Tooth Morphology, Isolation, and Access

Accessory[132] canals are minute canals that extend in a horizontal, vertical, or lateral direction from the pulp space to the periodontium. In 74% of cases they are found in the apical third of the root, in 11% in the middle third, and in 15% in the cervical third.223 Accessory canals contain connective tissue and vessels but do not supply the pulp with sufficient circulation to form a collateral source of blood flow. They are formed by the entrapment of periodontal vessels in Hertwig’s epithelial root sheath during calcification.46 They may play a significant role in the communication of disease processes, serving as avenues for the passage of irritants, primarily from the pulp to the[133] periodontium, although communication of inflammatory processes may occur form either tissue.
Based[133] on scanning electron microscopy (SEM) studies, the diameter of furcation openings in mandibular molars varies from 4 to 720 m. [7 mm] 224 The number of furcation canals ranges from none to more than 20 per specimen. Foramina on both the pulp chamber floor and the furcation surface were found in 36% of maxillary first molars, 12% of maxillary second molars, 32% of mandibular first molars, and 24% of mandibular second molars (Fig. 5-8). Mandibular teeth have a higher incidence of foramina involving both the pulp chamber floor and the furcation surface (56%) than do maxillary teeth (48%). No relationship was found between the incidence of accessory foramina and the occurrence of pulp chamber calcification or the distance from the chamber floor to the furcation. Radiographs usually fail to show the presence of furcation and lateral canals in the coronal portion of these roots. In one study involving 200 permanent molars, the pulp chamber floor was stained with 0.5% basic fuscin dye. Patent furcation canals were detected in 24% of maxillary and mandibular first molars, 20% of mandibular second molars, and 16% of maxillary second molars.87 Pulpal inflammation can communicate to the periodontium via these canals, and the result is furcation lesions in the absence of demonstrable periodontal furcation lesions may influence the viability of the coronal or radicular pulp tissue when these aberrant channels are present.84
 From[133] the early works of Preiswerk175 in 1912, Fasoli and Arlotta67 in 1913, and Hess and Zurcher90 in 1917, to more recent studies33,82,100,190 demonstrating the anatomic complexities of the root canal system, data indicate that a root with a tapering[136] canal and a single foramen is the exception rather than the rule. Investigators have shown multiple foramina, additional canals, fins, deltas, intercanal connections, loops, C-shaped canals, and furcation and lateral canals in most teeth (Fig. 5-9).* Consequently, complex anatomy must be considered the norm. The first premolar in Fig. 5-10, A, is a good example of complex anatomy. The extra root is not obvious in a pretreatment radiograph (Fig. 5-10, B). Figure 5-11 shows a cross-section of a similar tooth. This tooth has a fine, ribbon-shaped canal system instead of two distinct canals. Both these teeth present challenges for locating the canal and for achieving the previously stated objectives for root canal procedures.
* References 48, 53, 154, 170, 196, and 223.
Typically,[136] root canals take variable pathways throughout, coursing from the orifice to the apex. The pulp canal system is complex, and canals may branch, divide, and rejoin. Weine238 categorized[137] the root canal systems in any root into four basic types. Other studies,225 using cleared teeth in which the root canal systems had been stained with hematoxylin dye, found a much more complex canal system. In doing so the authors identified eight pulp space configurations, which can be briefly described as follows (Fig. 5-12):
Type[137] I: A single canal extends from the pulp chamber to the apex (1).
Type II: Two separate canals leave the pulp chamber and join short of the apex to form one canal (2-1).
Type III: One canal leaves the pulp chamber and divides into two in the root; the two then merge to exit as one canal (1-2-1).
Type IV: Two separate, distinct canals extend from the pulp chamber to the apex (2).
Type V: One canal leaves the pulp chamber and divides short of the apex into two separate, distinct canals with separate apical foramina (1-2).
Type VI: Two separate canals leave the pulp chamber, merge in the body of the root, and separate short of the apex to exit as two distinct canals (2-1-2).
Type VII: One canal leaves the pulp chamber, divides and then rejoins in the body of the root, and finally separates into two distinct canals short of the apex (1-2-1-2).
Type VIII: Three separate, distinct canals extend from the pulp chamber to the apex (3).
The anatomic variations present in these teeth are listed in Tables 5-1 and 5-2. The only tooth that showed all eight possible configurations was the maxillary second premolar.
The percentages of human permanent teeth with these variable canal configurations are presented in Table 5-3 and 5-4.
Similar observations have been described in large population studies with the exceptions that one canal was found in 23% of maxillary laterals, 55% of mesiobuccal roots of maxillary second molars.33,242 Differences in these studies, from the types described by Weine may be due to variations in the ethnic and racial populations studied. Other studies that were ethnic, racial, or gender-specific based have indicated wide variations in canal morphology, which sometimes appeared more often in specific teeth.* These authors concluded that gender, racial, and ethnic aspects should be considered in the pretreatment evaluation for root canal procedures (Fig. 5-13).
* References 82, 100, 190, 219, 229-231, and 236.
   One[137] well-organized ethnic variant is the higher incidence of single-rooted and C-shaped mandibular second molars in Native American and Asians populations (Fig. 5-14).62,63,130,131 However, this is not always the case, as the occurrence of two canals in the mesiobuccal root of maxillary first molars in Japanese patients is similar to that described for other ethnic groups.239 All this information makes it clear that the clinician is confronted daily with highly complex and variable root canal systems.
   Whenever[141] a root contains two canals that join to form one, the lingual/palatal canal generally is the one with direct access to the apex, although this may require radiographic verification. When one canal separates into two, the division is buccal and palatal/lingual, and the lingual canal generally splits from the main canal at a sharp angle, sometimes nearly a right angle (Fig. 5-19). One study197 recommended visualizing this configuration as a lower case letter “h.” The buccal canal is the straight-line portion of the h; the lingual canal exists about midroot at a sharp angel from the buccal canal. In these situations modification of the access to develop an unobstructed passage of instruments into the lingual canal is indicated.
The[142] classic concept of apical root anatomy is based on three anatomic and histologic landmarks in the apical region of a root: the apical constriction (AC), the cementodentinal junction (CDJ), and the apical foremen (AF). Kuttler’s description of the anatomy of the root apex has the root canal tapering from the canal orifice to the AC, which generally is 0.5 to 1.5 mm coronal to the AF (Fig. 5-20).109 The AC generally is considered the part of the root canal with the smallest diameter; it also is the reference point clinicians use most often as the apical termination for enlarging, shaping, cleaning, disinfecting, and filling. Violation of this area with instruments or filling materials is not recommended for long-term, successful outcomes.
