Oral Health Group
Feature

Keep Your Eye on the Prize: Predictable Root Canal Shaping with the Restored Tooth in Mind

May 1, 2012
by Elaine Sides, DMD


The restorative dentist is a multi-disciplined clinician and may embrace many aspects of dentistry including endodontics in their armamentarium. Restorative dentists realize that the restoration of endodontically-involved teeth needs a comprehensive approach to the complete and successful restoration of those teeth and are aware of the impact on the rest of the dentition and supporting structures.

The simple rules of endodontic cleaning and shaping have been clearly defined in scientific literature. In the past 15 years, the role of Nickel Titanium Rotary Instrumentation has revolutionized the process, while introducing several new variables such as file design, materials, instrumentation sequences, torsional strength and limits, and variable tapers, to name just a few.

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These NiTi instruments have been applied to the treatment of a vast array of canal shapes and various canal morphologies that clinicians are presented with each day. As the clinician gathers experience in endodontic therapy, some questions arise as well as the desire to search for better and more predictable tools and techniques. Each canal presents with its own challenges and the clinician will ask the questions, “What is this canal like,” “what challenges do I face,” “what tools do I need,” “what techniques will give me the best long-term clinical results?”

With the advent of newer products, there has been a tendency to treat endodontic cases using “off the rack” systems, consisting of pre-defined file sequences, matching gutta percha points or carriers, and even matching fiber posts, for a complete “endo-restorative continuum.”

Such a system-based approach has, as its goal, greater efficiency and operator confidence. However, caution needs to be exercised so that these goals do not morph solely into speed and simplicity. With improved, advanced technology and systems, many are puzzled by what appears to be a significant number of failures and are wondering if there is a way to measure the rates of success with these different techniques, materials and technologies.

With such complex root anatomy that clinicians face along with the dozens of variables in the instruments used to treat such anatomy, it is clear that a more “tailor-made” approach is needed for each endodontic treatment. The advent of micro-computed tomorgraphy, allowing more complete three-dimensional information on canal anatomy, underscores the need to take this approach.

Endodontic clinicians are keenly aware of the challenges facing each tooth individually. Like people, no two root canal systems are alike. Canal dimensions are different in different types of teeth. Large palatal canals call for different instrumentation strategies than tiny MB2 canals. The second canal within the mesial-bucal root is always smaller that the other canals and, so is often more difficult to clean and shape. Some canals are curved and need a thoughtful approach.

Dr. David Clark brings to our attention that rotary instruments and obturation points of gutta percha are round in nature because of their mechanical nature. They create anatomically appropriate shapes in round roots but fail in ovoid roots. He also points out that half of the root forms are ovoid.1

Further, a review of the endodontic literature indicates that the apical constriction and the 3 to 4mm of the root canal coronal to it, are larger than instruments sizes often recommended by some manufactures for cleaning and shaping procedures (ISO #20, #25, #30).2,3

Spangberg pointed out that all the emphasis on reducing the number of instruments and limiting apical preparation to small sizes does not produce clean apical preparation in diseased teeth.4

Since the apical dimensions of root canals range from very large to very small, we should seek instruments and techniques that can help the clinician determine when instrumentation to the optimum shape and the correct apical size has been achieved. These decisions must all be made with the end goal of preserving as much tooth structure as possible in order to improve the long term success. As Dr. David Clark recently reminded us, “the literature shows that the dentin in an endodontically treated tooth is not more brittle than in a vital tooth. In short, preservation of peri-cervical dentin and ferrule girth trump all other factors.”5-8

Once the decision has been made to allow the tooth anatomy to dictate our instrumentation strategy, and not the other way around, then sound knowledge of the properties of our tools, namely nickel titanium and its behavioural nuances, is essential. With almost perfect memory, NiTi metal is extremely flexible but the flip side is that when used to negotiate a severe curve, the file, having acute memory, wants to go straight. Secondly, a file rotating at 300 rpm is rotating five times every second and its actual surface speed is up to three times higher at its most coronal aspect. So the more fluted area on a file, the higher speed its surface is traveling, and of course the more canal wall that is engaged.

