CANAL PREPARATIONS

[Drsumitra]

 The mechanical objectives for endodontic canal preparation were brilliantly outlined almost 40 years ago.¹ When properly performed, these mechanical objectives promote the biological objectives for shaping canals, 3-dimensional (3-D) cleaning, and filling root canal systems (Figure 1).² During the following decades there has been the emergence of a staggering number of file brands, sequences, and hybrid techniques advocated for shaping canals. However, recent advances for endodontic canal preparation have focused on the concept "less is more."³ This article will describe a single-file technique for shaping the vast majority of canals, regardless of their length, diameter, or curvature.

ROTATION VERSUS RECIPROCATION 
By far, the greatest number of commercially available files utilized to shape root canals are manufactured from nickel-titanium (Ni-Ti) and are mechanically driven in continuous rotation. On the other hand, reciprocation, defined as any repetitive back-and-forth motion, has been clinically utilized to drive stainless steel files since 1958. Initially, all reciprocating motors and related handpieces rotated files in large equal angles of 90° clockwise (CW) and counterclockwise (CCW) rotation. Over time, virtually all reciprocating systems in the marketplace began to utilize smaller, yet equal, angles of CW/CCW rotation. Today, the M4 (SybronEndo), Endo-Eze AET (Ultradent Products), and Endo-Express (Essential Dental Systems) are examples of reciprocating systems that utilize small, equal30° angles of CW/CCW rotation.

When shaping canals, it should be appreciated that there are both advantages and disadvantages associated with utilizing continuous rotating versus a reciprocating movement. The greater tactile touch and efficiency gained when continuously rotating Ni-Ti files in smaller-diameter and more curved canals must be balanced with the inherent risks associated with torque and cyclic fatigue failures. Fortunately, these risks have been virtually eliminated due to continuous improvement in file designs, Ni-Ti alloy, and emphasis on sequential glide path management (GPM).⁴ Compared to reciprocation, continuous rotation utilizing well-designed active Ni-Ti files requires less inward pressure and improves hauling capacity augering debris out of a canal.⁵

On the other hand, a mechanical reciprocating movement has merit because it somewhat mimics manual movement and reduces the various risks associated with continuously rotating a file through canal curvatures. However, current motors that drive reciprocating shaping files through equal forward and reverse angles generally require multifile sequences to adequately prepare a canal. Further, systems that utilize small, equal CW/CCW angles have recognized limitations, including decreased cutting efficiency, more required inward pressure, and a limited capacity to auger debris out of a canal.⁶ As such, there has been a genuine desire to rethink reciprocation and optimize the motors and files that utilize this concept. 

Serendipitously, in about 1998, Dr. Ben Johnson and Professor Pierre Machtou co-discovered the unmistakable advantages of reciprocating Ni-Ti files utilizing unequal bidirectional movements. Subsequently, in the late 1990s, Professor Machtou and his endodontic residents extensively analyzed this novel unequal reciprocating movement using the entire series of not-yet-to-market ProTaper files. Starting with the end in mind, Dr. Ghassan Yared, a former student of Professor Machtou, performed exhaustive work to identify the precise unequal CW/CCW angles that would enable a single reciprocating 25/08 ProTaper file to optimally shape virtually any canal.⁷ Although this specific reciprocation technique stimulated considerable interest, this file was never designed to be used in this manner. Yet, Dr. Yared’s work rekindled interest to take this single-file concept closer to its full potential.