Reciprocation: A Safer and More Efficient Approach to Care

INTRODUCTION
When discussing ways to improve endo­dontics, anyone attempting to relate to the dentists doing the work should address the issues on several different planes. There is the plane of safety, generally referring to keeping the instruments intact, but also increasingly related to maintaining the strength of the root. There is the plane of convenience, relating to the time it takes to consistently perform the cleansing and shaping operations prior to obturating the canals. Closely related to the time required are the numbers of instruments needed to perform the task and how often they can be used that in turn relates to the practition­ers’ costs. To make things a bit more complicated, modifications in one plane have repercussions on other planes.

For example, one may prepare canals in a crown-down fashion, minimizing the amount of engagement any one instrument has with the canal walls, and increasing safety related to instrumentation. However, the greater taper imposed by crown-down preparations reduces the resistance of the root to the forces of occlusion, increasing the chances of vertical fracture. Alternatively, one may use fewer instruments, reducing costs, but creating risk of underpreparing the canal, particularly in the buccal and lingual dimension of highly oval canals. The time and cost to the dentist is reduced, but at the expense of having canals that are not thoroughly cleansed and placing greater stresses on the shaping files, thus increasing the chances of instrument separation.

From these 2 examples, we can see that modifications in technique increase the chances of some problems while solving others. The questions are whether or not these are the unalterable limitations that are imposed on us, and does any choice that we make produce burdens that simply replace the ones we had before?

CHALLENGING A PARADIGM
One way to address these questions is to challenge a basic principle of the paradigm of greater tapered rotating Ni-Ti instrumentation. It is a fact that rotation around a curve produces compressive and tensile stresses within the roots leading, at times, to instrument separation. This reality is one that is inseparable from rotating Ni-Ti (no pun intended). Heat-treated metals and newer Ni-Ti alloys are more resistant to breakage, but not immune to the cyclic fatigue. Nor are they immune to the torsional stresses that rapidly occur when the tip of the instrument binds apically.

Figure 1. Radiograph showing separation of rotary Ni-Ti instrument in the canal.	Figure 2. Radiograph of an extracted tooth demonstrating the much wider bucco-lingual extensions versus the mesio-distal width.

Figure 3. Photograph showing flat incorporated along the length of an engagement to reduce resistance and to create greater flexibility.

The problem of separation has been addressed by creating straight-line access, crown-down preparations, single usage files, using newer Ni-Ti alloys, and non-linear reciprocation that still produces 200 full rotations per minute. The frequency of breakage has declined and this has encouraged dentists to adopt these newer systems. However, what has not been understood is the opposite and equal impact that the rotating instruments have on the integrity of the root. The generated forces of rotation, either continuous or interrupted, that induce defects in the instrument when propagated, lead to instrument separation; this has been shown to induce defects in the dentin from the same cyclic fatigue and torsional stresses that plague the instruments with one major exception.^1-5 The dentist immediately knows when an instrument has separated, as confirmed by its clear presence in a radiograph (Figure 1). The dentinal microcracks produced by greater tapered rotating Ni-Ti instrumentation are not apparent clinically. Furthermore, the dentist does not generally receive negative information from their generation until years later, when the microcracks coalesce and propagate into a full blown vertical fracture; an event that appears to be increasingly common during the past 20 years. These negative events are not simply conjecture. Research has documented a reduced resistance to vertical fracture relating to the use of greater tapered rotating Ni-Ti instrumentation.^6-17

Research correlating such instrumentation to weaker roots is not a welcome insight. It implies that both greater tapered instrumentation and the rotation used to drive these instruments are responsible for producing defects that increase the incidence of vertical root fracture. The further implication is that we should seek ways that do not induce dentinal microcracks. The research has already documented a reduction, or complete elimination, of dentinal microcracks when hand instruments are used.¹⁷ That is not exactly a comforting thought for all the dentists who have increased their skills, saved time, and reduced hand fatigue by minimizing the use of K-files that made even simple endodontics a challenge. It is likely that no one who has used rotating Ni-Ti would want to return to these tedious procedures.

