Introduction
In 1925, Hess, a German dentist, wrote a book on the internal anatomy of human permanent teeth, by taking vulcanite impressions of the cleared canals and decalcifying the surrounding dentin. One of his most interesting findings was the presence of two canals in some mesiobuccal roots (MBRs) of the maxillary first and second molars. He also demonstrated that some of the canals in that root displayed anastomoses that made several curves before joining another canal near the apex. Hess made observations on other teeth as well, including the possibility of complex canal anatomy in the roots of teeth that were wider bucco-lingually than mesio-distally(8). Although it was hardly appreciated at the time, his work was truly an immense achievement which is verified by the fact that no one had ever attempted a similar compilation on the subject of tooth internal anatomy. When one realizes that from the 1970s to the present, hundreds of papers have been published that use aspects of his work, enabling clinicians to better treat endodontically involved teeth. The result of Hess’ efforts is that many millions of teeth have been saved, starting with his text.
As interesting as these conclusions were, very little attention was directed to them. At that time, endodontic treatment was firmly positioned on the lowest rung of the dental ladder. There were three reasons for this: 1) problems with the focal infection theory were centered around pulpless teeth, 2) teeth that were treated before the development of proper radiological principles but then were judged some years later radiographically to have latent infections, 3) there was a pervasive attitude by both physicians and dentists against saving teeth.
On the rare occasion when endodontic therapy was performed, it was usually for anterior teeth and some bicuspids with teeth, but curved canals and molars were almost never treated. Endodontics as a specialty was still 40 years away. Some of the pioneers of endodontics—Grossman, Rhine, Hine, Healey, Sommer, and others attempted to convince their contemporaries of the value of retaining involved teeth, but it was a difficult task. Molar teeth with necrotic pulps required four to eight, often difficult, appointments and there were frequent failures.
Terminology for Canal Configuration Studies
To understand the potentially different anatomy of the canals of the various teeth, some general terminology is necessary. According to Weine, there are four frequent configurations or pathways that canals may take as they traverse from the crown through the root to the apex (Fig 16). They are:
(1) type I—a single canal from the crown to the apical site of exiting
(2) type II—two canals in the crown which merge short of the apex to form a single exit site
(3) type III—two canals which remain separate and distinct to their sites of exiting
(4) type IV—a single canal in the crown that divides well short of the apex into two separate canals at the apex(9).
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| Figure 16A |
Types I, II, and III are found in many teeth and have been referred to in many articles. Type IV, however, was first discussed when studies on the mandibular first bicuspid were undertaken. This pathway has also been reported in maxillary second bicuspids, and MBRs of maxillary molars.
There are at least three other systems that have been proposed to classify canal configuration, but this is the most simple to understand and covers over 99.5% of human teeth. The remaining 0.5% usually will fit into this system, although not perfectly. For instance, a single canal that divides in midroot into two canals that merge short of the apex into a single canal again is very rare, perhaps 0.5%, but does exist. It would be classified as a Type I canal in this system. It is important to understand that this classification, as well as the others, takes into account only the main canals within the tooth. Auxiliary and lateral canals and small branches are not considered. Also, the comparatively few configuration studies in this review using other systems have been converted and listed according to the method stated above.
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| Figure 16B | Figure 16C | Figure 16D |
Early Studies on the MBR of Maxillary Molars
For over 40 years following the Hess study, few articles on canal configuration were published, generally in minor journals and with minimal attention. Then, in 1965, Rankine-Wilson and Henry of the Perth Endodontic Study Club, a group of sophisticated restorative dentists who were forced to perform endodontics on their own patients in the absence of specialists in southwestern Australia, published a study on mandibular anterior teeth in the JADA, indicating that incisors bifurcated in 40.5% of the teeth in the study and remained separate to the apex in 7.5%(10). With the interest in endodontic therapy now on the rise, the report did generate interest, but mandibular incisors did not require treatment very frequently so it had little effect on clinical procedures.
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| Figure 16E |
Weine and Rankine-Wilson met shortly thereafter when both were on the faculty of Northwestern University Dental School and discussed possible future studies in configuration. Although neither had ever read Hess’ work, both agreed that the MBR had more failures than any other roots treated endodontically. At that time, sophomore students radiographed, accessed, filed, and filled extracted teeth while mounted in a block, practicing for treating patients in the clinic. The Class of 1970, 52 students,each drilled down the mesial portion of the MBR on four teeth until the canal system was totally exposed. Obviously, in some cases the inexperienced sophomores drilled past the canals and not far enough in others. However, compilation of the data revealed that two canals were present in 51.5% (Type II and Type III) of the total of 204 teeth, an astonishing and unexpected total! Also noted was that 14% of the MBR had two canals that were separate and distinct to the apex (Type III), an even more significant finding(11).
