C-shaped root canal configuration: A review of literature.

The aim is to review and discuss the etiology, incidence, anatomic features, classification, diagnosis and management of the C-shaped canal configuration. C-shaped canal configuration is a variation that has a racial predilection and is commonly seen in mandibular second molars. The intricacies present in this variation of canal morphology can pose a challenge to the clinician during negotiation, debridement and obturation. Manual and electronic searches of literature were performed from 1979 to 2012, in Pub Med by crossing the keywords: C-shaped canals, mandibular second molar, mandibular first premolar, root canal morphology. Knowledge of the C-shaped canal configuration is essential to achieve success in endodontic therapy. Radiographic and clinical diagnoses can aid in identification and negotiation of the fan-shaped areas and intricacies of the C-shaped anatomy. Effective management of this anomalous canal configuration can be achieved with rotary and hand instrumentation assisted with sonics and ultrasonics. Modifications in the obturation techniques will ensure a 3-dimensional fill of the canal system and chamber retained restorations like amalgam or composites, serve as satisfactory post endodontic restorations.

INTRODUCTION

A thorough knowledge of the root canal anatomy and its variations is required for achieving success in root canal therapy, along with diagnosis, treatment planning and clinical expertise.[1234] One such variation of the root canal system is the C-shaped canal configuration. It is termed so because of the C-shaped cross-sectional anatomical configuration of the root and root canal.[5] This condition was described for the first time in literature by Cooke and Cox[6] in 1979, though Weine et al.,[7] reported that several clinicians had suggested its presence in lectures earlier. Manning in his description of root canal anatomy of mandibular second molars, stated that C-shaped roots and root canals were first documented in 1908 and 1911, after examination of the skeletal remains of members of the Neanderthal race, which were predecessors of the Mongoloid race that includes Asian populations.[8] Numerous incidence studies prove the racial predilection of this variation.[15791011121314151617181920212223242526272829] Though most commonly found in mandibular second molars,[15791011121314151617181920212223303132] the C-shaped canal configuration may also occur in mandibular premolars,[242526272833] maxillary molars,[334] and mandibular third molars.[329] The C-shaped canal configuration presents with variations in both the number and location of the canal(s), as the canal(s) courses from the coronal to the apical third.[2] The complexity of this canal configuration proves to be a challenge with respect to debridement and obturation and possibly the prognosis during root canal therapy.[24] Recognition of a C-shaped canal configuration before treatment can facilitate effective management, which will prevent irreparable damage that may put the tooth in severe jeopardy.[735] The aim of this article is to discuss the etiology, incidence, anatomic features, classification, diagnosis and management of the C-shaped canal configuration. Manual and electronic searches of literature were performed from 1979 to 2012, in PubMed by crossing the keywords: C-shaped canals, mandibular second molar, mandibular first premolar, root canal morphology.

ETIOLOGY

Ever since the identification of the C-shaped canal anatomy, various causes have been postulated for its formation. Earlier, teeth with C-shaped roots were likened to taurodents; however, the definition of teeth that fit this description has been changed.[8] Manning[8] also conjectured that the C-shaped anatomy with separate canals could be the result of age changes due to deposition of dentin on the walls of the canal. However, this theory was refuted as separate canals in roots with C-shaped anatomy were observed even in patients under 40 years of age.[12] The failure of fusion of Hertwig's epithelial sheath is the most lucid explanation for the formation of the C-shaped canal configuration.[36] Failure of the Hertwig's epithelial sheath to fuse on the buccal side will result in the formation of a lingual groove, and failure to fuse on the lingual would result in a buccal groove.[8] Hence, this fusion is not uniform and a thin interradicular ribbon connects the two roots together.[1037] Failure of the sheath to fuse on both the buccal and lingual sides will result in the formation of a conical or prism-shaped root.[8] Fusion is most likely to occur if the distance between the root canals is small.[32] Earlier, the irregular fusion of the Hertwig's epithelial sheath was attributed to trauma, such as radiation or chemical interference, but following the documentation of racial predilection, it is more likely to be of genetic origin.[838] The root canals in such teeth merge in a very wide, slot-like, single, continuous root canal morphology, which can assume the shape of a letter C.[39]

