Temporary Shell Proof-of-Concept Technique: Digital-Assisted Workflow to Enable Customized Immediate Function in Two Visits in Partially Edentulous Patients

The dental implant patient's expectations for implant rehabilitation have expanded. Patients no longer expect only improved function when replacing missing teeth, but they also anticipate a natural-appearing, fixed provisional restoration that can be delivered immediately.¹ Prosthetically driven diagnosis and treatment are necessary to achieve optimal implant positioning and an ideal prosthetic reconstruction.^2,3 In a digitized workflow, the primary challenge is transferring the soft-tissue contours and dental anatomic landmarks from the patient onto a 3-dimensional (3D) model generated by an implant planning software program. Digital developments have made it possible to fuse different sets of 3D imaging files (digital imaging and communications in medicine [DICOM]) and stereolithography (STL) files, resulting in the creation of a virtual dental patient. Superimposition and 3D rendering of the facial skeleton, soft tissue, and dentition provide a systematic method for evaluating all aspects of dentofacial anatomy, function, and esthetics in a more logical and interdisciplinary manner than the conventional approach.

The growing interest in minimally invasive implant placement, along with the option of delivering a prefabricated provisional prosthesis immediately, has precipitated the development of numerous 3D planning software programs.^4-8 A digital workflow in implant dentistry starts with acquisition of clinical data, including clinical photography, cone-beam computed tomography (CBCT) scans, and intraoral optical scanning (IOS) of the dental arches or extraoral optical scanning (EOS) of the patient's study casts. Technological advancements have significantly improved data acquisition, providing a highly realistic overview of bony and anatomic structures, as well as bone density, for enhanced predictability of implant stability during the virtual planning stage.⁹ In addition, the use of IOS to retrieve the surface scan of the residual dental arch and soft-tissue architecture gives the clinician a realistic view of the intraoral cavity. Integrating these comprehensive clinical data into a 3D visualization of the implant recipient site characteristics and neighboring anatomy provides the clinician with better insight into the surgical, prosthetic, and esthetic treatment requirements and, as such, may improve decision-making, increasing the predictability of the overall implant treatment.¹⁰

In the integrated digital workflow, the superimposition of the CBCT scan and optical surface scan (Figure 1), through matching of the resulting DICOM and STL data files, respectively, requires mutual landmarks in both scanning datasets. This represents the main limitation to the use of the integrated digital workflow in the fully edentulous patient, where fewer landmarks, such as teeth or restorations, exist. Ritter et al assessed the accuracy of this newly developed digital workflow on 16 patients through 1,792 measurements on teeth.¹¹ All data pairs were matched successfully, and mean deviation between CBCT and optical surface scanning data was between 0.03 mm (±0.33 mm) and 0.14 mm (±0.18) mm. Based on the results of this study, the registration and matching of 3D surface data and CBCT data is reliable and sufficiently accurate for implant and prosthetic planning.

Operators, however, should not blindly trust the transfer precision from the 3D virtual surgery, via the drill guide, to implant placement. Rather, an adequate learning curve and careful application of guided surgery protocols is necessary. Hence, the belief that guided implant surgery requires less training than conventional freehand implant placement is unfounded.¹⁰

Nevertheless, the digital integrated workflow using tooth-supported surgical templates, having a higher degree of accuracy when compared with bone- and mucosa-supported surgical templates, may reduce the 3D discrepancy between the virtual plan and the actual surgical placement of the implant.^7,12-14 Technically, the accuracy of this automatic matching workflow is 1 voxel size below the manual segmentation workflow (Nobel Biocare AB, Göteborg, Sweden, data on file), based on pairing of a minimum of three points on the surface of the patient CT/CBCT anatomy with the equivalent three points from the patient anatomy obtained by digital high-resolution optical scanning. Thus, the combination of IOS and CBCT images, by mutual superimposition and use of planning software, provides a complete and accurate 3D representation of both hard and soft tissues (Figure 2 and Figure 3).

The most current 3D implant planning software program used by the authors (DTX Studio^™, Nobel Biocare, nobelbiocare.com) automatically overlays the DICOM data from CT/CBCT of the patient with the STL data from the extraoral (EOS) or intraoral (IOS) high-resolution optical surface scan of the patient's intraoral anatomy using a proprietary algorithm process. Therefore, the patient dentition is scanned (STL files) and integrated with the craniofacial (hard tissue) model (DICOM files) to create a more accurate 3D model of the patient's hard- and soft-tissue anatomy.

The availability of tooth morphology in virtual libraries within the implant planning software streamlines the digital planning further by providing a virtual wax-up that may be used to visualize the ideal prosthetic set-up. The virtual diagnostic wax-up can be performed on the STL files of the patient's digital models, assisting the digital planning of the implant surgery.

The novel diagnostic planning program used by the authors allows for automated design of the missing tooth/teeth with the proper functional and esthetic shape, as well as placement of the missing tooth/teeth in occlusion with teeth in the opposing arch (Figure 4). This "smart tooth" set-up dramatically reduces the time it takes clinicians to create a prosthetically driven treatment plan. The "smart set-up algorithm" feature is not only a way to streamline the planning of implant position, but also produces a personalized CAD/CAM design of the interim restoration. This is achieved through generation of an STL file of the provisional restoration, which is automatically created after completing the 3D implant positioning using the DICOM files. The generated STL files of the provisional restoration are ready to be shared with any in-lab or in-house prosthetic planning software program, in which adjustment of the position of the transmucosal component, the occlusion, and interproximal contact points may be performed (Figure 5 and Figure 6).

