Probing and filing
The defining difference between probing and filing is that when filing the assumption is that the file is smaller than the canal. In probing, the file is larger than the canal.
Filing to me means circumferential filing, that is moving he file linearly while moving around the circumference of the canal. There are of course other ways of enlarging a canal, such as reaming, PQTP( Push-Quarter-Turn-Pull) filing, drilling etc.
Fine-Cut Files are unique in their design. To the untrained eye they look much like hedstrom files. They are not. The only resemblance is that both have a positive rake angle.
Fine-Cut Files differ in that the helical angle is consistantly low from shank to tip but the inter-flute distance varies. On a hedstrom file the inter-flute distance is the same and the helical angle varies. It gets greater as you approach the tip.
The result is that at the tip of a Hedstrom file, the weakest part of a file, the tortional stress is the greatest. This is why you must never get hedstrom files stuck in the canal. At the instant that the file breaks free, the torque created by the helical spiral gives the file a quick twist. Sometimes more than the metal of the file can take.
File-Cut Files are designed to get stuck and take it. In fact you can quite literally pound them into a tooth with a hammer and pull them out with a pair of pliers and they won't break. The reason is simple. Wire has tremendous tensile strength but poor torsional strength.
Furthermore, the included tip angle of a Fine-Cut File is a 60 degree cone.
When a pulp is irritated by caries, abrasion, restoration, occusal trauma etc. the pulp reacts by laying down reparative dentin and the inorganic part of the tooth grows inward like a plasterer laying down layer upon layer of plaster on walls until the room gets smaller.
The pulp eventually, totally obliterates itself coronal to apical. Therefore the oldest part of the reparative dentin and thus the most mineralized is farthest from the center. The most organic and the softest is nearest the center.
If there is a canal left, you can use a small file to work your way through. If there IS NO CANAL left, that presents a different problem.
Fine-Cut files can be pushed(literally forced) into the softer core of dentin and pulled out with each cycle of push-pull going deeper. The conical tip keeps the file centered and the fact that the files can get stuck and be pulled out allows us to progress down the canal.
Thus is is possible, even when there is literally no canal left, to work a file through the secondary dentin and follow the path left by the previous canal. Often this requires a slight modification of the technic whereby the file is screwed in slightly and then released and pulled out. Since the helical angle is lower, it translates what small amount of torque is applied into a lot of apical pressure. Much more than one could apply without buckling the file.
Push-Turn-Pull is really Push-Turn-Release-Pull. You turn until torsion is felt. Then let go of the file and release the torsion before you pull. You will be able to probe some pretty calcified canals this way.
This is an especially useful technique for the coronal portion, which is frequently difficult to probe.