K. S. Al-Fouzan
Dental Department, King Fahad National Guard Hospital, Riyadh, Kingdomof Saudi Arabia.

Introduction.
Athorough knowledge of the anatomy of teeth involved in root-canal treatment is essential for successful debridement and obturation of the root-canal system. Recognition of unusual canal configurations and variations are paramount, because it has been established that the rootwith a single tapering canal and apical foramen is the exception rather than the rule (Abou-Rass et al. 1980).
Of particular interest is the canal configuration of the mandibular second molar as a great deal of variation can occur, C-shaped canals are common (Cooke & Cox 1979) with the presence of a thin ￠n connecting the root canals being the main anatomical feature (Barril et al. 1989). Once recognized, the C-shaped canal provides a challenge with respect to debridement and obturation, especially because it is unclear whether the C-shaped orifice found on the ￡oor of the pulp chamber actually continues to the apical third of the root.
The shape and the number of roots are determined by Hertwig’s epithelial sheath, which bends in a horizontal plane below the amelocemental junction and fuses in the centre leaving openings for roots (Orban & Mueller 1929). Fused rootsmay formeither bycoalescenceowing to cementum deposition with time, or as a result of failure of Hertwig’s epithelial sheath to develop or fuse in the furcation area (Pederson 1949). A C-shaped canal appears when fusion of either the buccal or lingual aspect of the mesial and distal roots occurs. This fusion remains irregular, and the two roots stay connected by an interradicular ribbon (Barnett 1986, Barril et al. 1989).The pulp chamber has a single ribbon-shaped ori- ￠cewitha1808arc fromthe distal tomesial canal (Cohen & Burns 1994). The ￡oor of the pulp chamber is deep and has an unusual anatomical appearance (Barnett 1986).Two or three canals may be found inthe C-shaped groove, or the C-shape may be continuous throughout the length of the root.
The mandibular second molars with C-shaped canals vary in their configuration (Yang et al. 1988, Melton et al.1991). Clearly, the recognition of these configurations facilitates cleaning, shaping, and obturation (Rice & Gilbert 1987). Preoperative radiographs demonstrate close fused roots or images of two distinct roots. This occurs when the ￠n is thin and thus, not visible on the X-ray and makes clinical recognition of the C-shaped canal unlikely until access to the pulp chamber has been achieved (Barril et al. 1989). Radiographs takenwhilst negotiating the root-canal systemmay suggest suchanatomyas theycanreveal two characteristics: instruments tending to converge at the apex (Rice & Gilbert 1987) or instruments appearing both clinically and radiographically to be centered and appearing to be exiting the furcation (Fig.1).This cancause confusion and initiate a search for a perforation (Barril et al. 1989, Melton et al.1991).
The C-shaped canals were ￠first documented by Cooke & Cox (1979) in three case reports. Similar reports have since been published (Wells & Bernier 1984, Rabie 1985, Barnett 1986). Endodontic textbooks state that the Cshaped canal is not uncommon (Walton & Torabinejad 1996) and this is con￠rmed by studies in which frequencies ranging from 2.7 to 8% have been reported (Cooke & Cox 1979,Weine et al.1988). Although it is not speci￠- cally stated, these ￠gures probably relate to teeth of Caucasoid origin with an absence (Ainamo & Loe 1968, Tamse&Ka!e1981,Vertucci1984)ornearabsence (Weine etal.1988) of C-shaped canals. Investigations of the rootcanal anatomy of mandibular second molars conducted onJapanese (Kotoku1985), Chinese (Yang et al.1988) and Hong Kong Chinese (Walker 1988) populations showed a high incidence of C-shaped canals (31.5%). It has thus been established that this particular anatomy is more frequent in Asians than inother racial groups (Manning 1990).No dataareavailable onthe incidence of this canal configuration in the Saudi Arabian population.
This study was undertaken to provide information on the distribution of C-shaped canal anatomy, to establish a classi￠cation for the most common configurations, and to determine its frequency in a sample representing the Saudi population.

