• Two-Dimensional Radiographic Modality: For ideal site assessment for dental implants, a true evaluation and determination of the buccal-lingual available bone must be determined. Because periapical radiography anatomically compresses the width dimension into a two-dimensional radiograph, important information cannot be obtained. Therefore when attempting to estimate width distances in close approximation to maxillary and mandibular anatomic structures with two-dimensional radiographs, the implant clinician must be continuously conscious of the inherent inaccuracies associated with two-dimensional images.
• Identification of Vital Structures: When evaluating the position of vital structures with intraoral radiographs, extreme caution should be exercised. In the evaluation of the true location of the mental foramen, studies have shown less than 50% of periapical radiographs depict the correct location of the mental foramen.1 Other studies have concluded that, because of insufficient cortical bone around the mandibular canal, only 28% of periapical radiographs will accurately identify the mandibular canal.2 Therefore periapical radiographs exhibit high false-positives and false-negatives with respect to the identification of vital anatomic structures.
Periapical radiographs have many inherent disadvantages, the most notable being that they provide only a two-dimensional image of a three-dimensional object. The inability to determine the buccal-lingual bony dimensions is a major shortcoming with respect to implant treatment planning. These radiographs are of little value in determining quantity and quality of bone, identifying vital structures, and depicting the spatial relationship between structures within proposed implant sites. Thus, periapical radiographs should be limited to an initial evaluation of a proposed implant site, intraoperative evaluation, and postoperative assessment.
Panoramic radiography is a curved-plane tomographic radiographic technique used to depict the body of the mandible, maxilla, and the maxillary sinuses in a single image. Its convenience, speed, and ease have made this type of radiography a popular technique in evaluating the gross anatomy of the jaws. However, the implant clinician must understand the inherent fundamental limitations characteristic of this type of radiograph.
• Magnification/Distortion: All panoramic radiographs suffer from vertical and horizontal magnification, along with a tomographic section thickness that varies according to the anatomic position. Because the x-ray source exposes the jaws utilizing a negative angulation (~8%) to avoid superimposing the occipital bone/base of the skull over the anterior dental region, variable magnification will always be present on panoramic radiographs. Increased magnification stems from variances in patient positioning, focal object distance, and the relative location of the rotation center of the x-ray system and variations in normal anatomic form and size from one patient to the next. Zarch et al. have shown that 83% of panoramic measurements are underestimated, with the greatest magnification being present in the anterior region (Fig. 4.2).3
• Horizontal magnification: Horizontal magnification is determined by the position of the object within the focal trough. The degree of horizontal magnification depends on the distance of the object from the focal trough center and is influenced by the patient’s anatomy and positioning within the panoramic machine. In the anterior region the horizontal magnification will increase significantly as the object moves away from the focal trough. This results in anterior magnification being far greater and more variable than posterior magnification.
• Vertical magnification: Vertical magnification is determined by the differences between the x-ray source and object. Because the beam angle is directed at a negative (upward) angulation, structures positioned closer to the source are projected higher within the image in relation to structures positioned farther from the x-ray source. Therefore the spatial relationships between objects projected on a panoramic radiograph are inaccurate (see Fig. 4.3).
• Two-Dimensional Radiographic Modality: The panoramic radiograph is a two-dimensional (2-D) image depicting 3-dimensional (3-D) structures. Accordingly, it does not demonstrate the buccal-lingual dimension of maxillofacial structures; therefore bone width and vital structures cannot be determined. Additionally, it produces a flattened, spread-out image of curved structures, which results in significant distortion of the vital structures and their relationship in space.
• Identification of Vital Structures: Panoramic radiography does not exhibit an accurate assessment of bone quality/mineralization, and it does not truly identify and locate vital structures accurately.
