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Diagnosis in Endodontics Dr. Scott Weed, DDS

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EVERY DECISION WE MAKE IN CLINICAL DENTISTRY INVOLVES SOME DEGREE OF UNCERTAINTY WHETHER WE RECOGNIZE IT OR NOT. MOST PATIENTS DON'T APPRECIATE THE UNCERTAINTY WE FACE AND OFTEN DEMAND DOCTORS WHO ARE CONFIDENT AND CERTAIN, WHO ALWAYS HAVE THE CORRECT ANSWERS.

It is important to understand that many errors in judgment are a result of our human thinking processes—processes that are common to all rational beings. We must also recognize and embrace the inevitable uncertainties in clinical decision-making by applying principles of probability in ways most of us were never exposed to in school. Finally, we need to understand the nature of endodontic diseases and why our endodontic tests often fail us.

Part 1: Judgment under uncertainty
In the early seventies, many of the weaknesses of the human machinery used for making judgments in the face of uncertainty were delineated by Amos Tversky and Daniel Kahneman. They explained that people tend to utilize "heuristics," which are rules of thumb, and these heuristics can lead us to biased decisions. Heuristics are built-in, experience-based shortcuts we all rely upon to quickly process information.

However, because heuristics are so commonly used (subconsciously), they easily convince our brain that the conclusions they point us towards are automatically valid, and this is certainly not true.

Some well-known heuristics have been described in the cognitive science literature:

Anchoring Heuristic

For example, a patient reports of pain on the upper right with tooth #3 displaying a large area of coronal destruction due to caries may influence how we interpret a periapical radiograph of #3. As a specialist, I often get referrals where the patient or referral slip reports a periapical radiolucency ("my dentist says there's a ‘dark spot' on the X-ray") when none of my 2D or 3D images corroborate this. In these cases, the clinician probably allowed early information to "anchor" his/her thought process, leading to the overly concrete and specific visualization of a radiolucency on the imaging.

This is especially problematic for younger and less experienced clinicians. When I was in my residency, I would frequently discuss cases with undergraduate dental students who would tend to "see" radiolucencies on periapical radiography when I could appreciate no such radiographic feature. By way of clinical advice, on cases where you suspect subtle radiographic perturbations, scan other areas of the image and see if similar areas don't show up on adjacent teeth.

Availability Heuristic
We should be careful when we are interpreting the subjective and objective information we've collected from patients and at least recognize the availability heuristic. For example, pain on a tooth with recent restorative work very well could be pain of endodontic origin that requires endodontic therapy. However, in my practice, occlusion and overall life stress of the patient are large factors that I always consider, despite the very plausible explanation of pulp trauma from dental treatment. Failure to include other hypotheses in the diagnostic work-up generally leads to people getting root canals they didn't need.

Representativeness Heuristic
A good example of this is the terrible-looking root canal treatment. Often we see short root fills, missed canals, weak or incomplete fi lls and we infer disease from these features even in the absence of clear clinical findings or patient reports of symptoms. Combined with anchoring, we might even infer a periapical radiolucency on the image when in fact none exists.

I had a patient referred to me one year ago with complaint of pain on #9. Her dentist had recently performed root canal therapy on that tooth. When I examined her, it was clear her problems were coming from #10. However, she had two other root canal treatments that were supposedly causing her pain even though I couldn't elicit any responses from them with biting or percussion. It took 5 months of calcium hydroxide therapy on #10 for her symptoms to subside. Everything about this patient pointed to her being "a little crazy." However, I decided to perform a test by prescribing amitriptyline for one month. All her symptoms subsided.

Escalation of Commitment
I frequently encounter patients in my practice who have, in the recent past, invested in a new full or partial veneer crown, with the tooth now exhibiting signs of pulpal/periradicular disease. During my assessment, I might determine that the structural integrity of the tooth, among other factors, creates a poor prognosis for the tooth. I may even recommend extraction of the tooth and the placement of a titanium implant. Patients often choose to ignore this advice and move forward with endodontic therapy "because I've already put so much into this tooth." The faults of this line of reasoning are self-evident and center around what is called "escalation of commitment."

