Lateral cephalometric radiographs are most commonly used as a diagnostic tool in orthognathic surgery as well as orthodontic treatment. But the limitation of lateral cephalograms is its 2 dimensional nature whereas the human body is 3 dimensional. Conventional 2D lateral cephalograms have numerous drawbacks in terms of investigating the changes in the alveolar bone and roots, particularly in the anterior region, as a consequence of the midsagittal projection. Additionally its accuracy is questionable as it has projection errors.
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The use of computed tomography in 3D imaging of human body is available in the field of medicine since last 30 years. CT scanning is the three dimensional imaging technique giving quantitative assessments of the buccal and lingual cortical bone plates and labiolingual width of alveolar bone with elevated accuracy and precision. But the use of computed tomography in dentistry is limited because the amount of radiation exposure with this technology is very high.
Since the invention of Cone Beam Computed Tomography, the amount of radiation exposure in the patient is reduced. This enhances its use in obtaining the 3D images of the craniofacial structures. This technology helps in visualizing the hard and soft tissues of the craniofacial structures from various perspectives and helps in thorough diagnosis and treatment planning of orthognathic surgery and orthodontic patients. The principles of CBCT and its use in the field of orthodontics will be discussed in detail in this paper.
KEYWORDS: Cone beam computed tomography, Surgical orthodontics.
Orthodontics is a field, which places a significant amount of emphasis on the modification of abnormal craniofacial growth patterns, in addition to the correction of dental malrelationships. Successful orthodontic and surgical treatment of such anomalies naturally requires efficient and reliable imaging of the structures of the cranial complex. Ever since the advent ofÂ the Bolton cephalometer in 1931 , orthodontists have consistently used lateral cephalograms in evaluation of treatment as well as in diagnosis and treatment planning. In addition, postero-anterior, panoramic, occlusal and peri-apical views of the skull and teeth have been used as and when required to aid in the diagnosis. All these additional radiographic views add up to a significant quantity of radiation exposure to the patient, which can and should be avoided if possible. Also, the 2 dimensional nature of these conventional radiographic views imposes further limitations such as overlap, leading to lack of visualization of individual structures, errors due to projection, as well as the incapability to identify true skeletal asymmetries when present . Thus, it has been recognized for some time now that three- dimensional imaging of the skull is the need of the hour in orthodontics.
3D Computed tomography in Orthodontics
The use of computed tomography in 3D imaging of human body is available in the field of medicine since last 30 years. But the high radiation exposure and the prohibitive cost of this technology have till now precluded its use in orthodontics. However, recent advances in CT technology have seen a dramatic decrease in radiation as well as in cost, making it a viable and desirable alternative to traditional imaging. The newer CT machines can now perform a complete scan of the head in just a few seconds and provide the patient an effective dose of only 50 micro-Sieverts, compared with about 2000 from a conventional CT scan of the entire head . This follows the ALARA principle (As low as reasonably acceptable) for radiation exposure, of the American Dental Association.
Radiation exposures are further reduced when one believes that a single CT image can replace a number of conventional radiographs that are now considered essential for almost every orthodontic procedure. Thus, the routine use of CT scans for orthodontic diagnosis may not be very far away .
Cone Beam Computed Tomography (CBCT): Technique and Advantages
Conventional CT machines acquire image data by using either a single narrow X-ray beam or a thin broad fan-shaped X-ray beam. These X-ray beams rotate around the patient in a circular or spiral path as the patient moves through the scanning machine or as the rotating beam passes over the patient. A series of detectors register the attenuation of these X rays, and from the data gathered, the machine reconstructs the internal structure of the patient’s body . 3D data of the patient’s anatomical structures is stored in the form of Voxels. These can be thought of as tiny cubes arranged next to each other. The brightness of each cube represents the density of the corresponding anatomic structure. Obtaining the final 3D object from the raw data requires a time consuming process called rendering, which is achieved using computer algorithms .
However, a new digital imaging breakthrough, the NewTom QR 9000 Volume Scanner (Verona, Italy) is now available for clinical practice. This CT scanner uses a cone-shaped X-ray beam that is large enough to encompass the region of interest. It produces a much focused beam, minimizing scatter, thus reducing the absorbed radiation dose to 45 microSieverts . In contrast to conventional CT imaging the patient remains stationary throughout the procedure. In a single scan, the X-ray source and a reciprocating X-ray sensor rotate around the patient’s head and acquire 360 pictures (1 image per degree of rotation) in 17 seconds of exposure time. The 360 acquired images undergo a primary reconstruction to mathematically replicate the patient’s anatomy into a single 3 dimensional volume. Further, the software allows for reformatting and viewing the image data from any point of view in all 3 dimensions. Thus, from a single scan, frontal, lateral, panoramic and other views can be created. Additionally, the anatomy can be peeled away layer by layer to locate the desired section. A major advantage of CBCT-generated cephalograms is the ability to excise unwanted structures such as the cervical spine and occiput, avoiding superimposition of irrelevant structures, and providing a remarkably clear image of pertinent maxillo-facial structures .
