Use of Selective Open Architecture in Digital Restoration Fabrication

Capturing accurate impressions for indirect fixed restorations using elastomeric materials is difficult. In the office, impression material selection, tray selection, placement of the material into the tray and around the prepared teeth, undisturbed setting, proper removal after setting, and ideal handling during disinfection, packaging, and transport are among many potential causes of unsuccessful or distorted impressions. The dental literature indicates that the number of defective impressions sent to commercial dental laboratories for construction of fixed restorations is excessive. In a review of 193 impressions performed by 43 practitioners and sent to commercial laboratories for construction of FPDs, Samet et al¹ found problems that they deemed "disturbing." Ninety percent of the impressions had at least one defect, with greater than 50% having voids or tears at the finish line. Fifty-five percent of the impressions were in soft plastic disposable trays. These trays, when combined with high-viscosity impression materials, have been shown to allow distortion across both the prepared tooth and the arch.²

Assuming practitioners overcome the difficulties encountered in their offices, the next challenge is transferring the impression to the dental laboratory for fabrication of the restoration. Precise handling of the impression during cast fabrication, die sawing, die trimming, margin marking, and careful articulation are mandatory. Failure to accomplish any of these steps can compromise the final result.

In the authors' experience, assuming preparation and isolation are ideal, the Lava C.O.S. will eliminate all of the above sources of error. It allows the clinician to preview the preparation and isolation, enabling any necessary correction prior to scanning. Without impression material, problems with timing of the mix, void-free placement of the light-bodied material, tray seating, undisturbed setting, disinfection, packaging, transport, and all other materials issues are eliminated. Without the need for a tray, concerns regarding tray size selection, rigidity, tray contact with the teeth, or the separation of impression material from the tray are eliminated.

As the digitized replicas of the prepared teeth are transmitted electronically and models are created from this electronic data, all sources of laboratory error from pouring to articulation are eliminated. Initial studies of stereolithographic (SLA) model accuracy show it to be consistent with those created from vinyl polysiloxane (VPS) impressions.³ Stereolithography is the process by which photosensitive resin is laser "printed" to create a 3-D replica of the scanned arches. The fit of the crowns fabricated from these resin dies generated from electronic data are equal to those created from VPS impressions.⁴

Independent of the system type, reduced seating times of restorations due to accurate fit is likely the most profound and consistent benefit reported by early adopters, which confirms the accuracy of digital data capture and resulting model fabrication. Reviewed and published scientific studies for verification and validation of the accuracy of digital impressions vs conventional impressions and the restorations fabricated from each are needed.

Case 1

A 57-year-old male presented with the chief complaint of biting pressure and cold sensitivity in the left lower quadrant.

Clinical findings: Tooth No. 18 had a large occlusal amalgam restoration and failing margins. Tooth No. 19 had a large occlusal amalgam restoration, failing margins, and an enamel crack evident in the distal marginal ridge.

Radiographic findings: Teeth Nos. 18 and 19 had deep amalgam restorations and sound interproximal bone levels. Tooth No. 18 had evidence of a sedative base.

Pulpal diagnosis: Tooth No. 18 had a normal response to cold, slight sensitivity to biting pressure, and a normal response to percussion. Tooth No. 19 demonstrated an exaggerated response to cold and an acute response to biting pressure but a normal response to percussion.

Diagnosis: Teeth Nos. 18 and 19 had failing amalgam restorations and compromised structural integrity. Tooth No. 19 was cracked.

Plan: The plan involved the use of cast gold restorations with composite resin buildups to eliminate decay and to restore the structural integrity of the teeth.

Case 2

A 77-year-old male presented with the chief complaint of a missing upper left tooth, which had a removable replacement.

Clinical findings:Tooth No. 13 was missing. Flexible unilateral replacement supported entirely by the soft tissue. Tooth Nos. 12 and 14 provided stability. Gingival recession was evident on the palatal of teeth Nos. 12 and 14 secondary to trauma from a tissue-borne removable partial denture. Tooth No. 12 had been restored with a mesial-occlusal composite resin with evidence of a crack through the distal marginal ridge. Tooth No. 14 had been restored with a full gold crown.

