More Information, With Less Radiation by Drs. Jay Burton and Jack Fisher

Introduction
The benefits of using cone beam computed tomography (CBCT) in dentistry—and specifically in orthodontics—have become evident as more doctors adopt this technology. In our private practices, which are limited to orthodontics, CBCT has truly been a game-changer, and has pushed us to be more than just a "tooth mover." We honestly can't imagine practicing without it.

We now look at things we never thought we'd be seeing or considering while we work up a diagnosis and treatment plan for a patient. The technology provides considerably more information than traditional 2-D radiography can; we no longer need to guess or wonder what's going on in a blurry area of a panoramic radiograph.

Because of modern ultra-low-dose (ULD) technology, orthodontists who use CBCT gain that additional information while exposing patients to less radiation than traditional 2-D radiography. The photos you'll see in the accompanying case study, for example, were taken with the Planmeca ProMax Mid CBCT unit, with an average dose reduction of 77 percent when compared with standard protocols.1

Case study
Here's a situation where a CBCT scan could have made a significant difference if it had been taken sooner; the patient likely would have been able to keep her laterals and avoid canine substitution. Without a CBCT scan, you wouldn't recognize there may be a problem with the laterals, because things appear fine.

This case shows how the lack of appropriate imaging/monitoring can be disastrous for a patient. This could have been prevented had serial extractions or expansion been completed; the extractions would have provided the necessary room for the maxillary canines to redirect, thereby potentially saving the laterals. At the very least, periodic radiographs should have been taken to reveal whether damage to the maxillary laterals was occurring/continuing. Additionally, the patient should have received a referral to an orthodontist for monitoring. Unfortunately for this patient, none of this was done.

Changing practice for the better
We all care about providing the highest standard of care to our patients. In our practices, CBCT changes the treatment plan for every case, if for no other reason than looking at the alveolar boundaries (Fig. 9) before deciding on treatments and mechanics. CBCT changes the way we treat, what types of cases we will or won't treat, and can even change the timing of treatment, as far as staging is concerned and whether that treatment should be limited or comprehensive.

For those who use CBCT on a regular basis in their practices, the benefits are numerous. We've summarized some major ones below from an article by J.K. Mah et al:2

• Having a 3-D representation of the dental and craniofacial structures while doing diagnostic work-up and treatment planning. Before CBCT, the only 3-D records we had when treatment planning were the patient's stone models. With this technology, we have all dental and craniofacial structures available to us.
• The ability to reformat or reorient images in different angles and perspectives. This gives us better information with which to make treatment decisions. In the case presented, Figs. 3–6 (p. 43) show how the MPR view can be reoriented to reveal much more than any type of 2-D radiography ever could. Other examples of this type of use would be the localization of impacted or supernumerary teeth, TMJ tomographic images, normal panoramic and lateral cephalometric images, PA cephalometric and submentovertex images and alveolar boundary images (Fig. 10).
• Distortion-free images, with no magnification error. Because our profession is judged in millimeters, the one-to-one nature of these images is particularly helpful. The ability to do superimposition over one-to-one images allows for better information without the distortion found in 2-D radiography.
• The CBCT volume can be used to generate data for not only diagnostic purposes but also for medical modeling purposes, as well as the manufacture of certain appliances. .STL files can be printed and used then to plan orthognathic surgery and other complicated cases that may involve more difficult treatment mechanics.

We can now take a full-field-of-view CBCT (Fig. 11) with less radiation than is required for traditional 2-D radiography. Because of the lower radiation dose, one could make the argument that ULD/LD CBCT imaging is indicated in all patients.

Indications
Several types of cases show some benefit to the use of this technology; in many of them, indications wouldn't be known until after the CBCT has already been taken. One example of this would involve a supernumerary tooth that isn't present in a traditional panoramic radiograph, but becomes visible in a CBCT volume. Other examples include pathology, TMJ dysmorphology, alveolar boundaries, etc. (See "Indications for CBCT", at right.)