The study was published in Oral Surg, Oral Med, Oral Pathol in 1969 and created immediate reactions among educators as well as clinicians, many of them quite negative. Ingle had published the first edition of his textbook, Endodontics, in 1975(12), with triple the number of pages than in the older endodontic textbooks and many large, appealing drawings. However, the maxillary first molar was listed and drawn as having three roots and three canals. The books on root anatomy written by Wheeler, probably the most widely used texts in dental education, including the edition printed in 1976(13), listed the MBR as having a single canal, even though the drawing of the root from the proximal was over 50% wider than that of the distobuccal root (Table 1).
Further Studies of the MBR
Many subsequent studies(14-35) have been undertaken on the maxillary first and second molars and they have had more articles in the dental literature than any other teeth. If anything, the results seem to indicate that the number of two canals in the Weine et al. study was too low. This is especially true of the more recent reports which incorporated magnifying equipment for identification.
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| Figure 16F | Figure 16G | Figure 16H |
Two other studies, both from Europe, deserve additional discussion, those by Evenot from France, and by Zill from Germany. Both were the basis of theses for Ph.D. degrees and have many more pages and much more information than the other studies. Evenot reported on a great number of teeth (378) in his study and his use of the microscope in addition to radiographs had not been used in other studies until recently. He also reported the lowest number of a single canal in the MBRs studied up to that time and, hence, the highest number of teeth with two canals in the MBR(18). Zill used a different type of classification system than used by most of the studies referred to in this review and thus his statistics are a bit difficult to state. He did find two interesting statistics not proposed up to that time. They were that male patients had more teeth with two separate and distinct canals than did females (41.5% vs. 25.0%) and that the percentage of MBRs with two canals increased with the age of the patient(24).
Table 1. Configuration of MBR of Maxillary Molars, (1925-2002) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Hess8 (1925) | 513 | Vulcanite Impressions | 47.0 | <-------------- 53.0 ------------> | 0 | |
| Weine et al.11(1969) | 208 | Vertical sectioning | 48.5 | 37.5 | 14.0 | 0 |
| Pineda14 (1973) | 262 | Proximal radiographs | 39.3 | 12.2 | 35.7 | 12.8 |
| Green15 (1973)** | 100 | Vertical sectioning | 64.0 | 22.0 | 14.0 | |
| Seidberg et al.16 (1973) | 100 | Horizontal sectioning | 38.0 | 37.0 | 25.0 | |
| 100 | Clinical cases | 66.7 | <-------------- 33.3 ------------> | |||
| Pomerantz and Fishelberg17 (1974) | 71 | Decalcified and dyed | 71.8 | 16.9 | 11.3 | 0 |
| 100 | Clinical cases | 69.0 | 16.0 | 15.0 | 0 | |
| Evenot18 (1980) | 178, 208 | Radiographs, several microscopic | 28.8 | 23.5 | 38.8 | 8.8 |
| Hartwell and Bellizzi (l982)19 | 538 | Clinical cases | 81.4 | <-------------- 18.6 ------------> | 0 | |
| Vertucci20 (1984) | 100 | Decalcified and dyed | 45.0 | 37.0 | 18.0 | |
| Bjorndal and Skidmore21 (1987) | 85 | Acrylic casts | 41.1 | 40.0 | 18.9 | |
| Weller and Hartwell22 (1989) | 835 | Clinical cases | 61.0 | <-------------- 39.0 ------------> | ||
| Fogel et al.23 (1994) | 208 | Clinical cases | 28.9 | 39.4 | 31.7 | 0 |
| Zill24 (1997) | 105 | Radiographs and some sections | 1.0 | 41.0 | 39.0 | 19.0*** |
| Weine et al.25 (1999) | 300 | Radiographs, files in teeth | 42.0 | 24.2 | 30.4 | 3.4 |
| Stropko26 (1999)* | 1732 | Clinical cases with operating microscope | 16.5 | 31.9 | 44.3 | |
| Wolcott et al.27 (2002)** | 1193 | Clinical cases, initial | <-------------- 58.8 ------------> | |||
| Clinical cases, retreatment | <-------------- 67.4 ------------> | |||||
| *MBR studied in first, second, and third molars, only 72/92 4-canaled teeth could be filed **MBR studied in first and second molars ***German, “sonstige,” means “otherwise” | ||||||
While the first finding does not offer a clear rationality, the second may be explained by the canal in the MBR after eruption having a single, figure-eight shaped canal. With age, there is increased dentin deposition and the center point of the figure-eight eventually squeezes off the single canal into two in the MBRs.
After the first few studies were published on the MBR of the maxillary first molar, many more were undertaken using different methods of investigation and reporting different results. It is possible that some of these investigations were undertaken to disprove the frequency of two canals in the MBR, since it had been noted in precious few studies in the preceding years. However, the finding of over 50% of the teeth examined having two canals was a constant of in vitro reports. The early in vivo reports indicated fewer bicanaled MBRs. However, several of the very recent studies, including that by Stropko using an operating microscope and a large number of cases, have indicated that two canals may occur as often as in 80% or more of the teeth (Table 1)(26).
