Benefits of CBCT for orthodontic assessment

The benefits of CBCT for orthodontic assessment include accuracy of image geometry. Clinicians have learned to deal with the inherent image magnification and distortion that is part of 2-dimensional radiography. With lateral cephalograms, structures on the left side are magnified less than the same structures on the right because of proximity to the film. With panoramic imaging, the amounts of horizontal and vertical magnification vary at different rates as objects are displaced from the focal trough. However, CBCT offers the distinct advantage of 1:1 geometry, which allows accurate measurements of objects and dimensions. The accuracy and reliability of measurements from CBCT images have been demonstrated, allowing precise assessment of unerupted tooth sizes, bony dimensions in all 3 planes of space, and even soft-tissue anthropometric measurements—things that are all important in orthodontic diagnosis and treatment planning. Additionally, to allow use of our historic growth and normative data, it has been shown that landmarks can be located reliably on cephalometric images that are generated from the CBCT volumes.

Other benefits include the localization of ectopic teeth and the assessment of root resorption. The accurate localization of ectopic, impacted, and supernumerary teeth is vital to the development of a patient-specific treatment plan with the best chance of success. There seems to be little debate in the literature that CBCT is superior for localization compared with conventional imaging methods. One study indicated that this improved localization and space estimation does result in changes in diagnosis and treatment recommendations. Another study analyzed the “failed” treatment of 37 impacted canines, successfully delivering the canine in about 70% of these cases because of careful diagnosis and 3-dimensional imaging. Initially, failure occurred because of mistaken localization and directional traction in 40.5% of the patients. There is also increasing evidence that assessment of root resorption, both from ectopically erupting teeth and as a side effect of orthodontic treatment, can best be done with CBCT, since much resorption occurs in a slanted direction that is not readily imaged without the use of a tomographic technique.

In orthodontics, an asymmetric malocclusion is one of the most difficult problems to diagnose and treat. Before CBCT, the skeletal and dental contributions to the problem were assessed from clinical examinations, study models, and perhaps a posteroanterior cephalogram. Although these provided meaningful insight for a diagnosis, the CBCT volume allows direct measurement of the transverse dimensions and the relative positions of the teeth within the skeletal components, and has been judged to be superior to previous methods. We recently conducted a study using CBCT to objectively assess asymmetry and found that this method is potentially useful for clinicians.

Imaging of the temporomandibular joint has not been common practice for asymptomatic orthodontic patients. The view of the condyle and the fossa on a panoramic film has been used as a screening tool with subsequent specific imaging ordered for the temporomandibular joint if bony changes are noted. The CBCT volume used for orthodontic assessment will generally include the right and left temporomandibular joints, and therefore they are available for routine review. The orthodontist can screen for bony changes and get an indication of condylar position from this review. A recent review of nearly 200 consecutive orthodontic patients at the University of Minnesota showed that 18% had incidental temporomandibular joint findings noted by a radiologist that were significant enough for further follow-up or referral. A similar result for incidental temporomandibular joint findings was reported by researchers in North Carolina in an older, nonorthodontic population.

Traditional 2-dimensional cephalometric imaging has been limited in its ability to assess airway dimensions, and our view of the sinuses has been limited to the tomographic slice on the panoramic image. With CBCT imaging, 3-dimensional views of the airway and the sinuses are clearly visible and measurable. Although as a specialty we still struggle to understand the impact of the airway on the growth and development of our patients, we all understand that breathing has primary biologic importance and that significant postural adaptations to airway problems can cause undesirable growth changes. This is another item that was frequently noted by the radiologist in our study of incidental findings in orthodontic patients. Nearly half our patient population had sinus or airway findings noted, ranging from relatively minor sinusitis or polyps to complete opacification of the maxillary sinuses.

The need to assess the periodontal bone levels before orthodontic treatment has always been important and has been emphasized with the American Board of Orthodontics’ requirement to include a formal periodontal evaluation for all patients over the age of 18 years or for those with signs of periodontal disease. Imaging suggestions for this evaluation include a panoramic image supplemented with bitewing and anterior periapical radiographs, or a full-mouth series of radiographs including periapicals and bitewings, or images taken from a CBCT volume. The first 2 are supplemental images that require additional exposure, whereas the images from the CBCT are reconstructed as needed from the acquired volume with no additional exposure required. Misch et al reported that CBCT imaging provides a significant advantage over conventional radiographs for periodontal assessment because it allows for the measurement of buccal and lingual defects as well as interproximal defects. Other investigators have also found that CBCT-derived images offer advantages for periodontal assessment.

