The clinical practice of root canal treatment has changed markedly over the last 20 years. The introduction of rotary instrumentation, of various new obturation systems, of ultrasonically powered devices, and of magnification systems (in particular operating microscopes) has significantly increased the success rate of primary endodontic treatment and endodontic surgery [3, 4].

Perhaps, the most important difference today is that an endodontist’s manual dexterity is no longer the prime determinant for success. Obviously, individual skill always makes a difference, but with the use of modern endodontic techniques, levels of excellence are seen that were unheard of in the past [5, 6]. Nevertheless, a significant portion of the workload of endodontic specialists continues to be dedicated to redoing failed endodontic treatments.

Implantology can deliver high success rates [7] that might make extraction and replacement by implants seem a valid alternative to endodontic retreatment. However, it should be stressed that the two types of treatment have different goals: endodontic treatment aims to prevent or treat apical periodontitis, whereas implants are used to replace missing teeth [8]. This means that retreatment is the approach of choice for cases with endodontic failure, in which the tooth can be salvaged, both from restorative and periodontal standpoints.

In a recent US epidemiological study by Salehrabi and coworkers on a population of 4744 teeth that had undergone orthograde retreatment by endodontists, the success rate after 5 years was 89 % [9]. Earlier, Fristad and coworkers analyzed the success rate in 112 roots retreated by undergraduate students after 10–17 years, and after 20–27 years. The success rate at 10–17 years was 85.7 %, but 10 years later this had risen to 95.5 %, equaling the success rate found in cases of first treatment [10]. The study authors conclude that the persistence of asymptomatic periapical radiolucencies, especially those with overfill, should generally not be classified as failures, as many of them will heal after an extended observation period. This statement is of particular interest in increasingly easy access to cone beam computed tomography (CBCT). It has been shown that considerably more (asymptomatic) lesions are detected on CBCT scans rather than on periapical radiographs of endodontically treated teeth [11]. Likewise, in a systematic review, Torabinejad reports that endodontic surgery appears to offer more favorable initial success, whereas nonsurgical retreatment leads to a more favorable long-term outcome [12].

Planning endodontic retreatment requires a correct diagnosis using clinical and radiographic findings (see Chap. 2 in this book). It is necessary to understand the cause or causes that have led to the failure of the prior treatment and determine the feasibility of salvaging the tooth restoratively and periodontally. Prognostic factors of individual cases must be combined with a diagnosis to formulate a correct treatment plan.

The secret of achieving high success rates in retreatment undoubtedly lies in understanding typical causes of failure of the original endodontic treatment and will be discussed in the following section.

A root canal system may be highly complex, especially that of molar teeth (see Chap. 1). In many cases, complete mechanical treatment of such root canal systems is virtually impossible. Two different types of incomplete treatment of the root canal system exist:

Historically, it has been held that inadequate apical seal may be a cause of failure in nonsurgical endodontic treatment [13]. It is indeed impossible to remove all the bacteria, toxins, and pulp tissue from the root canal system [14], and therefore tissue fluid (via the apical foramen and other foramina) may reach an incompletely filled canal space so that bacterial contamination persists and apical periodontitis is the consequence. Thus, to achieve long-term success, the root canal system should be filled as completely as possible.

Inadequate seal may occur in four cases:

According to clinical studies, vertical root fractures account for approximately 3 % of failures in teeth treated endodontically [15] and for 11 % of failures in fixed prostheses after 18 years [16]; of teeth requiring extraction, up to 32 % displayed vertical root fractures [17]. The extracted, fractured teeth were mostly root canal treated and associated with posts and screws. Diagnosis may be straightforward if the fracture can be identified in the coronal portion of the tooth in question: typically probing reveals a very narrow periodontal pocket along the fracture line (Fig. 9.6).

Conversely, diagnosis is far from simple for a vertical root fracture that starts from the apical portion of the root. In these cases, there is initially no periodontal probing; however, a radiograph may show a radiolucent area along the lateral aspect of the root, or it is visible in a small field of view CBCT. In cases of a confirmed fracture connecting periodontal and pulpal spaces, nonsurgical retreatment is not recommended [18]. Endodontic surgery (see Chap. 10) may confirm or deny root fracture and provide means to improve the apical seal of the root canals (Fig. 9.7).

Loss of coronal seal is a frequent cause of failure some years after endodontic treatment. Magura and coworkers state that canals that have been obturated completely and exposed to the oral environment for 3 months or longer may need retreatment; if the root canal space had been prepared for a post, and if the root canal filling was incomplete, then the time lapse before clinically significant leakage occurs could be shorter [19].

In generating a treatment plan, the endodontist must next analyze the prognostic factors that may influence the treatment plan (Box 9.1).

To undertake retreatment without having clearly understood the causes responsible for failure of the original treatment is a serious mistake. While persisting or newly introduced microorganisms are the main cause of apical periodontitis [20], it is impossible to improve a situation without knowing where it went wrong in the technical aspects of treatment [21]. In difficult situations, it is not uncommon to think of the most complex and unusual causes (unsealed lateral canals, extra canals, etc.) and fail to see the obvious. The four causes of failure are listed above, and the specific cause or causes responsible for failure in individual cases must be determined. Naturally, an endodontist must have an in-depth knowledge of root canal anatomy for each molar, including possible anatomic variables, and he or she must perform a complete and in-depth clinical and radiographic assessment (see Chaps. 1 and 2 in this book).

