The Root Canal Anatomy of the Mandibular First Molar Tooth Report

Introduction

The various teeth in the oral cavity have vastly different root canal systems. Extensive knowledge of the morphology of these canals structuring by practitioners is very important if successful edodontic therapy is to be achieved. This essay seeks to analyze the root canal system and to this end, the anatomy of the root canal of the mandibular first molar has been detailed in the first part of the paper. The second part of the report analyzes the various anatomical considerations that have to be taken during preparation and the final part studies the various problems that arise during molar teeth instrumentation and how they can be handled.

Root canal anatomy of the mandibular first molar

In order to understand the morphology of the root canal of the mandibular first molar, we must first have knowledge of the basic components of a typical root canal1. The pulp cavity which houses the dentine cavity normally assumes the external shape of the tooth. Physiological and pathological factors however contribute to the modification of its shape and size through production of secondary and sometimes tertiary dentine and cementum material2. This pulp cavity comprises two parts; the pulp chamber which is found within the crown of the tooth and the root canal(s) which are located within the root of the tooth. Other features that contribute to the formation of the root canal include lateral, furcation and accessory canals which are related to the canal orifices alongside intercanal connections and apical foramina. The actual root canal originates as funnel shaped orifices normally found either at the cervical line or slightly distal to it. It then runs through the horns to end at the apical foramina which open at the surface of the root. These apical foramina are located at about 0 and 3mm from the center of the root apices3. A vast majority of root canals have curvatures towards the facial-lingual direction. These curvatures may occur gradually along the length of the root canal. The canal may also be straight for most of the length and then acquire a sharp curvature when approaching the apex4. In a few cases, the canals may acquire double‘s’ curvatures. Branches of the main chamber that houses the pulp communicate with the outer part of the roots. These are called accessory canals. Accessory canals present in a number of ways but the most important and most common are the lateral canals which extend horizontally towards the external surface of the root from the middle of the primary canal and the furcation canals which occur in the bifurcation or trifurcation of teeth with more than one foot. The lateral canals serve as passages for irritants away from the pulp and towards the periodontium.

Landmarks both histological and anatomical distinguish the apical root canal; these are: the cemento-dential junction, the apical constriction and the apical foramen. The root canal tapers from the orifices of the canal to the apical constriction commonly located about 0.5 to 1.5 mm within the apical foramen. The apical constriction is the region where the root canal acquires the smallest diameter. The cemento-dential junction is the region of the canal where the dentine and the cementum meet. The pulp tissue ends here marking the beginning of the periodontal tissues. The apical foramen is the region of the cemental canal which opens up to the external surface of the root. This apical foramen assumes the shape of a funnel and it is the region of the root canal with the largest diameter. The root canal tends to widen from the apical constriction towards the apical foramen. On average the length between the major and minor diameters is 0.5 mm in young children and 0.67 in older people. This length increases with age generally due to an increase in the accumulation of cementum. Canals take various pathways on the way to the apex; some canals may branch, others divide and there are those which end up rejoining. In total eight configurations of the pulp space components have been identified. These variations are listed as types 1-8 and are briefly described below:

• Type 1- Has just one canal all the way to the apex.
• Type 2- Has two canals which join to end up as one canal at the apex (2-1)
• Type 3- One canal which splits into two and later the two rejoin to end up as one canal at the apex (1-2-1)
• Type 4- Has two canals that run all the way to the apex
• Type 5- One canal splits into two (1-2) and then the two canals traverse the rest of the length to the apex.
• Type 6- Two canals join together then split to end up as two canals at the apex (2-1-2)
• Type 7- One canal splits into two canals which rejoin before splitting again towards the apex (1-2-1-2)
• Type 8- Has three canals from the pulp cavity all the way to the apex. This type of formation is however extremely rare.

