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 Table of Contents  
REVIEW ARTICLE
Year : 2013  |  Volume : 3  |  Issue : 1  |  Page : 1-9

The anatomy of the root apex: A review and clinical considerations in endodontics


1 Department of Conservative Dentistry, Division of Endodontics, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia
2 Department of Oral Rehabilitation, Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand

Date of Web Publication7-Aug-2013

Correspondence Address:
Nicholas P Chandler
School of Dentistry, University of Otago, P.O. Box 647, Dunedin 9054
New Zealand
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1658-5984.116273

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  Abstract 

Studies on the anatomy of the root apex are an area of interest to the endodontist; they have reported that the position of the apical constriction, apical foramen and the cemento-dentinal junction varies across the tooth types. These anatomical apical landmarks are considered extension limits for root canal instrumentation and filling. Achieving an optimum working length is thought essential for successful root canal treatment, so adopting any of these landmarks is associated with certain risks and benefits. The variability in the position of the apical constriction and apical foramen, for example, complicates their clinical detection, while the cemento-dentinal junction is a histological landmark that cannot be detected clinically. The radiographic apex does not always coincide with the anatomic apex of the tooth. The pre-operative status of the pulp must be considered while obtaining the working length. Most prognostic studies agree that extending the root filling to within 2-3 mm of the radiographic apex is associated with favorable treatment outcomes.

Keywords: Apical constriction, apical foramen, cemento-dentinal junction, endodontics, radiographic apex


How to cite this article:
Alothmani OS, Chandler NP, Friedlander LT. The anatomy of the root apex: A review and clinical considerations in endodontics. Saudi Endod J 2013;3:1-9

How to cite this URL:
Alothmani OS, Chandler NP, Friedlander LT. The anatomy of the root apex: A review and clinical considerations in endodontics. Saudi Endod J [serial online] 2013 [cited 2019 Dec 9];3:1-9. Available from: http://www.saudiendodj.com/text.asp?2013/3/1/1/116273


  Introduction Top


The search strategy for this review consisted of a combined electronic and manual search of references published in English. The former was carried out using Medline via the Ovid interface to November 2012 using the search terms: Apical foramen, apical constriction, cemento-dentinal junction, root apex, anatomic apex, radiographic apex, major diameter, minor diameter and outcome of root canal treatment. The latter search involved a cross-check of the electronic results with review articles and textbook chapters to identify all possible relevant publications.

The terminal part of a tooth root exhibits four distinct landmarks; the apical constriction (AC), apical foramen (AF), roots apex (anatomic and radiographic) and cemento-dentinal junction (CDJ). [1] While the AF is the main apical opening of the root canal, the AC is defined as the apical part of the root canal with the narrowest diameter. The anatomic apex differs from the radiographic apex in that the former is the root end as identified morphologically and the latter is identified radiographically. [2] The CDJ is the line of union between dentin and cementum at which pulpal tissue ends and periodontal tissue starts. [3]

The apical constriction

Variability in the position of the AC is well documented. The distance between the AC and the AF ranged between 0.4-1.2 mm, while its reported location in relation to the root apex ranged between 0.5-1.01 mm [Table 1]. The AC is mostly located either in dentin or at the CDJ level and less frequently in cementum. [4] Another study of 50 mandibular premolars, reported that the AC is always found coronal to the CDJ. [10] Hence, the AC and CDJ do not always coincide.
Table 1: The position of the apical constriction


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The shape of the AC in longitudinal sections has four possible configurations - single, tapered, multi-constricted and parallel; the first two were the most common. [7] Forty-eight percent of ACs had the single topography while the rest were tapered, multi-constricted or parallel. [12] A recent study evaluated the morphology of the AC in palatal roots of maxillary molars using micro-computed tomography and reported that the AC was present in 34% of the roots. The AC was considered present only if it had single or tapered morphologies. However, roots with flaring or parallel shapes were considered to lack any constriction. [13]

