Written by Richard Mounce, DDS Friday, 30 April 2004 19:00
Like many of my dental colleagues, I have occasionally done what I affectionately call the hallway dance. This is not some new rave dance, but the elation that comes from having some small clinical success that literally makes me want to get up and do a dance in the hallway. Getting out a stubborn silver cone (SC) has been the source of more than one hallway dance in my career. While my staff may think I've lost my marbles, I tend to look at every SC re-treatment as a possible opportunity to show my stuff.
SC root canal treatment (RCT) was most popular in the 1950s and 1960s and endured until the late 1980s. SCs are still seen clinically on a daily basis. They often present an endodontic treatment planning challenge. For example, is it indicated to leave an asymptomatic SC RCT alone if the tooth requires a new coronal restoration? What are the factors that would argue for its re-treatment or for it being left in place? Would it matter if the SC RCT under consideration was the terminal abutment of a long-span bridge? What is the chance that the SC RCT will fail later if no obvious pathology is present now and the patient is asymptomatic? Does the presence of the SC alone predispose the RCT to failure?
The purpose of this article is to address the indications for SC re-treatment, answer the questions above, and provide basic strategies for SC removal.
CLINICAL USE OF THE SILVER CONE
|Figure 1a. A single, long silver cone that was removed with bypassing and forceps via small K files. A solvent (chloroform) was used to dissolve the existing gutta-percha and sealer that remained around the cone. After bypassing, the cone was delivered with a forceps.||Figure 1b. Completed case.|
|Figure 2a. Multiple silver cones fractured within the same lower molar. This tooth represents a significant re-treatment challenge requiring multiple techniques. Of important note, this case would not be possible to treat without a microscope since the mesial cone mimics a separated instrument as does the apical cone in the distal root.||Figure 2b. Completed case.|
SCs conform to standardized ISO sizes and taper. Historically, they were rapidly accepted by dentists and employed due to their ease of placement and length control. The rigid yet flexible nature of SCs permitted accurate placement in small, curved canals such as the mesial-buccal roots of upper molars and mesial roots of lower molars. They can be wedged very tightly with significant frictional retention with or without cementation.
By today's standards, the average SC preparation would be considered underprepared. SCs may have given past operators a false sense of security since the cone could fit into root canal spaces with minimal preparation and radiographically extend to the apical foramen. In other words, SC RCTs often looked acceptable radiographically, but in actuality they did not obturate the root canal space from the canal orifice to the minor constriction of the apical foramen in 3 dimensions. As a result, they did not fulfill the biologic requirements of modern endodontic therapy. Historically, while some SC cases were clinically successful, many were not. The failure rate has been so high over time that in some quarters SC RCT is considered a temporary root canal treatment because of its corrosion, lack of 3-dimensional (3-D) obturation, and aforementioned poor success rate.
SC RCT technique suffered 4 major drawbacks:
(1) SCs are round; canals are not. SCs cannot be molded or thermally softened into all of the ramifications of the root canal system for 3-D obturation. If SCs provide a true seal at all, they do so over only a very small distance of the canal, which most often is not in the apical third. The net effect is that a high potential exists for bacteria, corrosion products, or debris, which can percolate into the apical tissues from the untreated space alongside the SC.
(2) Exposed to coronal microleakage, SCs corrode. Corrosion is most often a result of tissue fluids interacting with the SC to form silver salts, which are toxic to periapical bone. Once removed, the vast majority of SCs are in fact corroded. Any moisture left in the canal before obturation as a result of improper drying can also cause corrosion. Gingival tattoos have been reported as a result of severe corrosion of failing SCs. In this author's experience of re-treating numerous SCs, only one has ever been taken out of a tooth without evidence of corrosion. Such corrosion products will and often do migrate into the apical bone, with resultant inflammation and abscess formation. Interestingly enough, many asymptomatic and radiographically normal SC-treated teeth, once accessed, are covered with "soot", which is a combination of corrosion products along with dissolved bases, cements, sealers, liners, and buildup materials that were used as a restorative.
(3) Once placed, post space preparation is virtually impossible without disturbing the apical seal of a SC.
(4) Retreatment of failed SCs can be very simple, complex, or both in the various canals of the same tooth. Sometimes SCs seem literally to jump out from teeth with ease, while others are embedded irretrievably. It is often difficult to predict which SCs will allow straightforward removal and which will pose significant re-treatment challenges. That said, the larger the cone, the larger the canal, and the more voided space alongside the SC, generally the more simple and feasible is the removal. Conversely, narrow, significantly curved, and long SCs tend to pose much more difficulty, all things being equal.
SILVER CONE RE-TREATMENT
SC re-treatment is complicated by numerous factors (several among many are reported here), including the following:
(1) Whether the SC was amputated at the orifice level when it was placed and the amount of the SC's head emerging from the canal orifice.
(2) Whether a cementing medium was used.
(3) The size of the SC.
(4) How long and curved the canal is.
(5) How tightly wedged the SC is into the narrowing cross-sectional diameter of the canal.
(6) How long the SC is (ie, is it a tiny fragment or a complete cone extending the length of the canal?).
(7) If fragmented, where in the canal the fragment of the SC rests.
(8) If there are multiple SCs in the same canal.
(9) The location of the tooth (a second molar is harder to reach than a central incisor) and the patient's ability to open.
