocation are valid motions for shaping a root canal. “Ultimately,” he says, “the goal is to create a given shape. Rotation motion is unequivocally more efficient in cutting, however, reciprocation can provide a safe alternative for those who are more heavy-handed or less experienced.”

Tawil agrees. “Reciprocating files are great for new practitioners and students as they get used to how much pressure to put on files and how to get the tactile feedback from the handpiece and the files.” But he notes that experienced operators may find reciprocation handpiece performance less than smooth, and says they may notice a lack of tactile feedback. Some experts suggest the best strategy is to use both systems to complement one another. In fact, a few manufacturers have made this easier by introducing motor systems that accommodate both modes of operation.

File Design and Construction

Similar to Maynard’s watch spring, files are made of wire that is ground into various cross-sections, including triangles, squares, rectangles and rhomboids. It is then either twisted or machined to create flutes, lands and cutting edges at any number of tapers, pitches, helical angles and rake angles that are tailored to efficiently execute specific RCT chores, such as cutting, scraping and removing debris. Tips may be cutting, which can come in handy in calcifications, or noncutting, for safer canal navigation.

According to Nasseh, RCT efficacy, efficiency and safety are directly impacted by file design. “The optimal design,” he says, “allows safe and efficient cutting and debridement of the root canal walls, while reducing torque on both the file and the root canal walls.”

Nasseh admits that this is a tall order as each design has its pros and cons. The same is true of metallurgy. “For example,” he says, “non-heat-treated files have better cutting efficiency and torque resistance while heat-treated files have more flexibility and cyclic fatigue resistance. I believe, given the strengths of different file designs, metallurgy, and motion in specific parts of the root canal, perhaps hybridizing and blending files of different design and metallurgy under one comprehensive technique may help address various clinicians’ needs for safer and more predictable NiTi.”

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But it is, indeed, NiTi that is heralded as having the most recent impact on modern endodontics. Although one problem that has surfaced with NiTi files is shape memory, which can cause them to rebound — sometimes in the canal at inopportune times. They may also be more subject to flexural and torsional fatigue. Various heat treatments have been explored as ways of enhancing resistance to such pitfalls.

“Through the 1990s, companies played with the designs, tapers, flute angles, cross sections, etc,” says Tawil. “But the latest development in files is the change of the metallurgy itself. In the 2000s a new metal called control memory (CM) was introduced.”

CM technology involves a type of thermal processing that has been shown to enhance flexibility and fatigue resistance compared to conventional NiTi files.2 Says Tawil, “Basically, they made the martensitic phase of NiTi, which used to exist at super low temperature, available at room temperature. That was groundbreaking as we now can have super flexible memory-free files.”

Tawil explains that having a NiTi file without a shape memory makes it possible to negotiate extreme curvatures. “The challenge is that when a file is super flexible without a memory, its cutting efficiency tends to go down as it does not withstand pressure as previous files have,” he says. “So clinicians have to get used to putting less pressure on these files and be more patient with cutting. Thankfully, research has shown these new files to have a higher resistance to fatigue, so they can be used at a higher rpm to compensate and make their cutting efficiency clinically effective.”

In Roda’s opinion, a couple of important trends are evident. The first, he says, supports the concept of minimally invasive dentistry by removing as little dentin as possible. “The theory being,” he says, “that the less dentin removed, the less the root structure is weakened, and the less chance there is of root fracture.” To this end, he says that the maximum flute diameters of files are being reduced.

But there’s a catch.

“Canals must be cleaned, disinfected, and completely sealed, which are processes that are enhanced by using bigger preparations,” notes Roda.” The two goals are at odds with each other, but as canal irrigation improves, this is becoming less of an issue with smaller canal shapes.”

But Roda further observes that the smaller access preparations put greater demands on the instruments since they now need to negotiate multiple curvatures to reach the apex. “Using smaller files has upped the ante in terms of needing to resist file breakage, and the new metallurgy has made this less of a gamble,” he says. “File separation appears to occur less often than before, and while a better understanding of engine-driven instrumentation is the primary reason for this, there is no doubt that enhancements in file design and metallurgy are contributing to this trend.”

Another trend Roda reports is that of new file designs, which are increasing the amount of canal surface that is being contacted by rotating instruments. “These new files expand into wider areas of the canals but also constrict to fit into smaller spaces. In addition, they generally have smaller maximum flute diameters, so they can also help with dentin preservation.” Indeed, among some of the more recent innovations are files that expand and contract in response to body temperature. Another offers a flexible lattice-like construction, in which the center is hollow. These designs are said to enhance three-dimensional cleaning and shaping in canals.

No matter what types of files your customers favor, there is clearly plenty of sales potential in this continually evolving product category. By sharpening your knowledge on file basics and staying ahead of the curve on new developments, you’ll be well positioned to help your clients continue to shape success not only inside the canal but also in their practices.


Terms to Learn

  • Flute: Curved portion of a file that extends from cutting edge to cutting edge. Collects debris.
  • Helical angle: Angle of the file’s cutting edge in relation to its long axis.
  • ISO: International Organization for Standardization.
  • Ledging: Formation of a ledge in the curved portion of a canal wall when a file continues in a straight line.
  • Martensitic phase: Occurs at low temperatures when nickel-titanium transforms into its crystal structure from its higher-temperature cubic form, austenite.
  • Pitch: The distance between threads, or spirals, in a file. A constant pitch may cause files to screw into the canal. Many files have a variable pitch.
  • Radial land: The flat surface between the two cutting edges of the flutes. Provides blade support for a flat, planing cut.
  • Rake angle: The angle formed by the cutting edge and cross section in relation to the file shaft. A positive rake angle cuts, and a negative angle scrapes.
  • Separation: File breakage.
  • Taper: Progressive tapers may be fast, but are at greater risk of separation; variable taper is regarded as safer while also being efficient.
  • Zipping: When a file straightens inside a curved canal, resulting in over-enlargement of that area.

References

  1. Weeks S, Bahcall J. Continuous or reciprocating endodontic rotary files: Evidence-based clinical considerations. Available at: entalcetoday.com/courses/298/PDF/DTCE_Weeks_Oct_214_fnl.pdf. Accessed January 22, 2018.
  2. Acosta EC, Resende PD, Peixoto IF, Pereira ES, Buono VT, Bahia MG. Influence on cyclic flexural deformation on the torsional resistance of controlled memory and conventional nickel-titanium instruments. J Endod. 2017;43:613–618.

Featured Image by MAVOIMAGES/ISTOCK/GETTY IMAGES PLUS

From MENTOR. March 2018;9(3): 20-22,24-25.

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