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The Clinical Challenge in Endodontic Retreatment Drs. Richard E. Mounce and Gary Glassman

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With the advent of implants, at a general dental and specialist level, endodontics has been challenged to optimize results. As an alternative to implants, to the greatest degree possible, endodontics must offer patients predictable retention of natural teeth with failed root canal therapy and those with strategic value. Endodontic retreatment is predictable, economical, more convenient and far less invasive than implants. Literature-based success rates for endodontic treatment and retreatment are virtually identical to implant therapy.

It is our empirical bias that the clinical challenge in retreatment is not primarily the mechanical removal of posts, separated files and repair of perforations, etc., but rather the treatment planning skills required to appreciate which teeth should be retreated and which should be extracted. In essence to 1. Appreciate why the treatment failed, 2. Assess the risk factors present, 3. Identify solutions that can repair existing iatrogenic events, 4. Accomplish the above without creating new iatrogenic issues and 5. Give the patient a realistic expectation of predictable clinical success. This challenges the clinician to identify vertical fractures and non-restorable teeth. In addition, the clinician should optimally be able to identify teeth that even if not fractured now, are likely to fracture with time and/or for which it is not possible to create a proper coronal or apical seal. It is our bias that in the vast majority of cases it is possible to clearly identify the existing risk factors and provide the patient with a realistic expectation of long-term clinical success.

Non-surgical endodontic retreatment (NSER) should be thought of as a means to revise failed endodontic results and bring these teeth back into function. Doing so requires that the clinician revise the aspect of treatment that was deficient, repair existing defects, and create a coronal and apical seal. NSER has several key principles and requirements. These are:

1. It is imperative that the clinician uses a surgical operating microscope (SOM) (Global Surgical, St. Louis, Missouri). The visual and tactile control available with its use cannot be duplicated by other means. In a practical manner, many of the challenges that clinicians encounter are easily diagnosed and managed through the visual information provided by the SOM. For example, if a mid-root perforation has occurred, it is a simple matter to know where the perforation has occurred and have a much better concept of what retreatment strategies will be required for its repair. Sources of clinical root canal failure are almost always immediately evident once looking under the SOM at the particular clinical defect that has caused the failure. Given the relative lack of adaptation of the SOM by the general dental community, approximately five percent versus the penetration rates in the endodontic specialist community (more than 90 percent), has led to a entirely different perception of the clinical reality of root canal therapy, be that first time treatment or NSER. In short, while loupes are helpful, they are not a substitute for the visual and tactile acuity that is possible using the SOM. For example, in attempting to locate an MB2 canal in a failed upper first molar, using the SOM it is often possible to use a bur over the MB2 to uncover the canal in lieu of ultrasonics and in the process both save time and create exactly the same desired result as using ultrasonics. For those clinicians not using the SOM at higher powers, using a bur on the pulpal floor at any time would be strictly contraindicated.

2. Radiographically, it is essential to have at least two and often three angles of a failed root canal prior to starting. Starting with one radiographic angle, even if it is digital is likely to exclude important information that would otherwise be visible. These angles should be straight on, from the mesial and from the distal. Radiographic evaluation should be comprehensive in that many visual clues to the source of failure can be obtained and will be easily missed if diligent attention to detail is not given. For example, radiolucency in the chamber is indicative of a lack of coronal seal. Small fragments of rotary nickel titanium (RNT) files might remain after cleansing and shaping procedures and the clinician might not appreciate these fragments. A careful evaluation of the entire canal space can often identity these file shards (which at times can be entirely surrounded by obturation material and only recognized by their increased opacity relative to the material that surrounds them). Similarly, small fragments of hand K files, such as #6, #8 and #10 sizes can separate which can be almost barely visible, yet during canal negotiation can frustrate the clinician when the canal is not negotiable.

Appreciating these subtle radiographic findings will make interpretation of the clinical case much easier and lead to improved long-term success rather than misinterpreting the existing signs that would otherwise guide the clinician.

3.In assessing the source of the failed root canal, it should be appreciated that root canals fail for the following reasons: (On the causes of persistent apical periodontitis: a review Nair PN Int Endod J 2006; 39:249-81.) “...six biological factors lead to asymptomatic radiolucencies persisting after root canal treatment. These are: (i) intraradicular infection persisting in the complex apical root canal system; (ii) extraradicular infection, generally in the form of periapical actinomycosis; (iii) extruded root canal filling or other exogenous materials that cause a foreign body reaction; (iv) accumulation of endogenous cholesterol crystals that irritate periapical tissues; (v) true cystic lesions, and (vi) scar tissue healing of the lesion.” For the vast majority of failed cases in clinical endodontic practice the source of failure is related to intradicular infection in the form of uncleaned and unfilled space and a lack of coronal seal that leads to persistent bacterial contamination of the root canal space (Figure 1). With regard to expectations of retreatment, the practical matter in treatment becomes trying to sort out what has allowed the bacteria to gain entry into the tooth or what defect in the treatment has led to the bacteria that remain. As a practical matter, for example, a common finding is canals that have not been located, poor irrigation, canals that have not been prepared to the correct taper, working length and/or master apical diameter (or working width to use a different term to describe the master apical diameter) and a lack of coronal seal, even if the tooth has been otherwise well-treated.

