He was 47. He had been treated for pain in his lower right second molar three years earlier — root canal, post, crown — by a general dentist whose work I had no reason to doubt. The pain had returned eighteen months later. He had been referred back to the same general dentist, who had taken a periapical radiograph, seen no obvious lesion, prescribed a course of amoxicillin, and the pain had resolved. The pain had returned again four months later, and the general dentist had referred him to me.
The periapical radiograph he brought with him showed a tooth with three obturated canals — mesiolingual, distal, and what was visible as the mesiobuccal — and a moderately well-condensed filling material reaching to within two millimetres of the apex on each. The lamina dura looked intact. The periapical tissues looked unremarkable. On the radiograph alone, the case was a hard one. There was no obvious reason for the recurring pain.
I retreated it. The first visit was straightforward. Rubber dam. Access opening. Removal of the post under ultrasonics. Removal of the gutta-percha from the three obturated canals. The patient tolerated the work well. He was scheduled for completion two weeks later.
The second visit, under the operating microscope at sixteen-times magnification, was the visit at which the case stopped being a hard one and started being a routine one.
What the microscope showed
The mesiobuccal root of a lower second molar has, in approximately a third of the published anatomical studies, a second canal that lies palatal to the main mesiobuccal canal. It is small. It is often missed on a panoramic or periapical radiograph because it sits in the buccolingual plane the radiograph collapses. It is reachable, in most cases, only after removal of a small ledge of secondary dentine that obscures its orifice on the chamber floor. The first treatment, three years earlier, had not found it. The general dentist had filled the three canals he could see and finished the case. The fourth canal had retained necrotic tissue. The necrotic tissue had been the source of the pain.
The orifice of the fourth canal was visible at sixteen-times magnification as a small dark dot on the chamber floor, a millimetre or so palatal to the main mesiobuccal orifice that had been filled. With a small-tip ultrasonic instrument I removed the dentinal ledge. The canal opened. I irrigated. A small file passed to working length. The canal was approximately 14 mm long, narrower than the others, and contained necrotic pulp tissue and bacterial flora consistent with a long-standing untreated canal.
I obturated. The patient came back at six weeks. The pain had resolved. The radiograph at twelve months showed no developing periapical lesion. The tooth, three years later, is asymptomatic. The case took two visits at the second attempt to do what the first attempt had not finished.
What the case was about
I am not writing this to embarrass the general dentist who did the original treatment. I do not know who he was. The case was referred to me through a general practice that was not his. The original treatment was, on the radiograph alone, defensible. He did the work that the imaging he had supported. He did not, as far as I can tell from the documentation, use an operating microscope or surgical loupes above 4× magnification. The fourth canal was, on the imaging he had and the magnification he was using, not visible.
This is the part of the case that matters. The published prevalence of a second mesiobuccal canal in lower second molars, across the anatomical literature, is high enough that any clinician treating one of those teeth should be looking for it. The looking, in 2026, requires magnification. The 4× loupes that were standard equipment in general practice a decade ago are not, in my reading, adequate for this question. The 8× or 16× operating microscope is the equipment the published outcome data on endodontic retreatment is built on.
I am not arguing that general dentists who do not use microscopes should not do endodontics. I am arguing that endodontic procedures in molar teeth — first or second, upper or lower — without microscope or high-magnification loupes are being performed without the equipment the modern standard of care presupposes. The patient who has the procedure done that way is not necessarily going to have a poor outcome. The patient is being treated under a protocol that has a known failure mode the equipment would have caught. Some of those cases will succeed. Some will, three years later, arrive in my chair for retreatment with a missed canal that the microscope finds in twenty minutes.
What this changes
For the patient: a question worth asking before agreeing to molar endodontics is whether the clinician uses magnification, and what magnification. A clinician who routinely operates at 16× under an operating microscope is not, on this dimension, the same clinician as one who uses 2.5× loupes. The procedures look the same on the appointment book. They are not the same procedures.
For the clinician: the equipment we use shapes the outcomes our patients get. There is no version of modern molar endodontics that does not include high magnification. The cost of the microscope is real. The cost is not the patient’s. The procedure is the procedure that the published outcome data supports, and the equipment is part of the procedure.
For me, on this case: the patient sent me a card at Christmas the year after I finished the retreatment. He did not know that the case I had finished was a case the original treatment had not. He knew the pain had resolved. The clinical record knows the difference. That is fine. The card is on a shelf in the practice. The radiograph at twelve months is in the file. The fourth canal is filled. The patient does not know what it is for him not to know. That is what good work, mostly, looks like from the patient’s chair.
For the upstream procedural decision on whether the tooth should have been root-canal-treated at all rather than treated with vital pulp therapy in the first place, see vital pulp therapy vs root canal. For the save-versus-replace framework if the question on the table is whether the tooth is salvageable at all, see when to save a tooth and when to replace it. For the consult-side reflection on declining a procedure a patient wanted but did not need, see the consult I didn’t take. For the patient-side reflection on what good international treatment outcomes have in common, see the patient who got it right.