In the authors' experiences, it is highly beneficial for clinicians to have a thorough understanding of the interplay between the implant position and the prosthetic emergence of the provisional restoration. Once the dental technician or restorative doctor has determined the final tooth shape, the STL file of the interim restoration is imported back into the surgical planning software for a final inspection (Figure 7 and Figure 8).

The digitally designed, prefabricated provisional restoration has an ovate pontic shape with an open transmucosal portion, with two proximal wings fitting the occlusal or cingulum surface of the adjacent teeth to interdigitate precisely, thus placing the provisional restoration in the correct position as digitally planned. This temporary shell design with proximal wings may be relined and fitted onto the abutment connected to the implant that was just placed with a CAD/CAM surgical guide template (Figure 9). A five-axis milling machine fabricates the temporary shell from a multilayered polymethyl methacrylate (PMMA) CAD/CAM material that confers the provisional restoration with a natural esthetic appearance (Figure 10 and Figure 11). An auto-polymerizing polyurethane resin is used to reline the temporary shell and improve the fit, ensuring accurate positioning onto the abutment surface. The emergence profile of the provisional restoration is adapted by trimming the resin remnants, polishing the surface, and, lastly, removing the proximal wings used to index the restoration.

The virtual articulator embedded in the restorative software is effective for designing a "non-occluding" provisional restoration, eliminating any static and/or dynamic contacts in the case of an implant-supported single crown. For a multiple implant-supported provisional restoration, the occlusal scheme is virtually designed and adjusted to obtain light contacts in centric relation, equally distributed on the masticatory surface, and, in lateral excursion, contact is achieved with group function guidance.

When the recipient site anatomy allows for prosthetically driven implant positioning, the overall accuracy of the digital workflow results in an easy fitting of the temporary shell onto the prosthetic abutment. Moreover, it enhances the clinician's ability to provide the patient with immediate function using a digitally prefabricated, screw-retained provisional restoration (Figure 12 and Figure 13).

The modern digital workflow utilizing a temporary shell to deliver a provisional prosthesis in two patient visits is summarized in Table 1.

Computer-guided implant surgery may help clinicians perform successful implant therapy while avoiding elevation of large flaps or eliminating flaps altogether, resulting in less pain, discomfort, and swelling to patients.⁷ The digital workflow also has been reported to be more efficient than the conventional workflow in terms of cost and time,¹⁵ and it attains higher levels of acceptance by patients.^10,16 However, an adequate and multidisciplinary learning curve is still important because the overall accuracy of the digital workflow depends on the cumulative amount of interactive errors involved, from dataset acquisition, to performing the surgical procedure, to delivery of a fixed restoration.

Moreover, clinicians must be aware that computer-driven flapless surgery often overlooks the ideal location of implants relative to important soft-tissue anatomic features, such as the thickness, width, and position of keratinized mucosa.¹⁷ New technologies that combine CT/CBCT data (DICOM) with information on the soft tissues and crown morphology, obtained through digital high-resolution optical scanners, are encouraging, as they are expected to positively impact the outcome of guided surgery and immediate prosthetic rehabilitation protocols, contributing to an improved integration of all available digital datasets.^18-22

Regardless of the degree of difficulty, surgical implant positioning and placement must be guided by the design of the future prosthesis (ie, prosthetically driven), as recommended in a recent consensus statement,²³ and, more importantly, driven by functional and esthetic expectations of the patient. The use of a digital workflow may be the optimal method to visualize the future dental arrangement before performing the actual treatment. With diagnostic data obtained from CBCT and EOS or IOS scanning, a digital pathway for virtual diagnostic waxing, a virtual prosthetically driven surgical plan, a digitally designed and produced surgical template, and an implant-supported digitally designed and fabricated interim restoration can be produced using an open-architecture CAD/CAM milling system.

Figure 14 through Figure 20 demonstrate this technique for the restoration of an upper right central incisor.

The novel integration of 3D digital, surgical, and prosthetic planning software programs provide a comprehensive and dynamic interplay between the surgical and restorative aspects of implant treatments. This facilitates interpretation of the clinical data, enables incorporation of diagnostic information, and potentially leads to improvements in clinical outcomes. This novel digital workflow offers a time-saving solution for clinicians to provide their patients with immediate function while minimizing the need for chairside adjustments of the provisional restoration. The use of a digitally milled, prefabricated temporary shell can improve treatment efficiency, providing a faster time-to-delivery of the prosthesis and increasing the reliability of the immediate function for partially edentate patients.

The authors have no commercial or financial interest in the products or companies mentioned in this article.

Alessandro Pozzi, DDS, PhD
Professor
Oral Sciences, Italy
Adjunct Associate Professor
Restorative Sciences
Dental College of Georgia
Augusta University
Augusta, Georgia
Private Practice
Rome Italy

Lorenzo Arcuri, DDS, PhD
Student
University of Rome Tor Vergata.

Peter K. Moy, DMD
Nobel Biocare Endowed Chair
Surgical Implant Dentistry
Clinical Professor
Department of Oral & Maxillofacial Surgery
UCLA School of Dentistry
Los Angeles, California