As an important and directly related aside, it is a common observation that the source of much failure in endodontics is a lack of diligent negotiation of canals with small hand files to achieve patency. In essence, at no time in the process did the clinician intentionally and with patience use #6, #8 and #10 hand K files to discover the curvature, calcification and anatomical variations that were always present within the root. As a result, lacking the correct tactile knowledge of the root anatomy, poorly matched RNT file sequences are used which often lead to iatrogenic outcomes.

The steps in retreatment are universal irrespective of the type of failure and clinical challenges. These steps are:

1. Achievement of straight-line access. Many times the source of endodontic failure can be determined easily once the access is made. Often the previous access was far too small for the given canal anatomy. This is especially true in the pursuit of the MB2 canal of upper molar teeth. The MB root of upper first molars is the root that fails most commonly. The MB2 lies off a straight line between the MB1 and the palatal canal. Most often it has not been located because the correct lateral wall access extension has not been made to uncover the MB2. The exact sequence of files to negotiate the MB2 is detailed below. It is important to realize that the exploration of the MB2 in retreatment and first time orthograde endodontic treatment is exactly the same.

In NSER, retaining the crown is always of secondary importance relative to achieving unrestricted canal access. The coronal restoration can always be remade, but compromising the apical cleansing and shaping to preserve the crown will compromise the long-term result and is to be avoided at all costs. While not all crowns must be removed, most do. Crown removal will expose many fractures, unset restoratives, missed canals, caries and other issues that if left undetected represent compromises to the cleaning, shaping and obturation quality that would otherwise be possible. Crowns are made to fit the existing occlusion, they are irrelevant to the apical location of canals. As a result, it is often necessary to expand access to well beyond what would appear necessarily just looking at the occlusal surface of the tooth.

2. The cervical dentinal triangle must be removed. This is done prior to attempting to remove gutta percha be this in the carrier base or master cone based form. Using a RNT file rotated at higher speeds up and away from the furcation to remove the CDT is ideal. This motion will minimize any chances of furcal perforation.

3. The orifice shape should be ideal before moving apically. It is contraindicated to attempt to remove gutta percha without having an ideal visual and tactile command over the orifice and canal below.

4. Gutta percha removal is passive, gentle and sequential. As a first step, heat should be used to remove as much gutta percha as possible using a downpack motion similar to the downpack of obturation materials in the SystemB technique. An excellent source of heat for this purpose is the Elements Obturation Unit* and its accompanying SystemB heat tips. The Fine or Fine Medium heat tip can be used in the SystemB downpack motion to great effect. This action alone can often clear approximately half of the gutta percha from the canal.

Subsequently, a RNT can be used to clear the remaining bulk of gutta percha. Insertion of the RNT is passive and gentle and done at enhanced speeds. In practice the RNT is rotated at between 900 and 1500 rpm. Insertion is highly controlled. The RNT is never allowed to drop into the canal without advancing the file by intention. When undue resistance is encountered the clinician should back the file out of the canal. If the clinician keeps an awareness of the expected true working length and does not keep pushing to move the RNT apically when it will not advance easily, a host of iatrogenic issues can be avoided (canal transportation, file separation, perforation, etc.) In purely clinical terms, if the clinician is advancing a RNT file at 18mm into a canal which gives resistance to file advancement and the expected true working length is 21mm, the clinician should stop the attempted advancement and switch to the third tier of strategies for gutta percha evacuation, the use of chloroform.

Chloroform is inserted one drop at a time to create a slurry in the canal and soften the apical remnants of the previous root canal filling. The hand K files used to create the slurry should be precurved and their length and diameter matched to the particular indication. Specifically, if the gutta percha that should be dissolved is at 18-21mm, the hand K file should be 21mm and precurved to negotiate any canal curvatures present. For an average canal, if patency can be achieved easily, the gutta percha softens easily and a minimal number of successive single drops of chloroform are needed. It has value to use paper points to soak up excessive gutta percha that will form in the resulting slurry. At all costs, the gutta percha slurry should be kept within the tooth and prevented from being extruded beyond the apical tissues. This can be accomplished by carefully monitoring how deep the files are within the roots at all times and preventing excessive amounts of chloroform (more than one drop at a time) being placed in the canal. Use of the SOM makes this easy to monitor and accomplish.