Retreatment of Failing MBRs
The first determination to make when treating a failure of endodontic therapy on a maxillary first molar is the root(s) involved. By far, the MBR is the most frequent, perhaps by a ratio of 5 to 1, or more. The distobuccal, smallest of the three roots, usually has a gentle curve that is prepared and filled rather easily. The palatal is more curved than the distobuccal but not as much as the MBR, generally to the buccal direction. However, the canal is much wider than the buccals and, therefore, easier to prepare and fill than the MBR. But the keys to determination in evaluation of the MBR root are the preoperative radiographs. In addition to the normal straight-on view, another view must be taken from the distal so that the distal and palatal roots overlap or the distal is mesial to the palatal. This puts the MBR into a mild profile, and, if present, will show the canal filling not centered in the root, but to the mesial. Post-filling films must include an angle from the distal (Fig. 17, a).
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| Figure 17A | Figure 17B |
Although not as reliable an indicator as the film from the distal, another hint that two canals are present in the MBR and should be located is that the tooth is shorter than average from occlusal to apex, that is less than 19 millimeters. Such teeth have a tendency to be wider bucco-lingually than teeth that are longer, thus permitting essentially similar periodontal ligament attachment regardless of length (Fig. 17, b). It is usually best to attempt location and position of the mesiolingual canal before making a decision on the retreatment of the mesiobuccal canal. If the latter is obviously inadequate, both canals in that root should be treated. If the mesiolingual canal merges with the mesiobuccal canal, some advise that the latter may be left alone. Weine advises retreatment of the mesiobuccal and treatment of the mesiolingual whenever retreating a failure of the MBR and has demonstrated cases where a type II canal system was shown to be present, but a sinus tract did not clear up until both canals were prepared(9).
Names of the Canals in Maxillary Molars
Some authors and/or clinicians have referred to the second canal in the MBR as the “MB2.” This is neither an accurate nor a correct name and has no basis as compared to the references to any of the other canals in the dental arch. The teeth with multiple canals have always had an important basis in common—the direction for entering the root is the opposite to the name of the canal. In bicanaled bicuspids, the buccal canal is entered from the lingual and the lingual canal from the buccal. In the three-canaled mandibular molars, the distal canal is entered from the mesial, the mesiobuccal from the distolingual, and the mesiolingual from the distobuccal.
In the maxillary molars, the palatal canal is entered from the buccal, the distobuccal from the mesiolingual, and the mesiobuccal from the distolingual. Examination of the typical mesial roots from the 1969 report(11) indicated that the correct path towards entering the second canal in the mesial root is from the distobuccal, hence it should be referred to as the mesiolingual. As was mentioned and will be discussed in the next section, the maxillary second molars very rarely may have two palatal canals. These would never be called P1 and P2, but, more properly, the mesiolingual and the distolingual. The same conditions apply to the mesiobuccal root.
In the first English edition of his text, Castellucci states that rather than using MB2 as the name of this canal, it is “more appropriately named the mesiolingual canal”(28). Failure to gain entrance to this canal often occurs because the operator fails to consider this initial curve to the distobuccal and the small file continuously bends when pressure is exerted as the instrument tip bypasses the angle of the orifice. Weller and Hartwell(22) and Fogel(23) et al. also refer to the second canal in the mesial root as the mesiolingual.
Studies of the Maxillary Second Molar
With so many multicanaled MBRs found in maxillary first molars, it was not too long before this root in second molars started to receive attention. Several of the earlier reports on the first molar had concomitant results listed for the second molar, too (Table 2). Subsequent studies on the second molar only are listed in Table 2(17,30-33,36). Many endodontists started finding a second mesiobuccal canal in the first molar, but had great difficulty in locating an additional canal in the second molar, probably due to the position as most posterior tooth and inclination to the distal. However, the statistics are probably correct. Still, the in-vitro investigations depict a much higher percentage of average number of canals than do the in-vivo reports. At this time, when many maxillary first molars have four canals filled frequently, the percentage of treating four canals in the second molar still lags far behind.
The maxillary second molar has many more canal configurations than does the first molar. One, two, three, or four roots may be present with two, three, or four canals, whereas the first molar has three or four roots in virtually every instance, always with three or four canals. The spread of the roots in the first molar is much greater than in the second molar and the position of the canal orifices also may differ, particularly the distobuccal in the second molar being more to the mesiolingual in some teeth.
