CAD/CAM Fabrication of Definitive Implant Prostheses

A digital workflow from planning to implant placement to final restoration

By Harold S. Baumgarten, DMD; and Alexander Wünsche, CDT

Osseointegrated dental implants, as developed by P.I. Branemark,¹ were originally designed to replace teeth in a severely compromised edentulous dental arch. The restorations that were fabricated were termed fixed-removable or hybrid dentures. Essentially, a fixed prosthesis was designed using a removable prosthodontic protocol. It was composed of a cast gold framework, denture teeth, and a soft-tissue-colored denture base and was fixated to transmucosal abutments using gold prosthetic screws. More recently, milled titanium bars have replaced the cast gold framework.² Over time, maintenance of the restoration can become a vexing problem, as denture teeth may stain, wear, or fracture.³

Today, more durable esthetic materials can be used to fabricate a hybrid denture. This case report illustrates the digital surgical planning for a cone-beam computed tomography (CBCT) guided implant placement, as well as the CAD/CAM fabrication of a hybrid denture composed of zirconia and feldspathic porcelain.

Case Overview

A 68-year-old female patient presented with complaints of loose bridgework. Tooth No. 8 was a pontic on a transitional removable partial denture. The crown on the implant in site No. 30 was mobile and in infraocclusion. Radiographic and clinical examination revealed large areas of caries, cement washout of the maxillary right and left posterior bridges, and multiple periapical radiolucencies (Figure 1 and Figure 2). The treatment plan called for extraction of all remaining maxillary teeth, as well as teeth Nos. 20 and 21.

Surgical Phase

At the time of tooth extraction, sockets were debrided and grafted with a bovine xenograft (Bio-Oss^®, Geistlich Pharma North America, geistlich-na.com). An immediate maxillary denture was inserted at this time.

While the grafted sockets were healing, a new maxillary denture was fabricated that satisfied the patient's esthetic and phonetic requirements. Implants were placed in the lower left posterior quadrant and subsequently provisionalized (Figure 3).

The new denture was duplicated in a mixture of 30% barium sulfate to acrylic resin powder by weight and utilized as a radiographic stent. A CBCT was done while the patient wore the radiopaque duplicate denture. The digital imaging and communications in medicine (DICOM) files from the CBCT were imported into implant planning software (SIMPLANT^®, DENTSPLY Implants, dentsplyimplants.com). The surgical plan was developed and then sent to the software vendor for fabrication of a soft-tissue-supported surgical guide for use in a flapless surgical procedure. The surgical guide was made compatible with the Tapered Navigator^® surgical kit (Biomet 3i, biomet3i.com) (Figure 4).

The high degree of accuracy of fully CT guided surgery has had a major influence on both the surgical and prosthetic approaches to treatment. CT guided surgery has been shown to be more precise than either manually placed implants or implants placed using a pilot guide.⁴ However, there is still a margin of error between the treatment plan and the actual placement.^5,6 Therefore, the surgeon is required to plan the distance from the implant to vital structures in a similar manner to that used for manual surgical techniques.⁷ It also precludes the fabrication of a completed provisional restoration based on the surgical plan. As a result, the provisional restoration is completed by luting the temporary cylinders to the prosthesis intraorally⁸ after the implants have been placed.

After the patient was anesthetized with 2% lidocaine with 1:100,000 epinephrine, the surgical guide was fixated with three 2-mm diameter bone screws. The osteotomies were prepared and the implants placed through the surgical guide (Figure 5). Low-profile abutments were then placed into the implants. The maxillary denture was converted to an immediate-load hybrid denture by incorporating titanium temporary cylinders into the denture; the cylinders were then screwed into the low-profile abutments. The palate and labial flanges were removed (Figure 6). Once the implants had integrated (eight weeks post-placement) the restorative procedures were begun.

Restorative Phase

CAD/CAM has been used to restore edentulous arches with implant-supported restorations.⁹ One of the reasons is that the fit between the implants and the restoration is better than that of metal alloy castings.^10,11

In this case, the restorative phase began in a traditional manner. Abutment-level transfer impressions were made using abutment-level transfer copings. A soft-tissue master cast was poured, and a cast verification index was made on the master cast (Figure 7). This is arguably the most important phase of the prosthetic procedure. Because the final restoration was to be milled from a monolithic block of zirconia, there would be no opportunity to reassemble the framework in case of a misfit. Thus, the cast must be verified to be an exact duplicate of the patient. The verification index was then tried in the patient's mouth, and the one-screw test¹ was used to ensure a precise fit. It should be noted that if all of the cylinders of the index do not fit precisely, the impression must be remade.

