The Reverse Scan Body Protocol: Completing the Digital Workflow

The use of intraoral scanners for acquiring implant positions, designing prototypes, and fabricating definitive zirconia implant fixed complete dentures is gaining popularity.^1-12 While intraoral scanning is effective at capturing full-arch implant scans in completely edentulous patients, the procedure is technique sensitive. The absence of anatomical landmarks, such as teeth, in fully edentulous patients makes the alignment of digital standard tessellation language (STL) files challenging.^1-6 To counteract the lack of adequate landmarks needed to stitch the scan, the double digital scanning technique utilizes fiducial markers to help overcome the superimposition limitations of an intraoral scanner. While this technique is efficient for the maxillary workflow, the lack of adequate residual ridge and absence of a palate makes this workflow difficult for scanning the edentulous mandible.² Nevertheless, the use of fiducial markers with intraoral scanning does aid in data acquisition of edentulous jaws and assists in merging multiple digital scans.^7-12 Other variables, such as the presence of saliva, sensor fogging, ambient light, and lack of keratinized mucosa, coupled with advanced mandibular resorption, also may hinder adequate data acquisition with an intraoral scanner when scanning a completely edentulous mandible.^3,4

Recently, extraoral techniques have provided an alternative approach for the digital acquisition of implant positions in fully edentulous patients.^13,14 The "reverse scan body protocol" digitally simulates the traditional back-pouring technique that has long been utilized in analog workflows. Papaspyridakos et al reported 100% fit with this protocol when milling a prototype, as well as the definitive prosthesis, demonstrating it to be a predictable and efficient workflow for fabricating a full-arch prosthesis.¹³

This article discusses two cases, one featuring an edentulous maxilla and the other an edentulous mandible, in which an extraoral scanning technique was used to capture implant positions, design a prototype, and fabricate the definitive monolithic zirconia prosthesis to restore function and esthetics. The cases were treated with the Straumann^® Pro Arch (Straumann, straumann.com) concept.

It should be noted that before starting the fabrication of the final prosthesis using this extraoral scanning protocol, the clinician should evaluate the fixed provisional prosthesis by torque testing the abutments to ensure they are not loose and assessing the accurate fit of the interim prosthesis.

A 50-year-old man presented to the authors' clinic with a maxillary removable partial denture (RPD) and terminal dentition (Figure 1 through Figure 3). Digital workflows were used to plan, design, and execute guided surgery and conversion of an immediate-load prosthesis for a maxillary rehabilitation (Figure 4 and Figure 5). After 6 months of healing, the implants were osseointegrated and screw-retained abutments were torqued. The conversion prosthesis was evaluated for proper fit, clinically, visually, and radiographically, and was also assessed for appropriate esthetics, phonetics, and vertical dimension of occlusion, which aids in the design for a prototype try-in. The extraoral reverse scan technique, as described in the following steps, was used for the digital acquisition for the implant positions to enable fabrication of a prototype prosthesis.

The sequence of scans is as follows:

Step 1: The conversion prosthesis is scanned intraorally, as is the opposing arch and the patient's occlusion (Figure 6, left). These scans provide the desired tooth position and vertical dimension, which had been established and verified for the patient.

Step 2: The prosthesis is then removed, and the edentulous arch is scanned to capture the soft-tissue topography (Figure 6, center). The scanning of the soft tissues reveals the healed residual ridge anatomy, aiding the technician in designing hygienic yet intimate intaglio contours of the prototype and, subsequently, the definitive prosthesis.

Step 3: The extraoral scan is executed (Figure 6, right). Reverse scan bodies (Straumann^® RevEX, Straumann) are attached to the provisional prosthesis, which is scanned extraorally. This scan provides the implant positions for the digital master cast.

The scans taken in steps 1 through 3 were then imported into a computer-aided design (CAD) software (DentalCAD, exocad, exocad.com) and aligned to each other to digitally articulate and cross-mount the STL files (Figure 7). An implant library for the reverse scan bodies was used to inform the software which implants were used for this case and to allow for the connection of the prosthesis to the respective titanium base. To verify that the STL file of the implant positions was accurately captured, a digital technique¹⁵ was used to design and mill a verification jig, which was then seated and evaluated intraorally (Figure 8 and Figure 9).

Once the verification jig was confirmed to have a complete and passive seat, an acrylic resin prototype was designed, milled, and delivered to the patient (Figure 10). The prototype was evaluated to confirm the desired esthetics, phonetics, and occlusion. The patient's ability to access the intaglio surface of the prosthesis for adequate hygiene was confirmed prior to completing the definitive prosthesis.

