Criteria determining when a root canal system is ready to be filled are discussed from the clinical (diagnosis and prognosis), theoretical (moisture, sealers, medicaments and timing) and technical (cleaning and shaping) aspects. The ability to increase the probability of successful endodontic treatment relies on: an accurate technical diagnosis, proper instrumentation, shaping, disinfection, placement of medicaments, and the ability to judge healing responses. There are various other factors and criteria which may be beneficial in determining when a root canal system is ready to be filled, but have not been included. Finally, although biological systems are often limited by nature, the capability to provide predictable and precise criteria as to the most ideal circumstances when root canal system is ready to be filled will be discussed.
Key Words: Criteria, obturation, root canal treatment.
Successful endodontic treatment has become increasingly driven by evidence-based healthcare, as it is the continually expanding evidence based concepts of clinical, theoretical and technical aspects which help determine criteria as to when a root canal system is ready to be filled. This paper discusses current criteria which determine the ideal time to place a root filling. Areas of consideration include: clinical evaluation, theoretical based concepts to consider prior to obturation, and technical aspects regarding proper cleaning and shaping.
Criteria that determine when a root canal system is ready to be filled
Part 1 – Clinical Aspects
Diagnosis: Clinical Significance
An important aspect in determining when a root canal system is ready to be filled begins with identifying the cause of endodontic pathology, along with an accurate diagnosis of pulpal and periradicular tissues; clinically, this is accomplished through a series of investigations and comprehensive special tests . An initial diagnosis and continued evaluation is critical, as one ideal aspect to determine when a root canal system is ready to be filled is if the patient is, or after primary phase treatment has become, asymptomatic . The basis for this criterion is that symptoms of acute and chronic pain are often indicators of an existing disease process, associated with an offending tooth . Moreover, pain is often highly indicative of continued inflammation and irritation of associated tissues; as a result of stimulating chemicals released during the inflammatory response . A main etiological factor for pain is the direct virulence factors of bacteria and the host’s response to these micro-organisms. Through the work of Stanley & Fitzgerald in their 1965 study of microbiology of pulp exposure in rats, it was determined that a key component causing a pulp and periradicular inflammatory response is directly related to levels of bacterial species . Further, the importance of a pre-operative evaluation cannot be underestimated as it is of considerable significance as it allows for the clinician to monitor and assess for modes of healing responses based on signs and symptoms. Therefore, a clinical diagnosis of a symptomless patient is ideal; as it is more likely that the diversity of remaining bacteria are not viable to maintain nutritional relationships and as a result their survival is less favourable . Although to complicate this issue, it has been found that patients with mild, or significant symptoms have been shown to become asymptomatic upon obturation . However, filling a root canal known to be infected is risky, as Ingle and Zeldow have found an increase in post-operative discomfort from filling infected root canals . Therefore, conflicting studies do provide different aspects to determining the criteria as to when a root canal system is ready to be filled. Although, clearly the goal of endodontic treatment is dependent on reversing or eliminating signs and symptoms associated with a bacterial infection . Thus, an asymptomatic patient maybe ideal; however, it is not an absolute requirement and the more important criterion is to clinically monitor and evaluate a patient prior to obturation, to assess for signs and symptoms of healing.
Prognosis: Evaluation of Success
Prior to filling a root canal system, the prognosis of the tooth should be re-evaluated. Generally, this is related to the initial treatment planning phases during the determination of the; strategic value, patient circumstances, periodontal health and restorability factors associated with a tooth . After initial phase treatment further review is often necessary in assessing and providing an updated prognosis for the clinician to better evaluate the most appropriate course of treatment prior to obturation; however, it must be remembered that in the era of evidence based research, determining the prognosis of endodontic treatment needs to be taken with caution, as often studies are limited due to lack of: standardization, variable treatment, and methodologies involved . Furthermore to complicate matters, technical skill, procedural errors and the coronal restoration all influence the outcome of endodontic therapy . Thus, it may be possible using evidence based dentistry criteria to provide patients with the statistical value for the prognosis of a given tooth (e.g. healed rate, or complete healing ranging from 73-90% for those cases with apical periodontitis) . Although, It is not always possible to assume that a particular tooth based on clinical signs and symptoms has the same chance of healing . Therefore, providing patients with a long term prognosis, along with using that knowledge in evaluating the most appropriate clinical action is beneficial; however, it must be remembered that both studies and biological systems have limitations and are not fully predictable.
