We know that certain canals are likely to be curved and this it is often just good judgement to assume that they are curved.
If a canal is curved to the mesial or distal, since xrays are taken from the buccal, it is usually fairly easy to see that canals are curved and which way.
If they are curved buccaly or lingually(palatally) it is more difficult but by taking xrays of certain suspected teeth from a slight angle, it is possible to minimize errors.
If a canal is curved and a straight file is inserted, it will bind before it gets to the apex.
If we take a curved file, it will insert without binding if the plane of the curve of the file matches the plane of the curve of the canal.
Inserting a straight file while initial probing we can often tell if a canal is curved consistently or if it is straight for a while and THEN curves and for the most part, where the curve starts.
If while circumferential filing with one size the file does not bind and with the next size it binds but can be forced further after the bind this almost always signifies a curve.
I distinguish initial probing from "continuation" probing only on the basis that in initial probing, one does not know the size of the canal or indeed even if it is open.
K-files have a negative rake and thus are not as likely to get stuck irretrieveably but they have the disadvantage of being end cutting and thus one has to be careful not to create a mechanical canal but cutting out the side of a curved canal.
Hedstrom files if turned while probing in a tight canal can screw themselves in so tightly that they cannot even be unscrewed without breaking. Thus they should only be done with a push(very gently) getting them stuck and pulling them straight out. With care this can be done with most hedstrom files. It is howevere extremely risky with some files that have steep helical angles on the smaller files, which is, of course, the very files you are likely to be probing with.
Fine-Cut files have a uniquely low helical angle that allows them to be forcefully stuck and pulled out without creating enough torque to break them. They are thus ideally suited for initial probing ANd because the tips have a 60 degree tip angle which guides it around most curved without procurving.
Since it is almost certain to result in a file getting stuck when initial probing, and because rotary files do not last but about 540 degrees of turn once stuck, I do not recommend that any initial probing be done with rotary files, either NiTi or stainless.
If the tip of a file can be inserted into an existing orifice, it can be opened using either rotary instrumentation or push-pull instrumentation. But in the case of rotary, eventually the canal will be smaller than the file and rotary filing must stop at that point.
Since Fine-Cut files are designed to get stuck and be pulled out safely, crown-down orifice opening can safely be done with them.
Generally, I will start with a file that I estimate will procede to within 3mm of the apex abut not go through. Usually this is a size 25. In younger teeth, sometimes I will use a 30. In older teeth, it is conceivable that even a size 15 would not penetrate the apex.
Crown down is carried to the point where the file binds. If it binds too far from the apex then a smaller file can be used. The danger is that if the canal curves drastically and THAT is the cause of the binding, the smaller files can be broken by bending them too sharply.
If probing can enlarge a canal, then by continuing the probing with larger files can actually prepare a canal completely. I.e. once a canal has been probed to a depth with a size 15, repeating with a 20, 25 etc will with the same motion and method, enlarge the canal. In fact this is what we did for most of the early days of endo. It was called Push Quarter Turn Pull Filing or simply PQTP filing. The file was screwed in a quarter turn and removed.
But as we found, this took a lot of time and was instrumental in plugging a lot of canals with debris.
Using rotary tapered files, following one size with another was the first concept and resulted in a lot of breakage.
Subsequent methods used larger tapers on each subsequent file and thus the files would bind near the top and when the entire length of the file began to bind, the next size and larger taper were used. This reduced the breakage because the filing was stopped before the tip bound.
Since Fine-Cut hand files have a conical tip and can be stuck without danger of either breakage or drilling a new canal, the method is simply to push the file into a canal and then pulling it out, wiping it off and repeating until the next large file fits to depth.
This method is often still used for dressing the apex to size and a round shape for fitting a gutta percha point or finalizing the apex.
As a method of enlarging and cleaning the whole canal it is very slow and thus circumferential filing is much preferred. It is much faster but does not guarantee a round canal.
Since the cutting that is done with PQTP is actually done with the side of the file, this is actually more properly called reaming.
This is done only with Fine-Cut files because K-files will not progress down the canal and hedstrom files would break.
There is a modification of the push-pull technique wherein the file is pushed then turned clockwise until binding occurs, using the spirals to vector the tip of the file apically. As long as the file is not turned and pulled at the same time, it is a safe way of enlarging a canal although it is generally only used in cases there the canal is SO tight that push-pull can make no progress.
LightSpeed files are actually small augers with a safe tip on the end of a flexible shaft of Nickel Titanium. The tip is only about a millimeter in height and only half of that does the cutting. It is football shaped.
It leaves a cylindrical hole.
Its proponents recommend that a series of cylindrical holes be drilled getting smaller as the apex is approached.
The actual method STARTS at the apex with a small size and then each consecutively larger cylinder is further from the apex. Since the increments are less than a millimeter in height, although the canal actually is a stack of cylinders it appears for all practial purposes, conical.
This 'stack' can be around a curve because each cylinder is short and the neck of the file is smaller than the head and it made of flexible NiTi.
The advantage of NiTi in curved canals is that reaming can be done and the file acts as a flexible shaft since with each rotation the file must bend and then bend back this results in hundreds of flexures per minute depending on the rotational speed.
The only problem with this is that the files when flexed around a curve will exert their force to the outside of the curve. This usually is not a problem unless the canal is ribbon or ovoid shaped. Until the canal is made the size of the file, there is no cleansing on the inside of the curve since the file flexes outward.
This is usually not a problem unless the maximum dimension of the oval is to great to make a round canal without perforating.
Another problem is when a single canal splits into two. Since the NiTi rotary will always follow the path of least resistance, it is often necessary to file the second branch by hand.
And of course the big problem with rotary filing is the possibility of getting a file stuck. The greater the lateral pressure placed on a file, the greater danger of breakage. This is enhanced when a canal is 'keyholed' that is where a flat or ribbon shaped canal is prepped round on one end and then the file slips into an unprepared portion and sticks suddenly.
Since the file does not move in linear but in rotational oscillation, the in and out linear oscillation has to be done by hand by the dentist.
Sonics and Ultrasonics do neither linear nor rotational oscillation. They accomplish their function by a whipping action of the tip of the file. They too must be moved linearly to make them cut, or more properly to make them cut uniformly. If simply inserted to depth and turned on they will leave a ledge by their whipping action.
Sonics are those whose oscillations are below the range of human hearing in the range of 7000 cycles per second to 12000 cycles per second.
Ultra Sonics are above the human hearing range in the area of 25,000 to 30,000 cycles per second. All this means is that ledging occurs more rapidly.
The Fine Cut Handpiece actually creates the linear oscillation needed to make files cut. Each cycle is about 1.2mm in length which is slightly greater than the interflute distance of Fine-Cut files.
The optimum speed is about 17,000 cycles per minute, far from sonic even. But since actual cutting is done on each stroke, the 280 strokes means that it cuts at least 50-100 times faster than you can cut by hand or with many of the sonic systems on the market.
This means less stress and better control since the dentist only steers the file controlling the depth and lateral pressure.
The speed in cutting means that crown down orifice opening can now be done as a routine procedure without the tip of the file coming out of the orifice and buckling which was a hazard with other methods.
The speed with which cutting can be done means that smaller files can be used which are more flexible and require less pre-curving.