Restoratively Driven, Minimally Invasive Endodontics

Dr. John A. Khademi

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INTRODUCTION
Ursula Franklin, PhD, a professor, physicist, and educator, writes, “Every tool shapes the task. Any task tends to be structured by the available tools. It can appear that the available tools represent the best, or even the only way, to deal with a situation.”1,2 Thus, the tools or instruments that are available shape and constrain an approach to a problem and may actually define the problem. However, new tools and instruments may redefine the problem and, thus, the approach to the problem.

We had already begun redefining and reconceptualizing endodontic access in the 1990s with the introduction of the first generation of rotary NiTi files (Figure 1). These first-generation rotary files were large and quite stiff by today’s standards, yet still allowed a completely different conceptualization of straight-line access (SLA) when compared to the legacy concepts using hand filing and instruments. Writing in 2001, I quoted a Walton textbook from 1996 on access: “Access preparation is the most important phase of the technical aspect of root canal treatment.”3,4 In 2014, Dr. L. Stephen Buchanan commented in The Art of Endodontics hands-on lab manual (Dental Education Laboratories, 2014) that “the quality of the access cavity you cut will greatly influence the experience you have through the rest of the procedure…if you try to do rotary instrumentation through a [traditional] access prep, you will be punished severely.” More recently, Buchanan writes, “Errors accumulate during procedures. That’s the reason botching the access at the start of an RCT is so much more devastating than, say, problems that come from misfitting a gutta-percha cone just before finishing the case.”5 The importance of endodontic access continues to this day with Pathways of the Pulp, which states, “Access to the complex root canal system is the first and arguably the most important phase of any nonsurgical root canal procedure.”6

Figure 1. Early reconceptualization of straight-line access (SLA), based on first-generation rotary NiTi files of the 1990s. These files were quite large and stiff by today’s standards, yet they allowed for a radical change in the traditional concept of SLA. SLA to the MB canal in this maxillary molar in this case from 1999 was from the distal and palatal (b), while SLA to the palatal canal (f) was from the mesial and buccal. These are the straight-line orifice projection angles of canals, and they show the straight-line insertion paths of rotary files. In today’s access lexicon, these projection angles taken together form the canal convergence profile.11,12 Access extensions were largely the opposite of traditional access extensions.3,11 Best Practices in Endodontics: A Desk Reference (Quintessence, 2015) has an extensive section on modern access and peri-cervical dentin (PCD) preserving concepts and the associated lexicon.

Treating the Compromised Tooth vs the Goals of Endodontics
So, clearly, there is no shortage of commentary on the importance of endodontic access, yet most textbooks present “ideal” endodontic access concepts and techniques as if we were treating caries-free, unrestored teeth with large canals and visible, patent orifices. With few exceptions, the clinician performing endodontic treatment is working with a compromised tooth—compromised by a crown placed over one or more restorative materials that may have a base; compromised by decay; and/or compromised by the patient’s ability to open, requiring the clinician to work through the orifice between the lips, as opposed to a laboratory bench, where access is often studied. Visibility is compromised by inadequate light and angulation, and canal orifices are frequently constricted due to calcific metamorphosis (calcification) and may be blocked by pulp stones and dentinal shelves. Add in subgingival decay, changes in tooth angulation from drift, and chamber and orifice position masked by a crown, along with an isolation difficulty, and one begins to wonder how the long-held goals of endodontics could possibly be accomplished. As it turns out, our best evidence is that these goals are not accomplished.

Importance of Minimally Invasive Access Openings
The quintessential goals of endodontic procedures have been stated to be the elimination of all organic substrate and bacteria filling the root canal systems,7 as the purpose of endodontics has been stated to be the prevention or treatment of apical periodontitis.8 Over the decades, 2 broad schools of thought have emerged, prescribing the intervention parameters that are thought to best achieve these treatment objectives to predict long-term outcomes. Thus, the focus has always been on eliminating the bacteria, often at the expense of sound coronal and radicular tooth structure. However, observations of the stream of long-term, 20-plus-year-surviving cases present a completely different picture. The surviving cases, as a group, are devoid of evidence of any of these traditionally required procedural objectives from either camp. Many are coming to believe that better predictors of long-term outcomes, and the real problems and complications with endodontically treated teeth, are not bacterial in origin but structural failures, often as a direct result of loss of tooth structure from endodontic procedures themselves. In a 2014 article by Ellen Meyer, John C. Kois, DMD, MSD, writes, “Creating more conservative access openings can reduce the risk of tooth loss related to weakened tooth structure. Significant problems are more related to the loss of tooth structure more than the endodontic procedures themselves.”9 Since the dawn of endodontics, the endodontic triad for success of shaping, cleaning, and packing has been central to all competing camps in endodontics. This intense focus on these legacy objectives has blocked endodontics from seeing possibilities with the technology available in the state-of-the-art practice.