Table 2. Configuration of the MBR of Maxillary Second Molar, (1972-2002) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Pineda and Kuttler29 (1972) | 294 | Proximal radiographs | 64.6 | 8.2 | 12.8 | 14.4 |
| Nosonwitz and Brenner30 (1973) | 161 | Clinical cases | 68.9 | 25.5 | 5.6 | 0 |
| Pomeranz and Fishelberg17 (1974) | 29 | Clinical cases | 62.1 | 13.8 | 24.1 | 0 |
| Vertucci20 (1984) | 100 | Decalcified and dyed | 71.0 | 17.0 | 12.0 | 0 |
| Kulild and Peters31 (1990) | 32 | Access and bur penetration | 21.8 | <-------------- 78.2 ------------> | 0 | |
| 32 | Sectioning and microscope | 6.3 | <-------------- 93.7 ------------> | 0 | ||
| Eskoz and Weine32 (1995) | 67 | Radiographs, files in place | 59.7 | 20.9 | 16.4 | 3.0 |
| Wolcott et al.27 (2002) | 689 | Clinical cases, initial | <-------------- 35.3 ------------> | |||
| Clinical cases, retreatment | <-------------- 43.6 ------------> | |||||
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| Figure 18A | Figure 18B |
The maxillary second molars also may have two palatal roots, in one of two distinctly differing formations, a condition almost never seen in the first molar. In one of the types, the palatal roots are rather far from each other and the tooth resembles an upside down card-table, easily discernible on the radiograph (Fig. 18, a). In the other condition, the palatal roots are much closer together, difficult to see on the radiograph, and often are noticed only when examining the floor of the chamber of the tooth (Fig. 18, b).
Table 3. Configuration of the Mandibular First Bicuspid (1972-1992) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Pineda and Kuttler29 (1972) | 202 | Proximal radiographs | 69.3 | 1.5 | 1.5 | 0.9 |
| Green15 (1973)** | 50 | Vertical sectioning | 86.0 | <-------------- 10.0 ------------> | ||
| Zillich and Dowson33 (1973)*** | 1393 | Radiographs | 69.3 | <-------------- 2.7 ------------> | ||
| Vertucci20 (1984) | 400 | Decalcified and dyed | 70.0 | 4.0 | 1.5 | 0.5 |
| Baisden et al.34 (1992) | 106 | Transverse sectioning | 76.4 | 0 | 0 | 23.6 |
| *reported 0.9% had 3 separate canals **did not report on 3 canaled teeth ***reported on first and second bicuspid; 0.4% of the teeth had 3 canals | ||||||
Studies of the Mandibular Bicuspids
Next to the MBR of maxillary molars, the configuration studies on the mandibular first bicuspid have been the most illuminating and useful. (Table 3)(15,20,29,33,34). For many years it was thought that the mandibular first bicuspid had only a single canal, although there were a few examples of those teeth being bicanaled in rare instances. Of all the teeth studied for additional canals, this tooth seems the most mystifying for failure to diagnose that more than a single canal is present immediately upon examining properly angled radiographs. The key sign is that the occlusal portion of the canal seems large and well developed, but suddenly disappears in midroot (Fig. 19, a). That site is obviously the point of separation of the canals into two, and, sometimes, even three canals to their apices.
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| Figure 19A |
The older studies on this tooth listed Type I systems on 69%-86% of the teeth examined and some percentage of Types II, III, and IV. A recent study, by Baisden et al.(34), reported that the tooth only had either a single canal or a bifurcated canal system. The net result of these studies indicates that more and more mandibular first bicuspids being treated are found to have more than a single canal, but rather a Type IV system, the most complicated of the systems to treat. This finding occurs in a much higher number than had been assumed—perhaps as high as 30%—and the Baisden study certainly should result in fewer failures in the future. In 1973, Zillich and Dowson reported on this tooth, studying 1393 teeth—a huge number—and reported that 69.3% had a type I system. Of the remaining bicuspids, 22.7% had 2 canals and 0.4 had three canals(33). Both mandibular bicuspids may have two or three canals, but this finding occurs much less frequently in the second bicuspid than it does in the first.
All of the studies indicating two canals in this tooth stress the need for a wide preparation buccolingually. The buccal canal is penetrated from the lingual whereas the lingual canal is found from the buccal. The height of the division from one canal to two usually will determine the outlook for the treatment. When the division is towards the occlusal, the treatment is not complicated. However, in cases where the site of division approaches the apex, treatment is very difficult.
In general the mandibular second bicuspid is one of the easiest teeth to treat endodontically, with a single canal well centered in the crown and extending to the apex. Even in single-canaled mandibular first bicuspids the access is more difficult because it is the only tooth in which the long axes of the crown and the root meet at an angle (Fig. 19, b).
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| Figure 19B | Figure 19C | Figure 19D |
Studies of the Mandibular First Molar
Shortly after the first study of the MBR of the maxillary first molar, Skidmore and Bjorndal reported on the canal configurations in the mandibular first molar(35). Bjorndal had investigated many of the human teeth and, with aid from his graduate students, reported not only the configuration but also the lengths and widths of the permanent teeth in a booklet, Anatomy and Morphology of Human Teeth, second edition(35). Many hours were invested in this complex undertaking. Bjorndal and Hess probably should be given co-credit as the fathers of modern canal configuration reporting (Tables 4-7 and 9).
