Variable permanent mandibular first molar: Review of literature.

INTRODUCTION: The success of root canal therapy depends on the locations of all the canals, thourough debridement and proper sealing. At times the clinicians are challenged with variations in morphology of root canal. This review article attempts to list out all the variations of permanent mandibular first molar published so for in the literature.
MATERIALS AND METHODS: An exhaustive search was undertaken using PUBMED database to identify published literature from 1900 to 2010 relating to the root canal morphology of permanent first molar by using key words. The selected artcles were obtained and reviewed.
RESULTS: Total ninty seven articles were selected out of which 50 were original article and forty seven were case reports. The incidence of third canal in mesial root was 0.95% to 15%. The incidence of three rooted mandibular first molar was 3% to 33%. Only ninety cases reported with c-shape canal configuration. Incidence of Taurodintism without congenital disorder was very rare.
CONCLUSION: The root canal treatment requires proper knowlegde of variations in root canal morphology in order to recognise, disinfect and seal all portal of exit. This can be accomplished with proper diagnosis using newer modes, modification in access preparation, use of operating microscope, enhanced methods of disinfecting and sealing of all canals.

INTRODUCTION

For root canal treatment to be successful, it is necessary to locate all root canals, debride them thoroughly and seal them completely with an inert root filling material. The clinician should be aware of the internal morphology of permanent teeth and the possible variations which may be encountered.[1] The mandibular first molar is the first posterior tooth that erupts and is the tooth that most often requires root canal treatment. This tooth usually has two roots but occasionally, it has three with two or three canals in the mesial root and one, two, or three canals in the distal root.[2]

The mandibular first molar generally has two separate roots with a round, or more frequently elliptical, canal in the distal root and two canals in the mesial root [Figure 1]. The Distal canal (s) is normally straight all the way to the apex, oval or flattened in cross- section, but quite large which makes instrumentation easy. Often the most apical 1-2 mm of this canal curves up to 90 degrees distally, but this is seldom a clinical problem.

Figure 1

Diagram illustrating the mesial and distal root of mandibular molar

Regarding mesial root in 90%, they remain separate as far as the foramen; in the remaining 10%, they join together at a common foramen. The canals of the mesial root take a more curved course with a mesial orientation immediately below the orifice and then distal in the rest of the root canal.[3]

Root canal morphology and configuration have been classified by Weine et al,[4] Vertucci[5] and Gulabivala et al,[6] [Figure 2].

Figure 2

Root canal configuration as described by Weine and Vertucci et al.

Previous in vitro and in vivo reports have indicated that mandibular molars can have more than three root canals. The permanent mandibular molar have been reported to have three roots, mesial and distal roots with three canal each or molar with 5, 6, or more canals. The purpose of this study was to perform a review of literature related to variations in roots and root canal morphology of permanent mandibular first molar.

MATERIALS AND METHODS

An exhaustive search was undertaken to identify published literature related to the root canal morphology and its variation of permanent mandibular first molar. The database was searched via the pubmed search engine http://www.ncbi..nlm.nih.gov/sites/entrez?db = pubmed by using following search criteria: Root canal morphology of mandibular first molar, Middle mesial canal of mandibular first molar, Radix Entomolaris, Radix Paramolaris, C-shape morphology of mandibular first molar, Taurodontism. A similar search was also undertaken in Google, journals, and reference lists. The search included all years up to December 2010. Obtained articles were divided into case reports and study articles which were then allocated to each variation. The abstracts were collected. The relevance of the abstract was studied and accordingly full article were obtained and reviewed.

Results

Tables 1 and 2 presents case reports and studies of middle mesial canal of mandibular first molar. The incidence of middle mesial canal were approximately 0.95% to 15%. Tables 3 and 4 presents studies and case reports concerning distal root. The incidence of third root (radix entomolaris) was 3% to 33%.

Table 1

Case reports of a third canal in the mesial root of mandibular first mandibular first molar

Table 2

Prevalence of a third canal in the mesial root of mandibular first molars according to different authors

Table 3

Prevalence of three rooted mandibular first molars- survey of available studies

Table 4

Previous reports concerning radix entomolaris and number of distal canals

Table 5 presents case report of c-shape canals in mandibular first molar.