The roots of the mandibular lower first molar usually have 3-6 canals; the mesial root generally has two to three canals while the distal root may either have one, two or three canals. If there is only one canal in the mesial root, it is referred to as the mesial canal. Whenever multiple canals occur they are named mesiobuccal, mesiolingual and medial-mesial canals. The middle-mesial canal is however rare and occurs in the developmental ridge between the mesiobuccal and mesiolingual canals. If there is only one canal present in the distal root, it is named the distal canal. Whenever more than one canal happen in the distal root, they are referred to as the distolingual, distobuccal and the middle distal canals depending on their particular location. The canal(s) in the distal root are always straight from where they originate in the pulp cavity all the way to about 1-2mm from the apex where they curve up to 90 degrees distally. The distal canals sometimes present with a mesial curvature but this curvature is not acute. The distal canals have oval or flattened cross sections and compared to other canals they are quite large; a factor that makes them easily accessible by instruments. Both the mesiobuccal and mesiolingual canals present with curvatures along their whole extents with the curvatures characteristically peaking in the region towards the apex. These canals majorly curve distally as well as having minor curvatures bucally or lingually at the same time. The canals in the distal root often start together and divide a few millimeters below the floor of the pulp chamber. The canals in the mesial root and the corresponding canals in the distal root can combine before they reach the apex.

The accessory canals of this tooth present in three forms. First is where the furcation canal stretches from the pulp cavity down to the intraducular area. This presentation manifests in 13% of the entire first mandibular molar tooth population. About 23% of the manifestations present as a lateral canal extending from the coronal third of a key root canal and lengthens to the furcation region. Of this presentation, 80% occur in the distal root. Finally, about 10% of molar first teeth have both furcation and lateral canals.

The floor of the pulp cavity from where these root canals originate takes various shapes in the mandibular molar. The most common presentation is the rectangular floor followed by the triangular floor and the least common happens to be the oval shaped chamber floor.

Race and ethnicity heavily influence the number and morphology of the mandibular molar tooth root canals. For instance, the three rooted mandibular molar has over time been given particular attention with surveys revealing that the mongoloid and chinese populations have a higher incidence of this structure as compared to other races.

Anatomical considerations during root canal preparation

For a successful root canal procedure, primary shaping is of utmost importance. This shaping and cleaning has to meet four basic criteria. First, the root canal preparation should assume a funnel shape tapering from the apex of the root to the coronal access cavity. Secondly, the foramen should be kept as small as is practically possible. The apical foramen should also remain in its origin positioning in relation to both the surface of the root and the bone and finally, the preparation should follow the shape of the canal as much as possible considering the concavities and the curvatures.

Various anatomical considerations have to be made during the preparation including the original structure of the root canal (i.e. length, curvature, number of canals and diameter), the location of the apical terminus and the thickness of the dentine and cementum that form the boundaries of the root canal. The major objective of the procedure is to gain direct access to the canal system while at the same time maintain the natural structure of the tooth.

Before starting the root canal procedure, the dentist should be able to determine the possible number of canals present. The floor of the pulp chamber and the wall of the cavity serve as important guides in establishing the kind of morphology that is present in the root canal system. In the identification of the pulp chamber boundaries and the orifices of the root canal, it has been established that the most important marker is the junction between the cementum and enamel. Six laws have been established to help determine the positioning and morphology of the root canals. These laws are briefly explained below.

1. Law of symmetry 1: If a line is drawn in a messiodistal direction through the floor of the pulp chamber, the orifices of the canals will be equidistant from this line. The only exceptions to this rule are the maxillary molars.
2. Law of symmetry 2: The canal orifices will lie across the floor of the pulp chamber in a line that runs perpendicular to a line drawn in the messiodistal direction through this floor. This presents in all teeth except for maxillary molars.
3. Law of color change: The floor of the pulp chamber is always darker compared to its walls.
4. Law of the orifices location 1: The openings of the root canals are always located in the region where the floor and walls meet.
5. Law of the orifices location 2: The openings of the root canals are always located at the angles in the region where the floor and the walls meet.
6. Law of the orifices location 3: The openings of the root canals are always located at the terminus of the lines that indicate the fusion of the root during development.

