Permanent human molars exhibit a wide range of variations in root number and their morphology. Even the most stable among them, the mandibular first permanent molar (MFPM), sometimes has three roots instead of the usual two (Figure 1). In this case there is one mesial and two distal roots. The distobuccal (DB) one corresponds to a regular distal root and the distolingual (DL) one to a supernumerary root, radix entomolaris (RE).
The RE is morphologically more variable than the mesial and distal roots (1). It varies from a rudimentary extension to a well-formed root approaching the length of the distal root. Moreover, it is often curved, especially in the radiographically unseen buccolingual plane, and may be separate or non-separate in relation to the distal root. However, this root invariably contains a single canal with a nearly round canal cross-section (2-5).
The prevalence of the MFPM with an RE in European populations (Dutch, Finn, English, German) ranges from 0.7% to 3.4% (6-9). The RE is, however, much more frequent in Asian populations (Chinese, Japanese, Thai) where the prevalence exceeds 20% (4, 10-17). In addition, the RE appears to be more frequent in individuals affected by Turner syndrome than in healthy individuals (18).
An RE represents a diagnostic and therapeutic challenge, particularly in endodontics, periodontology and oral surgery. It may be the cause of endodontic failure if not identified before or during treatment. In addition, complications may arise during cleaning and shaping of a severely curved root canal in the RE (straightening and ledging of the root canal, perforation, deformation of the apical foramen or instrument fracture). On the other hand, the RE appears to be a local factor that contributes to the progression of advanced periodontal disease (19). Lastly, during extraction of a tooth an RE can be easily fractured because of its apical gracility and curvature.
The aim of this case report is to present the endodontic management of a MFPM with an RE in a Caucasian patient. In the present case, the patient was initially treated by a general dentist and then referred to an endodontic office because a ledge was created during instrumentation of a buccolingually curved RE canal. Additionally, this paper highlights the root anatomy of such teeth and reviews the usefulness and limitations of dental radiography in RE diagnosis.
A 14-year-old Caucasian patient was referred by his general dentist to the Department of Operative Dentistry and Endodontics, University Medical Centre Ljubljana for continuation of endodontic management of the right MFPM (tooth 46). The referring dentist reported that she identified four root canals, but was not able to establish patency of the DL canal. There was asthma in the patient's medical history. He had been receiving an oral antihistamine, an inhaled corticosteroid, and an inhaled short-acting beta2-agonist.
Clinical examination revealed a temporary mesio-occlusal restoration. The tooth was not sensitive to percussion and had normal mobility. The probing depth was less than 4 mm at all six locations around the tooth. The neighbouring second permanent molar was about to emerge into the oral cavity. An intraoperative periapical radiograph, made by the referring dentist, clearly showed three separate roots of this tooth and radiolucency associated with the mesial root (Figure 2). A supernumerary root was identified as an RE and a diagnosis of asymptomatic apical periodontitis was made. The root canal treatment was completed in two visits.
The inferior alveolar nerve was anesthetized using 2% mepivacaine with epinephrine (Scandonest 2% Special; Septodont, Saint-Maur des Fossés, France). After removal of the temporary restoration, the missing mesial wall of the access cavity was reconstructed with Fuji IX glass-ionomer cement (GC, Tokyo, Japan) and a rubber dam (Hygenic Dental Dam, Coltène Whaledent, Langenau, Germany) was placed in position. The operative field was disinfected with 2% chlorhexidine. Inspection of the pulp chamber floor revealed mesiolingual (ML), mesiobuccal (MB), distolingual (DL), and distobuccal (DB) root canal orifices (Figure 3). Initial canal exploration and enlargement was performed by K-files in sizes 8 through 15 (Dentsply Maillefer, Ballaigues, Switzerland). A ledge was identified in the apical third of the DL (RE) canal. The root canals were coronally enlarged and relocated with SX file from ProTaper Universal series (Dentsply Maillefer, Ballaigues, Switzerland) to obtain a straight-line entry for the instruments. Using a size 10 Kerr file with a distinct curve at the tip, it was possible to bypass the ledge and reach the apical terminus of the RE. The working lengths were determined electronically using an AFA Apexfinder (EIE Ananlytic Technology, Orange, USA). Readings for the DL (RE) canal and the DB canal (in the distal root) were 22.0 mm and 22.5 mm, respectively (with the DB cusp as a reference point). Clinically, this case was classified as type III RE according to De Moor et al. (20). Root canals were shaped with ProTaper Universal rotary instruments, except the apical third of the RE canal which was prepared manually with pre-curved stainless steel files. Attempts to completely eliminate the ledge by filing against the lingual wall of the canal were not successful. Thus, the presence of internal canal irregularities did not allow the use of flexible Nickel-titanium instruments for further enlargement of the apical third of the RE canal. During instrumentation root canals were ultrasonically irrigated with 20% disodium edetate (Calcinase, Lege Artis, Dettenhausen, Germany) and 2.5% sodium hypochlorite used in an alternating manner. An interappointment calcium hydroxide dressing (Calxyl, OCO-Präparate, Dirmstein, Germany) was placed in the canals, and sealed with Fuji Triage glass-ionomer cement (GC, Tokyo, Japan).