As with primary endodontic treatment, the presence of a preoperatively detected periradicular radiolucency in retreatment cases markedly reduces the success rate [21, 22]. In a critical review of literature published between 1970 and 2004 that sought to explore the level of evidence relating to the outcome of endodontic retreatment, Paik and coworkers [23] found that all included studies agreed in that the success rate was inferior in the presence of a periradicular lesion prior to retreatment; they also found that this difference varied greatly:

This prognostic factor is certainly one of the most problematic one. When dealing with the endodontic failure of a molar that has been treated satisfactorily from the endodontic standpoint (correct working length, appropriate shape, density of filling material), the most common error is to embark upon retreatment in the hope of improving the quality without taking into account that a possible cause may be a vertical fracture as discussed before. Chevigny and coworkers report an 88 % success rate for retreatment in cases where prior treatment was unsatisfactory, but only 66 % where prior treatment was adequate [22].

The presence of ledges, perforations, false pathways, stripping, internal or external transportation of the foramen, mineralizations, broken instruments, apical resorption, or internal resorption are all factors that complicate retreatment and may jeopardize its success (Fig. 9.8). In a study on 452 teeth retreated after 2 years, Gorni and coworkers [21] found a significant difference in terms of success rate in cases in which the root canal morphology was altered, and in the presence of a periradicular lesion: they report 84.4 % success in the absence of iatrogenic alterations versus only 40 % in the presence of alterations of the root canal system.

It is important to distinguish between two different periodontal problems. One is primary: the patient is affected by periodontal disease of some degree of severity; he or she has numerous affected sites, and one of these is the molar scheduled for retreatment. As was already seen, in this case, a careful periodontal evaluation is necessary to evaluate whether the tooth can be salvaged, or whether it would be better to extract it and place an implant. The second eventuality is that in which there is a primary endodontic lesion with secondary periodontal issues, often as a consequence of iatrogenic damage (perforation, stripping, etc.; see Fig. 9.9). In these cases, the success rate is significantly reduced [24].

As stated above, an evaluation of epithelial attachment must be made, even before radiography. Narrow probing reaching the apical third is very probably the consequence of a vertical root fracture. The presence of a single periodontal pocket, possibly in the furcation of a molar retreatment tooth, casts serious doubt on the possibility of salvaging the tooth for a long term. Another sign that points toward a vertical fracture is the presence of sinus tracts in unusual locations, for example, closer to the marginal gingival or on both the buccal and lingual aspects [25].

Lack of coronal seal at the time of retreatment undoubtedly complicates the already complex RCS infection. Kwang and Abbott [26] state that “The restoration-tooth interface is a potential pathway for bacteria to enter teeth and infect the root canal systems.” Complete removal of an existing restoration insertion of a leakage-proof temporary buildup is often the only way to ascertain that no new infection will occur during endodontic retreatment. Infection in retreatment cases may or may not be more difficult to eradicate, because it is sustained by streptococci, enterococci (e.g., Enterococcus faecalis) and yeasts [20, 27] existing in a biofilm, enabling them to develop effective protection against disinfectants [27, 28]. More recently, however, evidence has emerged that the microbial flora at the apicalmost portion of the root canal is similar in retreatment cases and initial treatment; this suggests that the inability to disinfect all canal space rather than specific microorganisms plays a role in failing endodontic treatment [29].

Because retreatment procedures frequently involve thinned root walls and altered anatomy, the endodontist’s level of experience is important, both in the initial phases of evaluating the case and formulating the treatment plan, and in the subsequent technical phases of executing the endodontic retreatment [30]. An experienced endodontist will be able to remember 15–20 other cases similar to the one being tackled, the difficulties encountered, how they were overcome, the end result, and the long-term outcome. A beginner in the specialty has the advantage of open-mindedness, but will require additional time to prepare to avoid irreparable damage of compromised teeth. Chapter 8 of this book provides a detailed discussion of decision-making in endodontics.

Whether in nonsurgical or surgical retreatment, both the operating microscope and ultrasonic tips are irreplaceable devices that can make a difference, significantly raising the treatment’s success rate. Setzer reported a success rate at 1 year of 88 % in surgical endodontic treatments performed with modern techniques, but without the help of enlargement, or only with magnifying loupes. The success rate rose to 94 % if surgery was performed under the operating microscope [31].

It is very important to have a consultation appointment with the patient, during which the endodontist illustrates costs and benefits together with any mishaps that could occur during treatment. The various phases of the procedure should be briefly explained and an estimate of costs given. The endodontist is under the obligation to list any alternative treatments, pointing out their advantages and disadvantages. It is imperative to understand the value of the tooth for the patient. A patient must never be urged into a decision: he or she must be sure that retreatment is the right choice and a good economic investment. To undertake retreatment in a nonmotivated patient is very stressful for the endodontist, and in consequence, it will be difficult to achieve a satisfactory outcome.

Nonsurgical retreatment is in principle considered the treatment of first choice, in particular, if this better addresses the cause of the treatment failure. Only when attempted nonsurgical retreatment is not feasible or was performed with no satisfactory result, should one select surgical treatment (see Chap. 10 in this book).

The clinical strategy in root canal treatment is to shape, clean, and fill the root canal system. Thus, it seems reasonable to assume that when nonsurgical retreatment addresses the entire canal space, cleaning it and densely filling it, success rates comparable to those possible with the primary endodontic treatment may be achieved. Studies point out better outcomes of surgical retreatment if it is preceded by nonsurgical retreatment that disinfects and seals the canal system successfully [32].

With this premise in mind, there are some cases in which surgical retreatment is indicated as first choice:

We may conclude that retreatment is indicated when there is a periradicular lesion that developed after the first endodontic treatment, when the lesion is enlarged or appears unchanged for more than 4 years, when the tooth is symptomatic, when the coronal seal has been lost for longer than 3 months, and when prosthetic restoration is necessary in the presence of an inadequate endodontic treatment, even without periradicular lesions and/or symptoms.