In horizontal sections, the labio-lingual dimension of the canal at the AC was larger than the mesio-distal dimension by 0.05 mm. [9] The most common shape of root canal at the AC was circular, although oval and irregular outlines were also seen. [5],[9] The AC of maxillary central incisors exhibited an average 0.165 mm slope with only 10% representing a flat morphology. [14] Out of 93 canals, 41 canals had a slot-like constriction while 52 canals had a point constriction. [15]

The apical foramen

Deviation of the AF from the root apex is common, with a reported frequency ranging from 17-100% [Table 2]. On the other hand, the average distance between the AF and the root apex was found to be less than 1 mm [Table 3].
Table 2: Frequency of apical foramen deviation from tip of root apex


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Table 3: Distance between the apical foramen and tip of root apex


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A higher frequency of AF deviation has been associated with aging and deposition of cementum. [4],[19],[22] Deviation also differed according to tooth type. Only 1-5% of maxillary central incisors had their AF at a distance of more than 1 mm from the root apex, [21],[33] but a higher frequency of AF deviation in posterior teeth has been observed. This was attributed to more cementum deposition at the apices of posterior teeth as an adaptation to higher occlusal loads and compensation for enamel attrition. [7],[23],[27] Excluding molars, the average AF deviation for maxillary and mandibular teeth was similar. [18] Another study found that AF deviation in mandibular teeth was more than for maxillary teeth. [27] However, another study by the same group found that the opposite was true. [30]

The cemento-dentinal junction

During tooth development, cementum deposition follows that of dentin, resulting in a line of delineation separating the two tissue types. [34] The line of union was difficult to recognize histologically and considered imaginary. [35] However, several later studies have reported the microscopic appearance of the CDJ. [4],[10],[22],[26],[33],[36],[37] Out of 268 teeth, only 37 teeth did not display the CDJ, even at higher magnification. Both sides of the CDJ ended at the same level in only 53% of a younger teeth group and 60% of an older teeth group. [4] Another study reported that both levels of the CDJ coincided in only 5% of the teeth. The level of the CDJ at the left side of the canal was more coronal than on the right side, and it was more coronal in maxillary canines than in maxillary incisors. [26] Histological examination of 122 mandibular premolars revealed that neither side of the CDJ coincided. [38] Variability in the position of the CDJ has been documented. The CDJ was coronal to the AF by at least 0.3 mm while the maximum distance reported between the CDJ and root apex was 2.5 mm [Table 4].
Table 4: Position of the cemento-dentinal junction


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Importance of achieving optimum working length

The aim of root canal treatment (RCT) is to resolve and/or prevent apical periodontitis. [39] The association between microbial infection of the root canal system and the development of apical periodontitis has been well established. [40],[41] RCT usually involves mechanical instrumentation of the root canal system accompanied by sodium hypochlorite (NaOCl) irrigation. [42],[43],[44],[45],[46] Three-dimensional filling of the root canal system [47] and the provision of a well-sealed coronal restoration are concluding steps of treatment. [48] Management of infected root canals differs from those containing vital, inflamed tissue because they require adequate disinfection prior to root filling. This is generally achieved by the application of calcium hydroxide as an intra-canal medicament for at least one week. [49] In vital teeth, calcium hydroxide is not necessary because the radicular part of the inflamed vital pulp remains infection-free as long as it is vital. [50] The level of root canal instrumentation and filling should allow complete debridement and sealing of the root canal system without violating the surrounding tissue or leaving critical parts of the apical region unclean and/or unsealed. [51]

Consequences of over-instrumentation

Over-instrumentation in this context implies the extension of instruments into the periapical tissue beyond the boundaries of the root canal system. Endodontic instruments should ideally be kept within the root canal system to minimize post-operative discomfort. [52] Over-instrumentation may lead to discomfort as a result of the acute inflammatory response elicited from mechanical damage to the periapical tissue. Development of pain will depend on the intensity of the acute inflammation, which is in turn proportional to the extent of tissue damage. [53],[54]