Many[144] investigators who have evaluated apical and periradicular tissues after root canal procedures concluded that the most favorable prognosis was obtained when procedures were terminated at the AC, and the worst prognosis was produced by treatment that extended beyond the AC.113-116,180 Procedures terminated more than 2 mm from the AC had second worst prognosis. These findings occurred with root canal procedures in teeth with both vital and necrotic pulps and when bacteria were present beyond the AF. Sealer or gutta-percha (or both) in the periradicular tissues, lateral canals, and apical ramifications may cause a severe inflammatory reaction. However, it is difficult to locate the AC clinically, which is why some studies direct clinicians to terminate all procedures at or beyond the radiographic apex, thereby filling all apical ramifications and lateral canals.186 This dictate is empirically based, and CBCT evaluation of procedures previously thought to be successful has allowed identification of more posttreatment disease.158
ACCESS[151] CAVITY PREPARATIONS
Anterior Teeth
The following discussion outlines the steps for maxillary and mandibular anterior teeth. Tooth-specific access concerns are illustrated and discussed in the section Morphology of and Access Cavity Preparations for Individual Teeth, later in the chapter.
External Outline Form
Once caries and restorations have been removed as necessary to establish sound tooth margins, an initial external outline opening is cut on the lingual surface of the anterior tooth. This step may also be performed during the removal of caries and restorations. For an intact tooth, cutting commences at the center of the lingual or palatal surface of the anatomic crown (Fig. 5-38, A). A #2 or #4 round bur or a tapered fissure bur may be used to penetrate the enamel and slightly into the dentin (approximately 1 mm). An outline form is developed that is similar in geometry to an ideal access shape for the particular anterior tooth (Fig. 5-38, B); it is half to three quarters the projected final size of the access cavity. Because most of this step involves removal of enamel, the high-speed handpiece is used for cutting efficiency. The bur is directed perpendicular to the lingual surface as the external outline opening is created (Fig. 5-38, C).
Penetration of the Pulp Chamber Roof
Experienced clinicians usually penetrate the pulp chamber roof using a high-speed handpiece; however, less experienced clinicians may find the increased tactile sensation of a slow-speed handpiece a safer option. Continuing with the same round or tapered fissure bur, the angle of the bur is rotated from perpendicular to the lingual/palatal surface to parallel to the long axis of the root (see Fig. 5-38, D). Penetration into the tooth continues along the root’s long axis until the roof of the pulp chamber is penetrated; frequently, a drop into the chamber effect is felt when this occurs. Measuring the distance from the incisal edge to the roof of the pulp chamber on a dimensionally accurate pretreatment radiograph may serve as guide in limiting penetration and possibly preventing a perforation. If the drop-in effect is not felt at this depth, an endodontic explorer can be used to probe the depth of the access, using magnification or the DOM. Often a small opening into the chamber is present, or the dentin is very thin and the explorer penetrates into the chamber. The depth and angle of penetration should be accessed for any division away from the long axis of the root in both the mesiodistal and buccolingual dimensions, and the penetration angle should be realigned if necessary. Angled radiographs can be used to assess progress at any time if any confusion or doubt exists. A little caution and concern at this stage can prevent a mishap.
Removal of the Chamber Roof
Once the pulp chamber has been penetrated, the remaining roof is removed by catching the end of a round bur under the lip of the dentin roof and cutting on the bur’s withdrawal stroke (see Fig. 5-38, E) Because each tooth has a unique pulp chamber anatomy, working in this manner enables the internal pulp anatomy to dictate the external outline form of the access opening. In teeth with irreversible pulpitis, pulp tissue hemorrhage can impair vision during this process. In such cases, as soon as sufficient roof structure has been removed to allow instrument access, the coronal pulp should be amputated at the orifice level with a spoon or round bur and the chamber irrigated copiously with sodium hypochlorite. If the hemorrhage continues, a tentative canal length can be established by measuring the pretreatment radiograph. A small broach coated with[152] a chelating agent then can be used loosely in the canal to grasp and sever the pulp at a more apical level. Copious irrigation with sodium hypochlorite helps control hemorrhage. Subsequently, the chamber roof, including the pulp horns, is removed and all internal walls are flared to the lingual surface of the tooth. Complete roof removal is confirmed with a #17 operative explorer if no “catches” are identified as the explorer tip is withdrawn from the pulp chamber along the mesial, distal, and facial walls.
Removal of the Lingual Shoulder and Coronal Flaring of the Orifice
Once the orifice or orifices have been identified and confirmed, the lingual shoulder or ledge is removed. This is a shelf of dentin that extends from the cingulum to a point approximately 2 mm apical to the orifice (Fig. 5-39). Its removal improves straight-line access and allows for more intimate contact of files with the canal’s walls for effective shaping and cleaning. In addition, its removal from mandibular anterior teeth may often expose an extra orifice and canal.
The contemporary approach to flaring the orifice involves the use of rotary nickel-titanium (NiTi) orifice openers that allow rapid, safe removal of the lingual ledge, following the manufacturer’s directions for use (DFUs). When used properly, these openers allow refinement of the orifice shape or help to enhance straight-line access to the canal with minimal removal of dentin. Although there may be subtle differences in these instruments from manufacturer to manufacturer, proper application achieves the same objective. A time-tested and traditional way to achieve the same goal involves the use of a tapered, safety-tip diamond or carbide bur or a Gates-Glidden bur. However, use of these instruments may result in excessive removal of cervical dentin. When a fine, safety-tip diamond bur is used, the tip is placed approximately 2 mm apical to the canal orifice and inclined to the lingual during rotation to slope the lingual shoulder. The bur should be placed so as to avoid putting a bevel on the incisal edge of the access preparation (Fig. 5-40). When Gates-Glidden burs are used, the largest that can be placed passively 2 mm apical to the orifice is used first. During rotation, gentle pressure is applied on the bur as it cuts against the lingual shoulder and then is withdrawn. The size of these burs can be increased sequentially, depending on the size of the canal, with repeated shaping of the lingual wall until the lingual should of dentin has been eliminated in anterior teeth. During this process, the orifice is often concomitantly flared so that it is contiguous with all walls of the access preparation. If this is not achieved, use of the orifice openers is recommended.
