Root canal files

- What do they look like? - pictures, an explanation of their component parts. | Hand vs. rotary files. | What the patient experiences. | Issues and concerns with reusing files between patients - sterilization, file breakage.

Root canal files.

This page gives details, and provides insights that a patient might be interested in, about root canal files.

These are the small instruments that a dentist uses during a tooth's endodontic therapy to clean and shape each of its root canals.

What do root canal files look like?

Since a picture is worth a thousand words, take a look at our graphic below. It shows two root canal files.

A picture of two root canal files.

Root canal files.

As you might guess from our labeling, there are a couple of different parts that are important to the endodontic process.

Here's an explanation of each of them, and what role they play.

The parts of a root canal file.

A = The working end.

This is the metal part of the file that's inserted into your tooth's root canal for the process of cleaning and shaping it. It's made out of a flexible metal alloy, with steel or nickel-titanium being the ones most commonly used.

A closeup of the cutting edges of a root canal file.
These tools are actual mini files.

Spiraling down the surface of the metal shaft of a file is a series of grooves (it's "flutes"). It's this pattern's design, and the degree of sharpness and roughness it possesses, that allows the instrument to create the friction needed to clean and shape the walls of a tooth's canal.

 
Animation of the filing action of a root canal file inside a tooth.

The primary cutting action of a file is on the up stroke.

Kinds of root canal files.

A dentist has a few different types of "files" to choose from. They include K-files, Hedstrom files (H-files), reamers and barbed broaches.

Each type has its own characteristic design (often a function of its method of manufacture), and that dictates how it's most efficiently used. As examples:

  • Some files are primarily intended for use as rasps.

    When used, the dentist will insert the file to a chosen depth inside the tooth. Then as they draw it back out, they'll scrape it against the walls of the canal so to abrade them. (A pulling-planing action.)

  • Others are designed to be used with a reaming motion.

    The dentist will first wedge the file snugly in the canal. They'll then twist it a fraction of a turn (so its cutting edges engage the walls of the canal). They'll then draw it back out, removing a bit of tooth structure as they do. (A twisting-cutting action.)

 
A closeup of barbed broach that has snagged necrotic pulp tissue.

Pulp tissue snagged on a barbed broach.

  • An entirely different type of hand instrument is the barbed broach.

    Per its name, it's not unlike a single strand of barbed wire with a handle. When it's inserted into a canal, its sharp barbs snag and dislodge tissue and debris (see picture).

Section references - Torabinejad

What you'll feel.

During your procedure you'll of course be numbed up. But even so, you may get a sense of what's going on (pressure, vibrations, strokes of the file) as your dentist creates the various motions described above.

Additional characteristics of the working end of root canal files.

  • The type of metal chosen. - Two important characteristics of any alloy chosen for making root canal files is that it's both flexible and strong.

    Flexibility helps the file to passively follow the gradual curves that canals frequently have. Good strength characteristics are needed because it's important that the file doesn't break easily (separate) as it's being used (a situation that can result in case failure).

    In recent decades, nickel-titanium alloy has been introduced to the field of dentistry. And much of the current mechanization of the root canal process (see rotary files section below) has been possible because of the superelasticity and resistance fatigue that this alloy offers. (Hargreaves)

  • File size and taper. - You may have noticed in our pictures that root canal files are tapered. And they also come in different sizes (diameters).

    The idea is that a dentist will use a series of files as they clean and shape the canal at hand, starting with small ones and progressing on up to larger sizes. As each file is used in turn, the internal shape of the canal is gradually opened up (widened and flared) just a little bit more.

  • File length. - Just like teeth, root canal files come in different lengths. 21mm files are typically long enough for use with back teeth and lower incisors. The roots of upper front teeth and lower eyeteeth are usually larger, so a longer file is needed for them (25mm is common, even 31mm is available).

    Your dentist choosing the shortest length file that is appropriate means that you won't have to open nearly as wide during your appointment. That's actually a pretty big deal, both for your comfort and your dentist's ease and quickness of work.

Section references - Hargreaves

A picture of two root canal files.

B = File measurement markings.

The dark rings on the shaft of the files in our picture are calibration markings.

They offer your dentist a way to conveniently and quickly know the length measurement from the tip of the instrument to that point. (Marked in whole number millimeters. The first marking you see on the 21mm files in our picture, the one right above the "B", is at the 16mm point from their tip).

Why they're important.

Using these markings, the dentist knows precisely how far into the tooth and canal the file extends. That's critical because their goal will be one of cleaning the canal all of the way to its end but not beyond the end of the root. (Doing so helps to minimize postoperative complications.)

C = Movable plastic stopper.

The yellow items you see on the shafts of the files in our picture are movable plastic stoppers. The idea is that the dentist slides the stopper to that point on the file's measuring scale (the lines labeled "B") that correlates with the needed "working length" for the file (the length of file needed to clean the full length of the canal).

They're just markers, not bumpers.

Since the stoppers are movable, they're just markers rather than fixed bumpers (that would limit the amount of file length that can enter the tooth). Their presence simply provides a quick visual indication for the dentist as to how much of the needed working length has already been extended into the canal.

A picture of two root canal files.

D = The file handle.

Obviously the portion of the file we have labeled with a "D" is its handle. This is the part that the dentist holds with their fingers as they work it with combination up-and-down and twisting motions.

