CC
A 14-year-old female is referred for exposure and bracketing of her impacted canines.
HPI
The patient’s parent states that she had her primary canines removed at a younger age, with subsequent failure of her permanent canines to erupt. Her orthodontist has recommended exposure and bracketing of the impacted canines to facilitate correct eruption and to avoid possible root resorption of her adjacent lateral incisors. The patient is otherwise asymptomatic.
PMHX/PDHX/medications/allergies/SH/FH
Noncontributory.
Examination
General. The patient is well developed, well nourished, and in no apparent distress.
Intraoral. Teeth #6 and #11 are not visible in the mouth. There is a questionable bulge of the maxillary anterior palate on the left and right sides; however, definitive localization of the impacted canines by palpation is not readily apparent.
Maxillary canines are the second most frequently impacted teeth, after the third molars. The prevalence is 1% to 3% of the population. Approximately 60% to 85% of these impactions are palatal. Impacted canines are seen more commonly in females.
Imaging
Panoramic imaging revealed that teeth #6 and #11 were impacted and in close association with the adjacent lateral incisors ( Fig. 6.1 ). The third molars, also present, were full bone impactions. Given these findings, the risks and benefits of additional imaging were discussed with the patient’s parents, who agreed to a cone-beam computed tomography (CBCT) scan to evaluate the maxillary canines ( Fig. 6.2 ).
In approximately 38% to 67% of cases, impacted canines can cause varying degrees of resorption of adjacent teeth, especially the lateral incisor. Root resorption can be difficult to diagnose with traditional two-dimensional (2D) radiography, especially when the canine is in a direct palatal or facial position to the lateral incisor roots.
Two-dimensional imaging for surgical or orthodontic planning has several limitations, such as image magnification and distortion, superimposition of structures, and misinterpretation. Three-dimensional (3D) imaging allows the surgeon to determine the best clinical approach and reduces the invasiveness of surgery. Additionally, it allows the orthodontist to determine what orthodontic force vector should be applied to move the canine efficiently, thus reducing involvement of adjacent teeth.
CBCT offers a 3D view that can provide more accurate information about the size, shape, angulation, associated pathology (cysts, tumors, resorption of adjacent teeth), and relationship to adjacent structures (inferior alveolar nerve canal, sinus). CBCT software allows anatomic entities in the 3D image to be differentiated by assigning each a color (known as a mask ). The masks can be turned off, allowing the clinician a better appreciation of the anatomy. This type of reconstruction can be time-consuming, but it can be referred to third-party companies.
Assessment
Impacted maxillary canines needing surgically assisted exposure and bracketing for orthodontic correction.
Treatment
The precise location of the maxillary canines was determined. No readily apparent resorption of the lateral incisors was noted. (It is possible to underestimate root resorption, owing to inadequate visualization secondary to the limitations of CBCT, such as selecting a large field of view [FOV], which diminishes the resolution of the image.) The 3D reconstruction served two important purposes. It allowed the surgeon to easily appreciate the anatomy, and it also provided a visual aid that enabled the patient to easily understand the anatomic configuration of her problem; this in turn facilitated discussion of the procedure and its risks and benefits with the patient and her parents. Subsequently, the impacted canines were exposed and bracketed without incident in a standard fashion. (See Chapter 28.)
Complications
Clinicians must abide by the “as low as reasonably achievable” principle when ordering an imaging modality for a patient. Exposing the patient to the radiation must provide an image with a diagnostic value that is greater than the detriment the radiation exposure may cause. Not every patient requires CBCT because the technique does expose the patient to radiation and results in increased cost. The American Dental Association Council on Scientific Affairs suggests that CBCT use should be based on professional judgment, and clinicians must optimize technical factors, such as using the smallest FOV possible for diagnostic purposes and using appropriate personal protective shielding.
Although there was a sixfold increase in medical radiation exposure between 1980 and 2006 in the United States, radiation exposure per capita decreased 20% between 2006 and 2016. However, ionizing radiation is also found in the natural environment in the form of cosmic rays or radon, which contributes to overall exposure. At doses used in diagnostic and interventional procedures, ionizing radiation may cause DNA damage and increase the risk for future cancer. The probability of effects arising from ionizing radiation (e.g., future cancer, cataracts) is a function of the total radiation dose, although the severity of such effects is also influenced by other factors, such as genetics. The linear no-threshold (LNT) model is the most widely used theoretical dose–response model that assumes that any exposure to ionizing radiation can induce future cancer. However, the accuracy of the model has been called into question recently. This model proposes that there is no threshold dose for radiation-induced cancer, and a small dose (0.1 mSv) is associated with an increased cancer risk. The most credible studies comparing radiation dose–response with carcinogenesis mainly involve doses of 1 Sv or less, which are magnitudes greater than those encountered in diagnostic imaging. How the LNT model applies to low-dose radiation exposure is still unclear and requires further investigation.
