Getting the nomenclature of endodontic mechanics right

However, cutting the dentin is not our goal. It is the removal of dentin that is basic to shaping, and cutting merely makes an indentation without removal.

To drive this point home, consider the action of shaving your face or legs. Hair is removed because the cutting blade is at right angles to the plane of motion, resulting in the shaving of the hair and its consequent removal. If the blade were in the same plane as the plane of motion, we would be slicing into our faces or legs. We would be cutting, but we would not be producing removable shavings.

 

As long as the K-files are used with a motion that is in the same plane as the flutes, we will be cutting without removal. With this knowledge, we have a better appreciation of exactly what we want the instruments to accomplish, namely shaving dentin away rather than making indentations into the dentin without any removal of the dentin. Now obviously, K-files remove dentin from the walls of the canal, but it doesn’t occur until the instrument is pulled from the tooth. Upon the pull stroke, the blades are finally at more or less a right angle to the plane of motion and will now shave the dentin from the walls of the canal.

At best the horizontal component of motion, otherwise known as watch winding, accomplishes little more than engaging and disengaging the dentin. Not until the pull stroke is employed is dentin shaved away.

Once one understands what is going on, the K-files can rightfully be looked at as incorporating inefficient designs for what we want to accomplish. The more the flutes engage the walls of the canal, the greater the resistance to apical negotiation. One of the features of horizontal flutes is that more can be crowded into the working length (usually 16 mm), compounding the problem of increased resistance along length.

Added liabilities include the increased stiffness of the instrument as the number of flutes increase, a function that reflects the number of twists incorporated along the shank of the instrument. Given the predominant watch-winding motion, the K-file represents an instrument that engages more, shaves dentin away far less efficiently and is stiffer, producing poor tactile perception.

An instrument that is stiffer is more likely to spring back to the straight position rather than record the curve it is negotiating through, a fact that causes the K-file to selectively work against the outside wall of curved canals upon the pull stroke, leading to canal transportation in the apical third.

Until we get our nomenclature correct, substituting dentinal shavings for cutting, we will tend to confuse just what we want from the instruments we use. Once we understand what we want to accomplish, we immediately understand why the design and utilization of K-files defeat our goals for efficient undistorted shaping. We also understand why reamers both unrelieved (Fig. 2) and relieved with fewer more vertically oriented flutes fulfil our more accurate definition of what we want instruments to accomplish.

Unlike the K-files, the more vertically oriented flutes used with the same watch-winding motion will now not only cut dentin but shave it away. Where the K-files incorporate a design that leads to a viscous cycle of increasing engagement and resistance along length, the reamers are immediately and constantly removing dentinal shavings in a virtuous cycle.

To begin with, the reamers have significantly less engagement along length than the K-files, but immediately upon employing a watch-winding motion the vertically oriented blades (Fig. 3) are removing dentin. As the instruments travel apically, the dentin is continuously being shaved away, compensating for the deeper penetration.

Using more flexible instruments that cut more effectively keeps the instruments more centered in the canal on the down stroke. On the upstroke where the K-files are designed to work against the outer wall, the relieved reamers’ increased flexibility combined with the vertically oriented flutes makes them poor shavers of dentin against all the walls of the canal and the outer walls of curved canals in particular, reducing the potential for canal distortion in the apical third.

Watch-winding, whether used correctly with reamers or incorrectly applied with K-files, has the benefit of reducing the arc of motion to a point where torsional stress and cyclic fatigue are generally not issues that lead to instrument separation.

With the incorporation of rotary NiTi, both torsional stress and cyclic fatigue become factors that can lead to instrument separation. With separation directly related to greater canal curvature, and the increasing tip size and taper of the rotary NiTi instruments, the dentist is motivated to limit the canal preparation to dimensions that are likely to lead to inadequate irrigation and obturation.

Another way to look at cutting and shaving is to understand that cutting is always a component of shaving, but shaving is not necessarily a component of cutting unless the blades are at right angles to the plane of motion. With this understanding, a lot of the confusion that surrounds the term “cutting” is removed. What we should get from this discussion is that cutting is insufficient to accomplish our goals and we should concentrate on those techniques that emphasize shaving that results in shaping (Figs. 4–6).

From a point of view of the larger picture, schools would encourage the understanding of important endodontic mechanical concepts if they adopt language that clearly and accurately reflects what we want to accomplish. Teaching via indoctrination rather than critical thinking makes for muddled imagery that can take us off into useless tangents that result in uninformed decision-making that is ultimately bad for the dentist and his or her patients.

About the author

Barry Lee Musikant, DMD, is a partner in the largest endodontic practice in Manhattan, Musikant’s 35-plus years of practice experience have established him as one of the top authorities in endodontics.

Editorial note: This article was originally published in Endo Tribune Vol. 6 No. 5, May 2011.