Dr. John West discusses “Minimally Invasive” versus “Maximally Appropriate”

The premise

The concept of minimally invasive can be misleading in the world of endodontics. An example of misuse of the phrase “minimally invasive” in endodontics would refer to making an access that did not predictably achieve of the Three Fs of endodontic treatment mechanics: “Find, Follow, and Finish.” “Find” refers to discovering the root canal system’s orifi. “Follow” refers to navigating an endodontic file from canal orifice to the canal terminus. “Finish” means to (1) clean and disinfect the root canal labyrinth; (2) prepare an appropriate radicular shape that allows for a predictable conefit; (3) safe, controlled obturation with a material that can be removed if ever required; and (4) coronal seal placement to prevent future microleakage. A literal “minimally invasive” interpretation might suggest that the clinician should design the smallest access possible. That would mean an access with a quarter round bur! There would be no chance to “Find” all the canals. But, of course, almost all the coronal tooth structure remains. If the canals were ever found through such a miniscule access, then a truly minimally shaped canal would be the size of a No. 6 file. There would be no chance to clean, no chance to disinfect, no chance for an intentional preparation design, and no chance for a 3D seal. Taken at its face value, “minimally invasive endodontics” (MIE) is an inaccurate distinction or, at least, a deceiving one. The suggestion is that smaller is better. Sometimes it is. But smaller (i.e., MIE) is only better if the desired outcomes of 3D endodontics are consistently and reproducibly achieved. If the access and shaping are too small, smaller is not better. The term “maximally appropriate endodontics” (MAE) is, to me, a more accurate and outcome-related term.

The language

The term minimally invasive is one of the current dental buzzwords like same-day dentistry or the use of the word platform in business. Sometimes I get tired of hearing these words because they are just that: buzzwords. As always, current, popular buzzwords will all fade over time and will be replaced with new buzzwords, such as maximally appropriate; dentistry now; patient-centered value; mindfulness; or, in the case of endodontics, a word such as endopreneur (a buzzword that I made up and trademarked — just because I could). Consumer-targeted buzzwords are followed by new words that feed the consumer’s insatiable thirst for the latest and greatest. In the future, today’s buzzwords will be considered old-fashioned and out-of-date. Oh, how the pendulum swings!

MIE begs the question of how the role of ferrule fits into the invasive versus appropriate equation. Which distinction produces the best choice for both the dentist and the patient?

And so goes the ferrule and MIE. The balance between preservation of tooth structure (conservative) and appropriate (thoughtful) radicular preparations that produce predictable 3D cleaning and 3D- controlled obturation are always on the endodontic clinician’s mind. For example, to extend the MB access of a maxillary second molar’s MB complex (mesiobuccal and mesiopalatal) in order to slide into the orifice’s distal orientation may not be considered a ninja-type access. A “conservative” access without sufficient extension for entering, a glidepath, and safe rotary instruments can make endodontic treatment technically impossible or even dangerous. What would you do if it were you being treated? Would you tell your endodontist to save that precious MB dentin, or would you say, “Make it easy on yourself?” If you are like me, you would want your tooth’s attachment apparatus no matter what. With modern cements, if you make a new crown with a little less tooth structure, it will still predictably remain cemented. You would also tell your endodontist, who is designing your endodontic access cavity, that the critical part of the posterior teeth’s ferrule is not the mesial and distal; it is the buccal and lingual. If you are the patient, tell the endodontist, “Make it easy on yourself. Make sure you treat my endodontic anatomy.” This is the theme, thinking, and technique of MAE.

The thought process of MIE has been used in recent years to describe the desired shape or taper and width of the radicular funnel endodontic preparation. Advocates of MIE always show a buccal view of a posterior tooth and suggest it is overprepared and that the tooth is weakened. In many cases, their intentions are right and noble. We value the precious gift of dentin. We all want as much remaining dentin after our preps as possible, including in coronal restorative preparations. Every bit of tooth helps the retention of the restorative and the longevity of the tooth. And, as our patient populations’ age increases, we are going to need to be predictable for longer and longer periods of time.

What really matters

In endodontic posterior accesses, the critical measurement is not the mesial-distal remaining tooth structure; it is the buccal-lingual tooth structure that represents the resistance to forces, and it is this dimension that needs to be robust. Therefore, when a dentist evaluates remaining ferrule, considering the MIE philosophy, the more significant measurement should be made in the axial view of a 3D CBCT image. MAE-thinking understands that the buccal-lingual ferrule dimension is the critical measurement for access evaluation, not the mesial-distal, as many MIE evaluations suggest.

