Authors: McSpadden J.
Title: Advanced geometries in endodontic micro files: the rationale.
Journal: The NT Company
Date: Jan 1996
Citation: (end of citation)
Category: Access and Canal Instrumentation
Evidence-based Ranking: 5
Purpose/Objective: To describe concepts of advanced rotary endodontic file designs.

Discussion: The following are considerations for designing an ideal endodontic file: 1. Rake angle is the angle at which the blade meets the surface (either negative or positive). Negative rake angles cause the blade to scrape rather than cut dentin. Overly positive rake angles tend to bind into the surface without forming a chip that can be dislodged. The ideal is a slightly positive rake angle. 2. Flute design (cross sectional design) should allow effective channeling of debris through the spiral of the flute and avoiding compression of the debris that frequently cause instrument failure. 3. Helical angle or the angle at which the blade spirals around the shaft of the instrument is important in effective cutting and chip removal. Too many spirals tend to compress the chips and cause them to be channeled out too slowly while too few spirals cause the chips to accumulate too quickly before they can be removed. 4. Blade support in the form of radial lands (a radial flat surface that projects peripherally as far as the blade) are effective in preventing canal transportation by deflecting the instrument around curvatures and preventing formation of cracks by adding mass behind the cutting edge. 5. Frictional resistance can be minimized by reducing the surface circumference, which is accomplished by recessing the radial land. 6. Notch phenomenon is the concentration of stress at the deepest and smallest radius of the instrument groove. The ideal file should have the greatest depth of flute most distal to the cutting edge to be most resistant to failure. 7. Working surface (taper) variations were developed to maximize the cutting efficiency by minimizing the surface of the instrument in contact with the canal walls. The ideal file sequence should have numerous taper changes, each having the same size tip to maximize cutting efficiency. 8. The ideal file utilizes more peripheral strength or thickness of the instrument lying between the deepest parts of each groove. File is only as strong as the weakest part. 9. The use of a faceted tip (negative rakes which become positive rakes at the periphery) minimizes stress on the tip of the instrument and maximizes cutting efficiency for small canals. This tip eliminates the burnishing action of a non-cutting tip that may lead to failure. 10. Balanced forces of the working surface allow cutting to occur on the canal wall opposite the pressure surfaces. Pressure surfaces are usually on the inside of the curvature at the angle of the curvature and on the outside of the curvature apically.

ReviewerÂ’s comments: This is an article that describes the process followed in designing the Quantec Series 2000 rotary instrument. The design considerations discussed also applies to characteristics of other rotary files. The main weakness of the article is that the statements (although they appear theoretically sound) are not backed by any kind of scientific evidence.