What the Microscope Finds

When a root canal fails, the assumption is usually that something was done badly. Occasionally that is true. Far more often, the treatment that was done was performed competently on the canals that were found, and the problem is a canal that was not.

The anatomy nobody can see

An upper first molar is conventionally described as having three roots and three canals. In reality, somewhere between 60 and 90 per cent of them have a fourth — a second canal in the mesiobuccal root, known as MB2.

It is small, it frequently branches off at an angle, and it is very often covered by a shelf of secondary dentine laid down over decades. Without magnification and direct illumination, it is not that it is difficult to find. It is that there is nothing visible to find.

A canal that is never opened is never disinfected. The bacteria in it are unaffected by everything done to the canals beside it.

Lower incisors have a second canal around 40 per cent of the time. Lower first molars regularly have a fourth. These are not exotic variants; they are the normal range of human anatomy.

Why a radiograph does not settle it

A periapical radiograph compresses three dimensions into two. An untreated MB2 sits directly behind the treated mesiobuccal canal from the angle the film is taken at, so it is hidden by the root filling in front of it.

A small-field CBCT resolves this immediately. On the axial slice, the untreated canal is simply visible as a dark circle beside the filled one. It is often the first time a patient has been shown an unambiguous explanation for six years of intermittent symptoms.

What retreatment involves

Removing the existing filling material, finding the missed anatomy, shaping and disinfecting the whole system, and re-sealing it. The negotiation of a calcified canal under the microscope is slow work — it is not unusual to spend forty minutes on a single orifice.

Our success rate on retreatment cases referred for extraction is 94 per cent. That number is not a reflection of technique so much as of the fact that the original problem was usually findable, given magnification and time.

The case for magnification as standard

I use the microscope for every case, including the straightforward ones, for the same reason a surgeon does not decide case by case whether to wash their hands.

The molars that turn into retreatments in six years are not the ones that looked difficult on the day. They are the routine ones where a fourth canal was present, was not visible, and was therefore not there as far as anybody knew.