The method implementing the fully guided digital concept (SafetyCrown) is designed to offer patients in need of a single implant-supported restoration in the posterior area a safe and time-preserving treatment option, combined with the benefits of immediate restoration and the one-abutment/one-time concept.

For the method itself, only three major visits are required. Figure 1 illustrates the workflow of the treatment method. It can also be implemented in the restoration of multiple implants, and tooth-retained fixed restorations. Complete diagnostics based on functional, periodontal, and full-mouth examinations as well as radiographic diagnostics are conducted in the planning phase. After patient information and shared decision-making, the SafetyCrown-workflow can commence.

Fig. 1
figure 1

Visualization of the SafetyCrown-workflow, from the treatment planning to the insertion of the final crown

Visit 1

Full-arch intraoral optical impressions of the upper and lower jaws are prepared using an intraoral scanner. The individual shade of the adjacent teeth is selected and documented for an esthetically pleasing restoration. Furthermore, a cone-beam computer tomography (CBCT) is performed to assess the individual bone volume and for fully guided implant planning. The field-of-view (FOV) should be limited to the smallest FOV. Special focus should be placed on expectable artifacts limiting the possibility of precise matching.

Preparation/laboratory

Virtual models of the upper and lower jaws are generated based on the intraoral optical impressions and exported as standard tessellation language (stl) files. The CBCT data are exported as a Digital Imaging and Communications in Medicine (DICOM) dataset. For three-dimensional implant planning, a software with the possibility of exporting the planned implant position as an stl file can be used. The CoDiagnostiX software is used (Dental Wings, Montreal, Canada) by our working group. The DICOM-dataset is imported and matched with the virtual model. As a prosthodontic setup, a tooth is virtually placed in the edentulous area prior to the implant for ideal alignment of the screw channel (Fig. 2A). In functionally challenging occlusion patterns, an individual prosthodontic setup can be created in a laboratory computer-aided design/computer-aided manufacturing (CAD/CAM) software (Exocad 3.0, exocad GmbH, Darmstadt, Germany) and can be imported into the implant planning software. To fabricate the abutment, the implant position must be exported alongside the implant index. This can be represented via the placement of a scan body into the virtual model (Fig. 2B). In the next step, the exported model is imported into the laboratory-side CAD/CAM software (Exocad 3.0). An individual abutment is designed using tooth-colored, high-strength zirconia (LavaPlus, 3M, Saint Paul, Minnesota). To achieve a supragingival cementation line, the abutment is formed freely within the software, and the margin is not limited to the gingival height (Fig. 2C). After milling, sintering, and polishing according to the manufacturer’s specifications, it is adhesively bonded to a titanium base. Therefore, the titanium base as well as the zirconia abutment’s interface is sandblasted (Al2O3, 50 µm, 2 bar for titanium base, 1 bar for zirconia) and cleaned using an ultrasonic bath and 90% ethanol. Afterwards, a phosphate primer (Monobond Plus, Ivoclar Vivadent AG, Schaan, Liechtenstein) is applied to both surfaces and the zirconia abutment is adhesively bonded to the titanium base using a self-curing resin (Multilink-Hybrid-Abutment, Ivoclar Vivadent AG; Fig. 2D). In addition to manufacturing the abutment, a drilling guide is digitally designed within the implant planning software and fabricated using a three-dimensional (3D) printer or by milling. The drilling template must visualize the planned implant index position for correct orientation. Figure 2 illustrates the preparation steps as well as the individual abutment and implant drilling guide.

Fig. 2
figure 2

Preparation phase. A Planned implant aligned centrally in the axis of the tooth setup. B Virtual planning export. A scanbody represents the planned implant in position and index. C Design of the individual abutment with 1-mm supragingival cementation line. D Milled, polished and adhesively luted individual abutment on titanium base. E Fully navigated implant drilling template

Visit 2

Implant placement is performed under local anesthesia. A full flap is elevated, and the crestal bone is exposed (Fig. 3A). Implant osteotomy is performed following the manufacturer’s guidelines, using a drilling guide aiming for sufficient primary stability of 35–50 Ncm. Figure 3B shows a properly aligned implant in terms of height and index. The abutment is tried in, and the screw is tightened. Small discrepancies in the vertical position, angle, and rotation are tolerable and can be compensated for by the temporary and final restorations. The soft tissue is adapted, and suturing is performed (Fig. 3C). The screw channel is provisionally closed using sterile Teflon tape. Afterwards, an intraoral optical impression of the abutment, opposing dentition, and buccal bite is taken (Fig. 3D). A chairside PMMA crown (Telio CAD, Ivoclar Vivadent AG) is designed (Fig. 3E) milled (MCXL, Dentsply Sirona, Bensheim, Germany) and provisionally cemented using Temp Bond NE (KerrHawe SA, Bioggio, Switzerland). Performing as an immediate restoration, it is assured that there are no occluding, dynamic, or approximal contact points (Fig. 3F).

