Written by Gregori M. Kurtzman, DDS, and Douglas F. Dompkowski, DDS Thursday, 13 March 2014 12:41
Dentistry is undergoing a technology revolution, and traditional methods are being displaced at an increasing pace for new, more precise techniques. A growing number of restorations at the dental labs are utilizing digital technology in the form of model scanning, virtual design, and CAD/CAM milling. The future indicates that this will further expand, and practitioners will continue to use more of this technology in their offices.
Chairside digital impressions have now been utilized for fabrication of indirect restorations for more than 25 years, allowing clinicians to scan preparations and to fabricate inlays, onlays, and crowns on natural teeth while the patient waits. In recent years, this has permitted those practitioners who owned a CEREC (Sirona Dental Systems), Lava C.O.S. (3M ESPE), or other similar system to scan their preparations and digitally send the data files to the lab for fabrication of the restorations. These were either made on virtual models (similar to how restorations were made in-office with the CAD/CAM milling units like CEREC), or models were fabricated through printing or milling. Actual models allow the laboratory teams to offer more options for restorations than can be afforded with restorations that are just CAD/CAM milled. With respect to implant restorations, metal or zirconia abutments can be CAD/CAM milled, and then full-contour, all-ceramic restorations can be milled to fit the abutment or a coping designed. These can be designed and milled to accept layered porcelain to finish the desired restoration with optimal aesthetics. When a screw-retained restoration is desired, the substructure can be virtually designed and milled, then placed on the CAD/CAM generated model, and finished by the application of ceramics.
Digital Impressions Increase Accuracy
The next step in digital impressions has been able to replace the need to take either an open- or closed-tray impression for implant restorations. This decreases inaccuracies with implants in relation to adjacent implants that will be part of the same restoration; this, in the past, has required the use of verification stents. Passive fit is critical with implants since they do not have the periodontal ligament (PDL) that is found around natural teeth. With regard to a bridge on natural teeth, if the orientation of the abutments is off slightly in relation to the other units in the bridge (marginal fit is good), the PDL will accommodate this, allowing the tooth to shift slightly, bringing the units into alignment within a day or 2. Unfortunately, implants do not have a PDL, and if there is tension (lack of passive fit) upon insertion, crestal bone loss is typically observed throughout time; this, in turn, may lead to further complications that range from periodontal issues to loss of the fixture. Thus, it is imperative to capture the implants in relation to each other so that a true passive fit can be achieved.
Intraoral implant scan bodies have been developed which allow the practitioner to place these into the implants and scan the implants—as well as adjacent natural teeth—intraorally. The scanning orients the implant virtually to position the analog of the implant in the vertical axis (positioning of the implant’s platform), angulation, and circumferentially (orientation of the implant’s connector). This information can then be sent via the Internet to the dental laboratory team for fabrication of the desired restorations.
CAD/CAM Milling Increases Accuracy of Fit
CAD/CAM milling is not new to dentistry and was used first with restorations for natural teeth and then expanded to use for dental implant restorations. CAD/CAM affords for more precise fitting implant restorations that are stronger than traditional cast UCLA-type bases, which have been the standard in the past. Implant restorations that will connect multiple implants either in a fixed prosthesis (cemented or screw-retained bridge) or overdenture bar, when CAD/CAM milling is utilized, allows for structures that are free of microporosity, which is commonly found in casting and eliminates the need for soldering or laser welding that may fail under loading at a later date.
The new technologies that will be addressed in this case presentation utilize digital impressions for capture of the information intraorally, eliminating the need for traditional impressions with their potential inaccuracies, and fabrication of CAD/CAM milled restorations for precise, strong implant restorations.
Diagnosis and Treatment Planning
An 84-year-old patient presented with a failing fixed PFM bridge on the lower right posterior involving teeth Nos. 29 (lower right second premolar) and 31 (lower second molar) with a pontic space fitting a molar between the abutments (Figure 1). The anterior abutment had extensive decay and was nonrestorable. The posterior abutment demonstrated sound crown margins, with no decay noted in the radiograph. The patient expressed a desire for only a fixed prothesis approach. The treatment plan formulated would section the bridge at the connector between the molar abutment and pontic, preserve the abutment crown on No. 31, extract the root at No. 29, and place implants at sites Nos. 29 and 30. If adequate insertion torque would be found to be present, immediate screw-retained provisionals would be placed for the healing period before final restorations were fabricated.
