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ABSTRACT
The relationship between obturated root canal fillings and the external surface of the root was examined to determine if a correlation exists between asymmetric root canal obturations and unfilled canals. Extracted, root canal treated teeth were selected and digitally radiographed from different angles and the distances from the mesial and distal edges of the root filling to the external root surface were recorded. Obturations were classified as being asymmetric when these distances differed by 0.3mm or greater. Next, the roots were sectioned at the corresponding level and viewed under magnification by a blinded evaluator. The presence or absence of a missed canal was recorded. There was a statistically significant positive relationship between asymmetric root canal obturations and the presence of missed canals. We propose that symmetry of a root canal filling should routinely be included in the radiographic assessment of treatment quality.
Bacteria are the primary etiologic agents in pulpal and periapical infection.1-3 Therefore, the aim of successful endodontic therapy should be to thoroughly debride the canal system of infected pulp tissue and microorganisms and to seal the canal space to prevent reinfection of the pulp cavity.4 A missed canal is neither debrided nor thoroughly sealed, and thus may result in the development or persistence of periapical inflammation. In an extensive study evaluating 1,229 endodontic treatment outcomes at the University of Washington, presence of a missed canal was related to three percent of all failures at a two-year recall.5 A lower incidence of missed canals in the Washington study would be expected due to the supervision by endodontic faculty. Hoen, in a clinical study, found that 42 percent of the 337 failing root canals presenting for nonsurgical retreatment had missed canals,6 and Rud and Andreasen found that a missed canal accounted for 10 percent of failures following endodontic surgery.7 Hence, an unfilled, missed canal may be a significant cause of treatment failure.
Symmetry of the root canal filling material within the radiographic root outline seems reasonable to expect. The consistent association of the canal space shape relative to the external surface of the root was noted as far back as 1925 by Hess.8 Wheeler9 has stated that, “the dental pulp is centered in the dentin body of the tooth; the anatomical outline form of the pulp cavity will be a reflection of the outer form of the dentin body, whether it be viewed faciolingually or mesiodistally in cross section.” This association is generally believed to be correct8-10 and suggests that missed canals will result in asymmetric root canal fillings. This is in agreement with Hoen’s investigation6 in which 89 percent of cases with asymmetric obturations had missed canals that were located clinically.
The purpose of this investigation was to examine the relationship between obturated root canal fillings and the external surface of the root and to determine if a correlation exists between asymmetric root canal obturations and unfilled canals.
MATERIALS AND METHODS
One hundred and sixty extracted teeth collected from the dental school at the University of Detroit Mercy were used. There was no information available regarding the causes and times of the extractions. All teeth had been instrumented and filled in vitro by general dentists while participating in an endodontic course on nickel-titanium rotary instrumentation and obturation. All teeth were prepared using Profile GT (Tulsa Dental Products, Tulsa, OK) rotary instruments and filled with a variety of obturation techniques, including Thermafil Plus (Tulsa Dental Products), GT obturators (Tulsa Dental Products), Obtura II (Obtura Co., Fenton MO), and cold lateral compaction of gutta-percha. The sample investigated included 40 maxillary premolars with single or fused roots, 40 mandibular premolars with single or fused roots, 40 maxillary molars with single or fused mesiobuccal roots and 40 mandibular molars with single or fused distal roots. For the maxillary and mandibular molars, only the mesiobuccal and distal roots, respectively, were studied.
All teeth were digitally radiographed using a Schick CDR #2 sensor (Schick Technologies, Inc., Long Island City, NY). The sensor was suspended by a Rinn stand positioned 30cm from a Gendex GX-770 dental x-ray unit (Gendex Corporation, Des Plaines, Il) set at 70kVp, 15mA, and four impulses. The head of the X-ray unit was positioned at both 0 horizontal and vertical angulation to the sensor. Teeth were suspended by their crowns in a mounting device (PanaVise, Sparks, NV) and placed such that the roots were laying in front of, and parallel to the sensor. Two radiographs of each tooth were taken. The first radiograph was taken perpendicular to the buccal surface of the tooth (straight-on) and the second was taken with a 20 shift in the horizontal angle of the tooth to either the mesial or distal. Precise horizontal shifts were assured by means of two compasses: one compass being fixed to the bottom of the mounting device and the other to the counter top.
For each pair of digital radiographic images (0, 20), the measurement electronic enhancement tool in the Schick CDR program was used to determine the position of the obturating material relative to the mesial and distal external root surface at a level 5 mm coronal to the root apex. These measurements can be made to 0.1mm. Subsequently, each of the roots was sectioned perpendicular to the long axis of the root at the corresponding 5mm level. A grit 180, 4″ x .012″ x 1/2″ diamond wafering blade (Mager Scientific Inc., Dexter MI) in an Isomet low-speed saw (Buehler Ltd., Evanston IL) was used for sectioning. A single blinded evaluator then viewed the resected surfaces under 16X magnification of an operating microscope (Global Surgical Corporation, St. Louis, MO) and recorded the number of filled and unfilled canals. Methylene blue dye was used to aid in visual interpretation. Digital photographs were then taken of each of the specimens.
