Yared’s work rekindled interest to take this single-file concept closer to its full potential. 7 Although this specific reciprocation technique stimulated considerable interest, this file was never designed to be used in this manner. 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. 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. Ben Johnson and Professor Pierre Machtou co-discovered the unmistakable advantages of reciprocating Ni-Ti files utilizing unequal bidirectional movements. 6 As such, there has been a genuine desire to rethink reciprocation and optimize the motors and files that utilize this concept. 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. However, current motors that drive reciprocating shaping files through equal forward and reverse angles generally require multifile sequences to adequately prepare 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. 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. 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). 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. When shaping canals, it should be appreciated that there are both advantages and disadvantages associated with utilizing continuous rotating versus a reciprocating movement. A ♜T image demonstrates the importance of treating root canal systems. Today, the M4 (SybronEndo), Endo-Eze AET (Ultradent Products), and Endo-Express (Essential Dental Systems) are examples of reciprocating systems that utilize small, equal 30° angles of CW/CCW rotation. Over time, virtually all reciprocating systems in the marketplace began to utilize smaller, yet equal, angles of CW/CCW rotation. Initially, all reciprocating motors and related handpieces rotated files in large equal angles of 90° clockwise (CW) and counterclockwise (CCW) 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. 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. 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. 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. 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). The mechanical objectives for endodontic canal preparation were brilliantly outlined almost 40 years ago.
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