• Visibility of mandibular canal: Lindh has shown that the mandibular canal cortical walls were visible in only 36.7% of panoramic radiographs.4
• Mental foramen location: Yosue et al, in evaluation of the mental foramen, concluded that over 50% of radiographs will not depict the true location of the mental foramen.1
• Linear measurements: Sonic et al have shown an inaccuracy rate of 24% in determination of linear measurements for bone assessment with respect to vital structures.5
• Anterior loops: Studies completed by Kuzmanovic et al of anterior loops (mental nerve courses anteriorly to the mental foramen), concluded panoramic radiographs exhibit a high incidence of false positives and false negatives, making them totally inaccurate.6
• Location of septa: In evaluation of maxillary sinus floor bony septa by Krenmair et al, correct identification and location has been shown to be approximately 21.3%.7
• Identification of accessory foramina: Accessory (double) foramina have been shown to accurately identified in less than 50% of panoramic radiographs.8
Although panoramic radiographs have historically been the gold standard in evaluating potential implant sites, many disadvantages are associated with these types of radiographs. A lower resolution prevents evaluation of the fine detail that is required for the assessment of osseous structures and anatomy. The magnification in the horizontal and vertical planes is nonuniform; thus linear measurements are inaccurate (Fig. 4.3). Often the image has superimposition of real, double, and ghost images, which result in difficulty in visualizing anatomic and pathologic details. The true positions of important vital structures, which are crucial in dental implant treatment, are not easily seen or incorrectly depicted. Therefore panoramic radiographs have value for initial evaluation; however, they predispose the implant clinician to many surgical, prosthetic, and medicolegal complications.
Cone Beam Tomography
To overcome some of the disadvantages of two-dimensional radiographs and conventional medical CT scanners, a new type of computed tomography specific for dental applications has been developed. This type of advanced tomography is termed cone beam volumetric tomography (CBVT) or cone beam computed tomography (CBCT). In the past, conventional computerized tomography, when used for dental implant treatment planning, has been underutilized due to concerns related to potentially high radiation dose and lower resolution. Because of the low radiation dose inherent with cone beam technology, the limitations of medical computerized tomography have been overcome. Additionally, this scanning technology has advantages including potential “in-office” installation and use, which allow the clinician and patient the convenience of onsite scanning capabilities and treatment planning. In addition, for clinicians not wishing to invest in a personal CBCT unit, many major cities have specialized dental scanning centers for referral for appropriate CBCT imaging.
Today, CBCT imaging has become the gold standard for dental implant treatment planning. However, many implant clinicians lack the background and knowledge in evaluating and treatment planning with CBCT, thus predisposing to possible complications. Therefore the implant clinician must have a thorough understanding of inherent disadvantages of CBCT scans along with knowledge of applied head and neck anatomy, anatomic variants, incidental findings, and pathologic conditions with respect to implant treatment planning.
CBCT Technology Complications
The x-ray sensor receives the x-rays and converts them into electrical data that are then converted to various images via special computer programs. There exist two types of sensors used today in CBCT technology: (1) image intensifiers (IIs) with charged coupling devices (CCD) and (2) flat panel detectors (FPDs). Image intensifiers have many disadvantages in comparison to flat panel detectors including poorer resolution, larger size, and a higher patient radiation dose requirement. Flat panel detectors, although more expensive than image intensifiers, produce images with much higher quality and resolution. Most FPDs used today in CBCT units utilize cesium iodide (CsI) as the scintillator crystal screen. Cesium iodide scintillators produce the highest spatial resolution possible among various CBCT screens.
The unit element in the 3-D image is termed the voxel, which is analogous to the 2-D pixel. Images composed of multiple voxels are stacked in rows or columns that are isotropic (i.e., they have equal dimensions in the x, y, and z planes) and range in size from 0.075 to 0.6 mm. Each individual voxel is assigned a grayscale value that corresponds to the anatomic structures attenuation value. The smaller the voxel size, the greater the resolution and quality of the image, but also the greater the resultant radiation dose. A voxel size of 0.2 to 0.3 mm is considered ideal because it allows for an equitable trade-off between image quality and absorbed radiation dose (Fig. 4.4).
Spatial resolution is measured in lines/millimeter (lp/mm) and relates to the ability to distinguish two anatomically close objects. On a CBCT image, the higher the spatial resolution, the greater the ability to delineate two different objects from one another. Normally, CBCT scanners (voxel size 0.075–0.6 mm) are most commonly associated with higher spatial resolution than medical grade scanners (voxel size 0.6–1 mm). However, decreased spatial resolution on CBCT images may result from: (1) the use of a higher voxel size (> 0.4) (i.e., use of voxel sizes > 0.3 mm for implants is not recommended due to the lower spatial resolution), (2) decreased radiation (kVp or mA), which results in increased noise, (3) metallic restorations resulting in artifacts, and (4) increased focal spot size.
