Famed mystery novelist Agatha Christie wrote that there was only one man in the world her fictional detective Hercule Poirot feared, his dentist. Sure enough, in her novel, the dentist was murdered off. In real life, I was woken from my complacency two weeks ago with a recurrent toothache. The dentist advised a root canal operation.
While she poked about in my mouth, I noticed her using something familiar, a differential probe, familiar to me from my college electronics lab days. It measures the voltage (potential difference) between any two points. In the image below, the clip is connected to one point and the needle is used to touch the other point whose potential is to be measured.
A differential electrical probe
In her version, instead of the crocodile clip shown in the image above, there was a metal hook which she hung from my lower lip. The other end had a hook which was connected to a tiny needle like file which she inserted in the tooth canal. As she twisted the file in, the machine started beeping until finally it emitted a long continuous beep at which point she removed the file and measured the depth on a scale.
I asked her what the device was for and she replied that it was to measure the depth of the root canal.
Adaptive Apex Locators like the one used on me to measure the root canal depth
Later I did a little reading into how this worked. In school, we all learnt the Ohm’s Law.
V = I x R
When voltage (V) is applied across any material, it tries to pass an electrical current (I) through the material. The amount of current that actually passes through is determined by something called the resistance (R) which is a property of the material. The exact relation is given by the equation above. So to measure the resistance of something, we apply a small voltage across it and measure the current coming through it. By dividing the two, we get the resistance.
R = V/I
As you would expect, tooth enamel is an electrical insulator and has a very high resistance. However, at the bottom of the canal, there is a tiny hole through which the conductive material at the end of the canal is connected to the periodontal ligament. This is a bunch of fiberous tissue containing nerves and runs through the jawbone and holds the tooth in place. The metal hook on the lower lip is thus placed to
Tooth structure showing the periodontal ligament
What this means is that while the metal file is descending in the canal, the resistance it measures depends on how far it is from the canal terminal. When it touches the terminal, the resistance abruptly falls which the machine detects and gives out a sustained beep.
This is the basic idea behind measuring the depth of a tooth’s root canal. In practice, contemporary instruments make the measurement more reliable by measuring at 2 frequencies and considering capacitance effects.
Interestingly, when I asked about the elaborate dentist chair she used, she admitted that though it was assembled in India, the parts had to be imported. Why, I wonder?