Straight-[152]Line Access Determination
After removal of the lingual shoulder and flaring of the orifice straight-line access must be determined. Ideally, a small intracanal file can reach the apical foramen or the first point of canal curvature with no deflections. Unnecessary deflection of the file can result in numerous consequences related to loss of instrument control. Deflected instruments function under more stress than those with minimal or no deflection pressure and are more susceptible to separation during enlargement and shaping (Fig. 5-41). Deflected instruments also lack access to critical areas of the canal and therefore do not function effectively. Without straight-line access, procedural errors (e.g., ledging, transportation, and zipping) may occur, but this is primarily seen with the use of hand files or larger NiTi instruments (Fig. 5-42).
   If the lingual shoulder has been removed properly and a file still binds on the incisal edge, the access cavity should be extended further incisally until the file is not deflected (Fig. 5-43). The final position of the incisal wall of the access cavity is determined by two factors: (1) complete removal of the pulp horns and (2) straight-line access.
Visual Inspection of the Access Cavity
Appropriate magnification and illumination should be used to inspect and evaluate the completed access cavity. Although this can[153] be done during any stage of the preparation, it should always be done at this point. The axial walls at their junction with the orifice must be inspected for grooves that might indicated an additional canal. The orifice and coronal portion of the canal must be evaluated for a bifurcation.
Refinement[153] and Smoothing of Restorative Margins
The final step in the preparation of an access cavity is to refine and smooth the cavosurface margins. Rough or irregular margins can contribute to coronal leakage through a permanent or temporary restoration. Proper restorative margins are important because anterior teeth may not require a crown as the final restoration. Smooth cavosurface margins allow the placement of a composite resin restoration with the precision necessary to minimize coronal leakage. Such leakage could jeopardize the success of the root canal procedure.
   Another factor to consider regarding the access margin of a maxillary anterior tooth is that the final composite resin restoration will be placed on a functional tooth surface. The incisal edges of the mandibular anterior teeth slide over these maxillary lingual surfaces during excursive jaw movements. Therefore, the restorative margins of an access cavity in maxillary anterior teeth should be created to allow a bulk of restorative material at the margin. Butt joint margins are indicated, rather than beveled margins, which produce thin composite edges that can fracture under functional loads and ultimately result in coronal leakage. If the anterior tooth requires a crown as the final restoration, the cavosurface margin becomes a less critical factor, although if it is not restored in a timely fashion, breakdown and leakage may occur.
Individual Anterior Teeth
See the figures in the section Morphology of and Access Cavity Preparations for Individual Teeth, later in this chaper.
Posterior Teeth
Preparing access cavities on posterior teeth is similar to the process for anterior teeth, but significant differences warrant a separate discussion.84 Posterior teeth requiring root canal procedures typically have been heavily restored or the carious process was extensive. Such conditions, along with the complex pulp anatomy and position of posterior teeth in the oral cavity, can make the access process challenging.
External[153] Outline Form
Removal of caries and existing restorations from a posterior tooth requiring a root canal procedure often results in the development of an acceptable access outline form. However, if the tooth is intact, the access starting location must be determined for an intact tooth. The pulp chamber of posterior teeth is positioned in the center of the tooth at the level of the CEJ. Therefore, in maxillary premolars, the point of entry that determines the external outline form is on the central groove between the cusp tips (Fig. 5-44). Crowns of mandibular premolars are titled lingually relative to their roots (Fig. 5-45); therefore, the starting location must be adjusted to compensate for this tilt (Fig. 5-46). In mandibular first premolar, the starting location is halfway up the lingual incline of the buccal cusp on a line connecting the cusp tips. Mandibular second premolars require less of an adjustment because they have less lingual inclination. The starting location for this tooth is one third the way up the lingual incline of the buccal cusp on a line connecting the buccal cusp tip and the lingual groove between the lingual cusps.
   To determine the starting location for molar access cavity preparations, the mesial-distal and apical-coronal boundary limitations for this outline must be determined (Fig. 5-47). Evaluation of bite-wing radiographs is an accurate method of assessing the mesiodistal extensions of the pulp chamber (Fig. 5-48). The mesial boundary for both the maxillary and mandibular molars is a line connecting the mesial cusp tips. Pulp chamber are rarely found mesial to this imaginary line. A good initial distal boundary for maxillary molars is the oblique ridge. For mandibular molars, the initial distal boundary is a line connecting the buccal and lingual grooves. For molars the correct starting location is on the central groove halfway between the mesial and distal boundaries.
   Penetration through the enamel into the dentin (approximately 1 mm) is achieved with a #2 or #4 round bur for premolars and a #4 or #6 round bur for molars. A tapered fissure bur may be used instead of round burs. The bur is directed perpendicular to the occlusal table, and an initial outline shape is created at about half to three fourths its projected final size. The premolar shape is oval and widest in the buccolingual dimension. The molar shape is also oval initially; it is widest in a buccolingual dimension for maxillary molars and in a mesiodistal direction for mandibular molars. The final outline shape for molars is approximately triangular (for three canals) or rhomboid (for four canals); however, the canal orifices dictate the position of the corners of these geometric shapes. Therefore, until the orifices have been located, the initial outline form should be left as roughly oval.
Penetration of the Pulp Chamber Roof
Once initial penetration into the pulp chamber has been achieved, the angle of penetration changes from perpendicular to the occlusal table to an angle appropriate for penetration through the roof of the pulp chamber. In premolars the angle is parallel to the long axis of the root or roots, both in the mesiodistal and buccolingual directions. Failure to analyze this penetration angle carefully can result in gouging or perforation because premolar roots often are tilted relative to the occlusal plane. In molars the penetration angle should be toward the largest canal because the pulp chamber space usually is largest just occlusal to the orifice of this canal. Therefore, in maxillary molars, the penetration angle is toward the palatal orifice, and in mandibular molars, it is toward the distal orifice (Fig. 5-49).
   As with anterior teeth, penetration is limited to the distance measured on a pretreatment radiograph to just penetrate the roof of the pulp chamber. If the drop-in effect is not felt at this depth, a careful evaluation of the angle of penetration is necessary before going deeper into the chamber. In multirooted posterior teeth, lateral and furcation perforations may occur rapidly without attention to 3D detail during this penetration. As with anterior teeth, aggressive probing with an endodontic explorer at any time during the penetration often can help locate the pulp chamber.
Removal[156] of the Chamber Roof
The bur of choice is used to remove the roof of the pulp chamber completely, including all pulp horns (Fig. 5-50, A and B). Visibility problems caused by pulpal hemorrhage should be handled as described in the previous section for anterior teeth. The goal is to funnel the corners of the access cavity directly into the orifices, and a safety-tip diamond or carbide bur (Multi Bur, Dentsply Tulsa Dental Specialties) performs this task nicely (Fig 5-50, C and D); it can be set on the pulp floor and the entire axial wall shaped at one time, with little or no apical pressure (Fig. 5-51). These burs are passed between the orifices along the axial walls to remove the roof, taper the internal walls, and create the desired external outline shape simultaneously.
