Complete Digital Restoration: Implant-Supported Prosthesis Using Rapid Prototyping and a Model-Free Approach

George Michelinakis, DDS, MSc, MPhil; Dimitrios Nikolidakis, DDS, MSc, PhD; and Dimitrios Apostolakis, DDS, MSc, MSc

Increased accuracy in digital intraoral scanning combined with improved monolithic material properties has in recent years created a paradigm shift in the restoration of single and partial fixed dental prostheses on implants.^1,2 The use of a completely digital workflow without the need for a stone cast has been shown to provide restorations with superior marginal and internal fit and low complication rates compared to restorations produced with a conventional workflow on teeth^3,4 and implant-supported prostheses of limited span.^2,5-7

Regarding the restoration of a completely edentulous arch with an implant-supported fixed prosthesis, the conventional workflow utilizing an elastomeric impression and a metal-ceramic implant-retained fixed denture is still considered by many the treatment of choice. Re­cent evidence, however, suggests that complete-arch intraoral digital impressions may be equally accurate to conventional impressions.^8,9 Various clinical factors, such as the distance between implants,^10-12 depth of implant placement,^13,14 and shape and material of implant scanbodies,^15,16 have been found to influence scanning accuracy.

Monolithic materials such as polymethyl methacrylate (PMMA) and cubic zirconia are instrumental in the development of a complete digital workflow. Provisional and final implant-supported restorations from these materials are manufactured using rapid prototyping techniques. A recent Delphi study by Sanz et al reported that by the year 2030, protocols in the manufacturing of prosthetic frameworks will have greatly shifted toward 3D printing and subtractive milling.¹⁷ The mechanical properties and esthetic parameters of these materials are, therefore, paramount to long-term restoration survival and success.

For 3D-printed PMMA provisional restorations, various parameters such as printing orientation¹⁸ and filler particle proportion¹⁹ have been found to influence mechanical properties in in vitro studies. Regarding monolithic complete-arch implant-supported zirconia restorations with minimal gingival or facial veneering, only medium-term (up to 5 years) follow-up studies that are retrospective in nature are available and indicate success rates of 98% to 99%.^20-22 Minor complications, including debonding or fracture of titanium cylinders, were reported in monolithic zirconia complete-arch fixed prostheses as opposed to porcelain-veneered zirconia fixed prostheses, where the incidence of porcelain fracture was considerably higher.²³ In a recent review article, certain steps in the design and manufacturing process of monolithic implant-supported zirconia complete-arch restorations are proposed to enhance long-term success and survival.²⁴ From a laboratory point of view, a high-quality zirconia material should be selected, and strict adherence to spatial prerequisites should be followed, such as maintaining ≥12 mm interocclusal space, having ≥3 mm of material around screw-access holes, and using a limited mandibular (<14 mm) and maxillary (<8 mm) cantilever. Slow heating and cooling protocols are also essential to avoid crack initiations within the material.²⁴ From a clinical standpoint, a milled or 3D-printed acrylic prototype should be used to verify fit, occlusion, and esthetics in an effort to minimize adjustments in the final monolithic reconstruction.

The purpose of this clinical report is to present the transition of a mandibular arch from a state of periodontally compromised dentition to that of a fixed prosthesis supported by six intraosseous implants using a completely digital, cast-free approach.

Clinical Report

A 45-year-old male patient had previously undergone maxillary rehabilitation with an implant-supported complete-arch fixed prosthesis (Figure 1).²⁵ During consultation, he expressed the desire to have his failing, periodontally compromised mandibular natural dentition restored with a complete implant-supported fixed bridge. He requested fixed interim rehabilitation for the entire duration of the treatment.

Following an intraoral scan (TRIOS^® 3, 3Shape, 3shape.com) and a CBCT scan, the decision was made to rehabilitate the mandible with an implant-supported fixed prosthesis on six implants. In the planning stage, the remaining mandibular teeth were virtually extracted (Figure 2) except for the canines and third molars, which were maintained and digitally prepared in a CAD software (DWOS, Dental Wings, dentalwings.com). A complete-arch digital diagnostic wax-up was prepared (Figure 3) and a provisional fixed prosthesis, which would be retained by the strategically kept teeth, was designed and milled out of a PMMA disc (Delta, Techim, techimgroup.com). The remaining mandibular teeth were extracted and the strategically maintained teeth were prepared (Figure 4). The fiber-reinforced PMMA provisional (Interlig^®, Angelus, angelus.ind.br) was relined chairside and fitted (Figure 5).

After a 3-month healing period, CBCT scans of the mandible with and without the provisional in place were acquired, and six implants were virtually planned for insertion using treatment planning software (Blue Sky Plan, BlueSkyBio, blueskybio.com) following prosthetically driven planning as per the existing provisional mandibular prosthesis (Figure 6). An immediate-load complete-arch prosthesis was also milled from the same PMMA disc and reinforced with glass fibers (Interlig) in the laboratory (Figure 7). A pilot-guided surgical stent was also 3D-printed using polylactic acid material (PolyLite^™ PLA, Polymaker, us.polymaker.com) to fit on the prepared mandibular canines and third molars, and six implants (Straumann STL/RN, Straumann, straumann.com) were inserted using a flapless surgical procedure (Figure 8). The immediate-load provisional prosthesis was attached to temporary titanium cylinders using flowable resin with the aid of the remaining mandibular teeth. The canine teeth were then extracted, occlusion was adjusted, and the prosthesis was delivered to the patient (Figure 9).

