Author Information
Seminars in Musculoskeletal Radiology
Cone Beam CT Imaging Anatomy of the Jaw and Dentition
Kathleen Dhont, MD 1, 2; Anja Bernaerts, MD 1; Charlotte Vanhoenacker, MD 2; Filip Vanhoenacker, MD, PhD 3, 4, 5; Bert De Foer MD, PhD 1
1. Department of Radiology, GZA Hospitals, Antwerp, Belgium 2. Department of Radiology, UZ Gasthuisberg, Leuven, Belgium 3. Department of Radiology, AZ Sint-Maarten, Mechelen, Belgium
4. Department of Radiology, Antwerp University Hospital, Edegem, and Faculty of Medicine and Health Sciences, University of Antwerp, Belgium
5. Faculty of Medicine and Health Sciences, Ghent University, Belgium
Address for correspondence and reprint requests:
Anja Bernaerts
GZA Hospitals Antwerp Department of Radiology
Oosterveldlaan 24, 2610 Wilrijk, Belgium E-mail: [email protected]
KEYWORDS
4Cone-beam computed tomography (CBCT) – 4Teeth anatomy –
4 Jaw anatomy
CBCT IMAGING ANATOMY OF THE JAW AND DENTITION
ABSTRACT
Knowledge of dental, maxillary and mandibular anatomy and the use of correct nomenclature is of utmost importance in the evaluation of a mandibulofacial and/or maxillofacial imaging data set. The use of the correct diagnostic imaging tool tailored to the patient’s needs is of equal importance. This chapter highlights imaging anatomy and cross-sectional imaging modalities mainly focusing on CBCT of the mandibulofacial and maxillofacial region.
INTRODUCTION
Imaging has become indispensable in dentistry. Radiology is essential in detecting the presence and extent of disease, treatment planning, and follow-up in dental and maxillofacial pathology. Since the first dental radiograph in 1896, dental imaging has undergone a significant evolution. Several intraoral and extraoral radiographical imaging methods are available, but this information still is based upon bidimensional geometric projections. These techniques hence suffer from superposition due to the curved configuration of the mandible and maxilla, allowing them to conceal a lesion or an anatomical structure of interest. 3D cross-sectional imaging is able to overcome these obstacles and results in superior diagnostic capabilities. It allows accurate information about bone morphology and structure as well as information about the location of critical anatomical structures such as neurovascular canals, paranasal sinuses, and the nasal cavity.
Both, multi-detector computed tomography (MDCT) and cone-beam computed tomography (CBCT), are essential cross-sectional imaging modalities in dentistry. CBCT is an advanced digital imaging technology that uses volumetric scanning. It generates 3D data of a region of
interest at a lower radiation dose, a lower cost and a higher spatial resolution compared to MDCT. It is essential to note that not all patients need 3D imaging. Any imaging modality should follow the important basic principle of radiologic safety, which is the ALARA rule - As Low As Reasonable Achievable. This rule is based on the minimization of radiation dose for each patient. Before scanning a patient, there needs to be a clear objective, and radiation exposure is only justified if the proposed study will provide clear additional information. 3D imaging should not be used on a routine basis nor for screening purposes.1 The American Academy of Oral and Maxillofacial Radiology (AAOMR)2, as well as the European Academy of Dental and Maxillo-Facial Radiology (EADMFR)3, have established guidelines regarding justification and optimization of the use of CBCT for diagnosis, treatment planning, and follow-up of patients with conditions affecting the dental and maxillofacial region.
Knowledge of dental, mandibular and maxillary anatomy and anatomical landmarks as well of anatomical variants is essential to ensure precise surgical procedures and to safeguard patient’s vital structures. The use of adequate anatomical terminology is of utmost importance to correctly describe normal anatomical structures and to effectively report abnormal findings in a radiological report. This article provides an overview of the use of CBCT in dental imaging. It also describes dental anatomy and nomenclature, allowing the radiologist to communicate confidently and accurately with regard to dentition.
