Reaming(Rotary Filing)

Reaming simply means that cutting is done with the instrument rotating.

All cutting is done by the side of the instrument as it bears against the side wall of the canal.

There are two ways that enough pressure can be exerted to allow cutting.

First, the instrument can be flexed against the wall of the canal.

The stiffness of the instrument determines how much pressure can be exerted.

In turn how much pressure is NEEDED is determined by the sharpness of the cutting edges which in turn is determined by the rake angle of the blade.

If the rake is positive, it takes very little pressure but there is a risk of the file digging in and causing a torsional failure.

If the rake is negative, it takes a lot of pressure and more of a smear layer is created but it is not likely to get stuck.

In this method the canal may or may not end up shaped corono-apically conical.

It may end up hyperbolic in shape, that is like the bell of a trumpet.

Mathematically a hyperbola is formed when two lines diverge until at infinity they are parallel.

The other way is to use the Morse Taper to create lateral pressure.

With this technique the canal ends up shaped exactly like the shape of the instrument itself, i.e. a cone.

When this happens as soon as the instrument bottoms out, the entire surface of the instrument is in contact with the walls of the canal.

Each successively larger instrument wedges further coronally and as apical pressure is exerted, cutting occurs creating a new cone of the same shape but a larger size.

This technique is fraught with risk because if too much pressure is exerted, the instrument binds and since it is rotating, it exerts torsional pressure, which is the type that metal can LEAST resist.

Since NiTi is little more able to resist this type of stress than stainless steel, it is very important not to have this happen.

Some manufacturers have sought to reduce torsion by only allowing a small portion of the reamer to contact at any time, even as apical pressure is exerted.

This is done by the expedient of producing a succession of files with increasing Morse tapers.

Thus if a 1 per cent Morse taper is followed by a 2 per cent Morse taper etc., the reamer will bind first at the coronal end and the tip will float freely out of contact with tooth structure.

As cutting occurs, more and more of the instrument surface contacts tooth structure as the canal becomes the same shape as the instrument.

This prevents the most fragile part of the instrument, the tip, from receiving torque as long as cutting is stopped just short of the bottom.

This creates a uniform taper while minimizing torque, which is one of the major goals of techniques that condense gutta percha.

In general, if an instrument cuts on the end, this is technically not reaming but drilling or augering but some so called reamers do have end cutting capabilities and because an instrument is called a reamer does not guarantee that it is not end cutting.