Management of Persistently Symptomatic Teeth in Primary Root Canal Treatment: Advice on Maximizing Successful Outcomes

Controllable factors

Procedural reasons for non-healing include insufficient aseptic control, suboptimal access cavity design resulting in incomplete removal of coronal pulp, missed canals, inadequate chemo-mechanical debridement and extruded exogenous material.¹¹

Uncontrollable factors

Factors that are beyond the control of the treating clinician can often be identified during the initial assessment. This includes unrestorable teeth or those with root fractures, and complex endodontic lesions and anatomy.

The presence of these factors does not equate to non-healing as the process of healing is complex and depends on the ability of organisms to induce disease, and the host-immune response to the disease process.

Studies have shown that endogenous cholesterol released by the degenerating inflammatory cells present can result in a foreign body-type reaction. Cholesterol crystals irritate the tissues and are detrimental to post-treatment healing when present in large numbers.¹¹ Furthermore, patient factors such as systemic conditions (e.g. diabetes), can impair healing and reduce the chances of success of root canal treatment.¹²

Restorability assessment

Although patients are often keen to avoid extractions, ensuring that a tooth is restorable prior to undertaking endodontic treatment is key to ensuring success. Teeth that are unrestorable should not be root treated as an inability to achieve a post-operative coronal seal will inevitably result in re-infection and recurrence of symptoms.¹³

The amount of coronal tooth structure remaining and the structural integrity of this should be carefully assessed. Where extra-coronal restorations are required post-treatment, the clinician should assess whether there is enough of a ‘ferrule’, which is defined as 1.5–2 mm of dentine coronal to the margin of the restoration. This reduces the stresses within the tooth, protects the underlying tooth structure and reduces restoration failure.¹⁴

The periodontal status, to include the presence of periodontal pocketing and associated tooth mobility, should also be taken into consideration. It is important to note that mobility can occur in teeth that show periodontal stability, owing to the large extent of a peri-apical lesion. Therefore, the mobility should be carefully assessed and the distinction made. Single isolated periodontal pockets may be an indication of vertical root fractures. In single-rooted teeth, extraction will be required, and multi-rooted teeth will require either extraction or consideration of a hemi-section or root resection (Figures 1 and 2).¹⁵

Indices such as the Tooth Restorability Index and the Dental Practicality Index can aid clinicians in taking a structured approach to restorability assessment.^16,17 Restorability assessment allows a clear conversation with patients regarding the prognosis of the tooth in question and is a vital part of the consent process.

Endodontic assessment

Studies have shown that root-treated teeth with associated apical periodontitis may harbour residual infection within the complex apical anatomy. Residual microbes have been found mainly within uninstrumented spaces within the main canals, within the isthmus joining two canals, such as the mesial roots of mandibular first molars, and also within accessory/lateral canals. This can result in persistent intra-radicular infection, thereby preventing the resolution of endodontic lesions.¹¹

Radiographic examination plays a pivotal role in the endodontic assessment. Plain film radiography techniques, such as the use of the parallax technique, whereby two long cone peri-apical radiographs are taken at differing angles, can help to locate additional canals and visualize the anatomy. For example, where a canal appears to disappear on a radiograph in the mid or apical third, this suggests that the canal bifurcates and taking a parallax view can allow identification of this pre-operatively. Furthermore, in cases where symptoms do not resolve despite the outcome appearing favourable radiographically, a cone beam computed tomography (CBCT) scan can be useful in detecting further canals that may have been missed in the initial treatment.¹⁸ CBCT scans can also be used pre-operatively and are becoming more common as part of the initial assessment, to allow assessment of the canal anatomy and the presence and extent of apical lesions (Figure 3).

There are several key clinical factors that have been shown to reduce the prognosis of root canal treatment and are useful to note when undertaking an endodontic assessment of a tooth. These are listed in Table 2 and an awareness of these is useful when determining the prognosis of each clinical case.

The Royal College of Surgeons released guidelines to aid clinicians in determining the complexity of restorative treatment. Factors that contribute to the highest complexity within root canal assessment include a root curvature greater than 40°, canals that appear non-negotiable throughout their entire length, iatrogenic damage, pathological resorption, difficult root anatomy and peri-radicular surgery.²⁰ The British Endodontic Society also has an online case assessment tool to help clinicians in assessing the risks and difficulties of individual cases.²¹

General dental practitioners may consider referral to a specialist in these complex cases if it is beyond their scope. A complexity assessment of a tooth prior to undertaking root canal treatment is highly recommended.