Figure 2. (a) A placement bend is routinely and easily imparted to a stainless steel K-file. Here, a #10 stainless steel K-file is negotiated to length and establishes a preliminary glidepath, and canal lengths of 24 to 25 mm on the lower molar shown in Figure 5. (b) Once a #10 stainless steel K-file is negotiated to length, rotary preparation can commence. An SS White V-Taper2 14/V.03 heat-treated file is dipped into a readily accessible canal, typically the palatal of a maxillary molar, or here, the distal of the lower molar. This changes the temperature of the file to allow and hold a placement bend. (c) The straight file is placed against the mirror to begin the placement bend. (d) The file is pushed against the mirror. (e) The mirror is rotated to overbend the file and finish imparting the placement bend. (f) The file can now be “hooked” into the mesial canals in a similar fashion to steel files. This is done without activating the handpiece. (g) The file is visually placed into the orifice (here, the ML) and advanced to very light resistance without activating the handpiece. (h) The handpiece is raised to straighten the projection angle of the canal. Note the angle of the contra-angle head on the left of the frame has changed from (g) to (h). The handpiece is then activated and the file is allowed to advance down the canal. A pecking motion is not used. The file will advance on its own with only very light engagement pressure until advancement slows or stops. (i) The file is removed and carefully inspected for unwinding under high magnification. Heat-treated files will unwind considerably before separating.
Figure 3. A stepped access is performed where a wider cut is made through restorative material, and stepped down at the depth dentin is encountered. The access through the crown is 3 mm, (a) and the access through dentin is ~2 mm. These smaller accesses, coupled with CBCT, still allow discovery of multiple portals-of-negotiation (PONs) of the root canal system. Image (b) shows the MB/MB2, and (c) shows 3 DB PONs in the fused DB/P root. The postoperative PA and CBCT (e) show 6 obturated canals in this fused-root maxillary molar. The 5-year followup (f) demonstrates normal PDL space, even with the convergent access, small shapes, and very small apical sizes.

Endodontics of the Past and the Future
Progress in a domain is generally hindered by trying to take the past into account as a way to move forward, and endodontics is no exception. Endodontics has fixated on clinical treatment objectives and end points directed toward the removal of pulpal remnants and bacteria believed to be the etiologic agent of endodontic disease: “…namely that to achieve predicable success in endodontic practice, root canal systems must be cleaned and shaped—cleaned of their organic remnants and shaped to receive a three-dimensional hermetic fill of the entire root canal space.”10 This all-consuming pursuit of what is believed to be required for disease prevention and elimination, an assumption that persists and permeates the specialty of endodontics and dominates our science, operates at cross-purposes with long-term tooth retention. Competing camps debate the merits of their particular intervention parameters while, at best, paying lip service to the new paradigm of dentin preservation that is taking hold in endodontics. We expect discussion of these legacy objectives and misguided debates about how to best accomplish the vestigial goals of the endodontic triad to continue until, as a maxim from the early 1900s, attributed to Max Planck, “the opponents [of current thinking] eventually die and a new generation grows up that is familiar with current thinking—that preservation of the tooth starts with preservation of tooth structure.”

Buchanan first began to address this issue with the introduction of GT files with coronally limited diameters in the mid-90s. David Clark and I began to address this issue of creating smaller and more tooth-sparing access openings, leveraging changes in SLA allowed by rotary NiTi files. However, there were limits in the tools that shaped and constrained the task. In 2001, I wrote:

Figure 4. Pictured above: Drs. Charles Maupin (left) and L. Stephen Buchanan (right). Dynamic guided access (DGA), as introduced earlier this year, brings another level of precision and predictability to endodontic access. No longer is endodontic access constrained by vision or instrument flexibility. This allows wildly unconventional access to the root canal system. This advancement—along with active irrigation, like with Sonendo GentleWave—merits a completely fresh look at the entire process of file-based endodontic treatment.

With the advent of nickel titanium instruments and their hyperflexibility, one might mistakenly conclude that minimizing instrument flex is of lesser importance. In fact, when using rotary nickel titanium instruments, straight-line-access and minimizing instrument flex is of increased importance. This is because conventional stainless steel files can be pre-curved and “hooked” into canals. If a rotary nickel titanium file is curved or bent, it is ruined and must be discarded.3

Technological Advancements Change the Practice of Endodontics
Times have changed. The convergence of 3 pieces of technology has allowed a complete shift in the practice of endodontics: the microscope; the low-dose, focused-field CBCT scanner; and root-form-appropriate, heat-treated NiTi instruments.12 Heat-treated NiTi allows placement bends to be imparted to rotary files, which changes the constraints on the task. Access designs that would have resulted in instrument fracture and/or excessive coronal enlargement are now commonplace. The specific technique is reviewed in Figure 2.

Figure 5. The completion of the necrotic, calcified lower molar in Figure 2, treated in 2 visits with minimal access and shapes, given the degree of calcification. Shapes in the mesial root were done with the SS White VTaper2 14/V.03, and the distal 17/V.04. CBCT scans show obturant to the root ends on booth roots. Short-term follow-up is WNL.

