Looking at the process of sealing and filling with the warm gutta percha technique from the outside, stripping away all the rhetoric this is what is really going on.
In order to successfully condense gutta percha it must be warmed to make it flow.
When a heated plugger is applied to gutta percha it is in a root canal which is a heat sink warmed to 98.6 degrees F.
The heat is dissipated by the body and thus it is only feasable to get GP to flow approximately 2mm ahead of the plugger.
Therefore if GP is going to be condensed at the apex, the canal must be large enough to accommodate the plugger size that is chosen.
The smaller the plugger, the less heat can be applied.
If the plugger is smaller than the canal, or the GP is too molten the GP will flow around the plugger and thus less pressure in an apical direction can be applied.
If a canal is not round, not enough pressure can be applied to adapt it apical to the plugger.
Thus a round canal is a requirement.
There are various ways of inserting GP but here is the most common method, step by step.
An evenly tapered round canal is prepared terminating in a small opening at the apex. This opening can be as small as #8 or it can be whatever size is necessary to get the canal clean.
Generally warm gutta percha advocates are also advocates of filling completely to the apex and allowing a small amount of cement to escape creating a puff.
The canal is prepared such that a plugger, usually one the same size as a #40 file is able to reach to within one mm of the end of the canal.
GP points are tried in until tug-back is felt when the point is at apical length indicating that the point fits tightly in the apical area.
Thus if the apex is prepared to a size 20, a size 20 GP point will provide tug-back.
It is important to note that the canal must be larger than 20 everywhere except at the apex otherwise the point will bind somewhere other than the apex and a false interpretation will occur.
Some cement is introduced into the apical area (usually the apical 4-5mm either with a small file or with a lentulo spiral).
This cement is either ZOE or AH-26, a resin.
The previously fitted point is inserted through the cement and wedged at the apex.
A heated (not warmed) plugger is inserted to about 5mm from the apex and the GP point is seared off at that point, the excess being removed.
A warmed plugger (usually a larger one) is introduced and apical pressure is applied warming the gp but not melting it at the apex.
The pressure causes the coronal end of the 5mm gp plug to adapt to the walls of the prep thus creating hydraulic pressure on the cement surrounding the gp.
This pressure causes the cement to escape first of all through any descrepancy between the gp and the apical foramen but also through any lateral canals in the apical five mm.
At this point many of the techniques differ.
The objective is basically to back-fill from the 5mm point to the orifice to the canal.
In general there is little likelyhood that there are lateral canals in this portion so the urgency to apply pressure is not as great.
Some simply back-fill with a large GP point coated with cement.
Some inject molten GP with a special syringe and some simply back-fill with cement injected with a syringe.
There are two advantages of this approach.
With the apex being plugged with non-molten gutta percha, any excess is cement.
A lot of pressure can be applied to force cement into lateral canals.
Back filling is only incidental to the technique and can be accomplished in many ways at the discretion of the operator.