CONE BEAM CT
Although CBCT was initially developed for angiographic applications, it was subsequently adapted for dental imaging. It was first introduced in Europe in the late 1990s. In 2001 the Food and Drug Administration (FDA) approved CBCT for clinical use in the United States.4
The principle of CBCT is closely related to MDCT. However, the technical characteristics of CBCT pose two significant advantages of CBCT over MDCT: the volumetric nature of the information5 and relatively limited dosimetry.
DATA ACQUISITION
In conventional CT-imaging narrow, one-dimensional ‘fan’ beam shaped x-ray beams are directed at multiple rows of linear detectors opposite the x-ray source. Data acquisition requires the x-ray source to rotate 360 degrees around the region of interest (ROI). It also requires translation in the z-plane of the gantry in order to make many rotations around the patient and achieve multiple 2D-projections to eventually construct a volume of images composed of multiple axial sections (4Fig. 1a). In multidetector helical CT-scanning, multiple cross-sections are made at the same time, and the scan time is made shorter by having the X-ray source make a continuous spiral movement around the patient instead of individual rotations.6
CBCT uses a divergent ‘cone’ shaped beam that is projected on a larger 2D area flat panel detector (FPD).7 The CBCT unit makes a single rotation of 180 to 360 degrees around the patient, in which way it obtains multiple 2D projections referred to as basis projection images or raw data.8 (4Fig. 1b). The head of the patient is stabilized to avoid movement during the acquisition of the data volume. These raw data deliver a 3D volume equal to the many adjacent images that are obtained with conventional and helicala classical CT using special reconstruction techniques. The number of images will be determined by the degree of rotation and the time of acquisition. Scan times vary from 5-40 seconds, comparable to panoramic radiography. Radiation may be pulsed or continuous.9
The smallest picture element of a 2D image is called a pixel. Pixels are represented as squares, with a fixed height and width — the smaller the pixel, the better the quality of the
picture. The smallest picture element of an acquired 3D volume is called a voxel. A voxel is represented as a cube with a certain height, width, and depth. Each three-dimensional voxel represents a specific x-ray absorption. MDCT voxels are non-isotropic, meaning that two sides of the voxels are equal, but that the third side has a selectable width, determined by the distance the patient moves through the gantry in the z-plane.10 The third side determines the thickness of each image slice and is usually 1.0 to 100.0 mm. CBCT voxels are isotropic, which means that all the sides of the voxel are equal with identical resolution in all dimensions. To differentiate between small structures, the voxel size needs to be smaller than the desired structures that needs to be visualized. The small field of view units can use a voxel size down to 0.075 mm, which enables visualization of minimal changes to small structures.
Other voxel sizes available are in the range of 0.2 to 0.6 mm.11 The smaller the voxel size, the higher the spatial resolution, but the amount of radiation exposure will also increase.
RADIATION DOSE
Radiation dose produced by CBCT depends on various machine parameters such as field of view (FOV), peak kilovoltage (kVp), milliamperage (mA), number of basis images, scan time, continuous or pulsed radiation, and degree of rotation.5,12 Some of these parameters are machine specific and will be different according to each manufacturer or CBCT unit, and others are clinician dependent.13 All of them will influence the amount of radiation absorbed by the patient. The effective dose of radiation in CBCT is difficult to generalize because the large exposure range varies from 5 up to 1073 μSv according to the chosen FOV and the model of CBCT equipment.14-16 In general, most studies report a significant lower dose for CBCT compared to MDCT.15,17-19 Mean adult effective doses are 84µSv for small FOV, 177µSv for medium FOV and 212 µSv for large FOV.16 In comparison, the effective radiation
dose for conventional MDCT ranges from 474 to 1160 μSv.15 The average effective dose for a conventional set of dental radiographs is 35,81µSv.20
IMAGE DETECTION
Current CBCT units use a flat panel detector (FPD).5 FPDs are a newer digital capture system that offers a greater dynamic range and does not experience distortion at the periphery of the images. Measurements will be accurate in the center of the volume as well as in the outer edges of the volume.21 The size of the detector dictates the field of view (FOV) capabilities of each unit. FOV refers to the area of the patient that will be irradiated and that will be included in the data volume. Depending upon the type of detector and the beam-geometry, the FOV can be classified as small or limited, medium, and large.22 Small FOV scans cover approximately five teeth (5 cm diameter) and are generally used in endodontics due to the possibility of using a minimal voxel size and thus producing images with the highest spatial resolution. Medium field of view images generally covers one arch or both dental arches and are approximately 6-11 cm in height. A medium field of view is indicated in implant planning and the evaluation of the temporomandibular joint. The large field of view images may range from 11 to 24 cm and covers most of the craniofacial skeleton. It is generally used for specific cases with skeletal asymmetry or anomaly where surgery is planned. Because of the large area irradiated, the main disadvantage of the large FOV is radiation exposure. It is essential always to use the smallest possible FOV focused on the region of interest to avoid unnecessary radiation exposure.
