First of all let me state that before one instruments a canal near the apex that there should be at least one radiograph (digital or film) with an instrument in the canal near the apex.
The reason for this is very simple. It is much easier to see that a canal curves and which way if the metal instrument is in the canal. It is all too easy to mistake one canal that goes straight for another that curves if they are superimposed.
There are basically three ways one can determine apical length. For academic as well as practical reasons, let me be very clear here. My definition of apical length has only a slight resemblance to the actual length of a tooth or a root canal.
Apical length is the amount of file or needle that must be inserted into a canal to reach the apex. The apex again is defined as the hole in the end of the root.
It is entirely possible for a root to be longer than its canal or shorter than its canal. If for instance, a canal curves within a root, it will be longer than the root itself. And if the canal exits short of the end of a root, it will be shorter than the root.
When doing endo we really do not care how long the root is, only how much instrument to insert.
When referring to canal length and instrument length I will refer frequently to apex minus x millimeters or apex PLUS x millimeters, rarely to the actual length.
I have often used the story that dentists think differently than most other people. I will illustrate.
If you tell a machinist to drill a hole in piece of metal 10mm deep the first thing he will ask you is plus or minuse how much? Because he KNOWS he is not perfect and wants to be within a certain RANGE of correctness.
We dentists tend to think in absolutes. "The margin is perfect or it isn't." "This denture fits or it doesn't"
So what is the RANGE of closeness needed for canal length determination? My answer is plus or minus 0.2mm. Why? Because that is safely within the width of the periodontal ligament. And inserting a file into that space is safe. Going beyond that is damaging to bone. Going short leaves debris in the canal.
So the question becomes, can we in fact, get that close?
That in turn depends on the accuracy of our measuring method.
Since we know that we can measure a file placed in the canal accurately down to hundredths of millimeters by the use of a digital caliper the only question is, how much distortion do we have in our xray.
By the way, if a file is placed into a canal, a hemostat is locked onto it lying across(hopefully) two cusps, we can measure extremely accurately. Using a rubber stop for measurement is asking for guesswork due to tilting etc. of the rubber stop.
The big question is, how do we minimize the innacuracies caused by xray distortion?
For this we turn to the machinists mind....and a bit of logic.
Let us assume that xrays MAY be distorted a maximum of 20%. (one really has to distort and xray to get more than that.)
If we know that when we took the xray, we had exactly 15.56mm of file in the canal beyond our landmark(s) and then measure the distance from the tip of the file on the xray to what we perceive as the apex, then any distortion will be confined to that area.
If we assume that the maximum distortion is 20% and the distance to the apex is 5mm then the length could be 5mm plus or minus 20% or 1mm. Which does not fall within our criterion of plus or minus 0.2mm.
Therefore to get an accurate measurement we need one xray with a file within 1mm of the apex( 20% of 1 being 0.2)
So if we take our first xray and the file is 5mm away, we could simply add 4mm to the file length, reinsert it to that depth and take another radiograph.
The only problem with that is that if the xray were really elongated by 20% and we were REALLY only 4mm from the apex, adding 4mm might put us out the apex.
So to be technically correct we have to add 80% of the perceived shortage and remeasure.
If we are already past the apex, we need to subtract 120% of the amount.
Usually with one and perhaps two xrays we usually can get a measurment that is accurate to within 0.2mm.
This method works with straight or curved canals, with film xrays or digital xrays.
The only gotcha is that if the file is too small, or your xrays too poor, you may not clearly see the end of the file and thus create errors in your measurement. To avoid pixel size problems in digital xrays, this dictates a minimum of a size 15 file.
Using an electronic apex locator to determine apical length can lead to problems if one does not also have a radiograph with the file near the apex.