From 1972 when I first began to be aware of cutting edges on files until 1987 I knew that there had to be a better way of cutting dentin.
At that time there were K-files and Hedstrom files. K-Files were twisted and Hedstrom files were ground. Hedstrom files broke easy and K-files were dull. That was the extent of the explanations you could get from endo instructors. If you went to one school you used Hedstroms. If you went to another, you used K-files. It was that simple.
A turning point for me was an article in Scientific American magazine in 1979. The title was The Mechanisms of Abrasive Machining. It wasn't about files. It wasn't even about dentistry. But it dealt with HOW TOOLS CUT.
This introduced me to the subject of rake angles. The rake angle being defined as the 'angle between the cutting edge and a perpendicular to the surface being cut'. If the cutting tool was ahead of the perpendicular it was, by definition, negative. If it was behind, it was positive. The difference is between peeling an apple with a knife or scraping a carrot with a knife.
K-files had a negative rake , which means that it does not dig in when you cut dentin. Which is good if you donŐt want your files to get stuck. So you use K-files when probing a tight canal and use Hedstrom files to enlarge it.
Along the way, I conceived what it is about a hedstrom file that makes it vulnerable to breakage. It's not the metal. It is the design.
The main characteristic of a hedstrom file that makes it fragile is it's steep helical angle. The steeper the helical angle the more torque is created when a file gets stuck and is pulled on. If you can keep the helical angle low then the blades will cut themselves free before the file twists off.
The problem is that by the very geometry of the file, if you keep a fixed inter-flute distance, the helical angle INCREASES as you get closer to the tip. The smaller the file, the weaker the tip, the steeper the helical angle, the more torque. It's no wonder files break.
Fine-Cut files have a truly unique design. The interflute distance decreases as you move toward the tip. The helical angle stays the same. The key point of this design is to minimize the torque at the tip of the file.
The result is a file that if it gets stuck will pull out without breaking. Thus is can be used for probing as well as circumferential filing.
Fine-Cut files come in two forms, hand files and SPeed Files. SPeed files have a special head that fits into either the Sonic Adapter, the STS or the new Fine-Cut Endo Contra-Angle. SPeed files allow the file to float to minimize the chances of lateral force that could cause zipping.