The relationship between radiographic symmetry and the presence or absence of missed canals was evaluated statistically using Pearson Chi-Square analysis.
RESULTS
Table 1 shows the number and incidence of unfilled canals in each tooth group. Overall, 27 (16.9 percent) of the 160 roots examined had missed canals 5mm from the apex, and the relationship between asymmetry and the presence of an unfilled canal was statistically significant (P< 0.05) (tables 2,3). By tooth group, five (12.5 percent) of the mandibular premolars and 18 (45 percent) of the mesiobuccal roots of maxillary molars had missed canals, and the relationship between asymmetry and the presence of an unfilled canal in these teeth was statistically significant (P<0.05). Two (five percent) of the maxillary premolars and two (five percent) of the distal roots of mandibular molars had missed canals, and the relationship between asymmetry and the presence of an unfilled canal in these teeth was not statistically significant (P>0.05).
DISCUSSION
It has been demonstrated that 93 percent of lateral canals, and 98 percent of apical ramifications are present in the apical 3mm of the root.11,12 We chose to investigate the root canal 5mm from the apex in order to avoid this complex area of anatomy. At this level, we found an overall statistically significant relationship between a radiographically asymmetric root canal filling and the presence of an unfilled, missed canal. However, our results were not statistically significant for the maxillary premolars or for the distal roots of mandibular molars. This was likely due to the small sample size. For example, of the 40 maxillary premolars and 40 distal roots of mandibular molars, only two teeth from each group had missed canals.
Microscopic examination o
f the sectioned teeth revealed a high frequency of the presence of fins, crevices, isthmuses and dumbbell-shaped canals (Fig. 1).
Lateral canals may also have been present. In some circumstances, it was difficult to determine whether the unfilled portion was a missed canal as opposed to an unfilled extension of an incompletely obturated canal. As we had a single examiner evaluate all of the tooth sections, it is reasonable to assume that a different evaluator would have interpreted some of the results differently. The has been clearly demonstrated by Goldman et al13 in the interpretation of radiographs. Evaluating multiple tooth sections may have enabled our evaluator to more decisively determine the presence of a missed canal.
In some multirooted teeth, particularly the maxillary molars, it was difficult to precisely delineate the external root surface due to overlap of the roots on the radiographic image. Even with careful manipulation of the brightness and contrast features of the Schick software, differences up to 0.2mm were frequently found when measurements were repeated. For this reason, root canal obturations were considered radiographically asymmetric only when the distance from the edge of the obturating material to the root edge on the mesial side differed from the edge of the obturating material to the root edge on the distal side by 0.3mm or more. This number is more sensitive than the 0.5mm Hoen6 used in his clinical study.
Attention should be drawn to our “false positives”, where asymmetry was noted, yet no missed canal was found. This occurred in 20 teeth (12.5 percent) when the radiographs were taken straight-on, and in 17 (10.6 percent) when the radiographs were taken with a 20 horizontal shift of the tooth. There may be a number of explanations for this occurrence. As described by Seltzer et al,14 the thickness of the cementum may vary widely from one side of the tooth to the other. As the usual thickness of cementum is up to 0.30mm, it is reasonable to conclude that an uneven deposition of cementum on the roots would give an asymmetric radiographic appearance of the root canal filling. Another explanation may be the presence of an oblong- or irregular-shaped canal that is incompletely filled and the obturating material is off to one side (Fig. 2). Finally, using 0.3mm of asymmetry as a predictor of a missed canal may be too sensitive. The ideal measure could only be determined with an extensive study whereby an ROC curve for sensitivity and specificity can be plotted to determine the most ideal measure.
The concept of taking two different angled radiographs was introduced by Richards15 in 1953, and its importance is highlighted in the results of this study. Of the 27 teeth that presented with a missed canal, only nine (33.3 percent) could be predicted from a straight-on radiograph. From an angled radiograph (20 horizontal shift), missed canals could be predicted on 15 teeth (55.6 percent). However, when combining the information provided from both radiographs, a missed canal could be predicted on 21 teeth (total 77.8 percent).
In conclusion, this study revealed a statistically significant relationship between the presence of asymmetric root canal obturations and the presence of unfilled, missed canals. We propose that symmetry of a root canal filling should routinely be included in the radiographic assessment of treatment quality.
Dr. Partnoy is in private practice limited to endodontics, Endodontic Specialists, in Toronto, ON. Dr. Vokal is an endodontic resident at the University of Detroit Mercy, and Dr. Hoen is graduate director, Department of Endodontics at the University of Detroit Mercy.
Oral Health welcomes this original article.
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Table 1
| Tooth type | Missed Number of Canals | Percentage of Missed Canals | |
| Maxillary biscupid | 2 | 5.0 | |
| Maxillary molar (mesiobuccal root) | 18 | 45.0 | |
| Mandibular biscupid | 5 | 12.5 | |
| Mandibular molar (distal root) | 2 | 5.0 |
