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Year : 2015  |  Volume : 3  |  Issue : 3  |  Page : 57-63

A brief review of the methods used to determine the curvature of root canals

1 Department of Conservative Dentistry, College of Dentistry, Ziauddin University, Karachi, Pakistan
2 Department of Oral Biology, College of Dentistry, Ziauddin University, Karachi, Pakistan
3 Department of Fixed and Removable Prosthodontics, College of Dentistry, Ziauddin University, Karachi, Pakistan

Date of Web Publication30-Oct-2015

Correspondence Address:
Dr. Haroon Rashid
College of Dentistry, Ziauddin University, Karachi
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2321-4619.168733

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Successful endodontic therapy is largely dependent on a triad of access cavity, canal preparation, and three-dimensional hermetically sealed obturation of the canals. Canal preparation is the most vital part of the triad that can be very challenging due to the complex morphology of the root canal system. Clinicians quite frequently encounter severe canal curvatures of different degrees within the roots that lead to a variety of problems including ledge formation, separation of instruments, canal blockage, and tear-drop transportation at the apex or perforation. Anatomical variations within the complex root canal morphology are the commonest cause of endodontic treatment failure. It is, therefore, essential to have a thorough knowledge about the internal and external morphologies of teeth. The aim of the current paper is to review the methods used to determine the root canal curvature and its management.

Keywords: Canal blockage, curved canals, root canal curvature, root canal morphology

How to cite this article:
Balani P, Niazi F, Rashid H. A brief review of the methods used to determine the curvature of root canals. J Res Dent 2015;3:57-63

How to cite this URL:
Balani P, Niazi F, Rashid H. A brief review of the methods used to determine the curvature of root canals. J Res Dent [serial online] 2015 [cited 2021 Apr 15];3:57-63. Available from: http://www.jresdent.org/text.asp?2015/3/3/57/168733

  Introduction Top

The main purpose of endodontic therapy is to treat diseased (vital or necrotic) dental pulp so that the function and appearance of the treated natural tooth can be maintained.[1] The therapy involves the removal of diseased dental pulpal tissue, preparing the root canals along with proper irrigation solutions, and then sealing them subsequently using an inert filling material.[2] Once the canals are sealed, a coronal seal must be provided so that bacterial ingress from the coronal portion may be prohibited.[3] The literature states that an ideal canal preparation is one in which the original canal morphology is maintained during the preparation procedure, along with the flare taper shape from the coronal to the apical region and thus, preserving the apical foramen.[2] This, however, may not be always possible due to the complexity of the root canal morphology. Common challenges that endodontists usually encounter during endodontic therapy are:

  • Accessing all the canals without encountering a procedural error
  • Maintaining the adequate working length and obturating the canal to its full working length
  • Preparing the canals by maintaining the adequate size and geometries of canals in all directions.

Unfortunately, the root canal morphology is not always as straight and simple as it appears on the radiographs. Various curves are present along the length of the canal and the preparation of these curved root canals becomes very challenging for a clinician. These curved canals may also restrict the mechanical and chemical preparation of the curvature or may lead to some procedural errors affecting the prognosis. The classification of root canal curvatures is enumerated in [Table 1]. Preoperative assessment of the root canal morphology is thus necessary so that the complexity, the degree of curvature, and radius of the root canals are determined to an extent. This will significantly reduce the occurrence of the procedural errors and the excess removal of tooth structure from the inner curvature, resulting in stripping or zip formation.[4],[5]
Table 1: Classification of root curvature[1],[3],[4]

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In the past few decades, only the angle of the canal curvature was the focus for categorizing the root canal morphology and the curvature. The canal was classified as either straight (if the angle was 5° or less), moderately curved (if the angle was 10-20°), or severely curved (if the angle was >20°). Later, it was proposed that the degree, position, and severity of the canal curvature also play an important role.[1]

Also, it is also important to choose the correct instruments and instrumentation techniques as the final outcome of endodontic treatment in curved canals depends largely on the flexibility of the instruments used, diameter of the instrument, and technique of the instrumentation.[6] The common challenge that a practitioner may encounter during the treatment of complex canals are:[7]

  • Negotiating the root canal curvature
  • Enlarging the canal space by maintaining the original internal anatomy of the canal
  • Creating a taper-shaped canal to optimize irrigation and obturation.

