Some Thoughts on Root Canal Therapy

Statutory warning:

1.     This post is likely to be of interest to dentists only. It also uses technical terminology without making any attempt at explanation for the general reader. Read at your own discretion.

2.     As a general practitioner I have no access to in vitro or in vivo laboratory tests to validate the claims I am about to make in the course of this article, at least one of which is a direct challenge to received dental wisdom. I am, however, approaching the topic from the point of view of logic and practical experience – the latter reinforced by the fact that endodontics forms the overwhelming majority of my dental practice.

Back when I was a student in dental college, one of the standard tropes of root canal treatment was the enlargement of each canal to the upper limit of the file or reamer set being used; usually that meant No. 40 for a posterior tooth canal or 80 for an anterior tooth canal. The teachers would insist on it. Even the textbook we were using at the time stated, and I quote, “the canal should be enlarged to at least two sizes larger than the first reamer that binds.”

Now, obviously, the idea behind this enlargement was twofold:

1.     To remove enough dentine from the canal wall to be relatively confident of having removed bacterial inclusions in the dentinal tubules and

2.     To shape the canals to the appropriate preformed gutta percha cones (of course this is more true now with NiTi rotary instruments with preformed F1/F2/F3 cones, but the idea is the same even with the old stainless steel instruments and obturation using the lateral condensation technique and the 15-40 and 55-80 GP points.)

The problem was, unfortunately, that the concept of enlarging in every case to “at least two sizes larger”, let alone to 40 or 80, was utterly faulty; and we found out the problems on our own, right away.

Just think about it. Once you’ve shaped a canal to take a GP cone, any additional enlargement is just a way of creating additional space making it more difficult to fit your GP cone, and forces you to enlarge even further to fit the next larger GP cone in the series, and so on. Allied to that was the insistence on the step-back technique, something I always eschewed because I know no better way to create ledging and hence failure of obturation than that step-back technique. But either way, additional enlargement will result in only one thing – the removal of additional radicular dentine that you frankly cannot afford to lose.

But what about the bacteria in the dentinal tubules? There are two possibilities, logically speaking (and, as I shall repeatedly stress in the course of this article logic is a rara avis where RCT tropes and received wisdom are concerned).

The first is that the pulp of the tooth being root treated is vital, or non-vital but uninfected, and in that case, assuming that one’s instruments are sterile, there shouldn’t be any pathological bacteria in the root canal in significant numbers anyway. In any case there will be organic material in the tubules, primarily odontoblastic processes. These cannot be removed and will degenerate in time.

The second is that the tooth is non-vital and infected/gangrenous. In this case, there will probably be some bacteria in the dentinal tubules – but these bacteria are largely irrelevant. The primary point of failure, if any, of the treatment will come from the root tip. Properly obturated canals will seal off the tubules and prevent the bacteria from communicating with any pathological foci at the root tip, unless it is an endodontic-periodontic lesion, in which case the periodontic therapy will in any case form part of the treatment process. Even with an endo-perio lesion, any bacteria that manage to get through dentine and cementum to the periodontal ligament will be in very small numbers, and likely to be a smaller problem than bacteria from other sources – for instance, blood borne bacteria, or bacterial remnants in the periapical region.

Therefore, whether the tooth is vital or otherwise, the idea of enlarging in order to remove bacteria from the dentinal tubules is invalid.

So, from an entirely logical point of view, the canal should not be enlarged more than the minimum required to achieve removal of the canal contents and irrigation of the root tip adequate to achieve sterilisation.

However, since said logic was missing from our training, we kept enlarging the canals beyond the required level, which led to two perfectly predictable sequelae:

1.     Excessive removal of radicular dentine, leading to weakening of root structure, gouging and transfer of the apical foramen (due to the stiffness of the larger files/reamers, this was an exceedingly common occurrence), and subsequent splitting of the root with vertical fracture.

2.     Separation of instruments at the root tip region because of attempting to push thicker reamers beyond curvatures.

As I said, these were predictable sequelae, and yet they kept happening because everyone kept doing the same thing, over and over. If repeating an already failed action and expecting a different outcome can be called insanity, then what we were doing was insane.

However, that was more than fifteen years ago, and things have changed for the better now, haven’t they?

Actually, no.