The concentration of ionizing radiation in a specific volume of air is a measure of radiation exposure and is expressed in roentgens (R). The amount of radiation absorbed by a specific tissue is measured in grays (Gy) or rads. The effective dose is measured in sieverts (Sv) (1 Sv = 1000 mSv = 1,000,000 μSv), which provides a quantification of the potential radiobiologic detriment caused by radiation and takes into account tissue-weighting factors defined by the International Commission on Radiological Protection (IRCP). Calculating the effective dose allows for comparison across different imaging modalities and from partial and whole-body exposure. The IRCP estimates a 4% to 5% increased relative risk of fatal cancer after an average person receives a whole-body radiation dose of 1 Sv. Some models predict that 1 in 1000 persons exposed to 10 mSv (10,000 μSv) will develop cancer as a result of that single exposure.
For comparison purposes, consider the following estimated effective doses:
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Single chest x-ray: 0.1 mSv
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United States coast-to-coast roundtrip flight: 0.03 mSv
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Annual individual radiation dose from the natural background: 3 mSv
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Intraoral radiograph: 0.043 mSv (43 μSv)
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Computed tomography (CT) scan of the head and neck: 1.4 mSv
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CT scan of the chest: 5.4 mSv
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CT angiography (noncardiac): 5.4 mSv
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CBCT (maxillofacial, standard settings): 0.176 mSv
The radiation risk with many newer CBCT machines is lower than that for the most common intraoral full mouth series; therefore, it may be possible, when indicated, to use CBCT with select intraoral images as an option for dental treatment planning in the future.
Currently, the results for the use of CBCT for caries detection are mixed. CBCT for this purpose is limited to nonrestored teeth and may be used diagnostically for occlusal caries and deep lesions into the dentin. Because of beam hardening, CBCT for caries detection has a high sensitivity. For periodontics, CBCT promises to be superior to 2D imaging for the visualization of bone topography and lesion architecture (intrabony defects and furcations) and measurement of alveolar crest height, but defect width, depth, and type (vertical vs horizontal defects) were similar in both CBCT and 2D imaging methods. Restorations within the dentition can obscure views of the alveolar crest.
CBCT for endodontic is gaining in popularity. It has been shown to be useful for detecting apical lesions and root fractures, canal identification, characterizing internal and external resorption, facilitating apical surgery, and assessing outcomes of root canal treatment. In orthodontics, CBCT can be used to evaluate the shape and function of the maxillofacial complex, and it provides a powerful tool for the visualization of root angulations.
Adjacent anatomy outside the region of interest is usually captured with CBCT. Given the volume of tissue that is exposed and readily available for review, there is a moral, ethical, and legal responsibility attached to the interpretation of the volumetric data set. Because of the complexity of the anatomy of the maxillofacial area, review of the images by an appropriately trained practitioner is prudent.
Computed tomographic scanners consist of an x-ray source and detector mounted on a rotating gantry. A divergent cone-shaped source of radiation is directed through a defined region of interest (ROI) while the residual attenuated radiation beam is projected onto an area x-ray detector on the opposite side. The x-ray source and detector rotate around the rotation center within the center of the ROI. The detector records the residual x-rays after attenuation by the patient’s tissues, which is known as the raw data. These raw data are reconstructed by a computer algorithm to generate a volumetric data set that can be used to provide primary reconstruction images into the three orthogonal planes: axial, sagittal, and coronal.
The most common algorithm for reconstructing 3D objects (cone-beam reconstruction) from cone-beam projections is the Feldkamp, Davis, and Kress (FDK) method, which is used by many research groups and commercial vendors. This algorithm has some limitations, such as distortion in the noncentral transverse plane, resolution degradation in the longitudinal direction, and a high computational time required to perform reconstructions. To address these problems, other algorithms and cone-beam geometries are being developed that will likely be incorporated into future machines. One such algorithm is the Neural Network Feldkamp-Davis-Kress (NN-FDK) method. This algorithm uses machine learning to improve reconstruction accuracy and computational efficiency even in cases with high-noise, low projection angles, and large cone angles.
Gaêta-Araujo et al. describe the features of 279 currently and formerly available CBCT models in 2020 as well as a recommended CBCT feature standardization method.
Discussion
Dedicated CBCT of the maxillofacial region has created a revolution in all fields of dentistry and has expanded the role of imaging from diagnosis to image guidance for many surgical procedures. CBCT has eliminated some of the inherent limitations of 2D images, such as magnification, distortion, superimposition, and misrepresentation.
CBCT uses a cone beam–shaped source of ionizing radiation, and the beam is directed through the middle of the area of interest (FOV). The beam covers the entire FOV; therefore, only one rotation of the gantry is required. Traditional medical CT uses a fan-shaped beam to acquire individual image slices (each slice requires a separate scan), which are then stacked to obtain a 3D representation. CBCT usually results in a lower dose of radiation than CT, but doses vary widely among different systems and among different imaging protocols (slice thickness, FOV, mAs, kVp, scan time). It is recommended that clinicians use appropriate selection criteria, along with imaging protocols that use the minimal doses that ensure acceptable diagnostic qualities ( Table 6.1 ).