Table 4. Configuration of the Mesial Root of the Mandibular First Molar (1971-1988) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Skidmore and Bjorndal35(1971) | 45 | Acrylic casts | 6.7 | 55.6 | 37.7 | 0 |
| Pineda and Kuttler29 (1972) | 300 | Proximal radiographs | 12.8 | 30.2 | 50.4 | 6.6 |
| Green15 (1973)* | 100 | Vertical sectioning | 12.8 | 48.5 | 37.7 | 1.0 |
| Vertucci20 (1984) | 100 | Decalcified and dyed | 27.0 | 38.0 | 26.0 | 9.0 |
| *sections of both mandibular first and second molars | ||||||
Table 5. Configuration of the Distal Root of the Mandibular First Molar (1971-1973) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Skidmore and Bjorndal35(1971) | 45 | Acrylic casts | 71.1 | 17.7 | 11.1 | 0 |
| Pineda and Kuttler29 (1972) | 300 | Proximal radiographs | 73.0 | 12.7 | 5.7 | 8.6 |
| Vertucci20 (1984) | 100 | Decalcified and dyed | 89.3 | 4.9 | 2.9 | 2.9 |
Table 6. Configuration of the Mesial Root of the Mandibular Second Molar (1972-1988) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Pineda and Kuttler29 (1972) | 300 | Proximal radiographs | 58.0 | 20.6 | 13.8 | 7.6 |
| Vertucci20 (1984) | 100 | Decalcified and dyed | 27.0 | 38.0 | 26.0 | 9.0 |
| Bjorndal and Skidmore21 (1987) | 60 | Acrylic casts | 25.0 | 43.0 | 32.0 | 0 |
| Weine et al.36 (1988)* | 72 | Radiographs, files in place | 4.0 | 52.0 | 40.0 | 0 |
| *4.0% C-shaped teeth were found | ||||||
Table 7. Configuration of the C-shaped Mandibular Second Molar (1972-1998) | |||
| Author(s)(Year) | Number of Teeth | Method of study | % of Second Molars that were C-shaped |
| Pineda and Kuttler29 (1972) | 300 | Proximal radiographs | none noted |
| Cooke and Cox37 (1979) | * | Clinical cases | 8% |
| Tamse and Kaffe38 (1981) | 508 | Radiographic | 10.9 |
| Bjorndal and Skidmore21 (1987) | 60 | Acrylic casts | 13 |
| Melton et al.39 (1991) | 15 — known to be C-shaped | 8—polyester resin casts 7—cross sectioned, 3 levels | 100 |
| Weine et al.40 (1998) | 399 | Retrospective—clinical cases | 6.2 |
| 412 | Prospective—clinical cases | 8.9 | |
| *no number reported | |||
In their evaluation, Skidmore and Bjorndal stated that four canals were present in the mandibular first molar in 28% of the cases, separate and distinct in 11% and merging short of the apex in 17.5%. They listed three roots to be present in 2.2% of the teeth studies and recommended a larger, more rectangular access preparation than had been shown in Ingle’s first edition(12), which was largely adopted by the endodontic community soon thereafter. This change from a smallish triangular form allowed for the added room to locate the fourth canal, if present (Fig. 20).
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| Figure 20 |
The mandibular first molar, being the first permanent tooth to erupt, probably is the most common tooth to have pulpal damage but early in the 20th century it was usually extracted when involved. For the past 40 years, it has been treated more and more as endodontic techniques have become more predictable. For this reason, it is important for any dentist treating this tooth to be well informed as to the configuration possibilities and some excellent studies have been reported(20,29).
According to the studies on human teeth, the mandibular first molar is one of the few teeth where the normal configurations differ between the European/American and the Asian population. Most of the studies reported in this review are from the former group. A further discussion of some of the types prevalent in the Asian population will be presented later in this report.
Studies of the Mandibular Second Molar—A Tooth With Multiple Variations
The mandibular second molar has the greatest variation of configurations among the molar teeth. One of the most egregious errors is the often-felt opinion that the mesial root has a fairly high percentage of only one canal (Table 6)(20,35,36). In the study of Weine et al.(36) this was clearly stated as a source of failure on this root and, furthermore, when only one canal is located, radiographs of several angles with files in place, mesial and distal, should be taken to verify that only one centered canal is present. In addition, the distal root may have several variations and, in some more rare cases, a completely different variant is present, the so-called C-shape. This will be discussed in further detail in the next section.