Adequate access

Adequate access allows removal of all of the coronal pulp tissues within the pulp canal chamber, including the pulp horns. It also ensures that all the root canals within the tooth have been identified and allows instrumentation along the entire length. Research has shown that incorrect access results in instrumentation of only the coronal aspect of the root canal. Achieving straight line access is important to improve instrumentation of the apical portion and decreases the chance of instrument fracture.²³ Understanding of the root canal anatomy is required when determining how to access a tooth. The shape of the access cavity should reflect the shape of the pulp chamber and research by Krasner and Rankow determined ‘laws’ of anatomy that can aid in the identification of canal orifices (Figures 4–8).²⁴

Micro-organisms are the aetiological factor of endodontic disease, and the success of treatment relies on adequate disinfection. Disinfection ensures that all pulpal remnants and micro-organisms are removed from the canal prior to obturation. This is achieved via mechanical debridement using endodontic files, and the use of chemical irrigants. An ideal irrigant should provide lubrication for files, dissolve the organic matter and smear layer, and disinfect the root canal system, while also being biocompatible.²⁵ Rubber dam should be used to ensure aseptic control, as well as provide protection of the airway and soft tissues. In addition, research has shown a significant improvement in endodontic outcomes with the use of rubber dams.²⁶

Sodium hypochlorite (NaOCl) is considered the gold standard for endodontic irrigation because it has a broad spectrum of antimicrobial activity and organic tissue dissolution properties. The concentration used ranges from 0.5–6%, with higher concentrations showing increased ability to dissolve organic tissue and a faster rate of antimicrobial action. Lower concentrations should therefore be used in higher volumes and higher frequencies to ensure continuous replacement. A dwell time of 30 minutes has been suggested for NaOCl to allow sufficient bathing and debridement of the canals.²⁵ Clinicians should therefore be mindful that there is adequate time for disinfection when using newer one-step file systems that can reduce treatment times, therefore reducing the duration of irrigant activity.

Ethylenediaminetetraacetic acid (EDTA) is a chelating agent that is effective in removing the smear layer and inorganic debris. The smear layer prevents penetration of the irrigant into the dentinal tubules, which contain bacteria and nutrients that allow for bacterial growth. EDTA has positive synergistic effects when used with NaOCl and is particularly effective in endodontic re-treatment cases and in the elimination of Enterococcus faecalis. EDTA 17% is often used as a penultimate rinse for 1 minute and repeated alternation with NaOCl should be avoided to prevent erosion of dentine, which can compromise tooth structure.^25,27 Direct contact between EDTA and chlorhexidine should be avoided as a white precipitate can form.²⁵

Chlorhexidine is a broad spectrum disinfecting agent and is bactericidal at higher concentrations by decomposing bacterial cell membranes. Use of chlorhexidine alone is not recommended, and it is often used in combination with other irrigants, although should not come in direct contact with NaOCl as they react and precipitate forms that blocks canals. There is evidence to suggest that it has pre-carcinogenic properties.²⁵

It has been shown that bacteria in apical aspects of the root canal were associated more with the canal wall. Therefore, ensuring adequate chemo-mechanical disinfection of the canal wall in the apical region is vital (Table 3).²⁸ A 27- or 30-gauge side-venting needle is advised, which reduces the risk of extrusion because the irrigant does not exit further than 1–2 mm beyond the needle tip.²⁵ The root canal should therefore be adequately instrumented to an appropriate working length to allow irrigant penetration to the apical regions. Furthermore, the efficacy of the irrigant can be improved by manual agitation of the irrigation needle or the use of ‘GP-pumping’, whereby a gutta-percha (GP) cone is used to agitate the irrigant within the canal. Ultrasonic activation and heating of the chemical irrigant are alternative methods that can be used.²⁵

Intra-canal medication and multiple appointments

Calcium hydroxide is a widely used intra-canal medicament owing to its antibacterial properties and ability to dissolve tissue and neutralize endotoxins. Calcium hydroxide destroys the bacterial cell membrane components owing to its high alkalinity, with a pH around 12.5, acts as a physical barrier for bacterial proliferation and prevents coronal leakage.²⁹ In cases of persistent apical periodontitis, calcium hydroxide may be left within the root canal for longer periods of time to help promote healing. The use of intra-canal medication also helps to dry persistently wet canals.³⁰ The addition of iodine to calcium hydroxide-based medicaments, such as Vitapex (Neo Dental International, USA), enhances antibacterial properties and promotes hard tissue formation. It is recommended in the treatment of resorption, traumatized teeth and extensive apical lesions, although it can be difficult to remove (Figure 9).^31,32

Antibiotic-based medicaments have also been used widely, often during the initial access in cases of symptomatic pulpitis. An example of this is Odontopaste (ADM Australian Dental Manufacturing, Australia), which is a zinc oxide-based dressing containing clindamycin, alongside a steroid-based anti-inflammatory agent and calcium hydroxide. Ledermix (Ozdent Dental Products, Australia) is another antibiotic-containing medication that includes demeclocycline alongside a corticosteroid, triamcinolone acetonide.

