Why Endodontic Cases Fail

Things that come to mind include missed canals, insufficient instrumentation, inadequate obturation, poor restorations and fractures. Missed canals result from assumptions based on limited x-ray angulation. Missed canals and insufficient instrumentation both result in leaving pulp tissue and a source of nutrition for bacteria that may be present. Inadequate obturation and poor restorations are a cause of leakage and potentiate future breakdown in the form of reinfection and decay. Fractures often resulting from poor restorations are most often a terminal form of leakage that requires extraction.

By categorizing the causes of endodontic failure, we take the first steps in reducing its occurrence. What are the best steps we can take to make sure we are not missing canals? X-rays at different angles will often expose the ligaments of canals that we did not see with straight on x-rays. An x-ray of a molar taken from the distal will displace the mesio-buccal and disto-buccal canals mesially separating them from their lingual counterparts. Taken from a mesial angle, the mesio-buccal and disto-buccal canals will be displaced distally if indeed there is a differentiation to be made. The schools have made rules to define where the displacement occurs dependent upon the original x-ray angle. I prefer to think of the x-rays as a flashlight concentrated on two fingers held together vertically, one behind the other. If the source of light is coming from the right it displaces the forward finger to the left. If the source of light is from the left the forward finger is displaced to the right. It’s that simple and requires no memorization of rules.

A more thorough way to check for missed canals is to take a CBCT. I am all for this approach when contemplating retreatments. For example, if a patient presents with a radiographic area on the mb root of a previously treated maxillary molar, my first instinct is to think missing mb2, but I want to confirm that suspicion with a CBCT. To retreat the mb root on the assumption that an mb2 is present can lead to needless weakening of the root especially if an mb2 is not present. The added advantage of a CBCT is the more complete picture of bone loss the coronal view provides demonstrating whether or not the buccal and lingual cortical bone plates remain intact. If not, the expectation of success is significantly reduced.

Insufficient instrumentation I believe stems from two likely causes. One, we have a tendency to think in 2 dimensions with the mesio-distal width of the canal representing one dimension and the length of the canal from orifice to apex representing the other dimension. If we produce a white line representing obturation of these two dimensions, the work not only can look excellent and often is, but leaves the possibility that all the pulp tissue and associated bacteria have not been removed in the bucco-lingual plane. With the advent of micro-ct scans, we now know that the bucco-lingual dimensions of the canal are often far wider than their mesio-distal counterparts. Unless we actively appreciate the third bucco-lingual dimension, we are likely to limit instrumentation to what is typically the thinner mesio-distal plane producing impressive x-ray results, despite the inadequate attention paid to the pulp tissue and bacteria that may still be residing in thin isthmuses as well as buccal and lingual extensions of highly oval canals.

 The other reason for inadequate instrumentation of the bucco-lingual dimension is the fear that rotary instruments will separate if aggressively applied to the buccal and lingual extensions. A list of precautions to prevent instrument separation within a canal includes staying centered with minimal deviation from that centered position. I can understand the incentive to stay centered. Centered instrumentation will produce a beautiful mesio-distal x-ray result. Crown down instrumentation assumes that larger coronal preparations will encompass more of the bucco-lingual dimensions producing a greater amount of cleansing and debridement. That is a correct assumption, but the price of that assumption is a needlessly weakened tooth.

The way around this dichotomy is the use of stainless steel reamers, discussed in a previous post, in a 30o oscillating handpiece that virtually eliminates instrument separation under the most strenuous applications to the canal walls. The thinnest stainless steel reamers, oscillating at 3000-4000 cycles per minute can be fearlessly applied to the buccal and lingual walls. In fact a uniform removal of dentin will produce a three-dimensional shape that mirrors the original canal anatomy in larger form. It is cleansed three-dimensionally through a process that I like to call “internal routing”.  That cleansing process is accentuated by the sonic action of the oscillating handpiece not only removing a uniform amount of dentin from all the canal walls, but activating the irrigants to maximize their effectiveness. For me, this is the best way to create a glide path that will then be further enlarged via a rotary system that also has been designed to prevent separations. 

Obturation is far more predictable when the canals have been three-dimensionally cleansed. I make no attempt to maximize the amount of gutta percha in the canal. What I do want is a canal flooded with an epoxy-resin cement that bonds physically and chemically to both the master gutta percha point and the canal walls and is dimensionally stable. The purpose of a well fitted gutta percha point is to prevent its extension beyond the confines of the root. It acts as a carrier and driver of the cement. The buccal and lingual spaces, wider than their mesio-distal counterparts will be filled with varying thicknesses of sealer. Given the cement’s resistance to fluid degradation this presents no down sides. What is needed is a returnable path to the apex should the tooth require future retreatment. The single gutta percha point accurately placed to the apex provides that pathway.

 The final step of restoration in a sense is the most crucial. I have seen too many cases where the tooth subsequently fractured, rendering it now unrestorable because the restoration after treatment was never done. Typically, in posterior situations we are best off with full coverage, excellent margins and a stable ferrule. The shear forces are often too great to leave it with simply a filled access. It is one of the more depressing things to see a patient returning several months later with the complaint that something is moving only to find the tooth now split and now a candidate for an implant.

So the goal is thorough debridement and cleansing coupled to the conservation of tooth structure leaving a tooth more resistant to fracture and reinforcing that goal with a strong restoration. Knowing what causes endodontic failures is the best way to devise methods that prevent it.

 Regards, Barry