Like all phases of dentistry, the goal of endodontics is to save our natural dentition to function in the mastication of our food. The key word here is function and to maximize that functioning over as long a period as possible. With this in mind, the mechanics of endodontics can have a direct impact on that longevity.
Missing canals, separating instruments within the canals, and removing excess tooth structure beyond the need for removing pulp tissue weakens the teeth’s resistance to vertical fracture and reduces the chances of success. Knowing that these events in treatment reduce our rates of success, it is important for us to examine the systems we employ and how they potentially impact the endodontic procedures we are doing.
One of the mainstays of endodontic instrumentation is the employment of myriad rotating systems, whether used in continuous or interrupted rotations. As has been amply described in the literature, rotating instruments have the potential to separate within a canal due to excess torsional stress, cyclic fatigue, or some combination of both.
These instruments most often separate in the apical third, preventing effective irrigation and the subsequent use of larger instruments to more completely remove pulpal remnants, leaving bacteria-laden pulp that prevents the resolution of an infected site.
The steps needed to reduce the incidence of separation when performing greater tapered rotary endodontics include straight-line access and crown-down preparations, both of which remove sound, supportive dentin. From the perspective of the instruments, these steps make sense since they reduce the incidence of instrument separation. However, from the perspective of the tooth, these steps may prevent breakage of an instrument within the confines of the canal, but they unalterably weaken the tooth’s resistance to vertical fracture.
Stated succinctly, rotary techniques require steps that weaken the tooth for the benefit of the instrument. With the widespread use of rotary systems, the removal of excess tooth structure is not so much thought of as a needless sacrifice, but rather a routine safety step that must be incorporated. This type of mindset redirects the impact of the excessive removal of dentin away from the tooth and concentrates on its beneficial effects for the rotary instruments.
A Safer Alternative
I would not be detailing the negative impact on the excess removal of dentin if I weren’t offering an approach that eliminates the need for making these compromises in the first place. Instruments break in rotation due to excess torsional stress and cyclic fatigue. If the instruments no longer go through full rotations, torsional stress and cyclic fatigue can be reduced to a level that no longer leads to breakage. How is this done?
By substituting a 30° handpiece oscillating at 3,000 to 4,000 cycles per minute, we limit the arc of motion to approximately 1/12 of a circle. Note that the high oscillating frequency does not lead to either excess torsional stress or cyclic fatigue. The 30° oscillating handpiece uses our thinnest .02 tapered instruments, eliminating practically all hand fatigue from the very beginning of the instrumentation process.
Once the arc of motion is confined to 30°, a cascading of fortunate events follows. Knowing the instruments are now invulnerable to breakage, they may be vigorously applied to all the walls of the canal, leading to more complete tissue removal in those canals that are highly oval and have thin isthmus-like anatomy associated with them.
I call this process internal routing, where instruments thinner than the canal at least in one dimension lean against all the canal walls. I have no need for excessively straight-line access, nor do I require crown-down preparations. Far more dentin is preserved while more pulp tissue is removed. No tradeoffs are required, and the tooth remains stronger and more resistant to vertical fracture.
Several other fortunate events occur due to the short arcs of motion that are now employed. I have no need for instruments of greater taper, as .02 tapers suffice. Even with this minimal taper, these instruments in their thinnest configurations tend to be larger than the diameter of the canals in the mesio-distal plane.
For those dentists who do endodontics, how often do you see a canal in an adult wide in the mesio-distal plane? From my experience, very rarely. Much more frequently, canals are so thin in the mesio-distal plane that they are barely visible on x-ray.
If canals are so thin to begin with, there is no rationale for using instruments of greater taper other than the safety issue that arises when employing rotation. But we are using 30° oscillations rather than full rotations, either continuous or interrupted.
With the .02 taper more than adequate for thorough endodontic instrumentation, we can eliminate expensive greater tapered NiTi and employ instruments using twisted stainless steel, which is far less vulnerable to breakage, retains its cutting edge longer, and exhibits more than enough flexibility to open canals anywhere from a minimum of 25 through 40.
The instruments are more than strong enough, particularly when confined to a 30° arc of motion to be used several times before replacement. They will dull out and be replaced long before they are ever subject to separation when used in the 30° reciprocating handpiece.
From the perspective of irrigation, the thin tapered instruments are removing far less tooth structure. Consequently, the ratio of irrigant to debris is much higher, resulting in a loose, watery solution rather than dense, cakey debris that can be impacted buccally and lingually when shaping canals with greater tapered instruments.
One should also be aware that the 3,000 to 4,000 cycles per minute generated by the oscillating handpiece activates the irrigants to make them more effective. Logically, the greater the ratio of irrigant to debris, the more effective the irrigant.
Fine points on maximizing the 30° oscillating approach to canal cleansing include the shape of the .02 tapered stainless steel instruments in the configuration of reamers rather than files. Reamers have half the number of flutes along their 16 mm of working length compared to 30 flutes along the same 16 mm for a file.
With half the number of flutes, the reamer encounters half the resistance along the length of the canal. The flutes are also twice as vertical as those on a file, allowing the reamer to shave dentin away from the canal walls with the first clockwise motion. This immediately decreases the engagement of the reamers with the canal wall, thereby reducing the resistance as the reamer negotiates to the apex.
Understanding the benefits of an instrument design that reduces engagement along the length of the treatment, these stainless steel twisted reamers incorporate a flat along their entire working length, producing two vertical blades that remove dentin in both the clockwise and counterclockwise motions generated by the 30° oscillating handpiece.
With minimal engagement to begin with, the instruments most often negotiate to the apex with minimal resistance as well as being applied both bucco-lingually and mesio-distally to remove tissue three-dimensionally.
This is the approach we are teaching our students. It is safer than any other approach while being highly efficient in terms of results and time requirements. It requires a relatively inexpensive oscillating handpiece. It does not require an expensive torque-sensing adjustable motor, nor the skills one would need in mastering techniques where the instruments remain vulnerable to separation.
That alone, the elimination of separation anxiety, is a worthy goal. That we actually leave a stronger tooth at the end of the process is the much greater goal that this approach provides.
For those dentists who would like to learn what we are teaching our students, we are developing a continuous education course to meet those requests. Please let us know if you are interested in learning more.
Disclosure: Dr. Musikant is president of essential dental systems, which manufactures relieved reamers (SafeSiders) and epoxy resin cement (EZ-Fill).