A resolution to the problems and the alternatives mentioned to date is the added insight from the research that engine-driven instrumentation does not induce dentinal defects if the amplitude of motion is small. A study using the self-adjusting file (SAF) demonstrated that an amplitude of motion of 0.4 mm oscillating at 5,000 cycles per minute did not induce dentinal microcracks.¹⁰ A 30/02 relieved stainless steel reamer in a 30° reciprocating handpiece oscillating at 3,000 to 4,000 cycles per minute generates an arc of motion of 0.104 mm or 75% less than the SAF that has been shown not to induce dentinal defects. Thinner tipped instruments produce even smaller amplitudes of motion.

From this information, one can conclude that, un­like with rotation, continuous or interrupted, 30° reciprocation does not induce dentinal defects. Such small amplitudes of motion protect both the integrity of the canals and the instruments. Confined to short arcs of motion, these instruments are virtually invulnerable to breakage.

Knowing that instruments are virtually invulnerable to breakage changes the mindset of a dentist. If one’s primary concern is preventing separation, shaping the canal in 3 dimensions is of secondary importance. We may comfort ourselves when we see a beautifully tapered obturated preparation on our mesiodistal periapical radiographs (equating that with a job well done), but we should be aware that, in most canals, the buccolingual extensions are far wider than the original mesiodistal width of the root (Figure 2). The result is a preparation in the mesiodistal plane with a taper greater than needed and an inadequate buccolingual preparation that often does not even touch the wider extensions of pulpal tissue.

This mindset changes when using properly designed instruments in a 30° to 45° reciprocating handpiece. Dentists intuitively know that short manual arcs of motion are unlikely to result in instrument separation. There is even less likelihood for separation when 30° engine-driven reciprocation is employed, simply because such a small amplitude is difficult to achieve manually. The strict limitations on the amplitude of motion give the dentist the freedom to employ the instruments with high frequencies of oscillations. The result is the rapid shaping of canals along their entire length. Of more importance, it gives the dentist the freedom to use these instruments vigorously against all the walls of the canal. This freedom is not one accorded rotary Ni-Ti that must stay centered carefully while avoiding aggressive pressure against the buccal and lingual extensions. This point is accentuated by the fact that a crown-down preparation is necessary when using rotary Ni-Ti to protect against the excessive stresses of instrumentation, even when limited to the mesiodistal plane. Conditioned to such precautions, it is the rare dentist who will venture to use rotary Ni-Ti instruments aggressively in the buccolingual plane. The result, as confirmed in many research studies, is inadequate removal of tissue in the buccolingual extensions of oval canals.^18,19

Figures 4 to 6. Representative before and after radiographs, using the clinical techniques mentioned in this article.

In contrast, 30° to 45° reciprocation protects the instruments against excessive torsional stresses and cyclic fatigue that lead to separation, while protecting the tooth structure that is vulnerable to the same stresses affecting the instruments.
We are clear on what constitutes a safer means of motion. Let’s now concentrate on the design of the instruments that are used with such short arcs of motion.

IMPORTANCE OF INSTRUMENT DESIGN AND TECHNIQUE
From the start, we have replaced K-files with modified reamers (SafeSiders [Essential Dental Systems]) as a more practical designed instrument to create the glide path. Given the history of 02-tapered stainless steel K-files creating blockages in the apical third of canals that result in a loss of length, and then the distortions that are incurred when attempting to regain that lost length, one becomes aware of the shortcomings of K-files.

What many of us are not aware of is that there are far better alternatives that are not only more efficient, but are well adapted to the use of the 30° to 45° reciprocating handpiece. The author recommends relieved, vertically fluted 02-tapered stainless steel instruments. With about one half the number of flutes along the same 16 mm of working length, this results in a more prominent vertical orientation of the flutes. Used manually, with a watch-winding motion, the relieved reamer negotiates the full apical length with significantly less resistance than a comparably sized K-file. A flat incorporated along length reduces the level of engagement, further reducing resistance (Figure 3). The vertical flute orientation is far less likely to impact debris apically than the predominantly horizontal flutes along the length of the K-file. More significantly, the first clockwise motion of the relieved reamer shaves dentin away from the canal walls, immediately reducing engagement along length, further reducing resistance. When attached to the 30° to 45° reciprocating handpiece, the modified reamers simulate manual usage at a much higher frequency, thus providing rapid shaping of the canal at its earliest stages.

