Endodontic Instruments: Choosing the Right Stainless Steel Hand File

Monday, June 10, 2013

 Endodontic Instruments: Choosing the Right Stainless Steel Hand FileStainless steel hand files (SSHFs) are a vital part of the endodontic armamentarium. Traditionally, along with Gates Glidden Drills, these instruments are the primary means of shaping root canal systems.

In developing countries, and to some degree in North America, there is still a significant contingent of doctors using SSHFs as their primary means of shaping canals in lieu of rotary nickel titanium (RNT) instruments. Regardless of whether one shapes canals with SSHFs solely or with RNT instruments, when used appropriately, SSHFs play a major role in the negotiation, initial shaping and prevention of iatrogenic events in endodontic therapy.

As a practical matter, there are a bewildering array of hand file designs, indications, lengths, sizes and geometries available to the clinician. Even for the veteran endodontist, knowing what SSHF to use for a given purpose and when can be challenging, as there are an infinite combination of SSHFs and canal anatomies to negotiate and shape.

Making matters more complex, there are no clear-cut answers for some SSHF questions. For example, does it matter if a K file is safe-ended in the smaller tip sizes #6-15, especially during reciprocation? To what is the optimal size a canal should be prepared by hand before RNT files are brought in? Is a glide path prepared with SSHFs superior to one prepared by RNT instruments designed for this purpose? When is a nickel titanium hand file more appropriate than one made of stainless steel?

Among many such possible questions, this article will discuss the clinically relevant and important distinction between hand K files and K flex type instruments. I will also offer a perspective on the questions posed above.

In the most general terms, a K file (Mani K files for example, Fig. 1A) is made from a square stainless steel blank, making it stiffer than a K flex type instrument (Examples of a K file and a flexible K file from Mani are shown in Fig. 1, above at left). This single difference makes the clinical application and characteristics of these two file types dramatically different.

K files are “universal instruments” in that they can be used for both negotiation of canals—although they are not as stiff as carbon steel files specially designed for this purpose—and also for initial enlargement of canals to make way for RNT instruments. Given their stiffness due to their square cross section, K files can also be used to bypass canal blockages or other canal transportations.

SSHFs should be used once and discarded. As a full time endodontist, I find an average molar requires approximately 2-3 packs of SSHFs. Using new instruments improves cutting efficiency and assures optimal tactile command. It is neither economical nor clinically advisable to attempt sterilization and reuse.

Clinically, in my hands, these files are always pre-curved before insertion and when a hand K file will reach the apical terminus and remains tightly bound in the canal, reciprocation is a viable and efficient alternative to manual use.

Fig. 2: Completed case negotiated with Mani hand file K files initially and whose glide path before RNT files was shaped with Mani Flexile files.

Fig. 2: Completed case negotiated with Mani hand file K files initially and whose glide path before RNT files was shaped with Mani Flexile files.

Once inserted, it has value to observe whether K files (and K Flex type instruments) emerge curved from the canals. If curved when withdrawn, the curvature imparted onto the hand file provides an impression of the canal shape before, during and after canal shaping with RNT files. Such knowledge allows the clinician to assess the complexity of the case in terms of the initial shape and evolving prepared shape, as well as the ideal sequence of RNT files needed to shape the tooth.

For example, if a hand K file is removed from a canal with a three-dimensional curvature imposed upon it, the canal is complex and consideration should be given to using a step back approach rather than a crown down enlargement method.

Safe ended K files can be purchased, but as mentioned above, especially in the smaller hand file sizes it is a matter of personal preference whether the file should be safe ended or have a standard cutting tip, especially when used in reciprocation because it is very difficult, except with gross misuse, to ledge, block, and transport canals with SSHFs at the smaller end of the size range (#06-#15 K files).

K flex type instruments are much more flexible than their K file equivalents. They are not useful in canal negotiation per se, especially in calcified canals, as they buckle far too easily when encountering resistance. Alternatively, if the canal has been scouted and has a minimal diameter—for example, the diameter of a #10 SSHF—K Flex instruments can shape curved canals relatively efficiently. In general, this means K Flex instruments are used more for shaping curved canals that have already been negotiated to the apex and have some minimal size after initial negotiation.

Given the aforementioned flexibility of K flex type files, why, when and how are nickel titanium hand files (NTHFs) indicated? The answer is multifactorial, but ultimately the answer relates to both the design of the NTHF and the anatomy at hand. The flexibility of NTHFs, when blended with an efficient cutting design, provides functionality that potentially surpasses those of K flex type instruments used alone, hence the popularity of existing NTHFs in various markets. Irrespective of the above, the functionality of NTHFs versus K flex type and K type SSHFs is a matter of personal preference. There is no one size fits all means to shape canals.

In summary, K files are useful for negotiation and initial canal shaping. They are relatively stiff, a useful attribute for the above indications. K flex type files are flexible and can shape curved canals efficiently, especially when creating a glide path. Single use of SSHFs is recommended. I welcome your feedback.

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