Contrast resolution is defined as the ability to differentiate tissues of different radiodensities. In implant dentistry, the ability to produce different shades of gray is important for a clearly diagnostic image. Because CBCT images utilize less radiation and are produced with lower kVp (peak kilovoltage) and mA (milliamperage) settings in comparison to MDCT units, dental CBCT images are associated with slightly higher image contrast, modifiable through software settings. Dental CBCT images generally have increased noise and image scatter compared to medical units. To minimize noise and scatter, a smaller FOV may be used. However, smaller FOVs are usually associated with slightly higher radiation settings.
The quality of CBCT images is directly related to the number of shades of gray (bit depth). Currently, CBCT units produce up to 16-bit images, which corresponds to (216) 65,536 shades of gray. However, computer monitors may display up to only 8 bits (28) 256 shades of gray. To increase the quality of the image, the monitor brightness and contrast may be adjusted to display 8 bits per image.
Bone Density: MDCT.
Medical CT data permit differentiating between tissues that have a physical density of less than 1%. In contrast, conventional radiography requires a minimum of 10% difference in physical density to be seen.9 Each medical CT image is composed of pixels and voxels, which are characterized by a given numeric value, which reflect the x-ray beam attenuation. These values are directly affected by the density and thickness of the tissue. The Hounsfield units (HU), or CT numbers, correlate with the density of the medical CT image and range in value from −1000 (air) to +3000 (enamel). A specific shade of gray or density number is assigned to each CT number, which ultimately forms the image. The correlation of these CT numbers has been used to associate the density of the area of interest with various bone densities used for surgical and prosthetic treatment planning. Thus the gray values depicted on medical CT images are considered true attenuation x-ray values (HU) (Fig. 4.5).
Bone Density: Dental CBCT.
When evaluating dental CBCT images in regards to bone density, there does not exist a direct correlation (accuracy of measurement) as compared with medical CT. Most dental CBCT systems inherently have an increased variation and inconsistency with density estimates. The density estimates of gray levels (brightness values) are not true attenuation values (HU); thus, inaccuracies in bone density estimates result.10 This is mainly due to the high level of noise in the acquired images and the slight inconsistencies in the sensitivity of the CBCT detectors. Dental imaging software frequently provides attenuation values (HU); however, such values should be recognized as approximations lacking the precision of HU values derived from medical CT units.
Because metallic objects in the oral cavity are associated with higher attenuation coefficients than soft tissue, dental CBCT images inherently are predisposed to these artifacts. One of the most common types of artifacts is termed beam hardening. Beam hardening occurs when x-rays travel through the bone/implant, resulting in more low-energy photons being absorbed than high-energy photons. Because of this the image will have compromised image quality.11 The titanium alloy surface is highly susceptible to these types of artifacts because of the high-density nature of the metal. This results in inaccuracies, especially when viewing periimplant bone levels. Conventional intraoral images will not exhibit these beam-hardening artifacts and may appropriately be used to better evaluate the quality and quantity of bone mesial and distal to an implant when beam-hardening artifacts may obliterate visualization of interproximal bone, especially when multiple implants are present in the same quadrant. Additionally, higher-density materials commonly found in the oral cavity (e.g., amalgam, gold) will lead to complete absorption of the beam and beam-hardening artifacts.12 There exist two types of beam-hardening artifacts that result in linear areas of dark bands or streaks between dense objects and cupping artifacts. Cupping artifacts occur when x-rays pass through the center of a highly dense object and are absorbed more than the peripheral x-rays. This results in an image in which a uniformly dense object appears to be less dense (darker, lower CT numbers) at its center and appears as a “cup” (Fig. 4.6).
Motion artifacts are usually the result of patient movement and result in the inaccurate depiction of bony landmarks, measurements, and implants.13 Patient movements and incorrect patient positioning create blurring problems, double density line artifacts adjacent to major bony structures that result in nondiagnostic images. Patients should be instructed to not move and avoid swallowing throughout the scan. The motion blurring causes “double contours” of anatomic structures that result in decreased scan quality and spatial resolution. This may lead to improper implant placement and possible damage to neural structures.14 Motion-related artifacts may be decreased by using sit-down CBCT units or head restraints, or by decreasing scanning times (Fig. 4.7).