Three weeks later, the tooth was asymptomatic and was reaccessed for final obturation. After ultrasonically enhanced irrigation, the root canals could be obturated using Roeko gutta-percha cones (Coltène Whaledent, Langenau, Germany), AH Plus sealer (Dentsply De Trey, Konstanz, Germany), and a cold lateral condensation technique (Figures 4 and 5). The final restoration was made using a resin composite (ExciTE F and Tetric EvoCeram; Ivoclar Vivadent, Schaan, Liechtenstein). Twelve months later the patient was recalled for a follow-up. At the clinical examination the tooth was asymptomatic and the radiographic examination revealed an almost complete resolution of periapical radiolucencies (Figure 6).
Generally speaking, periapical radiography is not a perfect tool for detection of supernumerary roots, mostly because radiographs are two-dimensional representations of the three-dimensional root complex. Partial or complete overlapping of the RE and the main distal root can appear on standard parallel views because both roots are mostly situated in the same buccolingual plane. In addition, the RE is usually smaller than the distal root and is therefore not always easily visualized radiographically. An additional angled radiograph may be necessary to confirm its presence. According to the SLOB (Same Lingual, Opposite Buccal) rule, the lingually positioned root (RE) appears to move in the direction of the central ray deviation, while the buccally positioned root (distal root) appears to move in the opposite direction. Therefore, if a 25° mesioradial projection is employed, the RE will appear to wander mesially, and can also be successfully removed from any overlapping by the distal root (21).
Currently, despite its limitations, periapical radiography is considered to be an essential imaging method for detection of the RE. Preoperative radiographs should be evaluated with care, preferably using a viewing box. The light surrounding the radiograph may be blocked and a magnifying lens may be used for inspection. Under such conditions, the RE should be readily evident radiographically in over 90% of cases (22). An RE may also be depicted in a panoramic radiograph. However, Harada et al. (15) demonstrated that only about 16% of REs found with periapical films can be detected on panoramic radiographs.
According to Krasner and Rankow (23) the orifices of the root canals are equidistant from an imaginary line drawn mesiodistally through the pulp chamber floor (1st law of symmetry) and lie on a line perpendicular to it (2nd law of symmetry). The permanent maxillary molars as well as the MFPMs with an RE are an exception to these laws. In the latter, the 1st law of symmetry does not apply because the RE orifice lies at a greater distance from the imaginary split of the pulp chamber floor than the orifice of the canal in the distal root (Figure 3). Moreover, the RE orifice is located distolingually, lingually or mesiolingually from the orifice of the canal in the distal root (24). Therefore, the 2nd law of symmetry may (Figure 3) or may not apply to MFPMs with an RE.
Measurements of Gu et al. (2) indicate that the resulting shape of the pulp chamber floor resembles that of a trapezoid with an average DL angle of 75°. One must keep this in mind when creating the access cavity, which should not be triangular or quadrangular, as appropriate for the two-rooted MFPMs, but rather trapezoidal with an extended DL corner (Figures 3 and 5). In looking for the RE orifice, dark developmental lines on the pulp chamber floor may serve as natural guides. According to the study by Gu et al. (2), the RE orifice is located approximately 3 mm from both adjacent orifices (ML and DB orifices). These authors also reported that the mean distance between the DL (RE) and DB orifices is significantly greater than between the ML and MB orifices (2.9 mm vs. 2.5 mm, p < 0.01).
