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CBCT: Clinical Applications in Orthodontics



Traditionally, a number of different types of X-rays are taken to provide localized visualization of craniofacial structures. For example, orthodontists traditionally prescribed periapical, bite-wing, panoramic, and cephalometric X-rays for a complete survey as part of the initial and final records. In addition, corrected tomographic images of TMJ were obtained for the patients with temporomandibular disorders (TMD). With the advent of cone-beam computed tomography (CBCT) technology, it is now possible to view the whole skull without having to take multiple plain film images. Three-dimensional imaging has a great potential as a diagnostic tool in facilitating treatment planning and evaluation. Compared to the conventional plain film images CBCT provides 1:1 image with great spatial resolution (Mah and Hatcher, 2004), panoramic image without any distortion, orthogonal cephalometric images with superior linear measurement accuracy (Kumar et. al., 2008, Berco et. al., 2009, and Brown et. al., 2009), and accurate TMJ images (Hilgers et. al., 2005 and Honey et. al., 2007). Because CBCT allows for better hard tissue differentiation, it can also be used to clearly view any craniofacial fractures (Shintaku et. al., 2009).

After secondary reconstruction through various softwares, one can make accurate and reliable measurements (Baumgaertel et. al., 2009 and Chien et. al., 2009) on 3D volume and/or multiplanar reformatted images, such as sagittal or coronal section slices (Scarfe et. al., 2006).
Compared to multislice CT, it provides adequate image quality while minimizing metallic artifacts and significantly reducing the effective radiation dose (Suomalainen et. al., 2009). However, compared to CT, CBCT images lack soft tissue differentiation because of the low radiation applied (Hauret and Hodez, 2009). Therefore, CBCT images may not provide clear contrast of face and neck soft tissues.

Perhaps, the greatest benefit of the CBCT image is our ability to reorient the 3D volumetric image. Because the volumetric data obtained from CBCT consists of isotropic (cuboidal) voxels, the volume image can be reoriented (Scarfe et. al., 2006). Similar to the idea of standardizing the head posture by using the cephalostat in 2D lateral cephalometric x-rays, we can reorient the CBCT volumetric data in a standardized manner by using a set of reference planes. Dr. Heon Jae Cho at the University of the Pacific recently developed a three dimensional cephalometric analysis, through which the craniofacial structures can be evaluated in all three planes of space (Cho, 2009). Finally, we now can accurately analyze any transverse or vertical asymmetries of the maxilla and the mandible.
Many clinicians or patients may have some concerns on the radiation issue of CBCT. Considering the average natural background radiation in the U.S. is approximately 8µSv/day and the CBCT effective dosage is approximately 40µSv (for new generation i-CAT and NewTom 3G), one exposure from CBCT is equivalent to five days of natural background radiation (Brooks, 2008). It is clear that the benefits from CBCT images easily outweigh the potential risks of additional five days of natural background radiation.

At the University of the Pacific, Arthur A. Dugoni School of Dentistry, the orthodontic department has a protocol on viewing all CBCT images. Due to a generous donation from Dr. Ron Redmond, we are currently using the i-CAT CBCT machine with variable field of view (FOV) sizes. Two 3D softwares used at the Pacific are InViVoDental (Anatomage, San Jose, CA) and Dolphin 3D (Dolphin Imaging, Chatsworth, CA). Using InVivoDental, the CBCT images are reoriented to obtain 3D volumetric images, cephalometric, panoramic, TMJ, and transverse molar relationship images. Reorientation is important because we can evaluate a skull for any true skeletal asymmetries and make measurements that are not affected by improper head postures. For example, only after an accurate reorientation of the skull, you can conclude for sure that the double borders of the mandible observed on an orthogonal lateral cephalogram indeed represent true asymmetry of the mandible. Therefore, reorientation should be done before constructing any images from CBCT. Reorientation is performed by using the nasion, R/L frontozygomatic points, orbitale, and temporal fossa point as reference landmarks (Fig. 1) (Cho HJ, 2009). InVivoDental is currently developing the 3D analysis as part of the software to incorporate Dr. Cho’s 3D cephalometric analysis. For now, we can manually make individual measurements of Cho’s 3D analysis (Fig. 2).
Not only do we use CBCT images for diagnostic purposes, we are using them to aid in treatment planning. InVivoDental has features to plan for implants or microimplants (Fig. 3). For instance, before placing a microimplant on buccal cortical bone, one can measure the interradicular space and cortical bone thickness to evaluate for safety and stability of the implant (Park and Cho, 2009). In addition, it is possible to simulate implant placement (Fig. 4).