CBCT images are susceptible to streak artifacts caused by x-rays traveling through objects with a high atomic number (metallic restorations). Streak artifacts usually are seen as light and dark lines that arise from the source object, resulting in images with decreased quality and obscuring of anatomic structures (Fig. 4.9).
Another disadvantage inherent with CBCT images is scatter, which is most commonly produced from metallic restorations. This is caused by photons (x-rays) that are deflected from their original path by metallic objects. When these deflected photons reach the sensor (detector), the intensity of the signal is magnified in a nonuniform magnitude. The end result is an image with decreased resolution and image quality, which ultimately leads to inaccuracies in the reconstructed CT number or voxel density.15 The field of view (FOV) of the CBCT is proportional to the amount of scattering; thus smaller FOVs are associated with less scattering. CBCT images have inherently greater scatter radiation than medical-grade CT images (Fig. 4.9).16
There are two types of noise associated with CBCT images: additive (results from electrical noise) and photon-count (quantum noise). Because CBCT operate at much lower amperage (mA) settings than MDCT scanners, CBCT images are associated with greater quantum noise. The noise is displayed as a “graining” of the image and is the result of inconsistent distribution of the signal, which results in inconsistent attenuation (gray) values in the projection images (see Fig. 4.8).
Bone Dehiscence on 3-D Reformatted Images.
MDCT and dental CBCT data have the ability to be reformatted by software algorithms to represent 3-D images by only projecting the voxels that represent the surface of the object (surface rendering), resulting in a “pseudo-3D rendering of the facial skeleton.” The pixels are illuminated on the screen as if a light source is present in the front of the object. The closer the pixels, the brighter they appear. This shading effect allows the object to be projected as a 3-D object with depth. However, some 3-D images appear to have large voids or no bone present on the surface because the software averages volume elements, and the voids appear when the software attempts to reconstruct portions of the image covered by a very thin layer of bone. When evaluating the cross-sectional images, bone will be present. This is a direct result of the reformatting process, which usually selects a higher HU reformation, resulting in decreased scatter on the 3-D image. Therefore the implant clinician should be aware 3-D images do not accurately depict the quantity and quality bone, but provide only a stylized representation of the facial skeleton (Fig. 4.10).
Scanning Technique Complications
The patient should be positioned within the CBCT unit as per manufacturer’s recommendations. When taking the scan, the teeth should be slightly separated so that the different arches may be easily differentiated upon reformation. Cotton rolls, tongue depressors, or a bite registration may be used. Additionally, cotton rolls may be placed in the vestibule to separate the lips and cheeks from the buccal mucosa. This will allow for a more accurate representation of the contour and thickness of the gingival tissues.
Position of the Scanning Template.
The position of the scanning template/radiographic markers in the mouth during CBCT examination is crucial for the accuracy of fabrication of the surgical template. First, it is recommended that an index be used to position the scanning template in the correct position. The ideal index includes a radiolucent bite registration, which will allow for the teeth to be separated and maintain the patient in centric relation. This will prevent inaccuracies and help stabilize the template in the mouth. Additionally, denture adhesive should be used with the CBCT template to keep it in the ideal position.
When fabricating mucosa-supported surgical guides, the thickness of the mucosa may have a direct effect on the accuracy of the of the planning of the implant sites. Increased mucosa thickness may lead to inaccurate placement of the mucosa-borne guides during the surgical placement procedure. Vasek showed a 1.0-mm buccal mucosa thickness may result in a buccal-lingual deviation of over 0.41 mm.17 This will inevitably cause inaccurate measurements and possible misalignment of the surgical guide when placing the implants. When significant atrophy is present in the premaxilla, usually there will be excessive tissue thickness, which will result in rocking of the template.
CBCT Anatomic Radiographic
The role of CBCT is rapidly emerging in all aspects of diagnosis and treatment planning with dental implants. Because of varying FOVs, the implant clinician is placed in a position to evaluate maxillofacial areas that they may not be familiar with. Therefore it is crucial the implant clinician be able to interpret anatomic structures and pathology outside their primary area of interest. In radiology, an incidental finding is defined as an unexpected discovery found on a radiologic examination performed for an unrelated reason. Unfortunately, many normal anatomic variants, developmental anomalies, and imaging artifacts may be misidentified as possible pathologic conditions by inexperienced clinicians.18 This may lead to unnecessary concern and stress for patients and embarrassment for the clinician. Additionally, possible significant pathologies may exist that go undiagnosed. This problem results in many professional, ethical, clinical, and potential legal issues for the implant clinician.