After a 3-month osseointegration period, the third molars were also extracted, the provisional was removed, and intraoral scanbodies (CARES^® Mono, Straumann) were hand-tightened onto the implants (Figure 10). An intraoral mandibular digital impression (Figure 11) was obtained using an intraoral scanner (TRIOS 3) following the manufacturer's instructions. To register the occlusal vertical dimension (OVD) and maximum intercuspation that was already established with the provisional, the PMMA prosthesis was sectioned in half along the midline. Centric occlusion and OVD were stabilized in the right side of the prosthesis using a silicone bite registration medium (Prestige^® Bite, Vannini Dental, vanninidental.com), and a digital impression on the left side was performed with the scanbodies attached to the implants (Figure 12). The same procedure was repeated for the opposite side (Figure 13). During the bilateral bite registration process, the scanbodies that did not interfere with the centric occlusion were left on the implants. This aided in enabling the intraoral scanner to capture the bite more accurately, as each scanbody provided a fixed landmark.²⁵ The complete digital bite registration was finalized in the intraoral scanning software (Figure 14).

To verify the accuracy of the mandibular digital implant impression, a metal bar was designed on titanium bases (Variobase^® cylindrical, Straumann) (Figure 15) and 3D-printed using a selective laser melting device (ProX 100, 3D Systems, 3dsystems.com). The fit was verified intraorally using the Sheffield screw test and periapical x-rays (Figure 16).

A 3D-printed PMMA complete-arch prosthesis on titanium bases (Variobase cylindrical) was then designed and manufactured using a digital light processing printer (D20+, Rapid Shape, Dental Wings). This prototype was secured onto the implants intraorally and the final occlusal, phonetic, and esthetic adjustments were made (Figure 17). The purpose of this clinical step was to ensure that only minimal adjustments would be needed on the final monolithic prosthesis. The prototype, together with the opposing dentition and centric occlusion, were digitized intraorally, with the clinician making sure to capture as much keratinized soft tissue and as many mandibular anatomical landmarks, such as the retromolar pads, as possible.

Using a laboratory CAD software (DWOS), the digital implant impression and the prototype scans were superimposed and merged into a single digital file using identical keratinized soft-tissue landmarks on both STL files and a best-fit alignment algorithm (Figure 18). A final monolithic zirconia complete-arch prosthesis was milled from a disc (Katana^™ HTML, Kuraray Noritake, katanazirconia.com), stained, glazed, and cemented intraorally on the titanium bases (Figure 19). Screw-access channels were covered with polytetrafluoroethylene (PTFE) tape and composite resin (Filtek^™ Supreme, 3M Oral Care, 3m.com). An occlusal guard was also fitted. The post-placement panoramic x-ray revealed the excellent fit of the prosthesis (Figure 20).

Discussion

Using a model-free approach in complete-arch restorations can be challenging for both the clinician and laboratory technician. Intraoral digital implant impressions in edentulous patients have been found to be affected by various technical and clinical factors.²⁶ Increased interimplant distance and the lack of fixed reference landmarks have been shown to influence scanning accuracy.¹⁰ The use of an auxiliary device to introduce artificial landmarks between implants to facilitate intraoral scanning has been presented but in vivo validation is lacking.²⁷ In the present clinical report, implant spacing in the arch was symmetrical, with edentulous spans between implants comprising no more than one tooth; therefore, no auxiliary scanning device was used. Complete-arch digitization was also aided by the presence of keratinized and immobile soft tissue between the implants that served as reference points.

The merging of different data sets from a CBCT and an intraoral scanning device or the merging of different intraoral scans can be prone to errors, especially when reference points between the data sets are missing. Recently, a technique was reported for merging a digital implant scan with that of an implant-supported maxillary provisional restoration to produce a prototype.²⁸ The palatal rugae can provide adequate reference landmarks for superimposition of scans using a best-fit alignment algorithm. However, in a fully edentulous mandible, the process is more challenging for laboratory technicians. The presence of thick, keratinized, and immobile soft tissue between implants can be beneficial in this process.

Fabrication of a 3D-printed working cast was avoided and a monolithic zirconia restoration was utilized. 3D-printed casts from a stereolithography printer were recently shown to have a mean deviation of 59 μm in implant analogue position,²⁹ thus a cast-free approach can help mitigate this dimensional error.

The correct OVD of the patient already established by his natural dentition was maintained by the subsequent tooth- and implant-retained provisional restorations. Through this process, the initial tooth morphology was preserved and copied in the final monolithic restoration, allowing for quicker patient accommodation to the final prosthesis.

Summary

This article presented a method of restoring a mandibular arch with monolithic fixed interim and final implant-supported prostheses using rapid prototyping techniques, intraoral digital scanning, and a model-free approach. The merits of this technique include increased accuracy of fit, greater patient comfort, and reduced treatment time due to fewer time-consuming laboratory steps. The article also described a technique to verify the accuracy of intraoral digital scanning of multiple implants and a method to acquire the digital bite registration using the fixed implant-supported provisional.

Acknowledgment

The authors thank Emmanuil Pavlakis, CDT, for the laboratory work in this case.

About the Authors

George Michelinakis, DDS, MSc, MPhil
Private Practice in Prosthodontics and Esthetic and Implant Dentistry,Private Practice, Heraklion, Crete, Greece

Dimitrios Nikolidakis, DDS, MSc, PhD
Private Practice in Periodontics, Heraklion, Crete, Greece
Dimitrios Apostolakis, DDS, MSc, MSc
Private Practice in Dental Radiology, Heraklion Crete, Greece

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