Ethical approval
This experimental study was approved by the Ethics Committee of the Hannover Medical School (approval number 9815_BO_K_2021). The 1964 Helsinki Declaration and its later amendments or comparable ethical standards were complied within in this study. All patients provided written consent for the use of their data.
Computed tomography (CT) data selection
We randomly selected 100 datasets from the radiographic records of the Department of Oral and Maxillofacial Surgery at the Hannover Medical School. The datasets were selected by chance out of an unsorted file folder with several thousand datasets with oral and maxillofacial disease patterns. These datasets were screened in alphabetical order until 25 datasets matching the following inclusion criteria were selected for the study:
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a CT scan with at least a 1-mm slice thickness
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complete maxillary dentition up to the first molar
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minimal artifacts due to metallic restorations or other structures
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written consent for the scientific usage of datasets by the respective patients.
All included datasets were anonymized and randomized using serial numbering.
Digital workflow
A comprehensive digital workflow (Fig. 2) was prepared for the clinical application with the Individual Patient Solution Implants® Preprosthetic (KLS Martin Group, Tuttlingen, Germany). Digital dental arches aligned to the sizes of commercially available impression trays (sizes 1–5 and XS-XL) were designed. The dental arch size of the patient reconstructed using this implant system was measured in advance, and a suitable temporary prosthesis was fabricated corresponding to the implant system. The material of the temporary prosthesis can be chosen according to the indication and expectation of the patient [e.g., titanium or plastics (Fig. 3)]. For this study, a simplified protocol was used, and only the digital datasets of the temporary prostheses were used for all analyses.
Preparation of virtual models
All 25 datasets were imported using the DICOM format into the Materialize Mimics Suite software (Materialize, Leuven, Belgium). Afterimage smoothing and segmentation, individual arch data were separated, if necessary, and the mandibular arch data were discarded. The left maxillary quadrant dentition was virtually removed, and the resulting partially edentulous maxilla image was exported as a standard tessellation language (STL) file (Fig. 4).
Fabrication of temporary prosthesis in dental laboratory
Three-dimensional models of the selected 25 partial edentulous maxillae were printed using additive manufacturing with a 3D printer (Ultimaker BV, Utrecht, Netherlands). Afterward, teeth wax-up of the temporary prosthesis was performed by a dental technician using acrylic resin teeth and wax (Fig. 5). The teeth were positioned according to the tooth-bearing quadrant of the upper jaw. No occlusion contacts were planned. Later, these 3D models with waxed-up teeth were digitalized using a conventional model scanner (S600 Arti, Zirkonzahn GmbH, Gais, Switzerland) (Fig. 6).
Manufacturing of the digital temporary prosthesis
All 25 partially edentulous maxillary STL files were imported into the Geomagic Freeform® software (3D Systems, Rock Hill, South Carolina, USA). The image surfaces were smoothed, and virtual dental arches of the fitting sizes were inserted in the edentulous space. These virtual dental arches were replicated from one healthy artifact-free dental CT dataset several years ago and virtually adapted to different standard impression tray sizes (0–4), with 0 being the smallest and 4 being the largest size of dental arches. These virtual dental arch sizes were developed to fit most patients’ original dental arch sizes. The fitting size digital dental arches were chosen, and the dental arches were cut to the correct length to fit the defects. Then the cut dental archer was inserted digitally to reconstruct the patients’ dental arches. The resulting images of reconstructed maxillae were exported as STL files (Fig. 7).
Comparison of conventionally fabricated and digitally constructed temporary prostheses with the original dentition
The respective STL files of conventionally and digitally constructed dentitions were superimposed over the original STL files of the unmodified maxillae (original teeth still in place). This superimposition was performed using the unmodified right maxillary quadrant as a guide. A heat map was used to monitor the superimposition process (Fig. 8).
After sufficient fusion, the corresponding predefined six points (incisal edges of 22 and 23, buccal and palatal tooth cusp tips of 24, and mesiobuccal and distopalatal tooth cusps of 26) were marked on the natural and reconstructed dentition. The difference between the original and reconstructed dentition was measured at three different points (z, x, y), defining vector L. The length of the connecting vector L was calculated in millimeters using the following formula:
$$\left( {\left| {\vec{L}} \right| = L = \sqrt {L_{x}^{2} + L_{y}^{2} + L_{z}^{2} } } \right)$$
The resulting L values were used for further analysis (Fig. 9). For all 3D analyses, GOM inspect suite (GOM inspect 2019, GOM GmbH, Braunschweig, Germany) was used.
Statistical analysis
The obtained data were analyzed using SigmaPlot 13.0 software (Systat Software Inc., San Jose, California, USA). The Shapiro–Wilk-Test was used to test for normality. The paired t-test was performed for normally distributed values, the Wilcoxon signed-rank test for non-normally distributed values to check for significance, and a P-value of < 0.05 was considered statistically significant.