Part 2 – Theoretical Aspects
Significance of a Dry Root Canal System: Theoretical Basis
The theory behind eliminating all moisture and serous exudate prior to obturation is an important criterion which must be considered. This is because both moisture and liquids can negatively affect sealing ability . Furthermore, both micro-organisms which can remain inside root canals (in dentine tubules) even after vigorous chemical-mechanical preparation and apical leakage, have the potential to cause endodontic failure . Thus, for practitioners to minimize the risk of a compromised seal, the theoretical basis of drying a root canal must be discussed.
Significance of a Dry Root Canal System: Purulence & Weeping
Purulent exudate from a root canal system is a clear sign of infection; however, a more elusive condition is persistent serous exudate . This exudate described as a persistent clear or reddish discharged (associated with periapical pathosis) from a root canal system, is commonly regarded as the “Weeping Canal” .
Purulent infections are generally treated by removal of irritants, drainage, irrigation, intra-canal medicaments, or antibiotics for rapidly spreading infections . On the other hand, weeping canals which may be the result of a large apical foramen, over-instrumentation or a patent foramen; may require further dressing with calcium hydroxide . The use of calcium hydroxide as an intra-canal medicament is based on the theory that the pH of the periapical tissues of a weeping canal is acidic. Thus, by changing the root canal system to a more basic environment, alkaline phosphatise becomes activated which plays an important role in hard tissue formation . This is often successful, as packed calcium hydroxide (due to its high pH) against a point of seepage may succeed in desiccating or necrotizing an exudating surface . Thus, allowing for the root canal system to be properly dried and free of moisture or serous contaminates, prior to the root filling. Clinically, both purulent and weeping canals are of significance, as an important criterion in determining when a root canal system is ready to be filled, is the ability to adequately dry the root canal system for obturation .
Significance of a Dry Root Canal System: ‘The Seal is the Deal’
A dry canal is ideal, as the chemical reactions of particular sealers are often impeded by aqueous media like saliva or tissue fluids . This is further compounded by the slow setting reaction of certain sealers (eg. Roth’s 811), as slow but continued tissue fluid association interferes with setting mechanisms. Thus affecting sealer properties and consequently the quality of root canal sealing .
Several studies have explored the affects of moisture and its influence on achieving a competent seal via obturation. Negm et al. , and Horning et al.  both found that moisture and liquids can adversely affect sealing ability; thus, increasing the potential risk of micro-leakage. Furthermore, Roggendorf et al.  were able to demonstrate, through penetration tests with Methylene blue dye, many important facts. Firstly, a direct relationship exists between the degree of dryness within the root canal system and an improved apical seal . This was achieved through the use of a higher number of paper points and additional drying methods such as a heated probe . Also, it was demonstrated that if root canal systems was sealed with the presence of moisture a greater degree of leakage was detected . Thus, prior to obturation the significance of confirming that all serous exudate and moisture is eliminated to the highest degree possible is an important criterion, as it allows for a better seal.
Finally, to demonstrate the significance of an adequate seal and its effects on periapical health, one must consider both the root canal filling and the coronal restoration for comparison. Many studies have demonstrated conflicting results in the attempt to determine if a root filling or the coronal restoration has greater significance in the impact of periapical health . Through the work of Ray & Trope 1995, it was found that the quality of the restoration had a greater impact on periapical status, rather than the quality of the root filling . In contrast, Torstad et al. 2000, found a correlation suggesting that the quality of the restoration is less significant when compared to the quality of the root filling . Further, findings by Kirkevang et al. 2000, found that the periapical health of the root-filled teeth depends equally on the coronal restoration and root filling . Thus, it is generally regarded that all modes of sealing are important, and the ability to maximize and understand theoretical concepts pertaining to a dry root canal should not be overlooked, and all efforts should be made to dry the canal as best as possible prior to obturation.
