Is the Adage “Instruments Shape, Irrigants Clean” Still Relevant in Endodontics Today?
For decades one of the rotary instrumentation theories to which most endodontists have subscribed states that root canal instruments only prepare and shape the root canal space for obturation while endodontic irrigants alone are used to disinfect the root canal space prior to obturation. As a result, shaping has largely been considered an attribute of instrumentation and disinfection a function of irrigants. While this theory is still widely adhered to, endodontists have long known that instruments are capable of cleaning quite efficiently-perhaps even more efficiently than irrigants-if they actually touch the root canal walls and remove a layer of dentin and its associated biofilm and tissues. Thus, shaping and disinfecting would be highly efficient if the root canal space could be enlarged with ever-increasing file sizes, similar to removal of decay with increasingly larger round burs, until clean dentin is reached.
This strategy was attempted around 30 years ago with the advent of files with very large tapers (0.08, 0.10, and 0.12). The problem that emerged from this experiment was the excessive hollowing out of teeth with over-instrumentation and the potential weakening of the remaining tooth structure through shaping to disinfect. About 15 years later, the pendulum swung back to the use of a minimally prepared root canal space (0.02, 0.03) that relied increasingly on chemical disinfection of the canal and calcium hydroxide therapy in multiple visits. This chemical disinfection was necessary because most root canal walls were not being adequately touched by conventional instruments and were being inadequately scraped with these minimally shaped and tapered preparations. It is important to realize that regarding total tissue volume reduction in a root canal, the most efficient method is afforded by direct touch of the sharp blades of a file or rotary instrument.1,2 This can occur through direct contact during instrumentation or with the aid of modern 3-dimensional (3D) instruments that can achieve virtual diameters greater than their core diameters through metal expansion or eccentric designs.
In recent years, expensive chairside equipment has been developed and marketed with the promise of providing improved irrigation and disinfection of these minimally prepared root canal spaces. The increase in cost and complexity of care per procedure, however, has precluded this approach from gaining wide acceptance. More recently, the use of conventional ultrasonic energy in passive and continuous ultrasonic irrigation has been shown to be as effective as the more expensive irrigation modalities.3 The addition of eccentric or 3D files to ultrasonic irrigation and disinfection using tapers limited to a range of 0.04 to 0.06 may prove to be an optimal solution for safe and effective instrumentation and irrigation that combines the disinfection qualities of activated irrigants with instrument tapers capable of shaping and disinfecting large portions of the root canal. This combination of ultrasonics and eccentric files may help improve clinical efficiency and allow instruments to clean and shape canals while enhancing the disinfecting qualities of conventional irrigants.
I first heard the old endodontic adage "instruments shape, irrigants clean" almost 50 years ago at dental school, and it is still referred to today. But does it still have relevance? There are two concepts that can be examined to determine if this saying is still meaningful.
The first concept is the answer to the question, "Why do we shape a root canal?" The answer is to create a shape that will: (1) allow a disinfectant solution (sodium hypochlorite) to work most effectively and (2) expedite obturation. In other words, a clinician shapes a root canal to make the endodontic treatment straightforward.
The other concept is that of an endodontic-restorative continuum. This refers to the intimate relationship that exists between the endodontics and the final restoration. Endodontics and restorative dentistry should not be considered separate entities. Rather, they should be seen as an intimate continuum. A dentist who is placing a post in a tooth is working in the exact same space as the clinician who performed the root canal.
Understanding these two concepts allows us to address whether the "old adage" is still relevant. I believe this axiom is still applicable today because performing root canal therapy is not about radiodontics, or how a case looks. Endodontics is really about saving the tooth and doing so in such a manner that the tooth can be completely restored. Teeth, following endodontic treatment, can last a lifetime if they are restored properly and good hygiene is practiced.
Endodontics means little if the tooth has been destroyed during the process of said root canal treatment. The key to the long-term retention of the endodontically treated tooth arguably is the preservation of critical tooth structure in the coronal third of the root (when performing the root canal) and the ferrule effect.