In infected cases, the periapical inflammatory reaction might be further compounded if over-instrumentation leads to the extrusion of microbes and infected debris. It has been shown that extruded dentin and cementum chips, possibly infected, provoke varying inflammatory responses in the periapical tissue. [55] Microscopic, culture and molecular methods have disclosed microbes in apical granulomas. [56],[57],[58],[59] These pathogens may gain access to the periapical tissue through over-instrumentation, with Actinomyces israelii and Propionibacterium propionicum, known for their abilities in establishing extra-radicular infections. [60]

Over-instrumentation also promotes over-filling [61] which can result in mechanical and temporary chemical irritation of the periapical tissue. [50] In a histological study, all six overfilled cases showed severe inflammatory reactions in the periapical region but none of the patients experienced pain. [62] Furthermore, overfilling was associated with contamination of the apical area and possible foreign body reaction. [63]

Consequences of under-instrumentation

Under-instrumentation refers to cleaning and shaping the root canal to a level shorter than desired, leaving regions of the apical part of the canal without proper debridement.

Achieving optimum instrumentation depth might be more critical during the treatment of infected canals than when managing vital inflamed pulps. [64] The likelihood of an unfavorable outcome of root-filled teeth with inaccessible ACs was 5.3 times higher than when instrumentation could be extended to the level of the AC. The presence of a pre-operative radiolucency related to the teeth of the former group increased the chances of an unfavorable outcome by 3.4 fold. [65] Under-instrumentation may lead to loss of working length and accumulation of infected debris apically. [50] Build-up of apical debris should be prevented; although it is yet to be proven that it impairs or prevents healing. [51]

Optimal working length

There is general agreement that maintaining instrumentation and root filling within the root canal is desired. [51],[66],[67] Several apical reference points have been suggested as the furthest apical extension for instrumentation and root filling. These include the CDJ, AF, AC and the radiographic apex. Siqueira [50] suggested that the pre-operative status of the pulp should also be taken into consideration during working length determination.

CDJ as the ideal apical termination point

The CDJ is the ideal termination point for RCT. [68],[69] Sealing the root canal system at this point would theoretically, prevent microbial escape into periapical tissues and block entry of tissue fluids into the canal space. [3],[34] However, the CDJ is a histological point that cannot be located clinically and its appearance varies from tooth to tooth. [4],[10],[26],[33],[35],[36],[62],[66],[70],[71],[72] Moreover, Saad and Al-Yahya [38] demonstrated that the CDJ of some teeth were located inside the root canal. Such findings preclude the adoption of the CDJ as an end-point for root canal treatment procedures.

AF as the ideal apical termination point

Cleaning and filling the root canal to the AF has been proposed. [35],[70] The AF is a reliable apical landmark [73] and limiting cleaning and shaping short of the AF guarantees that the entire procedure is performed inside the root canal regardless of the position or existence of the AC. [51] Nonetheless, accurate location of the AF is only possible histologically. [33] Periapical radiographs frequently failed to identify the position of buccally- or lingually-deviated AFs. [18],[23],[74] The position of the AF was reported to affect the accuracy of electronic apex locators. [29],[75] Inflammatory root resorption associated with infected canals might eliminate the AF. [1] The AF is, therefore, not a reliable reference point for working length determination. [26],[51]

It has been suggested to extend root canal instrumentation to 1 mm short of the radiographic apex, which would ensure closer proximity to position of the AF. [21] However, continuous cementum deposition alters the position of the radiographic apex to the AF. [76] Meanwhile, depending on a subjective level short of the radiographic apex to which disinfection and root filling are extended to do not necessarily guarantee that the entire canal is treated and does not absolutely prevent over-instrumentation. [1],[23],[50]