The more that the canal wall is engaged, the more stress on the instument, as well as increased cyclic fatigue, especially while instrumenting curved roots. As one of the pioneers of NiTi Rotary design, Dr. John McSpadden pointed out, “The torque required to rotate a file varies directly with the surface area of the file’s engagement in the canal.” Attention should also be given to the file diameter at the point of curvature, especially if the canal curves in the coronal or mid-root area. Dr. McSpadden further notes that, “The smaller the diameter of a file, the longer it can rotate around a curvature without fatigue failure.” He points out in his book “Mastering Endodontic Instrumentation”: that “larger tapered files commonly used in minor mid-root or coronal curvatures have diameters that can cause severe compression and tension stresses that can lead to fatigue failure.”9

To avoid instrument fracture during instrumentation, it is important to understand the reasons why files fracture, namely excessive torsional forces and fatigue caused by rotation of a file around a curvature in the canal, or a combination of both of these.

An important factor in instrument selection would be intuitive design. If our goal was to know the tooth inside out, before, during and after treatment, then it would follow that our instruments should afford us the luxury of the canal dimensions as they are being shaped in real time. For example if a .04 or.06 tapered file is taken to working length, thought must be given to how wide the canal can be enlarged without risking perforation and removing unnecessary tooth structure. As Baugh et al noted, “Ignoring science for the sake of speed and simplicity may place the final outcome for our patients in jeopardy.”10

This article will focus on two newer file designs, one a series of smaller size instruments designed for glide path negotiation, and a second more aggressive file design with a revolutionary new type of NiTi material.

A recent arrival to the NiTi world is the Xplorer file (Clinical Research Dental, London, ON/Clinicians Choice) (Fig. 1). This file is aptly named as all instruments have a 10mm (X) fluted cutting zone while most rotary files have 16mm or greater fluted zone. This result in a reduction in stress on the file at any point in the treatment, because less of the file is “engaged,” which in turn reduces that chance of breakage. Thus the canal centre and location is much easier to maintain, as the fluted portion is usually less than the canal length. The smaller fluted portion also naturally makes the instrument stronger along its full length, as there is at least 6mm less fluted area than most files. The shaft above the fluted portion is parallel enhancing sight lines during treatment as well as debris removal.

The Xplorer files are a series
of three nickel titanium glide path instruments designed to be used sequentially after a #10 file ISO hand file passes easily from orifice to apex. These unique files assist the operator in becoming aware of the shape dimensions of the canal, therefore assisting the operator file selection.

Once a #10 ISO hand file can pass easily to the apex and you have established your working length, you are ready to move to the XPlorers files to complete the glide-path phase. The first Xplorer instrument is a size 15 (ISO) with a .01 taper, the first of its kind but with half the taper of a traditional stainless steel hand file. This file has a triangular cross section, and a 10mm cutting zone. This design allows the file to easily glide down the canal increasing the diameter of the glide path very efficiently from a size 10 to a 15. The 10mm cutting zone minimizes engagement of the file in the canal, thereby reducing stress on this small instrument. The triangular cross section gives this first file added flexibility in order to easily negotiate curves or complicated anatomy (Figs. 2-4) .

The next two files in the Xplorer sizes are a size 20 with a .01 taper, then a size 20 with a .02 taper. These files have a square cross section, for added strength, and a 10mm cutting zone. They are run at 400 rpm (for reciprocating motors, refer to manufacturers’ instructions). The sizes are colour coded on the handle as the 15/01 is silver white colour, the size 20 file being gold (ISO yellow) in colour with one stripe for the .01 taper and two for the .02 taper. (Figs. 5-7) .