  Determining the Root Canal Curvature Top

Curvature of the root canal system should be determined preoperatively to avoid procedural errors and subsequent treatment failure. The following methods can be used for root canal curvature determination:

Periapical radiographs

These can be used to assess the root curvature but may lead to misinterpretation since the radiographs produce a two-dimensional image of a three-dimensional object [5] and thus, curvatures that are present buccolingually may not be visible. The majority of the canals do have some curvature on the different planes and thus, it is not possible to demonstrate them solely on the basis of radiographs.[4]

Cone beam computed tomography

CBCT is a new advancement in the field of radiology as described by Atria et al.[8] and Moshiri et al.,[9] and is specifically used for detailed three-dimensional imaging of oral and maxillofacial structures. The technique reduces the incidence of false negative results as it overcomes the limitations of the conventional radiograph such as image distortion, anatomic superimposition, and the compression of three-dimensional objects into two-dimensional images. CBCT helps in assessing the true size, extent, nature, and position of the lesions as compared to conventional radiography, that is, periapical radiographs or orthopantomogram (OPG).[10] Periapical pathology can be detected sooner as compared to other radiological approaches,[10] as the lesions that are present in the cancellous bone can only be detected using CBCT.[11] CBCT can be divided into large, medium, and small limited units based on the size of the field of view (describing the scan volume of CBCT machine), and depends on the detector size and shape, beam projection geometry, and ability to collimate the beam.[12] Unlike medial computed tomography (CT) scanner that has fan-shaped beam of x-rays, CBCT projects pyramid- or cone-shaped x-ray beam. The position of the patient depends on the manufacturer of the system and he/she can be in a supine, standing, or sitting position and the x-ray source and scanner makes a complete or half-rotation around the patient's head to capture the field of view. The images are then visualized using computer software at different anatomic planes.[10]

The radiation dose of the CBCT is much less than the medical scanner or conventional radiograph. The effective dose of one CBCT unit is equivalent to the dose of two or three standard periapical radiographic exposures.[8] CBCT provides a better view of root canal morphology as compared to radiographs. For example, the buccolingual curvatures that are missed in radiographs can be seen in a CBCT image.

The radius of root curvature can be determined through CBCT measured by the circumcenter using Planimp software (CDT Informatics, Cuiabá, MT, Brazil, 3D imaging system) based on the three mathematical points. Two semi-straight lines of 6 mm are drawn and the midpoint of the lines is determined. Perpendicular lines from the midpoint of each primary semi straight lines are drawn until they meet at a central point that is termed the circumcenter. The distance between the circumcenter and the midpoint of each semi-straight line will actually determine the magnitude of the canal curvature.[2] The smaller the radius, the greater the curvature and thus more complex the root canal structure.[13]

According to this method, curvature can be classified as:

  • Small radius (r < 4 mm): Severe curvature
  • Intermediary radius (r > 4 and r < 8 mm): Moderate curve
  • Large radius (r > 8 mm): Mild radius.[2]

Schneider's method

Using this method, a mid-point is marked on the file at the level of the canal orifice. A straight line is drawn parallel to the image and that point is labeled as point A. Another second point is marked where the flare starts to deviate that is labeled point B. A third point is marked at the apical foramen and is termed point C and the angle formed by the intersection of these lines is measured [Figure 1]. If the angle is less than 5°, the canal is straight; if the angle is 5-20°, the canal is moderately curved; and if the angle is greater than 20°, the canal is classified as a severely curved canal.[14],[15]
Figure 1: Diagrammatic representation of Schneider's method

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Lutein method

Lutein et al.[16] modified Schneider's method by using two lines drawn by the identification of four geometric points. Point A is first marked at the center of the canal orifices and then point B is marked 2 mm below the orifices in the long axis of the canal. A first primary line is drawn joining point A and point B and then point C is marked 1 mm coronal to the apical foramen. Point D is marked at the apical foramen then a second primary line is drawn joining these two lines [Figure 2]. The angle formed by intersection of the two lines is measured as in the Schneider method.[17]
Figure 2: Diagrammatic representation of Lutein method

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Cunningham's and Senia's method

This approach is different as it focuses on multiple root curvatures, that is, S-shaped canals, and the angle is measured separately at the coronal and apical ends. Point A is first drawn at the center of the orifices and then Point B is marked where the deviation or curve of the canal starts and a line is drawn joining these two lines. Point C is then marked where the canal again changes its direction or the deviation starts and point C is joined with point B. Point D is finally marked at the apical area and joined with point C [Figure 3].
Figure 3: Diagrammatic representation of Cunningham's and Senia's method

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The angle formed by the intersection of lines through points A and B and then points B and C is named angle X while the angle formed by the intersection of lines through points B and C and points C and D is named angle Y.[17]

Weine's method

Weine [18] described another method for the determination of root canal curvature similar to Schneider's method [Figure 4] but showed the differences in the angles according to curvature of the canal. In this method, a straight line is drawn from the canal orifices to the point of curvature and a second line is drawn from the apex for the apical curvature and the angle is measured at the point of intersection between the two lines.[15]
Figure 4: Diagrammatic representation of Weine's method

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  Determining the Horizontal Dimension Top

Determining the horizontal dimension of the root canal is one of the challenging factors since the horizontal dimension varies at different vertical dimensions. These dimensions are often known as the "forgotten dimension" as very few studies have been carried out for determining the horizontal dimension.