INCIDENCE

The C-shaped canal configuration shows an ethnic predilection. As can be evidenced from [Table 1], it has frequently been reported in countries belonging to the Asian continent. East Asian population groups like Chinese (0.6%-41.27%) and Koreans (31.3%-45.5%) display a high prevalence of this variant.[914152122232728] Among the South Asian countries, Burmese population showed a prevalence of 22.4%, which was much higher than the Indian, Thai or Sri Lankan population.[11131718202426] Higher incidence of C-shaped anatomy was documented in Lebanese population (19.1%) as compared to the other West Asian population groups (Iranian, Jordanian, Saudi Arabian).[5101219] An incidence of 10% was reported in Sudanese population from the African continent.[16]

Table 1

Incidence of C-shaped root canal configuration

Canal configuration has been shown to have a high prevalence in mandibular second molars with a percentage ranging between 2.7%-45.5%.[15791011121314151617181920212223303132] Incidence studies in mandibular premolars have been reported in Chinese, Indian and Iranian population, with the highest frequency being reported in the Chinese population (29.7%).[2425262733] The C-shaped variation in canal anatomy has also been reported in maxillary first molars (0.12%), maxillary third molars (4.7%), mandibular third molars (3.5%-4%) and mandibular second premolars (1%).[3282934] Singla & Aggarwal[40] reported finding a C-shaped canal configuration in the palatal canal of a maxillary second molar. The possibility of finding C-shaped canal anatomy in maxillary lateral incisors with type III radicular groove has also been documented.[41] There is no correlation of C-shaped canal configuration with gender and also with age and tooth position. Bilateral occurrence of C-shaped canals has been reported in a percentage of 70%-81%.[2242]

ANATOMIC FEATURES

The following are the pertinent features in relation to the external root anatomy and configuration of the pulp chamber and the root canal system usually found in C-shaped mandibular molars, though similar features may be found in C-shaped maxillary molars and mandibular premolars.

Roots

A conical or square configuration is characteristic of roots having a C-shaped canal.[843] The root configuration of molars having this canal shape may be represented by fusion of either the facial or lingual aspect of the mesial and distal roots. The roots display an occluso-apical groove on the buccal or lingual surface, which represent the line of fusion between mesial and distal roots. The surface opposite this radicular groove is convex.[2] Peiris et al.,[22] referred to the C-shaped roots of mandibular second molars as being ‘gutter-shaped’.

Pulp chamber

The pulp chambers of teeth with C-shaped canals mostly have greater apico-occlusal width with a low bifurcation.[13] This results in a deep pulp chamber floor, which has uncommon anatomical configuration.[37] The connecting slit that gives the tooth its name of “C-shaped” may have closure to the buccal or lingual. If the buccal portion of the mesial and distal roots is fused, the slit goes through the area of fusion, and so the “C” is closed to the lingual. If the lingual portion of the roots is fused, then the “C” is closed to the buccal.[7]

Root canal system

The root canal system of C-shaped canals shows broad, fan-shaped communications from the coronal to the apical third of the canal. The canal(s) change shape from the coronal aspect of the root. For example: A continuous C-shaped canal would change to a semicolon configuration in the midroot and then becomes continuous C-shape in the apical third of the root or vice versa.[2] Accessory and lateral canals, inter-canal communications and apical delta can be found in a prevalence of 11-41%, in the apical region of C-shaped canals.[44] Cross-sectional C-shaped canal morphology reveals thinner lingual walls than buccal with the lowest value measured being 0.26 mm.[4] The varied intricacies in the C-shaped canal anatomy are outlined in the section on classification.