Identification of All Canal Orifices
In posterior teeth with multiple canals, the canal orifices play an important role in determining the final extensions of the external outline form of the access cavity. Ideally, the orifices are located at the corners of the final preparation to facilitate all of the root canal procedures (Fig. 5-52). Internally, the access cavity should have all orifices positioned entirely on the pulp floor and should not extend into an axial wall. Extension of an orifice into the axial wall creates a “mouse hole” effect (Fig. 5-53), which indicates internal underextension and impedes straight-line access. In such cases the orifice must be repositioned onto the pulp floor without interference from axial walls.
Removal of the Cervical Dentin Bulges and Orifice and Coronal Flaring
In posterior teeth the internal impediments to an ideal access opening are the cervical dentin ledges or bulges and the natural coronal canal constriction.84 The cervical bulges are shelves of dentin that frequently overhang orifices in posterior teeth, restricting access into root canals and accentuating existing canal[157] curvatures.117 They can develop from mesial, distal, buccal, and lingual walls inward. These bulges can be removed safely with burs or ultrasonic instruments. The removal instruments should be placed at the orifice level, and light pressure should be used to cut laterally toward the dentin bulge to remove the overhanging ledge (Fig. 5-54). After removal of the ledge, the orifice and constricted coronal portion of the canal can be flared with NiTi orifice openers, Gates-Glidden burs, or large, tapered rotary instruments (.10/.12), which are used in a sweeping upward motion with minimal lateral pressure away from the furcation. As the orifice is enlarged, it should be tapered and blended into the axial wall so that an explorer can slide down the corner of the external outline form, down the axial wall, and into the orifice without encountering any obstructions (see Fig. 5-54).
Straight[157]-Line Access Determination
Straight-line access is paramount to successful shaping, especially given the complexity of the root canal systems in posterior teeth. Files must have unimpeded access to the apical foramen or the first point of canal curvature to perform properly. Therefore, each canal must have straight-line access, and adjustments must be made accordingly (see Fig. 5-54, O).
Visual Inspection of the Pulp Chamber Floor
The floor and walls must be inspected, using appropriate magnification and illumination, to ensure that all canal orifices are visible and no roof overhangs are present (Fig. 5-55).
Refinement and Smoothing of the Restorative Margins
In both temporary and interim permanent restorations, the restorative margins should be refined and smoothed to minimize the potential for coronal leakage. The final permanent restoration of choice for posterior teeth that have undergone a root canal procedure is generally a crown or onlay, although this may vary, depending on the opposing dentition and patient function.
Individual Posterior Teeth
See the figures in the section Morphology of and Access Cavity Preparations for Individual Teeth, later in the chapter.
Maxillary[175] First Molar
The maxillary first molar is the largest tooth in volume and one of the most complex in root and canal anatomy (Fig. 5-103).50 The pulp chamber is widest in the buccolingual dimension, and four pulp horns are present (mesiobuccal, mesiopalatal, distobuccal, and distopalatal). The pulp chamber’s cervical outline form has a rhomboid shape, sometimes with rounded corners. The mesiobuccal angle is an acute angle; the distobuccal angle is an obtuse angle; and the palatal angles are basically right angles. The palatal canal orifice is centered palatally; the distobuccal orifice is near the obtuse angle of the pulp chamber floor; and the main mesiobuccal canal orifice is buccal and mesial to the distobuccal orifice and is positioned within the acute angle of the pulp chamber. The mesiopalatal canal orifice (also referred to as the MB-2) is located palatal and mesial to the mesiobuccal orifice. A line drawn to connect the three main canal orifices—the mesiobuccal orifice, distobuccal orifice, and palatal orifice—forms a triangle, known as the molar triangle.
The three individual roots of the maxillary first molar (i.e., mesiobuccal root, distobuccal root, and palatal root) form a tripod. The palatal root is the longest, has the largest diameter, and generally offers the easiest access. It can contain one, two, or[177] three root canals (see Table 5-13 online at Expert Consult site). The palatal root often curves buccally at the apical one third, which may not be obvious on a standard periapical radiograph. From its orifice the palatal canal is flat, ribbonlike, and wider in a mesiodistal direction. The distobuccal root is conical and may have one or two canals (see Table 5-14 online at the Expert Consult site). From its orifice, the canal (or canals) first is oval and then becomes round as it approaches the apical third of the root. The mesiobuccal root has generated more research and clinical investigation than any other root in the mouth.202 It may have one, two, or three root canals (see Table 5-15 online at the Expert Consult site). A single mesiobuccal canal is oval and wider buccolingually; two or three canals are more circular. In general, a concavity exists on the distal aspect of the mesiobuccal root, which makes this wall thin. The μCT scans for the maxillary first molar can be see in Fig. 5-104. (See Video 5-6 online at the Expert Consult site for rotational views of these teeth.) The DOM has been used to study the location and pathway of the mesiopalatal canal in maxillary first and second molars.80 The clinician must always keep in mind that the location of this canal varies greatly; it generally is located mesial to or directly on a line between the mesiobuccal and palatal orifices, within 3.5 mm palatally and 2 mm mesially of the mesiobuccal orifice (Figs. 5-105 and 5-106). These authors80 found that not all mesioopalatal [mesiopalatal] orifices lead to a true canal. A true orifice for this canal was present in only 84% of molars in which a second orifice was identified (Fig. 5-107).205
   Negotiation[177] of the mesiopalatal canal often is difficult; a ledge of dentin covers its orifice, the orifice has a mesiobuccal inclination on the pulp floor, and the canal’s pathway often takes one or two abrupt curves in the coronal part of the r  oot. Most of these obstructions can be eliminated by troughing or countersinking with ultrasonic tips mesially and apically along the mesiobuccal pulpal groove (Figs. 5-108 through 5-111). This procedure may shift the canal mesially, meaning that the access wall also must be moved father mesially. Troughing can easily be 0.5 to 3 mm deep, and care must be taken to avoid furcal wall perforation in this root. Apical to the troughing level. the canal may be straight or may curve sharply to the distobuccal, buccal, or palatal. [Punctuations sic]
   Because the maxillary first molar almost always has four canals, the access cavity has a rhomboid shape, with the corners corresponding to the four orifices (see Fig. 5-108). One study demonstrated that the access cavity should not extend into the mesial marginal ridge.246 Distally, the preparation can invade the mesial portion of the oblique ridge, but it should be parallel to a line connecting the mesiobuccal and distobuccal orifices and not to the buccal surfaces of the tooth.