Clinical and Laboratory Protocols
Local anesthetic was administered, and then carbides and diamonds were utilized to section the bridge, leaving the abutment crown on tooth No. 31. The mesial contact was polished with a rubber wheel intraorally. Next, an incision was made, and a full-thickness flap was elevated to expose the root of No. 29 and the crest of site No. 30. The remaining root at No. 29 was atraumatically extracted using elevators and periotomes. Implant surgical drills were used sequentially to prepare the sites to accommodate Biodenta bone level root form implants to a depth of 12.0 mm and a 4.1-mm diameter. The lingual concavity necessitated placement of the implant at site No. 30 in a more mesial position. Implants were placed using a surgical handpiece until the handpiece torque reached 40 Ncm. Then, final seating was performed with a hand wrench, yielding a final insertion torque of greater than 50 Ncm (Figure 2).
|Figure 1. CBCT radiograph; the patient presented with a failing bridge abutment and fractured PFM bridge.||Figure 2. Biodenta bone level implants placed into the 2 sites with placement heads still attached.|
|Figure 3. Periapical radiograph of the Biodenta implants with screw-retained provisional splinted restorations, at 4 months post-insertion and ready for initiation of the final restorations.||Figure 4. Periapical radiograph of the scan bodies upon the Biodenta implants to verify full seating of the heads into the implants.|
|Figure 5. Scan bodies placed intraorally on the 2 Biodenta implants with the flat aspect of the scan head facing buccally.|
|Figure 6. TRIOS intraoral scanner (DentaSwiss) attached to a laptop, ready for scanning.|
Titanium provisional temporary abutments were placed on both implants, and the vertical height of each abutment was adjusted with a carbide bur. The provisional heads were coated with a dual-cure resin (Breeze [PENTRON]) and light cured, providing an intimate bond to the metal. A light-cured composite resin designed for temporary restorations (Revotec [GC America]) was used to achieve the general shape and contours of the splinted provisional crowns. The provisional restoration was removed intraorally, and a flowable composite (Flow-It [PENTRON]) was applied gingivally and buccally to complete the contours, and then light cured. The provisional restoration was contoured with finishing carbides followed by composite polishers (ComposiPro [Brasseler USA]). The provisional was returned to the mouth and the abutment screws tightened to 25 Ncm. Screw access was filled with a piece of polytetrafluoroethylene (PTFE) tape (Teflon plumber’s tape) and sealed with Cavit (3M ESPE). Occlusion was checked and adjusted, keeping the provisional restoration out of occlusion. The patient was told that the provisional was placed for aesthetics and was instructed to avoid chewing on it.
At 4 months post-implant placement and immediate provisionalization, the patient presented for initiation of the final restoration. It was noted that a portion of the provisional on the distal-buccal of No. 30 had chipped. A radiograph was taken to check integration (Figure 3). The provisional was removed and intraoral scan bodies (DentaSwiss) matched to fit a Biodenta B2 platform were inserted into the implants. Then, a radiograph was taken to verify full connection of the part with the implant (Figure 4). The flat area of the scan body was oriented toward the buccal, per the manufacturer’s instructions (Figure 5).
Next, the TRIOS intraoral scanner (DentaSwiss) (Figure 6) was set up, and scanning began with the area of interest capturing the buccal, occlusal, and lingual aspects to scan the entire arch (Figure 7). The opposing arch was then scanned, capturing the entire arch. The patient was then asked to occlude, and 2 scans of the articulated teeth were captured (each 3 to 4 units in the posterior bilaterally). It is not necessary to scan the entire occluded arch, as the software only requires those 2 scans to virtually occlude the maxillary and mandibular virtual arches.
The TRIOS scanner captures 3,000 2-D images per second, stitching the images together automatically as the scanning progresses, rendering up to one thousand 3-D images. Powder is not required, making the scanning process less cumbersome and eliminating the potential for the powder to affect the captured image. The live visualization on the TRIOS screen assists in indicating what areas of imaging have not been adequately captured, thus necessitating another pass over that section. The digital impression is then digitally sent to the lab team for processing and design. The lab team then designs the substructure virtually, and this is communicated via the Internet to the DentaSwiss Service Center for CAD/CAM fabrication of the models as well as the restoration substructure or abutments.