Uses of 3-dimensional computed tomography in Orthodontics
- Assessment of alveolar bone
The alveolar bone height is particularly important in adults and periodontally compromised patients. Assessment of available bone is necessary prior to arch expansion or labial movement of incisors. Surface irregularities due to ectopic teeth, bone dehiscences, salivary gland invaginations and other abnormalities can also be visualized in three- dimensional images. A new resource for occlusal assessment is the lingual view-as if the clinician were looking from the back of the patient’s head into the oral cavity.
- Impacted tooth position
Impaction (or failure of eruption) of teeth is a common orthodontic problem, which requires precise localization for the purpose of surgical exposure and guidance into the oral cavity. Conventional views such as the occlusal and periapical views cannot precisely locate such teeth. CT scans with 3 dimensional reconstructions provide an excellent means to accurately locate such teeth. In such a study done on a 21 year old girl, by Ravinder et al. , an impacted maxillary left canine was accurately localized, and revealed to be in a horizontal, palatal position. This was done, by obtaining various views, such as plain axial, sagittal CT slices, as well as superior, sagittal and superior- oblique views of the maxillary dentition. Walker, Enciso and Mah  have also reported the advantages of 3D imaging in the management of impacted canines. In addition, cysts of the jaws, supernumeraries and ectopic/buried teeth can also be visualized using this technique.
- Temporomandibular Joint Assessment
Coronal, sagittal and axial views of the temporomandibular joint obtained from the CT scan can be correlated with the occlusal views. Functional shift of the joints can be occasionally detected as differences between the left and right TMJ views. In addition, 3D CT studies on patients who underwent orthognathic surgery, have allowed better evaluation of post surgical condylar resorption .
- Surgical patients including syndromes and clefts
Surgical planning for patients with jaw asymmetry, e.g. Hemifacial Microsomia can benefit from 3D imaging. This allows measurement of true jaw dimensions without the customary problems of magnification, superimposition and distortion, inherent in 2 D cephalograms. Use of virtual “cutting tools” and “collision tools” to plan out surgery on the 3D images, means that orthognathic surgery as well as distraction osteogenesis can be carried out with a far greater degree of precision, leading to more predictable results. 
- Facial Analysis
A conventional photograph is a simple two- dimensional representation that is not correlated with the supporting skeleton. The 3D volume can provide any frontal, lateral or user-defined view of the face, and by altering the translucency of the image, one can determine the exact relationship of the soft tissues to the skeleton. This has major implications in the planning of tooth movements, orthodontic extractions, orthognathic surgery, and other therapies that could alter facial appearance.
- Tongue size and Posture
Volume measurements of the tongue could provide a more objective assessment of size, to aid in the diagnosis of arch-width discrepancies and open bites.
- Airway assessment
Volume measurements of the airway could evaluate patency, particularly in patients suspected of adenoid hypertrophy, mouth-breathing or obstructive sleep apnea. Turbinates and nasal morphology can also be evidently seen in CT scans. This would mark a significant improvement over the use of 2 dimensional lateral cephalograms.
- Root resorption
3D CT images can show areas of root resorption on central and lateral incisors adjacent to impacted canine teeth. Walker, Enciso and Mah  showed that incisor resorption adjacent to impacted canines is present in 66.7% of lateral incisors and 11.1 % of central incisors. A correlation was found between the proximity of impacted canines to the incisors and their resorption. Current CT machines may have too low resolution to detect early stages of root resorption as a result of orthodontic movement, but this may be possible in the future .
- Planning for placement of dental implants
Osseo-integrated implants may be used in orthodontics either for the prosthetic replacement of missing teeth, or as stationary anchorage to facilitate tooth movement. Optimal spacing as well as correct root angulations of adjacent teeth must be achieved in order to successfully place dental implants . Cone beam CT scanning could be used to accurately assess space availability, root angulations, as well as the quality of alveolar bone at the implant site. This would replace the use of panoramic and peri-apical radiographs currently used for the purpose.
- Cephalometric Analysis
Conventional 2D cephalometric measurements can also be carried out, by rendering a 2D projection of the 3 D data, resembling a radiograph. For bilateral cephalometric landmarks, the computer can calculate the midpoint between them. Certainly, new cephalometric landmarks and analyses based on 3D data shall be developed in the near future.
3D computed tomography represents the cutting edge of orthodontic imaging and diagnostic capability. While mainstream orthodontists are still living and practicing in a 2D world, orthodontic residents in many universities are becoming 3D sense. The several distinct advantages of 3D CT imaging, with ever-decreasing radiation doses, mean that this is where the future of orthodontic imaging lies.
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