IMAGE RECONSTRUCTION
During the reconstruction phase, the raw data 2D images obtained during acquisition are used for secondary reconstruction in the orthogonal planes. As mentioned, a primary difference between CBCT and MDCT is the isotropic nature of the data acquisition and subsequent reconstruction in CBCT. The reconstruction is most frequently based on a modified Feldkamp algorithm, which is a 3D adaptation of the filtered back-projection method used in fan-beam 2D reconstructions.23 The data reconstruction can also be based on algebraic reconstruction techniques (ART), which is more expense computational, but may help counteract beam hardening artifacts when metal objects are present.24
Maybe, the most crucial benefit of CBCT in dental imaging is the possibility to create images imitating those generally employed in clinical settings. CBCT data sets can be reconstructed to provide, for example, oblique, curved reformations, and cross-sectional reformations, all of which can be used to evaluate precise anatomic structures for diagnostic and treatment planning purposes. Because of the isotropic nature of the images, all measurements calculated in the different reconstructed images are free from distortion and magnification. All reconstructions in the different planes display the same high imaging quality.25
ADVANTAGES OF CBCT
IMAGE ACCURACY/HIGH SPATIAL RESOLUTION
Voxel size can range from 0.6 mm to as low as 0.075mm.11 This produces a sub-millimeter resolution, exceeding the highest resolution of conventional CT, which is precise enough for the evaluation of minimal changes to small structures.
DOSE REDUCTION
Large exposure ranges make CBCT doses difficult to generalize.14,16 Several studies have reported that CBCT showed a significantly lower absorbed radiation dose than conventional MDCT.15,17-19 Although it should be noted that there are studies using low-dose MDCT protocols which report overlap in radiation dose with CBCT.18,26,27 A CBCT study may replace all conventional radiographs, but a CBCT study generally still emits 2 to 4 times more radiation than a standard dental radiographic series, so it is still important to assess the need for 3D imaging.20,28
REDUCED METAL ARTIFACT
Metallic structures produce fewer or less prominent streak artifacts in CBCT than MDCT.
This may be attributed to the cone-beam geometry or the lower energetic spectra. 29 Availability of additional artifact suppression algorithms has also contributed to less pronounced metal artifacts, mainly in secondary reconstructions intended for the visualization of jaws and teeth.30
DISPLAY MODES UNIQUE TO MAXILLOFACIAL IMAGING
Besides providing images in orthogonal planes, CBCT data sets can be segmented nonorthogonal to generate distortion-free reconstructed images to accentuate precise anatomic structures or generate images commonly used in clinical practice. These features are essential, considering the intricate oral and maxillofacial anatomy. Measurements calculated on-screen are free from distortion and magnification. Furthermore, accurate 3D visualization of the dataset is available.
LIMITATIONS OF CBCT
POOR SOFT-TISSUE CONTRAST/LOW CONTRAST RESOLUTION
The most significant disadvantage of CBCT is that it has only limited contrast resolution, compared to conventional CT. The low contrast resolution is due to relatively high scatter radiation, the divergence of the cone-shaped x-ray beam, and artifacts inherent to the FPD.5 Therefore, CBCT is not sufficient for soft tissue evaluation. Only bone, calcified structures, and airway spaces are reliably visualized. Nevertheless, the most important dentomaxillofacial diagnostic requirement is the acquisition of a 3D high spatial resolution data set for adequate depiction of small bony structures.
