Root canal re-treatment. Part 1: why and when?

Root canal treatment offers a predictable outcome in the majority of cases; however, some treatments will fail, necessitating further intervention, which may include extraction. This article is the first in a two-part series and summarizes how the outcome of root canal treatment (RCT) is assessed and, in the event of failure, how to determine when re-treatment is the most appropriate option, with a focus on suitability for re-treatment in general dental practice. The second part of the series will provide practical clinical advice and solutions for the management of re-treatment, maximizing the likelihood of success.¹

Why might root treatment fail?

There may several reasons for failure, which can be categorized as technical (procedural) errors, biological factors or others.

Technical errors

Technical errors may occur at any stage of RCT, broadly classified as access, shaping, disinfection, obturation and restoration. Pre-operative assessment by way of a thorough history, examination and appropriate imaging is critical in identifying potential iatrogenic errors (Figure 1). This will enable appropriate planning, and if appropriate, referral for tier 2 or 3 treatment.²

Both ledge creation (24.8%) and perforations (2.7%) are frequently encountered when revising an endodontic case (Figure 2).³ Such errors are more common in curved canals, with up to 58.4% and 7.8% of curved canals having ledges and perforations, respectively.³ File fracture is also more common in curved canals, and an increased incidence is seen in rotary files compared with hand files (1.68% vs 0.25%).⁴ File fracture most commonly occurs in the apical third (82.7%), creating a significant challenge for file retrieval, establishment of patency and satisfactory disinfection prior to obturation.⁴

Vertucci⁵ highlighted the complexities and varying configurations of root canal systems, with 93% of upper first permanent molars having four canals and 42% mandibular incisors have a second canal.⁶ Therefore, inadequate detection and endodontic management of a canal can contribute to persistent symptoms and failure of treatment. Other challenges, such as the presence of significant calcification and excessive root curvature can be identified pre-operatively via imaging to allow referral of difficult cases. Not achieving canal patency may reduce success two-fold.⁷

An aseptic technique is critical in endodontics, and the use of rubber dam has been associated with significantly higher success rates in both primary and secondary RCT.⁸ It is beneficial to establish which irrigant has been used previously, and whether a rubber dam had been used. Rubber dam is becoming ‘the norm’, with a recent survey finding 88%, 87% and 83% of UK general dental practitioners always use rubber dam for posterior, premolar and anterior endodontic treatment, respectively.⁹ In addition to preventing re-infection of canals, rubber dam use allows the use of an appropriate irrigant, such as sodium hypochlorite (NaOCl). Disinfection of canals is an essential component of treatment, and which may be subject to failure if inadequately undertaken.^10,11 There is considerable variation of technique in primary dental care throughout the UK, with a lack of ‘activation’ (which is discussed further in Part 2) recently reported, although sodium hypochlorite is now used by the majority of practitioners.¹² Systematic reviews have found similar efficacy and bacterial reduction following the use of NaOCl or chlorhexidine digluconate (CHX).^13,14 NaOCl use is advised at a minimum concentration of 1%, while CHX is bactericidal only in concentrations >2%, and therefore mouthwashes containing CHX, which are typically 0.12–0.2%, are inadequate.^15,16 NaOCl has the additional benefit of achieving pulpal dissolution providing a strength of 1% or greater is used, which may enable better access of irrigant into lateral canals. Lower concentrations, or other non-bactericidal irrigants, such as local anaesthetic, distilled water or saline, will not eliminate bacteria and will allow their persistence.¹⁵ Optimal concentrations of 1–5% NaOCl are suggested, but towards the lower end of this range, it is recommended to use higher volumes, activation techniques and longer periods of time to account for reduced effectiveness.^15,17

CHX and NaOCl should not be used in combination owing to the formation of parachloroanaline. This is a carcinogenic and staining, insoluble precipitate that is challenging to remove and can block canals and tubules,¹⁸ reducing the outcome for treatment.

An important and frequently overlooked aspect of disinfection is ‘activation’ of the irrigant.¹

Once prepared and disinfected, obturation technique can also contribute to failure. Extrusion of filling material can contribute to apical periodontitis or apical abscess formation and has been associated with a 62% reduced success rate.⁷ Filling short will also negatively affect outcome (although less so than over-extended root fillings), with every 1 mm of un-instrumented canal short of the apex resulting in success rate reducing by 12% for non-vital cases.⁷

The coronal restoration placed following completion may be equally or more important than the obturation itself in terms of tooth survival,¹⁹ with a high-quality coronal restoration being shown to increase success 11-fold.⁷ Restoration as soon after definitive obturation as possible helps to minimize fracture risk and microbial microleakage. The loss of a marginal ridge in posterior endodontically treated teeth (ETT) would normally necessitate cuspal protection through the placement of an indirect/direct crown or onlay.²⁰ In addition to reducing the risk of catastrophic fracture, such restorations will improve coronal seal.

Biological factors

A resistant microflora may result in failure.²¹ Residual species found in failing ETT with apical periodontitis were found to be different from species associated with untreated teeth.^10,15,22 These are less susceptible to antimicrobial agents, including non-setting calcium hydroxide, and may persist within canals, although appropriate irrigation techniques may minimize this risk. A meticulous aseptic preparation and obturation technique can help minimize the risk of nosocomial infection, for example, disinfecting Gutta Percha (GP) points prior to obturation can reduce their bacterial load significantly.^23,24

Other aspects relating to biological factors for failure include developmental defects (e.g. grooves), lateral or furcation canals, medical conditions affecting immune response and the presence of a peri-apical lesion or patent sinus.⁷ The presence of an extra-radicular biofilm may be manageable only by endodontic microsurgery as non-surgical RCT is unable to reach these surfaces.²⁵ These are all important factors to consider when discussing options with patients, presenting the option of orthograde RCT and discussing the likely success. For example, the location of an infection may indicate the presence of a lateral canal, which can be accessed through scouting (e.g. using a size 10k file with a curve placed in the tip) and the use of ‘activated’ NaOCl at an appropriate concentration (Figure 3).