One of the major reasons why researchers were content to believe that only one canal in the mesial root is a dominant feature of the mandibular second molar is that there is a high percentage of two canals which merge short of the apex in that root (Type II system). In that case, if only one of the two canals is found, generally the mesiolingual, but it is prepared and filled adequately, that root will have a decent chance for success. But in case the two canals are separate to the apex (Type III), then the chances of success are severely decreased.
The mandibular second molar has a few very unusual configurations, including a second canal in the distal root and, very rarely, a second distal root (Fig. 21). Viewing the preoperative radiographs of this tooth with magnification is mandatory because of the many variants that are possible.
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| Figure 21 |
The C-shaped configuration of the mandibular second molar (CMnd2M) was first reported by Cooke and Cox(37) in 1979, who stated that of the second molars treated in the late 1970 at the Dental School of Washington University (St. Louis), 8% were CMnd2Ms. This was quite a surprise because of 300 mandibular second molars investigated by Pineda and Kuttler in 1972(29), no CMnd2Ms were reported and no single-rooted teeth were seen. Similar results were reported by Vertucci(20). In succeeding years, other studies were reported noting the CMnd2Ms, with similar frequencies to the original Cooke-Cox report(38-41). There have been a few papers indicating the finding of a C-shape on other teeth beside the second molar(43,44).
The tooth is referred to as C-shaped because the canals present (usually three, but may be two or four, on occasion) have a continuous septum at the orifice area that may or may not extend to the apex. If cut in cross section, the septum is that of the letter “C.” Generally the closed area of the “C” is to the lingual, but it may be to the buccal. The canals are usually far from each other at the orifice, but merge near the site of exiting. In some cases only two of the three canals merge (Fig. 22).
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| Figure 22A | Figure 22B | Figure 22C |
In 1981, Tamse and Kaffe reported on conical mandibular second molars in 10.9% of 508 teeth surveyed, demonstrating radiographs that did indicate some, but perhaps not all, as being CMnd2Ms(38). Melton et al.(39) studied only 15 mandibular second molars, but all were known to be C-shaped and made a number of interesting findings. In general, C-shaped teeth have one of two root configurations, a single root or two roots with an attached segment to allow for the septum to traverse from the mesial to the distal. They also found that not all of the “C” sections were continuous, but that some had interruptions. Also, not all of the canals merged at the apical portion, but instead one canal was separated from the others at the site of exiting.
The study involving the most teeth was reported by Weine and Members of the Arizona Endodontic Association(40). Three hundred ninety-nine second molars were evaluated retrospectively by radiographs and 412 were examined while they were being treated, including radiographs with files in place and postoperatively. It is of interest to note that fewer CMnd2Ms were recorded retrospectively (25/399, 6.2%) than prospectively (37/412, 8.9%). This probably occurred because the operators were looking for the specific variant in the prospective portion of the study.
It is obvious that this variant—the C-shape—has multiple shapes and configurations, easily the most complicated of any single tooth. For this reason, Weine et al.(40) suggested that larger sample sizes should be obtained and studied to clarify some of the situations noted and examine for more possibilities. They recommended that endodontic study clubs or other smaller organizations pool their cases to gain larger number of teeth for study, just as the mandibular incisors were investigated by a small group.
The configuration offers some difficulty in filling because of the slit-like canal present, as opposed to the normally occurring oval or roundish canal. Several of the reports warned that the condition should be considered early in treatment, lest irreparable damage occurs in the narrow slits present(43,44). Manning wrote two papers on the variants in the C-shape and gave two interesting conclusions. He said that the slit portion of the C-shape should be filed and filled with great care to avoid perforation through the narrow walls. He further stated that studies of the Chinese and the Hong Kong Chinese had greater number of C-shaped roots than did Caucasians.
Studies of the Maxillary Bicuspids
The maxillary bicuspids have a number of potential configurations, but somehow do not present as much of a problem during treatment as do the MBRs, C-shaped molars, or bicanaled mandibular incisors. Perhaps it is because the teeth are easier to radiograph or that the patients are able to open widely enough to allow for better examination than with the maxillary molars. The first bicuspid often has only a single root, usually with Type II or III configurations, but rarely just a single canal. It may have two roots, each with a single canal. It also may have three separate and distinct roots, although rarely, which may be difficult to recognize and thus cause difficulties early in the treatment (Fig. 23). The three-rooted tooth resembles the common maxillary second molar which has three separate canals. Two types of this condition are seen, one where the buccal canals separate near the end of the crown with the other, more difficult type, having canals separating further toward the apex, similar to a Type IV system, making preparation and filling more complicated.
| |
| Figure 23 |
The maxillary second bicuspids also may have one or two roots, but with this tooth the two canaled combination may be slightly more common than in the first bicuspid. Type I, II, and III canal systems may be present in the tooth with a single root. However, this tooth may have two complex configurations—a Type IV canal system that has the position of division fairly deeply in the root (Fig. 24) and three separate and distinct canals. The latter is even more rare than in the first bicuspid (Table 8).