INCREASED SCAN TIME
Scan time ranges from 5 to 40 seconds making it vulnerable to motion artifacts, caused by patient's involuntary movements. Small motions can cause blurring, and significant movement can cause double images or ghost images.31 These artifacts result in poor overall image quality. Since the spatial resolution is very high, even small motions can have a detrimental effect on image quality, and thus proper patient stabilization is crucial. In the past decade, scan time to acquire a volume data set has diminished in the high end CBCT machines to a time range of about 15 to 20 seconds.
IMAGE ANALYSIS AND RECONSTRUCTION
Correct interpretation of a CBCT examination is done by evaluation of the images reconstructed in three orthogonal planes. Additionally, panoramic and parasagittal multiplanar reconstructions (MPR) are made along the curve of the mandibula mandible or maxilla at regular intervals. Thickness and interval can be adjusted according to the indication.
Volume rendering (VR) techniques provide a true three-dimensional representation of the scanned volume. Relationships of various anatomic features and anatomical details, such as the mental foramina, are sometimes better visualized by 3D images than on cross-sectional
reformatted images. Routinely, a set of at least three 3D images are enclosed in each dental CBCT examination. 3D rendering is for visualization purposes only, not for diagnosis and analysis.32
INDICATIONS OF CBCT
CBCT has numerous indications in dentomaxillofacial imaging and head and neck imaging.
The main indication for dental imaging is the visualization of pathologies or anatomical variants for planning therapeutic procedures. The same rules apply for CBCT as for other imaging modalities: the medical benefit of the patient must be greater than the possible adverse effects resulting from radiation exposure. A CBCT study is only justified if the desired information cannot be obtained with conventional radiography, which is associated with a lower dose. Following is a summary of some of the most frequent indications of CBCT focused on dental imaging.
DENTAL IMPLANTS
Replacement of missing teeth by dental implants demands accurate assessment of the implant site for the successful placement of implants and to avoid injury to adjacent vital structures.
CBCT is the modality of choice for the preoperative assessment by providing accurate information about bone density, height, and width of the planned implant site and its relation to vital structures while delivering low radiation exposure.33 CBCT can be employed in postsurgical assessments of bone grafts and the implant's position in the alveolus.34
TEETH IMPACTION AND STRUCTURAL TEETH ANOMALIES
CBCT has proven its worth in the diagnosis and orientation of impacted or supernumerary teeth.35 CBCT offers precise knowledge of the exact location of the tooth in the jaw and its
relation to other teeth and surrounding anatomical structures, such as the relationship of an impacted mandibular third molar and the inferior alveolar nerve, required before surgical removal. For tooth agenesis, a 3D image is not strictly required. Although, when being treated orthodontically, the use of 3D imaging may be a valuable option.36
(PERI-APICAL) INFECTION OR INFLAMMATION
The most common dental pathologies are inflammatory lesions of the pulp and periapical areas. CBCT, when compared with 2D imaging, is superior in detecting apical periodontitis and periapical lesions.37 CBCT can also precisely demonstrate lesion location relative to the maxillary sinus or the mandibular canal.
TUMOR AND TUMOR-LIKE CONDITIONS
CBCT is being used to examine the precise location and extent of odontogenic and non- odontogenic tumors or cysts of the jaws.38 The exact size of a lesion, its relative density (radiolucent, radiopaque, or a combination of the two), internal structure, the extent of a lesion's relationship to teeth, root resorption, cortical expansion and erosion, and the presence of multiple lesions can all be evaluated.39 This aids diagnosis as well as surgical treatment planning by using three-dimensional information.
(DENTAL) TRAUMA
The diagnosis of a simple dental fracture or jaw fracture can be achieved with plain radiographs. Initial evaluation of a complex jaw fracture may also be performed with plain films. CBCT is advised for detecting radiographically occult fractures, or fractures suspected based on secondary signs, such as a sinus air-fluid level, and for defining fracture displacement prior to surgical reduction and fixation.36 Compared to plain radiographs, CBCT
images are significantly better for diagnosing specific aspects of dentoalveolar trauma, especially horizontal root fractures.40 CBCT can often provide sufficient information for diagnosis in one scan. It is useful for the identification of the fracture, defect morphology, and the relative location of relevant anatomical structures, all information required prior to surgery.