Incidental findings on CBCT scans have been well documented in the literature. The exact frequency of incidental findings varies from study to study depending on age, gender, race, and FOV. Price et al showed a high incidence (3.2 findings/scan) of incidental findings with approximately 16% requiring intervention or referral.19 These incidental findings ranged from common benign findings to significant pathologic conditions. Miles reported a minimum of two reportable findings per CBCT and also showed a high incidence of periapical lesions that went undetected on conventional radiographs.20 Cha determined after evaluation of 500 scans an incidence of 24.6% of incidental findings, most in the airway region.21 Arnheiter showed patients 40 to 49 years old had the largest percentage of reportable incidental findings (70%), with patients aged 20 to 29 years old with the lowest percentage (40%).22
Obtaining a Radiology Report.
Radiology reports immediately after CBCT exams, prior to surgery, minimize the liability that may present to the implant clinician. Formal radiology reports may be obtained from many sources, preferably from an appropriately qualified, board-certified maxillofacial radiologist. Unfortunately, the geographic distribution of maxillofacial radiologists is not uniform within states or regions within a state, and a careful search will be required. Several, but not all, states require that the report be made by a maxillofacial radiologist licensed in the state, and it is therefore crucial to check with your local dental board or dental practice act to determine if in-state licensure is required. The implant clinician must be able to recognize and evaluate variations from normal and refer for appropriate medical consultation any significant incidental finding that may be contained in the radiology report.
Use of the Smallest FOV as Possible.
Ideally, the smallest FOV should be utilized for scans when treatment planning for dental implants (Fig. 4.11). A smaller FOV (~Mid FOV) will reduce radiation dose to the patient, thus adhering to the ALARA (As Low As Reasonably Achievable) principle. However, caution should be exercised to not take an inadequate FOV that includes insufficient view of the anatomic area of concern. The most common anatomic area for this to occur is the maxillary posterior region because many practitioners will set the limits of the scan superiorly/coronally to exclude the maxillary ostium. When placing implants or bone grafting in the posterior maxilla area, confirming the patency of the ostium is important to minimize complications associated with an obstructed osteomeatal complex.
Normal Radiographic Anatomy
Due to the complex nature of implant treatment and the potential for complications throughout the surgical and prosthetic phases, the clinician must have a thorough understanding of the normal anatomy of the maxillofacial region. Traditional dental education has focused on the interpretation of conventional 2-D radiographic images for diagnosis, but with the introduction and rise of CBCT images, a deeper understanding of anatomy is necessary to examine the patient’s structures in three dimensions. This section of the chapter will address the basic radiographic anatomy as viewed in the three planes (axial, coronal, sagittal) typically seen on a CBCT image.
The position of the mandibular canal as it courses through the mandible from posterior to anterior is highly variable. Although the pathway of the inferior alveolar nerve and the mental nerve have been well described in the literature, it is paramount that the implant clinician have a clear understanding of their anatomic features and variations. When evaluating the intraosseous course of the mandibular canal buccal-lingually and inferior-superiorly within the mandible, many variations exist based on gender, ethnicity, amount of bone resorption, and age.
Buccal-Lingual Mandibular Canal Locations
In the posterior region of the mandible, the inferior alveolar nerve enters the mandibular foramen on the lingual surface of the mandible and progresses anteriorly in the body of the mandible. In between the mandibular canal and the mental foramen, the buccal-lingual position is extremely variable. Studies have shown the buccal-lingual location is dependent on such variables as the amount of bone resorption, age, and ethnicity.23 The buccal-lingual position of the mandibular canal is easily depicted on cross-sectional images after canal location is verified and highlighted (Fig. 4.12).
The intraosseous path of the mandibular canal is variable in the buccal-lingual position within the mandible, and a comprehensive radiographic survey (CBCT) ideally should be completed prior to implant osteotomy initiation to determine the anatomic path. A 2-millimeter safety zone between the implant and the mandibular canal should always be adhered to. Attempting to place an implant buccal or lingual to the neurovascular bundle may result in neurosensory impairment.