Another anatomical consideration to be made in the cleaning and shaping of the root canal is the size of the pulp cavity. This cavity generally decreases in size with age due to the fact that the formation of dentine is not standardized throughout life and that it forms more rapidly on the roof and floor than on the walls for teeth in the posterior region. As a result, calcification of the root canal may occur with the pulp chamber ending up assuming a flattened appearance. Calcified canals are particularly very difficult to investigate and they require proper illumination and magnification in order to identify them. Proper investigation of the floor and walls of the pulp chamber will give clues pertaining to the kind of canal system present as well as the positioning of the orifices. If there is only one canal, it will be located in the middle of the preparation. Should a canal be located at off-center, then there is a chance that another canal exists on the opposite side.

The morphology and relationship of the canals to each other should also be given consideration during the preparation. If there are multiple canals, they will definitely be smaller than if a single canal was present. The relationship between the canals is significant because the closer the canals are to each other the greater the chances of them joining along the length of the root. Whenever the root canals join, the palatal or lingual canal will present a straight line access in relation to the apex while the buccal canal will have a less direct access to the apex. The location at which the two canals join will usually have a smaller diameter than the rest of the canal. This poses a problem especially if the canals split up again towards the apex because during filling, this constriction will occasion the creation of voids in the apical third. Some canals also begin as a single canal but end up splitting into two. In the incidence of such a split, the two divisions are named buccal and lingual based on their presentation. The lingual canal generally breaks away from the main canal at a very sharp slant; sometimes at almost 90 degrees. The buccal canal exits from the main canal in a straighter angle. Initial identification of the two canals will help guide the modifications that are necessary to achieve unobstructed instrumentation.

The length of the canal and the positioning of landmarks will also help to determine the working length of the preparation. The most important landmark in this part of the root canal procedure is the apical constriction. Being the part of the canal with the smallest diameter, it serves as a logical guide to where the obturation and preparation should end. At this part of the canal, pulp tissue converts into periapical tissue which is avascular. The apical foramen needs to be kept unobstructed to avoid the obstruction of unwanted matter to the periapex. Keeping the foramen open also ensures that the irrigation solution is able to find way into the apical third. Care should therefore be taken not to widen its diameter.

Therefore, ending the procedure at this region helps in the eradication of pathogenic microbes.

The number of roots of a tooth should also be considered during the preparation. For instance, the maxillary molars will have three roots, the mesial, the distal and the palatal roots. The palatal and distal roots can have as many as two canals each while the mesial root can have up to three canals. The ability to identify the various roots on a tooth will help in complete preparation and obturation hence preventing cases of recurrent pain post operatively.

Common problems during molar teeth instrumentation

Perforations and Strip perforation

The molar tooth root canal has numerous curvatures especially in the messiobuccal and the messiolingual canals. These curvatures present complexities during preparation particularly when using instruments which have a sharp cutting edge and operate in a circular motion. If adequate care is not taken in these circumstances, perforations may occur which may later cause destruction of the cementum and sometimes may end up causing infection and irritation of the periodontal ligament. In the root canal procedure, perforations may end up leaving some parts of the original root canal underprepared in the event that the region apical of the perforation is not accessible.

The mesial root of the molar tooth is divided into two areas basically referred to as the mesial and distal parts. The mesial region of this root has a thick layer of dentine that is only touched by the endodonic instruments during preparation. This region is referred to as the safety zone. The distal area on the other hand usually presents as a straight layer of dentin commonly referred to as the danger zone. During instrumentation this danger zone becomes exposed to strip perforations if adequate care is not taken. The cleaning and primary shaping processes are designed to provide easy identification of the foramen, facilitate disinfection and enable access by the material used for obturation. Modern instrumentation methods encourage a progressive preparation of the root canal from the crown downwards. The flare technique is particularly encouraged for preparation of the cervical third. Unfortunately this method calls for transportation of the canal towards both the safety and danger zones alike. This leads to undesirable episodes as well as strip perforations of the root which can lead to inflammatory problems and later the dilapidation of supporting structures. The incidence of strip perforations also leads to alveolar bone loss.