Table 8. Configuration of the Maxillary Second Bicuspids (1972-1976) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Pineda and Kuttler29 | 250 | Proximal radiographs | 98.9 | 0 | 1.2 | 0 |
| Vertucci et al45 (1974)* | 100 | Decalcified and dyed | 48.0 | 27.0 | 17.0 | 6.0 |
| *2 teeth had 3 canals | ||||||
 |  |  |  |
| Figure 24A | Figure 24B | Figure 24C | Figure 24D |
Vertucci(20,45-47) has been the most prolific investigator of configuration in these teeth. He has also described combinations of canal systems that are more complex than the Type I, II ,III, IV description used in this review. Vertucci obtained his specimens from exodontists in the southeastern portion of the US and many were extracted to allow for replacement with dentures, rather than with restorative and/or pulpal problems. Even so, the specimens are quite beautiful and have interesting curvatures and anastomoses.
Table 9. Configuration of Mandibular Incisors (1965-1997) | ||||||
| Author(s)(Year) | Number of Teeth | Method of study | %Type I | Type II | Type III | Type IV |
| Rankine-Wilson and Henry10 (1965) | 111 | Radiographs, files in place | 59.5 | 35.1 | 5.4 | |
| Pineda and Kuttler29 (1972) | 356 | Radiographs, proximal view | 98.3 | 0.5 | 1.2 | 0 |
| Vertucci47 (1974) | 100 centrals | Decalcified and dyed | 92.0 | 5.0 | 3.0 | 0 |
| 100 laterals | 93.0 | 5.0 | 2.0 | 0 | ||
| Bjorndal and Skidmore21 (1987) | 277 | Acrylic casts | 63.9 | 25.9 | 2.2 | 7.9 |
| Miyashita et al.48 (1997) | 1085 | Decalcified and dyed | 87.6 | 9.3 | 1.4 | 1.7 |
Studies of the Mandibular Incisors
Most dentists, and even many endodontists, do not realize that it was on these teeth that the first definitive study of canal configuration was performed. Many general dentists, similarly, do not appreciate the difficulty that these teeth offer during endodontic treatment, thinking that they are “merely small maxillary centrals.” Nothing could be further from the truth and can only lead to disaster. As stated earlier in this review, because these teeth are not treated endodontically very often, the complexities of the bi-canal possibilities coupled with the narrowness and depressions on the proximal surfaces are not considered during therapy (Table 9). Miyashita et al.(48), reported on the types of canal systems in these teeth as an introduction to their article on general aspects of mandibular incisors shape, canals, external appearance, and accessory canals, which is of considerable interest to anyone treating these complex teeth. Furthermore, the difficulty in taking useful angled radiographs to indicate the possibility of more than a Type I systems is very difficult to obtain because of the mesio-distal narrowness of the teeth and the ease with which they appear overlapped in angled views. The study by Rankine-Wilson and Henry emphasized the need for placing the access in the incisal edge to give best approach to the two canaled teeth (Fig. 25)(10). The mandibular incisors should always be treated with great care.
| |
| Figure 25 |
Studies of Teeth With Five or More Canals
Periodically, reports crop up in the endodontic literature demonstrating teeth with five, and sometimes even more, filled canals. Because these cases are in-vivo and the teeth were treated in the hope that they may remain in the mouth, no sectioning to verify individual canals is ever made. Demonstration of five and more canals in-vitro with file placement has been reported only very rarely.
In 1982, Weine published an article in which three canals were filled in the mesial root of a mandibular first molar(49). He stated that although it would appear that the mesial root had three definite canals, such was not the case. The tooth had been treated originally when the patient was only 10 years old and the canals obviously quite large. The preoperative radiographs appeared to show two (poorly) filled canals that had merged short of at the apex. After opening the tooth, he found that the two canals that had been filled were both in the mesiobuccal segment of the tooth. Opening the chamber floor further, a definite path to the mesiolingual was opened and enlarged. Both of the previously filled canals were prepared, as was the last-found mesiolingual and all three separate canals filled into a common site of exiting near the apex. He postulated that this did not constitute three separate canals in the mesial root, but rather a single, very wide canal that permitted preparation and filling in more than two sites. In 1997, DeGrood and Cunningham published a case report that demonstrated three canals filled in the mesial root that was identical(50). This was a Type II canal in the system used for this review. In 1989, Fabra-Campos also published a similar paper, except that one mesial canal had a separate exit but the other two had a common exit(51). This was a Type III canal. In 2004, Baugh and Wallace described the treatment of a mandibular first molar with two separate and distinct canals in the distal root and two separate mesial roots, one of which showed two canal fillings, merging short of the apex(52). Pomeranz et al. were early investigators on wide canals in the mesial roots of mandibular molars which were able to be filled as three canals(53). In a recent case report, Ferguson et al. described the filling of three canals in the MBR of a maxillary first molar for the first time(54). The postoperative angled radiograph shows only two sites of exit, one to the buccal and one to the lingual, clearly a Type III system. The patient was young—only 18 years old—and a photo of the access to the canals indicated the two orifices to the lingual much closer than the middle orifice to the buccal.
