Incommensurability in Endodontics: The Role of the Endodontic Triad

John A. Khademi, DDS, MS, and David Clark, DDS


Scientific progress is generally hindered by trying to take the past into account as a way to move forward. Einstein once said that “we can’t solve problems by using the same kind of thinking we used when we created them.” General Relativity is not an outgrowth of Newtonian Mechanics. The Heliocentric Model (planets orbit the sun) is not an outgrowth of the Geocentric Model (Earth is the center of the universe). These advances in understanding required an abandonment of the core principles of previous models. They are complete departures from prior models, based on an entirely different set of principles and objectives, and result in scientific incommensurability: not speaking the same language. For at least 50 years, the Endodontic Triad for Success1 of shaping, cleaning, and packing has been the only theory, mistaken for reality. For scientific theories such as the Endodontic Triad to be discussed, the proponents of competing viewpoints must agree on the list of biological objectives that any candidate viewpoint must solve. Competing camps may then debate the merits of their particular intervention parameters. Endodontics has fixated on clinical treatment objectives and end-points directed toward removal of the 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 3-dimensional hermetic filled of the entire root canal space.”2 This zealous 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 normal science—operates at cross-purposes with long-term tooth retention.

The idea of normal science was introduced more than 50 years ago by physicist and philosopher of science Thomas Kuhn in the landmark book The Structure of Scientific Revolutions.3 In it, he challenged the prevailing notion of scientific progress being “development-by-accumulation” which he called normal science: “…the activity in which most scientists inevitably spend almost all their time, is predicated on the assumption that the scientific community knows what the world is like. Much of the success of the enterprise derives from the community’s willingness to defend that assumption, if necessary at cost.”3 Normal science is puzzle-solving. In endodontics, the prevailing puzzle is how to best cure or heal lesions, as success or failure is said to depend on it.4,5 Thus, a statement like “Although well intended, this shaping shift, alone, will never fulfill the biological objectives for success,”1 presupposes that we all know, and agree upon what those objectives are. What if we don’t know, or don’t agree? Whose view of success or failure? What if these viewpoints are incommensurable—that we’re not speaking the same language of success or failure?

What we see depends on our point of view. Endodontists have one viewpoint: endodontic success. A decade after Kuhn wrote his landmark piece, a charismatic Dr. Herbert Schilder turned the endodontic world upside down with his landmark piece—a viewpoint embodied as Five Mechanical Objectives for endodontic success.2 There were no studies that evaluated the consequences of weakening of the tooth that convenience-driven straight-line access and obturation-driven shapes created. Nor were there patient-centered outcome studies that validated improved outcome with this incredible cost in lost tooth structure. This viewpoint started and ended with The Look of the gutta-percha in setting sealer thought to be important and improve endodontic success.6 This was never demonstrated.

Restorative dentists have a different viewpoint: a patient in tears over the death sentence of an “excellent” root canal, post, buildup, and crown is something every endodontist needs to experience, 100 times. These broken teeth are summarily extracted, and no report is sent to the endodontist. If endodontists could track their cases throughout more than 40 years of practice like restorative dentists do, watching fat endodontics break at 7 to 10 years and seeing the “anomalies”—the skinny endodontics, which is endo­dontics that has no business working at all—seemingly “ignoring the physical and biologic requirements for endodontics success,”2 and yet succeeding at 40 years, endodontists would view things differently. These shifts in points of view come with the accumulation of these anomalies that dominate the surviving stream of cases in combination with a better understanding of the basic biology likely to be responsible for outcome. Why do these old beliefs persist?

We think we know something. Knowledge blocks learning. Our ideas are sticky. Once we have a theory, even one formed on the basis of weak evidence, we are not likely to change our minds.7 The Geocentric Model persisted for another thousand years, at least partially due to a religious viewpoint. Knowledge required Earth to be the center of the universe, lest one face an Inquisition. Those defending the Geocentric model required increasingly complex and fantastical explanations to account for discrepancies in the planetary observations, while holding fixed the central belief of a motionless earth. The endodontists defending these beliefs of improved outcome hold fixed a knowledge of the Endodontic Triad for Success and confuse achieving this central belief via a process-outcome (what we do as clinicians) with patient-centered outcomes (what matters to the patient).6 We have difficulty interpreting subsequent information that contradicts these beliefs, even if this new information is obviously more accurate.7,8 Holding this knowledge fixed requires fantastical explanations for the surviving stream of 20-plus-year-old cases, none of which should have worked at all (see Figure 2 in the clinical article by Khademi et al).