Another indication would be for focusing on the anomalies of dentition—impacted teeth, supernumerary teeth and wisdom teeth, specifically. Based on our conversations with doctors who've purchased a CBCT unit, this has been one of the main reasons that orthodontists specifically have adopted the technology. One more indication getting some attention, given the advent and use of skeletally fixated expanders, are those with transverse discrepancies. Decisions whether to use traditional rapid maxillary expanders or skeletally fixated expanders can now be made based on the amount of bone on the buccal of the teeth to which the appliance would be attached.

This leads us to the issue of dentoalveolar boundaries, which is significant because we can now consider the limitations to treatment—specifically in the AP and transverse dimensions. Examples include cases that may need extractions because of bimaxillary protrusion.

We can now consider the amount of bone available in the direction the teeth are to be moved, and even simulate the tooth movement revealing exactly how much bone would be present if that treatment plan were carried through. We must also consider now the possibility that in many cases there is a lack of bone in the direction we would like to move the teeth, and whether treatment is still indicated.

Alveolar boundary conditions (Figs. 12 and 13) are becoming an important consideration, given our new access to this information. Before the use of CBCT, it was impossible to evaluate boundary conditions radiographically. We do not plan any case without evaluating whether bone is available in the region where tooth movement is to occur, and specifically available in the direction that the movement is to take place.

Radiation concerns
One of the biggest areas of concern about CBCT technology has been the amount of radiation patients are exposed to—particularly in orthodontics, where most patients are children who are more susceptible to ionizing radiation.

Research published in 2008 reported that during a 2-D radiograph, the average radiation exposure is 14–24 microsieverts. The exposure during a lateral cephalometric head film and a PA cephalometric head film was about 5 microsieverts each.3

Typical radiographic imaging in most offices vs. our imaging protocol are compared in Table 1, which includes imaging during the three points in treatment we generally take radiographs: initial records, progress records and final records. With ULD CBCT, radiation levels can be reduced by as much as one-half from what's associated with traditional 2-D radiography.

When thinking about radiation exposure, diagnostic quality must stay at the forefront of the discussion. Some dentists are refining the ALARA principle ("as low as reasonably achievable") and instead referring to ALADA ("as low as diagnostically acceptable"). Their point is that just because a lower radiation dose may be achievable, it doesn't necessarily mean that it's diagnostically acceptable. Depending upon the indication of the scan, clinicians must use the appropriate field of view, mA and KVP settings, as well as resolution, to obtain what would be a diagnostically acceptable image.4

In our offices, initial and final records generally are taken at a resolution of 600µ, or 0.6mm voxel size. For progress records, we'll take a dental-only scan at 400µ, or 0.4mm, to evaluate tips and torques for bracket repositions. A 3-D volume as a progress record provides the added benefit of not only mesial/distal tip (as in 2-D radiography), but also the ability to look at the buccolingual angulation/torque as well.

Conclusion
As Bob Dylan sang, "For the times they are a-changin'." Because of the additional information and lower radiation associated with ULD 3-D radiography, we consider it safe to recommend its routine use for all orthodontic patients. We would choose it for the treatment of our own children. In fact, we believe that soon the use of 2-D radiography will come under fire because of the higher radiation doses patients receive from its use.

References
1. Ludlow, J.B., & Koivisto, J. (2014). Dosimetry of Orthodontic Diagnostic FOVs Using Low Dose CBCT protocol. University of North Carolina/University of Helsinki. Charlottesville: UNC/UH.
2. Mah, e. a. (2011, March). Advanced Applications of Cone Beam Computed Tomography in Orthodontics. Seminars in Orthodontics, 17(1), 57-71.
3. Ludlow JB, Davies-Ludlow LE, & White SC. (2008). Patient risk related to common dental radiographic examinations. J Am Dent Assoc(139), 1237-1243.
4. Jaju PP, P., & Jaju, S. (2015). Cone-beam computed tomography: Time to move from ALARA to ALADA. Imaging Science in Dentistry, 45(4), 263-265.

Next up: Using CBCT in your practice
Drs. Jay Burton and Jack Fisher's next article for Orthotown will discuss specifics about the effective use of CBCT technology in your office—multiplanar reconstruction, rendered images and which images are important for treatment decisions.