Results
The digital scanner provided an STL file of appropriate resolution for metal additive manufacturing or 3D printing. The indirect onlays were successfully printed in cobalt–chromium with the morphology, dimensions and fit that were clinically acceptable for cementation. Cementation was completed without issue and with suitable retention, similar to previous investigations.6 The marginal adaptation was generally acceptable, except for one area, owing to an open margin. The surface finish was generally acceptable, but could be improved in some places, especially on the occlusal surface.
Discussion
This preliminary investigation suggests that the work- flow for additively manufacturing cobalt–chromium onlays exclusive of digital design is possible. For predictability, the scanner and printer are required to have a sufficient resolution. The print quality of the definitive restoration is dependent upon the quality of the provisional restoration; therefore, a highly morphologically accurate, ideally adapted and polished provisional restoration is necessary, as polishing of cobalt–chromium is challenging. The tooth preparation seemed appropriate for the material.
A novel workflow (Fig. 11) has been proposed that provides a simple, efficient and inexpensive alternative to the traditional workflow. The new workflow circumvents the need for digital design, which would greatly reduce the time and cost, compared with the traditional workflow. The approach may also provide a more sustainable treatment option.
This study investigated cobalt–chromium indirect restorations, but other metal powders currently used in additive manufacturing, including stainless steel and titanium, could be employed.7 In addition, the novel workflow could be applied to non-metal, aesthetic materials such as zirconia and lithium disilicate. The use of zirconia was also explored in this investigation with a single unit, using the same workflow, and achieved similar results but with milling rather than additive manufacturing (Figs. 12 & 13).
This study had a small sample size and limited assessment, as the restorations were placed on stand-alone extracted teeth and only assessed through photographs, radiographs and a clinical post-cementation checklist. Further studies are required with larger sample sizes, adjacent teeth, antagonists, other materials, physical testing and clinical evaluation.
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