CBCT Enhances Facially Driven Digital Implant Design

INTRODUCTION
Implant placement is no longer a novel treatment for dentists or patients. Although many treatment plans involving implant placement and restorations can be done using conventional radiographs and traditional surgical techniques, there are distinct advantages in incorporating cone beam computed tomography (CBCT) techniques in the treatment-planning phase.

The use of a 3-D CBCT scan in implantology is essential to avoid vital anatomy¹ or to navigate challenging anatomical structures, such as nerves, blood vessels, concavities in the mandible and maxilla, and maxillary sinuses. The use of 2-D periapical films and panoramic films can aid in identifying these structures, but, due to inherent distortion in these radiographic methods, it is difficult to know their location with accuracy.^2-4 Additionally, CBCT imaging may reveal pathologies that may affect implant placement at the site of interest or adjacent sites,³ the design of the prosthesis, and the long-term success of the treatment. It is important to note that in any instance in which a CBCT is ordered, the entire scan must be read and interpreted. (Clinicians may be liable for missed diagnoses or misinterpretations of the scan, even if they are not within their scope of practice. It is highly advisable to refer any questionable scans to an oral radiologist.)

This article illustrates the advantages of using CBCT images to digitally design implant treatment plans to ensure a more predictable prosthetically driven outcome and pa-tient satisfaction.

Figure 2. Digital wax-up of the new smile overlaid on existing dentition.

CBCT Imaging
To facilitate the use of the CBCT in digitally designing a patient’s ideal outcome, it is important that the scan be taken with the teeth separated using a bite stick or cotton rolls to ensure that the maxillary and mandibular teeth are not superimposed over one another. This allows the clinician to visualize the anatomy and incisal edges of the individual teeth. However, newer software capabilities allow the CBCT to be taken with the teeth in occlusion as long as there is bite registration material (fully polymerized) between the teeth. The software will be able to identify the anatomy of the individual teeth and separate the arches.

The CBCT images are then imported and processed into planning software. Using the software, each individual tooth can be outlined and identified as separate entities (also known as a “mask”) using different colors. Once each mask has been created, masked teeth can be “hidden” (turned off) or “seen” (turned on) in the arch as required. The creation of the masks provides the clinician with a baseline of the occlusal plane and incisal edges as the patient presents preoperatively (Figure 1). When creating a digital wax-up of the new smile, the clinician can make comparisons with the pre-existing form. For example, in a case with a severely worn dentition, when it is desirable to establish a new incisal edge position, the newly sized teeth can be overlaid on the masked teeth and compared (Figure 2). Similarly, newly created digital teeth can be added into the correct positions at the desired angulations for function and aesthetics (Figure 3). Essentially, this is the beginning of the digital wax-up design phase.

Figure 4. Lips in repose.

Figure 5a. Regular smile.	Figure 5b. High smile line.

Manipulating the Data to Create a Facially Driven Treatment Plan
The CBCT scan is invaluable when creating a treatment plan and is used most often to guide a bone-driven plan. However, the CBCT scan can also be used to aid in the creation of a facially driven plan so that when the implants are placed, not only are they in the correct bone position, but the final aesthetics of the face and teeth are optimized as well.

CBCTs will not capture soft tissue. As such, to create a facially driven treatment plan, it is imperative that high-quality frontal and profile photos of the patient are taken. These include:

• Lips in repose. Have the patient pronounce the letter M and note the amount of incisal edge that shows when he or she finishes annunciating (Figure 4).
• Regular smile and high smile. This will help determine the amount of lip translation (lip movement) (Figure 5).
• Photos in maximum intercuspation (MIP) or in a closed vertical position while using lip retractors (Figure 6).
• Photos with patients in the corrected vertical dimension of occlusion (VDO) with a bite registration in place while using lip retractors (Figure 7).
• Additionally, the patient should bring in historical photos of his or her smile to help in the planning phase.

Ideally, these should be photos with a 1:1 magnification ratio so that measurements can be made and correlated between images. The information that is evaluated includes:

• The length of the upper lip.
• The amount of lip translation.
• The number of teeth showing in the buccal corridor.
• The position of the facial midline.
• The profile of the patient/nasolabial angle (Figure 8).