Mandibular premolars

All C-shaped premolars have shallow or deep radicular grooves located on the external mesiolingual surface of the root.[274546] These grooves usually start 3 mm below the cementoenamel junction (CEJ) and may or not extend to the apex. The mean depth of the grooves at some areas on the root can be around 1.5 mm.[46] This must be taken into consideration during cleaning and shaping. The morphology of C-shaped canals in mandibular first premolars may present one of the following four features: Continuous C-shaped canal only, semilunar buccal canal only, combination of continuous C-shape and semilunarbuccal canal or C-shaped canal interrupted by non-C-shaped canal.[27]

CLASSIFICATION

The clinician should not assume a fixed spatial relationship, if a C-shape is identified, but rather be aware of other possibilities.[7] To facilitate a better understanding of the root canal anatomy of C-shaped canals the numerous classifications were proposed. The earliest classification were proposed by Manning[1] and Melton et al.[2] However, there was no clear distinction between the categories in Melton's classification.[47] Besides, Melton's and Manning's classifications describe only the appearances of the canal orifices, but fail to describe how the C-shaped configuration may vary along the root length.[44748] . Various other classifications were then proposed by Al-Fouzan[12], Fan et al.,[35] Fan et al.,[47] Gao et al.,[49] and Min et al.[50]

DIAGNOSIS

Preoperative radiographic diagnosis

A preoperative radiograph usually provides various clues in the identification of any variation in root canal morphology. However, there are differences in opinions on the value of a preoperative radiograph in diagnosing a C-shaped case. Cooke & Cox[6] were of the opinion that it is not possible to diagnose C-shaped canals on preoperative radiographs. Conversely, some investigators described four radiographic characteristics that can allow prediction of the existence of this anatomical condition: radicular fusion, radicular proximity, a large distal canal or a blurred image of a third canal in between.[1012] Hence, a C-shaped root in a mandibular second molar may present radiographically as a single-fused root or as two distinct roots with a communication.[1113] When the communication or fin connecting the two roots is very thin, it is not visible on the radiograph and may thus give the appearance of two distinct roots.[10] The radiograph may also reveal a large and deep pulp chamber, usually found in C-shaped molars.[1013] Fan et al.,[35] divided the radiographic appearance that the C-shaped teeth are present with three types [Figure 1a]. In type I, the C-shaped canal system actually appears as two distinct canals, because the isthmus that links the mesial and distal “main” canals is very thin and hence is not detected radiographically. In the radiographic type II, the mesial and distal canals assume their own individual course to the apex. Thus, there are apparently two distinct canals on the radiograph. In type III, one canal continues its course to the apex giving the image of a distinct canal whereas the other(s) proceeds very close to or within the fused area, that is, the “web” between the two main roots in the apical third. Hence, the canal may seem to exit into the groove radiographically. Wang et al.,[23] reported a higher incidence in the recognition of C-shaped canals using a combination of radiography and clinical examination under the microscope (41.27%) than using the radiography (34.64%) or clinical examination (39.18%) alone.

Figure 1

(a) Radiographic classification of C-shaped canal configuration. (b) Classification of pulp chamber floor of teeth with C-shaped canal configuration

Preoperative clinical diagnosis

The crown morphology of teeth with C-shaped anatomy does not present with any special features that can aid in the diagnosis. A longitudinal groove on lingual or buccal surface of the root with a C-shaped anatomy may be present.[47] Such narrow grooves may predispose the tooth to localized periodontal disease, which may be the first diagnostic indication.[13]

Clinical diagnosis following access cavity preparation

Some investigators asserted that since radiographic diagnosis is difficult, clinical diagnosis of C-shaped canals can be established only following access to the pulp chamber.[231] The pulpal floor in C-shaped teeth can vary from peninsula like with a continuous C-shaped orifice to non C-shaped floors as per the classification given by Min et al., [Figure 1b].[50] However, when a C-shaped canal orifice is observed, say, under the operating microscope, one cannot assume that such a shape continues throughout its length.[47] Fan et al.,[47] stated that for mandibular second molar to qualify as having a C-shaped canal system, it has to exhibit all the following three features:

Fused roots;

A longitudinal groove on lingual or buccal surface of the root;

At least one cross-section of the canal should belong to the C1, C2, or C3 configuration, as per Fan's anatomic classification [Figure 2].