Another great feature of InVivoDental is the Anatomodel. Anatomodel provides 3D photographic image and Invisalign-like dentoalveolar setup. Just as the ClinCheck (Align Technology, Santa Clara, CA) tooth setup, any clinician can perform an individual tooth setup and even a surgical setup for either maxillary or mandibular jaw. After the setup, you can then simulate soft-tissue prediction viewed from all three dimensions.
Invisalign-surgery patients demonstrate a great usage of CBCT. The following 26-year-old male was treated by one of the authors (RLB) who presented with a history of previous orthodontic treatment as an adolescent (Figs. 5-9). His chief complaint was “chin deviation and Invisalign to straighten out teeth.” His problem list included Class III mandibular prognathism with asymmetrical jaw deviating 5mm to the left and a posterior crossbite on the left side. His profile was straight, with a slightly concave appearance; his lips were competent, and he had normal vertical facial proportions. Dentally, he had asymmetric Class III molars and canines (Right side: full cusp Class III; Left side: ½ cusp Class III) with the lower dental midline deviating 4mm to the left. He had a 2mm of anterior crossbite, and 2mm of reverse overbite. The patient had 1-2mm of upper and lower anterior crowding.

The Cho 3D analysis was utilized to analyze the etiology of the skeletal chin asymmetry. Comparison of the lengths of the mandible (Co-Pog), ramus heights (Co-Go), and mandibular bodies showed that the lower jaw is deviated to the left, and the right mandibular body length is significantly greater than that of the left. This revealed the intra-mandibular component of mandibular asymmetry (Fig. 10). Because of the limited volume size of the initial CBCT image, it was not possible to locate the cranial base structures (Na and R & L FZP) and therefore could not analyze any extra-mandibular component of mandibular asymmetry.

Treatment goals for this patient were to level, align, and coordinate the arches, reduce mandibular prognathism, correct mandibular skeletal asymmetry, establish a Class I canine and molar relationships, achieve an overbite-overjet relationship of 2mm, coincide upper and lower dental midline, and eliminate the anterior and posterior crossbite.

Invisalign appliances (Align Technology, Santa Clara, CA) were used to level, align, decompensate, and coordinate arches for the pre-surgical occlusion set-up with 21 upper stages and 18 lower stages. The pre-surgical phase took 12 months. For four months, Class II elastics were worn with aligners I to gain additional negative overjet and flaring labially of the lower anterior teeth.
The Dolphin software prediction (VTO) called for a mandibular bilateral split osteotomy with 5mm setback and rotation of 4mm to the right. Before the surgery, impressions for stone models were taken to check for a proper occlusal relationship (including sagittal, transverse relationships, midlines, and premature contacts). The BSSO setback surgical procedure was performed using rigid fixation.
On the first post-surgical visit after a few days of the surgery, the patient presented with an open bite with a premature contact on the right second molar after removal of IMF. By using the InVivoDental, the mandible was simulated to move into centric occlusion (Fig. 11). When the dentition came into maximum intercuspation at centric occlusion, it was demonstrated that the right condyle will be displaced posteriorly. This suggested that right side inter-segment rigid fixation was done with displaced right proximal segment. Upon removal of the IMF, muscular attachments were allowed to bring the proximal segment including condyle into optimal position causing displacement of distal segment causing the open bite on the right side. Visual observation of the 3D volumetric simulation eliminated exploratory secondary operation to find out the possible cause of the open-bite on the right side. Upon agreement with the surgeon, we decided to start the “segmental management” protocol.
Since the first post-surgical visit, the patient started the ‘segmental management’ by wearing light Class III vertical elastics along with the aligners (Fig. 12). While the aligners hold the teeth together, the light vertical and Class III force vectors were prescribed to settle the open bite and achieve a Class I occlusion by guiding the distal segment into a proper position without adversely affecting the condylar position. In a few weeks the open bite, sagittal occlusal relationship, and dental midlines greatly improved.