IMAGING ANATOMY
Reporting dental CBCT examinations requires essential knowledge of dental anatomy and systematic analysis of all related structures. Knowledge and interpretation of normal dental anatomy are essential in recognition of disease.
TEETH
ANATOMY
Teeth are the hardest substances in the human body. There are two sets of dentition: the primary (deciduous) and permanent. The primary dentition consists of 20 teeth: 10 per jaw, and 5 per quadrant. Each quadrant consists of a central incisor, lateral incisor, canine, first molar, and second molar (4Fig. 2a). There are age ranges for the development, eruption, and shedding of primary teeth.41 The permanent teeth develop within the jawbone and resorb the roots of the primary teeth as they migrate toward the oral cavity. The permanent dentition gradually replaces the primary dentition. The permanent dentition consists of 32 teeth:16 per jaw, and 8 per quadrant. Each quadrant consists of a central incisor, lateral incisor, canine, two premolars, and three molars (4Fig. 2b). The incisors and canines are named the anterior teeth and the premolars and molars the posterior teeth.
The part of the tooth erupted above the gum line is called the crown. The portion or portions of the tooth within the alveolus are called the root or rootlets. The crown and root join at the cementoenamel junction (CEJ), which is also called the cervical line. Teeth are composed of multiple components (4Fig. 3). The majority of the tooth consists of dentin, which has a radiologic opacity similar to cortical bone. In the region of the crown, the dentin is covered by a highly radiopaque mineralized surface called enamel. Enamel has by far the highest opacity of natural tissues. The root dentin is surrounded by a thin layer of cementum. Dentin is
hypoattenuating relative to the enamel and iso-attenuating relative to the cementum, which cannot be differentiated radiographically. The pulp cavity is centrally located and contains the neurovascular elements of the tooth. It is the most radiolucent structure of the tooth.42 The pulp cavity consists of two continuous spaces. The pulp chamber is mainly in the crown portion, and root canal is in the root portion.
The molars in the mandible generally have two roots, whereas molars in the maxilla consist of three roots. The first premolar of the maxilla generally has two roots, whereas the remaining teeth have a single root. Roots are named according to their location in the alveolar process:
buccal, lingual, mesial, distal, mesiobuccal, distobuccal. There is a wide variety in the number of roots, the number of root canals per root, and in root canal morphology among populations and regions and even in different individuals within the same population.43 The success of endodontic treatment depends on the precise knowledge and identification of anatomic variations or additional root canals. Cone-beam CT offers the added value of high spatial resolution and, therefore, better visualization of the root canal system. Root canal morphology has been classified using different ways by several investigators in the literature. Weine et al.
classifies it into four types depending on the pattern of division of the main root canal along its course from the floor of the pulp chamber to the root apex.44 Vertucci also classified the root canal morphology more descriptively into eight different types.45 The classification by Vertucci has been widely used by many researchers to classify the canal system of different teeth. However, because of the high spatial resolution of cone-beam CT, there has been an increase in the number of reports on even more complex root canal morphology, for which new classification systems are proposed.46,47
NOMENCLATURE
Dentists use somewhat different terminology than medical doctors to designate the location of an abnormality within the dental arch (4Fig. 4). The outer surface of the tooth is referred to as vestibular and is subdivided into labial for the anterior dental arch and buccal for the posterior dental arch. The inner surface of the tooth is referred to as oral and is subdivided into palatal for the maxillary dental arch and lingual for the mandibular dental arch. The surface of the tooth facing towards adjacent teeth is called the proximal surface. The proximal surface may be either called mesial if facing towards the midline, or either distal if facing away from the midline. The direction towards the crown of the tooth is referred to as occlusal for the premolars and molars and incisal for the incisors and canines. The direction toward the root tip(s) or apex(es) is referred to as apical.