From a practical point of view, after the thinnest instrument is taken to length manually, it is most often immediately attached to the reciprocating handpiece where the primary goal of reaching the apex is already accomplished, and the added task of shaping the wider buccolingual extensions is our next most important goal. Distortion is not a concern because 02-tapered modified stainless steel reamers (with a tip size of 06, 08, 10, and 15) are highly flexible with the ability to adapt without distortion to multiple curves. An important question is to what extent the initial 02-tapered reamers can shape the canals without distortion. Here one must understand that there is a great difference between shaping a curved canal with a series of instruments, starting with highly flexible thin-tipped 02-tapered instruments, and entering the canal initially with a greater tipped 02-tapered instrument. The former approach creates an increasing well-demarcated pathway for the marginally stiffer instruments to follow, with each instrument in the sequence defining the pathway for the next larger instrument. The latter must make its pathway on its own; a task not well suited for an instrument that is too stiff to faithfully follow the original anatomy without prior canal preparation.

The advances to date include: 30° to 45° reciprocation oscillating at 3,000 to 4,000 cycles per minute as a more practical form of motion in protecting both the shaping instruments and the strength and quantity of the remaining dentin; the emphasis on 02 tapered shaping rarely exceeding preparations encompassing an 04 taper; the retention of more tooth structure in mesiodistal plane; a greater cleansing action in the buccolingual plane; the elimination of hand fatigue in glide path creation; and, the multiple usage of instruments with associated cost savings and an overall reduction in procedural stress.

While it is hard to single out any one single attribute as the most important, perhaps we should consider the implications of incorporating a technique that virtually eliminates instrument breakage. We know that an instrument designed with vertical flutes is adaptable to the 30° reciprocating handpiece. The modified reamers used to rapidly create the glide path without hand fatigue are quite efficient in this regard. This insight is the reason why dentists no longer need to experience hand fatigue when establishing the glide path. However, once the glide path is reached (shaped to a 20/02 preparation) we can advantageously employ a system that works more vigorously in the buccolingual plane when used in a reciprocating handpiece and requires only 2 more instruments to establish a final shape that is reflective of a 30/04 preparation.

I say reflective because, unlike 04-tapered rotary Ni-Ti, which imposes a 30/04 conical shape in 2 planes with the buccolingual preparation often underprepared, the new system (Tango-Endo [Essential Dental Systems]) adapted to a latch-type reciprocating handpiece is designed to extend the buccolingual preparation to encompass all the tissue that may be present. This can only be done effectively because the canals have been opened apically to a 20/02 preparation prior to their use. However, with this precondition in place, canals often highly oval and isthmus-like in the buccolingual plane are readily cleansed more thoroughly. Indeed, the canals are tapered along their entire length, but unlike rotary Ni-Ti preparations, they are not necessarily conical in cross section. Rather, it is far more likely that the mesiodistal taper will be minimal, reflecting their original shape and the thinness of the root in this plane with a greater taper in the buccolingual plane reflecting the broader expanse of tissue that is likely present.

CLOSING COMMENTS
The approaches advocated herein are compatible with the anatomy of the roots and the distribution of pulp tissue as it is actually found. By encouraging the use of techniques that preserve both the integrity of the roots qualitatively and quantitatively and the integrity of the instruments, the author is providing rational alternatives to techniques that, though widely applied, are being critically reviewed in the research as damaging to the remaining tooth structure. Practical alternatives (Figures 4 to 6) that are far less expensive, safer, and more efficient result from years of clinical experience and a thorough exposure to the research that throws out challenges that have yet to be effectively met by those producing these instruments. In the author’s opinion, what is clearly needed is a major change in the thinking that has prevailed throughout the past 20 years.

References

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Dr. Musikant has lectured worldwide in more than 150 locations as well as co-authored more than 200 dental articles published in major dental journals. As a partner in a New York City endodontic practice, his 30-plus years of clinical experience have crafted him into one of the top authorities in endodontics. He can be reached at (800) 223-5394, via email at info@essentialseminars.org, or via the website essentialseminars.org.

Disclosure: Dr. Musikant is president of Essential Dental Systems. Some of the products mentioned in this article are manufactured by Essential Dental Systems.