AC as the ideal apical termination point

The AC would represent the smallest apical dimension possible for root canal instrumentation. [5] Termination at this point would result in the least amount of tissue damage and consequently, minimal repair would be needed. [62] The quality guidelines of the European Society of Endodontology (2006) recommend that working length determination should be as close as possible to the AC. [77] In addition, the divergent shape of the canal apical to the AC would be difficult to adequately clean and seal. [4],[5],[36] The most favorable histological response at the periapical region was seen when instrumentation and filling ended at the level of the AC. [62] Sjögren et al., [78] concluded that instrumentation to the AC resulted in 90% healing in infected teeth. However, the method of identifying the AC was not clear and many teeth were prepared 1 mm short of the radiographic apex if the AF could not be identified radiographically. Teeth where instrumentation could be extended to the level of the AC were found to have better treatment outcomes than those in which the AC could not be negotiated. [76] Kuttler [4] recommended that all root canal procedures should terminate 0.5 mm short of the AF, as this point is considered to be the nearest to the AC. To remain close to the AC, a range of 0.5-1.5 mm short of the radiographic apex was recommended as an appropriate working length depending on the specific root being treated. [24]

Adopting the AC as an apical landmark has several limitations as restricting instrumentation to this position risks leaving diseased tissue apical to the AC. [79] Moreover, the AC could not be histologically identified in many teeth. [13],[80] Inflammatory processes associated with a necrotic pulp may lead to root resorption and consequently, loss of the AC. [1] When the AC was absent or poorly developed the Endocater (Hygenic Corp, Akron, OH, USA), a single-frequency impedance-based electronic apex locator, gave readings beyond the apex. [8] Nonetheless, the accuracy of the Root ZX (J. Morita Corp, Osaka, Japan), an impedance ratio-based apex locator, was not influenced by root resorption and it was able to detect the narrowest part of the canals within regions with resorption lacunae. [81],[82]

Attempting to confine root canal procedures to the level of the AC is further complicated by the variability of its position and topography. [5],[7],[9],[11],[13],[51] Clinically, setting the working length 1 mm short of the radiographic apex may position the file exactly at the AC in 22%, 35% and 11% of anteriors, premolars and molars, respectively. [83] Unfortunately, continuous cementum deposition alters the relation of the radiographic apex to the AC. [76] This implies that the use of a single 'average' distance to locate the AC is unreliable and unrealistic. [10],[24],[66] The 'unevenness' of the AC indicates that using it as an apical end-point would result in a working length that is long and short at the same time. [14]

Ricucci and Langeland [62] advocated the need for good anatomical knowledge, careful interpretation of a pre-operative radiograph accompanied with tactile sensation and visualization of bleeding on the tips of instruments to aid location of the AC. Further studies are needed to clarify the suitability of the AC as the termination point for cleaning and shaping. [10],[84]

Radiographic apex as the ideal apical termination point Schilder [47],[52] recommended extending instrumentation and root filling to the level of the radiographic apex, presumably to include all apical ramifications in the disinfection and root filling procedures. However, complete instrumentation and filling of lateral canals was practically impossible. [85] Simon [1] suggested instrumentation to the radiographic apex and then stepping back to create an apical stop for the root filling. No long-term results are available to support this concept.

Adopting the radiographic apex as a reference point would result in under- or over-instrumentation because the AF is usually not located at the radiographic apex. [79],[28] When the file was inserted to the level of radiographic apex in vitro, 50% of the teeth had files extending beyond the AF. [69] Files terminating at the radiographic apex in vivo had actually extended beyond the AF in most cases. [86]

Apical limit of instrumentation and root filling for vital teeth

Preservation of the apical pulp tissue has been recommended to achieve a better outcome in vital immature permanent teeth. This vital tissue has been considered essential for apical closure by newly-deposited hard tissue after RCT. [51],[66],[87],[88] In support of the preservation of the apical vital pulp are prognostic studies indicating that the maintenance of the apical 2-3 mm of vital pulp provides favorable results and that both over-instrumentation and overfilling in these teeth has a negative impact on treatment outcomes. [64],[78],[87],[89],[90],[92] However, the vitality of the residual apical pulp tissue was not always maintained. [93] As reviewed by Siqueira, [50] the current evidence indicates that the preservation of the pulp stump is not essential for apical integrity.

