Rotary file design is evolving. This 3-part article series was written to provide an understanding of rotary design concepts to enable clinicians to maximize their endodontic skills for any technique available today and use advancements as they become available without dependency on advocacy claims and trial and error experience. Such an understanding has its foundation in asking relevant questions that are discussed over the series.
Despite the advantages of rotary systems, file separation remains frustrating and all too frequent. Interestingly, however, fear of separation has led to the creation of advocated techniques. Some of the techniques derived with this fear in mind can subsequently limit the comprehensive benefits of rotary files, and it is frustrating for the clinician to find that even when conscientiously followed, these techniques can still lead to instrument failure. In addition, recommendations derived to avoid separation may limit file efficacy. For example, we are often instructed never to rotate a file more than 350 rpm, yet in many circumstances 1,200 rpm can be more than 4 times as effective with less threat of complications. Understanding the ramifications of file design relative to canal anatomy enables the dentist to achieve exceptional treatment results consistently with the least iatrogenic potential.
(1) What Are the Terms I Need to Know When Comparing the Physical Properties of Files?
The success of using instruments while preventing failure depends on how the material, design, and technique relate to the forces exerted on the instruments. To understand fully how the file reacts to applied forces, terms have been defined to quantify the actions and reactions to these forces. Common terms related to forces exerted on files are as follows:
•Stress—the deforming force measured across a given area.
•Stress concentration point—an abrupt change in the geometric shape of a file, such as a notch, will result in a higher stress at that point than along the surface of the file where the shape is more continuous.
•Strain—the amount of deformation a file undergoes.
•Elastic limit—a set quantity that represents the maximal strain, which when applied to a file allows the file to return to its original dimensions. The residual internal forces after strain is removed return to zero.
•Elastic deformation—the reversible deformation that does not exceed the elastic limit.
•Shape memory—the elastic limit is substantially higher than is typical of conventional metals.
•Plastic deformation—permanent bond displacement caused by exceeding the elastic limit. The file does not return to its original dimensions after strain is removed.
•Plastic limit—the point at which the plastic deformed file breaks.
(2) Why Nickel Titanium?
Manual stainless steel files provide excellent tactile control and sharp, long-lasting cutting surfaces. However, due to the inherent limited flexibility of stainless steel, manual preparation of curved canals is problematic, while mechanized use risks separation or canal transportation (Table).
|Table. A Relative Comparison of the Properties of NiTi and Stainless Steel|
|Effective modulus||Approximately 48 GigaPascal||193 GigaPascal|
|Density||6.45 g/cm³||38.03 g/cm³|
|Ultimate tensile strength||Approximately 1,240 MPa||760 MPa|
|Coefficient thermal expansion||6.6 to 11.0 x 10-6 cm/cm/˚C||17.3 x 10-6 cm/cm/˚C|
|Resistivity||80 to 100 µm-ohm/cm||72 µm-ohm/cm|
Nickel titanium alloy has a unique ability to negotiate curvatures during continuous rotation without undergoing the permanent plastic deformation or failure of stainless steel files. Nickel titanium was first studied relative to stainless steel in an article published in 1988.1 In 1991, the first commercially available nickel titanium manual and rotary files were introduced by NT Co (Chattanooga, Tenn). In 1994, NT Co also introduced the first series of nickel titanium files with multiple non-conventional tapers, the McXIM Series, which had 6 graduating tapers ranging from 0.02 taper to 0.05. The graduating tapers were designed to reduce stress by limiting the file’s engagement during instrumentation. Nickel titanium rotary files have become widely accepted by the profession as a result of these initial successes.
As a super-elastic metal, the application of stress to nickel titanium does not result in the usual proportional strain other metals undergo. When stress is initially applied to nickel titanium, the result is proportional strain. However, the strain remains essentially the same as the application of additional stress reaches a specific level (forming what is termed loading plateau). Eventually, the application of more stress results in more strain that will increase until the file breaks. This unusual property is the result of a molecular crystalline phase transformation. External stresses transform the austenitic crystalline form of nickel titanium into the martensitic crystalline structure that can accommodate greater stress without increasing the strain. Due to its unique crystalline structure, a nickel titanium file has shape memory, or the ability to return to its original shape after being deformed. Nickel titanium alloys were the first and are currently the only readily available, economically feasible material that have the flexibility and toughness necessary for routine use as effective rotary endodontic files in curved canals.
(3) Are Nickel Titanium Files Always Advantageous Over Stainless Steel Files During Rotary Instrumentation?