3D superimposition of immediate post-surgery i-CAT to two weeks postsurgery i-CAT showed that the distal segment was guided to a posterior and superior direction while the right proximal segment moved medially (Fig. 13).

This allowed the open bite to settle and the occlusion to come into class I occlusion while the condyles were seated in the center of the mandibular fossa. 3D superimposition of pre-surgery i-CAT to two weeks post-surgery i-CAT illustrates the asymmetrical set-back of the distal segment and stable proximal segment position (Fig. 14). In conclusion, Class III vertical elastics, which were light enough not to disturb the optimal position of proximal segments but strong enough to cause strains at peri-screw bone tissue, were started. Slowly but surely light elastics repositioned the distal segment into optimal dental relationship over the 8 weeks after surgery. One year retention records show that the final occlusion showed more settling and coincident dental midlines. Class III and midline correction remained stable while the canines and premolars settled in slightly further, especially on the left side (Fig 15).

Final i-CAT to 1 year retention in both 2D superimposition and 3D superimposition showed very well retained dentition and skeletal correction (Fig 16, 17, 18). On the final i-CAT, Cho’s 3D analysis showed that the asymmetrical mandibular BSSO setback surgery corrected the asymmetrical chin by reducing the length of the right side’s mandibular body length (Fig 19). The ramus heights and left side’s mandibular body length remained relatively unchanged. Ultimately, the total mandibular body lengths became similar through the surgery.
The Dolphin VTO and InVivoDental with the Anatomodel soft-tissue prediction both produced a good prediction when they were compared to the actual result (Figs. 20 & 21). The Anatomodel provides a very useful tool, which allows a doctor to set up both the dentition and the jaws to simulate pre-surgical orthodontics and the actual surgery (Figs. 22-25). Furthermore, it can also generate a 3D soft-tissue prediction, allowing a clinician to view the soft tissue changes from the frontal view, which is very useful for the patients with transverse skeletal asymmetries. 3D soft-tissue prediction from the frontal view for this patient generated a satisfactory result when it was compared to the actual changes (Fig. 21).

So far Anatomodel lacks the ability to differentiate the gingival margin due to the limitation of the CBCT technology (Fig. 26). Recently 3D intra-oral scanners improved their ability to accurately scan the dentition and the gingival tissue. With 3D intra-oral scanners combined with CBCT, it will soon be possible to improve the bone and the gingival tissue demarcation. In the near future, it may be possible to avoid taking any intraoral impressions, and instead, retrieve patient models from CBCT and intra-oral scans. Moreover, it will be possible to perform a model surgery, which is currently done on a set of stone models, through CBCT technology.
*Full time faculty at the Department of Orthodontics, Arthur A. Dugoni School of Dentistry, University of the Pacific, San Francisco, California.
Dr. Joorok Park – Assistant Professor
Dr. Robert Boyd – Frederick T. West Professor and Chairperson
Dr. Heon Jae Cho – Associate Professor and Program director
Dr. Sheldon Baumrind – Professor and Director of Craniofacial Research Instrumentation Laboratory
Dr. Hee Soo Oh – Assistant Professor Dr. Thomas Indresano – Chair of Oral and Maxillofacial Surgery

References
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  3. Berco M, Rigali PH Jr, Miner RM, DeLuca S, Anderson NK, Will LA. Accuracy and reliability of linear cephalometric measurements from cone-beam computed tomography scans of a dry human skull. Am J Orthod Dentofacial Orthop. 2009 Jul;136(1):17.e1-9.
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  15. Park J, Cho HJ. Three-dimensional evaluation of interradicular spaces and cortical bone thickness for the placement and initial stability of microimplants in adults. Am J Orthod Dentofacial Orthop. 2009 Sep:136(3):314.e1-314.e12.
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