Figure 1. Radiographwith instruments in root canals for working length determination. (A) Instruments tend to converge at the apex and (B) instruments appear to perforate the furcation.

One hundred and ￠fty-one mandibular second molars scheduled for root-canal treatment were examined over a 1-year period in the Endodontic Department, King Fahad National Guard Hospital, Riyadh, Saudi Arabia. The incidence of C-shaped canalswas revealedfollowing radiographic and clinical examination. Three preoperative radiographswere takenat a constant target-￠lmdistance and angulation by utilizing the extension cone paralleling device (Rinn Corp., Elgin, III, USA): onewith a908 angulation to the tooth ina buccolingual direction and another two at a mesial and distal angulation of approximately 208 to allow better visualization of the buccolingual anatomy.The radiographs were examined on a viewer using a peripheral block and a 6_magnifying lens.The numberand the positionof root canalswere noted. Three postoperative radiographs using the same exposure geometry were taken to con￠rm canal configuration.
Clinical examination of the pulp chamber and canal orifices was carried out and root canals were investigated with size 10 K-￠les (Kerr Co., Romulus, MI, USA); radiographs were taken to con￠rm canal morphology. Once C-shaped anatomy was recognized, teeth were categorizedusingamodi￠cationof anexisting classi￠cation (Melton et al.1991).

• Category I: continuous C-shaped canal running from the pulp chamber to the apex (Fig. 2).
  
• Category II:‘semicolon’ (;) shaped orifice inwhich dentine separated a main C-shaped canal from onemesial distinct canal (Fig. 3).
  
• Category III: subdivision I, C-shaped orifice inthe coronal third that divided into two or more discrete and separate canals that joined apically (Fig. 4); subdivision II, C-shaped orifice in the coronal third that divided into two or more discrete and separate canals in the mid-root to the apex (Fig. 5); and subdivision III, C-shaped orifice that divided into two or more discrete and separate canals in the coronal third to the apex (Fig. 6).

Figure 2. Category I: (A) preoperative radiograph of the mandibular second molar showing fused roots and (B) completed root filling using lateral condensation of gutta-percha showing true C-shaped canal.



Figure 3. Category II: (A) preoperative radiograph showing a radicular fusion, a large distal canal, a narrow mesial canal, and a blurred image of a third canal in centre; (B) postoperative radiograph. Note the presence of dentine separating a mesial canal from the rest of the C-shaped canal.



Figure 4. (A) Preoperative radiograph of a mandibular second molar and (B) postoperative radiograph. Note the apical fusion of the three canals.



Figure 5. (A) Preoperative radiograph of a mandibular second molar and (B) postoperative radiograph. Note the main C-shaped canal splits near themid-root into three canals.



Figure 6. (A) Preoperative radiograph of a mandibular second molar and (B) postopertive radiograph. Note the main C-shaped canal splits in the coronal third into three canals.

Results.
Of the151molars,16 (10.6%) exhibited C-shaped canals. Five molars presented a continuous C-shaped canal (category I), and three had a semicolon shape (category II) with a mesial canal located on the buccal or lingual side (depending on where the fusion occurred). In one instance, this canal swung back and merged with the distal canal, thus yielding a single foramen; in the other two cases, it remained distinctwith itsownportal of exit. The remaining eight were considered as category III, amongst those: two were subdivision I; two were subdivision II; and the remaining four were subdivision III.