Root canal treatment can be carried out as a single-visit approach or a multiple-visit approach. Research has shown that both approaches are equally effective, with no evidence to suggest that a particular approach is preferred.³⁴ However, advantages of multiple appointments include allowing the use of intra-canal medication, more time for irrigant delivery and it allows the clinician the opportunity to assess healing and resolution of symptoms or sinus tracts prior to obturation.

Therefore, in cases with large apical lesions, persistently exudative canals and persistent sinus or symptoms, the use of intra-canal medication over multiple appointments should be considered to promote healing.

Recapitulation and apical patency

The most complex anatomy is within the apical portion of the root canal and instrumentation of the apical portion is required to ensure adequate disinfection. Ensuring apical patency via continuous recapitulation involves passively passing a small flexible K-file, 0.5–1 mm past the apical constriction, without widening the diameter. Instrumentation of the root canal should be up to the working length, which is advised to be 1–2 mm short of the radiographic apex. However, recapitulation past this point with a small file, ensures that the canal is not blocked with debris during instrumentation, which helps to avoid ledge formation, perforations and apical transportation, and ensures that apical patency is maintained. Care should be taken not to widen the apical constriction as this iatrogenic damage can result in loss of an apical seal.³⁴ The use of glide path files has been shown to be beneficial in maintaining original root canal anatomy and decreasing apical extrusion.³⁵

Apical seal and preventing extrusion

Apical gauging involves measuring the size of the apical constriction prior to obturation. The master cone GP point should have ‘tug back’ to ensure an adequate apical seal, which can help to prevent extrusion. The size of the GP point at the apex can be checked using a GP gauge and can be adjusted to give a more accurate fit using apical gauge rulers. Although large extruded pieces of GP can be well encapsulated and tolerated by the body, fine particles can lead to a localized tissue response and aggregation of inflammatory cells. This excess material, contaminated with chemicals that can irritate the tissues, results in a foreign body reaction and impairs apical healing.¹¹ Research has shown that success rates drop by 62% in overextruded cases.³⁶

Other materials used in endodontic treatments, such as paper points and cotton wool, can be pushed into the apical tissues during instrumentation. These contain cellulose fibres that cannot be degraded by the body and so remain within the tissues with the potential to induce a foreign body reaction. Further materials found within apical biopsies include amalgam, endodontic sealers and calcium salts from extruded calcium hydroxide.¹¹ Care should be taken when removing coronal restorations to prevent displacement of materials into the root canal space and clinicians may find that sponges are easier to retrieve in comparison to cotton wool. Copious irrigation with careful suction should be used and files should be regularly cleaned.

Obturation and coronal seal

There are several techniques available for obturation, including cold lateral condensation, warm vertical or lateral compaction, single-cone obturation and carrier-based techniques. There is little difference in the long-term outcome of these different methods and generally the technique used is down to clinician preference.³⁶ The aim of obturation is to ensure the root filling is well condensed, with no voids and within 2 mm of the radiographic apex.¹³ Research has shown that the success rates of root canal treatment falls by 12% for every millimetre that the canal is underfilled. Underfilled canals suggest inadequate preparation of the canal walls, which may still harbour bacterial colonies.³⁶

Following obturation, the placement of an effective coronal seal immediately is essential to prevent re-infection and allow healing (Tables 4 and 5).¹³

Dual obturation and apicectomy

A dry canal should be achieved prior to obturation. If a canal is persistently exudative following multiple appointments, it may indicate a cystic lesion. A ‘true cyst’ is self-sustaining in nature, and less likely to resolve with non-surgical treatment alone, resulting in persistent apical periodontitis.¹¹ Alternative treatment options, such as dual obturation with apicectomy may be considered. This is discussed below, alongside a clinical case outlining the treatment performed.

The Royal College of Surgeons released updated guidelines for peri-radicular surgery in 2020. Indications for peri-radicular surgery include a persistently exudative canal ‘despite repeated chemo-mechanical debridement’ (Table 6). Peri-radicular surgery can help to resolve microbial infection as it allows access to the apical tissues and can by-pass apical anatomical complications.⁸