The root canal system
The root canal system of human teeth may be extremely complex. Still, in spite of the irregularities, all groups of teeth have a general pattern in their root canal morphology. Virtually all teeth have main root canals and it is the main canals that we instrument and obturate during endodontic treatment. Lateral and accessory canals are influenced by irrigants and antiseptics, and only in rare instances will the outcome of the treatment depend on whether an accessory canal is actually obturated or not. This is the case even if an endodontic lesion clearly originates from a lateral root canal. If adequate chemomechanical instrumentation and disinfection are carried out and the main root canalis filled, the lateral periodontitis will heal even if the lateral canal is not filled. This is an extremely important concept in endodontics, and without this concept, root canal treatment has no logical foundation. Therefore, the root canal morphology of the teeth as it pertains to clinical endodontic therapy will be discussed.
Maxillary Central Incisor
The access cavity is prepared with long-shank round burs and the point of entry is on the pal-atal surface of the tooth, slightly incisal to the singulum. The bur is held at an angle of about 30° to the long axis of the tooth and the long shank is helpful in aligning the bur correctly.When an opening to the pulp chamber has been achieved, the access cavity is enlarged mainly with outward strokes of the bur to reflect the size and shape of the pulp chamber. It is very important to include the pulp horns in the cavity so that all tissue and discolored dentin are removed to prevent discoloration of the tooth. A round bur no. 2 is used for this purpose.Also, the root canal has a definite cervical constriction that should be removed before the actual root canal instrumentation begins. This is done with a long, tapered, diamond-coated bur, a Gates-Glidden, or similar bur. The apical part of the root canal is circular in shape and a circular apical box may be readily prepared in this tooth.
Maxillary Lateral Incisor
The access cavity is prepared with long-shank, round burs as described for the maxillary central incisor. The lateral incisor typically has a wide root canal in a narrow root. Thus, the diameter of the canal in the apical 5 mm of the root is generally wider than both in the maxillary central incisor and the maxillary canine. However, the most apical part of the canal has a fairly round shape, so that a circular apical box may be prepared in lateral incisors. Still it must be understood that the canal of these teeth should be enlarged considerably more in the apical area than would normally be expected judging by the size of their roots.
The root of the maxillary lateral incisor is frequently curved apically, often in a palatal direction so that the curve may not be apparent in a radiograph immediately. Although the curved canals are generally narrower than the canals in lateral incisors with straight roots, they are still wide and may be difficult to instrument adequately. As a result, historically speaking, endodontic treatment of the maxillary lateral incisor has failed more than in any other tooth. However, the use of flexible nickel-titanium instruments has made it easier to instrument wide, curved canals without perforations, canal transportation, or other mishaps. These malformations may allow penetration of bacteria to the pulp and endodontic treatment of these teeth will frequently be necessary. The practical approach will depend on the clinical and radiographic findings and the degree of irregularities of the tooth. Often a surgical intervention with retrograde fillings to support the best possible orthograde obturation of the root canals will be necessary.
The access cavity is prepared with long-shank, round burs as described for the maxillary central incisor. The maxillary canine is the longest tooth in the dentition and teeth 30 mm and longer are occasionally seen. The root canal is straight and
circular in shape and only a very slight apical curvature of the root is sometimes seen. A circular apical box may be readily prepared, and this tooth lends itself exceptionally well to treatment with the standardized technique. On extremely rare occasions, the maxillary canine may have two root canals.
Maxillary First Premolar
The maxillary first premolar has, as a rule, two root canals. The pulp chamber is elongated in a buccopalatal direction and the orifices of the canals are located slightly centrally to the buccal and lingual cusp tips. The access cavity is prepared with a long-shank, round bur directed at the long axis of the tooth with the point of entry in the middle of the occlusal central groove. When the pulp chamber has been penetrated, the bur is used in the normal manner with outward strokes to remove the tooth structure overhanging the pulp chamber. The shape of the floor of the pulp chamber will usually indicate the number of canals. If one canal is present, it is located centrally in the tooth. If two canals are present, they are usually further apart, i.e., further buccally and palatally than perhaps would be expected. Teeth with three canals have two buccal canals and one palatal canal. Most maxillary first premolars have a distinct concavity in the mesial root surface, increasing the risk of mesiocervical perforations during access preparation. The cusps of this tooth are often weakened when endodontic treatment is indicated, and their height should be reduced or they should be strengthened with a bonded restoration prior to endodontic treatment to prevent uncontrolled crown-root fractures.
The root canals of the maxillary first premolar vary considerably in width, and the variations are not necessarily related to the diameter of the root. Endodontically, this is a difficult tooth, and more failures are found with it than with any other tooth except the maxillary lateral incisor. To improve upon this, it is necessary to understand and accept how wide the apical part of the root canals can be in the maxillary first premolar and to instrument them accordingly. This requires both patience and skills in the narrow and often curved roots of this tooth. However, a circular apical box can most often be prepared and the use of flexible nickel-titanium instruments is helpful.