Radiographic findings: There was a normal periapical appearance of teeth Nos. 12 and 14. The maxillary sinus location prevented implant placement without grafting. Bone loss was evident on distal tooth No. 14.

Diagnosis: Tooth No. 13 was missing, requiring replacement for dental arch stability and esthetics.

Plan: The plan involved the use of a fixed partial denture, Lava Zirconia, abutments being teeth Nos. 12 and 14, and a pontic tooth No. 13.

In both cases, the author followed the same protocol used in conventional techniques to the point of completion of the preparations and placement of two retraction cords. The teeth to be scanned were then isolated with an OptraGate® (Ivoclar Vivadent, https://www.ivoclarvivadent.com) and dry angles. A very light application of titanium dioxide powder was placed per manufacturer to enable the Lava C.O.S. to record points of reflectivity. As with VPS impressions, dryness is essential. In each case, full-arch scans of both the operative quadrant and opposing arches were accomplished (Figure 1, 2, 3, 4, 5, 6).

Scanning with the Lava C.O.S. is considerably different than impressioning. The operator focuses on the computer monitor to ensure complete data capture. This is a learned skill; repetition will generate confidence and improve technique. The scanner records the interocclusal relation with the patient in maximal intercuspal position and displays the articulated case on the monitor. A touch screen enables the operator to view the scans from every angle. After a review of the scans, the electronic prescriptions were completed and all data captured was transmitted via wireless router to the authorized design laboratory for completion of virtual articulator selection, digital margin marking, digital die trimming, SLA model manufacture, and CAD/CAM or traditional restoration fabrication.

The so-called digital dental highway is changing the landscape of how dentistry is practiced and how laboratories provide support. The influx of new materials, technology, and state-of-the-art manufacturing methods in the dental community are giving dental laboratories new choices, but also forcing them to make tough decisions. While they may want to jump on the digital highway, most laboratories are not in a position to make the significant financial investments in the necessary capital equipment and subsequent hiring or retraining of staff. Until they are ready, their best solution initially is to partner with an entity that has made those investments and already has the staff, equipment, and experience in place.

The digital workflow essentially consists of hardware and software that digitize the patient's mouth, special software to design restorations, and finally milling or printing equipment that produces the restoration or facsimile of it that can be used with traditional laboratory fabrication techniques. Until recently the systems used in the digital workflow have fallen into two categories: "open" architecture systems and "closed" architecture systems.

3M ESPE's Lava brand originally was an example of a closed architecture system. 3M ESPE Lava model scanners only work with 3M ESPE Lava restoration design software, which only works with 3M ESPE Lava milling systems. The advantages of such closed architecture systems are simplicity and one-stop support for resolving issues. However, users are limited to only the materials and features offered by that manufacturer.

Open architecture systems work with an industry-standard digital file that allows a model scan from Company A to be used to design a restoration with software from Company B, which can then be fabricated with a mill or printer from Company C. This process offers virtually unlimited flexibility with regard to case input, design, manufacturing methods, and materials. However, the process is slightly more complex, and more significantly, technicians may be talking to three or more companies to find solutions to problems that may occur.

3M ESPE recently adopted a "Selective Open Architecture" initiative in which the company is validating the interoperability between the 3M ESPE Lava system and other manufacturers' scanners, design software, and milling or printing systems. 3M ESPE is ensuring that a scan performed using one system can be designed in another, and milled or printed in potentially yet another, with at least one step in the process involving a 3M ESPE product. It is an ambitious task. However, once the systems have been validated, dentists and technicians can be confident that these systems should work together every bit as well as the "closed" architecture systems perform.

Cases are delivered with an e-mail announcement and appear within the 3M ESPE C.O.S. Laboratory Software. The software is used to set the bite plane of the model, virtually die cut the operative teeth, and identify the margins so the model fabrication facility can print the working and solid model forms. The software provides four different views of the prepared teeth: a virtually created scan model, a margin profile view, photographs of the prepared teeth, and 3-D photographs. This combination provides perspectives to identify margins in a level of detail that traditional methods cannot offer. The entire process typically takes 4 to 10 minutes per preparation, depending on the quality of the scan and preparation.