Most of the radiographs demonstrating five or more filled canals show four or less sites of exiting at the apex. This can be explained as a canal being quite wide or the pulp becoming necrotic when the patient was quite young and allowing for multiple sites of filling. These reports are interesting to read and we all should endeavor to treat any canal that can be located. However, these findings of an unusual number of canals (five or more), as compared with the total number of teeth treated, still remain an extremely small group.
Role of Ethnicity in Configuration Studies
Most of the studies listed in Table 1 through 9 have statistics derived from teeth accumulated from two ethnic groups. They are from the Europeans, largely Caucasians, and the African-Americans. Both of these groups have large populations in Europe, the Americas, and Australia, and, in the case of the African-Americans, Africa as well. However, this leaves out a very huge percentage of the world population, particularly the Asians. In these days of multi-national companies and super-sonic travel, dentists and especially endodontists should be aware of the deviations from Tables 1 through 9 that might occur when a patient of an alternative ethnic group seeks treatment.
Walker(55-57), who was on the Dental Faculty in Hong Kong, and, therefore, had access to the Chinese population from the southern portion of China, wrote several illuminating articles, listing some of the differences that this group had as compared to other groups. In general these studies were on mandibular molars and bicuspids. Other studies of Asians were also reported(58,59). Weine et al.(25) investigated the MBR of maxillary first molars of an exclusively Japanese sub-population and reported few differences from those with the European/African background.
It still would seem that studies of Asians, including Asian Indians, the people of the eastern former Russian republics, and Oceania, would be interesting and useful to anyone whose patients from those areas require endodontic treatment. Perhaps these groups will be studied in the future.
Effect of Preparation Methods on Configuration Studies
Tables 1 through 9 not only supply statistics on the numbers and configuration of the canals indicated, but they state the methods used in the investigations. The easiest, useful technique is the sectioning method. A high number of teeth may be investigated and evaluated relatively quickly. The technique may be by vertical or horizontal cutting and it may be used to grind away tooth structure(11) or cut through it horizontally(16). In the latter case with various levels of cutting, for example every 1 millimeter, there is the allowance for repositioning the segments to demonstrate the variance at any level. The disadvantage for the grinding method, as stated earlier, is that it is possible to grind through too far, removing the most, or even the entire canal, or grind too little. In either error, the critical midcanal area is lost for demonstration.
Another popular method has a clinical orientation, using radiographs with files in place to demonstrate the relationship of the files in any canal, in vitro. The extracted tooth may be rotated in several planes to disclose positions not always available in vivo. This method was used in the Rankine-Wilson and Henry study(10) and works well when study clubs combine results for large numbers of teeth. The possible disadvantage is that the files have stiffness and, if traversing a very curved canal, will tend to take a central path rather than detail the true curvatures. The injection methods demonstrate this condition very well. Another radiographic technique combines the use of x-rays and sectioning of roots to allow for views from the proximal without the placement of files. Other than the palatal root of the maxillary first molar, all of the other roots in the mouth are either wider bucco-lingually or roundish. By taking radiographs of sectioned roots from the proximal, the wider dimension, the more significant view is obtained and there is no overlap of adjacent roots. This method was used by Pineda and Kuttler(29) to survey a very large number—7,275—of teeth. Many teeth may be examined in a relatively short period of time, but the assessment of the interior of the teeth—where the canals are—may not be accurate.
A very frequent, but time-consuming technique, is the decalcifying and dyeing method, which probably gives the best results for student use and for long-range retention of the specimens. The contrasts seen in the preserved roots show examples of the unusual relationship of the curving canals, not seen in either the grinding or files-in-place methods. Vertucci(20, 45-47), who studied most of the teeth in the arches, including anteriors, used this method and because of the many anastomoses picked up by the dyes, applied a different and more complicated configuration coding. Hess’ studies used a vulcanite impression technique, which differs from the decalcifying-dyeing method, but his results look very similar(8) to Vertucci’s.
The major problem for the articles by Vertucci is that because many of the patients supplying the teeth were young and had extractions for prosthetic rather than pulpal/periapical disease, the canals were relatively wide. This is not typical of the types of cases seen in most endodontic practices because of the absence of reparative dentin that occurs in response to decay and/or restorations. The canals in his specimens show many exotic curvatures. Several other reports using the dyeing mechanism also are replete with multiple lateral canals and curvatures. These are rarely demonstrated following treatment of patients, especially of middle-aged or older, using any of the popular canal filling techniques.
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