If all canals were straight, stainless steel files would have results as good as or better than nickel titanium. Work-hardened stainless steel files have more torsional strength and maintain their sharp edges longer than nickel titanium. Unfortunately, few canals are entirely straight, limiting the practicality of stainless steel as a rotary file. Even minor curvatures can cause excessive stresses on stainless steel files, resulting in canal transportation or separation. Nickel titanium offers no advantage for instruments with large diameters and tapers due to a lack of appreciable flexibility, and hence are unnecessary. The advantage of stainless steel rotary files of larger diameters and tapers can certainly complement the use of nickel titanium files. Stainless steel rotary files are being introduced in lieu of nickel titanium files in larger sizes and tapers. Stainless steel files in such systems are only available in sizes that do not require functional flexibility (Figure 1).
|Figure 1. The FKG stainless steel rotary files are available in sizes for which NiTi files would lack any appreciable flexibility or advantage.|
(4) Are There Other Alloys That Offer Advantages as Rotary Files?
Other alloys have been developed that are suitable for rotary files and might have properties that are advantageous over those of nickel titanium. The problem is economic. In order to be feasible, alloys must have applications aside from endodontics to help offset production costs, which are otherwise prohibitive.
One alloy having considerable potential and economic feasibility is a nickel titanium niobium alloy having a substantially higher loading plateau, making it tougher than either stainless steel or nickel titanium. It has a sharper, more durable cutting edge and enhanced fracture resistance. Somewhat stiffer than the conventional NiTi alloys but more flexible than stainless steel, it is particularly advantageous for rotary activation of smaller files. The flexibility is sufficient to negotiate acute curvatures with minimum canal transportation yet stiff enough to withstand the pressure for optimal feeding into small canals.
Other titanium alloys that contain molybdenum and zirconium increase stability, workability, or corrosion resistance. In time, it will become known if the economic feasibility of these and other alloys will eventually provide a better alternative to the present material.
(5) Why Rotary Instrumentation?
Mechanical rotation possesses the enhanced ability to collect and remove debris from the canal system compared to manual filing. Hand instrumentation can push debris laterally into canal intricacies or apically through the foramen when using techniques that commonly use piston-like file insertions or counterclockwise directional motion. Conversely, continuous clockwise rotation will convey debris only in a coronal direction.
Mechanical rotation provides a more constant 360º engagement of the file tip in the canal, subsequently forcing it to track the canal, reducing transportation. Tracking the canal preserves tooth structure while more effectively cleaning and shaping. Continuous rotation reduces the time required for instrumentation. Obviously, a file constantly rotating from 200 to 2,000 rpm shapes canals far more rapidly than hand instrumentation, creating tremendous efficiency relative to manual techniques.
(6) Why Do We Need to Know Anything About Instrument Design?
The capabilities of files (made of the same material) are entirely dependent on design. No one aspect of file design is indicative of the file’s overall usefulness. Optimizing one design feature often compromises another. Design effectiveness is measured by cutting ability, operational torque, torque at breakage, flexibility, screwing-in forces, lack of transportation, and tip mechanics. File design quality is determined by how efficiently it meets the requirements of the canal anatomy with respect to its torsion resistance and fatigue failure.
Limitations of the initial nickel titanium file designs were largely due to an attempt to adapt the more easily manufactured hand file designs and technique concepts to these new rotary instruments. In using any file design, understanding the rudimentary physics involved is imperative to take full advantage of its benefits and minimize risks. Regardless of file design and the technique used, there are certain considerations that provide the understanding for using rotary instrumentation to its fullest advantage. It is important that anyone using rotary files be able to recognize the warning signs of possible complications and have strategies for file use to maximize efficiency. This can only be accomplished by thoroughly understanding the function of design, and this series of articles will go far toward aiding this purpose.
(7) Is a Particular Technique Important?
Canal anatomy, file design, and file dimensions dictate appropriate instrument use. Often, techniques for particular files are the result of subjective concepts recommended for the sake of simplicity. As such, the capabilities of files become confused with the capabilities of what has inappropriately become their associated recommended technique. How well a file performs while following a specific technique should not be the measure of a file’s effectiveness. Rather, how well the capabilities of a file can address the requirements of the canal anatomy should be the true measure. Since canal anatomies vary, techniques to clean and enlarge the canal effectively may include modifications of the manufacturers’ recommendations and may include different instrument types known as hybrid techniques. All of the above go a very long way toward explaining why endodontists rarely perform treatment the exact same way every time in various canal anatomies.
Part 2 of this series will further address the questions relevant to understanding the specific design characteristics of various files and their significance.
1. Walia HM, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endod. 1988;14:346-351.