An Extended 02 Tapered Glide Path is Far Safer Than Greater Tapered Rotary Preparations

This post relates to a study posted in the March 2018 issue of the JOE titled, Effect of the Glide Path Establisment on the Torque Generation to the Files during Instrumentation: An In Vitro Measurement by Ang Won Kwak, et al.

The claimed advantages of rotary NiTi over the use of stainless steel are centered around their greater flexibility giving them the ability to maintain the original canal anatomy during the shaping of the canal producing less procedural errors and more favorable results than stainless steel. The previous statement is true when a comparison is made for the way greater tapered preparations are made. Greater tapered preparations are generally shaped by greater tapered rotary NiTi instruments or they can be shaped with stainless steel using a step back manual technique. Given the increased flexibility of NiTi it is completely logical that NiTi instrumentation will produce less distortion when the canals are exposed to greater tapered preparations. The central question that is germane to canal distortion is whether or not greater tapered preparations (beyond 02 tapers) are necessary.

At 02 tapers, instruments through at least a 20 are more than flexible enough to follow a significantly glide path established by even thinner tipped 02 tapered stainless steel reamers resulting in minimal distortion. The fact that rotary systems call for a minimum preparation of a glide path to 20 using stainless steel 02 tapered instruments supports their non-distorting shaping ability. The general wisdom is that preparations beyond 20 must be done with rotary NiTi. That wisdom is questionable when the shaping continues to be done with 02 tapered stainless steel relieved reamers confined to a 30o arc of motion generated either manually using a handpiece oscillating at 3000-4000 cycles per minute. 30o arcs of motion virtually prevent undue stresses from both cyclic fatigue and torsional stress leaving the instruments predictably intact. With separation no longer a concern each instrument in the oscillating sequence can be vigorously applied to all the canal walls in a uniform fashion resulting in instrumentation that is far more likely to remove pulp tissue from all the thin recesses that NiTi is incapable of reaching.

Each instrument in the sequence further defines a glide path that the next larger instrument can faithfully follow. Unlike rotary NiTi that imparts the conical configuration of the shaping instrument, the 02 tapered oscillating stainless steel relieved reamers continue to work against all the canal walls producing shapes that reflects the original canal anatomy in larger form. Because the canal preparation is often wider than the shaping instrument, it is rarely necessary for larger instruments than a 30/02. These instruments target pulp tissue in three dimensions while preserving far more tooth structure in the mesio-distal plane.

The interpretation of canal distortion is somewhat evasive as it pertains to greater tapered rotary preparations. A 12 mm canal from orifice to apex with an unprepared mesio-distal diameter of 06 mm with minimal taper at the canal orifice is opened to a 25/04 preparation widening the mesio-distal diameter to 0.73 mm or approximately 12 times the original mesio-distal canal diameter far beyond what is necessary to remove the pulp tissue from the canal needlessly weakening the tooth in the process. Yet because the original pulpal curve remains, this gross over enlargement is not considered a distortion when, from a quantitative perspective it is. Rotary instrumentation with greater tapers would sacrifice more tooth structure and further weaken the teeth.

Let’s examine why a glide path for subsequent greater tapered rotary instrumentation is considered a crucial preparatory step. “ A glide path of sufficient size may reduce the torsional stress of rotating the NiTi file, reducing the potential risk of torsional fracture and increasing the lifespan of the NiTi file. Creating a glide path has some other advantage, such as decreasing root canal transportation and producing less apically extruded debris.” Furthermore, “the magnitude of the torque generated within the NiTi instrument during canal instrumentation is influenced by the contact area between the file and the canal wall, the applied apical force, and the preoperative canal volume. Decreasing the contact area and increasing the total volume of canal by preparing a glide path may reduce the torque generation and stress to dentin.”

This study compared the amount of torque generated in canals with and without glide path establishment. From the collected data, the authors concluded that greater tapered rotary instrumentation is far less risky to both the instruments and teeth when a glide path is first established. Please keep this in mind for further conclusions that will be stated after a brief discussion of the results.

“Based on present results, the total torque generated during instrumentation was significantly reduced with a glide path. This means that the glide path establishment will lower the stress concentration on the file as well as on the root dentin….The increased volume of the canal lumen by glide path establishment should reduce torque generation during instrumentation because part of the work required for preparing the canal into the final shape had been performed by the glide path procedure.”

“Although glide path establishment reduced the total torque sustained by each system during instrumentation, for the maximum torque generated, there was no significant difference between systems examined regarding the presence or absence of glide path establishment. It might be reasoned that the maximum torque is largely affected by the stress generated when the file passes through the canal curvature rather than the straight part of the root canal. It was noted that the maximum torque was observed at about the fifth or sixth stroke in the pecking motion which was when the file reach the maximum curve in the canal.”

“During the canal shaping procedure, torque is generated within the material of the instrument as an inevitable consequence and the generated torque transmits to the root dentin. Part of the torque is consumed to remove dentin from the canal wall. Excess torque may give negative effects not only to the NiTi file such as torsional fracture, but also to the dentin, resulting in dentinal crack formation.”

When I read this paper, it became clear that significant stress to the shaping instruments and the canal walls only occurs with the introduction of rotations whether continuous or interrupted further exacerbated by instruments of greater taper. Together they entail contact with the canal walls with potentially damaging impact on both. Equally clear is the fact that damaging consequences don’t occur until the introduction of greater tapered rotary instrumentation. Yet, we appear to be insistent to include a step beyond the glide path, requiring greater tapered rotary NiTi instrumentation. Why? If I limit the instrumentation to a glide path created by relieved 02 tapered stainless steel reamers confined to short arcs of motion either applied manually with a watch winding motion or via a 30o handpiece oscillating at 3000-4000 cycles per minute, I can readily prepare the canal space to a minimum of 30/02 in the mesio-distal plane and extend the canal preparation to the full bucco-lingual extent without concern for either instrument separation or canal wall damage. Most canals are far thinner than a 30/02 mesio-distally and the internal routing potential of thin stainless steel relieved reamers assures a more complete removal of pulp tissue three dimensionally with minimal loss of dentin.

By eliminating rotary endodontics most often done with greater tapered NiTi instruments, we virtually eliminate instrument separation, the production of dentinal defects, centered shaping and single instrument usage. Oscillating 02 tapered stainless steel relieved twisted instruments shape the canal dictated by the original anatomy resulting in the preservation of tooth structure both quantitatively and qualitatively, a goal that is achieved by simpler safer and more affordable alternatives.

Regards, Barry