Chairside CAD/CAM Goes to the Dental Laboratory

Joel Benk, DDS; Larry Lindke

The creation of a dental restoration requires two components to be successful: a scientific component and an artistic one. Before the advent of in-office computer-aided design/computer-aided manufacturing (CAD/CAM) restoration systems such as CEREC® 3D (Sirona Dental Systems, Inc, Charlotte, NC), indirect cosmetic restorations were primarily the domain of the dental laboratory. CAD/CAM dentistry and its associated machines, computers, and programs represent the scientific principles of restorative technology. The majority of the parameters that dentists deal with here, as in most science, are absolutes. The CAD/CAM system can produce a crown coping slightly larger than the preparation stump to allow for the exact amount (in microns) of space required by the bonding agent. Dentists can show the system where the opposing and neighboring dentition are located, and it will, in a matter of milliseconds, propose an occlusal anatomy that fits and functions as naturally as the real thing. While machines are excellent in dealing with parameters, values, and measurements, they still cannot take the place of the individual artistry that a skilled clinician can accomplish in an esthetically ideal restoration. CAD/CAM systems have freed the laboratory technician from repetitive, labor-intensive fabrication tasks, giving the technician the freedom to concentrate more on the art and esthetics of the restoration.

Case Presentation

A 47-year-old man presented with an old, failing porcelain-fused-to-metal (PFM) crown on tooth No. 7. He was also unhappy with the obvious dark line at the margin when he smiled. He wanted a new tooth without the dark margin but also wanted to maintain the look and character of his natural smile.

The dentist wished to control the restoration in terms of fit and occlusal form. A chairside CAD/CAM system fulfilled these wishes by allowing the crown to be designed, fabricated, and tried-in at the dental operatory. For most CEREC restorations, the formula is simply “mill and place.” The restorations meet both the clinician’s and patient’s esthetic approval “right out of the milling machine” and, therefore, require no application of stains, glazes, or layering porcelains. However, there will be instances where the esthetic demands of the patient may preclude the use of the “mill and place” CAD/CAM technique. This is a case in point.

The patient requested that the restoration look like and blend in perfectly with his neighboring natural dentition. From a clinical standpoint, a single-tooth restoration is the most difficult to match. Therefore, the services of a dental laboratory and a highly skilled ceramist were employed to ensure the most realistic results.

At the Dental Office
Before the existing PFM crown (Figure 1) was removed from tooth No. 7, an optical impression of its occlusal surface was taken with the CEREC 3D camera (Figure 2) and stored in the digital image catalog. This “before” image would serve as reference data, enabling the CEREC 3D system to accurately correlate the design of the new crown onto the preparation. The old PFM was then cut with a beaver bur (Figure 3) and removed with a crown-removal instrument (Figure 4), exposing the existing crown preparation. After cleaning the stump (Figure 5) and minimal refinement with a rounded diamond bur, an optical impression was taken and stored in the digital image catalog (Figure 6). A digital photograph of the crown preparation (along with its corresponding shade tab) was also taken and later e-mailed to the laboratory to help assist the technician with selecting appropriate shades of layering porcelain (Figure 7).^1-3 Using the digital image of both the crown preparation and existing PFM, the CAD/CAM software automatically proposes a design for the final restoration (Figure 8). Once satisfied with the design of the virtual restoration, it is automatically milled out of a single, solid monochromatic block of all-ceramic material and tried-in. The patient was fitted with a temporary and sent home, and the restoration was sent to the laboratory for porcelain build-up and characterization.

At the Laboratory

The monochromatic all-ceramic CAD/CAM crown was received by the laboratory technician along with the patient’s preoperative and crown preparation photographs. Using these photographs and notes from the clinician, the laboratory technician created a color map of the neighboring dentition as well as the tooth being restored to determine how different porcelain colors would be applied to create the desired final shade (Figure 9). Then the component colors, such as effect enamel and effect chroma, were selected (Figure 10 ). Next, the CEREC crown was cut back to allow for the addition of layering ceramics.⁴ The amount of reduction is determined primarily by the amount of color change required; sometimes this means cutting back the incisal third, incisal half, or even all the way down to the gingival margin.⁴ This case required cutting back a little more than the incisal half. Once cutback was accomplished, porcelain was applied and fired in stages (Figure 11). Overextension of the incisal was necessary to compensate for the shrinking that occurs during firing (Figure 12). Additional porcelain was applied to the lingual marginal ridges and cingulum to create lifelike tooth anatomy (Figure 13). The VITA® (Vident) block used to fabricate this crown is specifically formulated for use with Vident layering ceramics (Figure 14). When fired, the all-ceramic restoration and the layering ceramics fuse together to create a unified “monobloc” structure.

After the final porcelain/enamel firing (Figure 15), the surface of the restoration was glazed and polished mechanically. The surface of an esthetic restoration is just as important as the color pigments because of the way surface textures react to light. Two studies done at the dental schools of the University of Oklahoma and University of Minnesota revealed that the smoothest surface is accomplished by first glazing the restoration and then coming back to polish it.⁵ Glazing only imparts a shine that appears to be smooth, but is not; “glazed” and “smooth” are not necessarily synonymous with each other. A restoration can be highly glazed and shiny but not very smooth under magnification. Conversely, it can have a smooth surface, yet little to no shine effect. The combination of glazing the surface and then mechanically polishing it to achieve the desired surface luster and smoothness is the ideal protocol. But this can only be done effectively if the restoration has first been overlaid or veneered with colored porcelains. Simply adding stains (surface colorant) to the enamel is not sufficient, due to the risk of removing some of the colorant during mechanical polishing. Incorporating the color below the enamel layers, as was done with this restoration, ensures that no color change will occur with mechanical polishing. The final result was a restoration that not only looks and feels like natural dentition, but one that also mimics the light-reactive properties of natural dentition (Figure 16).

Conclusion

Because the CEREC crown featured in this case was milled from a VITA block—a single, solid block of industrially manufactured machinable ceramic material—microflaws or cracks were not a concern.⁶ Furthermore, certain variables that are attributed to entirely laboratory-fabricated restorations were eliminated by using the CEREC 3D plus laboratory cutback and esthetic porcelain application technique described herein. For example, there is no issue with shrinkage of impression material, nor is there concern for shrinkage of stone used to pour the models—these steps are eliminated with this technique. The accuracy of the fit is predetermined at the dentist’s office, so the dentist has the confidence of knowing that the margins are ideal before he sends the restoration to the laboratory for build-up and characterization. Of course, the original and primary intent of chairside CAD/CAM restorations is to design, fabricate, and place all-ceramic indirect restorations in a single patient visit, thereby eliminating the need for any laboratory intervention. However, this case study demonstrates that there are certain unique case circumstances that can benefit greatly by employing a synergistic combination of both traditional laboratory-based restorative techniques and chairside CAD/CAM.

References

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4. Besler U, Magne P. In: Bonded Porcelain Restorations in the Anterior Dentition: A Biomimetic Approach. Quintessence Publishing. Hanover Park, Ill; 2002:306-323.

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