Table 5

Case reports of c-shape canal in Mandibular molar

Tables 6 and 7 presents studies and case reports of taurodontism in mandibular first molar.

Table 6

Studies done on taurodontism of mandibular molars

Table 7

Case reports on taurodontism

DISCUSSION

Variations in morphology of mandibular first molars

In 35% of cases, four canals are present. The distal root contains two canals, one in the buccal and the other in the lingual position. The second distal canal is sometimes found in a separate root in the distolingual position. Sometimes, the “extra” canal is found in the mesial root, which therefore contains three canals. This is the middle mesial canal [MMC][92][Figure 3].

Figure 3

Mandibular first molar depicting three root canal orifices on mesial root

In addition, cases with three canals in the distal root and two in the mesial root,[5859] two canals in the mesial root and three canals in three distal roots,[93] two canals in the disto-lingual root,[67] two roots and one canal in each,[94] Four canals in mesial root of mandibular molar,[95] molar with seven canals: 2 mesiobuccal, 2 mesiolingual and 3 distal canals[96] were reported.

Extra canal phenomenon

It has been postulated that secondary dentine apposition during tooth maturation will form a dentinal vertical partition inside the canal cavity, thus creating the root canals. The third root canal may be created inside the root cavity of the mandibular molar by this process.[1112] The larger mesio distal dimension of the single distal root, compared to that of the single mesial root, may account for the rare incidence of the third canal created by dentine apposition in a single distal root.[106063] Other possible reasons for the presence of extra root canals include role of external factors during odontogenesis, penetrance of an atavistic gene and more importantly racial genetic factors.[20]

Middle mesial canal (MMC) in the permanent mandibular first molar

The presence of an independent middle mesial canal with a separate orifice and a separate apical foramen was reported in 1974 by Vertucci and Williams[22] and Barker et al.[97]

In 1981, Pomeranz et al,[22] presented a more comprehensive report in which they discussed the in vivo occurrence, instrumentation, and obturation of the middle mesial root canal system of mandibular first and second molars in 12 clinical cases. They classified three separate morphological possibilities in the mesial root [Figure 4]:

Figure 4

Pomeranz et al. classification of middle mesial canal. (a) and (b) are classified as confluent where MMC joins Mesiolingual and Mesiobuccal respectively. (c) Represents independent MMC

The middle mesial canal was classified as a fin when at any stage during debridement; the instrument could pass freely between the mesiobuccal or mesiolingual canal and the middle mesial canal.

The middle mesial canal was classified as confluent when the prepared canal originated as a separate orifice but apically joined the mesiobuccal or mesiolingual canal.

The middle mesial canal was classified as independent when the prepared canal originated as a separate orifice and terminated as a separate foramen.

In 1982, Weine[7] reported a case of a mandibular molar in which a separate middle mesial root canal was located when the case was retreated. The presence of a third canal in the mesial root of mandibular first molars has been reported to have an incidence rate of 1 to 15%.[20] In 1985 and 1989, Fabra-Campos displayed middle mesial canal in 2.1% and 2.6% of the teeth respectively[1819] A radiographic study of extracted teeth reported mandibular first molars with three mesial canals in 13.3% of specimens, four mesial canals in 3.3% of specimens, and three distal canals in 1.7% of specimens.[25] Evangelos G[95] has demonstrated four canals in the mesial root of mandibular first molar performed under operating microscope. Aminsobhani M[98] reported 21 cases of mandibular first molar with three mesial canals of which 13 molars presented with vertucci's canal configuration – 3-2; 4 molars 3-1 and other 4 molars with 3-2-1 canal configuration. Faramarzi F reported a case of mandibular first molar with three mesial canals and broken instrument removal.[99]

According to Mortman,[8] the third mesial canal is not an extra canal but rather the sequelae of instrumenting the isthmus between the mesiobuccal and mesiolingual canals. The isthmus is located in between 54% and 89% of cases, most frequently between 4mm and 6mm from the apical foramen.

According to Von Arx,[100] isthmuses in the mesial root of mandibular first molars may be classified into 5 types [Figure 5]: Type I is two separate canals, type II is two separate canals joined by an isthmus, type III is three canals joined by an isthmus, type IV is two elongated canals that join in the centre and type V is a single, very broad and elongated canal.