Inferior-Superior Mandibular Canal Locations
The vertical position of the mandibular canal below the apices of the natural teeth within the mandible is also highly variable. Therefore, generalizations cannot be made as to a constant inferior-superior position because the distance of the canal to the root apices is not consistent.24 An early classification of the vertical positions of the course of the alveolar nerve was reported by Carter and Keen.25 They described three distinct types: (1) in close approximation to the apices of the teeth, (2) a large nerve approximately in the middle of the mandible with individual nerve fibers supplying the mandibular teeth, and (3) a nerve trunk close to the inferior cortical plate with large plexuses to the mandibular teeth. After the mandibular canal is located and drawn on the reconstructed panoramic image using CBCT viewing software, the vertical position of the intraosseous path may be determined by scrolling through the cross-sectional images. The vertical position is then easily seen on individual cross sections or CBCT-generated reconstructed panoramic images (Fig. 4.13).
The intraosseous path of the mandibular canal is variable in the inferior-superior position within the mandible, and a comprehensive radiographic survey (CBCT) ideally should be completed prior to implant osteotomy. Special care should be exercised in type 1 nerves because their close approximation to the root apexes results in compromised bone height for implant placement. Immediate implant placement in the mandibular posterior when a type 1 nerve exists is not recommended. Type 3 nerves are most favorable for implant placement in the posterior mandible because the mandibular canal is positioned low in the mandible, therefore having increased available bone in height.
The inferior alveolar canal (IAC), or mandibular canal (MC), contains the neurovascular bundle, which consists of the inferior alveolar nerve, artery, vein, and lymphatic vessels. The inferior alveolar nerve bundle enters the mandibular foramen, where it transverses anteroinferiorly from lingual to buccal within the body of the mandible. A 3-D evaluation of the MC position is recommended when implant placement is going to be completed in proximity to the nerve. The most accurate assessment of the anatomic position is with CBCT because images may be enhanced via viewing software adjustments for contrast, brightness, and gray scale to help depict the anatomic location of the MC.
Radiographically, the MC appears as a linear, radiolucent shadow with or without inferior and superior radiopaque borders. Studies have shown the total length to be approximately 62.5 mm, with slightly longer measurements in males (+~2.5 mm).26 The average diameter of the MC is approximately 2.0 to 3.4 mm with the diameter being the greatest in the posterior near the mandibular foramen (entrance of inferior alveolar nerve on the lingual surface of the ramus).27 The mandibular canal becomes more ovoid as it progresses anterior towards the mental foramen.28 Location is variable depending on the patient’s race, gender, and amount of bone resorption. Usually, the MC is located on a bony ledge, the lingula, which is located on the medial surface of the ramus. Studies have shown the foramen to be located approximately 19.7 mm from the anterior border of the ramus.29 The CBCT data are used with appropriate viewing software to identify and trace the MC. The depiction of the MC enables the implant clinician to assess the position in various multiplanar and 3-D reformations. Initially, the MC is most easily drawn on the CBCT reconstructed panoramic view with location confirmation on the cross-sectional images. In most cases, the endpoints are first identified (e.g., mandibular foramen, mental foramen), then the location of the MC is extrapolated between these two landmarks.
In many instances the mandibular canal may not be easily depicted on the CBCT image; thus identification can be extremely challenging. The visibility of the MC varies significantly, even within the same individual.
The mandibular canal walls usually are not made up of compact bone, showing only a coalescence of trabecular bone with varying degrees of density.2 This complicates the determination and location of the true identification of the canal. Studies have shown that the unreliability of identifying the entire MC course is a direct result of minimal to no dense cortical plates surrounding the nerve bundle, which has been shown to occur in approximately 30% of cases. The MC has an increased wall density in the posterior region (mandibular foramen > third molar region) in comparison to the anterior region.30
With CBCT, images are susceptible to noise and artifacts, with resulting low contrast. Because of these inherent quality issues, distinguishing the mandibular canal from other aspects of the internal trabecular components of the mandibular image may be difficult. Thus, the clinician should be be proficient at utilizing available tools within the software programs to be able to identify the mandibular canal.