A truly unexpected result from our study of CBCT incidental findings in orthodontic patients was the 10% frequency of significant endodontic findings: apical periodontitis, apical radiolucency, internal or external root resorption, or retained root tips. These are important items to assess before final orthodontic planning—items that could dramatically alter the treatment plan. I was convinced that this high degree of endodontic involvement was most likely a statistical anomaly since it did not correspond with my clinical experience; however, Price et al recently reported a similar prevalence of endodontic findings in a different population, lending additional support to the result.

Recent reports have suggested that certain regions are more desirable as placement sites for temporary skeletal anchorage devices. These recommen-dations are based on average cortical bone thickness and bone depth determined from CBCT images of skulls or patients. Although these general recommen-dations are helpful, they do not provide patient-specific information. When patients have CBCT imaging as part of their initial record set, areas that might be considered as placement sites for temporary skeletal anchorage devices can be individually assessed for bone quality without the cost or the inconvenience of additional imaging.

In addition to the items listed above, there are reports that suggest future benefits of CBCT imaging related to risk management. The bone density measured on CT imaging has been correlated with the risk of neurosensory disturbance after sagittal split mandibular advancement. Although 1 limitation of CBCT is that bone density in Hounsfield units is not as standardized as medical computed tomography, the use of fractal dimension analysis of CBCT images has recently been described as a promising tool for detecting bone changes caused by bisphosphonates.

Recently, the fabrication of custom lingual orthodontic appliances has been demonstrated by using CBCT image data with existing technology to virtually plan a patient’s treatment and manufacture the custom appliances with 3-dimensional printing technology. Such advances appear to be rapid, and promise efficient and effective treatment that is specific for each patient. Orametrix (Richardson, Tex) has been using CBCT technology for the last several years to provide the data necessary for planning and executing technology-assisted treatment through its SureSmile system.

Costs of CBCT for orthodontic assessment

The general argument against using CBCT as a standard imaging protocol for comprehensive orthodontic treatment centers on the radiation burden to patients. Most current recommendations are that CBCT should be used as an adjunct imaging technique when conventional 2-dimensional imaging proves to be inadequate. Comparison of effective radiation dose levels is difficult because of the many CBCT machines now available and the fact that new scanning protocols for the machines are constantly being implemented based on software modifications. In addition, the International Commission on Radiological Protection released updated guidelines in 2007 that added salivary glands, oral mucosa, and airway tissues to the dose equation, and this raised effective dose calculations from 32% to 422%. Therefore, comparisons of various imaging protocols and machines should be done by using those guidelines and not the previous 1990 guidelines.

The question of primary importance is the radiation burden of a CBCT image relative to a conventional lateral cephalogram, a panoramic radiograph, and any supplemental films that are required. To answer this question specifically for our facilities at the University of Minnesota, we conducted dosimetry testing of our CBCT machine and our conventional 2-dimensional digital radiography equipment. We found that the CBCT imaging normally used for comprehensive orthodontic patients was about 65 μSv compared with about 26 μSv for a lateral cephalogram and a panoramic image taken on our digital machine. Subsequent to our testing, a new low-dose scan protocol has been added to the CBCT machine that provides the needed orthodontic diagnostic information for an estimated 35 to 40 μSv (based on our data adjusted for reduced milliampere-second exposure).

These rapid advances in CBCT technology have resulted in 3-dimensional images that have about 2% or less of annual background radiation, with only slightly more than conventional orthodontic imaging without any supplemental radiographs. If full-mouth intraoral radiographs are taken to assess the periodontal status of adults, CBCT imaging typically reduces the patient dose.

There is little published information regarding the financial cost of CBCT technology used for orthodontics. From my personal experience, the transition to CBCT imaging for orthodontic assessment did not add to the patient cost of treatment in our university clinic or our private practice. Obviously, an investment must be made in the equipment, and many practitioners have difficulty justifying the return on this investment, since efficiency and income are not directly affected. However, in my opinion, the confidence gained in treatment decisions and the greater ability for patients to visualize problems dramatically improves my practice.