It should be emphasized that this step is integral in establishing the relationship and aesthetics of the teeth to the face. Assessing the frontal and profile pictures of the patient helps the clinician determine the position of the teeth, taking into consideration the natural movement of the lips as well as the soft-tissue display. In this way, the digital wax-up can be modified to optimize facial aesthetics.

Figure 7. The bite in the corrected vertical dimension of occlusion with a bite registration.	Figure 8. The profile of a patient showing a nasolabial angle.

Figure 9a. A dental implant placed in an ideal restorative position.

Planning Implant Placement
Once the facially driven design has been created, the implant placement can be planned. The software has a library of different implants and components from all of the manufacturers, which allows the dentist to choose the desired implant size, length, and abutments for each designated implant tooth.⁴ At this stage, the clinician evaluates the patient’s anatomy and hard-tissue structures, including the restorative space, bone width, bone height, and bone density, to aid in the selection and position of the implants.

It is essential that the implant is placed in the best volume and quality of bone yet emerges through an ideal restorative position—the central fossa of the posterior teeth or the cingulum of the anterior teeth (Figure 9).

Figure 10. Measuring teeth in relation to the high smile line.

Visualizing Bone Reduction
To optimize aesthetics, it is best to hide the prosthetic margin. As such, the prosthetic margin should be designed to be apical to the high smile line (Figure 10). Thus, the implants must also be planned apical to this high smile line position. Sometimes, bone reduction is necessary.

Once the implants are planned, the software can calculate the amount of bone reduction that is required. At this point, the facially and prosthetically driven digital design has been completed. Now, the final surgical guides, templates, and provisional prosthesis are fabricated to help the practitioner clinically reproduce the designed treatment in a predictable fashion.

IN SUMMARY
This clinical case illustrates the value of utilizing CBCT technology to carefully plan implant placement and the final aesthetic outcome of a patient’s treatment (Figure 11). To design the ideal patient outcome,⁴ the current CBCT software programs afford the dentist the ability to merge the clinical photographs with the radiographic images. In this way, the concept of a facially and prosthetically driven treatment plan is supported, as opposed to one that is restoratively driven based on the placement of the implants without thought given to the desired final outcome of the case.

References

1. Rugani P, Kirnbauer B, Arnetzl GV, et al. Cone beam computerized tomography: basics for digital planning in oral surgery and implantology. Int J Comput Dent. 2009;12:131-145.
2. Benavides E, Rios HF, Ganz SD, et al. Use of cone beam computed tomography in implant dentistry: the International Congress of Oral Implantologists consensus report. Implant Dent. 2012;21:78-86.
3. Moshfeghi M, Tavakoli MA, Hosseini ET, et al. Analysis of linear measurement accuracy obtained by cone beam computed tomography (CBCT-NewTom VG). Dent Res J (Isfahan). 2012:9(suppl 1):S57-S62.
4. Chakrapani S, Sirisha K, Srilalitha A, et al. Choice of diagnostic and therapeutic imaging in periodontics and implantology. J Indian Soc Periodontol. 2013;17:711-718.

Dr. Wong graduated from the University of Toronto with her doctor of DDS degree in 1996 and received her certificate in prosthodontics in 2007 from the University of Michigan. She is the only dentist that has attained a combination of the US board certification in implant dentistry (Diplomate from the American Board of Oral Implantology [ABOI]), US board certification in prosthodontics (Diplomate of American Board of Prosthodontics), and Canadian board certification in prosthodontics (Fellow of the Royal College of Dentists of Canada). She is a Diplomate of the International Congress of Oral Implantologists, and holds fellowships with the AGD; the American Academy of Implant Dentistry (AAID); and the Misch International Implant Institute Canada, for which she is also a faculty member. She is a past president of the ABOI and is currently the secretary for the AAID and president of the Association of Prosthodontists of Ontario. Dr. Wong is also founder and director of the Toronto Implant Institute, Inc. She lectures internationally on implant dentistry and continues to practice both the surgical and prosthetic phases of implantology in Toronto. She can be reached at (647) 748-3550 or by visiting drnataliewong.com.

Disclosures: Dr. Wong reports no disclosures.

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