As with most glide path instruments, it is recommended that these be used with a file lubricant. FileEze (Ultradent, South Jordan, UT/Clinical Research Dental, London, ON) (Fig. 8) is preferred as it is an aqueous based gel, which is easily flushed from the canal, and it has a non-peroxide emulsifier, which will not interfere with the setting of any resin sealers. Liberal irrigation with frequent exchange is also highly recommended with full strength sodium hypochlorite. The resultant size 20/.02 taper glide path avoids the canal transportation frequency associated with instrumenting canals with any apical curvature with stainless steel instruments.

Now that you have a path to the apex, you can then choose the file you wish to be your patent file. This is a hand file that passes easily to the apex without binding and without the typical hand instrument motion (watch-winding).

Another recent advance in rotary NiTi instrumentation is the new Typhoon file (Clinician’s Choice/Clinical Research Dental, London, ON) made with CM wire (Fig. 9). The CM wire used in the manufacture of these instruments is a new paradigm in Nickel Titanium. Whereas traditional NiTi has a perfect memory, the memory of the CM wire is controlled or has been removed by a patented thermo-mechanical process, which allows the instrument to follow the canal without creating undesired, disproportionate lateral forces on the canal walls. These Typhoon files with their higher torsional strength, and resistance to cyclic fatigue, two critical factors in controlling or eliminating file separation, allow for more conservation of tooth structure during instrumentation as the dentin truly determines the route of the file, which follows the path of least resistance. As a result, complex anatomy is instrumented and enlarges in a far more controlled manner.

This is in fact borne out in recent studies, one of which stated “The results of the present study indicated that NiTi instruments made from CM Wire were nearly 300-800% more resistant to fatigue failure than instruments made from conventional NiTi wire.”11

The Typhoon design includes a triangular cross section, variable pitch and helical angle, and a no-cutting tip. The more aggressive nature of this file is totally offset by the inherent safety of the Controlled Memory Wire it is made from. When such a file with no memory is taken down a curved root, it will intuitively follow the path of the tooth and WILL NOT try to go straight, resulting in the aforementioned balanced lateral forces. This is much more conducive to respecting the canal path of the tooth (Fig. 10). Another advantage of the Typhoon is that CM Wire has the ability to pre-curve your rotary NiTi instrument, which cannot be done with traditional NiTi (Fig. 11). This is especially advantageous in teeth with limited access, i.e. second molars, and pre-curving also affords the ability to get around ledges in the canal.(Figs. 12-14). These files are quite robust, with higher torsional strength and tremendous resistance to fatigue. They are much less likely to separate; they will tend to unwind, not break, if torsional limits are exceeded.

The Typhoon files range in size from 20-45, and come in 21 or 25mm lengths. The 20-35 sizes are available in .04 and .06 taper while 40 and 45 come in .04 taper only. The Typhoon files are designed to be used in a crown down technique.

Once working length, apical and canal size, and a patency file selection have been achieved, you will then be able to determine which Typhoon file you will want to stop with. After determination of the aforementioned, the next step is to use the Typhoon files to clean and shape the canals to the desired outcome (Figs. 15-18). It is highly recommended that with any filing of the canal, lubricant (FileEZ, Ultradent/Clinician’s Choice) and a rinse (sodium hypochlorite) be used. After each file and rinse the patent file should be inserted into the canal to ensure that patency was maintained. For example, you may wish, depending on the canal, to start with a 35/.06 taper, 30/.04 taper, 25/.06 taper and 20/.04 taper (Fig. 19).

Once the desired shape has been achieved, all debris removed and the canal dried, the canals are ready for obturation. Since we have already established that no two canals are the same and one half of the canals are not perfectly symmetrical, a warm and thermoplasticized gutta percha vertical condensation technique was used in Figures 20-24. (Alpha and Beta systems Clinicians Choice/Clinical Research Den­tal) (Fig. 25).