DENTAL NOTATION
When associating information to a specific tooth, tooth notation is crucial in correctly communicating the information between radiologists and dental professionals. The two primary dental notation systems used to number teeth are the FDI (Fédération Dentaire Internationale) numbering system and the ADA (American Dental Association) Universal numbering system, with the ADA system more commonly used in the United States.48
The American Dental Association (ADA) Universal numbering system uses uppercase letters A to T for primary teeth and numbers 1 to 32 for permanent teeth (4Fig. 5). Starting with the letter A for the right upper second molar for primary teeth or number 1 for the right upper third molar for permanent teeth, the numbering progresses through the jaws in a clock-wise counter, ending with the letter T for the right lower second molar for primary teeth or the number 32 for the right lower third molar for permanent teeth.
The FDI (Fédération Dentaire Internationale) numbering system, also called the ISO 3950 system (International Standards Organization Designation System), uses a two-digit system (4Fig. 5). The number for each tooth is composed of 2 separate digits. In adults, the first digit indicates the quadrant in a clockwise counter: The number 1 stands for the upper right quadrant of the jaw bone, 2 for the upper left quadrant, 3 for the lower left quadrant, and 4 for the lower right quadrant. The second digit indicates the number of the tooth within each quadrant, starting with the central incisor (1) and ending within the third molar (8). In children, the upper right quadrant is indicated by the number 5, the upper left quadrant by 6, the lower left quadrant by 7, and the lower right quadrant by 8.
ALVEOLAR PROCESS
The thickened ridge of the jaw serving as support for the teeth is called the alveolar process.
The alveolar process contains the bony tooth sockets or dental alveoli that firmly hold the root portion of the tooth. The compact bone that is the lining of the tooth socket is called the lamina dura (4Fig. 3). The integrity of the lamina dura should be evaluated when studying images for pathological lesions. The periodontal ligament is the connective tissue that attaches the cementum of the root portion of the tooth to the lamina dura of the tooth socket. The periodontal ligament is seen as a thin radiolucent layer, hardly visible on CT. A widened, clearly visible periodontal space is typically indicative of pathology. The alveolar bone between the roots of adjacent teeth is called interdental bone. The alveolar bone between the multi-rooted teeth is called interradicular bone (4Fig. 3). The presence of interradicular bone loss is one of the findings that can lead to a diagnosis of advanced periodontitis and potentially to a less-favorable diagnosis for the affected tooth.49
MAXILLA
Several surgical procedures are performed on the maxilla such as dental implant placement, orthognathic surgery, supernumerary or impacted teeth removal, cyst or tumor operations, and endodontic or periodontal surgery. Practitioners should be careful concerning some critical structures in the maxilla such as the maxillary sinus, the nasopalatine canal, the anterior superior alveolar canal, and its accessory canals. These canals contain neurovascular bundles, and injury could lead to per operative or postoperative complications. These structures are well visualized on cone-beam CT imaging due to its high spatial resolution. Increasing the knowledge of exact anatomy can improve clinical outcome.
MAXILLARY SINUS
The maxillary sinus can extend between the roots of the molars and premolars. Due to the close relationship with these structures, the maxillary sinus can be easily affected by inflammatory conditions or cystic lesions of the adjacent teeth. Extraction of a tooth or endodontic surgery can cause perforation of the floor of the maxillary sinus or root displacement into the maxillary sinus. Sinusitis can appear as a result of both iatrogenic perforation and the spread of periapical infection into the sinus.
NASOPALATINE CANAL
The nasopalatine canal is situated in the midline of the anterior maxilla posterior to the central incisors and connects the palate to the floor of the nasal cavity. It contains the nasopalatine nerve and the terminal branch of the descending nasopalatine artery. The oral aperture in the midline of the anterior palatine bone, just dorsal to the roots of the upper central incisors, is called the incisive foramen. The opening in the nasal fossa is called the nasopalatine foramen and can be single or multiple.50 The position of the nasopalatine canal is one of the most
essential factors for implant placement in the premaxillary region. Placement of implants into the nasopalatine canal may lead to many complications such as bleeding during the operation, postoperative short-term sensory disorder, non-osseointegration of the implant, and the formation of nasopalatine canal cyst.51,52 There is an important variability observed in the anatomy and morphology of the nasopalatine canal.53 Therefore, detailed imaging of the relevant region is essential to prepare a suitable surgical intervention program.