I remember an accounting professor in college state this bold axiom: "Sunk costs are never relevant in future decisions." His classic example involved the decision of whether to continue a relationship with one's girlfriend/boyfriend or not. To consider how much time had already been put into the relationship serves only to confuse the situation and shouldn't have any weight on the decision of whether to move forward with the relationship today.

Part 2: The probability of diagnostic science
As part of our dental educations, we all took a course or had a section on biostatistics. We were taught the basics of frequentist statistics, championed a century ago by RA Fisher and later by Neyman, Pearson and others. These calculations assume that we "observe" large repetitions of scenarios that we actually don't observe. I remember my professor saying, "If we ran this experiment thousands of times . . . " But we didn't run it thousands of times. This brand of statistics doesn't necessarily characterize the types of things we, as clinicians, want to know.

For example, in the real clinical world, I'm not so interested in the probability of a patient with pulpitis experiencing cold sensitivity; I'm interested in knowing the probability that a patient who reports cold sensitivity has pulpitis and needs interventional pulp therapy. It is critical to understand the differences in direction of logic between these examples. Mathematically we write: P(A|B) ≠ P(B|A). This would read: The probability of event A given observation B is NOT the same as the probability of observation B given event A.

To clarify and drive home this point, consider this simple example: Suppose you see an American citizen. What is the probability that citizen is a U.S. Senator? Now, suppose you see a U.S. Senator. What is the probability that Senator is an American citizen? Huge difference in the answers.

The type of probability we're interested in as clinicians is what is known as conditional probability. A good example of conditional probability can be shown with this short story.

There was a hit-and-run accident that happened in the late evening hours. The only witness reports to the police that is was a blue taxicab that struck a pedestrian. As the law enforcement officials check out her report and test her reliability at cab color identification, they find that her accuracy is only about 80 percent. What are the odds that it was a blue taxi that hit the pedestrian?

We can't actually answer this yet. We need a key piece of information: are there any other kinds of taxis in the town? It turns out there are two cab companies, one green and one blue. The green taxis make up 85 percent of the taxis in town, blue the other 15 percent. It's critical to note that this distribution of taxis in town is a very important part of the equation. This is known as the base rate.

Clinicians often fail to take into account base rates. I've frequently had dentists show me radiographs of maxillary molars and ask if the mesiobuccal root is cracked. I usually point out that the "crack" they are "seeing" is actually the superposition of the periodontal ligament of the palatal root. Then I talk about the frequency, or base rate, of cracks of that shape and location, which rate is close to zero. "That's not how teeth crack."

The English friar, Thomas Bayes, introduced Bayes' Theorem in the 18th century. It involves calculating posterior probabilities (the chance of an event after applying new observations and data) using prior probabilities and accumulated evidence. Prior probability is the chances of an event before it has occurred. What is the likelihood that the sun will come up tomorrow? It hasn't happened yet. But the prior information we have available to us (our life experience and scientific knowledge of the solar system) indicates that the sun will rise tomorrow with exceedingly high probability, almost complete certainty. We can't have absolute certainty since it still hasn't happened yet!

I encourage the reader to spend some time on the Internet researching the principles of Bayes' Theorem. As you do, you won't be surprised to learn that in our hit-and-run example above, the chances of it being a blue taxi, as the eyewitness stated, aren't 80 percent. In fact, the probability that it was a blue taxi, utilizing Bayes Theorem to calculate it, is actually only 41 percent. So this 80 percent accurate witness is more likely to be wrong than right!

When I lecture on this topic, I get a lot of questions from audience members that sound like this: "What's this got to do with endodontic diagnosis?" Go to Dentaltown.com and search for the thread in the EndoFiles section titled: "So what does 80 years of Lit have to say for Ms. Smith." This illustrates beautifully what the real-world problems are stemming from the failure to understand conditional probability. School gave us a lot of answers that served us well during written examinations. The clinical reality provides endless examples of where these answers are insufficient.