In such canals, the clinicians would never place a RNT file to the apical extent of the negotiation without assuring themselves that they had both achieved apical patency and had a confirmed true working length. In the clinical scenario described here, the clinician would monitor when they reach the estimated working length and attempt to feel for a tangible “pop” with hand K files. Once the “pop” is felt the clinician would then place an electronic apex locator onto the hand K file after drying the canal and get an electronic apex locator length. Cleansing and shaping would proceed as per usual at this point in the process.

6.5. We use the Twisted File* (TF) for its simplicity, cutting efficiency, flexibility, fracture resistance and ability to shape the canal to larger tapers with fewer files than ever before as well as prepare larger apical diameters as desired (Figure 2). In the clinical scenario above once the true working length and patency were established in the MB2 canal, a final canal taper of .08/25 would be prepared. In roots with and without previous gutta percha obturation, the .08 can be most often inserted to the true working length in 3-4 insertions and is followed by the .06/30 TF, .06/35 TF and the .04/40 if desired. In addition, it is noteworthy that TF is available in a .04/50 variety. The TF sequence described here is the essence of simplicity and efficiency because in addition to the larger tapered preparation possible with TF, the achievement of larger apical diameters occurs in a single insertion of the .06/30 TF, .06/35 TF and higher as needed.

After preparation to the enlarged master apical diameter and taper with TF, the canal can be easily obturated with a bonded technique such as RealSeal* master cones with SystemB or the RealSeal One bonded obturators.* In more than a theoretical way, a bonded obturation material can help resist the coronal leakage. Gutta percha has no ability to prevent the migration of bacteria, does not bond to sealer, does not bond to dentin and requires the use of a coronal filling to protect it against bacterial contamination. RealSeal has been shown in a number of in vitro and in vivo studies to resist coronal leakage in a statistically significant manner relative to gutta percha, a definite indicator of improved long term success relative to cases completed with a poor coronal seal obturated with gutta percha (Figure 3, page 58).

7. In the clinical case example above, to address the MB2 canal, once dentin is removed over the MB2, its initial management is critical relative to accomplishing the goals of cleaning and shaping of this delicate space. Specifically, it is imperative in the MB2 to begin the exploration of the canal with a precurved #6 hand K file inserted with the most gentle touch possible. At all costs the clinician must avoid forcing debris apically or compacting pulp tissue into the apical regions of the canal. If the hand K file resists advancement, it should be removed and the canal irrigated and a new #6 hand K file inserted. If the canal will allow insertion, it should be taken apically. For an average MB2 it will take anywhere between one to three packs of #6 hand K files to gain apical patency. Once the #6 hand K file reaches the true working length, a #8 hand file is inserted and the canal enlarged. This sequence of files is repeated until the canal is open and negotiable to the diameter of a #15 hand K file. This enlargement is much simpler and more efficient with the reciprocating M4 Safety Handpiece.* While a comprehensive discussion of the M4 is outside the scope of this paper, once the hand K file reaches the apex, the M4 is placed onto the file under the rubber dam. With M4 reciprocation, an MB2 can be enlarged from a #6-15 hand K file diameter in approximately two minutes or less. Figure 4-6.

Clinical decisions that should be made in treatment planning and revision of failed root canals are presented above. Emphasis on restorability and the source of failure is indicated. Risk assessment in retreatment is essential. Prior to access, a set of clinical solutions is decided upon to deal with the risk factors present. Achieving patency, removing obturation materials passively, preparing the correct final taper and master apical diameter are all key components of long term retreatment success. We welcome your feedback.

*SybronEndo, Orange, California
Author’s Bios
Dr. Richard E. Mounce is the author of the non-fiction book Dead Stuck, “one man’s stories of adventure, parenting, and marriage told without heaping platitudes of political correctness.” Pacific Sky Publishing. DeadStuck.com. Dr. Mounce lectures globally and is widely published. He is in private practice in Endodontics in Vancouver, WA, USA.

Dr. Gary Glassman graduated from the University of Toronto, Faculty of Dentistry in 1984 and was awarded the James B. Willmott Scholarship, the Mosby Scholarship and the George Hare Endodontic Scholarship. A graduate of the Endodontology Program at Temple University in 1987, he received the Louis I. Grossman Study Club Award for academic and clinical proficiency in Endodontics. The author of numerous publications, Dr. Glassman lectures globally on endodontics and is on staff at the University of Toronto, Faculty of Dentistry in the graduate department of endodontics. Gary is a fellow and endodontic examiner for the Royal College of Dentists of Canada, and the endodontic editor for Oral Health dental journal. He maintains a private practice, Endodontic Specialists in Toronto, Ontario, Canada. He can be reached through his Web site www.rootcanals.ca.
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