HYBRID TECHNIQUE

[drmithila]

Written by Barry L. Musikant, DMD, and Young Bui, DDS

Using instruments limited to a short arc of motion provides the dentist with several advantages,4 the most obvious ones being the virtual elimination of torsional stress and cyclic fatigue,3 the 2 factors that lead to rotary Ni-Ti separation.5 This has been such a consistent problem for rotary Ni-Ti that the industry continues to introduce newer systems, each supposedly having an incrementally increased resistance to fracture. The problem cannot be switched off like a lightbulb, because fatigue and torsional stress6 are indigenous to the weakness of all metals including Ni-Ti when used in rotation. In that regard, some rotary systems are now incorporating what is best described as hybrid rotation or hybrid reciprocation, depending upon your point of view. By that, we mean that for each motion of 150° there is a compensating motion of 30° in the opposite direction. Reciprocation is occurring, but there is still a net increase of 120° per 2 oscillations. With a frequency of 600 cycles/minute, that still amounts to 200 full rotations per minute, producing a similar degree of rotations that have been in existence since their introduction. As we know, rotations around a curve lead to cyclic fatigue7 and an increasing instance of separation as the tip size and taper of the instruments increase and the curvature of the canal becomes more acute.

Up to this point, we are emphasizing the safest method of instrument utilization: short arcs of motion either manually or in a 30° reciprocating handpiece. Of at least equal importance is the design of the instruments. Used predominantly with a horizontal watch winding motion, the blades remove dentin most effectively when the cutting edges of the flutes are more or less at right angles to the plane of motion. To understand this concept, think of shaving one’s face with a razor blade. It works quite well because the cutting edge is at a right angle to the plane of motion.8 That is why the blade is on a T; if the blade were in the same plane as the plane of motion, we would be slicing into our face, removing nothing but blood, which we all agree is not our goal. Yet, when we use K-file,9 we are using instruments whose flutes are more or less in the same plane as the plane of motion only to pull up after engagement to shave away the dentin. While this second application of motion removes dentin, it does so at the expense of straightening the outer wall of curved canals on the upstroke while the horizontal flutes along length tend to impact dentin upon the downstroke.

Please note the difference in the angles of flutes along the shank of a K-file file compared to the angle of the flutes along the length of the relieved reamer. It is the vertical orientation of the flutes along the length of the relieved reamer that account for their great efficiency when used in shaping canals (Figure 5).

When we use relieved reamers,10 instruments with a flat along their entire working length and vertically oriented flutes with the customary watch-winding stroke, we are immediately shaving dentin away, freeing up the instrument for further apical negotiation while reducing the engagement along length, the exact opposite that occurs when using an instrument designed as a file. In other words, the relieved reamer is the proper design to maximize the penetration of the instrument to length with the least resistance encountered during the process.

To sum up where we are at the moment, we are establishing the fact that K-files represent an incorrect instrument design that nullifies the benefit of being used with the correct motion. Rotary Ni-Ti, an instrument correctly designed as a reamer, is unfortunately utilized incorrectly. Relieved reamers, on the other hand, represent both the correct design and correct utilization of instruments to accomplish the task of cleansing and shaping canals.