Figure 2

Fan et al.,'s anatomic classification of C-shaped canal configuration

Clinical and radiographic diagnoses during working length determination

Working length radiographs are more helpful than preoperative and final radiographs in diagnosing C-shaped canals.[31] In a true C-shaped canal, (single canal running from the orifice to the apex) it is possible to pass an instrument from the mesial to the distal aspect without obstruction.[13] In the semicolon type, (one distinct canal and a buccal or lingual C-shaped canal) whenever an instrument was inserted into any side of the C-shaped canal, it always ends in the distal foramen of the tooth and a file introduced in this canal could probe the whole extension of the C.[51] When negotiating the C-shaped canal, instruments may be clinically centered.[2] Radiographically, the instruments may either converge at the apex or may appear to be exiting the furcation, thus adding to the confusion and troublesome task of determining whether a perforation has occurred.[24751]

MANAGEMENT

The high percentage of canal irregularities, such as accessory to lateral canals, and apical delta in a C-shaped canal makes it difficult to clean and seal the entire canal system adequately.[844] The wide fins and small surface area of these canals preclude complete debridement using traditional hand instrumentation techniques, which can lead to failure of root canal therapy.[2] Therefore, careful location and negotiation of the canals and the meticulous mechanical and chemical debridement of the pulp tissue should be carried out in order to successfully treat a C-shaped canal.[15]

Location and negotiation of canals

Modifications in the access cavity designs may be required for teeth with C-shape configuration to facilitate location and negotiation of the complete canal system.[52] When the orifice is continuous C-shape or arc like Mesiobuccal-Distal (MB-D), the number of canals can vary from one to three; when the orifice is oval or flat, the number of canals may be one or two; and when the orifice is round, there is usually only one canal below the orifice.[50] Hence, for continuous C-shape orifice, 3 initial files are inserted, one at either end and one in the middle. When the orifice is oval, two files are inserted, that is, one file at each end of the orifice and when the orifice is round, one initial file is inserted.[53] Calcifications present in the pulp chamber may disguise the C-shape of the canal system. In such cases, several orifices may be probed that link up on further instrumentation.[13] There are also chances of missing out on canals because of bifurcation, dentin fusion, and curvatures.[53] Exploration should be carried out with small size endodontic files, such as a no. 8, 10, 15 K-file with a small, abrupt apically placed curve, to ensure that these irregularities are not missed.[50]

Cleaning and shaping

In order to access all the irregularities in the C-shaped canal system, the orifice portion of the slit can be widened with Gates Glidden drills.[54] However, C1 (continuous C type) and C2 (semicolon type) configurations always have a narrow isthmus and care should be taken to avoid perforation during their preparation.[35] It has been suggested that for such narrow, interconnecting isthmus areas, Gates Glidden drills should not be used and cleaning should be carried out by using a size 25 instrument or smaller.[4355] There is a high risk of root perforation at the thinner lingual walls of C-shaped canals during cleaning and shaping. The Abou-Rass et al.,'s anti-curvature filing technique has been recommended to avoid danger zones that are frequently present at mesiolingual walls.[4] However, Jin et al.,[15] reported that the tooth structure of the groove area in C-shaped canal was thicker than that of the danger zone of normal second mandibular molar and hence, the C-shaped canals were as safe from strip perforation as other teeth in the limitation of this study. Extreme caution must be taken to prevent strip perforation during cleaning and shaping of C-shaped mandibular premolars, which display thin dentinal walls in the radicular groove area.[27]

Very few studies have evaluated the effectiveness of cleaning procedures in relation to C-shaped root canals [Table 2]. Though nickel-titanium rotary instruments seem to be safe in such canals, further enlargement to an apical dimension greater than size 30 (0.06 taper) is not recommended.[56] After instrumentation by NiTi rotary instruments, K-files or H-files could be passively introduced into the canal, and filling could be specifically directed towards the isthmus areas to obtain better debridement in clinical practice.[57] The recently developed self-adjusting file (SAF) system has been reported to be more efficacious than the protaper system for shaping of C-shaped canals.[58]