Discussion.
Previously reported di!erences in the root-canal anatomy of mandibular second molars may be owing to the population fromwhich the teethwere chosen. If a Caucasian population were studied, then predominantly tworooted mandibular second molars could be expected (Ainamo & Loe 1968,Tamse & Ka!e 1981,Vertucci 1984, Weine et al. 1988), whereas if an Asian or Mongoloid population were examined then a higher number of teeth would be single-rooted (Kotoku 1985, Walker 1988). Single-rooted mandibular second molars have been reported less frequently (Kotoku 1985, Yang et al. 1988,Walker1988,Weine et al.1988), and the canal types have not been fully investigated.
A radiographic study (Weine et al. 1988) of three single- rooted specimens found one to have a single canal and two to be C-shaped, and photographs from another study (Walker 1988) of selected specimens that were cleared showed the complex anatomy characteristic of single-rooted specimens. In the current investigation, a variety of canal types was found, with category I representing almost one-third of the sample.
Cooke & Cox (1979) stated that it was impossible to diagnose C-shaped canals on the preoperative radiograph. In this study, almost all preoperative radiographs of category I, II, and III (subdivision I) showed common characteristics that provided a typical image and allowed identi￠cation of this anatomical condition. In fact,most radiographs revealed radicular fusionorproximity, a large distal canal, a narrow mesial canal, and a blurred image of a middle third canal (Fig. 3A).
Weine et al. (1988) advised placing ￠files inthe canals to determine canal configuration of the mandibular second molar. However, using a radiograph showing ￠les set to the canal terminus to diagnose and to determine canal morphology may not give the results expected. In some instances, it may be di⁄cult to distinguish between C-shaped canal or one with single or three canals joiningapically. Thus, itwas necessary to con￠rm the diagnosis by exploring the access cavity. Two types of root-canal orifices were observed.

1. A complete C-shaped orifice located either on the buccal or lingual aspect of the ￡oor, that extended from the usual locationof the distal canal to the usual location of the mesial canals (Fig.7).
2. Two or three orifices: a C-shaped orifice extending from distal to mesiobuccal ina buccal position separated with dentine from an independent mesiolingual orifice, or from distal to mesiolingual in a lingual positionwith an independent mesiobuccal orifice. Some teeth had onemesialand one distal orifice, and one orifice between the two (Fig. 8).

Figure 7. C-shaped chamber floor showing a complete C-shaped root-canal orifice in the formof a deep trough connecting the distal, mesiobuccal andmesiolingual canal orifices.



Figure 8. C-shaped chamber ?floor of mandibular second molar with mesial, distal, andmidbuccal canal orifices are seen.

In the present study, all root canals that conformed to the general structure of a‘C’and occurred in a C-shaped root were described as C-shaped root canals, whether or not a separate canal or orificewas observed.The classi￠ cation system of canal types described by Melton et al. (1991) was modi￠ed and used to describe the canal system in C-shaped roots. In type I canal, a single canal is present fromorifice totheapex; thishasbeen described as a true C-shaped canal. Other more complex canal types were found in C-shaped roots; category III (subdivision III), the type with three canals, which did not necessarily combine, occurred most frequently.
The root-canal anatomyof teeth froma Chinese population with the C-shaped canals has been reported previously (Yang et al.1988), when ￠rst and second molars were examined using a transparent technique (Robertson et al. 1980). The C-shaped roots occurred in 31.5% of second molars. They found that roots with separate canal orifices formed separate canals as well as true Cshaped canals.The latter, which were described as roots having a C-shaped canal from the orifice to the apex, occurred in 7.4% of the sample of mandibular second molars. Manning (1990) found that, amongst 19 teeth, three (10%) had true C-shapes. Haddad et al. (1999) reported that true C-shaped canals with a single swath of canal were the exception rather than the rule. In the present study, from approximately the same number of teeth, ￠ve teeth exhibited this form.
In this study, all patients showing category III con￠- guration were under 40 years of age. This is in contrast to the observation of Manning (1990) that age-related deposition of dentine formed separate canals. Manning (1990) also reported that category III (subdivision III) systems occurredmost frequently,which is inagreement with this study. Melton et al. (1991) reported that the Cshaped canals had various configuration at di!erent root levels; this was particularly true in all category III canals.
The study of the incidence of C-shaped canals in the mandibular second molars using radiographic techniques might appear to have certain disadvantages. Only a 2D image of the tooth is seen and conceivably both roots and root canals can bemissed. However, clinically it remains the only noninvasive method available, and bystrictlyadhering to the radiographic criteria for determining the incidence of C-shaped canals in themandibular second molars, the ￠ndings of this study were in good accordwiththose of other recent studies using various techniques.
The frequency obtained in this investigation (10.6%) lies between the 31.5% frequency reported byYang et al. (1988) and the 2.7% frequency reported byWeine et al. (1988).The geographical position of Saudi Arabia in particular and theMiddle East, ingeneral,may be the explanation for this phenomenon. However, further studies would be necessary to con￠rm this phenomenon.

