Case Selection and Instrumentation: A Traffic-Light Approach to Endodontics

How general dentists can decide which root canal cases to treat, which to refer, and how to shape the ones they keep.

One of the most valuable habits in endodontics has nothing to do with files or irrigants. It happens before you ever pick up a handpiece: deciding whether this is a case you should be doing at all. Case difficulty assessment can feel laborious, so I like to simplify it into something everyone recognizes instantly, a traffic light. Green means go. Yellow means proceed with caution. Red means stop and think hard, because this case probably belongs with a specialist.

I believe this framework does more than protect patients. It gives general dentists and endodontists a common language for talking about cases, and that shared vocabulary strengthens the referral relationship in both directions. The general dentist knows which cases to keep and which to send, and the specialist receives the cases where their skills and equipment actually change the outcome. Everybody benefits, most of all the patient.

Green-light cases are the ones where you can clearly see the canal on the radiograph, the access is straightforward, and the anatomy holds no surprises. Think of anterior teeth and premolars with wide, visible canals and uncomplicated roots. If you can trace the canal from orifice to apex on the preoperative film, you are usually looking at a green case.

These cases do not need to be seen by a specialist. In fact, I would encourage general dentists to tackle them confidently. They are the ideal cases to build experience with, and they are where anyone starting out in endodontics should begin.

Yellow-light cases still show you the canal, but the picture starts to change. The canals are smaller. You begin to see a little calcification, often coronally where caries or deep restorations have provoked the pulp. Molars with multiple canals, teeth with angulations or rotations that complicate access, and heavily restored teeth all tend to sit in this category.

The difficulty here is rarely dramatic; it is cumulative. Simply locating the canals may sound very basic, but in a calcified pulp chamber it can be genuinely time-consuming, and the access itself deserves extra care because it is easier to go astray when landmarks are harder to see. My advice for calcified cases is to approach the access with caution and patience. Most yellow cases are still very manageable for a general dentist; they just demand more time and more deliberate technique than a green case.

One factor worth keeping on your radar: patients who take statins on a regular basis. Studies have reported a higher chance of pulp calcification in these patients, which can quietly move a tooth from green towards yellow before you have even looked inside.

Red-light cases are intended for experts, and the extreme end of the category is challenging even for them. These are the cases where I strongly recommend that general dentists refer.

What does red look like? Canals you can barely see at all, or cannot see, on the radiograph. Canals that are narrow and twisting, or that appear and then vanish as they converge or calcify. To be clear, a canal that seems to disappear does not automatically make a case impossible, but it is a factor that should make you stop and reassess before committing.

Retreatments populate much of this category: teeth with cast posts or long fiber posts that are difficult and risky to remove, separated instruments, and canals that a previous operator has ledged, blocked, or transported. I have seen a case where a dentist attempted a canal and ledged it; I managed to bypass the ledge and finish the case, but it took considerable time and it illustrates the point. Ledged, blocked, and transported canals can sometimes be worked out with small hand files, used patiently and in conjunction with copious irrigation, but it is slow, technique-sensitive work with no guaranteed outcome.

This is also where honesty matters, with yourself and with your patient. Know when to stop. If you are partway into a case and find yourself second-guessing whether it is within your ability, that instinct is usually correct. Some of these cases have taken me, working as a specialist, a great deal of time to process and finish. Referring them is not a failure; it is good case selection. And when the canal can only be found with a microscope or confirmed with CBCT, the referral practically justifies itself.

Some red cases are better solved surgically than through the crown. When a tooth has a well-fitting post and the risk of removal outweighs the benefit, or when a transported canal cannot realistically be renegotiated, surgical retreatment lets you address the apical problem directly while leaving the coronal restoration alone. I have treated cases like these with an apicoectomy under the microscope, including a tooth that had been root-treated ten to fifteen years earlier with a long post and an apical lesion, and at follow-up around six months later the lesions were healing or gone entirely.

Surgery is also worth considering when hunting for a deeply calcified canal would force you to remove so much dentin that you compromise the structure of the tooth. In those situations, drilling ever deeper in search of a canal you cannot see can do more harm than the disease you are trying to treat. And for the most severe cases, part of honest planning is acknowledging when extraction and replacement should at least be on the table.

Case selection decides whether you start. Instrumentation decides how you finish. My shaping protocol follows the same traffic-light logic, and it starts the same way in every case: with a 25/.04 rotary file taken into the canal a maximum of three strokes at a time, until resistance.

Only then do I measure working length. The reason for that order is simple: working length tends to get shorter as shaping straightens the canal. By opening the canal first and then measuring, the length you record is very close to the true final working length, so you are not chasing a moving target for the rest of the case.

From there, the traffic light guides the sequence. In a green case, the canal accepts the 25/.04 readily, and you can carry the preparation through the full file sequence to your finishing size. In a yellow case, you proceed the same way but expect to earn each millimeter, with a shorter sequence taken patiently to resistance. In a red case, the rotary file should not lead; negotiate the canal first with a small hand file, establish a reliable glide path, and only then bring the rotaries in.

Whatever the color of the case, I finish with a file of at least size 30, and often 35. The reason is anatomical: the average apical foramen is around a size 25. If your final file is a 25, then in most cases you have not actually achieved mechanical debridement of the apical canal walls; you have merely matched them. Going at least one size beyond is what lets the instrument genuinely plane the walls. This is what I do for my own patients.

There is a second anatomical problem that no tapered rotary file fully solves: canals are not perfect circles. Histological sections show debris packed into the fins, isthmuses, and lateral irregularities that a round instrument spinning in the centre of the canal simply never touches. That residual debris matters, because it can harbor the infection behind persistent symptoms and treatment failure.

For this reason I also recommend finishing with a dedicated finisher file. The finisher's job is to clean up the sides, the walls and irregular recesses the shaping files missed, and because it is not cutting a new shape, it does so without transporting the canal or affecting your final preparation. It becomes especially valuable in oval canals, C-shaped canals, and any tooth with a lot of isthmus anatomy, where the gap between the round file and the real canal is largest.

Assess every case before you commit, and be honest about which light it shows. Treat the greens with confidence, take the yellows slowly and deliberately, and refer the reds to someone equipped for them, remembering that some of them are best solved surgically. Once you are in the canal, open first, measure second, finish at least a size beyond the foramen, and let a finisher file deal with the anatomy your shaping files cannot reach.

Good endodontics is not about treating every case that walks in. It is about knowing which ones to treat, and treating those well.