The long-term success of endodontic therapy is not in a particular system but in the successful cleaning, shaping, obturation and coronal sealing of the tooth. The research has clearly shown that it is critical to seal the orifice of the canals to prevent microleakage, thereby increasing the long-term prognosis of the tooth.12,13

It is not always possible to do the build-up at the same appointment as the obturation, so PermaFlo Purple (Ultradent, South Jordan, UT/Clinical Research Dental, London, ON) (Fig. 26) is highly recommended. With its ease of use, its ability to seal the furcation and coronal area, it is a must in the endodontic armamentarium.14

The future is exciting for the restorative dentist with all the proven tools and some dependable techniques to help provide a solid base for the final restoration ensuring clinical success.

Typhoon files are a revolutionary improvement to existing rotary systems and the endodontic clinician will be able to discern that these files will assist in achieving long-term predictable clinical results.

As our practices of dentistry continually evolve, offering our patients improved long-term treatment outcomes, we will also benefit from the satisfaction of knowing we have done the best for them as we continually strive to be the best we can be. OH

Dr. Elaine Sides received her DMD from the University of Saskatchewan in 1982. She is an Adjunct faculty member of the University of Alabama, Birmingham Campus Dental School. Dr. Sides is in private practice in the Birmingham, Alabama area where much of her practice is focused on endodontics.

Special thanks goes to Mr. Brian Bevan from Clinical Research Dental who helped with this article’s publication and also to Mr. Charles Sides, Charles Sides Photography, Birmingham, Alabama for the clinical photographs in this publication.

Oral Health welcomes this
original article.

REFERENCES:

1. Clark, David DDS, “Biomimetic Endodontics: The Final Evolution?” Dentistry Today, July 2007.

2. Baugh, Dean, DDS and Wallace, James, DDS,MDS,MSD,MS,”The Role of Apical Instru­mentation in Root Canal Treatment: A Review of the Literature:JOE-Vol. 31,Number 5, May 2005 and

3. Spangberg L. “The Wonderful World of Rotary Root Canal Preparation”, Oral Surg.,Oral Med.,Oral Path.,Oral Rad.,Endod.,2001:92:479.

4. Spangberg L., “The Wonderful World of Rotary Root Canal Preparation”, Oral Surg.,Oral Med.,Oral Path.,Oral Rad.,Endod.,2001:92:479

5. Clark, David DDS, “Biomimetic Endodontics: The Final Evolution?: Dentistry Today, July 2007 and Endodontics June 2009.

6. Christine M. Sedgley, BDS, MDSC,FRACDS and Herold H. ser, MDSC,PHD, “Are Endodontically Treated Teeth More Brittle?”, JOE-Vol.18.No.7,July 1992.

7. S, Palamara JE, Messer HH, “A Comparison of the effects of the two canal preparations on root fracture susceptibility and fracture pattern.”, J ENDO 205:31;4:283-287.

8. Hang TJ, Schilder, H, Nathanson D. “Effects of moisture content and endodontic treatment on some mechanical properties of human dentin.” J ENDOD 1992;18(5);209-215.

9. McSpadden, John T. DDS “Mastering Endodontic Instrumentation”, copyright 2007-published by Cloudland Institute.

10. Baugh, Dean, DDS and Wallace, James DDS,MDS,MSD,MS,”The Role of Apical Instrumen­tation in Root Canal Treatment: A Review of the Literature”: JOE,-Vol.31, No.5, May 2005.

11. Ya Shen, DDS, PHD, etal, “Fatigue Testing of Controlled Memory Wire Nickle-Titanium Rotary Instruments” JOE Vol.37, No.7, July 2011.

12. Helig I. Endodontic Failure Caused by Inadequate Restorative Procedures: Review and Treatment Recommendations. JPD 2002;87(6):674-678

13. Saunders WP., Saunders EM. Coronal Leakage as a Cause of Failure in Root-Canal Therapy: a review. Endodontics and Dental Traumatology 1994:10(3):105-108.

14. Glassman, Gary DDS, FRCD(C), Boksman, Len DDS, BSC, FADI, FICD, “Ensuring Endodontic Success: Tips for Clinical Predictability”. Oral Health 2009:18-28.