Armen Mirzayan, MA, DDS Private Practice Los Angeles, California Phone: 213.553.4535 Web site: www.estheticdentistry.net

Whether a direct or indirect approach is taken when faced with a patient requiring restorative treatment, creating and designing natural occlusal surfaces is not only a challenge, but also important to achieve proper bite registration, occlusion, and masticatory function. The indirect restoration described in this article features a computer-aided design/computer-aided manufacture (CAD/CAM)-generated restoration that is milled, finished, and placed in a single patient visit using the CEREC 3D chairside restoration system with a newly developed biogeneric tooth modeling function that is built into the system's latest software (Sirona Dental Systems, LLC). The biogeneric tooth modeling function allows dental professionals to generate all-ceramic indirect restorations (onlays, inlays, and full crowns) with anatomically correct occlusal surfaces at the touch of a button.¹

Biogeneric Tooth Modeling

The CEREC 3D system's biogeneric tooth modeling software was developed by Dr. Albert Mehl of the Department of Restorative Dentistry at Ludwig Maximilians University in Munich and Dr. Volker Blanz of the Max Plank Institute for Computer Science in Saarbruecken, Germany.^2-4 These university professors stored the measurements of several thousand natural teeth in a database. They then identified and categorized common features and structures, such as cusp tips, fissures, marginal ridges, and cusp slopes. These data provided the basis for computing an "average tooth" embodying the median characteristics of all the naturally occurring teeth scanned. Following this, the differences were computed between the individual database teeth and the "average tooth," and an analysis identified the most frequently occurring deviations. A probability analysis revealed that approximately 20 deviations are sufficient to describe 85% of the natural variability of tooth surfaces. In simpler terms, the combination of the average tooth and the 20 deviations permits a very effective description of occlusal morphology. As this process is based exclusively on data derived from natural teeth, Professors Mehl and Blanz have chosen the designation "biogeneric tooth model."

Figure 1—Preoperative photograph of fractured amalgam filling on tooth No. 19.	Figure 2—A bite registration is necessary so that the CAD/CAM designing software can take the occlusal morphology of the opposing tooth into account when it proposes a restoration design.	Figure 3—Bite registration with CAD/CAM imaging powder.
Figure 4—Prepared tooth with soft-tissue laser retraction.	Figure 5—The prepared tooth and neighboring dentition were powdered for optical impression.

Patient Presentation

A 35-year-old man presented to the office as an emergency patient, having suffered a fracture of the distal lingual cusp of tooth No. 19 after biting on hard food (Figure 1). The tooth had an existing distal-occlusal amalgam restoration. Radiographs and pulp testing revealed that the tooth was vital. The patient requested immediate treatment. An all-ceramic, single-visit CAD/CAM onlay was recommended, as it would allow the pa­tient to be fitted with a permanent restoration, with no need for a temporary or a second appointment.

Preparation, Powdering, and Picturing

Before the tooth was prepared, a static bite registration was taken so that the CEREC 3D software program would have an occlusal reference of the case stored in its memory. To take a static registration, a small amount of registration material was placed on the tooth to be restored before it was reduced (Figure 2). The patient was told to bite down and then release. The resultant registration was lightly dusted with imaging powder, and digitally captured using the CAD/CAM system's built-in infrared camera (Figure 3). This powder makes the teeth visible to the camera. The bite registration image is stored in the CAD/CAM system's memory, and when it is time to design the restoration, the software will automatically take the occlusal morphology of the antagonist tooth into account when it calculates and proposes the occlusal surface of the re­storation. This process ensures functional occlusion between the existing tooth and the new all-ceramic re­storation.

Because most CAD/CAM systems rely on a digital scan or "picture" of the prepared tooth as a model from which to construct the actual restoration, the accuracy of the preparation is essential to achieving an ideally fitting restoration.^5-8 Each CAD/CAM system has its own set of preparation guidelines. In this case, the preparation was achieved as conservatively as possible using an Occlusal Reduction Bur (Meisinger USA, LLC) to create a 2-mm occlusal reduction and a 1.5-mm reduction at the shoulder. The axial walls were smoothed with an approximately 6° taper, leading to a flat cavity floor.

Once all of the decay and remaining filling material was removed, the preparation was cleaned and prepared for powdering (Figure 4). First, a soft-tissue laser was used to facilitate re­traction so the margins could be clearly "read" by the CAD/CAM system's infrared camera. Use of the laser re­sults in exceptionally clean preparation margins without the influence of a retraction cord, blood, or other fluids.⁹ Next, an extremely thin coating of contrasting powder was applied to the entire preparation, the remaining tooth structure, and the neighboring mesial and distal teeth (Figure 5). The image captured by the camera is referred to as a "digital impression." The system's intelligent restoration designing software will use this digital impression when it proposes a design for the all-ceramic restoration that will be milled from a single, solid block of ceramic during the milling stage.

