How does a dentist determine the length of a tooth's root canals?

The dentist or endodontist performing a tooth's endodontic therapy will need to determine the length of the tooth's root canal system. In general, their approach will be one where they will want to clean as much of the tooth's root canal space as is possible without their instruments extending beyond the confines of the tooth.

Dentists have a few different techniques available to them that they use collectively to make this determination. A measurement will need to be made for each individual root's root canal(s). A tooth typically has between one and three roots, each of which will have at least one, and possibly two or more, root canals.

How precise will the dentist's measurement be?

A canal's measurement is typically calculated to the nearest 1/2 millimeter (about 1/50th of an inch). The measurement is made by reading markings scribed on the root canal files that are used to treat the patient's case.

Methods dentists use to measure the length of a tooth's root canal.

A dentist, or endodontist, will use the following techniques to help them determine the length of a tooth's individual root canals. Each of these techniques has its own strengths and weaknesses, so they are typically used in combination. Just as a reminder, this portion of the root canal treatment process is performed after the patient's tooth has been anesthetized ("numbed up"). The patient doesn't feel a thing.

A) Tactile root canal measurements:

When starting with a tooth's treatment, a dentist will typically take a look at one of the x-rays that have already been taken and from it make a rough estimate of the tooth's length. They will then insert a file in the tooth to about that length. In many cases (and especially noticeable to endodontists and those dentists who perform a lot of endodontic therapy) the feel of the file in the canal may give a hint that it has reached its terminus.

B) Radiographic root canal measurements:

Traditionally, the measurement of a tooth has been established/confirmed/documented by way of the dentist taking a x-ray of the tooth with file(s) positioned in its canal(s). Since root canal files are metal, they will show up distinctly on a x-ray. The dentist will evaluate the x-ray and, if needed, adjust their length calculations accordingly.

Modern radiographic techniques have made this type of length determination much quicker than it used to be. Traditionally, using film-based radiology, a dentist would need to expose the x-ray and then develop it before they could read it. This process would take several minutes. Nowadays, with the advent of filmless digital radiography, the dentist can take a x-ray image and view it immediately.

C) Electronic root canal measurements:

In recent years, electronic length-measuring devices have come into frequent usage. They work on the principle of impedance (the opposition to current flow in an alternating current circuit). The dentist will clip a lead from the measuring device to the metal root canal file that has been inserted into the tooth. They will also place a lead that makes contact with the patient. Collectively the needed circuit is created. As the dentist positions the file further and further into the root canal, the electronic device signals as the canal's terminus is approached.

All of this may sound unpleasant to some but in practice it isn't. In fact it's desirable. You won't feel a thing and in those cases where this type of apparatus is applicable, it's very quick and accurate.