Significance of Intra-Canal Medicaments: Calcium Hydroxide (CaOH2)
The procedure of dressing a root canal system with calcium hydroxide after removal of necrotic and non-vital tissue (between visits) is an important criterion of consideration, prior to obturation.
Currently, calcium hydroxide is one of the most commonly used intra-canal medicaments, due to its antimicrobial effect within the root canal system, which is caused by its dissociation of hydroxyl ions and the subsequent inhibition of bacterial enzymes . Moreover, the placement of calcium hydroxide as an intra-canal medicament is based on the fact that it has been found unfeasible to completely disinfect all root canal systems with cleaning, shaping and irrigation alone ; thus proving beneficial as an antimicrobial to decrease bacterial loads. Although, this conclusion must be compared to Sathorn & Parashos’s systematic review and meta-analysis of the antibacterial efficacy of calcium hydroxide as an intra-canal medicament . As no significant difference (P=0.12) of bacterial positive canals was found between placement of pre-medicaments and post-medicaments ; however, this conclusion must be evaluated with caution (as should be done with all evidence based research conclusions), as consideration to experimental procedures, such as the skill of operators and other clinical circumstances need to be considered. Thus, although there is conflicting evidence to calcium hydroxide’s use, prior to obturation, its’ placement may still prove beneficial if multiple visits are required; to inhibit the growth of bacteria between appointments . Therefore, the theoretical benefits of calcium hydroxide along with the clinical circumstances must be considered to its’ efficacy of placement, prior to obturation.
Part 3 – Technical Aspects
Technical Objectives: Cleaning and Shaping
The chemomechanincal preparation including both mechanical instrumentation and antibacterial irrigation, with the goal of eliminating micro-organisms from the root canal system; is a final key principle which must be fulfilled prior to obturation .
Mechanical instrumentation is an important procedural stage, prior to obturation, as it facilitates space for cleaning and placement of obturation materials . This technical aspect of mechanical instrumentation of a root canal system has undergone and continues to rapidly change, as new techniques and instruments are developed to enhance the ability of a practitioner to shape and clean a root canal system. Moreover, from a historical aspect there are many mechanical techniques to instrument a root canal; however, the emphasis of the mechanical instrumentation will focus on the relatively recent crown-down technique via the use of nickel titanium (NiTi) rotary, and stainless steel instruments for shaping and preparing a root canal system.
Irrigation is another critical aspect which must be given consideration, prior to obturation; as the physical properties of an irrigating solution to act as a lubricant, along with the additional benefit or removing the smear layer and susceptible bacteria,[28, 29] are influential as successful root canal therapy relies on proper disinfection . Therefore, Sodium hypochlorite (NaOCl) and Ethylenediamene tetraacetic acid (EDTA) will be discussed, for their advantageous role as an irrigation solution prior to the filling of a root canal system.
Technical Objectives: Principles of Mechanical Instrumentation
To achieve the technical goals of canal preparation of shaping a root canal system with the aim of eliminating microbiological insults and to aid in the placement of a root filling, key procedural steps must be followed. As Morgan et al.  reiterated the general agreement that exists regarding key principles of root canal instrumentation.
- The preparation should have a continuously tapering conical shape with the narrowest cross-section at its apex
A continuously tapering shape is an important criterion, as it aids access for irrigating solutions and provides a contour for resistance during packing of filling materials .
- The apical foramen should remain in its original position
If the apical foramen becomes displaced through iatrogenic errors, this can often cause and lead to further difficulties during packing of gutta-percha .
- Instruments should be used in sequential small to large sizes and should be pre-curved before insertion into curved canals
Similar to potential iatrogenic errors which can be inflicted via mechanical preparation near the apical foramen; if instruments are not pre-curved the potential of technical errors exist, such as zipping, ledging, or perforation.
