Probably the strongest correlation we can make in endodontics is that short-term success is directly tied to the degree we eliminate bacteria from the pulpal space. It is reasonable to assume that in an infected case we are not removing 100% of the bacteria so it is reasonable to assume we must reach some threshold of reduction that tips the balance in favor of healing. It is also possible that our anti-bacterial and fixative agents as well as the sealers we use are neutralizing enough remaining bacteria to advance healing. Our clearest criteria for healing in non-vital teeth are the resolution of periapical areas that were present as a result of bacterial activity.
Long-term success requires the same reduction in bacterial as short-term success, but is dependent upon leaving enough tooth structure to survive function and the replacement of tooth structure when enough is not present after endodontics is performed. In either case, leakage must be prevented and the functional forces distributed through the tooth and restoration to prevent fracture must be in place. To date, we don’t know what the threshold limits are for the reduction in bacteria required for resolution of infection. Indeed it may vary with individuals and the type of bacteria present. We also don’t know the optimum design of the final restoration and the subtly different requirements it may have with reduced amounts of remaining tooth structure. That too most likely varies with the individuals that may be bruxers, clenchers and prone to eating a hard diet.
It would be nice to have answers to these questions, but in their absence we are left with common sense that can review what we do clinically and correlate it to the data coming from relevant research. Let’s take reduction in bacteria. Common sense tells us that we must find the canals harboring the bacteria. We are routinely going to be better at that if our access is adequate for full observation of the floor. There is also no getting away from the reality that the use of the microscope that gives us both a magnified field and excellent illumination is going to make us better at this initial task. One may choose to perform endodontics without the benefit of a microscope using loops and directed illumination and this is better than working without any magnification and illumination, but it does not compare to working under the scope at 8, 10 and 15 power. One can limit endodontic procedures to those teeth that have simpler anatomy, but given the degree of bifurcating and trifurcating teeth there is no guarantee that that determination is an accurate one. Furthermore, the greater the magnification the more accurate our assessment of the cross-sectional shape of the pulp and our ability to see variations as we travel more apically in the process of troughing through calcified secondary dentin.
Common sense in endodontic instrumentation is open to conjecture. From one point of view, minimal preparations leave more dentin and a stronger tooth. From another common sense point of view, we want to maximize the amount of bacterial removed knowing that a wider preparation will allow the penetration of a greater volume of antibacterial irrigants. Perhaps resolution of these conflicting goals is concluding that bacterial reduction is the primary goal and we must attempt to achieve it with the least amount of dentin removed. According to the research we have available, a minimum apical space of 30 is required for effective irrigation. That doesn’t necessarily mean a preparation of 35, 40 or higher is better. But it also doesn’t mean that a preparation of 30 into a canal that is far wider in the bucco-lingual dimension is adequate to chemically remove the tissue that is ensconced in the buccal and lingual extensions of highly oval canals or in the thin isthmuses joining canals. If the apical preparation of a 30 is to be adequate, it must have the ability to extend its reach in the bucco-lingual plane. An apical preparation of a 30 in the mesio-distal plane is likely more than sufficient given its thin mesio-distal dimensions prior to cleansing and shaping.
Common sense tells us that if we start with highly oval and isthmus-like anatomy, cleansing and shaping that reflects that anatomy will be better. We will be extending our preparation bucco-lingually and at the same time reducing the amount of dentin removed in the mesio-distal plane to a fraction of what is presently being done. Now common sense has to shift into a different gear. Given what we want the instruments to do, can we adapt them to accomplish our goals? I don’t see how that is possible.
If we are limited to greater tapered rotary NiTi instruments used either in continuous or interrupted rotation, we are dependent upon crown-down preparations (even if only one instrument is involved), preflaring and the establishment of straight-line access all of which sacrifice mesio-distal dentin for the expressed purpose of keeping the instruments intact. The last goal of shaping should be the concern of the integrity of the instruments doing the shaping. Their integrity should be a given without ever requiring the sacrifice of dentin. That is not the case today and that is why we remove too much tooth structure in the mesio-distal plane through a combination of preflaring, crown-down preparations and straight-line access. We not only remove too much tooth structure, but we are highly encouraged to stay centered imparting a highly conical shape to pulpal spaces that were originally far thinner in the mesio-distal plane before preparation. The result is thin walls in the mesio-distal plane increasing the chances of strip perforation on the furcal sides of roots where significant concavities are routinely present. The net result is over preparation in the mesio-distal plane and inadequate removal of tissue in the bucco-lingual plane.
Have we reached the end of common sense and must we stay with what has already been developed despite its obvious deleterious consequences? From my perspective that is not the situation at all. We can employ 02 tapered relieved stainless steel reamers (with a flat along their working length) with an initial tip size of 06, instruments that are the thinnest available, highly flexible while still retaining enough “body” to effectively work uniformly against all the canal walls. We can use these instruments immediately after manually gaining full length to the apex with the thinnest 02 tapered stainless steel reamer in a 30-45º reciprocating handpiece oscillating at 3000-4000 cycles per minute virtually eliminating instrument separation and the hand fatigue usually associated with the creation of the glide path. The 30-45º arc of motion minimizes the torsional stresses and cyclic fatigue that plague rotary instruments allowing them to be used vigorously in the bucco-lingual plane. Separation is no longer a concern. As a result thin 02 tapered instruments are shaping canals to sizes that reflect the original canal anatomy in larger form. They are not imposing conical shapes in pulpal spaces that were not conical to start with.
We have now created an environment that is beneficial to the tooth with its reduced sacrifice of dentin and its extended ability to remove tissue. The instruments are not prone to breakage allowing their multiple usage that saves significant amounts of money and the mental peace of mind that comes from knowing the instruments will remain intact is priceless.