Another of the standard claims – I want to say myths – of endodontic treatment I’ve come across is the idea that all canals must be filled to the root tip, and not even a millimetre or two less. This particular trope has a bizarre longevity; as recently as October of 2011, I attended an endodontic seminar in town where the idea was repeated, forcefully, by the speaker (this same speaker was guilty of claiming that open dressings should not be resorted to; anyone who’s drained an alveolar abscess will recognise how silly that bit of advice is). From our endodontic textbooks of the 1990s to the current age, the lesson is the same: fill the canal to the tip of the root. Overfill if you must, but underfilling is a cardinal sin.

Rubbish.

First, there’s practical experience. Anyone who’s done endodontics is familiar with the canals which have been obturated only part-way, sometimes up to a curve in the canal in the apical third, or even less; canals which by the fill-to-the-apex rule should have failed long ago yet which are perfectly asymptomatic and radiographically problem-free decades after they were obturated. This is a perfect example of not fixing something which isn’t broken; the obturations may not be aesthetic (more on that in a moment) but they do the job of providing an adequate seal, which is what they are meant to do.

Let me pause here a moment to repeat the primary rationale of root canal treatment, a rationale which seems to have been lost sight of by almost all “eminent endodontists”  (like “eminent economists”, the ideas of “eminent endodontists” set my teeth on edge). The purpose of root canal therapy is to provide an adequate radicular seal; no more, no less. (I’ll explain in a bit why I said radicular and not apical seal.) It doesn’t matter at all if the obturation doesn’t fit the conventional definition of what a properly obturated canal is supposed to look like, as long as it does its job. We are in the business of providing healthcare, not intraradicular beauty treatments.

The thing is, you may not be aware of that fact if you listen to the average endodontics advice. The same seminar I mentioned earlier included radiographs of teeth, with the speaker asking which we would rather have – the ones filled exactly to the apex, with beautifully curved canal preparations flowing like rivers perfectly following the shape of the root, or less aesthetically obturated canals which were also filled up to the same general area but perhaps a millimetre less?

The answer, of course, is that it didn’t matter a damn whether it was the “aesthetic” canal obturation or the other one – the important thing was whether either or both solved the problem of providing a viable radicular seal.

I will now commit an act of endodontic heresy and claim that, all other factors being equal, a slightly short obturation is to be preferred to one exactly extending to the apex, let alone one that is overfilled. Let me explain why.

Even the most ardent advocate of obturation to the apex admits that it’s impossible to limit the obturation to the apical foramen. Some amount of sealant, inevitably, will leak into the periodontal membrane, if not into the alveolar bone. Also, there’s a high likelihood of protrusion of part of the GP cone, silver point, or other obturation material into the periradicular tissue.

Now, said protrusion is, obviously, a source of trauma to the tissue. It causes an injury. Remember that the tissue at the root tip (PD membrane or alveolar bone) has very little scope for trauma avoidance; and inflammatory pain after an obturation is so common an occurrence that one can almost expect it after a posterior RCT in a high occulsal stress area. This damaged tissue may recover entirely, but also it may not.

What happens of it does not – if it heals by scarring and leaves some devitalised tissue in the periapical region?

If we have a scenario where the tissue recovers only partially, we are throwing the door open for subsequent colonisation of that damaged tissue by bacteria (via bacteremia, or subsequent periodontal disease) and the formation of an alveolar abscess. Even the most perfect root canal obturation will not prevent the appearance of that abscess; in fact, the “perfect” root canal obturation is the cause of that abscess.

However, if we fill the root short of the apex, as long as we seal the root adequately, we are running no such risk. If the tooth pulp was vital, the pulpal stump will maintain its vitality since its blood supply will not be affected (and this is why half-filled vital roots remain successes decades after the event). If the tooth pulp was nonvital, as long as sterilisation has been achieved, the obturation short of the apex will detract in no significant way from RCT success. It will also absolutely guard against the possibility of extrusion of material into the periapical region.

There is another consideration. There is always a high chance of secondary canals in the apical region, forming an anastomosis. These canals are impossible to instrument, let along obturate. Usually they are even invisible to radiographs. Even a “perfectly filled” root canal will leave these canals unfilled, and hence, there is no such thing as a “perfectly filled” root canal. The obturation that stops a millimetre short of the apex will achieve the exact same level of radicular seal as the obturation that extends to, or beyond, the apex, and will cause substantially less secondary trauma.

These, then, are the conclusions I draw from this article:

1.     Do not overprepare or over-enlarge the root canal.

2.     Do not obturate precisely to the apex. A slightly shorter obturation does as well, or better.

Of course it’s not going to be all that aesthetic, but it works.

At least, it works for me.

Copyright B Purkayastha 2012