References.

Abou-Rass M, Frank L, Glick DH (1980) The anticurvature method to prepare the curved root canal. Journal of the American Dental Association101,792-4.
AinamoJ, Loe H (1968) A stereomicroscopic investigation of the anatomy of the root apices of 910 maxillary and mandibular teeth. Odontologiska ForeningensTidskrift 76, 417-26.
Barnett F (1986) Mandibular molar with C-shaped canal. Endodontics and DentalTraumatology 2,79-81.
Barril I, Cochet JY, Ricci C (1989) Le traitement des canaux presentant une con?guration dite en'C'. Revue Francaise D'endodontie 8, 47-58.
Cohen S, Burns R (1994) Pathways of the Pulp,7th edn. St. Louis, USA: CV Mosby, pp.184-9.
CookeHG, Cox FL (1979) C-shaped canal configurations inmandibular molars. Journal of the American Dental Association 99,836-9.
Haddad GY, NehmeWB, Ounsi HF (1999) Diagnosis, classification, and frequencyof C-Shaped canals inmandibular second molars in the Lebanese population. Journal of Endodontics 25, 268-71.
Kotoku K (1985) Morphological studies on the roots of the Japanese mandibular second molars. Shikwa Gakuho 85,43-64.
Manning SA (1990) Root canal anatomy of mandibular second molars. Part II. C-shaped canals. International Endodontic Journal 23, 40-5.
Melton DC, Krell KV, FullerMW(1991) Anatomical and histological features of C-shaped canals in mandibular second molars. Journal of Endodontics17,384-8.
Orban B, Mueller E (1929) The development of bifurcation of multirooted teeth. Journal of theAmerican Dental Association 16, 297-319.
Pederson PO(1949) The EastGreenland Eskimodentition: Numerical variations and anatomy. Bianco Lunos Bogtrykkeri, Copenhagen.
Rabie G (1985) Mandibular molar with merging mesiobuccal and distal root canal. Endodontics and DentalTraumatology1, 191-4.
Rice RT, Gilbert BO (1987) An unusual canal configuration in mandibular first molar. Journal of Endodontics13,513-5.
Robertson D, Leeb J, McKee M et al. (1980) Aclearing technique for the study of root canal systems. Journal of Endodontics 6, 421-4.
TamseA, Ka!e I (1981) Radiographic surveyof the prevalence of conical lower second molars. InternationalEndodontic Journal 14,188-90.
Vertucci FJ (1984) Root canal anatomyof thehumanpermanent teeth. Oral Surgery, Oral Medicine and Oral Pathology 58, 589-99.
Walker RT (1988) Root form and canal anatomy of mandibular second molars in a Southern Chinese population. Journal of Endodontics14,325-9.
Walton R, Torabinejad M (1996) Principles and Practice of Endodontics, 2nd edn. Philadelphia, USA: W.B. Saunders Co., pp.177-8.
Weine FS, Pasiewicz RA, Rice RT (1988) Canal configuration of the mandibular second molar using a clinically oriented in vitro method. Journal of Endodontics14, 207-13.
WellsDW,BernierWE (1984) Asinglemesial canalandtwodistal canals in a mandibular second molar. Journal of Endodontics 10,400-3.
Yang ZP, Yang SF, Lin YL et al. (1988) C-shaped root canals in mandibular second molars in Chinese population. Endodontics and DentalTraumatology 4,160-3.