Case 1

The bite plane was set, dies were cut, and margins were identified. The 3-D photographs were extremely helpful in precisely identifying beveled margins (Figure 7). The margin marking software provides a tool that allows the user to select where the die should be ditched below the margin and where it should not. This was particularly useful given the supragingival margins on this case and allowed the laboratory to maintain emergence profile of the prepared teeth.

Case 2

The bite plane was set, dies were cut, and margins were identified for a three-unit Lava bridge (Figure 8).

Before submission to 3M ESPE, a conference call and a computer screen-sharing session were arranged with the dentist and laboratory to review margins on both cases. No changes were made, and the cases were returned to 3M ESPE for file splitting and model fabrication. The margins in both cases were clear and well-defined, as a result of the dentist's attention to detail during tooth preparations, tissue management techniques and the level of detail captured by the C.O.S.

Once margins are marked and the cases submitted to 3M ESPE, models typically arrive within 48 hours. Models for CAD/CAM-based restorations are shipped to the CAD/CAM facility. Traditional non-CAD/CAM porcelain-fused-to-metal (PFM) or pressed material restoration models are shipped directly to the laboratories completing the cases.

A few hours after submitting the margin-marking files, the digital model files were available for download from 3M ESPE via a web-interface case manager. While models are being fabricated, CAD/CAM restorations can be designed and fabricated. This parallel manufacturing is a major step forward in productivity.

Both cases were imported into 3Shape Dental Designer software using a proprietary 3M ESPE file format which includes case information such as patient name, shade, tooth number, and materials. The process requires no more than several mouse clicks.

Case 1

Full-contour wax crown designs were created starting with a default library tooth. Design adjustments were made with advanced shaping tools within the 3Shape software (Figure 9). 3Shape's Dental Designer software allows for multiple restorations, including multiple single units as well as bridges, on the same arch, to be designed simultaneously. This is a case that would have been extremely difficult to design using 3M ESPE's Lava Design software, which only allows for single unit or bridge designs.

Case 2

A scan anomaly was found on the distal wall of tooth No. 12, and software tools within the 3Shape design software were used to remove the anomaly (Figure 10). The three-unit bridge framework was designed using simple copings, a pontic, and connectors. Proper support of porcelain was ensured by using the software shaping tools to extend the marginal ridge and cusp areas (Figure 11).

A second conference call and screen-sharing session was scheduled for the dentist and laboratory to review the designs. Minor changes were made to the contact areas of the gold crowns based on the dentist's and laboratory's preferences (Figure 12).

Both case designs were exported from 3Shape's Dental Designer software for milling in 3M ESPE's Lava™ Form mill. Because Apex Dental Milling's workstations are networked together, copying and pasting of the exported files to the appropriate folder for 3M ESPE's software was a simple process. 3Shape laboratories can send cases to 3M ESPE milling centers via Lava™ Connect, an FTP utility, or within an e-mail.

The zirconia framework was imported into the Lava software and prepared for milling with other zirconia copings. The framework was milled (Figure 13), cut from the frame stock (Figure 14), shaded, and sintered.

For some reason, the wax patterns for the gold crowns would not properly import into the Lava software for milling. Due to time constraints, the wax patterns had to be milled in one of the other Apex Dental Milling open architecture milling systems. After removal from the stock material (Figure 15) and blending the sprues (Figure 16), the cases were forwarded to LeBeau Precision Aesthetics (gold castings) and Summit Dental Laboratory (Lava FPD) for finishing.

CAD/CAM in the dental industry is evolving. New scanners, design software, and mills or printers are being introduced regularly. New materials are under development, and software continues to be updated.

In the authors' opinion, the hardware and software technology involved in producing these cases is sound. Software improvements will continue, most likely improving design tools and tooth libraries, and adding protrusive and excursive motion simulations.

The ultimate proof of how quickly the industry can change: although the wax patterns couldn't be milled with the 3M ESPE Lava Form mill for one of these cases, the software fix became available two weeks after the substitute patterns were delivered to LeBeau. Both 3M ESPE and 3Shape detected minor issues in their software and corrected them. Those patterns have since been milled with the 3M ESPE mill. No changes to hardware or to equipment were required; only a few lines of software were changed. 3M ESPE and 3Shape would not have known about the issues with their software if the companies had not collaborated to validate their systems. Selective-open architecture is giving this milling center a new level of confidence when going from one CAD/CAM system to another and giving laboratories with 3Shape design software a solid path to offer Lava restorations to their doctors.