ANTERIOR SUPERIOR ALVEOLAR CANAL OR CANALIS SINUOSUS
The infraorbital nerve, which is a branch of the maxillary nerve, runs along the infraorbital canal and divides into 3 alveolar branches: the anterior, middle, and posterior superior alveolar nerves. The anterior superior alveolar (ASA) nerve innervates the anterior teeth and is running in its own tortuous canal – called the canalis sinuosus- from the infraorbital foramen medially in the anterior maxillary wall. Reaching the lateral wall of the nasal fossa, the canalis sinuosus turns anterior and follows the bony margin of the nasal fossa ending at the nasal septum in front of the incisive canal (4Fig. 6).54 The frequency of accessory canals coming from the canalis sinuosus reaches up to 50%. These accessory canals are most frequently found palatal to the anterior maxillary teeth (4Fig. 7).55,56 Contact with the neurovascular bundle of the canalis sinuosus or the accessory canals can compromise osteointegration and cause temporary or permanent paresthesia with bleeding in situ.57 CBCT plays an important role in the detection of these canals and their anatomical variations.
MANDIBLE
The mandible base is located below the alveolar process and contains the mandibular canal at the inner aspect of the mandible base. The mental foramen is the anterior opening of the mandibular canal on the body of the mandible in the premolar region. Placement of
mandibular implants or mandibular surgery can be problematic, especially in the proximity of neurovascular bundles. Complications can occur, such as postoperative sensory disturbances, neuropathic pain, lack of osseointegration of implants, edema, and excessive bleeding. The mental interforaminal region was considered as safe without significant risks of damage to the vital anatomic structures, although several reports have described complications following surgery in the anterior mandible.58–60 The mandibular incisive canal and the lingual foramen are the essential anatomic landmarks in this region. The risk of damaging vital anatomic structures should not be overlooked. Advanced cross-sectional imaging modalities, especially CBCT, is a suitable tool for observing and identifying these vital structures and their normal anatomic variations.
MANDIBULAR CANAL OR INFERIOR ALVEOLAR CANAL
The mandibular canal contains the inferior alveolar nerve and vessels. The inferior alveolar nerve is a branch of the third division of the fifth cranial nerve. Injury to the mandibular canal may result in paralysis or numbness of the chin and the edge of the mouth and loss of tooth vitality within a whole quadrant. Identification of the mandibular canal before surgery is essential to avoid damage during the placement of implants or extraction of third molars. The neurovascular bundle enters the canal at the mandibular foramen at the lingual side of the ramus, continues on the buccal surface of the body of the mandible, and exits through the mental foramen. The mental foramen may vary widely in number, size, location, shape, and direction of opening(4Fig. 8).61 In case of osteoporotic bone, the cortical lamina may not be visible on some slices and the exact position of the mandibular canal can be found as a cut-out in the cortical bone or located by means of interpolation between other slices where the canal is visible. 62 The mandibular canal is most often found as a single canal, but variations may include bifid, or in more rare cases, trifid mandibular canals.63 Naitoh et al suggested the
classification of bifid mandibular canals on CBCT into four different types: retromolar (in which the foramen of the accessory canal is located on the bony surface of the retromolar region), dental (in which the canal reaches to the root of the apex of the second or third molar) (4Fig. 9), forward (in which the canal arises from the superior wall of the mandibular canal and can be with or without confluence to the main mandibular canal) and buccolingual (in which the canal arises from the buccal or lingual wall of the mandibular canal).64 Each separated canal might contain a neurovascular bundle.