The mental foramen is an opening in the anterolateral aspect of the mandible, commonly in the apical space in the first and second premolar area; however, individuals may rarely exhibit the position of the foramen as anterior to the cuspid area and as far posteriorly as the bifurcation of the first molar. One of the two terminal branches of the inferior alveolar nerve is the mental nerve, which exits the mental foramen with sensory innervation to the chin, lip, and anterior gingiva. The mental foramen completes in formation after the 12th gestational week, when the mental nerve separates into several fascicles. If the mental nerve separates prior to the formation of the mental foramina, the formation of a accessory foramen may result.31 The mental foramen location, size, and number is highly variable with many dependent factors including gender, ethnic background, age, skeletal makeup.
The mental foramen may be most easily identified on axial, coronal and cross-sectional images. The relationship between the mental foramen and teeth or vital structures can be evaluated most easily on volumetric 3-D images.
The location of the mental foramen on 2-D periapical and conventional panoramic radiographs has been shown to be inaccurate because they do not show the true location in most cases. Additionally, when placing immediate implants in the premolar region, angulation and avoidance of the foramen should be noted because the mental foramen has been shown to be located coronal to the root apex of premolars in 25% to 38% of patients. In most cases when an implant is to be treatment planned in approximation of the foramen, a CBCT evaluation is recommended. A CBCT image will always provide excellent visualization of the mental foramen because it exits the buccal cortical plate regardless of how poorly the canal may be visualized prior to the exit point. The identification difficulty occurs when the canal is poorly visualized by an absence of cortication or in D3 or D4 bone quality, where very little internal trabeculation is visible. Such a bony pattern makes it difficult to trace the canal posterior from the mental foramen (Fig. 4.14).
Mandibular Ramus (Donor Site for Autogenous Grafting)
The mandibular ramus area has become a very popular donor site for autogenous onlay and trephine bone grafting. This anatomic area of the mandible is extremely variable in the amount of bone present, as well as the buccal-lingual and inferior-superior position of the mandibular canal. Most commonly, the lateral aspect of the ramus is harvested as a block graft, which is used for ridge augmentation procedures.
The mandibular ramus is quadrilateral and contains two surfaces, four borders, and two processes. The lateral surface is flat with two oblique ridges, the external and internal. The masseter muscle attaches on the entire lateral ramus surface. The medial surface gives rise to the lingula, which is the entrance of the inferior alveolar nerve and associated vessels. When present, the antegonial notch, anterior to the angle of the mandible, is significant for the presence of parafunction.
The relationship between the lateral cortical plate in the ramus area and the position of the mandibular canal is easily seen on cross-sectional images after nerve location identification. Additionally, 3-D images and bone models assist in the determination of the osseous morphology in this region to help the clinician select the most appropriate graft site.
Historically, standard 2-D radiographs for evaluation of the ramus area as a donor site included conventional panoramic images, in which the location of the external oblique and the mandibular canal may be noted. However, 2-D evaluation of this area can be very difficult to use for determination of the amount of bone present and position of the mandibular canal. With this procedure it is vital that the implant clinician be able to completely determine the exact position of the mandibular canal with respect to the external oblique ridge and the lateral cortical bone. Overestimation of available bone can result in increased morbidity, so a more accurate representation of this area is with the use of CBCT (Fig. 4.15).
Mandibular Symphysis (Implant Placement and Bone Donor Site)
The mandibular symphysis area is a common area for implant placement as well as a donor site for autogenous block grafting. This anatomic region has been shown to be one of the most ideal intraoral donor sites for bone harvesting. However, the mandibular symphysis is susceptible to nonuniform bone resorption and contains various anatomic variations that may lead to surgical complications.
The anterior surface of the mandible is termed the mandibular symphysis. A ridge divides the right and left side and inferiorly forms the triangular eminence of the mental protuberance. The elevated center of this depressed area forms the mental tubercle, which is the origin of the mentalis muscles. This area should be evaluated on cross-sectional, axial, and 3-D images.
Two-dimensional imaging of this area should only be used as a preliminary evaluation for bone quantity determination. Poor angulation, bony undercuts, and measurements cannot be determined with 2-D radiography. CBCT imaging is highly recommended to prevent implant malposition or overestimation of available bone for harvest procedures, which may lead to increased complications (Fig. 4.16).
Anatomic Variants of the Mandibular Anatomy
As the mental nerve proceeds anteriorly in the mandible, it may on occasion extend beyond the anterior boundary of the mental foramen. This endosteal curved loop is proximal to the mental foramen and exits distally through the mental foramen and is termed an “anterior loop.” Studies have shown a prevalence of approximately 35% to 50 %, with a mean distance of 1.16 mm anteriorly to the foramen.32 Clinically, an anterior loop may be determined by probing within the mental foramen in a posterior direction; however, this necessitates full reflection of the mental foramen.