The patient was sent home to test-drive the prototype prosthesis with instructions to evaluate the esthetics and function before fabrication of the final prosthesis. After the patient approved the prototype, a CAD/computer-aided manufacturing (CAD/CAM) monolithic zirconia prosthesis with titanium bases was fabricated and delivered as the definitive full-arch prosthesis (Figure 11 and Figure 12).

A 91-year-old woman presented to a private prosthodontic clinic with a maxillary RPD opposing a mandibular complete denture. The patient, who was in good medical condition, was unhappy with the mandibular prosthesis.

A comprehensive diagnostic work-up was done, and treatment options were discussed with the patient, who opted to receive a fixed implant solution for her edentulous mandible. After dual CBCT scanning, importing of DICOM files, and digital implant planning (coDiagnostiX^®, Straumann) were completed, four implants were digitally planned and placed with guided surgery using a mucosa-supported surgical template. After implant placement, conversion of the existing mandibular denture was performed, and the patient was restored with a fixed conversion prosthesis.

After 2 months of healing, the patient presented for assessment of osseointegration (Figure 13). All four implants were successfully osseointegrated and the patient was pleased with the function and esthetics of the provisional prosthesis. At the same visit, the conversion prosthesis was evaluated for proper fit, clinically, visually, and radiographically, and the multi-unit abutments were torque-tested to ensure they were not loose (Figure 14). The extraoral reverse scan technique, as described above, was used for the digital acquisition of the implant positions to allow CAD/CAM fabrication of a prosthesis prototype (Figure 15). The STL files from the extraoral reverse scan technique were imported into CAD software (exocad), and the prosthesis prototype was designed and milled. Additionally, a CAD/CAM verification jig was designed and milled to be tried-in simultaneously with the prototype to ensure quality control, as described above.¹⁵

At the second restorative visit, the prosthesis prototype was evaluated to confirm the desired esthetics, phonetics, occlusion, and accuracy of fit. After being deemed adequate, the prototype was scanned intraorally and the STL file was sent to the laboratory technician, who then fabricated the mandibular screw-retained definitive prosthesis (Ivotion^™, Ivoclar, ivoclar.com). At the next and final visit, this definitive prosthesis was delivered and torqued (Figure 16 and Figure 17).

A benefit of this complete digital workflow is the fabrication of digitally designed prototype prostheses, which simplifies the rehabilitation process by reducing the number of required appointments. Once the interim prosthesis is assessed for appropriate esthetics, phonetics, and vertical dimension of occlusion, this workflow maximizes the efficiency for the data acquisition appointment by affording the ability to capture the dentition, intaglio contour, and implant positions extraorally. The extraoral scanning averts the challenges with intraoral scanning and eliminates the need for fiducial markers for data merging or virtual cross-articulation of STL files. The STL file derived from the extraoral scanning is easily superimposed with the STL file from the intraoral scanning of the interim prosthesis and opposing dentition.

Clinicians may consider the inherent flexibility of the resin material used for the fabrication of the conversion prosthesis a limitation of this workflow, even though the conversion prostheses were designed with adequate thickness. Hence, the fabrication of a verification jig with reduced thickness may be considered to ensure that the implant positions of the digital file represent the actual clinical implant positions, and thus serves as a quality control step in this complete digital workflow. Once passive fit is assessed, the verified jig can be back-poured into a cast and aid in the cementation of the titanium inserts for both the prototype and definitive zirconia prostheses.

The reverse scan body protocol streamlines the digital workflow for the fabrication of a passive-fitting final prosthesis. The accuracy of fit of both the generated prototype and definitive prosthesis is crucial for long-term success. Additionally, this protocol can reduce the number of patient visits to as few as three. Clinicians should appreciate various reports in the literature that discuss the potential for complications with implant fixed complete dentures.^16-21

This article was commercially supported by Straumann.

E. Armand Bedrossian, DDS, MSD
Associate Professor, Division of Postgraduate Prosthodontics, University
of Washington; Private Practice in Prosthodontics, San Francisco, California; Fellow, American College of Prosthodontics; Diplomate, American Board of Prosthodontists

Panos Papaspyridakos, DDS, MS, PhD
Associate Professor, Division of Postgraduate Prosthodontics,
Tufts University School of Dental Medicine, Boston, Massachusetts

Edmond Bedrossian, DDS
Professor, Department of Oral and Maxillofacial Surgery, Dugoni School of Dentistry, University of the Pacific, San Francisco, California; Fellow, American College of Oral & Maxillofacial Surgeons; Diplomate,
American Board of Oral & Maxillofacial Surgeons

Christopher Gurries, DDS
Private Practice in Oral Surgery, San Francisco, California