I love this quote from the late physicist, E.T. Jaynes: "In scientific inference our job is always to do the best we can with whatever information we have; there is no advance guarantee that our information will be sufficient to lead us to the truth. But many of the supposed difficulties arise from an inexperienced user's failure to recognize and use the safety devices that probability theory as logic always provides. Unfortunately, the current literature offers no help here because its viewpoint . . . directs attention to other things . . . (emphasis mine)."

Part 3: Diseases of endodontic origin and diagnosis
Endodontology as a science teaches us many things about the dental and periradicular tissues that are valuable in our efforts to solve the problems patients present to us. Now that I have been practicing clinical endodontics full time, I've come to realize that this training fails me often. It's overly simplistic and tries to fit everything into neat and tidy boxes that simply don't exist in reality.

Patients often present with complex problems that require more than the simple tests we were all taught in school. The body can produce a tremendous amount of noise that confuses the signals we as clinicians seek when unraveling complaints of pain, sensitivity, pressure, and even esoteric reports. I'm not talking about the straightforward cases where the patient points to the tooth with the large cavity and reports temperature sensitivity. I'm also not talking about the painful tooth with corresponding unambiguous radiographic lucency. Those are the easy ones clinically, but they can actually harm our diagnostic abilities. They mislead us into thinking our diagnostic tests should produce specific results.

We've all had cases where it seems that none of our tests are helpful. These are the tough ones; these are the cases where we must be cautious to avoid errors in judgment and carefully weigh probabilities.

I recently saw a retired Navy SEAL who was referred for endodontic treatment on tooth #2. Upon interviewing the patient, he confirmed that he was experiencing pain in the area of #2. Upon further questioning, he admitted that the pain was actually vague and he wasn't certain it was #2 but that his general dentist “confi rmed” it was #2. My tests were very inconclusive. In my mind, candidates for disease included #2, #3 and #31. The posterior probability from my testing was pretty much even for those three: 33 percent each. So I decided to run one of my most powerful tests: the “come back next week” test.

If you take away nothing from this article, at least consider this: everything is a test. Tapping on teeth is a test. So is biting on a ToothSlooth, a wooden stick or a cotton roll (each can be useful in its own way). Initiating treatment is also a test. Our pre-operative diagnoses of vitality or necrosis are really just tentative diagnoses until we open the tooth and verify.

Legitimate diseases of endodontic origin are not self-limiting. They may take months or years to manifest in some cases, but they nearly always get worse. So waiting—observing—is a test. Now, when to observe and when to be more aggressive is usually dictated by risk tolerance of the patient.

An astronaut heading to the ISS for 9 months has no tolerance for risk. If I'm not certain and if the patient isn't in a lot of pain, I wait until I have more information.

So back to the SEAL . . . I had him return in one week and the symptoms had pointed to #31, the testing now implicated #31 (percussion and bite), and initiation of treatment of #31 revealed a necrotic distal pulp horn, necrotic distal canal, and hyperemic tissue in the mesial canals. Waiting was the test that made all the difference.

Summary You and I have each sat through many lectures on endodontic diagnosis. It's almost always useless information because they focus on things that help us with the cases we don't need help with: the easy ones. Moving into the big leagues requires a big-league mentality.

We must understand the pitfalls of our cognitive machinery if we are to take rational approaches to diagnostics. We also have to spend some time crawling the Internet for information relative to probability theory, specifically conditional probability. Conditional probability is not a topic that comes naturally to most people. The whole point of a differential diagnosis is to enumerate possibilities with associated probabilities. Finally, we need to realize that the tests we use in endodontic diagnosis aren't always that good at discriminating, especially for subtle disease. Applying the give it time test can help avoid costly and confusing problems.
                                                               
Dr. Scott Weed received his DDS from the University of the Pacific in San Francisco, California. Upon graduation from dental school, Dr. Weed was commissioned a lieutenant in the Navy and completed an Advanced Education in General Dentistry at the United States Naval Dental Center, Okinawa, Japan. He then practiced general dentistry for two years with his father, Dr. Robert F. Weed, in Fallon, Nevada. Dr. Weed left general practice and then completed a two-year specialty program in endodontics at the University of Southern California in Los Angeles, California. It was in Los Angeles that Dr. Weed conceived of the Reno Endodontic Continuum.

           

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