The classification of root canal according to horizontal dimension is as follows:

  • Round (the maximum initial working width is equal to the minimum initial working width)
  • Oval (the maximum initial working width is up to two times greater than the minimum initial working width)
  • Long oval (the maximum initial working width is up to four times more than the minimum initial working width)
  • Flattened (the maximum initial working width is more than four times greater than the minimum initial working width)
  • Irregular (cannot be defined by any of the above types);[19] to maintain the horizontal dimension at different levels of the root canal, circumferential filing should be used to prepare the canal.

  Management of Root Curvatures Top

Managing apical curvatures

The tooth at the apical third area is mostly curved and it is important to state that an attempt to straighten it should not be made or else treatment failure may be the outcome due to direct perforation, formation of ledges, and creation of teardrop foramen or foraminal rip.

To avoid these misfortunes, it is highly recommended that a straight line access is gained into the canals.[20] The preparation can then be started using a smaller diameter K file such as #08 or #10. These smaller diameter files can also be precurved in the direction of the apical curvature.[21] A chelating agent such as ethylenediaminetetraacetic acid (EDTA) must be used, along with copious irrigation of the canals with sodium hypochlorite, and once a file is withdrawn from the canal, it must be cleaned and recurved before it is reintroduced.[13] Segal [20] suggested that a reamer should be used instead of K-file since it is more flexible in nature and provides a perfect mirror appearance of the canal curvature. Once removed, it describes the degree, type, location, and direction of the curvature. However, due to its flexibility it may also lead to canal transportation.

Once an access cavity is prepared, the root canal preparation should be started with stainless steel files of smaller diameter with light passive movement to debride the pulpal tissues and negotiate the apical area. Stainless steel files with a larger diameter must be avoided as they may alter the actual internal anatomy of the canal. The diameter of the glide path is then increased with nickel-titanium (NiTi) hand files before the preparation of the canal with rotary NiTi file.[13],[17] NiTi rotary files are flexible and very promising; however, multiple curves in the canals may cause strain in these instruments leading to instrument separation or ledge formation.

Managing middle curvatures

Mid-canal curvatures are relatively difficult to handle, especially if the coronal third of the root is straight and if this is not adequately dealt with, it may lead to iatrogenic errors such as file separation, perforations, ledge formation, and blockage of canals decreasing the prognosis of the tooth.[4] The relationship between the degree of curvature and incidence of ledge formation is enumerated in [Table 2].[22] Preoperative assessment and the usage of correct instrumental technique are highly recommended. The two steps for better management of mid-root curvature are adequate access and good coronal third preparation. This will ensure greater volume of irrigant to reach the mid-portion of the canal and allow instrumentation without any restriction and thus, create an ideal platform for the preparation of mid-root curvature. Once the coronal third portion of the canal is prepared, the mid-portion is prepared using precurved files. The bend given on the file should be gentle as sharp acute bends increase the probability of file fracture. The precurved file helps in negotiating the canal and makes a glide path before rotary NiTi files are introduced for cleaning and shaping.[4],[13]
Table 2: Relationship between the degree of curvature and incidence of ledge formation[23]

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  Different Instrumental Techniques Top

The instruments used for canal preparation and instrumentation technique were first described by Fauchard.[23] Edward Maynard is considered as the pioneer of endodontic hand instruments while Oltramare was the first to introduce rotary instruments for the preparation of root canal.[24],[25],[26] Racer endodontic handpiece was the first one to be introduced that worked with vertical filing motion. Later on, Giromatichandpiece was introduced with a reciprocal 90° rotation. NiTi hand files familiarized by Walia et al.[27] and NiTi rotary instruments with more flexible file were introduced for better preparation and to avoid mishaps during canal preparation.[28] However, mechanical instrumentation remains an important phase of root canal treatment that should never be neglected. Several methods/techniques of canal instrumentation have been proposed.

Schilder [29] described the "concept of flow" and design objectives according to which the canal should be tapering with the apical foramen essentially as narrow as possible without any modification in its original position. Along with design objectives, he also emphasized the biological objectives suggesting the adequate removal of diseased tissue from the canal while making sure that the necrotic debris is not extruded from the apical foramen. Also, it was suggested that there should be sufficient space for irrigation and intracanal medicaments.

