Copy Existing Tooth Form to Retrofit to an RPD Accurately: An Ideal CAD/CAM Application

One of the most vexing and difficult procedures in restorative dentistry is that of retrofitting a crown under an existing, removable partial denture. Personally, I have tried just about every technique available. I have always found that the procedure is either very laborious–requiring many indexing steps–or the removable partial denture must accompany the impression to the dental laboratory for 2 to 7 days for the work to be accomplished. Obviously, this is not a viable option if the removable prosthesis is the only one that the patient has and it involves anterior teeth. There is usually adjustment of the crown or partial clasping to make it fit. The partial and/or the crown usually require extensive adjustment to get the clasp and/or the rest to engage and fully seat.
A great solution can be found in CAD/CAM technology. The strength, durability, and ease of application of these all-ceramic crowns have shown this technology to be the ideal solution for those challenging cases where fit and final form is paramount. Retrofitting the crown to an existing removable partial denture can also be done as a single-visit procedure, thus allowing the patient not to have the nuisance and fear of being without teeth. CAD/CAM technology makes it possible for us literally to copy an existing tooth form to accomplish the retrofitting procedure. This makes the way we perform this task, right in our own office, light years ahead of any lab technique to date.


Figure 1. Tooth No. 3, preoperative view.

CASE REPORT

A67-year-old male presented to our office with a chief complaint of sensitivity and pain in a tooth that was an abutment to an upper removable partial denture. He told us that he had noticed the tooth sensitivity for a few days. He found that it had become especially reactive to sweets and certain foods. His health history revealed multiple hip replacements, a slight stroke on the right side a year ago, type II diabetes, and an obvious aversion to the doctor’s office. He was scheduled to undergo yet another hip operation in the next week. If possible, he wanted the dental problem resolved right away, since he would not be able to return to the dental office during the extended period of time required to heal from his hip surgery. He further stated that the partial was only 3-years-old, and he was not interested in having it replaced any time soon.
The clinical exam found that tooth No. 3 had a large, broken-down amalgam extending over much of the surface area of the tooth. Recurrent decay was also present on the facial surface at the cervical aspect (Figure 1). The removable partial denture had an Akers clasp with a rest on the mesial constituting the main basis of support and retention on the posterior right. After discussing the options with the patient, which included having to consider a limited time frame for completion, he agreed to have a CAD/CAM crown (CEREC 3D [Sirona]) fabricated in our office immediately.
While the anesthesia was taking effect, an image of his existing tooth morphology was quickly acquired and stored in the proper software catalog. The tooth was then prepared with the subgingival margin being defined using a laser to “trough” the tissue rather than packing cord. Then we captured the second image of the prep. With the imaging finished, the focus now went to the software side of the procedure. The dentist has multiple options when deciding which design mode to utilize based on the end result desired. In this case, the correlation design process was chosen because of the need for an exact replica of the abutment tooth morphology due to the clasping.

Figure 2. CEREC preoperative correlate image.

Figure 3. The preparation of No. 3 before imaging.

Figure 4. A camera jig fabricated from a PVS material is used to align the preoperative and prep images.

Figure 5. Prep margin determined via the software.

Figure 6. Defining the shape of the tooth proposal to match the original tooth form.

Figure 7. CEREC proposal for the crown. Note the copied mesial rest.

Figure 8. Trial fit of crown immediately after milling. Figure 9. Post cementation showing a well-fitting removable partial denture clasp.

Figure 10. Buccal view of the removable partial denture in place.

Figure 11. No adjustments were made to achieve this fit of the removable partial denture.

Figure 12. Photo taken at 5.5 years postoperatively.

Figure 13. The restoration is not showing any signs of failure after more than 5 years in function.

Correlation is one of the fabrication modes of CEREC 3D software. It allows the operator to duplicate exactly the morphology and contours of an existing tooth or restoration, including rests, by having the computer overlay the “before” and “prep” pictures of any tooth in the mouth (Figures 2 and 3). Most applications in this design mode have either one or 2 adjacent teeth for the software to look at, but this case had neither mesial nor distal neighbors, which made it a more challenging procedure to complete successfully. To assist with correct image overlay, a quick set vinyl polysiloxane (VPS) was placed on a C-Stat camera stabilizer to index its position relative to the nearest tooth, which in this case was tooth No. 5 (Figure 4). The VPS registration allows the practitioner to easily line up the same image angles multiple times, which in this case ensured a good overlay (correlate), since the computer did not have any adjacent teeth to “look” for when putting the pictures together. The software is then able to stitch the images together (Figures 5 and 6), and the operator can toggle back and forth to verify how well the computer overlay was performed. Once that is accomplished, it is simply a matter of working through the design screens to produce a carbon copy, in virtual 3-D, of the original tooth form. The design was finished and then milled out, utilizing an IPS Empress CAD (Ivoclar Vivadent) ceramic block (Figure 7). It was then trial-seated for fit (Figure 8). The crown was subsequently polished, the porcelain etched, and the internal aspects treated with silane. A total-etch technique was used to prepare the tooth for bonding. A desensitizer (Gluma [Heraeus Kulzer]) was then applied, followed by the application of Prime & Bond NT (DENTSPLY Caulk). Finally, the luting procedure was accomplished with a dual-cured resin cement (Variolink II [Ivoclar Vivadent]). The existing removable partial denture and the occlusion required very little adjustment since both were literally copied from the preoperative tooth (Figures 9 to 11).
The crown has now been in function for more than 5.5 years and shows no signs of wear or failure (Figures 12 and 13).

CONCLUSION

By essentially giving the dentist an in-house dental laboratory, the CEREC 3D chairside CAD/CAM technology allows the practitioner to better handle certain dental challenges of an aging population who may be limited in their access to dental care. Retrofitting a crown under an existing removable partial denture prosthesis utilizing chairside CAD/CAM technology has been shown to be predictable, accurate, durable, and aesthetic.


Dr. Burt maintains a private practice in Center Valley, Pa. He is director of the Lehigh Valley Dental Education Center, where he teaches advanced CEREC and laser technologies. He is also a member of the International Society of Computerized Dentistry, the Academy of Laser Dentistry, and the International College of Dentists. Among his many professional affiliations, Dr. Burt is an ISCD and Patterson Certified Basic Trainer for CEREC. He can be reached at DiveBurt@aol.com.

Disclosure: Dr. Burt is paid as a basic trainer for Patterson Dental.