Figure 5

(a) Type I is two separate canals, (b) Type II is two separate canals joined by an isthmus, (c) Type III is three canals joined by an isthmus, (d) Type IV is two elongated canals that join in the centre and (e) Type V is a single, very broad and elongated canal

In order to easily locate these intermediate canals within the mesial root these four steps are suggested by Fabra-Campos[18]

Once the access cavity is made the dentinal protuberance which separates the entrance to the mesiobuccal and mesiolingual canals are removed with either ultrasonic tips or round bur

An explorer is used to explore through the groove connecting the mesiobuccal and mesiolingual canal to search for any possible intermediate depression. Also in teeth with vital pulp a bleeding spot can be observed which may indicate MMC

Catheterize the third canal by using a thin file (#08 or 10) in an alternating 45-deg rotating motion

Once the canal is located enlarge canal entrance. Most of the time this intermediate canal will join at the apical or middle third with either the mesiolingual or mesiobuccal canal, ending in one foramen. This intermediate canal joins more frequently with the mesiobuccal canal. Various authors[7101819] have suggested that younger patients had intermediate canals which were more easily found.

Fabro-Campos[19] in his case report suggested that the intermediate canal should not be enlarged as much as the main canal because of the danger of perforation. This in turn makes obturation of the canal more difficult as spreaders cannot be introduced to the correct level during lateral condensation techniques.

Finally diagnostic measures such as multiple preoperative radiographs, examination of pulp chamber floor with a sharp explorer, troughing of the grooves with ultrasonic tips, staining the pulp chamber floor, visualization of bleeding points and with use of magnification, the middle mesial canal can be detected and treated for the success of the root canal treatment.

Three rooted mandibular first molar (Supernumerary root)

Three-rooted mandibular first molars (3RM1) were first described in England by A. E. Taylor in 1899. Since then, investigators have reported 3RM1 in various frequencies in human populations, and it was noticed that 3RM1 was higher in most Asian populations.[101]

In a worldwide survey of 11,318 individuals from 286 prehistoric skeletal and recent populations, Turner and Benjamin found 3RM1 most common in Asian and Asian-derived populations, especially in the Arctic and North Asian populations (25-30%) and least common (1%) in European and African groups.[102]

Tratman (1950) pointed out that the incidence of distolingual root is more when it occurs on deciduous second molar and is only present on first molar when there are five cusps.[103] The incidence of the distolingual root on first molar in the sexes as well as its presence on the left or right side is variable in the populations investigated to date.[103104]

Anthropologically, the presence of a three-rooted lower permanent first molar is significant. It is more commonly noted in Mongoloids than Caucasians and Negroids.[105]

Radix entomolaris

The major variant in mandibular first molar is the presence of an additional third root; a supernumerary root which can be found distolingually. This macrostructure, which is first mentioned in the literature by Carabelli (1844), is called radix entomolaris (RE).[66] Some studies reported a bilateral occurrence of the RE from 50% to 67%.[106107]

In European populations it has been reported that a separate RE is present in the mandibular first molar with a maximum frequency of 3.4-4.2%.[333536107] In African populations a maximum of 3% is found.[42] In Eurasian and Indian populations the frequency is less than 5%.[34] In populations with Mongoloid traits, such as Chinese, Eskimo and American Indians, the RE occurs with a frequency of 5% to more than 40%.[343740-4246]

In addition to anthropological interest, the 3RM1 has a role as genetic marker[108] and also has significance in clinical dentistry.[109]

The RE is located distolingually, with its coronal third completely or partially fixed to the distal root. The dimensions of the RE can vary from a short conical extension to a ‘mature’ root with normal length and root canal. The distolingual root may be separate from or partially fused with the other roots.[70]

The RE could be classified in three groups on the basis of the curve of the root/root canal. This classification is based on a classification proposed by Ribeiro and Consolaro (1997) [Figure 6]: Type I refers to a straight root/root canal, type II to an initially curved entrance and the continuation as a straight root/root canals, type III to an initial curve in the coronal third of the root canal and a second buccally orientated curve starting from the middle to apical third.[110]