Maxillary Second Premolar
The maxillary second premolar usually has one root and one root canal. The pulp chamber is elongated in a buccopalatal direction, and the access cavity is prepared as described for the maxillary first premolar. The coronal part of a single root canal is ribbon-shaped. It tapers off and takes on a circular form in the apical third of the root. In about 10-15 % of the teeth, a single canal splits into two canals in the apical 3-4 mm of the root. When two canals are present, their orifices are located buccally and palatally, centrally to the cusp tips, as in the maxillary first premolar. The two canals may occasionally join apically, but as a rule they have separate apical foramina. When three canals are present, two are located buccally and one palatally. These canals are narrow and may be difficult both to locate and to instrument. Otherwise, the maxillary second premolar is easier to instrument than the first premolar, also when two canals are present, and in most instances the canals can be given a circular form in the apical area.
Maxillary First Molar and Second Molar
The pulp chamber of the maxillary molars is generally located in the mesial two-thirds of the crown. The access cavity is prepared with long-shank, round burs and the point of entry is the mesio-occlusal central groove ot the tooth. The orifice of the main mesiobuccal canal is located slightly palatally to the mesiobuccal cusp tip. The orifice of the palatal canal is found central-palatally in the mesial two-thirds of the crown. When a second mesiobuccal canal is present, its orifice will be seen near the main mesiobuccal canal on a straight line between this canal and the palatal canal. The distobuccal canal is located slightly distally to the midline between the mesial and distal surfaces of the crown and somewhat more palatally than the mesiobuccal canal. The pulp chamber has a triangular base with the canal orifices located in the corners of the triangle. In molars with their roots grouped closely together, as is most often seen in second molars, the orifice of the disto-buccal root canal will have “moved” in a mesi-opalatal direction along a line perpendicular to the line between the mesiobuccal and the palatal canals. In second molars with extremely narrow pulp chambers, the distobuccal canal may have moved so far mesially and palatally that the orifices of all four canals are located on a more or less straight line between the mesiobuccal and palatal canals. If the mesiobuccal cusp is weak, or if a crown restoration is planned subsequent to the endodontic treatment, this cusp should always be cut prior to treatment. This will greatly improve the visibility of the access cavity and facilitate the root canal instrumentation and obturation phases of the treatment.
As indicated above, the mesiobuccal root of about every other maxillary molar has two canals. The coronal part of the canal located furthest buccally is fairly wide and this canal is readily located. The orifice and coronal part of the second mesiobuccal canal may be extremely narrow, and as a result this canal is often overlooked in the clinical situation. Still, the success rate of endodontic treatment of this root is good, conceivably because the two canals in most instances have a common apical foramen. Obviously, all maxillary molars should be examined for a second mesiobuccal canal. Circular apical boxes may be prepared in most mesiobuccal roots, regardless of the presence of one or two canals.
The distobuccal root is generally straight and has one canal with a near-circular shape in the apical area. The distobuccal root canal is exceptionally well suited to being instrumented with a circular apical box.
The palatal root is most often straight, but may curve in a buccal direction in its apical part. This may not be readily detected radiographically. The palatal root canal is wide and appears to present few problems during the instrumentation phase. However, the canal may be ribbon-shaped and may be wider apically than is clinically apparent. Apical foramina with a largest diameter of more than 3 mm have been observed in mature teeth. However, most often, the palatal canal decreases in size in the apical 2-5 mm of the root and it takes on a circular shape in this area. Still, the apical part of the canal should be instrumented as wide as the diameter and a possible buccal curvature of the root allow. A circular apical box is then usually attained.
The access cavity is prepared with long-shank, round burs. The point of entry is the central area of the lingual surface of the crown. When two canals are present, there is a buccal and lingual canal with the lingual canal peripheral in the root. The access cavity, therefore, must be enlarged in a cervical direction to expose the lingual canal orifice. Also, it is often necessary to enlarge the access cavity in an incisal direction, even to a point where the incisal edge is broken, to facilitate adequate access to the root canals. Special attention is given to the pulp horns and a round bur no. 2 is used to clean these areas of the crown.
Apart from the fact that the mandibular central incisor is somewhat smaller than the lateral incisor, the teeth are quite similar. This applies to their root canal system as well. The root canal is ribbon- or hour glass-shaped, or two canals may be present that almost without exception join in a common apical foramen. Endodontically, the mandibular incisor is a difficult tooth with a rather high failure rate. The lingual canal is often overlooked, usually because of an inadequate access cavity. In teeth with two canals, the apical part of the canals may be given a cylindrical shape. In roots with a single canal, the apical part of the canal will usually be ribbon-shaped and much wider in a buccolingual direction than has generally been assumed. A longitudinal filing technique, therefore, should be used all the way to the working length. It is important in these teeth to pay special attention to the buccal and lingual walls of the canal because of its wide buccolingual diameter. A master point as large as possible (depending on the mesio-distal diameter of the canal) is then used, but lateral condensation to the working length may be necessary in these teeth.
