Cone beam CT-based imaging for evaluation of supernumeraries

This case report describes an approach for orthodontic and surgical treatment planning in the case of a mesiodens, a midline supernumerary tooth commonly seen in the maxillary arch, utilizing CBCT and 3-DVR. These steps show how the accurate transfer of critical anatomical information of the surgical site can ultimately increase treatment safety and predictability.

Introduction

CBCT is making big inroads into every discipline in our dental profession, expanding the horizons of clinical dental practice by adding a third dimension to craniofacial treatment planning.¹ As a result, there has been a rapid increase in the number of dental practitioners taking advantage of this technology, including specialists and generalists.

The addition of 3-DVR software to CBCT was designed specifically to create 3-D images of the mouth, face, jaw and skull, allowing CBCT users to import data about teeth and soft tissue. Specific 3-D images can also be selected, sliced and rotated to capture and view all crucial bones, nerve canals, joints and teeth necessary for dental treatments.

The significance of accurate orthodontic and surgical treatment planning in the case of supernumerary teeth as it pertains to critical anatomical landmarks such as the mandibular canal, maxillary sinus and adjacent teeth cannot be overstated. The amount of information provided by CBCT and 3-DVR, its accuracy and its applicability as far as enhancing patient care needs to be weighed against cost, convenience, availability and the expertise re-quired in understanding and reading the output.

Case report

Supernumerary teeth, because they are usually impacted and asymptomatic, are often fortuitous findings during the radiographic examination. Incidence reports of succedaneous supernumerary teeth range from 0.5% to just under 4%,² making them common enough that most practitioners have seen several in their career. Primary supernumeraries are reported to occur in one to two percent of the population.³

Impacted supernumeraries can be difficult to visualize because they are often slow to develop, with overlapping and superimposed structures obscuring them. This is particularly problematic when using film-based methods. Multiple films, taken at different orientations, are mentally reconstructed by the dentist. The resulting gestalt yields a presumptive localization that can be misleading or, at the least, incomplete.

For example, a patient was referred for a mesiodens removal. Prior to the availability of CBCT imaging, and based on the best available radiographic evidence at the time, that is, a panoramic radiograph, a frontal, lateral and two oblique cephalographs, the mesiodens appeared to be on the palatal side. After laying a flap and making an access opening in the bone, the surgeon was unable to locate the mesiodens.

The type of problem described above is obviated with CBCT. CBCT has given orthodontists the ability to routinely localize supernumerary teeth and adjacent structures in 3-D space at a radiation dose comparable to alternative imaging modalities that are currently in use.

CBCT

Computed tomography has been available in the medical community for decades. The size, high price, radiation dosage and cost have prohibited routine analysis of the maxillofacial region. The computers originally necessary to process the complex reconstruction algorithms took up the space of a typical dental office and required specialized operating conditions, staff and maintenance.

CBCT-based 3-D imaging was first introduced in Europe and Japan in the late 1990s.⁴ In the mid-1990s, utilizing different review methodologies, two manuscripts^5,6 forecast the future of cross-sectional 3-D imaging technologies in dentistry.

Today, the ubiquitous presence of small, powerful and affordable computers, combined with the development of large sensors that can intensify and transmutate X-ray energy into gray scale images, has put 3-D volumetric imaging of the head within the reach of many dental practitioners. And the radiation dose is a fraction of a medical CT.⁷

In similar fashion to film-based systems, CBCT systems utilize multiple exposures. However, with regard to complex reconstruction algorithms, it becomes necessary to make sense of the 448 pulsed exposures taken during the 8.9-second scan acquired by a CBCT like i-CAT 3-D imaging technology (Imaging Sciences International). Mental reconstruction must give way to technology in the face of sensory overload.

Both film-based and CBCT systems alike utilize 2-D displays of X-ray shadows, and all input goes through what is essentially a 2-D receptor—the eyes of the practitioner, but that’s where the similarities end. CBCT reconstruction images do not exhibit any of the magnification distortions inherent in planar imaging (either film-based or utilizing digital sensors), and with square voxels (pixels with depth; voxel is a fusion of the word volumetric and pixel) can improve measurements more accurately.

After a CBCT scan of a cylindrical area that includes the head, a grey-scale level is assigned to each of the voxels within that volume. In the case of a second generation CBCT like the i-CAT, these voxels are 0.3 mm sided cubes. These voxels can be rearranged to display straight or curvilinear 'slices' of regions of interest.

For example, by dragging points along a line in the axial plane (Fig. 1), one can approximate, separately, the outline of the maxilla and mandible. The program that comes with the CBCT examines all the voxels in the plane that this line defines—the mandible in this case—and 'unwraps' this vertical slice to display an image that looks like a panoramic radiograph (Fig. 2), but without the shadows of the superimposed spine, tongue, etc. The practitioner can refine the thickness of the slice on either side of this line, similar to the focal trough of a panoramic radiograph.

Right-angle sections of this 'focal trough' are also displayed (Fig. 3). These 'slices of a slice' can be placed anywhere and their thickness can be varied—down to the resolution of the machine (0.3 mm, in the case of the i-CAT).

Additional software tools have the capacity to triangulate along any combination of the coronal, axial, or sagittal axes and rotate about a 3-D rendering of that area.

Applying these techniques to the case mentioned above, it was clear that the mesiodens was tightly bounded by the roots of the centrals and the nasopalatine canal (Fig. 4). Note that the upper first bicuspids were removed as part of a Class II camouflage treatment plan and canines are being retracted (Fig. 2). Before distalizing the incisors, a midline diastema will be created to allow access to the mesiodens from the buccal.

3-DVR software was also utilized in this case (Fig. 5), visualizing the mesiodens triangulated by the upper central roots. By rotating the image, the surgeon is able to view both buccal and palatal angles, ultimately increasing the surgical treatment predictability, success and patient safety.

Conclusion

Outcomes assessment in this area of dentistry is difficult, primarily due to bias. Experiential differences can be attributed to differences among clinicians and/or interest groups rather than differences in treatment outcomes. Furthermore, dentists should weigh the benefits of CBCT against the consequences of surgical failure. Once cost-to-benefit analyses are conducted, the increase in cost associated with CBCT-based treatment planning must be justified from a consumer perspective. That is, the value associated with increased treatment safety, success and predictability.

While the authors strongly urge the scientific community to conduct further clinical studies to evaluate the sensitivity and specificity of CBCT as it pertains to orthodontics and associated oral surgery, in the present case mentioned above, by taking a CBCT-based study, the surgical treatment was optimized. It helped the oral surgeon resolve the ambiguity of the original radiographic studies—which were thorough, albeit with conventional imaging techniques. The final result did not compromise any adjacent teeth or anatomical landmarks and was easily integrated into the original treatment plan.

Editorial note: A complete list of references is available from the publisher.

Contact info

Dr Dov Almog can be reached at dalmog@vdcportal.com.

Dr Michael E. Spoon can be reached at mspoon@rochester.rr.com.