An Example of an Endodontic Micro-CT Scan That Opens Our Eyes

Micro CT studies have given us the ability to assess the claims made for innovative endodontic instrumentation systems. One such system is the XP-3D shapers. It has an undulating shaft that when rotating is able to contact a greater portion of oval canals, something centered straight-shaft systems are less capable of. The micro-CT scan shown above does not necessarily cleanse canals better. In fairness, the undulating rotating shank of the XP-3D instruments will in wide-open oval canals produce a more thorough debridement because of the greater chances of circumferential contact. However, pulpal anatomy like the case above is not an example of wide-open oval anatomy. Rather it has the common characteristics of many mandibular molars, more or less round mesio-buccal and messio-lingual canals and a thin flat isthmus connecting the two. This pulpally inhabited isthmus dominates the space between the mesio-buccal and mesio-lingual canals and requires cleansing to increase the chances of long term endodontic success.

 It is here that the XP-3D instruments are not up to the task. Fabricated from NiTi they are extremely flexible, a quality in these instances that does not increase its ability to penetrate narrow pulpal spaces. Upon contact with an isthmus narrower than the XP-3D, the instrument immediately deviates away. You can see the difference in cleansing potentials when comparing the debriding process in the two mesial canals versus the distal where the canal is essentially round throughout the its length. A complete distal preparation leaves far less pulp tissue simply because the pulpal anatomy is far less complex. 

 Does the undulating design of the instrument provide superior functionality in these simpler anatomical situations? In my opinion, no. Anatomy round in cross-section is easily addressed by any number of rotating or oscillating systems. If in cross-section an area of internal resorption produces a space similar to an arterial aneurism, that space may be debrided by simply bending a stainless steel reamer at about 45o at the tip and working it against the walls enhanced greatly by its use in the 30o oscillating handpiece. Tactile perception tells us the location of internal resportion and we can concentrate the oscillating motion of the reamer at precisely this point.

In terms of penetrating a narrow isthmus or thin buccal and lingual extensions, we need a combination of flexibility and durability, a resistance to immediately deviating upon first contact. We could just use a thicker instrument and apply significantly more lateral force, but dentin preservation is always of paramount importance. It would be a pyrrhic victory if we removed tissue in the isthmus by widening this space to the same extent as central mesio-buccal and mesio-lingual canals. The cross-sectional dentin is thinnest in the isthmus with a major concavity generally present on the furcal side of these roots. Thinning out the dentin here can lead to strip perforations, dentinal micro-cracks that can coalesce and propagate into full vertical fractures. 

What is needed in attaining penetration and cleansing of the isthmus with minimal loss of dentin are instruments that are extremely thin and highly flexible, but have sufficient body to resist immediate deflection. Furthermore, they must be used in a forceful, but non-fatiguing method that prevents instrument separation. For me, the solution is the use of thin stainless steel reamers (as thin as an 06 tip with an 02 taper) used in a 30o handpiece oscillating at 3000-4000 cycles per minute. I know from previous experience that the thinnest reamers used in rapid oscillations are quite effective in shaving dentin from canal walls as illustrated by the use of an 06 tipped 02 tapered reamer in a canal narrow enough to strongly resist apical negotiation. Yet, after attaining apical length and applying the 06 tipped 02 tapered reamer against all the canal walls, I readily skip sizes to a 10 tipped 02 tapered stainless steel reamer that encounters far less resistance in gaining full apical length.

These reamers are, in effect, internally routing, creating a space that mirrors the original canal anatomy in larger form. They are more than flexible enough to adapt to any canal curvatures that may be present with the limited arc of motion (1/12 of a full circle) virtually eliminating any chance of instrument separation. From a wider perspective, I can’t help noticing how a simpler technological answer, the use of reamers in an oscillating handpiece (technology that has been around for many decades) surpasses in function the highly touted NiTi product in function, cost and safety. While NiTi has its merits, the initial debriding of canals are done so much more efficiently with stainless steel reamers with an incorporated relief starting with a 15 and limited to a 30o arc of motion. Greater penetration, minimal potential to cause dentinal micro-cracks, multiple use of the instruments and a virtual elimination of instrument separation are gained by upgrading and slightly modifying existing technology. 

I believe rotary has its place. It often produces more rapid results and creates a space in at least the mesio-distal plane that accurately correlates the space created with the placement of a gutta percha point that duplicates the mesio-distal dimensions produced by the instruments. Having said that, I will only apply a rotary system that affords me sufficient confidence that they will not separate under function, a system I have discussed in previous posts. Added to those precautions is the realization that despite marketing messages from the major manufacturers not all endodontic cases are best handled in rotary. The oscillating system employing relieved reamers is always a safe fall back position that produces consistently effective results without the concern of separation and without the hidden detrimental effect of producing dentinal defects.

As I’ve learned over the years, the approach to education that is maturing in me even to this day is one where the first goal is truth accompanied by humility knowing that the truth is sometimes an elusive goal and one’s own bias can be construed as truth. In addition, empathy for others, putting oneself in another persons shoes, is an adjunct to humility that makes the goal of truth even more worthwhile. And hopefully, what it leads to is trust. Without trust there is cynicism and too often nihilism that stops progress in its tracks and dampens the humanity that is within all of us. 

Regards, Barry