Combining Observable Evidence, Clinical Experience and Common Sense In Endodontic Instrumentation: Part Two

Up to this point, I have focused on the data, clinical experience and common sense that substantiates the safer and more thorough results that can be attained using the 30o oscillating reamers. I offer a few results attained using this approach in highly curved canals where the chances of using rotary NiTi would encounter a very high risk of instrument separation.

Let’s now refocus on why 30o oscillating reamers are a common-sense alternative to rotary NiTi. The first most basic insecurity that requires resolution is the unpredictable incidence of separation of rotary NiTi. We know its a genuine problem when the same companies that produce rotary instruments now offer asymmetric reciprocation, a form of interrupted rotations, for the exclusive purpose of reducing instrument separation. In contrast, the 30o oscillating reamers, a technique that eliminates full rotations has been shown in this study that we published a while back to virtually prevent separations.

The rotary systems have had to go through 6 generations to date in an attempt to solve the problem of unpredictable instrument breakage, a state of insecurity that while reduced still exists. From an application point of view, rotary instruments are most safely used by following an extensive series of precautions that include staying centered with little deviation from the largest least resistant portions of the canal. The safety of the instrument is tied directly to inadequate cleansing of oval canals as has been documented in a series of research studies of which I include two below. Another precaution is crown-down preparations that remove excessive amounts of coronal dentin to limit the contact of the rotary instruments along the length of the canal reducing the chances of separation, but weakening the tooth increasing the chances of vertical fracture.?

Superior debridement should clearly be a goal of rotary NiTi and micro-CT scans of teeth previously instrumented with some of the newer rotary systems give us some insight into their efficiency in three-dimensional debridement. I believe that the micro-CT scans of these two systems speak for themselves in demonstrating that rotary instrumentation leaves a lot to be desired in what has been marketed as the paradigm improvement in endodontic instrumentation. (Two videos)

Recently to reduce the incidence of instrument separation, very conservative preparations have been suggested, reducing the incidence of instrument separation, the time required for the procedure and a reduced number of instruments that lower the costs to the dentist. These are all benefits to the dentist, but come at the expense of further reducing the debridement of oval canals and thin isthmuses. It was the goal of oscillating stainless steel relieved twisted reamers to eliminate separation as a cause of concern that then gave the dentist the freedom to vigorously apply the instruments to all the canal walls in a form of internal routing that provides for circumferential uniform cleansing that reflects the original canals anatomy in larger form. That I believe has been demonstrated in the data points I have posted above.

Now here is an attention grabbing twist that directly impacts the data that demonstrates an increasing resistance of the newer rotary NiTi instruments to separate. The majority of studies showing the increased resistance of the predominantly martensite heat treated NiTi instruments to cyclic fatigue have been carried out at room temperature on plastic blocks or metal forms that don’t require lubricants. In a paper published in the Journal of Endodontics Volume 42, Issue 5, May 2016, Pages 782-787, titled Basic Research Evidence for Reduced Fatigue Resistance of Contemporary Rotary Instruments Exposed to Body Temperature, the authors found the following results and concluded:

Results: For the tested size and at 20°C, Hyflex CM showed the highest resistance to fracture; no significant difference was found between TRUShape and Vortex Blue, whereas ProTaper Universal showed the lowest resistance to fracture. At 37°C, resistance to fatigue fracture was significantly reduced, up to 85%, for the tested instruments (P?<?.001); at that temperature, Hyflex CM and Vortex Blue had similar and higher fatigue resistance compared with TRUShape and ProTaper Universal.

Conclusions: Under the conditions of this study, using a novel testing design, immersion in water at simulated body temperature was associated with a marked decrease in the fatigue life of all rotary instruments tested.

In another study published in theJournal of Endodontics Volume 44, Issue 10, October 2018, Pages 1563-1566 and titled,Effects of Sodium Hypochlorite Concentration and Temperature on the Cyclic Fatigue Resistance of Heat-treated Nickel-titanium Rotary Instruments, the authors published the highlights, results and their conclusions.

• Increasing the temperature of the sodium hypochlorite (NaOCl) solution to body temperature and above significantly decreased ProTaper Gold's resistance to cyclic fatigue.
• The irrigant solution and simulated body temperature should be considered in future nickel-titanium instrument failure studies.
• The irrigant temperature and concentration were shown to play a vital role in the cyclic fatigue resistance of endodontic instruments manufactured with heat-treated NiTi alloy.

Results The number of cycles to fracture of the PTG F2 was highest in distilled water at 25°C and lowest in 5.25% NaOCl at 60°C. Changing the irrigating solution from distilled water to NaOCl and increasing the surrounding temperature reduced the fatigue resistance.

Conclusions NaOCl irrigating solution at different concentrations and temperatures influenced the cyclic fatigue resistance of PTG instruments. Future NiTi instrument failure studies should be conducted under simulated body temperature conditions in commonly used irrigating solutions.

The marketing efforts to sell rotary NiTi as a paradigm improvement over all other techniques that have the same advantages of evolving into more efficient and safer systems is in my mind isolated from the reality of their usage. That they are unquestionably more efficient than the manual use of K-files is indisputable, but that is not the comparison that need be made today. Stainless steel in the configuration of relieved twisted reamers and powered by a 30o oscillating handpiece have changed the comparison between rotary NiTi and what was thought to be the height of stainless steel usage, namely the manual use of K-files. Oscillating stainless steel relieved twisted reamers are virtually immune to breakage, can be used several times before replacement based on dullness alone, dramatically reducing costs and allowing them to be used far more aggressively knowing that confined to the short arc of motion generated by the 30o oscillating handpiece they will not distort even highly curved canals.

Hopefully, this extended discussion offers those who are open to reasonable points of view a set of insights that will help them make better clinical decisions. It is important for us to make clinical decisions based on what appears to be logical data and that for each of us is an individual process. I am more than happy to hear and discuss different points of view in the exercise of improving our endodontic techniques.

Regards, Barry