- Instrumentation should be confined within the root canal space
As over instrumentation and significant disruption of the apical foramen is one of the most common causes for phenomena associated with root canal instrumentation, such as wet canals, chronically sore cases and other mysterious failures .
- The canal walls should be free of irregularities and void of dentin shavings.
Finally, canal walls should be smooth and free of irregularities to allow for enhanced penetration of irrigating solution, as will be described below.
Technical Objectives: Crown down technique
The principles of mechanical instrumentation, as described above, cannot be over emphasized; however, it is the technique, in which a practitioner uses, not only allows for fulfilment of these criteria, but also provides an efficient and effective means of achieving proficient root canal shaping.
Crown down Technique. An important factor associated with the mechanical preparation of a root canal system is the use of a preparation technique which allows for a straight line path into the canal, greater access for irrigating solution, decreased instrument breakage, and better access to the apical area . Furthermore, with the expanding use of NiTi rotary instruments, which allow for a high degree of flexure movement, older techniques such as the step-back technique are beginning to be phased out ; being replaced by the step-down techniques which prepares a canal using large instruments at the canal orifice, followed by progressively smaller files and taper to mechanically shape the root canal system . Finally, it must be realized, as explained by Young et al. , that NiTi instruments via the crown down technique are not designed for initial negotiation and that small stainless steel instruments, because of their greater stiffness, should be used for path finding and to establish patency.
Technical Objectives: Sodium Hypochlorite
Irrigation of a root canal system is a critical step in endodontic treatment and through the use of chemical solutions, such as sodium hypochlorite (NaOCl), the ability to effectively clean a root canal system is enhanced. There are many intra-canal irrigating solutions; however, sodium hypochlorite remains one of the most popular due to its antimicrobial properties, and ability to dissolve necrotic tissue and provide lubrication of root canals systems during mechanical instrumentation [34-36]. Although, sodium hypochlorite holds many advantages, controversy does exist as to the most appropriate concentration (1-5.25%) of solution to be used during the irrigation process . Several studies have shown that at higher concentrations NaOCl becomes cytotoxic; however, this is reduced at lower concentrations, along with its antibacterial nature and ability to dissolve necrotic tissue . On the other hand, it has been shown that a reduction in the concentration of NaOCl from 5% to 0.5% does not significantly reduce bacterial species [39, 40]. Thus, a lower concentration along with the application of a larger volume of NaOCl solution is recommended for irrigation; as the use of NaOCl in concentrations greater than 1% does not appear warranted .
Technical Objectives: Ethylenediamene tetraacetic acid(EDTA)
The removal of the smear layer is another area of consideration, prior to obturation. This is achieved through the use of a chelating agent, such as EDTA, as it has the ability to remove inorganic components and leave organic components intact in a root canal system . The general premise behind EDTA’s use, which dissolves inorganic components, is to facilitate NaOCl, which dissolves organic components, during chemomechanical preparation. The application of EDTA must be considered, as prolonged use may result in excessive removal of both peritubular and intratubular dentin . Therefore, the general recommendation for its use, is irrigation via NaOCl flowed by (17%) EDTA for one minute and a final rinse with NaOCl . Thus, the combination of EDTA and NaOCl provides an effective means to remove the smear layer, which is created as a result of mechanical instrumentation. Therefore, the use of an inorganic solvent during chemomechanical preparation is advocated to increase the ability to clean and shape a root canal system, and effectively reduce the bacterial load, prior to the placement of a root canal filling.
Clearly, as discussed above, there is an array of fundamental criteria that must be considered prior to filling a root canal system, if a practitioner is able to provide the highest standard of care. Therefore, the establishment and fulfilment of the necessary criteria, of which many are heavily based on evidence based research, is significant as successful endodontic treatment is highly dependent on the execution of clinical, theoretical and technical principles. Thus in order to satisfy the presented criteria, the practitioner must perform a thorough clinical evaluation, understand theoretical based concepts, and be proficient in technical aspects with regards to cleaning and shaping.
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