The study was approved by the local ethical review board (IRB00001473; file reference: EK507122017). The study was performed in accordance with the Declaration of Helsinki. All participants gave their written informed consent before taking part.
Artificial partially edentulous mandibular models resembling bone quality D2 (Mandibula Typ A, GOS® GmbH, Northeim, Germany) according to Misch were used [14]. First, an alginate impression was taken and a plaster cast (type IV; Excalibur, Dr. Böhme & Schöps Dental GmbH, Goslar, Germany) of the model was manufactured. X-ray splints were fabricated on those plaster casts by thermoforming using a plastic sheet (Erkodur 1.5 mm; Erkodent GmbH, Pfalzgrafenweiler, Germany) attached to a refFIX™ disk which contained three titanium pins (RefPin, IVS Solution AG, Chemnitz, Germany). Next, a cone-beam computed tomography (CBCT, Accuitomo, J. Morita Corporation, Osaka, Japan) of the X-ray splint fixed on the mandibular model was carried out. The parameters were set up according to a previous study [15]: tube voltage, 60.0 kV; current, 3 mA; exposure time, 17.5 s; gantry angle, 0.0°; field of view, 80 × 80 × 80 mm; voxel size, 0.160 mm. All data were transformed in the Digital Imaging and Communications in Medicine (DICOM) format and were transferred to the planning software (coDiagnostiX™, Dental Wings, Chemnitz, Germany). The planning of the implant positions was performed according to clinical guidelines taking into account bone height and width, distance and angulation to the adjacent teeth, and the distance to the mental foramen (Fig. 1). On the basis of the planned implant positions, four templates were manufactured using the gonyX® device (Institute Straumann AG, Basel, Switzerland). In region 35, where the implant insertion was planned to be performed with an orientation template (half-guided), two different sleeves were inserted in the template. In order to assess the influence of the length of the orientation sleeve, either a long sleeve (length 10 mm, inner diameter 2.35 mm; steco-system-technik GmbH & Co. KG, Hamburg, Germany) or a short sleeve (length 4 mm, inner diameter 2 mm; 3D Diagnostix Incorporation, Boston, Massachusetts, USA) were inserted. In region 45, a sleeve for a full-guided implant insertion with an outer diameter of 5.5 mm and a length of 4 mm (titanium guided Z; steco-system-technik GmbH & Co. KG, Hamburg, Germany) was inserted into the template.
Two identical dummy implants according to the Xive® S Plus system (Dentsply Sirona Implants, Mannheim, Germany) were inserted in the artificial mandibles in region of tooth 35 (guided pilot drill) and region of tooth 45 (fully guided). The implants had a diameter of 3.8 mm and a length of 11 mm.
The implant cavities were prepared without irrigation following the protocol of the manufacturer. The Frios Unit S/i (W&H Dentalwerk Bürmoos GmbH, Bürmoos, Austria) was used as surgical motor. For the half-guided implant insertion, the Xive® surgical tray was used, and for the full-guided implant insertion, the Xive® GS tray with drill sleeves for each bur was used (both Dentsply Sirona Implants, Mannheim, Germany). Following the insertion of the implants, the temporary abutment was removed, and the appropriate cover screw was inserted. Additionally, the time required to prepare the implant cavity and to insert the implant was recorded. Next, the artificial mandibles with the appropriate template attached to the model were scanned in the CBCT applying the same parameters in the pre-operative scans.
In order to evaluate the mismatch and deviation between the inserted implant and the planned position, the i-Dixel OneVolumeViewer 2.6.0. software (J. Morita MFG. CORPORATION, Kyoto, Japan) was used as described previously [15]. The implant region was analyzed in the axial, sagittal, and coronal plane applying a zoom level of 400%. The angle between the longitudinal median axis of the sleeve and the longitudinal median axis of the implant was measured to assess the deviation in bucco-lingual and mesio-distal direction. Additionally, the mismatch between the median longitudinal axis of the sleeve and the implant at the artificial bone level was measured in both these directions. The three-dimensional position of the inserted implant compared with the planned position was assessed using the “treatment evaluation” tool of the coDiagnostiX™ software. First, the pre-operative and the post-operative three-dimensional scan of the mandibular model were matched by matching three congruent parts of each scan (Fig. 2) a precise adjustment performed manually by the examiner. Then, the three-dimensional mismatch between the planned and the inserted implant was calculated by the software (Fig. 3). In order to determine the direction of the deviation and mismatch, different algebraic signs were applied: when the direction was mesial, oral, or cranial, a negative sign was used; when the direction was distal, buccal, or caudal, a positive sign was used. The recorded values were collected in an Excel® chart (Microsoft Inc., Redmond, Washington, USA).
Statistical analysis
A statistical analysis was carried out with SPSS Statistic 25 (IBM Corporation, Armonk, New York, USA). The mean values and their standard deviations as well as the median values were computed. The Shapiro-Wilk and Kolmogorov-Smirnoff tests were used to test for a normal distribution. For the comparison of the implant insertion mode, the Mann-Whitney test was applied, and for the comparison of the templates, the Kruskal-Wallis test was applied. Statistical significance was set at α = 0.05. For the exact statistical significance, a Bonferroni adjustment was performed.