So incommensurability is the idea that with a change in viewpoint, the old ideas and assertions cannot be strictly compared with the new ones. Even if the same words are used, their meanings have changed. A new model is chosen to replace the old one, not because it is true, but more because of a change in viewpoint. Even as materials, instruments, and techniques have progressed, our thinking has held fixed, always directed at the same objectives, shaped and constrained by a 50-year-old study on gnotobiotic rats that wasn’t even about endodontics.9 We view the continued controversy as to the best minimum mechanical requirements for achieving disinfection as normal science. We see these as distractions: philosophical debates stemming on an agreed-upon, disinfection-based, biological model of endodontics derived from a Kochian, planktonic, acute-disease view of apical periodontitis. Thus elimination of bacteria has become the objective of endodontic treatment protocols. This focus often comes at the cost of competing considerations, including structural and restorative considerations, which are likely more important for long-term tooth preservation. We view the prevailing assumptions with marked skepticism.

We see endodontics as a branch of restorative dentistry whose primary purpose is the preservation of the natural dentition over the length of a patient’s life. Such a transition of vision is not merely pedantic in nature; it affects virtually every single facet in the study of endodontics, how we make clinical decisions, and how we interpret our outcomes.

The transition from a paradigm in crisis to a new one from which a new tradition of normal science can emerge is far from a cumulative process, one achieved by an articulation or extension of the old paradigm. Rather it is a reconstruction of the field from new fundamentals, a reconstruction that changes some of the field’s most elementary theoretical generalizations as well as many of its paradigm methods and applications.3

Incommensurability results in clinicians talking past each other, with prevailing discourse continuing to be centered on the biologic objectives as embodied by the Endodontic Triad for Success, and how these cannot be achieved without whittling away at the tooth. Part of the power of a model is what it predicts will happen. As important is what it predicts cannot happen, which is clearly contradicted by the actual 20-, 30-, 40-year-old surviving, successful cases. One of the greatest barriers in transition from one viewpoint to another is the inability or refusal to see beyond current ideas and thinking. Simply put, before there can be true creation, there must be absolute destruction. As Bertrand Russell has said: “A stupid man’s report of what a clever man says can never be accurate, because he unconsciously translates what he hears into something he can understand.” That these ideas are already being misinterpreted, misunderstood, and misrepresented is evidence of this transition to a new viewpoint and the incommensurability in these viewpoints.

Endodontists and endodontics have semantically bleached these concepts introduced more than a decade ago as minimally invasive endodontics, and associate small with sloppy. Clinicians who have adopted these ideas know that the prevailing “small = sloppy” mischaracterization could not be further from the truth, as working through small access and small shapes can be considerably more difficult. Larger access and shape are primarily for the convenience of the clinician. If they contribute to outcome, it is in the wrong direction. The Endodontic Triad for Success is a house of cards. There is no reason to believe that whittling away at something makes it stronger, and every reason to believe the opposite. Balance needs to be restored. We expect these mischaracterizations will continue until, as Max Planck said, “the opponents eventually die and a new generation grows up that is familiar with it.”


  1. Ruddle CJ. Endodontic triad for success: the role of minimally invasive technology. Dent Today. 2015;34:76-80.
  2. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am. 1974;18:269-296.
  3. Kuhn TS. The Structure of Scientific Revolutions. 3rd ed. Chicago, IL: The University of Chicago Press; 1996.
  4. Debelian G, Trope M. BT-Race—Biologic and conservative root canal instrumentation with the final restoration in mind. Endodontic Practice US. February 17, 2014. Accessed on June 1, 2016.
  5. West J. Endodontic predictability: “are you making the right decisions?” Dent Today. 2010;29:90-95.
  6. Ruddle CJ. Predictably successful endodontics. Dent Today. 2014;33:104-107.
  7. Taleb NN. The Black Swan: The Impact of the Highly Improbable. New York, NY: Random House; 2007.
  8. Kahneman D. Thinking, Fast and Slow. New York, NY: Farrar, Straus and Giroux; 2011.
  9. Kakehashi S, Stanley HR, Fitzgerald RJ. The effects of surgical exposures of dental pulps in germ-free and conventional laboratory rats. Oral Surg Oral Med Oral Pathol. 1965;20:340-349.

Dr. Khademi received his DDS from the University of California San Francisco and his certificate in endodontics and did his MS on digital imaging at the University of Iowa. He is in full-time private practice in Durango, Colo, and was an associate clinical professor in the department of maxillofacial imaging at University of Southern California and is an adjunct assistant professor at St. Louis University. He can be reached at

Disclosure: Dr. Khademi discloses a financial interest in SS White and Carestream Dental.

Dr. Clark maintains a private practice in Tacoma, Wash, and is the founder of the Academy of Microscope Enhanced Dentistry. He is also a course director at the Newport Coast Oral Facial Institute and the director of the Bioclear Learning Center in Tacoma. He can be reached at

Disclosure: Dr. Clark reports no disclosures.

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