Table 2

Studies evaluating the effectiveness of cleaning procedures in C-shaped canals

Because of the large area of canal space, it is doubtful that intracanal instruments can reach and debride the entire portion of the continuum, making irrigation procedures more significant.[7] It is recommended that cleaning of the C-shaped canal system with rotary instruments should be assisted by ultrasonic irrigation.[56] Besides the use of sonication and ultrasonication, the use of chemical agents for disinfection cannot be over emphasized in the treatment of C-shaped root canal system.[57] Some authors have advocated the use of calcium hydroxide as an intracanal medicament for a period of 7-10 days.[5154]

With regard to the management of C-shaped canal configuration in mandibular premolars, many complicating factors make them difficult to treat. Anatomically the diameter and width of mandibular first premolar is much smaller than mandibular second molars. The small size of mandibular first premolar limits the coronal access to the complex root canal system, which unlike the mandibular second molar, is found apically.[45]

Obturation

Obtaining a three dimensional fill of a C-shaped canal may prove to be a problem due to the various intricacies present within the root canal system. If a cold condensation technique is adopted for obturation, deeper penetration of condensation instruments in several sites will be necessary.[7] To ensure proper placement of the master cones in C-shaped canals, Barnett[37] recommended placing a large diameter file in the most distal portion of the canal, before seating the master cone in the mesial canal. The file is then withdrawn and the master cone of the distal canal is seated, followed by placement of accessory cones in the middle portion of the C-shaped canal. Studies have shown that following the cleaning and shaping, the remaining dentin thickness around the canals is usually 0.2 to 0.3 mm. The resultant forces of compaction during obturation can exceed the dentin canal resistance, which may result in root fracture and perforation of the root.[495455] In this regard, the thermoplasticized gutta-percha technique may prove to be more beneficial. The aim of this technique is to move the gutta-percha and sealer into the root canal system under a hydraulic force.[59] But in C-shaped canals the hydraulic forces can dramatically decrease and can seriously compromise the obturation quality due to the following reasons: (a) there are divergent areas that are frequently unshaped, which may offer resistance to obturating material flow (b) communications exist between the main canals of the C-shape through which the entrapped filling materials that should be captured between the apical tug back area and the level of condensation may pass from one canal to another. Walid's technique[54] aimed to overcome these problems. This technique involves placing the master points simultaneously in the C-shaped canal. A large plugger is placed on one of the seared master points while the other master point is down packed with a smaller plugger. This increases the resistance towards the passage of obturating material from one canal to another. The smaller plugger is then held in place while the other point is down packed. This offers backpressure on entrapped filling materials and enhances the seal. Ordinola- Zapata et al.,[60] analyzed gutta-percha filled area of C-shaped canals filled with Maggiore's modified MicroSeal technique with reference to the radiographic features. They found that the apical third was less accurately filled and concluded that C2 canals with a 60° angle could be less difficult to fill than a 120° or 150° C2 canal.

Post endodontic restorations

Post endodontic restoration of teeth with a C-shaped canal configuration may also be compromised due to the relatively small amount of dentin between the external surface of the root and the internal canal system.[2] In the study by Cheung & Cheung, none of the prepared canals had a canal wall greater than 1 mm on the furcal aspect after instrumentation.[56] At least 1 mm of sound tooth structure should be present around a post if needed, for resistance to root fracture.[61] Hence, prefabricated or cast posts increase the risk of creating a strip perforation.[2] Besides, no prefabricated post (circular or conical i.e. of a circular cross section) would fit the C-shaped canals.[5456] Since the floor of the pulp chamber is deep it can provide ample retention from the available undercuts.[3743] Chamber-retained, bonded amalgam or composite is a better choice as the core or as the final restoration in these teeth.[5456]

CONCLUSION

The C-shaped root canal configuration has an ethnic predilection and a high prevalence rate in mandibular second molars. Understanding the anatomical presentations of this variation will enable the clinician to manage these cases effectively.