Design and Function: How They Interplay in Endodontic Instrumentation

 Function should determine design and design should determine and enhance function. To make progress, we should first know exactly what proper function implies. In the case of endodontic instruments, the goals are to initially shave dentin away from all the canal wall uniformly, to do so while encountering as little resistance along the length of the canal consistent with the removal of dentin. This implies a reduction of both torsional stress and cyclic fatigue stresses to both the instrument and the canal walls. The combination of these goals is not easy to deliver. Efficient rapid dentin removal means the shaving blades of the flutes along the length of the instrument engage the canal walls more aggressively. The more aggressively they engage the canal walls, the greater the torsional stress and in curved canals the greater the cyclic fatigue. Consistent with Newton’s Third Law of motion stating that two interacting bodies (the instrument and the canal wall) will have an equal and opposite effect on each other means that increased stresses directed to the instruments will also be directed to the canal walls. This reciprocity has been demonstrated in over 100 studies that have recorded dentinal defects after rotary instrumentation. 

Going back to our original goals of efficient dentin removal with as little damage as possible to the instruments and the canal walls requires modifications in the design of the instruments that have not been incorporated to date. One way to diminish the engagement of the instrument with the canal wall is to include a helical flat along the length of the instrument. Less surface area in contact with the canal walls, a result of the helical flat, decreases engagement that in turn reduces resistance to negotiating apically. Since it is less engaging, doesn’t that mean it is less efficient at removing dentin, reducing its functionality? The helical flat doesn’t compromise the flutes’ ability to shave dentin away. It makes the entire instrument more flexible and provides a space for debris that prolongs the shaving efficiency of the rotating blades before the instrument must be removed from the canal and debrided. An added benefit of the helical flat is a brake on the rotary instruments tendency to be “sucked” into and beyond the apex, a common defect of most rotary systems that increases the incidence of instrument separation.

With less resistance and engagement and the unpredictable tendency for being aggressively drawn deeper into the canal highly diminished, it takes less apical pressure applied to the rotary instrument to negotiate to the apex. With less resistance, it takes fewer rotations to reach the apex in both straight and curved canals resulting in a reduction in torsional stress and cyclic fatigue, the two causes of instrument separation. Accordingly, these instruments function at 310 revolutions per minute, a slow speed that enhances the dentist’s control over their application in the canals. According to Newton’s Third Law, now that the instruments are significantly less likely to separate, dentinal defects are also less likely to be produced.

With an instrument less likely to separate, the use of these instruments are no longer constrained to being employed in a strictly centered position. They are routinely applied to the buccal and lingual walls of oval canals assuring removal of pulp tissue and bacteria in these extensions. I should make an observation that purposely reduces the actions of rotary instruments especially in the larger sizes, namely 30/02 and 30/04. While they can be applied aggressively against all the canal walls, I would hesitate to take them into thin isthmuses that have already been shaped in the glide path stage to a 15/02 or at most by the rotary 20/02. Why do I say this? Please remember that a thin isthmus connecting two canals of larger dimension is thin because it reflects the external anatomy of the root, typically a concave configuration that has little dentin between the canal wall and its external surface. To create a uniform shape buccally and lingually implies removing a higher percentage of dentin where the canal walls are thinnest, a self-defeating exercise that can lead at best to needlessly weakening of the root and at worst strip perforation that will lead to predictable failure.

 The incorporation of the helical flat is a design feature that enhances the functionality of the instrument in attaining the goals we desire, namely more efficient removal of dentin from the canal walls, greater resistance to separation, their safe usage in the buccal and lingual dimension when appropriate (which is quite often), reduced impact on the integrity of the canal walls and multiple usage of the instruments (recommended 4 uses prior to replacement). 

I’ve been intrigued by the fact that what appear to be minor changes or additions in design can have profound effects on function. This turned out to be true with the split-shank design of the Flexi-Post and Flexi-Flange as well as the bi-directional design of a cement applicator. The split-shank design allowed these posts to have the previously unattainable combination of high retention coupled to low insertional stress and an even distribution of functional stresses. The bi-directional spiral allows the dentist to thoroughly flood a canal with cement while preventing its extrusion beyond the apical confines of the root during its application. The incorporation of a helical flat is another example of a design implementation that beneficially affects function. 

Hope you are all safe out there and finding productive endeavors during this most trying of times. Health is number one and perhaps a truer understanding that we are best when we support each other in any constructive way we can think of. I believe humanity and science are the design features we must incorporate if we are going to be better after this experience than before.

Regards, Barry