Fig. 3
figure 3

Implant placement and immediate restoration. A A full flap is elevated and the crestal bone is exposed. B Shows a properly aligned implant in terms of height and index. C The definitive abutment in situ and wound closure. D Intraoral optical impression in the region of the abutment. E Design of the temporary PMMA restoration. F Temporarily seated PMMA restoration

Laboratory

During implant healing, the definitive restoration with fully functional contacts is fabricated based on the digital impression of the abutment after insertion. It is milled out of tooth-colored, high-strength zirconia (IPS e.max ZirCAD Prime, Ivoclar Vivadent AG). As an important step, the position of the screw channel is marked by stain on the crown surface, providing predictable access to the screw in case of complications (Fig. 4A).

Fig. 4
figure 4

Definitive restoration. A The screw channel of the abutment is marked on the surface of the crown by stain, providing predictable access to the screw in case of complication. B For cementation, a thin retraction cord is placed just under the soft tissue line. C, D Seated definitive restauration

Visit 3

After permitting sufficient osseointegration of the implant, the provisional crown is removed, and the abutment is cleaned. The screw is retightened to the specified torque of 35 Ncm and the screw channel is sealed using sterile Teflon tape and a thin layer of bright composite resin. For cementation, a thin retraction cord is placed just under the soft tissue line (Fig. 4B). After extraoral sandblasting (1 bar, Al2O3 50 μm) and cleaning in an ultrasonic bath with 90% ethanol, the crown is adhesively cemented (Panavia 21 TC, Kuraray Noritake, Tokyo, Japan). The supragingival cementation line in combination with the thin retraction cord facilitates the removal of excess cement and a safe cementation process. Figure 4C, D illustrates the final restoration. Fourteen months after implant placement, a stable peri-implant tissue is present with no signs of inflammation (Fig. 5).

Fig. 5
figure 5

Follow-up 14 months after implant placement. A stable peri-implant soft tissue is present with no signs of inflammation

Modification in case of multiple adjacent restorations

In case of several necessary adjacent restorations, it may be appropriate to fabricate the definitive crown part of the SafetyCrown together with the neighboring restorations. In these situations, the temporary restoration is removed after successful osseointegration, and the abutment is rescanned together with the adjacent preparations in an optical impression. The restorations can then be manufactured and seated simultaneously.

Retrospective pilot study for sample size calculation

Based on this concept, three patients with 4 restorations were successfully treated (female, 53 years, FDI 47; female, 43 years, FDI 36; male, 53 years with two reconstructions, FDI 24, 46). They were in good general health, were non-smokers with no active periodontitis and did not suffer from bruxism. Implants were placed between 11 and 15 months ago and subsequently restored according to the protocol 3 months later (mean observation time 11.2 months). Until today no technical or biological complication occurred.

After the treatment, we enrolled a retrospective survey based on a questionnaire. To quantify the answers, we used a Visual Analog Scale (VAS) with scores ranging from 0 to 100 represented by a horizontal bar of 10 cm length. The individual answer was given by a vertical line. The questions were composed to gather more information about the patients’ perspective (Patient related Outcome Measures; PROMs) towards this new treatment concept. The questionnaire was completed by the three patients treated according to the SafetyCrown concept. As a control group, we randomly selected four patients from our clinic who recently had received an implant-supported posterior single crown using a conventional workflow (submerged healing, second-stage surgery, impression, try-in, insertion of the screw-retained crown). Based on this pilot study, we performed a sample size calculation to enroll an RCT with PROMs as primary outcome.

This retrospective survey was approved by the Institutional Review Board of the Medical Faculty, RWTH Aachen University (EK 069/22) and was conducted in accordance with the Helsinki Declaration of 1964, as revised in 2013. All patients provided written informed consent prior to completing the questionnaire.

The following question was asked in both groups:

  1. 1.

    How stressful was the treatment process from the implant placement to the definitive restoration with the implant-supported crown? (0 = not stressful at all, 100 = very stressful)

Only SafetyCrown:

  1. 2.

    How important did you consider the immediate restoration of the implant with a temporary during the waiting period to the definitive crown? (0 = totally unimportant, 100 = very important)

  2. 3.

    How much did the temporary improve your chewing ability? (0 = no improvement, 100 = great improvement)

  3. 4.

    How important did you consider the omission of the second surgical intervention? (0 = totally unimportant, 100 = very important)

Only control:

  1. 5.

    How stressful did you consider the remaining tooth gap after implant placement for the waiting period to the definitive crown? (0 = not stressful at all, 100 = very stressful)

  2. 6.

    How much did the remaining tooth gap influence your chewing ability? (0 = no negative influence, 100 = great influence)

  3. 7.

    How stressful did you consider the second surgical intervention? (0 = not stressful at all, 100 = very stressful)

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