What may not be obvious to you is that the color of the handle, and the number embossed on it, indicates it's size (its diameter, both at specific points along its taper and at its tip).

File sizes.

Root canal files come in a series of graduated sizes (i.e. sizes 10, 15, 20, 25, etc..., increments of 5 are built into the scale for smaller sizes, increments of 10 for larger ones). The specific dimensions associated with each size are industry standards. (This same scale is used for gutta percha points, the material usually used to fill in and seal a tooth's cleaned-and-shaped root canals.)

The number itself indicates the diameter of the file at its tip. For example, size 25 = .25mm, which is about 1/100th of an inch. As you are no doubt aware, that's a pretty small number. And with increments of just .05mm between sizes, it can be difficult to tell much difference between two consecutive files (see picture).

The importance of hand files.

What we've described so far is root canal files that are hand, or more precisely "finger," instruments. And actually, the finger-manipulated nature of these instruments provides a distinct benefit.

As they are used, the dentist gains information about the internal anatomy of the tooth via their tactile senses. Something that wouldn't be possible if the instruments were "hand" held.

Newer isn't better.

Related to that advantage, although modern automated/mechanized root canal systems have been developed (see rotary files, discussed next), their use can never entirely replace the need for the use of individual finger instruments.

No tooth's root canal system can be, or should be attempted to be, completed via the use of rotary instruments alone (Ingle). Instead, anticipate that your dentist will probably use a mix of both.

Section references - Ingle

Rotary root canal files.

Nickel-titanium.

As mentioned above, the introduction of nickel-titanium alloy to the field of dentistry has resulted in significant changes in the way root canal treatment is routinely performed.

The advantages that it offers are that of superelasticity (the ability of the metal to retain its shape after being deformed) and high resistance to cyclic fatigue. Due to these characteristics, files made from this alloy can be successfully used in curved root canals using a continuously rotating motion. (Hargreaves)

That last sentence opens the door for further mechanization of the root canal process. It means that nickel-titanium files, driven in a rotating manner by a dental handpiece (engine-driven as opposed to finger-manipulated files), can be safely and predictably used, even in curved canals. That's a big deal.

What this means for you the patient.

Mechanization of the process has been a substantial game changer for the field of endodontics. It offers a way of adding speed and efficiency to the procedure, without compromising results.

Mechanization makes one-visit endodontic therapy easier to accomplish. The reduction in patient chair time it provides may result in a lower fee for the patient. And studies suggest that the use of rotary-file systems may result in fewer operator errors, a factor that might be important with non-specialists (general dentists). (Torabinejad)

As alluded to above, don't expect that your tooth's entire cleaning and shaping process will be completed just via the use of handpiece-driven files, because it shouldn't be. But for whatever aspect of your procedure it can be used, it should be a positive event for you and your tooth's work.

Section references - Torabinejad, Hargreaves

Appearance differences between finger and rotary files.

The metal shafts of rotary (engine-driven) root canal files generally look the same as hand files (they make use of calibration marks and movable stoppers too). The main exception would be that some rotary systems come in different sizes/tapers than is standard (the files are often fatter).

And, of course, rather than having a handle designed to be grasped by fingers, this portion of the file will have a shape that can be inserted into and latched securely in the handpiece that's used to generate its working motion.

What you'll notice when a handpiece and rotary files are used.

Engine-driven files are generally intended to be used with a passive action inside a canal (Ingle). So beyond the minor vibrations you notice from the handpiece running, what you experience should be a nonevent.

Some rotary systems work the file in a continuously rotating motion, others in a reciprocating one (back and forth action). Some handpieces even back the file off by reversing its rotation if it senses that it's being placed under too much torque (stress).

During your procedure you may get a sense of each of these motions as they're created. Also, since these drills are often electrically driven, you'll hear the gentle whine of it's motor.

Section references - Ingle


Can/Should root canal files be reused?

The policy of reusing root canal files between patients, or patient visits, is one that's set by a dentist for their office. There is no industry standard that applies to this issue.

File sterilization.

The component parts of files are designed with sterilization in mind. And while attention to detail is required, several studies have documented that effective cleansing is possible.

Typically there will be a series of steps that the files are subjected to (rinsing, scrubbing, ultrasonic cleaning and then sterilization). A frequent method of sterilization involves the use of a steam or chemical autoclave (a 20 minute cycle of 250°F/121°C at 15 psi), which is sufficient to kill all bacteria, spores, and viruses.

Section references - Torabinejad

Why shouldn't root canal files be reused?

File separation.

Inadequate sterilization typically isn't the issue of concern with reusing files between patients. Instead it's metal fatigue (caused by repeated use of the instrument) that results in file separation (breakage).

As mentioned (and linked) above, instrument separation is a procedural complication that can result in case failure. And while guidelines exist, knowing the precise status of any file, especially a used one, is impossible.

Root canal files as disposable instruments.

The potential for file breakage during use can be reduced by simply considering files (both hand and rotary types) to be disposable instruments. As a side benefit, this choice can help to increase the efficiency of the dentist's work. It's been shown that a root canal file looses roughly 50% of its cutting efficiency after its initial use. (Hargreaves)

With this approach, the dentist of course incurs an added expense. Considering that some mix of hand and rotary files would typically be needed to perform a tooth's work, our calculations suggest that the added cost for this group of instruments might run on the order of $40 to $50.

Section references - Hargreaves

 
 
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