Maxillary[179] Second Molar
Coronally, the maxillary second molar closely resembles the maxillary first molar (Fig. 5-112). The root and canal anatomy are similar to those of the first molar, although there are differences. The distinguishing morphologic feature of the maxillary[180] second molar is that its three roots are grouped closer together and are sometimes fused. Also, they generally are shorter than the roots of the first molar and not as curved. The second molar usually has one canal in each root; however, it may have two or three mesiobuccal canals, one or two distobuccal canals, or two palatal canals (see Tables 5-16 to 5-18 online at the Expert Consult site). Four canals are less likely to be present in the second molar than in the first molar. The μCT scans for the maxillary second molar can be seen in Fig. 5-113. (See Video 5-7 online at the Expert Consult site for rotational views of these teeth.) The three main orifices usually form a flat triangle and sometimes almost a straight line (Figs. 5-114 through 5-118). The mesiobuccal canal orifice is located more to the buccal and mesial than in the first molar; the distobuccal orifice approaches the midpoint between the mesiobuccal and palatal orifices250; and the palatal orifice usually is located[182] at the most palatal aspect of the root. In general, the canal orifices in the maxillary second molar are closer mesially to each other than they are in the maxillary first molar.
   The[182] floor of the pulp chamber is markedly convex, which gives the canal orifices a slight funnel shape. On occasion the canals curve into the chamber at a more horizontal angle, requiring removal of a lip of dentin so that a canal can be entered more in a direct line with the axis. Teeth with two canals usually have a buccal and a palatal canal of equal length and diameter (see Fig. 5-114, B). These parallel root canals are frequently superimposed radiographically, but they can be imaged by exposing the radiograph from a distal angle. To enhance radiographic visibility, especially when interference arises from the malar process, a more perpendicular and distoangular[183] radiograph may be exposed. When two roots are present, each root may have one canal, or the buccal root may have two canals that join before reaching a single foramen. One study found that two palatal roots and two palatal canals occur in 1.47% of these teeth.166
   When four canals are present, the access cavity preparation of the maxillary second molar has a rhomboid shape and is a smaller version of the access cavity for the maxillary first molar (see Fig. 5-115). If only three canals are present, the access cavity is a rounded triangle with the base to the buccal. As with the maxillary first molar, the mesial marginal ridge need not be invaded. Because the tendency in maxillary second molars is for the distobuccal orifice to move closer to a line connecting the mesiobuccal and palatal orifices, the triangle becomes more obtuse and the oblique ridge usually is not invaded.
   If only two canals are present, the access outline form is oval and widest in the buccolingual dimension. Its width corresponds to the mesiodistal width of the pulp chamber, and the oval usually is centered between the mesial pit and the mesial edge of the oblique ridge.
Maxillary Third Molar
Loss of the maxillary first and second molars often is the reason the third molar must be considered a strategic abutment (Fig. 5-119). Careful examination of the root morphology is important before treatment is determined. The radicular anatomy of the third molar is completely unpredictable, and it may be advisable to explore the root canal morphology to evaluate the likelihood and degree of success. Even so, many third molars have adequate root formation; given reasonable accessibility, they can serve well as functioning dentition after root canal therapy.
   The[184] root anatomy of the maxillary third molar varies greatly. This tooth can have one to four roots and one to six canals, and C-shaped canals also can occur. The third molar usually has three roots and three root canals (see Table 5-19 online at the Expert Consultant site). The tooth may be tipped significantly to the distal, the buccal, or both, which creates an even greater access problem than with the second molar. The μCT scans for the maxillary molar can be seen in Fig. 5-120. (See Video 5-8 online at the Expert Consultant site for rotational views of these teeth.)
   The access cavity form for the third molar can vary greatly. Because the tooth typically has one to three canals, the access preparation can be anything from an oval that is widest in the buccolingual dimension to a rounded triangle similar to that used for the maxillary second molar. All the canal orifices often lie nearly in a straight line as the mesiobuccal and palatal orifices. The resultant access cavity is an oval or highly obtuse triangle. (Figs. 5-121 and 5-122).
Mandibular Central and Lateral Incisors
The root canal systems and access cavities for the two mandibular incisors are so similar they are discussed together (Fig. 5-123). As with the maxillary incisors, a lingual shoulder must be eliminated to allow direct-line access. The shoulder conceals the orifice to a second canal that, if present, is found immediately beneath it. Unlike the maxillary incisors, the pulp outline of the mandibular incisors is wider labiolingually. At the CEJ the pulp outline is oval and wider labiolingually than mesiodistally. At midroot the canal outline is still oval, but the canal is more constricted and narrower labiolingually. Most mandibular incisors have a single root with what radiographically appears to be a long, narrow canal. However, it is a broad canal labiolingually. Often a dentinal bridge is present in the pulp chamber that divides the root into two canals. The two canals usually join and exit through a single apical foramen, but they may persist as two separate canals. On occasion one canal branches into two canals, which subsequently rejoin into a single canal before reaching the apex (see Table 5-20 online at[185] the Expert Consultant site). The scans of the mandibular central incisor can be seen in Fig. 5-124. The scan for the mandibular lateral incisors can be seen in Fig. 5-125. (See Videos 5-9 and 5-10 online at the Expert Consultant site for rotational views of these teeth.)
   One[185] study determined that a relationship existed between crown size and the incidence of bifid root canals in these teeth.234,235 Double root canals occur more often in teeth with a smaller index.
   The mandibular incisors, because of their small size and internal anatomy, may be the most difficult access cavities to prepare. The external outline form may be triangular or oval, depending on the prominence of the mesial and distal pulp horns (Figs. 5-216 through 5-218). When the form is triangular, the incisal base is short and the mesial and distal legs are long incisogingivally, creating a long, compressed triangle. Without prominent mesial and distal pulp horns, the oval external outline form also is narrow mesiodistally and long incisogingivally. One study152 determined that by age 40 years, the mandibular incisor pulp chamber has decreased in size sufficiently to justify an oval access cavity routinely. Complete removal of the lingual should is critical because this tooth often has two canals that are buccolingually oriented, and the lingual canal most often is missed. To avoid missing this canal, the access preparation is extended well into the cingulum gingivally. Because the lingual surface of this tooth is not involved with occlusal function, restoration of the access opening with butt joint junctions between the internal walls and the lingual surface is not required.