Re-treatment of failed SC treatment represents a large subspecialty within the field of endodontic re-treatment and is too large a topic to be covered in detail here. That said, a summary-style review of basic methods for its removal is instructive as to its characteristics and is detailed as follows:
(1) Keep as much of the head of the silver cone intact in the chamber as possible. Access into a bulk of amalgam or composite that surrounds a SC is best performed with an ultrasonic tip and done under a microscope so as not to touch the SC and to prevent the accidental removal of the protruding head of the SC above the canal orifice.
(2) If the SC does not want to come out easily with a pair of cotton pliers or forceps, don't force it or apply undue pressure. Ultimately, many of these points are removed with a very light and deft touch. Too much force simply severs the cones (especially small ones) at or below the canal orifice, then removal becomes very much like the extraction of a separated file, with its attendant clinical challenges.
(3) Many SCs can be removed with bypassing. The technique requires employing small K files placed slowly, deliberately, and gently alongside the SC and creating an ever larger space within the canal into which the SC can be delivered and moved coronally. The 6-20 K files (or larger) can often be placed successively into the more apical regions of the canal, making use of the space created by the discrepancy between the perfectly round SC and the irregular shape of the canal. If significant sealer is present, this bypassing may need to take place in the presence of a solvent like chloroform. Once a 10 file can spin freely alongside a SC, an increased mobility of the cone can often be observed, and the SC is usually ready to be delivered with a cotton plier or forceps. Most often, large SCs in large canals (palatal roots of upper molars and the distal roots of lower molars) can be delivered fairly easily with this method if sufficient head exists for a purchase. Narrow and delicate SCs possess greater frictional retention, are much harder to deliver, and often require additional advanced techniques beyond those described here.
In addition to the above, once the SC is bypassed and a 10-20 K file can spin freely alongside the SC, a Hedstrom file slid into the available space allows a purchase on the cone along its length to tease it upwards. While it is also possible to wrap multiple Hedstrom files around a SC to try to remove the cone, this is usually only practical in a very large canal.
(4) Using the cutting edge of a small spoon as a lever and applying a gentle upward force can often pry a relatively loose SC from a canal. If the SC resists coronal movement with gentle pressure, it is important to stop and begin another method of removal since the cone can easily be pinched off at the orifice with this method.
(5) Applying ultrasonic vibration to cotton pliers or forceps that are engaging a SC is another method possible for breaking both the cement seal and the SC's frictional retention. It may be necessary to vibrate the SC for as short as a few seconds to as long as several minutes to allow the cone to become loose if the method is to be successful at all.
(6) Consideration should be given to significant irrigation with sodium hypochlorite, attaining patency beyond the most apical extent of the previous SC level, and placement of calcium hydroxide in the canal for a 2-visit treatment. Adequate cleansing, shaping, and sterilization of the root canal space is enhanced by the antibacterial effect of both the hypochlorite and calcium hydroxide in combating the presence of corrosion products and debris that might prove difficult to remove from delicate apical anatomy in single-visit treatment.
(7) Advanced SC removal, while beyond the scope of this article, is highly predictable with the surgical operating microscope. It may require ultrasonics, the Cancellier technique and kit, the Ruddle post removal kit (both from SybronEndo), and special spoon excavators and forceps. If removal is not easily forthcoming, referral is indicated. It must be emphasized that removal of SCs is often a combination of all of these methods performed repeatedly. Figures 1a and 1b and Figures 2a and 2b illustrate 2 SC re-treatment cases that were addressed by the above methods.
With the above important information in mind, what do we do with SCs in clinical practice? Specifically, the following questions need to be addressed:
(1) Is it indicated to leave or re-treat an asymptomatic SC RCT if the tooth requires a new coronal restoration? What factors should be considered when deciding to re-treat the case or perhaps leave the SC RCT alone? If there is any indication whatsoever that there has been coronal leakage, apical pathology, clinical symptoms, and/or significant uncleaned and unfilled space within the canal system, then the tooth should be re-treated before the new restoration.
(2) Would it matter if the tooth under consideration in question No. 1 was the terminal abutment of a long-span bridge? In this author's opinion, yes. A strategically important tooth should be given greater consideration for re-treatment even in the absence of apical pathology and symptoms, rather than hoping the patient is subsequently pain-free. The risk of long-term failure and possible loss of the new bridge outweighs the inconvenience of a short-term re-treatment.
(3) What is the chance that the SC RCT will fail later if there is no obvious present pathology? All things being equal, fairly high. That said, while giving a definitive statistical number isn't possible, if a tooth did not need a new coronal restoration and lacked apical pathology, it can be observed indefinitely, assuming that the factors discussed above in question No. 1 are addressed first.
(4) Does the presence of the SC alone predispose the RCT to failure? To the degree that the SC will corrode in the presence of any moisture, yes. In addition, since there is uncleaned and unfilled space alongside the SC in the vast majority of SC RCTs, a significant etiology exists that encourages failure over the long term.
SC RCT fell out of favor when it became apparent that the technique did not fulfill the biologic objectives of root canal treatment. Proper diagnostic assessment of SC RCT and subsequent re-treatment of SC failures are essential for proper long-term management of this clinical finding and are a cornerstone for successful restoration of these teeth. As an endodontist, aside from the privilege of serving the patient, SCs have given me the pleasure of a hallway dance on more than one occasion when a particularly difficult SC sees the light of day, much to the delight of the patient, my staff, and me.
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