Other factors

Failure of RCT can also be attributed to root fracture, particularly if such a fracture permits contamination, leakage and prevents the formation of a seal. There is an association with both ETT and ETT restored with posts, and vertical root fractures,²⁶ which may make the tooth unrestorable and extraction may be required. ETT are more susceptible to fracture because of the loss of tooth structure resulting in reduced stiffness of up to 63% in cavities that cross a marginal ridge, and a further 5% when a tooth is accessed.²⁰ Furthermore, loss of the pulp reduces proprioception and accelerates the ‘ageing’ process of dentine, and the use of disinfectant chemicals can reduce collagen strength within dentine tubules.^27,28 Post placement may result in the further loss of tooth tissue and unfavourable loading. Cast posts are more likely to result in a catastrophic fracture than a fibre post, and in both cases, consideration must be given to occlusal loading and the presence of a ferrule.^29,30 Sacrifice of further tooth structure through the preparation of a post channel must be undertaken with caution.

Additionally, it is important to consider the possibility of persistent dento-alveolar pain that may occur in 5–10% of patients despite the presence of technically sound RCT and lack of detectable pathology.^31,32 This may result from a combination of peripheral and central sensitization.³³ Signs and symptoms that may suggest pain of this nature include a history of repeated treatments with no pain resolution, previous treatment by a clinician experienced in orofacial pain, higher levels of anxiety, history of painful orofacial treatment or no clear evidence of tissue damage or localized bone destruction.^34,35 In these cases, referral may be most appropriate before embarking on re-treatment, surgery or extraction.

Treatment options

Before commencing further treatment, a decision must be made on whether the treatment has failed. RCT should be assessed clinically and radiographically after 1 year³⁶ and beyond, if deemed necessary. Table 1 indicates favourable and unfavourable clinical and radiographic signs and symptoms. An apical abscess should be considered an early failure. If the apical radiolucency remains the same size, or has reduced, but not disappeared, the outcome is considered uncertain, and further review should be carried out annually for up to 4 years.³⁶ However, Figure 4 shows an example of healing beyond 4 years following re-treatment.

In the event of an unfavourable outcome, where persistent pain is excluded, further treatment is recommended in the form of extraction, orthograde root canal re-treatment or surgical approaches that may encompass treatments, such as apical surgery, perforation repairs or root resection. Failure of primary treatment should not necessarily condemn a tooth to extraction. Careful assessment can reveal the cause of failure, which can often be corrected by re-treatment. Where there is a requirement to place an indirect restoration, or undertake non-vital bleaching in a previously ETT, careful assessment is needed because the clinician may then be taking ‘ownership’ of the root canal filling.³⁶ A pragmatic approach in asymptomatic cases with careful patient consent may be indicated, but cases with significant deficiencies (e.g. missed canal) or failure (e.g. apical abscess) require intervention. It is important to ascertain which treatment option may be most favourable, balancing the risks of re-treatment with the risks of developing symptoms should re-treatment not be undertaken.

Is peri-radicular surgery a better option?

Where re-treatment has not yet been attempted, systematic reviews suggest that non-surgical re-treatment may have a more favourable long-term outcome compared to surgical endodontic treatment,^37,38 although the advent of modern tricalcium silicate cements and microsurgical techniques increase predictability of surgical approaches. Furthermore, in the event of symptomatic and progressive peri-radicular disease, there are certain circumstances when peri-radicular surgery may be a preferred treatment option. Table 2 summarizes when each approach may be preferred, and may be beneficial when discussing likely options with patients prior to referral.^36,39

Re-treatment in general dental practice and when to refer

Although the majority of cases may be successfully managed in general practice, it is important to recognize where onward referral is appropriate. The American Association of Endodontists (AAE) advises that complex cases may best be referred to a specialist for an opinion rather than extracted,⁴⁰ but finance and accessibility must be taken into consideration. The Restorative Commissioning Standard² recommends referral to practitioners with enhanced skills (level 2 complexity), or specialist care (level 3 complexity) as outlined in Table 3. The British Endodontic Society also provides a useful case assessment tool⁴¹ via smartphone application that allows assessment of the challenges and risks of individual cases by considering tooth and patient factors, and suggesting appropriate management.

Patients should be fully consented to re-treatment and involved in discussions regarding potential benefits (resolution of pain, apical healing, bone remodelling) and risks (failure, root fracture, perforation, inability to resolve pain, inability to improve existing restoration, instrument separation, finding teeth not amenable to predictable treatment), as well as other considerations, such as the time required, number of appointments and the expected prognosis and survival of the tooth. The success rate of re-treatment has been found to vary between 77% and 94%,^7,8,42 but an awareness of the individual clinical and radiographic factors that may lessen this is also important.

Conclusion

It is important to diagnose the reason for failed endodontic treatment and decide whether this can be addressed in general dental practice. Where this is possible, re-endodontic treatment can be predictably provided. Cases outlined as level II/III complexity should be referred to a dentist with enhanced skills or specialist, respectively. This would include cases where surgery is being considered as an option. Part 2¹ of this series outlines how to assess teeth pre-operatively and suggests common approaches for re-treatment.