While most dentists agree that the root canal system is a 3D system with webs, fins, and anastomoses, there are differences of opinion regarding how to clean this system in a 3D fashion, particularly fins, which are commonplace. One approach is to increase apical and canal diameter sizes in the hope of attaining clean dentinal shavings. A concern, however, is that sometimes this extra sizing may result in significant weakening of the tooth, thereby compromising the long-term prognosis of the restored tooth.4-7 On the other hand, a properly shaped canal with a constant taper can lead to enhanced ultrasonic cleaning that will effectively debride the webs and fins while not weakening the basic structure of the tooth. In the specialty of endodontics, one of the greatest advances in the past 30 years has been enhanced 3D cleaning through the use of piezoelectric ultrasonics and advanced sonics.
Thus, in conclusion, the expression "instruments shape, irrigants clean" most certainly still applies today.
The evidence-based answer to the question, "Do instruments shape and irrigants clean?" is "yes." Success in endodontic treatment generally relies on prevention or treatment of apical periodontitis. This means that microbial infection of the root canal system must be prevented or eliminated; that is, successful root canal treatment depends on successful infection control. Although this discussion is focused on mechanical and chemical control of infection, it is important to note that success in endodontic treatment is founded on proper diagnosis and a viable treatment plan.
If root canal anatomy were simple, the task of infection control might be relatively easy. More than a century ago a German dentist named Walter Hess created elegant reproductions of the pulps of teeth.8 These specimens revealed the vast complexity of the anatomy of pulp. The key to endodontics is treating all of this pulp tissue while not destroying the restorability and longevity of the tooth while also managing the microbial flora that seeks to survive in the complex geography of the pulp.
To achieve infection control, evidence supports the need for both mechanical instrumentation and irrigation of the pulp chamber and pulp canals. Neither instrumentation alone nor irrigation alone has been proven to be adequate to remove enough tissue, necrotic debris, and infective microorganisms to control infection.9-12 Neither method by itself is sufficient to remove enough antigenic material-eg, infective microorganisms, vital and necrotic organic tissues, infected dentin-to allow for endodontic success.
For more than 60 years dentists have used stainless steel hand files to negotiate and clean root canals. Stainless steel files are known to create canal preparation errors, such as ledges, transportation, and zips. These errors can result in the canal preparation not being centered on the original canal, thereby precluding irrigants from reaching bacteria in the apical portions of canals.13 This adversely impacts the goal of thorough debridement of the canal system. The development of nickel-titanium rotary files in the 1990s helped reduce these canal preparation errors, as these files created the opportunity to maintain canal preparations centered on the original canal. If a glide path, centered on the canal, can be carefully created with stainless steel files, judicious use of nickel-titanium rotary files can produce a tapered canal centered on the original canal. This tapered space allows irrigants to penetrate to the apical portions of canals and reach anatomical complexities to act upon antigenic materials. Appropriate and effective irrigants, delivered passively, are then used to dissolve, destroy, and remove infective elements.
Many antimicrobial irrigants have been proposed, studied, and largely rejected for endodontic use. The two irrigants that have survived thorough scrutiny and are widely used today are sodium hypochlorite and ethylenediaminetetraacetic acid (EDTA). With adequate soaking time, sufficient volume, and appropriate concentrations, sodium hypochlorite has strong antibacterial activity and dissolves both pulp remnants and the organic aspect of dentin. EDTA is used to remove the smear layer of dentin and to open dentin tubules, thus further reducing a possible antigenic load and allowing sealer to flow into the tubules.
Alternate delivery systems currently under development likely will improve the penetration of irrigants within the root canal system. To date, more data is needed to be able to identify the optimal new delivery system. For the short-term future, endodontic success likely will remain focused on instruments shaping and irrigants cleaning.
Allen Ali Nasseh, DDS, MMSc
Clinical Instructor, Department of Restorative Dentistry and Biomaterial Sciences, Harvard University School of Dental Medicine, Boston, Massachusetts; Private Practice, Boston, Massachusetts
Anne L. Koch, DMD
Adjunct Assistant Professor, Department of Endodontics, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Founder and Past Director, Postdoctoral Program in Endodontics and Microsurgery, Harvard School of Dental Medicine, Boston, Massachusetts
Robert A. Augsburger, MS, DDS, MSD
Chairman and Program Director (Interim), Department of Endodontics, Texas A&M University College of Dentistry, Dallas, Texas
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