Figure 6

Ribeiro and Consolaro's classification of RE: (a) type I refers to a straight root/root canal, (b) type II to an initially curved entrance and the continuation as a straight root/root canals, (c) type III to an initial curve in the coronal third of the root canal and a second buccally orientated curve starting from the middle to apical third

A classification by Carlsen and Alexandersen describes four different types of RE according to the location of the cervical part of the RE [Figure 7]: Types A, B, C and AC. Types A and B refer to a distally located cervical part of the RE with two normal and one normal distal root components, respectively. Type C refers to a mesially located cervical part while type AC refers to a central location between the distal and mesial root components.[109]

Figure 7

Carlsen and Alexandersen's classification of RE. (a & b) Types A and B refer to a distally located cervical part of the RE with two normal and one normal distal root components, respectively, (c) Type C refers to a mesially located cervical part, (d) AC refers to a central location, between the distal and mesial root component

Calberson et al,[70] described the clinical approach for the treatment of mandibular molar with radix entomolaris. These can be summarized as follows

An angled radiograph in addition to preoperative radiograph is essential for accurate diagnosis of RE.

Clinically, analyze the cervical morphology of roots by means of periodontal probing. Presence of an extra cusp or more prominent occlusal distal or distolingual lobe in combination with a cervical prominence or convexity can indicate the presence of an additional root.

The orifice of the RE can be located disto- to mesiolingually from the main canal or canals in the distal root. An extension of the triangular opening cavity to the (disto) lingual results in a more rectangular or trapezoidal outline form.

An initial relocation of the orifice to the lingual is indicated to achieve straight-line access.

A straight line access, initial root canal exploration with small files (size 10 or less) together with root canal length and curvature determination and the creation of a glide path before preparation are step-by-step actions that should be taken to avoid procedural errors.

Radix Paramolaris

Bolk[111] reported the occurrence of a buccally located additional root in mandibular first molar: The radix paramolaris (RP). This macrostructure is very rare and occurs less frequently than the RE. The prevalence of RP as observed by Visser,[112] was found to be 0% for the first mandibular molar, 0.5% for the second and 2% for the third molar. Other studies have however, reported RP in first mandibular molars.[11341]

The RP is located (mesio) buccally. As with the RE the dimensions of the RP can vary from a ‘mature’ root with a root canal to a short conical extension. This additional root can be separate or non separate.[112] Carlsen and Alexandersen describe two different types [Figure 8]: Types A and B. Type A refers to an RP in which the cervical part is located on the mesial root complex; type B refers to an RP in which the cervical part is located centrally between the mesial and distal root complexes.[114]

Figure 8

Carlsen and Alexandersen's classification of RP. (a) Type A refers to an RP in which the cervical part is located on the mesial root complex; (b) Type B refers to an RP in which the cervical part is located centrally, between the mesial and distal root complexes

C-Shaped mandibular first molar

The C-shaped canal was first documented in endodontic literature by Cooke and Cox in 1979,[115] is so named for the cross-sectional morphology of the root and root canal. Instead of having several discrete orifices the pulp chamber of the C-shaped canal is a single ribbon-shaped orifice with a 180° arc (or more), which in mandibular molars starts at the mesiolingual line angle and sweeps around the buccal to end at the distal aspect of the pulp chamber.[116]

Typically, this canal configuration is found in the teeth with fusion of the roots either on its buccal or lingual aspect. In such teeth, the floor of the pulp chamber is usually situated deeply and may assume an unusual anatomic appearance.[114] The main anatomic feature of C-shaped canals is the presence of a fin or web connecting the individual root canals.[117]

Investigations of root development in mouse molars showed that roots were formed by the meeting of dentine leaflets. The fusing of these leaflets was sometimes irregular, forming accessory canals and occasionally, especially in the third molars the leaflets failed to form. In the case of mandibular molars such failure of dentine leaflet formation resulted in a C-shaped root and root canal.[118]

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 floor of the pulp chamber actually continues to the apical third of the root.[119] Fused and C-shaped roots may present with narrow grooves that predispose to localized periodontal disease which may in fact be the first diagnostic indication of such anatomic variation.[120]

For the successful treatment of such C-shape canals following points should be taken into consideration.