An anterior loop is difficult to identify and cannot be determined accurately with 2-D radiography. High false-positive and false-negative results have been noted on conventional panoramic and periapical radiographs. To identify an anterior loop on a reformatted CBCT image, the mandibular canal must be highlighted, including the cross-sectional image depicting the mental foramen slice. The anterior part of the mental foramen is marked with a constant perpendicular line (line will remain constant throughout all the images). In sequential axial images, scrolling from superior to inferior, any component of the nerve anterior to the constant line is considered a anterior extension. If an anterior loop exists, the highlighted nerve will be anterior to the perpendicular line (Fig. 4.17).
Determining the presence of an anterior loop is critical when placing implants anterior to the mental foramen. Inability to establish the existence of an anterior loop may result in implant placement too close to the mental nerve, resulting in possible neurosensory impairment and related complications.
In approximately 6.6% to 12.4 % of patients, an accessory (double) foramen is present with an average diameter of 1.0 mm.33–35 Special care should be noted to evaluate for an accessory canal because it may contain components of one of the three branches of the mental nerve. Accessory foramens are believed to be the result of early branching of the inferior alveolar nerve, prior to exiting the mental foramen during the 12th week of gestation.36
The ideal technique to determine an accessory foramen is evaluation of coronal images along with evaluation of the 3-D image. In the coronal image, the mandibular foramen will be shown bifurcating into two canals, resulting in the presence of two foramina. The evaluation of 3-D images will easily depict two canals. Normally, accessory canals are located superior and distal to the mental foramen (Fig. 4.18).
In the majority of patients, small accessory foramina usually contain a small branch of the mental nerve, which is not problematic because of cross innervation. However, in some cases, a larger branch of the mental nerve (equal or larger size foramen) may exit the mental foramen. If a larger accessory foramen is present and resultant damage to the nerve exists, possible neurosensory impairment is possible. The larger accessory foramens are sometimes termed “double foramens.”
Lingual Concavities (Anterior/Posterior)
The trajectory/angulation of the mandible along with inherent undercuts pose a significant problem to the implant clinician. Lingual concavities may occur in the anterior region as an hourglass or constriction of the mandibular bone. Butera has shown the incidence to be approximately 4% of patients, which is most likely genetic or developmental in origin.37 In the posterior region, concavities are much more common, resulting in undercuts in approximately 35% of patients.38 Because of these undercuts, implant placement may be difficult and perforation of the lingual plate may result.
Anterior undercuts are most easily seen in cross-sectional and 3-D images (Fig. 4.19A).
In the anterior region, perforation of the bony plates of the mandible during implant osteotomies may lead to extensive bleeding from sublingual vessels. A significant plexus of sublingual and submental arteries may lead to life-threatening floor-of-the-mouth hematoma formation. Therefore, a thorough CBCT examination will, with interactive treatment planning, determine the exact location and angulation for safe implant placement.
Posterior undercuts are most easily seen in cross-sectional and 3-D images.
In the posterior region, overestimation of available bone is a common complication. If an implant osteotomy is completed in this area, perforation of the lingual plate may result, leading to possible bleeding and possible implant morbidity. Life-threatening lingual bleeding may occur as a result of blood vessel injury leading to bleeding into the soft tissues. Additionally, damage to the lingual nerve may occur upon perforation of the lingual cortical plate. Case reports have been published revealing loss of the implant body into the sublingual space when large undercuts are present (see Fig. 4.19B).
Hypomineralization of the Mandibular Canal
Studies have shown in 20.8% of CBCT scans the mandibular canal walls are hypomineralized.39 This often results in poor localization of the mandibular canal and is sometimes an early indication of osteopenia or osteoporosis. A thin cortical outline, poorly defined internal bony trabecular pattern, and variable density within the cortical layer are potential indications of osteoporosis/osteopenia. In 30% of cases, no superior cortical plate is present, which complicates the identification of the true location (see Fig. 4.14 C–D).
The brightness and contrast may be altered using imaging software to more clearly define the canal walls. Ideally, the mandibular canal may be seen easiest in the panoramic or cross-sectional images.