Restoration Design

With the digital impression displayed on screen, the margins were defined by tracing around the perimeter of the preparation. The software then displayed a green outline, which "snapped" to the edges to conform precisely to the shape of the margin. Once the margin was clearly defined, the software automatically designed and proposed a restoration. As the biogeneric software proposes a re­s­toration, it displays an animated, 3-dimensional, morphing model of the proposal on screen in relation to the preparation (Figures 6 and 7). Within 5 to 10 seconds, the software will finalize the model and present an ideal restoration design—including cusps, contours, and occlusal surfaces—based on its comprehensive database of biogeneric tooth morphology. Then, the interocclusal clearance in relation to the antagonist was inspected to verify proper occlusion before the restoration was milled.

Figure 6—The CAD/CAM software automatically proposes a restorative form based on the biogeneric tooth model.	Figure 7—Lingual view of the proposed restoration design.	Figure 8—Tooth No. 19 with CAD/CAM allceramic restoration cemented in place (immediate postoperative photograph).
Figure 9—Lingual view of CAD/CAM allceramic restoration on tooth No. 19 (immediate postoperative photograph).	Figure 10—12-Month postoperative photograph of tooth No. 19 with all-ceramic CAD/CAM restoration.

Fabrication and Cementation

After checking all aspects of final restoration design, the appropriate block of material was secured into the CAD/CAM milling chamber. In this case, a Vita TriLuxe shade 1M2C multicolored block (Vident) was chosen because of its high strength and esthetic properties. The all-ceramic restoration was milled to specification in approximately 9 minutes. After milling, the restoration was detached from its sprue and lightly finished with a few quick passes of a polishing wheel. The restoration was tried in and found to fit without any adjustments. After a simple application of stain and glaze and approximately 8 minutes in a porcelain oven, the restoration was bonded in place using Simplicity and Anchor bonding agents (Apex Dental Materials, Inc). A photograph taken minutes after bonding shows a distinct border between the restoration and the remaining natural tooth structure (Figures 8 and 9). This is caused by desiccation of the natural tooth because of the need to maintain a clean, dry operating field, and also because the imaging powder tends to wick the moisture out of the tooth, causing a shift in color and shade. However, after the all-ce­ramic restoration and natural dentition have had a chance to rehydrate, the result is extremely lifelike and it is difficult to visually discern the area where the patient's actual tooth structure ends and the all-ceramic restoration begins, as seen in the 12-month postoperative photograph (Figure 10).

Conclusion

The design of complex occlusal surfaces is of primary importance because of their critical function in the patient's stomatognathic system. CAD/CAM systems that use the biogeneric tooth model automatically generate restorations featuring proper occlusal design through the use of a database containing the occlusal anatomy of several thousand human teeth. This simplifies the production of restorations, reduces design time, and improves their quality. The re­sults are all-ceramic restorations that satisfy their individual case parameters regarding contact and separation during excursion with achievement of a balanced, harmonized, and healthy occlusal profile.

Disclosure

Dr. Mirzayan is a CEREC 3D trainer for Sirona Dental Systems, LLC.

References

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3. Mehl A, Blanz V, Hickel R. A new mathematical process for the calculation of average forms of teeth. J Prosthet Dent. 2005;94:561-566.
4. Dunn M. Biogeneric and user-friendly: the CEREC 3D software upgrade V3.00. Int J Comput Dent. 2007;10:109-117.
5. Luthardt RG, Bornemann G, Lemelson S, et al. An innovative method for evaluation of the 3-D internal fit of CAD/CAM crowns fabricated after direct optical versus indirect laser scan digitizing. Int J Prosthodont. 2004;17:680-685.
6. Luthardt RG, Loos R, Quaas S. Accuracy of intraoral data acquisition in comparison to the conventional impression. Int J Comput Dent. 2005; ­8:283-294.
7. Neumann CP, Tatarciuc MS, Vitalariu A, et al. CAD/CAM technology-elaboration of electronic model through optical impression in the fabrication of the dental prostheses by a computer assisted device [in Romanian]. Rev Med Chir Soc Med Nat Iasi. 2000;104:161-163.
8. Pfeiffer J. Dental CAD/CAM technologies: the optical impression (II). Int J Comput Dent. 1999; ­2:65-72.
9. Scott A. Use of an erbium laser in lieu of retraction cord: a modern technique. Gen Dent. 2005; ­53:116-119.

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