Mandibular Canine
The root canal system of the mandibular canine is very similar to that of the maxillary canine, except that the dimensions are smaller, the root and root canal outlines are narrower in the mesiodistal dimension, and the mandibular canine occasionally has two roots and two root canals located labially and lingually (Fig. 5-129) (see Table 5-21 online at the Expert Consultant site). The μCT scans of the mandibular canine can be seen in Fig. 5-130. (See Video 5-11 online at the Expert Consultant site for rotational views of these teeth.) A fourth type is seen in subsequent Figs. 5-131 to 5-135.222
   The root canal of the mandibular cuspid is narrow mesiodistally but usually very broad buccolingually. A lingual shoulder must be removed to gain access to the lingual wall of the root canal or to the entrance of a second canal. The lingual wall is almost slitlike compared with the larger buccal wall, which makes the canal a challenge to shape and clean.
   The access cavity for the mandibular canine is oval or slot shaped (see Fig. 5-131). The mesiodistal width corresponds to the mesiodistal width of the pulp chamber. The incisal extension can approach the incisal edge of the tooth for straight-line access, and the gingival extension must penetrate the cingulum to allow a search for a possible lingual canal.
Mandibular First Premolar
As a group, the mandibular premolars present anatomic challenges because of the extreme variations in their root canal morphology (Fig. 5-136).123,157,252 The root canal system of the mandibular first premolar is wider buccolingually than mesiodistally.253 Two pulp horns are present: a large, pointed buccal horn and a small, rounded lingual horn. At the cervical line the root and canal are oval; this shape tends to become round as the canal approaches the middle of the root. If two canals are present, they tend to be round from the pulp chamber to their foramen. In another anatomic variation, a single, broad, root canal may bifurcate into two separate root canals. Direct access to the buccal canal usually is possible, whereas the lingual canal may be quite difficult to find. The lingual canal tends to diverge from the main canal at a sharp angle. In addition, the lingual inclination of the crown tends to direct files buccally,[190] making location of a lingual canal orifice more difficult. To counter this situation, the clinician may need to extend the lingual wall of the access cavity farther lingually; this makes the lingual canal easier to locate. The mandibular first premolar sometimes may have three roots and three canals (see Table 5-22 online at the Expert Consultant site). Multiple studies have reported C-shaped canal anatomy in this tooth.13,65,67 The μCT scans for the mandibular first premolar can be seen in Fig. 5-137. (See Video 5-12 online at the Expert Consultant site for rotational views of these teeth.)
   The[190] oval external outline form of the mandibular first premolar typically is wider mesiodistally than its maxillary counterpart, making it more oval and less narrow (Figs. 5-138 through 5-141). Because of the lingual inclination of the crown, buccal extension can nearly approach the tip of the buccal cusp to achieve straight-line access. Lingual extension barely[192] invades the poorly developed lingual cusp incline. Mesiodistally the access preparation is centered between the cusp tips. Often the preparation must be modified to allow access to the complex root canal anatomy frequently seen in the apical half of the tooth root.
Mandibular[192] Second Premolar
The mandibular second premolar is similar to the first premolar, with the following differences: the lingual pulp horn usually is larger; the root and root canal are more often oval than round; the pulp chamber is wider buccolingually; and the separation of the pulp chamber and root canal normally is distinguishable compared with the more regular taper in the first premolar (Fig. 5-142). The canal morphology of the mandibular second premolar is similar to that of the first premolar with its many variations: two, three, and four canals and a lingually tipped crown. Fortunately, these variations are found less often in the second premolar (see Table 5-23 online at the Expert Consultant site). The μCT scans for the mandibular second premolar can be seen in Fig. 5-143. (See Video 5-13 online at the Expert Consultant site for rotational views of these teeth.) The access cavity form for the mandibular second premolar varies in[193] at least two ways in its external anatomy. First, because the crown typically has a smaller lingual inclination, less extension up the buccal cusp incline is required to achieve straight-line access. Second, the lingual half of the tooth is more fully developed; therefore, the lingual access extension typically is halfway up the lingual cusp incline.54 The mandibular second premolar can have two lingual cusps, sometimes of equal size. When this occurs, the access preparation is centered mesiodistally on a line connecting the buccal cusp and the lingual groove between the lingual cusp tips. When the mesiolingual cusp is larger than the distolingual cusp, the lingual extension of the oval outline form is just distal to the tip of the mesiolingual cusp (Figs. 5-144 through 5-149).
Mandibular[193] First Molar
The earliest permanent posterior tooth to erupt, the mandibular first molar seems to be the tooth that most often requires an endodontic procedure (vital pulp capping, pulpotomy, root canal);[195] therefore, its morphology has received a great deal of attention (Fig. 5-150).261 It often is extensively restored, and it is subjected to heavy occlusal stress. Consequently, the pulp chamber frequently has receded or is calcified. The tooth usually has two roots, but occasionally it has three, with two or three canals in the distal root (see Table 5-24 and 5-25 online at the Expert Consultation site). The μCT scans of the mandibular first molar can be seen in Fig. 5-151. (See Video 5-14 online at the Expert Consultant site for rotational views of these teeth.)
   The canals in the mesial root are the mesiobuccal and mesiolingual canals. A middle mesial (MM) canal sometimes is present in the developmental groove between the other mesial canals,148 but it may only represent a wide anastomosis between the two mesial canals.54 The incidence of an MM canal ranges from 1%223 to 15%77,209 The canals in the distal root include the distal canal (if only one canal is present) and the distobuccal and distolingual and middle distal canals (if more than one is present).75 The orifice to these canals are connected by a developmental groove. Orifices to all canals usually are located in the mesial two thirds of the crown, and the pulp chamber floor is roughly trapezoid or rhomboid. Usually four pulp horns are present.
   The presence of two separate distal roots is rare, but this does occur. In such cases the distolingual root is smaller than the distobuccal root and usually more curved. Also, the distolingual root often has a sharp apical hook toward the buccal side that is not obvious on radiographs (Fig. 5-152). The mesial root, the wider of the two roots, curves mesially from the cervical line to the middle third of the root and then angles distally to the apex. The buccal and lingual surfaces are convex throughout their length, whereas the distal surface of the mesial root and the mesial surface of the distal root have a root concavity, which makes the dentin wall very thin.