The access cavity is prepared with long-shank, round burs. The point of entry is the central area of the lingual surface, and the bur is held at a 30° angle to the long axis of the tooth until it has penetrated to the pulp chamber. The access cavity is then enlarged in an incisal direction to facilitate the location of a lingual second root canal, which, if present, splits off from the main canal in the midroot area and usually joins the main canal again 1 -5 mm from the apical foramen. Only occasionally do the two canals end in separate foramina, and two roots in this tooth is an infrequent occurrence. Thus, as a rule, the second canal does not influence the size and shape of the main root canal near the apex where the canal has a circular shape and can readily be given the shape of a circular apical box.
Mandibular First Premolar
The access cavity is prepared with long-shank, round burs. The point of entry is slightly buccal to the central occlusal groove. The crown of the mandibular first premolar often tilts in a lingual direction so that the occlusal surface has a lingual inclination. This must be taken into account during access cavity preparation or a buccal root perforation may occur. For the same reason, the buccal cusp may have to the reduced to allow adequate access to the root canals.
The mandibular first premolar most often has one straight root canal from the pulp chamber to the apical foramen. Five percent of these teeth have two uninterrupted root canals. The orifices of the canals are then situated buccally and lingually on the pulp chamber floor. In 15 % of these teeth, a second canal branches off from the main canal in a buccal or lingual direction in the middle or apical area of the root. The occurrence of a split root canal is suggested radiographically when the image of the canal suddenly decreases dramatically in width or seemingly disappears. Locating and instrumenting the branching second canal may be difficult. A stainless steel K-file with a curved tip should be used to probe the root canal wall in the area where the split appears to have taken place before any instrumentation of the canal is done. If the second canal is located, it should be opened up and instrumented first, and the patency of this canal should be constantly checked during the instrumentation of the main canal. With regard to shape and size of the root canals of the mandibular first premolar, they can readily be given a cylindrical shape apically.
Mandibular first premolars that have three, four, and even five root canals are occasionally seen.
Mandibular Second Premolars
The access cavity is prepared as described for the mandibular first premolar. The lingual tilt of the crown occurs in this tooth as well, although the lingual cusp is more developed than in the first premolar, reducing the lingual inclination of the occlusal surface. A split of the root canal below the pulp chamber floor is seen, but much less frequently than in the mandibular first premolar. On the whole, the mandibular second premolar is a tooth that gives few endodontic problems. The root canal is narrow in the apical area of the root and a cylindrical apical box can readily be prepared.
Mandibular First and Second Molar
The pulp chamber of the mandibular molar is located in the mesial two-thirds of the crown. The access cavity is prepared with long-shank, round burs and the point of entry is the mesial aspect of the central occlusal groove of the tooth. The orifice of the mesiobuccal canal is located directly underneath the tip of the mesiobuccal cusp, and this cusp should be reduced in height, if at all possible, to facilitate the locating and instrumentation of the mesiobuccal canal. The mesio-lingual canal is located between the lingual cusp and the central occlusal groove. In second molars, a ribbon-shaped common orifice for the two mesial canals may be present. The distal canal is located centrally in the tooth slightly distal to the buccal groove. When one distal canal is present, the floor of the pulp chamber has an approximately triangular shape with the three canals in the corners of the triangle. A rhomboidal shape of the pulp chamber floor either indicates a severely ribbon-shaped distal canal or two separate canals with their orifices at the distal corners of the floor.
The root canal morphology of the mesial root of mandibular molars is extremely irregular and quite unpredictable from one tooth to the next. If one canal is present, it may be ribbon- or hour glass-shaped and difficult to instrument. When two canals are present, they are usually connected by multiple bridges of tissue of varying widths. Also, the apical foramen may be ribbon-shaped with a largest diameter which is larger than the smallest diameter of the root. Still, one or two main root canals can always be recognized clinically, and in 40% of the roots, they end apically in one apical foramen, mostly in the second molar. Careful but determined use of Hedstrom files on the buccal and lingual canal walls is helpful during the debridement of these canals. Also, the mesial wall should be fairly vigorously instrumented with an outer-curve filing technique. However, the distal wall of the mesial canals must be treated with great care, first, because the root in this area is extremely thin, and second, because the distal wall borders on the inner curve of the canal since this root always curves in a distal direction. It is advantageous to use nickel-titanium instruments in curved root canals. Because of their flexibility, these instruments follow the curve and work centered in the canal. In spite of the irregularities of the root canal system of the mesial root of the mandibular molar, the success rate of endodontic treatment of this root is excellent. A cylindral apical box may be prepared in most instances, and a final instrument no. 35 is usually adequate.
In contrast to the mesial root, the distal root of the mandibular molar has a fairly uniform canal morphology. In most instances, one central canal with a circular shape apically is present. In some instances, the canal is ribbon-shaped coronally and it can be difficult with clinical means to determine whether the canal continues as a single canal to the foramen or whether it divides in the apical area. The rule then is that, when in doubt, a ribbon-shaped canal should always be prepared as if it were two separate canals, one distobuccal and one distolingual. Two master points are used and inserted from the peripheral corners of the canal and the rest of the ribbon is filled with accessory points and lateral condensation.