Radio graphically it may present as a single fused root or as two distinct roots.[6] Cooke and Cox stated that it is impossible to diagnose C–shaped canals on the radiographs.[115] Radiographs taken while probing the root canal system reveal two characteristics: Instruments tend to converge at the apex;[74] instruments appear clinically and radio graphically to perforate the furcation.[121]

Clinical recognition of C-shaped canals is based on specific anatomy of pulp chamber, difficult to control bleeding because of anastomosis,[122] large pulp chamber in occluso-apical dimension with deep lying bifurcation.[123]

After routine access cavity preparation and removal of pulp tissue the orifice portions of the slit must be widened considerably early in treatment but not too deeply toward the apex.[124]

In C-shaped molars the mesiolingual canal is separate and distinct from the apex, although it may be significantly shorter than mesiobuccal or distal canals. The mesiobuccal canal may merge with distal canal and both exit through a single foramen or both canal have separate portals of exit.[125]

An increased volume of irrigant and deeper penetration with small instruments using sonics or ultrasonics may allow for more cleansibility.[121]

Combination of lateral condensation and application of thermoplasticized gutta-percha is more appropriate.

If endodontic surgical intervention is indicated for a molar with C-shaped root canal anatomy, strong considerations should be given to extraction, retro filling and intentional re-plantation.[115]

If post placement for a crown core is desired, use of only the distal canal should be considered. Placement of posts or antirotational pins in the mesiolingual and mesiobuccal areas invites perforation.[117]

Taurodontism

Witkop defined taurodontism as “teeth with large pulp chamber in which the bifurcation or trifurcation are displaced apically, so that the chamber has greater apicao-occlusal height than in normal teeth and lacks the constriction at the level of the cementoenamel junction.”[89] [Figure 9]

Figure 9

Hypotaurodontism involving right mandibular first and second molar.

The term taurodontism was coined by Sir Arthur Keith to describe the “bull-like” condition in teeth in which the tall root trunk encloses a high pulp chamber and short roots.[126] The etiology of taurodontism is unclear. It is thought to be caused by the failure of Hertwig's epithelial sheath diaphragm to invaginate at the proper horizontal level, resulting in a tooth with short roots, elongated body, an enlarged pulp, and normal dentin. Previously, taurodontism was related to syndromes such as Down's and Klinefelter's. Today, it is considered as an anatomic variance that could occur in a normal population.[127]

A taurodont does not exhibit any unique morphologic clinical characteristics which may aid in its recognition. The radiographic examination is the only way to visualize a rectangular configuration of the pulp chamber. The apico-occlusal height of the pulp chamber varies depending upon the type of taurodontism.[128] Shaw has classified taurodontism into hypo, meso, and hyper taurodontism on the basis of the apical displacement of the floor of the pulp chamber.[129]

The teeth most frequently affected are the molars diagnosed most easily by radiograph. Premolars present a lower incidence and the mandibular teeth more commonly affected than the maxillary teeth.[90]

Witkop et al,(1988) used the following reference points on radiographs for diagnosis of taurodontism: A as the lowest point of the occlusal end of the pulp chamber, B as the level at which the furcation of the roots occur and C as the position of the apex of the roots. If AB≥’/2 AC the tooth was considered to be taurodontic [Figure 10].[130]

Figure 10

Witkop's criteria for diagnoses of taurodontism

A taurodont tooth shows wide variation in the size and shape of the pulp chamber, varying degrees of obliteration and canal configuration, apically positioned canal orifices and the potential for additional root canal systems. Therefore, root canal treatment becomes a challenge. Access opening for such tooth is no different compared to other mandibular molars. Because the pulp of a taurodont is usually voluminous in order to ensure complete removal of the necrotic pulp, 2.5% sodium hypochlorite should be used along with ultrasonics. Because of the complexity of the root canal anatomy and the proximity of the buccal orifices, complete filling of the root canal system in taurodontism is challenging. A modified filling technique has been proposed, which consists of combined lateral compaction in the apical region with vertical compaction of the elongated pulp chamber, using the system B device.[130]

CONCLUSION

The mandibular molar is the first permanent molar to erupt into the oral cavity and it is most often tooth requiring endodontic therapy. The knowledge of root canal system and the most common variations should be kept in mind before starting root canal therapy. With the recent innovations of various operating aids, the coming era can witness more variations in the root canal morphology of permanent mandibular first molar.