   The mesial canal orifices usually are well separated in the main pulp chamber and connected by a developmental groove.40 The mesiobuccal orifice is commonly under the mesiobuccal cusp, whereas the mesiolingual orifice generally is found just lingual to the central groove. On occasion an middle mesial canal orifice is present in the groove between the two mesial orifices (Figs. 5-152 through 5-159). The clinician must always check for such an orifice after shaping and cleaning the main root canals. A bur is used to remove any protuberance from the mesial axial wall that wound prevent direct access to the developmental groove between the two mesial orifices. Magnification is a tremendous aid during the exploration of this developmental groove with the sharp tip of an endodontic explorer. If a depression or orifice is located, the groove can be troughed with ultrasonic tips, at the expense of the mesial aspect, until a small file can negotiate the space.
   When only one distal canal is present, the orifice is oval buccolingually and the opening generally is located distal to the buccal groove. This orifice usually can be explored from the mesial with either an endodontic explorer or a small K-file. If the file tip takes a sharp turn in a distobuccal or distolingual direction, the clinician should search for yet another orifice; in rare cases a mesiodistal canal orifice is present.
   If three main root canals are present in this tooth, each is oval in the cervical and middle thirds of the root and round in the apical third. If two canals (distobuccal and distolingual) are present in the distal root, they usually are more round than oval for their entire length. The mesial root canals usually are curved, with the greatest curvature in the mesiobuccal canal. This canal can have a significant curvature in the buccolingual plane that may not be apparent on radiographs. Such a curvature usually can be detected with precurved pathfinder instruments.
   Multiple accessory foramina may be located in the furcation of the mandibular molars. These foramina usually are impossible to clean and shape directly; they are rarely seen, except occasionally on a posttreatment radiograph if they have been filled with root canal sealer or thermoplastic filling material. Because sodium hypochlorite solutions can dissolve organic debris, the pulp chamber should be thoroughly exposed to allow the solution to reach the tiny openings. Fractures occasionally occur, which are quite visible with magnification, on proximal marginal ridges and extend down the root or under the lingual cusps.
   The access cavity for the mandibular first molar typically is trapezoid or rhomboid, regardless of the number of canals present.247 When four or more canals are present, the corners of the trapezoid or rhombus should correspond to the positions of the main orifices. Mesially the access need not invade the marginal ridge, except if access to the tooth itself is compromised. Distal extension must allow straight-line access to the distal canal or canals. The buccal wall forms a straight connection between the mesiobuccal and distobuccal orifices, and the lingual wall connects the mesiolingual and distolingual orifices without bowing.
   A[198] variation in root morphology is the presence of an extra distolingual root.203 Usually this root has a type 1 canal configuration. Two thirds of the first mandibular molars found in a Chinese population had this variation.251 Similarly, this distolingual root occurred in 4% of mandibular first molars of a Kuwaiti population.159 These results confirm the observation that East Asian populations have more three-rooted mandibular first molars than do other racial groups.8
   Mandibular molars, mainly first molars, may also have an additional root located lingually or buccally. Although this is a rare occurrence in Caucasian populations, it is more common in Asian populations.220 The radix entomolaris (RE)is a supernumerary root located distolingually in mandibular molars (see Fig. 5-158),10,32 whereas the radix paramolaris (RP)is an extra root located mesiobuccally. Each root usually contains a single root canal. The orifice of the RE is located distolingually to mesiolingually from the main canal or canals of the distal root; the orifice of the RP is located mesiobuccally to distobuccally from the main mesial root canals. A dark line or groove from the main root canal on the pulp chamber floor leads to these[199] orifices.32 These anatomic variations present definite challenges to therapy because of their orifice inclination and root canal curvature. The canal can be straight, may have a coronal curvature, or can have separate coronal and apical curvatures.52
Mandibular[199] Second Molar
The mandibular second molar is somewhat smaller coronally than the first molar and tends to be more symmetric (Fig. 5-160). This tooth is identified by the proximity of its roots. The two roots often sweep distally in a gradual curve, with the apices close together. In some cases only one root is present. The degree of canal curvature and the configuration were studied in the mesial roots of 100 randomly selected mandibular first and second molars; 100% of the specimens showed curvature in both buccolingual and mesiodistal views.45
The pulp chamber and canal orifices of the mandibular second molar generally are not as large as those of the first molar. This tooth may have one, two, three, or four root canals (see Table 5-26 online at Expert Consultant site). The μCT scans for the mandibular second molar can be seen in Fig. 5-161. (See Video 5-15 online at the Expert Consultant site for rotational views of these teeth.) Figs. 5-162 through 5-166 provide additional variations and views of this tooth.
The two mesial orifices are located closer together. In some mandibular second molars with single or fused roots, a file placed in the mesiobuccal canal may appear to be in the distal canal. This happens because the two canals sometimes are connected by a semicircular slit, a variation of the C-shaped canal137,234,262 that often occurs in this tooth. The distal aspect of the mesial root and the mesial aspect of the distal root have concavities.
Mandibular second molars may have one to six canals, although the most prevalent configurations are two, three, and four canals (see Fig. 5-162). When three canals are present, the access cavity is similar to that for the mandibular first molar, although perhaps a bit more triangular and less rhomboid. The distal orifice is less often ribbon shaped buccolingually; therefore, the buccal and lingual walls converge more aggressively distally to form a triangle. The second molar may have only two canals, one mesial and one distal, in which case the orifices are nearly equal in size and line up in the buccolingual center of the tooth. The access cavity for a two-canal second molar is rectangular, wide mesiodistally and narrow buccolingually. The access cavity for a single-canal mandibular second molar is oval and is lined up in the center of the occlusal surface.
Mandibular Third Molar
The mandibular third molar is anatomically unpredictable and must be evaluated on the basis of its root formation (Fig. 5-167).110 Fused short, severely curved, or malformed roots often support well-formed crowns. This tooth may have one to four roots and one to six canals (see Table 5-27 online at the Expert Consultant site). The μCT scans for the mandibular third molar can be seen in Fig. 5-168. (See Video 5-16 online at the Expert Consultant site for rotational views of these teeth.) Additional variations are seen in Figs. 5-169 through 5-171.
C-shaped canals also can occur (Fig. 5-172). Most of these teeth can be successfully root treated, regardless of anatomic irregularities; however, the long-term prognosis is determined by the root surface volume in contact with bone. The clinician must weigh the benefit of treatment against the prognosis.
The anatomy of the mandibular third molar is unpredictable; therefore, the access cavity can take any of several shapes. When three or more canals are present, a traditional rounded triangle or rhomboid shape is typical. When two canals are present, a rectangular shape is used. For single-canal molars, an oval shape is customary.