Apical limit of instrumentation and root filling for infected teeth

While the apical 3 mm is the most critical, [1] the entire infected root canal should be disinfected thoroughly. [50],[88] Micro-organisms located at the apical part of the canal have better accessibility to periapical tissue. This would allow them to acquire nutrition and exert harmful effects on the surrounding structures. [54] The apical parts of infected root canals exhibiting periapical lesions will harbor abundant bacteria. [94],[95],[96],[97],[98] Bacteria have been detected at the level of the apical foramen [59] and persisted in areas inaccessible to instrumentation in the apical part of roots after single-visit treatment. [99] Nonetheless, bacterial profiles of the apical part of infected canals showed a diversity of microorganisms similar to the coronal part. [100] Hence, the whole root canal must be disinfected to the level of the AF because bacteria are able to colonize the entire canal and thrive, especially apically.

The effect of over-instrumentation on the outcome of RCT in infected teeth is yet to be fully explored. Over-instrumentation reduced the healing rate of infected root canals. [92],[101] Even when the level of instrumentation was limited to the apex level, a higher frequency of apical periodontitis was observed on follow-up. [102] The prognosis of infected root canals was reported to be improved when they were instrumented close to their apices. [64],[65],[78]

Many studies concluded that terminating the root filling within 2-3 mm of the radiographic apex results in the best long-term prognosis and overfilling or underfilling adversely affected the outcome. [64],[78],[90],[91],[92],[101],[102],[103],[104] On the other hand, it has been suggested that the extent of root filling does not influence the apical status of infected root canals. [105],[106],[107] Overfilled teeth showed signs of apical healing at longer follow-up periods because the extruded material delays, but does not prevent, healing. [108] In support of this finding is that resorption of excess sealer was evident with longer follow-up observations. [109],[110]

It must be kept in mind that the levels of instrumentation and root filling are related factors and that most commonly; the root canal will be filled to the level it is instrumented to. Thus, separating the effect of both factors might not be possible. Nevertheless, it seems that they both exert a significant influence on the outcome of treatment, regardless of the pre-operative pulp status of the tooth. [111]

Apical limit of instrumentation and root filling for non-surgical re-treatment cases

Over-instrumentation reduced the healing rate of conventional non-surgical re-treatment cases and led to development of new apical lesions. [112] Overfilling was found to result in lower healing rates of conventional non-surgical re-treatment cases. [112],[113] This was in contrast to the findings reported in other studies, in which the outcome of conventional non-surgical re-treatment was not influenced by the extent of the root filling. [78],[114] In fact, Sjögren et al. [78] identified the quality of root filling to be more important for achieving optimum results after conventional non-surgical re-treatment. Analyzing the results of the Toronto study showed that the extent of the root filling significantly affected the treatment outcome in the Phases I and II studies. [115] When the sample size was almost doubled in Phases III and IV, this factor was eliminated. [116] Others demonstrated that overfilled re-treated teeth may show signs of apical healing when the follow-up period was extended. [117]


  Conclusion Top


The apical part of the root includes several landmarks including the AC, AF and CDJ. Adopting any of these landmarks as an apical limit of instrumentation and root filling has its advantages and disadvantages. Attaining an optimum working length is a pre-requisite for favorable root canal treatment outcome. Variability in the positions of the AC, AF and CDJ complicates their clinical detection. The apical limit of instrumentation and root filling could differ according to the pre-operative status of the pulp. Root canal treatment outcome studies reported that extending the root filling to within 2-3 mm from the radiographic apex is strongly associated with favorable prognosis.

 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]


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