MANDIBULAR INCISIVE CANAL
The mandibular incisive canal is described as a prolongation of the mandibular canal anterior to the mental foramen containing a neurovascular bundle.65 The incisive nerve is one of the terminal branches of the inferior alveolar nerve. The incisive canal gradually narrows while progressing from the distal to the most anterior part of the mandible towards the midline (4Fig. 10). There are variations in measurement of the length and the course of the mandibular incisive canal, which can be determined with CBCT imaging.66
LINGUAL CANAL
The sublingual artery is a branch of the lingual artery, of which branches supply the floor of the mouth. These vascular branches, along with branches of the lingual nerve, enter the mandible through bony canals called lingual canals. The number and location of these lingual canals are highly variable, though there are two characteristic locations: one is in the mandibular midline area (median lingual canal), and the other is in the premolar region (lateral lingual canal) (4Fig. 11).67 The vessels entering the mandible through these bony canals can cause life threatening complications such as sublingual hematoma formation, upper airway obstruction or excessive bleeding when they are injured. Therefore, the distance from
the lingual canals to the alveolar crest is clinically relevant to implant surgery, for it may limit the length of the implant to be placed.68
CONCLUSION:
CBCT imaging surpassed the obstacles of 2D imaging, offering practitioners 3D analysis of high quality, sub-millimeter resolution images. Nevertheless, CBCT imaging should not be used on a routine basis or for screening purposes, but as a complementary tool for specific cases. Therefore, clinical information is critical in selecting the proper imaging technique. The diagnostic efficiency of the use of CBCT in dental imaging is essentially based on the healthy knowledge of the anatomy of the entire acquired image volume and the complete understanding of the concept of multiplanar reformatting.
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FIGURES :
(a)
(b)
Fig 1. Comparison of conventional fan-beam CT and cone-beam computed tomography data acquisition. (a), Conventional CT imaging uses a one-dimensional fan-beam shaped x-ray beam. The x-ray source makes multiple rotations and a translation in the z-plane to achieve multiple adjacent 2D slices. (b), Cone-beam CT imaging uses a cone-beam shaped x-ray beam. The CBCT unit makes a single rotation around the patient in which it achieves multiple 2D projections to create a 3D volume.
(a)
(b)
Fig 2. Drawings illustrating the two different sets of dentitions: (a) the primary or deciduous dentition in which each quadrant consists of two incisors, a canine, and two molars. (b) the permanent dentition in which each quadrant consists of two incisors, a canine, two premolars, and three molars.
Fig 3. Tooth anatomy. Drawing showing the normal anatomy of the tooth and corresponding cone-beam CT image.
Fig 4. Positional and relational terms relating to the dental arch. The outer surface of the tooth is referred to as vestibular and is subdivided into labial for the anterior dental arch and buccal for the posterior dental arch. The inner surface of the tooth is referred to as oral and is subdivided into palatal for the maxillary dental arch and lingual for the mandibular dental arch. The surface of the tooth facing towards adjacent teeth is called the proximal surface. The proximal surface may be either called mesial if facing towards the midline, or either distal if facing away from the midline.
(a)
(b)
Fig 5. Tooth numbering. (a), Tooth numbering in primary dentition according to the FDI numbering system and the ADA Universal Numbering system. (b), Tooth numbering in permanent dentition according to the FDI numbering system and the ADA Universal Numbering system.
Fig 6. Axial CBCT image at the level of the anterior nasal spine showing the anterior superior alveolar (ASA) canal (arrowheads), or canalis sinuosus, on both sides with an implant apex just posterior to the canal on the right side (arrow).
(a)
(b)
Fig 7. (a), Axial CBCT at the level of the nasopalatine canal showing 2 accessory canals of the canalis sinuousus (arrows), located posterior the extraction socket of the central incisors.
(b), Cross-sectional image demonstrates the vertical position of the accessory canals (arrowheads) and its close relationship of to the extraction socket.
Fig 8. Anterior view of a 3D volume rendered and surface shaded CBCT demonstrating the mental foramen (arrows) on both sides. An accessory mental foramen can be seen on the right side (arrowhead).
(a)
(b)
Fig 9. (a), Panaromic Panoramic CBCT reformation through the left mandibular body shows the protrusion of the third molar roots into the mandibular canal (arrow) and the branching of an accessory mandibular canal (dental type according to the classification of Naitoh et al) towards the second molar tooth (arrowhead) verified by the cross-sectional image (b).
Fig 10. Axial CBCT at the level of the inferior alveolar canal (arrows) also displays the mandibular incisive canal on the left side (arrowhead).
(a)
(b)
Fig 11. (a), Posterior view of a 3D volume rendered and surface shaded CBCT shows not one a single but 2 median lingual canals (small arrowheads) as well as 2 lateral lingual canals (large arrowheads). (b), Midline cross sectional CBCT image confirms the presence of both median lingual canals and foramina (arrowheads).