Teeth with C-Shaped Root Canal Systems
The main cause for C-shaped roots and canals is the failure of Hertwig’s epithelial root sheath to fuse on either the buccal or lingual root surface (Fig. 5-173). The C-shaped canal system can assume many variations in its morphology (Fig. 5-174, A). Further cross section of types I, II, and III are seen in Fig. 5-174, B to D. The original classification133 has been modified and has produced a more detailed description of C-shaped root and canal morphology. The C-shaped canal configuration can vary along the root depth so that the appearance of the orifices may not be good predictors of the actual canal anatomy.62,64,137,234,262
Category[199] I (C1): The shape is an uninterrupted “C” with no separation or division (see Fig. 5-163, A).
Category II (C2): The canal shape resembles a semicolon resulting from a discontinuation of the “C” outline (see Fig.[201] 5-163, B), but either angle α or β should be no less than 60 degrees.
Category III (C3): Two or three separate canals (see Fig. 5-163, C and D) and both angles, α and β, are less than 60 degrees.
Category IV (C4): Only one round or oval canal is in the cross-section (see Fig. 5-163, E).
Category V (C5): No canal lumen can be observed (is usually seen near the apex only) (see Fig. 5-163, F).
   The C-shaped root canal system was first reported in 1979 in a maxillary molar.43 Most C-shaped canals occur in the mandibular second molar (Fig. 5-175 through 5-180),234 but they also have been reported in the mandibular first molar,146 the maxillary first and second molars, and the mandibular first premolar.* One study reported the incidence of C-shaped canal anatomy in maxillary first molars as 2 in 2175 (0.092%); this study also determined that the DB and palatal orifices were connected by a common groove (see Fig. 5-176, B).53 Investigators who examined 309 Chinese maxillary second molars found C-shaped root canals in 4.9%.254
* References 62, 63, 65, 66, 146, 150, 252, 253, 261, and 262.
   C-[202]shaped mandibular molars are so named because of the cross-sectional morphology of their fused roots and their root canals.[203] Instead of having several discrete orifices, the pulp chamber of a molar with a C-shaped root canal system is a single, ribbon-shaped orifice with an arc of 180 degree or more. It starts at the mesiolingual line angle and sweeps around either to the buccal or the lingual to end at the distal aspect of the pulp chamber (see Fig. 5-176, A). Below the orifice, the root structure can show a wide range of anatomic variations. These can be classified into two basic types: those with a single, ribbon-like, C-shaped canal from orifice to apex, and those with three or more distinct canals below the usual C-shaped orifice. Fortunately, molars with a single swath of canal are the exception rather than the rule. More common is the second type, with discrete canals that take unusual forms.43 Other investigators determined that C-shaped canals in mandibular second molars can vary in shape and number along the root length.136
   One study reported a case of a mandibular first molar with a normal mesiolingual orifice and a C-shaped groove that ran continuously from the mesiobuccal orifice along the buccal wall to the distal canal orifice.18 The groove ran continuously down the root to the apical third, where it divided into two canals. Other researchers reported a C-shaped groove in a mandibular first molar that extended from the DL to the DB orifice and across the buccal surface to the MB orifice.26 The ML orifice remained separate. Four separate apical foramina were noted. One investigator evaluated 811 endodontically treated mandibular second molars and found that 7.6% had C-shaped canals.239 Several variants of the C-shaped canal morphology[204] were noted, the most common being two or three canals that merged and exited as one canal.239
Significant[204] ethnic variation can be seen in the incidence of C-shaped root canal systems. This anatomy is much more common in Asians than in the Caucasian population. Investigators in Japan106 and China254 found a 32% incidence of C-shaped canals. Other found the occurrence of C-shaped canals in a Chinese population to be 23% in mandibular first molars and 32% in mandibular second molars.254,255,257 Another study found a 19% rate in mandibular second molars in Lebanese subjects85 and an 11% rate in a Saudi Arabian population.6 Mandibular molars in a Burmese population were found to have C-shaped canals in 22% of teeth.82 Another investigation found[205] that 33% of Koreans has a C-shaped canal morphology in mandibular second molars.105,189
Four types of pulpal floors were found in mandibular second molars137:
Type I: A peninsular-like floor with continuous C-shaped orifice (see Fig. 5-174).
Type II: A buccal, striplike dentin connection between the peninsula-like floor and the buccal wall of the pulp chamber that separates the C-shaped groove into mesial (M) and distal (D) orifices. Sometimes the mesial orifice is separated into a mesiobuccal (MB) orifice and a mesiolingual (ML) orifice by another striplike dentin connection between the peninsula-like floor and the mesial wall of the pulp chamber (most common) (see Fig. 5-174).
Type III: Only one mesial, striplike dentin connection between the peninsula-like floor and the M wall, which separates the C-shaped groove into a small ML orifice and a large MB-D orifice. The MB-D orifice was formed by the merging of the MB orifice and the D orifice (second most common) (see Fig. 5-174).
Type IV: Non-C-shaped floors. One distal canal orifice and one oval or two round mesial canal orifices are present (least common) (see Fig. 5-174).
Not all C-shaped mandibular second molars with C-shaped canal systems have a C-shaped pulpal floor. This makes diagnosis difficult. However, radiographically a tooth with a C-shaped canal system always has a fused root with a longitudinal groove in the middle of the root.63 Furthermore, there are three types of C-shaped canal systems in mandibular second molars (see Fig. 1-175).73
Type I: (merging type): Canals merge to one main canal before exiting at the apical foramen
Type II: (symmetric type): Separated mesial and distal canals in each root exit as separate canals
Type III: (asymmetric type): Separated mesial and distal canals, with the distal canal having a long isthmus across the furcation area
The minimal wall thickness in the middle and apical parts of type III and in the apical part of type II makes these regions danger zones for canal enlargement procedures (see Fig. 5-176).73
 Another study on mandibular molars found that there is a higher risk of root perforation at the thinner lingual walls of C-shaped canals during shaping and after canal preparation procedures. Both buccal and lingual canal walls were frequently narrower at the mesial locations.36
The access cavity for teeth with a C-shaped root canal system varies considerably and depends on the pulp morphology of the specific tooth. Teeth with C-shaped anatomy pose a considerable technical challenge; therefore, use of the DOM during all treatment phases is recommended.
6.[205] Al-Fougan KS: C-shaped root canals in mandibular second molars in a Saudi Arabian population, Int Endod J 34:499, 2002.
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