Once the cast was verified, the laboratory required the following: master casts and either an impression of the satisfactory immediate-load prosthesis or an occlusal rim for a denture wax-up. In this case, since the immediate-load hybrid denture was deemed satisfactory, the maxillary master cast was articulated against a mandibular master cast using the immediate-load hybrid denture and a bite registration. An impression of the hybrid denture was also sent to the laboratory. Until this point, the prosthetic procedure followed a traditional analog workflow.

In the dental laboratory, scan bodies were placed on the abutment analogs. These are the equivalent of digital impression copings. The cast with scan bodies was scanned in a desktop laboratory scanner (Figure 8). Once scanned, the scan bodies were aligned using a library of supported implant interfaces. This allows the design software to create a precise fit of the prosthesis to the abutment.

The cast of the immediate-load hybrid prosthesis was scanned (Figure 9). The opposing cast was scanned, and the three scanned casts were merged (Figure 10). Using the superimposed scan of the immediate-load prosthesis as a guide, library teeth from the database in the CAD system were selected and placed in their appropriate locations. Connectors, screw-access holes, and gingival tissues were designed (Figure 11). Once the design was finalized, the design file was sent to the CAM workstation where it was nested in a PMMA block. The CAM software computed the tool path for the milling machine, and a PMMA replica of the final restoration was milled. Metal interfaces were cemented into the PMMA replica and it was returned for try-in (Figure 12).

The main function of the PMMA replica is to verify the esthetics, function, and occlusion intraorally. If needed, the PMMA replica can be modified and the occlusion adjusted, and the replica can even be worn by the patient for a short period of time if desired (Figure 13). When all manual adjustments were completed, the PMMA could be rescanned and the adjustments transferred to the digital design.

The most common post-treatment complication of a porcelain-fused-to-zirconia restoration is porcelain chipping.^12,13 In this case, all of the occluding surfaces were to be in zirconia, while the labial aspects of the six anterior teeth were to be layered feldspathic porcelain. This would decrease the chance that occlusal forces would cause porcelain chipping. This cut-back was accomplished in the design software, which thus completed the design process. The final digital design was transferred to the CAM workstation, where it was nested in a zirconia block (Ceramill^® Zolid, Amann Girrbach, amanngirrbach.com) and then milled (Figure 14). The zirconia block was in a pre-sintered or green state. Fine detail was then carved into the milled zirconia block using diamond burs, diamond discs, and hand instruments.

After successful shaping, the restoration was pre-stained prior to sintering. This provided a backdrop for externally applied porcelain and stain. For a restoration of this size (full-arch), a long sintering cycle was used to avoid cracking of the zirconia. After sintering, the stain was noticeably visible in the teeth and gingival areas (Figure 15). Porcelain was then applied to the cut-back anterior teeth and the gingival areas. The restoration was then externally stained and glazed, with slow heating and cooling cycles used.^14,15

The final step in completing the restoration was to bond the titanium interfaces into the restoration. These interfaces were first screwed onto the master cast. Then, their external surface and the internal aspect of the glazed zirconia restoration were air-abraded with 110-micron aluminum oxide. A primer liquid was applied and air-dried for 60 seconds. The titanium interfaces were then bonded into the zirconia restoration with a dual-cure cement (Multilink^® Automix, Ivoclar Vivadent, ivoclarvivadent.us). After the cement cured, the excess was removed and the completed restoration was ready for insertion (Figure 16). The left mandibular bridge, comprising ceramo-metal over custom abutments, was inserted on the same visit. The right mandibular implant crown was replaced to level the plane of occlusion.

The completed restorations satisfied the patient's esthetic and phonetic requirements (Figure 17). The fit of the restorations, as well as the maintenance of crestal bone around the implants, could be observed radiographically (Figure 18). An occlusal guard would be fabricated to control forces due to nocturnal bruxism.

Conclusion

The processes and materials used in dentistry in large part are determined by manufacturing methods available to the treatment team. CAD/CAM technology has afforded the profession the ability to utilize esthetic and durable materials that were not available in the past. As demonstrated in this case, precise planning and surgery combined with a CAD/CAM-manufactured definitive restoration allowed the treatment team to provide a highly esthetic, strong, and long-lasting restoration for patients.

Acknowledgment

The authors would like to thank the staff at Zahntechnique Dental Laboratory in Miami, Florida and Alfred Nelson, CDT, and the staff at Amsterdam Dental Laboratory in Philadelphia, Pennsylvania for their support in this article.