There continues to be multiple, ongoing critiques of these changes in access design, shaping, and apical sizes. These issues are addressed by changes in workflow. The microscope allows the preparation of a much smaller and more precise access—dramatically smaller than a traditional access and often individualized per root or even per canal. A critique of these smaller accesses would point out that they hinder the clinician’s ability to locate clinically relevant anatomy; they have been a primary objection to the minimally invasive, restoratively driven strategy. Also, very small shapes are cut with coronally conservative, heat-treated NiTi files, and Ca(OH)2 is placed in the prepared canals. These root-form-appropriate, variable taper files allow for safe and effective instrumentation and obturation of canals without unnecessarily weakening the tooth. One might object that these smaller shapes hinder the clinician’s ability to “cleanse and shape the canal properly.” In addition, the in-office CBCT scanner is the last but crucial piece of the puzzle. Proper utilization of the CBCT scanner addresses these legitimate concerns and is a requirement in a state-of-the-art endodontic practice. CBCT imaging guides treatment in a way that complements the microscope. The access can be precisely extended, if needed, to address additional anatomy. Access extension is minimized, as the direction and distance is known, guided from the imaging study. It turns out that these widespread objections are simply not well founded, as the maxillary molar in Figure 3, with 6 treated systems, shows. Dynamic guidance is set to change all previously held notions of endodontic access and may require a complete abandonment of all previously held access principles (Figure 4).

CLOSING COMMENTS
To address concerns raised with conservative access and canal preparation, it is imperative to remember that these are the clinician’s ideas and needs in attempting to address the ideals of The Endodontic Triad. The amount of cleaning and shaping (if any) required for clinical and radiographic evidence of resolution of AP is simply not known a priori. When in doubt, CBCT often allows for a rapid 3- to 4-month semiquantitative assessment of periapical radiodensity change to infer that those requirements have been met. In the final analysis, the smaller access, shapes, and sizes can meet the biological threshold for resolution of symptoms and a resolution of radiographic evidence of AP, as Figures 3 and 5 show. Accomplishing the objectives set out by The Endodontic Triad have never been shown to improve radiographic resolution of AP, nor have they been tied to patient-centered outcomes in a meaningful timeframe.


References

  1. Franklin UM. Every Tool Shapes the Task: Communities and the Information Highway. Vancouver, British Columbia, Canada: Lazara Press; 1996.
  2. Franklin UM. The Real World of Technology. Toronto, Ontario, Canada: House of Anansi Press; 1999.
  3. Johnson WP, ed. Color Atlas of Endodontics. Philadelphia, PA: Saunders; 2002.
  4. Walton RE, Torabinejad M. Principles and Practice of Endodontics. 3rd ed. Philadelphia, PA: Saunders; 2002.
  5. Buchanan LS. Cutting endodontic access cavities—for long-term outcomes. Roots. 2016;12:22-26.
  6. Hargreaves KM, Berman LH, eds. Cohen’s Pathways of the Pulp. 11th ed. St. Louis, MO: Elsevier; 2016.
  7. Ruddle CJ. Predictably successful endodontics. Oral Health. 2016;106:20.
  8. Trope M, Bergenholtz G. Microbiological basis for endodontic treatment: can a maximal outcome be achieved in one visit? Endod Topics. 2002;1:40-53.
  9. Meyer E. Beyond failures: learning from clinical problems to improve your practice. Inside Dentistry. 2014;10:100-108.
  10. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am. 1974;18:269-296.
  11. Schwartz RS, Canakapalli V. Best Practices in Endodontics: A Desk Reference. Chicago, IL: Quintessence Publishing; 2015.
  12. Khademi J, Trudeau M, Narayan P, et al. Image-guided endodontics: the role of the endodontic triad. Dent Today. 2016;35:94-100.

Dr. Khademi received his DDS degree from the University of California, San Francisco, while his certificate in endodontics and his MS in digital imaging were received from the University of Iowa. He is in full-time private practice in Durango, Colo. Dr. Khademi was an associate clinical professor in the department of maxillofacial imaging at the University of Southern California and is an adjunct assistant professor at St. Louis University. In his “prior life,” he wrote software for laboratory automation, instrument control, and digital imaging. He lectures internationally about CBCT, clinical trial design and outcomes, and conventional endodontic techniques. As a Radiological Society of North America member for more than 25 years, his background in medical radiology allows him a perspective shared by very few dental professionals. He has contributed to many sections and chapters in textbooks and is the lead author of Advanced CBCT for Endodontics: Technical Considerations, Perception, and Decision-Making (Quintessence, 2017). He can be reached via email at jakhademi@gmail.com.

Disclosure: Dr. Khademi is a paid consultant for SS White and receives honoraria and lecture support from Carestream Dental.

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