Many techniques for canal preparation have been described. [Table 3] shows a summary of the different instrumental techniques used in endodontics. Standardized technique, the first formal technique for canal preparation was described by Ingle.[35] In this technique, it was recommended that each file should be introduced up to the full working length of the canals so that a taper could be created and the canals are then subsequently filled using a single cone of gutta-percha.[28]
Table 3: Summary of the different instrumentation techniques[28],[29],[30],[31],[32],[33],[34]

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Roane [36] in 1970 described a new instrumentation technique for canals with severe curvatures. The technique was termed as the "balanced forced instrumentation technique." Different instrumental motions were carried out to balance the action and reactions that took place during the canal preparation with specially designed stainless steel and NiTi hand files with modified tips. In balanced forced technique, the instruments are placed in the canal using very light, inward clockwise rotation (1/4 turn). Once the file moves in the apical direction, a counterclockwise movement (1/2 turn) is given while holding the file with a slight inward pressure. Cleaning or removal of the debris is accomplished only by an outward clockwise motion with no pressure. This sequence of instrumentation is continued until the working length is achieved.[37]

Circumferential filing and instrumentation of all walls are carried out equally during the root canal preparation. Hedstrom files are very effective for this technique. Anticurvature filing technique was first described by Abou-Rass and Jastrab.[38] Using that technique, the files are directed away from the danger zone in molars and toward the bulkiest portion of the root structure. This technique is very useful in cases where there is a chance of strip perforation into the furcation. With this method, the dental practitioner maintains digital control over the endodontic instrument. The walls on the opposite side from the curve are instrumented more than the inner walls, resulting in a decrease of the overall degree of canal curvature. In severely curved canals, the instrument should be modified from certain specific sites to avoid overcutting from the outer curve in the apical region and from the inner curve in case of mid-root curvature.[37]

Ideal canal preparation may not always be possible due to certain factors including complex root canal morphology, anatomical variations, microbiological variations, and iatrogenic mishaps. Endodontic mishaps are unfortunate and usually occur during endodontic procedures. They are either caused by the lack of skill, poor instrumental techniques, or due to complex and unpredictable morphology of the root canal.[22] Weine et al.[39],[40] and Gliackman and Dumsha [41] were the ones who described the iatrogenic mishaps that occur during the root canal procedure. The most common complication occurring during poor instrumentation technique is the formation of the ledge.[28] Ledge is actually an iatrogenic defect occurring at the outer surface of the walls of the canal during instrumentation, preventing access of the instrument toward the apex of the root. Since ledge formation hinders the instrumentation and chemical cleaning of the canals, it increases the probability of treatment failure and eventually results in periapical pathosis.[22] Ledge can be formed due to a number of reasons that may include failure of the clinician to assess root canal curvature preoperatively, failure to use a precurved file, inadequate irrigation, use of endodontic files of greater diameter in a curved canal, inadequate technique, and failure to use root canal instruments in a sequential manner.[30] In a study by Jafarzadeh et al., the frequency of ledge formation was found to usually increase if the canal curvature was greater than 20°[22] and the mesiobuccal and mesiolingual canals are said to be more frequently involved than distobuccal or distolingual canals.[31] The role of instrumentation technique and instrument material is also related to ledge formation and more incidences of ledge formation with reaming motion and step back technique have been reported.[42]

Another common mishap is canal perforation that could be access cavity perforation, furcal perforation, or root perforation (cervical, mid-root, or apical). Perforations usually occur due to over instrumentation or forceful instrumentation.[32] Cervical perforation occurs when not using Gates-Glidden burs properly or when large instruments are used for coronal flaring. The first sign that a clinician may encounter is bleeding in the floor that can be managed by filling the defect with mineral trioxide aggregate (MTA) and a temporary filling initially followed by permanent filling. Similarly, in case of a mid-root perforation, MTA may be used for the repair.

Apical perforation that is caused by using longer instruments can be recognized by bleeding or by sudden response from the patient. This defect is repaired by packing a small amount of MTA at the apex to form a barrier between the gutta-percha and the periapical area.[39] Other iatrogenic mishaps include zip formation, strip formation, instrument separation, and damage to the apical foramen if correct instrumentation technique is not used.[33],[34]

  Conclusion Top

Root canal treatment can be very challenging for an endodontist due to complex anatomy and the presence of severe root curvatures that causes hindrance during ideal preparation of the canal. The curvature may vary from gradual curvature of the entire canal, sharp curvature of the canal near the apex, or a gradual curvature of the canal with a straight apical ending. S- shaped canals (double curvature) may also occur and success in negotiating these canals depends on the size and construction of the canal, degree of curvature, size and flexibility of the instrument, along with the skills of the operator. Therefore, preoperative assessment of the horizontal and vertical variations of the canals should be done and a proper instrumental technique is very necessary to avoid procedural errors. Moreover, hand instrumentation is a time-tested, easy, and economic method for root canal treatment but care must be taken during their use to avoid problems like ledge formation, creation of zip, transportation, and instrument breakage. In severely curved canals, the use of rotary NiTi files after making a glide path with hand stainless steel files is recommended.

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Conflicts of interest

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  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3]

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