What's wrong with PQTP filing anyway?
Since K-files have a negative rake angle no matter which way you use them, they are essentially dull.
In order to get them to cut you have to use a lot of pressure against the walls of the canal.
But the limit of the amount of pressure you can apply is the flexibility of the file.
So the typical way this is done takes advantage of the Morse Taper Effect.
This is the same effect that is used to hold the lathe spindle on the shaft of a lathe and the same effect that will split a root is too much pressure is applied to a gutta percha plugger.
The idea is that when you seat a cone within a cone, you produce lateral pressure.
It is this lateral pressure that keeps the negative rake edge of the blade against the dentin and thus does both the cutting and the creation of the smear layer.
One of the saving graces as well as one of the banes of the K-files is the fact that it does NOT dig into dentin very well.
Let me explain.
If you have a tapered sharp file with a helical spiral and you seat it into a corresponding taper, it will bite in and get stuck.
If you continue to turn clockwise you will eventually cause the file to twist off.
If the file does not have a sharp edge i.e. like a K-file(negative rake angle), it will not bite in, will not twist off and can only cut by creating a compression wave.
If you push hard enough on a K-file it WILL dig in enough and WILL twist off.
This assumes that you turn it clock wise. But if you were to turn it counter clock wise it would screw itself OUT.
And if you were to push inward at the same time it would keep the 30 degree negative rake blades against the dentin and keep it cutting. In other words, the file wants to screw itself out and you keep pushing it in.
This is known as the Balanced Forces Concept and it attributed to Dr. Jim Roan.
The file that Dr. Roan helped develop is a ground K-file that has about a 60 degree included tip angle. Generally K-files have a much blunter point and tend to pack dentin debris ahead of them.
An argument has been made that K-files should not be turned counter clock wise because they would thus screw the debris ahead of them and thus pack debris.
Probably true to some degree but the extra pointy tip on the Roan files (Flex-R Files) to a great extent ameliorate this tendency.
One of the main characteristics of either PQTP or Balanced Forces is that the resultant canal is a Morse taper exactly the shape of the last file that was used in it.
This may or may not be desirable, depending upon the filling material you choose to use.
If you are using a cement fill it matters very little the shape of the final canal as long as it is clean and the needle can reach as far as you need to.
Canal shape becomes critical however if the canal is to be packed with gutta percha.
Since gutta percha has to be softened from the top down and packed into a canal, the canal must be evenly tapered from crown down to apex so that the packing pressure on the warm gutta percha above exerts increasing pressures on the material below thus keeping it in contact with the dentin walls as the gutta percha cools and shrinks.
If the canal is larger apically than coronally, that pressure dissipates and cooling and shrinkage can occur that would cause a gap to develop.
But the real critical point is that if there is a discrepancy between the taper of the canal and the taper of the plugger it will concentrate the lateral force at that point and split a root.
You should keep in mind that the effective taper of a plugger can change if the gutta percha laterally adjacent to the side of the plugger hardens.
One of the interesting side lights of the discussion of PQTP vs. circumferential filing is that circumferential filing automatically creates a hyperbolic shape, that is one that is wider at the top than the instrument used and PQTP or reaming creates a shape the same as the last file.
Thus as long as the plugger you use is no greater a Morse taper than you file, the canal will be almost ideal in shape for condensing gutta percha, that is, greater than a standard Morse taper the farther you get from the apex.
Trumpet shaped, if you will.
The only justification for the use of a larger than standard Morse taper is that
One further point about standardized K-files with a one per cent Morse taper; when you work a file to a given point in a canal you have created a well fitting male and female Morse taper.
When the next size file is inserted, it wedges 2.5 mm coronal to the position of the previous file. It is blunt ended and end cutting.
It is worked apically to the position of the previous file.
If there is any fluid in the apical 2.5 mm, the file creates hydraulic pressure which forces the fluid apically.
Furthermore, the debris scraped off the canal wall in the apical 2.5 mm is pushed apically by the file.
This creates two very important caveats about filing with K-files, recapitulation and irrigation.
If a series of files is used to enlarge say from 15 to 50 to a given depth each would pack the debris created into the apex.
Two techniques that are very similar have been devised and promoted over the years as a requirement of filing.
These techniques are recapitulation and its poor cousin, patency filing. Neither are really needed EXCEPT that they are actually compensations for a design flaw that is inherent in negative rake instruments.
Namely the tendency to create a smear layer and to pack debris ahead of themselves.
Recapitulation dictates that after each file is used, each of the smaller files is used in the original order to the maximum depth reached to avoid ledging and to remove the packed debris.
Patency filing is actually an abbreviated short cut for recapitulation.
In patency filing, only a small file is used.
Both are essentially to keep these blunt-ended dull files from ledging and packing debris.
Another side light of the PQTP filing system is that after each file is used the canal is irrigated, usually with a solution of sodium hypochlorite.
The reason usually given is that it sterilizes the canal.
Actually there is a hidden agenda here.
That is that if you are going to burnish bacteria into the lateral canals and dentinal tubules you ought to kill them first.
The second reason for the use of this caustic, tissue killing chemical in the canals is that it dissolves the organic matrix of the dentin chips that are packing into the apex, so as to keep them soft enough that they can be removed easily by patency filing or recapitulation.
Here are my ten top reasons not to use sodium hypochlorite.
Circumferential filing, positive rake instruments and washed field eliminates all of these problems.