Thermoplastic Gutta Percha
Gutta Percha can be almost liquified when heated.
That heat can be applied with a heated metal instrument as it is in the warm gutta percha method. It can also be applied through friction.
When a rotating cylinder or cone of metal is applied against gutta percha the friction causes it to melt.
How fast it melts is a function of the circumferential speed and the pressure.
The circumferential speed is the circumference times the rotations per second.
In a cylinder this circumferential speed is uniform from top to bottom of the cylinder.
But in a cone the circumferential speed varies from tip to base of the cone and thus since the cone speed is relatively constant the amount of heat varies with the circumference.
Furthermore since the thicker the cone, the less flexion there is, the heating effect is enhanced in the thicker parts of the cone, which in the case of the root canal is at the coronal end.
With the Mc Spadden Compactor and other systems like it a gutta percha point is inserted into the canal and the rotating point is inserted along side it in the canal.
The instrument is rotated as lateral pressure is applied.
Thus the gutta percha is liquified first at the coronal end and last at the apical end.
The principle of the Compactor is that a helical spiral is used and the rake angle is the exact opposite of a hedstrom file, which it resembles.
The difference being that the Compactor would, if used as a file, cut on the way IN and not on the way OUT.
Thus when the gutta percha is liquefied the rotating instrument acts as a worm-screw device forcing the gutta percha apically and the instrument is removed from the canal.
The down side of all this is that considerable judgement is required to determine just when the gutta percha is molten at the apical end based upon what is seen at the coronal end.
If the instrument is kept in too long, and the tip is too close to the apex, considerable apical force can be applied forcing molten gp through an apex. In defense of the technique however, the apex would have to be pretty large for this to happen.
There are modifications of this technique that use other methods to melt the gutta percha and the reverse screw is used simply as a method of driving it apically.
There is one product where gutta percha is melted and the instrument is dipped into the gutta percha, coating the instrument and then the instrument inserts the gutta percha.
Another product comes already on the instrument. It is heated and the rotating instrument forces the gutta percha into the canal.