Dr. Jerry Harrison's Corpus Christi Study Group
After taking my course a number of years ago Dr. Jerry Harrison did a presentation of what he had learned to his study group in Corpus Christi, TX.

Since he was not prepared to re-teach my course he devised this demonstration to illustrate what happens when a negative rake is used and again when a positive rake is used.

Remember, a negative rake angle is when the leading edge of a cutting tool is ahead of the perpendicular to the surface being cut.

A positive rake means that the blade is BEHIND the perpendicular.

Examples of negative rake instruments are K-files, K-Flex files, Flex-R files, Reamers, and most of the current crop of NiTi files. Examples of positive rake files are Hedstroms, Fine-Cut Files, and S-files.

He brought to the meeting a stick of butter, a butter knife and a pepper shaker.

The butter represents dentin, the knife an endo instrument and the pepper, the debris, bacteria and necrotic dentin on the surface of the dentin lining a canal.

He then covered the butter with pepper, applied the knife at a negative rake and attempted to remove the pepper.

It took several attempts because each stroke although it removed some pepper, burnished what was left further into the butter.

He then re-covered the butter with pepper and this time holding the knife at a positive rake, in one stroke removed all of the pepper.

When a negative rake is used, it burnishes as well as cuts. In the process it drives bacterial debris into the dentinal tubules, fins and lateral canals. This in turn necessitates removing more tooth structure to get at the bacteria, which in turn...well you get the idea.

When a positive rake is used, only light pressure need be applied and because the canal can be filled with water, the debris is scraped off, and immediately gets suspended in water and can be flushed out.

  1. To provide a graphic demonstration of the effects of cutting with a negative rake vs. cutting with a positive rake, Dr. Jerry Harrison brought to his study group meeting, a stick of butter, a butter knife and a pepper shaker.


  2. Knife Blade held at a Right Angle to Surface Being Cut


  3. Blade Held at a Positive Rake Angle (assuming a left to right cutting stroke)


  4. Blade Held at Negative Rake Angle (assuming a left to right cutting stroke)


  5. He then covered the stick of butter with pepper to represent bacterial debris and necrotic tissue


  6. He then applied the knife to the surface of the butter and moved it left to right starting the first cutting stroke with a Negative Rake


  7. At the end of the first stroke with Negative Rake notice how much butter has been removed and that pepper has been burnished into the butter


  8. Start of Second Stroke with Negative Rake


  9. End of Second Stroke with Negative Rake.

    Notice again the amount of butter removed and how the remaining pepper is burnished into the butter.


  10. Notice the Amount of Butter (tooth structure) Removal in order to remove all the pepper.


  11. He then covered the butter with pepper again to start over for the Positive Rake demonstration.


  12. First Stroke with Positive Rake showing angle of the blade and direction of movement.


  13. After just one stroke with blade held at a positive rake. Notice how completely the pepper has been removed and how little butter had to be removed to do it.