|Figure 7. The TRIOS intraoral scanner capturing the scan bodies and adjacent teeth intraorally.||Figure 8. A virtual model of the lower arch, showing placement of implant analogs designed from the data collected with the TRIOS intraoral scanner.|
|Figure 9. Virtual models were designed from the intraoral scans collected.||Figure 10. Substructure for the splinted, screw-retained crowns is virtually created with sufficient cutback to allow for ceramic overlay to be placed after milling.|
|Figure 11. Virtually designed splinted substructures off the virtual model shown from the buccal and occlusal aspects.||Figure 12. CAD/CAM fabricated lower model with implant analogs positioned.|
First the lab trims the virtual models, and the software places analogs into the positions determined by the scan body with respect to position and orientation (Figure 8). Next, the lab specifies the desired models required and creates these virtually (Figure 9). The abutments are then designed virtually for 2 splinted, screw-retained porcelain-fused-to-titanium (PFT) crowns, allowing for adequate cutback for ceramic placement following CAD/CAM milling of the substructure (Figures 10 and 11). The information is sent digitally to the DentaSwiss Service Center, wherein the models are 3-dimensionally printed and the substructure is milled with a computer numerical control (CNC) machine. Analogs are positioned into the lower CAD/CAM model in the proper position and orientation (Figure 12). The models and substructure are then returned to the lab to have ceramics applied and to finish the case.
The titanium splinted substructure for a screw-retained, PFT fixed prosthesis was designed to have polished metal in the soft-tissue sulcus, as this is better tolerated by the gingiva than ceramic (Figure 13). The ceramic used here (described below) exhibits excellent tissue biocompatibility. The lab team placed the CAD/CAM milled substructure onto the model and checked the fit as well as cutback to verify sufficient space for ceramic application (Figure 14). A porcelain designed to be fused to either titanium or zirconia (ceramics2in1 [Biodenta]) was then applied in layers and shaped to the contours and aesthetics required (Figure 15). Ceramic application to titanium can be challenging, due to the graying effect that occurs when titanium is fired, resulting in a dark oxide layer. This particular ceramic has a unique and innovative titanium opaquer that causes a sealing of the titanium surface and establishes a reliable bonding between metal and ceramics. The resulting restoration demonstrates high flexural strength and improved optics due to the absence of a separate bonding layer to achieve adhesion of the ceramic to the metal. As leucite crystals are avoided in the ceramic, a stable thermal expansion coefficient ensures a stabilized processing to both titanium and zirconia.
|Figure 13. CAD/CAM milled splinted substructure for a screw-retained prosthesis with cutback for ceramic overlay.||Figure 14. Articulated CAD/CAM models with CAD/CAM milled titanium screw-retained splinted substructure fixated to the analogs with reduction for ceramic overlay.|
|Figure 15. Porcelain (ceramics 2in1 [Biodenta]) was applied to the titanium milled substructure to develop the contours and aesthetics needed.||Figure 16. Buccal occluded view of the finished, splinted screw-retained prosthesis fabricated from a CAD/CAM milled titanium substructure with the layered porcelain on the models.|
|Figure 17. Occlusal view of the finished splinted screw-retained prosthesis fabricated from a CAD/CAM milled titanium substructure with overlaying porcelain on the model, showing the screw access openings.||Figure 18. Buccal occluded view of the finished splinted screw retained prosthesis fabricated from a CAD/CAM milled titanium substructure with the layered porcelain
The ceramic was placed in layers to develop the desired contours and aesthetics, firing the restoration as needed during the lab process. Finishing and polishing were performed to complete the restoration (Figures 16 and 17).
Delivery of the Prosthesis
The screw-retained provisional restoration was removed, and the final PFT, screw-retained splinted 2-unit bridge was tried in (Figures 18 and 19). The abutment screws were placed to finger tightness, and radiographs were taken to verify fit, as well as the fit of prosthesis to the implants (Figure 20). A torque wrench was utilized to tighten the abutment screws to 35 Ncm. PTFE tape was placed into the screw access holes and taped down over the abutment screws. A flowable resin (Artiste [PENTRON]) was placed over the PTFE tape and light cured to close the screw access holes. Occlusion was then checked carefully and adjusted.
|Figure 19. Occlusal view of the finished, splinted screw-retained prosthesis fabricated from a CAD/CAM milled titanium substructure with overlaying porcelain (ceramics2in1) intraorally.||Figure 20. Periapical radiograph of the splinted screw-retained prosthesis on the Biodenta implants, verifying fit.|
Digital technology has expanded into the dental implant treatment modality, allowing the restoring dentist to capture implant position via intraoral scanning and the lab to complete fabrication of the restorations virtually via CAD/CAM technology. The result is a more precise and stronger restoration avoiding the issues seen with traditional impressions and casting of restorations.
The authors would like to thank Igor Kachanovsky, CDT, at Lintec Dental Lab (Falls Church, Va), for the lab work shown in this article.
Disclosure: Dr. Kurtzman reports no disclosures.
Disclosure: Dr. Dompkowski reports no disclosures.
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