Digital data files that are electronically submitted to dental laboratories facilitate the use of Internet-based collaboration between the dentist and laboratory technicians. In a conference call, full screen views of all facets of the design process enable real-time input from all persons involved in the fabrication of the restorations, which is not possible with conventional techniques. Two such online meetings occurred in the illustrated cases. These meetings aided in the analysis of the margins of all preparations (especially helpful in ensuring identification of the beveled margins of the gold preparations), the development of the interproximal contact areas of the gold restorations, the zirconia substructure design for maximum porcelain support and esthetics, and the pontic site development in the SLA model.

The fit of the milled wax units for the 7/8 gold crowns was impressive. Marginal integrity on the SLA model was outstanding. Two sets of patterns were milled, with one modified by conventional wax additive technique to enhance the anatomic contour of the computer-generated unit, and the other cast as milled. In attempting modification, the laboratory technician found that the milled unit did not accept the heated wax well. There seemed to be a lack of coalescence of the two wax materials. The use of milled wax for construction of cast gold restorations is currently more labor and time intensive (and truly developmental) than the wax additive technique. CAD/CAM techniques should help overcome ever-increasing manpower shortages in dental laboratories if this technology advances to a level of clinical excellence that equals or exceeds current protocol. The design technician experienced some early limitations in the capabilities of the software to properly contour the interproximal contacts of the adjacent wax patterns using Lava software, but these were overcome with the software engineers' creating an open architecture between the C.O.S. data and 3Shape.

At the seating appointments, provisional restorations were removed and the prepared teeth thoroughly cleaned. The final restorations were then tried in. Their marginal fit, proximal contacts, occlusion, and esthetics (in the case of the ceramic FPD) were verified. Cementation was accomplished with RelyX™ Luting Cement (3M ESPE).

Case 1

All of the gold castings fit the prepared teeth well with margin integrity equal to conventionally fabricated restorations (Figure 17, 18, 19, 20). Minor adjustment of the occlusion was necessary for both sets of castings. The crowns that were not modified were selected for cementation.

Case 2

The Lava FPD required no adjustment at seating. Interproximal contacts were ideal and occlusal contact held shim stock on two of the three units. Marginal integrity was excellent with explorer assessment. Findings from the radiographic evaluation supported those from the clinical observation. As is evident in the photographs, esthetic excellence was accomplished (Figure 21, 22, 23, 24). In the authors' experience, this restoration was clearly equal to the best results obtained with conventional impression systems.

On a clinician's best days, after meticulous attention to the details necessary for ideal preparation and isolation, impression material can help create outstanding restorations. Digital systems can do the same, with the bonus that they eliminate the sources of many potential errors. The cases presented demonstrate that the move to computerization can enable outstanding clinical results. With advances in the use of the digital data captured in the scan process into new software applications, the efficiency and accuracy of restoration fabrication will continue to improve.

The authors would like to recognize the talents and efforts of the laboratory technicians at Apex Dental Milling, Ann Arbor, Michigan; Summit Dental Labs, Waco, Texas; and LeBeau Precision Aesthetics, Kent, Washington, in the creation of the restorations displayed in this article.

References

1. Samet N, Shohat M, Livny A, et al. A clinical evaluation of fixed partial denture impressions. J Prosthet Dent. 2005;94(2):112-117.

2. Cho GC, Chee WW. Distortion of disposable plastic stock trays when used with putty vinyl polysiloxane impression materials. J Prosthet Dent. 2004;92(4):354-358.

3. Ogledzki M, Wenzel K, Doherty E, Kugel G. Accuracy of 3M-Brontes stereolithography models compared to plaster models. J Dent Res. 89(Spec Iss A):1060, 2010.

4. Beuer F, Naumann M, Gernet W, Sorensen JA. Precision of fit: zirconia three-unit fixed dental prosthesis. Clin Oral Investig. 2009;13(3):343-349.