| The permanent mandibular first molar usually has two roots (one mesial and one distal) and three canals (two mesial and one distal), yet in a number of individuals a third root develops at the distolingual aspect. The prevalence of the distolingual (DL) root in permanent mandibular first molars is mostly less than 5% in whites and blacks, whereas in Mongoloid populations the prevalence is much higher. This root variation has great anthropological significance. As an important nonmetric dental trait, it was widely used for population comparisons and exploration of the racial affinity. Moreover, the DL root also has great clinical significance. The DL canal may be left untreated or treated improperly, and this may lead to treatment failure. The purposes of this study were: (1) to detect the prevalence of DL roots in permanent mandibular molars in a Chinese population by using cone-beam computed tomography (CBCT); (2) to investigate the pulp floor and root canal system in three-rooted mandibular first molars by using micro-computed tomography (micro-CT) scans; (3) to measure the root canal curvatures; (4) to quantitatively analyze the levels of the root apex and furcation, the root canal wideness, thickness, and taper at different levels; (5) to explore the anatomic features of the root apexes; (6) to measure the root surface area (RSA) at different attachment levels; and (7) to evaluate the effect of projecting angles on radiographic diagnosis of the DL root in permanent mandibular first molars.Experiment 1: A total of 500 Chinese patients'CBCT images were examined. The incidences and bilateral concurrence rates of DL roots were determined for the permanent mandibular first, second and third molars. The differences between sexes and sides were tested by using Chi-square tests. The antimeric correlations for occurrence of the trait were analyzed with the Spearman's rank order correlation test. The results showed the prevalence of three-rooted mandibular first molars was 31.98% (134/419 individuals) calculated by individuals, and 25.89% (217/838 teeth) calculated by teeth. The bilateral concurrence rate was 61.94%. A significant right-side predilection was found in the female group (P < 0.05) as well as for both sexes combined (P < 0.01). The Spearman's correlations coefficient between antimeres was 0.77 (P < 0.01). The prevalence of DL roots in the mandibular second and third molars were 2.02% (8/396 individuals) and 12.10% (19/157 individuals), respectively. The corresponding bilateral concurrence rate was 12.50% and 35.83%, and the bilateral correlation coefficient was 0.22 (P < 0.01) and 0.50 (P < 0.01), respectively.Experiment 2: A total of 122 extracted mandibular first molars were collected from a native Chinese population. After calculation of the occurrence frequency, 20 three-rooted and 25 two-rooted first molars were scanned and reconstructed three-dimensionally. The results showed that the mean interorifice distances from DL canal to DB and mesiolingual (ML) canal were 2.93 and 2.86 mm, respectively. The mesial root predominately contained a type 2-2 root canal, with a frequency of 65% in the three-rooted molar group and 64% in the two-rooted molar group. Type 1-1 canals were seen more frequently in the DL and DB roots of the three-rooted first molars, as well as in the distal roots of the two-rooted first molars. The incidences were 100% (20/20), 95% (19/20) and 72% (18/25), respectively. Accessory and lateral canals rarely occurred in the DL roots. The incidence was only 10% (2/20). Fucation accessory canal leaving from pulp floor and exiting on fucation region was not observed.Experiment 3: The specimens were reconstructed three-dimensionally by software Mimics 10.01 and shown in a parallel projection mode. The images of the root canals in clinical (CV) and proximal view (PV) were analyzed by software Image-Pro Plus (Media Cybernetics, Silver Spring, MD, USA). Schneider method and a modified Pruett method were used to measure the angles and radius of canal curvatures. The results showed that in the three-rooted molar group in a CV, the average angles of primary curvatures were 24.34 for the mesiobuccal (MB), 22.39 for the ML, 13.71 for the DB and 13.81 degrees for the DL canal. In a PV, the average angles were 16.60 for the DB and 36.06 degrees for the DL canal, respectively. Secondary curvatures were frequently seen in a CV (60%) for the DB canals with an average angle of 26.94 degrees. In a PV, the average central angle of curvature was 59.04 degrees for the DL canal, and the average radius and curve length was 6.17 and 5.73 mm, respectively. In general no statistical significant difference was found for canal curvatures in the mesial roots between the three- and two-rooted molar groups (P > 0.05).Experment 4: Twenty three-rooted mandibular first molars were reconstructed three-dimensionally by software Mimics 10.01. The levels of the root apex and furcation; the diameter, wall thickness and taper of the MB, ML, DB and DL canals were determined. The buccolingual/mesiodistal (BL/MD) diameter ratios (⊿C) and the prevalence of long oval canals (⊿C > 2) at different levels were calculated. The results showed that the level of distal furcations was significantly (P < 0.01) lower than that of MD furcations with an average of 1.09 mm. The buccal and lingual walls were significantly (P < 0.05) thicker than the mesial and distal for the MB, ML and DB canals. Among 4 canals, the DL canal had the smallest mean value of⊿C at each level. The average BL taper was greater than the MD for each canal except the middle portion of the DL canal.Experiment 5: The apical anatomy of the tooth models were analysed in software Mimics 10.01. The apical foramen (AF) of the DB canals were frequently located at the DL side, the percentage reached 50% (10/20), and those of the DL canals and distal canals (two-rooted molar group) were most often at the buccal and distal sides, with a frequency of 35% (7/20) and 50% (11/22), respectively. The mean diameter of the apical constriction (AC) of the DB canals were significantly larger than that of the DL canals (long diameter: 0.32 mm vs. 0.27 mm, P < 0.01; short diameter: 0.25 mm vs. 0.22 mm, P < 0.01). In three-rooted molar group, the mean distance between AC and AF was 0.67±0.32 mm, and the mean distance between AF and apex was 0.49±0.28 mm.Experiment 6: In Mimics 10.01, the apical RSA at each attachment level (0, 2, 4, 6, 8, 10 mm below the cemento-enamal junction, and the furcation levels) and its percentage to total RSA were measured and calculated. The net or percent RSA and corresponding attachment levels were fitted to 5 functions. The results showed that the average RSA of three-rooted mandibular molars was larger than that of the two-rooted molars (410.6 mm2 vs. 391.6 mm2, P > 0.05). The linear function could provide the most appropriate fit for the data. As the attachment loss reached 5.76 mm (two-rooted molar group) and 5.61 mm (three-rooted molar group), the remaining RSA was reduced to 50%.Experiment 7: Twenty-nine extracted three-rooted mandibular first molars were used in this study. Radiographs were taken for each specimen by using a radiovisiography system in the bucco-oral and proximal projections, and from a mesial and a distal projecting angle of 20 degrees, respectively. Wheather the image of the DL root apex was overlapped by that of the DB root was regarded as the diagnostic criteria. The results showed that a distal projecting angle of 20 degrees can best expose the DL root (89.66%), followed by a mesial projecting angle of 20 degrees (27.59%). The exposing rate was the lowest for the the bucco-oral projection (17.24%). A mesial added by a distal angled (20 degrees) radiographs could raise the exposing rate to 100%.In conclusions, the prevalence of the DL roots in the Chinese population is higher. Three-rooted mandibular first molars commonly have 4 separate canals with high incidences of accessory canals in the mesial and DB root.The geometric data of pulp ?oors are useful for locating the DLcanal. A better understanding of the canal curvature, root canal widness, thickness and taper is essential for successful root canal intrumentation. The data of apical anatomy is useful for determination of the working length, as well as for apical shaping and filling during root canal therapy. Micro-CT scanning and three-dimensional reconstruction are usefual techniques for pecise measurement of the RSA. The occurrence of the DL root does not significantly increase the total RSA. A distal projecting angle of 20 degrees can best expose the DL root. |
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