By Joseph C. Stern, D.D.S.
There have been few innovations in endodontics as revolutionary as nickel titanium (NiTi) rotary. There was a time when one would spend multiple visits hand filing a canal with stainless steel instruments. This proved to be tedious and quite tiring for the practitioner. That all changed with the advent of nickel titanium rotary instruments, and a torque controlled motor. The beauty of nickel titanium is its flexibility and efficiency. Stainless steel files became stiffer as the sizes increased. Where navigation of a curved canal with stainless steel instruments might be extremely difficult and time consuming, nickel titanium rotary, when used correctly, can allow for a seamless canal preparation. The extreme flexibility exhibited by these ‘heat treated’ nickel titanium instruments is the key (1,2,3,4,10). This flexibility allows for navigation of the most complex and curved root canals. With every great innovation however, comes a down side and NiTi rotary is no exception. The big disadvantage of this system is sudden file separation within the canal. To fully understand the positive and negative aspects of this great technology let us first take a closer look at nickel titanium itself.
NiTi is a ‘superelastic’ metallic alloy that, when flexed, undergoes an austenitic-martensitic transformation from its original structure making it extremely flexible (5,17,20,21). This transformation usually happens when the metal is stressed, such as during the instrumentation of the root canal. If however the NiTi file is stressed beyond its elastic limit, it will break (8). One of the unique characteristics of NiTi is its ‘shape memory’, allowing it to be deformed during usage and then return to its original shape if it is not stressed beyond its elastic envelope (13). NiTi files also have high elastic flexibility in bending and torsion as compared with stainless steel files. Flexibility is the hallmark of NiTi rotary, and it is this feature that allows it to overcome one of the greatest challenges of root canal instrumentation, namely following the sharp and unexpected curves in most canals. When NiTi rotary is used correctly, it is significantly faster and more efficient in the instrumentation of the root canal (1,2,3,4,10).
When NiTi files fracture, it is due to cyclic fatigue or torsional strain. Torsional fracture is when the tip of the instrument locks in a canal while the shaft continues to rotate, inevitably leading to fracture (5,6,7). Excessive force on the file during instrumentation causes the tip of the file to lock in a “tight spot”. Larger sized files tend to be more resistant to these torsional fractures as they don’t bind as easily in those “tight spots”.
The other cause of instrument separation is ‘cyclic fatigue’. Repeated bending of instruments in curved canals causes the metal to fatigue, leading the instrument to eventually fracture. Obviously the more curved a canal is, the higher the probability of the file separating due to cyclic fatigue. The larger the file size and taper, the lower it’s resistance to separation by cyclic fatigue. Cyclic fatigue occurs when a metal is subjected to repeated cycles of tension and compression which cause its structure to break down, ultimately leading to fracture (8,9). Torsional fatigue is the twisting of a metal about its longitudinal axis at one end, while the other end is in a fixed position (11). Cyclic fatigue is most apt to occur in a canal with an acute curve and a short radius of curvature (12) and is the leading cause of NiTi instrument separation. Increasing the resistance to file separation has always been the main focus in new NiTi rotary instrument design.
NiTi rotary files are designed with three objectives in mind:
- Cutting efficiency – The file should be able to loosen up a tight canal with relative ease.
- Flexibility – The file should be able to navigate the expected and unexpected curves in a root canal.
- Resistance to separation in curved and calcified canals.
11 Tips to Prevent File Separation
- Glide path- the fracture rate of rotary NiTi instruments can be greatly reduced by creating a glide path for the NiTi instrument tip (14,15,19). This allows the unobstructed penetration by the rotary file all the way to the working length. This is done by preflaring the canal orifice with a NiTi orifice opener which is placed only a couple millimeters into the canal. By opening the canal orifice we will have a much easier time placing both hand files and rotary files into the canal. The orifice opener eliminates the “triangle of dentin” which obstructs the orifice. Another very important component of establishing a glide path is the creation of a predictable path for your rotary file to reach all the way to the working length, which is created by first hand filing the canal to at least a 15 hand file. Before you ever place a rotary file in a canal you should first instrument the canal comfortably with an 8 , 10, and 15 hand file. This will in essence pave the pathway for your first rotary instrument and reduces the separation rate of rotary files and also cause less canal transportation as compared to preparations without any glide path. To quote Cliff Ruddle – “Whoever owns the glide path, wins the shaping game of endodontics”.
- Always use instruments in sequence! Don’t skip around. If you start with a size 8 file, don’t skip to a size 15 as your next file in the canal. This will place more stress on the larger size files and increase the odds of instrumentation errors.
- Recapitulation- after each usage of a rotary file make sure to re-enter the canal with a 10 or 15 hand file to your working length (WL). This is to make sure you haven’t clogged the apical portion of the canal with debris. This will preserve the ever important glide path. Without the glide path you will end up stressing the NiTi file, ultimately leading to fracture.
- Reuse of Niti Files – How many times is too many? File fatigue will depend on several variables, including instrument properties, canal morphology and operator skills (16). For example, working one calcified canal will stress a file more than working multiple ‘loose’ canals. Forcing a file in a tight or curved canal will fatigue it more than placing a file in a relatively straight and loose canal. Thus, there is no magic number of how many times a file can be used. It is prudent to often examine the cutting edges for wear and strains.
- A rotary file should be used in an ‘in-and-out’ (pecking) motion with light apical pressure. Never force a rotary file into a canal and never place heavy apical pressure as this will cause the file to get locked into the canal and separate.
- Copious irrigation and lubrication of the canal. Never file a dry canal as this will create excessive debris in the canal which ultimately will lead to canal blockage. In trying to force your way through a blockage the chances of file separation are significantly increased.
- Pulp chamber access: Straight line access into the pulp chamber as well as the root canal is crucial to help avoid coronal tension on the file (18). This puts less stress on the file and ultimately will prevent needless file separations. This also aids in glide path maintenance
- File inspection- one of the indicators that a file has been fatigued and on the cusp of breakage is when the flutes of that file begin to unwind. It should be a habit to always inspect the file for this unwinding. Once you notice the file flutes beginning to unwind it is time to toss it.
- Bite block- having the patient use a bite block during treatment is another great way to avoid file separation. When the patient’s range of opening is compromised, your access to the posterior teeth may be limited. A sudden closure, even slight, by the patient during instrumentation can lead to file fracture.
- Always keep the rotary file moving within a canal. Stopping the rotary midway down a canal will put more pressure and stress on the file predisposing it to fracturing because of cyclic fatigue. Constant movement of the rotary file within the canal will better distribute the forces and prevent breakage.
- Be prepared! Thoroughly inspect the radiograph before starting a case. Extra care in rotary file usage will be needed in cases that have calcified or curved roots. Never rush! Never blame the file! With proper technique and experience, separating files will become a more and more rare occurrence.
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Dr. Joseph C. Stern is a Diplomate of the American Board of Endodontics. He is a Clinical Assistant Professor in Endodontics at Touro College of Dental Medicine and lectures frequently on the subject of clinical endodontics. He has lectured at many local county dental societies, at the NJDA Annual Session in May 2019, and at the Greater New York Dental Meeting in 2020. He maintains a private practice at Clifton Endodontics.