Perforations which tend to be common in molar teeth operations can be repaired even though this depends on the extent of damage caused. The level of damage is assessed based on the proximity of injury to support structures. The types of perforations which require most attention are those that occur close to the periodontal ligament. These include the strip perforations of canals whereby the loss of attachment ends up predisposing the patient to infection and inflammation.

Perforations and strip perforations can be prevented in a number of ways. First, before instruments are introduced into the canal an assessment should be carried out on the suitability of the chosen instruments based on their mode of operation. For hand instruments a number of techniques have been established over time and it is up to individuals to choose the one(s) they feel they are most comfortable with and which they can use to achieve optimum results. Of these techniques, the most popular for instrumentation are the ‘crown-down pressureless technique’ and the ‘balanced force technique’. The crown-down presureless technique involves preflaring of the coronal canal before the dentine is removed starting from the crown towards the apical direction. These techniques require extended periods of practice for individuals to master and use effectively. For instruments with rotary systems, the prime consideration is the accuracy of calibration. Electron micrographs will help determine the exact location where the instruments can reach without causing any possible accidents. A general clear understanding of the morphology of various tooth canals will also go a long way in ensuring that rotary instruments are used safely. For instance, in a situation where two canals join, a rotary file if not used with this anatomical consideration in mind can separate as the instrument cuts through the sharp curvatures into the common region of the canal.

False working length

For a successful endodontic canal treatment, the proper working length has to be established. If the length of the root is not determined, incomplete instrumentation and under-filling of the canal may occur. Canals that are not properly filled may lead to unrelenting pain and discomfort occasioned by inflammation of the pulp tissues which are retained. A ledge may also develop in the root canal towards the apex resulting in the formation of a dead space which makes retreatment virtually impossible. Formation of the ledge may result in a continuous periradicular laceration. Improper determination of the length of the root may also result in apical perforation and consequently overfilling associated with possible infection and increased pain may arise.

Molar teeth generally tend to have curvatures especially in the mesiobuccal and mesiolingual canals. These curvatures lead to narrowing and obstruction of the canal due to deposition of dentine. This narrowing of the canal leads to a false sense of apical constriction which binds the file before it gets to the apex. This problem can be handled by preflaring of the canal. Preparing of the coronal part of the root canal before attempting to determine the working length helps in removing most of the obstruction in this part. As a result, there is a marked improvement in the detection of the apical constriction since the file then binds at the apex.

The preflaring process has a number of advantages which cannot be ignored. First, the need to precurve files is significantly reduced. This is because the general curvature of the canal is reduced after preflaring and even when files have to be precurved it is only by a very small angle. Preparation involves proper clearing of the pulp cavity toilet and therefore preflaring in essence provides for efficient removal of debris. Once the restrictive dentine is removed from the coronal region, bigger size files such as numbers 10 and 15 can easily be maneuvered within the canal hence reducing the need to use multiple sets of instruments. This coronal enlargement also helps provide straight-line access and more control in the preparation of the apical third. The enlargement of the coronal part of the canal ensures that most of the necrotic material is cleaned out thus reducing the chance of pushing this infected material apically. Preflaring also ensures that the amount of the solution used for disinfection is greatly increased hence augmenting the effectiveness of the solution. Finally and actually most importantly, preflaring allows for the accurate taking and maintenance of the working length.

By virtue of the curvatures in parts of the root canals and sometimes by the extra root present especially in the maxillary set, molar teeth need adequate access preparation if a successful procedure is to be carried out. Improper preparation and shaping may cause a number of problems during instrumentation. These include insufficient instrumentation and unsatisfactory obturation occasioned by the limited mobility. Another common problem that would arise is the uncalled for damage to tooth structure alongside defective restitutions. Poor perceptibility of the canal structure may also occasion perforations ending up with continuous post operative pain.

Conclusion

Various endodontic texts have been effectively analyzed in this paper and it has been revealed that for desirable results to be attained particularly in the root canal procedure, extensive knowledge of the anatomy of the system is vital.

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