The lingual wall of the pulp chamber has a dentinal shelf, which frequently hides the underlying RE orifice (2). Therefore, before negotiating the canal, it is necessary to remove the shelf of dentine from the lingual wall of the access cavity, to get a straight-line access to the canal itself. This can be done using Gates-Glidden drills, nickel-titanium rotary files with a large cone, or preferably with specially designed ultrasonic tips. Clearly, this preliminary step insures that the instruments will encounter a lesser curvature than that of the original RE canal.
Although most of the REs appear straight on the preoperative radiograph, they will often curve markedly towards the buccal. In two recent studies (25, 26) the RE canal was severely curved (>25°) buccolingually in 90.5% and 60.0% of the MFPMs examined, respectively. The RE canal possessed greater mean angle of curvature (32.06°) than the canals in both main roots (max. 24.34°) (26). Therefore, it is important to keep in mind the existence of this canal curvature and to use small, precurved hand instruments. The RE canal may also be inclined lingually, in confirmation of which the handle of the instrument introduced faces buccally.
De Moor et al. (20) proposed a clinically applicable classification of RE canals based on their buccolingual curvature and lingual inclination. Type I refers to a straight RE canal, which is parallel to the canal in the distal root, type II to a straight, but lingually inclined RE canal, and type III to a lingually inclined and buccoligually curved RE canal (Figure 1B). Song et al. (27) later added two additional morphological types: a small type, of which the RE canal is shorter than half the length of the canal in the distal root, and an even smaller conical type, in which the RE canal is radiographically unidentifiable. In Korean individuals, types II (47.5%) and III (40.5%) were the prevalent types, followed by type I (8.1%), small type (2.5%), and conical type (1.4%) (27).
In the presented case, RE was clearly depicted on the preoperative radiograph and the referring general dentist identified and instrumented all four root canals. The reason for referral to an endodontic office was ledging of the canal wall created during instrumentation of a buccolingually curved RE canal, most probably with a straight instrument of inadequate size. Although the RE appeared straight on the periapical radiograph (Figure 2), manual instrumentation of its canal revealed a pronounced buccolingual curvature located apically (type III RE). Management of the ledged RE canal has followed recommendations recently presented by Jafarzadeh and Abbott (28). Their recommendation was also taken into account not to use Nickel-titanium instruments for completion of the treatment unless the ledge has been completely eliminated. In accordance with the European Society of Endodontology Quality guidelines (29), the tooth was treated in two visits and dressed with an inter-appointment calcium hydroxide medicament.
In the presented case, the working length of the DB canal exceeded that of the DL (RE) canal by only 0.5 mm (measured from the same reference point). This is not an unusual occurrence for a PMFM. A recent CT study has shown that REs shorter than half of the length of the distal root represent less than 4% of all cases (27). Indeed, measurements on extracted MFPMs (14) and micro-CT images (30) have indicated that the RE is on average shorter than the distal root only by 1.5 mm and 0.9 mm, respectively.
An additional factor which should be considered during RE canal instrumentation is thickness of the radicular wall. During root canal instrumentation, perforation is most likely to occur at the middle level of the RE canal, 4 mm from the apical foramen, where the thickness of the mesial wall averages only 1.15 mm (30). Fortunately, the RE canal with its simple tubular shape, a nearly round cross-section, small natural taper and absence of lateral canals and apical ramifications may be cleaned satisfactorily without extensive removal of the radicular dentine (24). Obviously, due to relatively thin dentinal walls and curvature, an RE is not suitable for anchoring prosthetic dowels.
The prevalence of the mandibular first permanent molar (MFPM) with a distolingually-located supernumerary root radix entomolaris (RE) in European populations ranges from 0.7% to 3.4% according to literature. Therefore, when examining the preoperative radiograph of the MFPM, the clinician should always consider the possibility of an RE. To paraphrase the statement from Trope and Debelian (31), the clinician will never find more roots than he or she is looking for. The difficulties posed by this additional root during root canal instrumentation are primarily related to its lingual inclination, pronounced buccolingual curvature and relatively thin dentinal walls.