June 24, 1994...Santa Cruz, CA

Last Edited on Wednesday, January 14, 1998

• Why do files break?
• Why do smeared layers happen?
• Why do roots split?
• How does the "Balanced Forces Concept" really work?

Critical to learning the answers to these questions is an understanding of the Morse Taper Concept.

Those of you who have attended the Automated Endo course in the last few years have probably heard me use the term already.

Only recently have we begun to see the term appear in the literature and in endodontic file ads.

The technically correct terms have been replaced by the ad man's terms and instead of referring to a 1 per cent Morse Taper it is called a .02 taper.

The Morse Taper is a method used by machinists to describe the concept of a cone within a cone. I.e. it is a standardized way of describing how much the radius or diameter changes per unit length.

The most common example of a Morse Taper is the dental laboratory lathe.

Tools are attached to the rotating spindle via the Morse Taper Effect.

Since the rotating spindle is uniformly tapered and the corresponding recess in the tool is tapered the same, until they come into intimate contact, the spindle does not grab the tool.

When it does, the metal in the spindle compresses or the metal in the tool expands or more likely both.

To get the tool to hold on tight, you tap it onto the spindle.

To release it you tap it the other way.

The principle is the same as a log-splitter that is, the horizontal component of force gets very great as you approach parallelism due to the outward force vector.

Morse tapers are measured in percentage units that reflect the the shaft length relative to the RADIUS of the shaft.

• Thus, if for every inch of shaft the RADIUS increases .01 inch, this would by definition be a one per cent MORSE taper.
• If the RADIUS increases .04 inch for every inch of shaft length this would be a four per cent MORSE taper, etc.
• The smaller the per cent MORSE taper, the closer to parallelism and thus the tighter it holds.
• Typical lathes are around four to seven per cent.

In about 1958 the endodontic file manufacturers of the world got together and agreed that the old #1-#6 file sizes had to go. There was no consistency among manufacturers and the numbers were essentially meaningless.

So they made two important changes to the terminology and the standards. The first was that they agreed to a one per cent MORSE taper. The second was in the standardization of the sizes.

Since they agreed to make files a one per cent MORSE taper, that means that as you move up the shaft of a file from tip to shank, the RADIUS increases by .01mm and the since there are two radii in a diameter the DIAMETER increases by .02mm.

You have probably noticed several files entering the dental market place that touting the advantages of a 2, 3, 4 and 5 percent MORSE taper. ( Although they don't call it a MORSE taper, that's what it is. If you want to translate Morse Taper numbers into the more commonly advertised numbers, multiply the Morse Taper by 2.)

Other endodontic devices were also standardized, e.g. pluggers, paper points and gutta percha points.

The second important standard they agreed to nominal sizing and its terminology.

The NOMINAL ( named ) size would reflect the actual size at the D1 diameter, that is at a point one millimeter from the tip, would be the size in hundredths of a millimeter.

For example, the file that is .15mm in DIAMETER is called a Size 15 and a Size 50 would be .50mm in diameter at the D1 point.

It was further agreed to that files would be sold in increments of .05mm and thus we have our size 10,15,20,25,30 etc.

There are several implications for endodontic preparation hidden in these standards.

One very important point is that each file is in effect a SEGMENT of a cone that is part of a much longer cone.

That is, if you can visualize a standard one per cent Morse tapered endo file that is about 70 mm long that starts out at 0.10mm in DIAMETER at the tip and is about 1.5mm in diameter at the top.

If you move up this mythical file in increments of 2.5mm and cut a segment about 20mm in length you would have a representative file in our system of file sizes.

The first segment would be a size 10. Moving 2.5 mm toward the shank and then cutting a segment would make a size 15 and so forth.

To put it another way, if a canal is prepared to the apex with a size 15 file,it is in effect a male and female one percent Morse taper.

If the file were to be extended 2.5 mm out of the apex the apex would now be a size 20.

Or another. If a size 15 will wedge at the apex, a size 20 will wedge 2.5 mm from the apex.

A very important corollary to this concept is that if a size 15 will fit to a given depth and the size 20 will NOT go to that point, this is diagnostic of a curve because although it may fit it is binding because of the curve.

Or to put it still another way. If you cut off 2.5 mm of a file, you would have a file EXACTLY the next size.

And if you want intermediate sizes you would cut off less than 2.5mm.

A very important use of the Morse Taper concerns the technique of Push Quarter Turn Pull filing and the Balanced Forces concept.

(see also Push Quarter Turn Filing)

(see Balance Forces Concept)

E-mail your comments to drjack@BetterEndo.com