Nature gives us the rules of the endo endgame. Teeth remain in use for more than 100 years in the human body unless caries or trauma or restorations do not allow the teeth to sustain this 100-plus-year life. What is the width of Mother Nature’s canals at the canal orifice? It is the third-third-third rule. If nature has 2 times the dentin thickness of the canal, that would be appropriate for lifetime use, our preparations do not need to be narrower than nature’s. When the endodontic clinician shapes a calcified canal, for example, the finished mesial-distal preparation width should be anywhere from a fifth to a third of the width of the root at any given level. In this case, Mother Nature’s requirements are satisfied. The reward is endodontic predictability and root strength. Tapers generated by connecting the dots from the minimal apical constriction from a fifth to a third of the coronal root diameter will generate sufficient resistant form hydraulics to control a 3D obturation of the root canal system’s portals of exit.

The right direction

There is one aspect, however, about the MIE narrative that I really appreciate and has made me a better endodontist. For me, MIE has increased my awareness of the shape of my radicular preparations from access to apex. Before the “MIE movement,” my access and shaping designs were going essentially unnoticed, as far as critically evaluating remaining tooth structure, when my endodontics were finished. Now, because of article after article and lecture after lecture, I completely agree with a focus on ferrule and pericervical dentin preservation while being simultaneously mindful that my job is to heal or prevent lesions of endodontic origin (LEOs). Every patient’s tooth is different; we all treat a different mix of patients; we are all in different stages of dental life requiring different needs; and we all have different skill levels. There are also growing differences in technologies used for access, cleaning and disinfection, shaping, and obturation. When any part of the Endodontic Triad develops disruptive change, disruptive change affects the remainder of the Triad, which can, in turn, affect the part of the Triad that forced the change in the first place. This sequence is not a vicious cycle but, rather, a dependent cycle. The critical endodontic outcome — the “Look” of the final radiographic endodontic look — is evolving. This transformational Look is directly related to improvements in any part of the Endodontic Triad.¹

It is never fair to look at someone’s final endodontic image and pass judgment. We were not there. There may have been circumstances we are not aware of, such as the patient hardly being able to open his/her mouth, or the clinician was fatigued or behind in his/her schedule and decided the answer to being behind is to go faster. This is a bad choice, and it’s when mistakes happen.

Where does the pendulum swing from here?

So, what’s it all about? I believe the MIE/MAE pendulum will keep swinging left and right, depending greatly on what new technologies enable dentists to improve one or more parts of the time-tested Endodontic Triad: clean, shape, and pack. Others may call it something different, but, so far, Mother Nature’s rules are simple: Reduce the threshold of the disease source, and she will heal. As an endodontic clinician, we give Mother Nature her best chance by making shapes that can be cleaned and can be filled along all the root canal system’s walls and portals of exit. The trend is to make accesses and shapes smaller and smaller. Remember, smaller and smaller is not Mother Nature’s idea. She is quite content with shapes that are a fifth to a third of the root width in all dimensions. Under-shaped or unshaped canals tend to produce weak obturations and a lack of obturation control, which can be catastrophic. For example, in 2016, there was a $4.5 million verdict in a malpractice lawsuit for a case in which a clinician filled up the mandibular canal during endodontic obturation. In this case, it appears to have been a failure to design a resistance form, funnel-shaped preparation. Therefore, the filling material could not be contained within the root canal system and flowed unrestricted into the mandibular canal. Endodontic radicular design requires meticulous preparation and is particularly challenging since, in endodontics, we do it in the dark. We cannot directly make our preps. We cannot see and do at the same time. We now have instruments that can improve our predictability of success, but sometimes there is little room for error. Welcome to dentistry!

Without appropriate resistance form in all dimensions, overfills and overextensions of under-filled systems would undoubtedly increase. As dentists, this loss of confidence is everything. What had been fun would become feared. Ultimately, we value the three Cs: consistency, which leads to control, which leads to confidence. Armed with the knowingness that we can prove our endodontic preps are appropriately finished through the conefit, you can enjoy the procedure, and your patient will enjoy success.

And so, that’s it! Not too big, and not too small.

Just right — and appropriate.

This article was reprinted with permission from Dentistry Today.