Cases

The survey included all the cases who underwent static-guide-assisted oral implant placement surgery at Shimizu Dental Clinic (one facility in Japan) from March 1, 2014, to March 1, 2018, using one of the following CT scanning methods: (1) SCT method, (2) DCT method, or (3) MSCT method. One surgeon well-trained for static surgical guides (H.S.) decided the most suitable computer-guided system for each case according to the patient’s oral condition before the surgery. From March 2014 to September 2015, the DCT method was only used. In October 2015, the SCT and MSCT methods were introduced. Thereafter, an appropriate method was chosen from the DCT, SCT, and MSCT methods. The SCT method was only applied to cases with one or more untreated natural teeth (far from artifacts) in each of the 3 portions, anterior teeth, and right and left molars. From December 2016, the SCT and MSCT methods were applied except for the DCT method because the MSCT method gained credibility. The exclusion criteria were as follows: (1) cases in whom the guided surgery system could not automatically match the DICOM data between the pre- and post-operative CT scans and (2) those who did not provide their consent to participate in the study.

The research protocol was approved by the Okayama University Ethics Committee (Ethics Committee No. 14000046, Approval Number: 1806-031). Patients provided written informed consents with permission to use their data for scientific purposes.

Treatment steps

Treatment steps for the SCT method were as follows. A preoperative CT scan (Aquilion Lightning, Canon Medical Systems, Japan) was performed to obtain the DICOM data of maxillofacial region. Definitive impressions were taken using silicone impression materials (ImprintTM4 Penta™ Soft Tray, 3M EPSE, Aquasil Ultra, Dentsply Sirona, USA) to fabricate plaster cast model of intraoral morphology. The obtained plaster cast model was scanned with 3D desktop scanner (CARES® Scanner D7 plus, Straumann, Switzerland), then converted to STL data. The DICOM data of the maxillofacial region and STL data of the plaster cast model were imported to a simulation software program (coDiagnostiX®, Dental Wings Inc., Canada) and superimposed with reference to the surface morphology of the remaining teeth. The implant placement simulation and surgical guide design were performed based on the superimposed data. Surgical guide was designed to fit to the STL data of the intraoral morphology and fabricated using a 3D printer (CARES® P Series P40, Straumann, Switzerland).

In the DCT method, definitive impression was taken using silicone impression materials (ImprintTM4 Penta™ Soft Tray, 3 M EPSE, Aquasil Ultra, Dentsply Sirona, USA) to fabricate the intraoral plaster cast model. A dental technician fabricated a radiographic guide, by burying 6–8 gutta-percha on the plaster cast model. Two types of CT scan, with a radiographic guide and the patient wearing the radiographic guide, were performed (Aquilion Lightning, Canon Medical Systems, Japan), and their DICOM data were obtained. Two types of DICOM data were imported to the simulation software program (Nobel Clinician®, Nobel Biocare, Switzerland) and superimposed with reference to the gutta-percha points. After performing the implant placement simulation and surgical guide design, the surgical guide was fabricated based on the DICOM data of the radiographic guide using the optical shaping method (Nobel Biocare, Switzerland).

The MSCT method was performed according to the protocol previously described by Shimizu et al. [8]. Definitive impression was taken using silicone impression materials (ImprintTM4 Penta™ Soft Tray, 3 M EPSE, Aquasil Ultra, Dentsply Sirona, USA) to fabricate the plaster cast model of the intraoral morphology. The obtained plaster cast model was scanned using a 3D desktop scanner (CARES® Scanner D7 plus Straumann, Switzerland) and converted to STL data. Resin template for the CT scan was printed using a 3D printer (Form 2, Formlabs, USA) based on the obtained STL data of the intraoral morphology. Six glass ceramics markers were added on the occlusal surface of the resin template. Finally, CTMT was completed. CT scan (Aquilion Lightning, CANON MEDICAL SYSTEMS, Japan) of the patient wearing CTMT was performed. Two types of scanning, plaster cast model itself and CTMT attached plaster cast model, were performed (CARES® Scanner D7 plus, Straumann, Switzerland). Then, the obtained STL and DICOM data of the patient wearing the CTMT were imported to the simulation software program (coDiagnostiX®, Dental Wings Inc., Canada) and superimposed with reference to the glass ceramics markers and remaining teeth morphology. After implant placement simulation, the surgical guide design was performed based on the STL data of the intraoral morphology, and the surgical guide was fabricated by a 3D printer (CARES® P Series P40, Straumann, Switzerland).

The surgical guides were fabricated using each methodology, and their internal fittings were confirmed through the pre-formed inspection windows before surgery. During oral implant surgery, all the surgical guides were anchored using fixation pins. The implant surgeries were open-flap or flapless, according to the recommended guided surgery protocol of the manufacturer. Implant bodies were placed without the removal of the surgical guide. A postoperative CT scan was taken to confirm whether the actual implant position was clinically appropriate. Placed implants were selected from following systems: Brånemark System, NobelActive, NobelSpeedy, NobelReplace Straight, NobelReplace/Select Tapered, NobelReplace Conical Connection (Nobel Biocare, Switzerland), and Straumann Bone Level and Bone Level Tapered (Straumann, Switzerland).

Complications

The following events described in the medical records were considered as complications. (1) complications before surgery: (a) failure of the preoperative simulation for designing surgical guides due to artifacts, (2) complications during implant placement surgery: (a) unexpected changes in the implant placement surgery plans (unplanned bone augmentations or changes in implant body diameter or width) during the surgery, (b) fracture of surgical guide templates during the implant surgery (c) unexpected implant body exposure from the alveolar bone surface, (d) perforation into the maxillary sinus or nasal cavity during implant placement surgery, and (e) collision of the implant body with an adjacent tooth.

Observation factors

Alveolar bone quality according to Lekholm and Zarb (type 1/2/3/4) [10] and alveolar bone surface inclination (degree) was evaluated from a preoperative CT image. Bone surface inclination at the implant placement site in preoperative simulation was evaluated separately as buccolingual (bone surface inclination X) and mesiodistal inclination (bone surface inclination Y) (Fig. 1).

Fig. 1

Measurement method of alveolar bone surface inclination. a A buccolingual virtual plane (p-1) was set through the implant body axis in the axial slice at the entry of the implant body in the preoperative simulation. b Buccolingual bone surface inclination X (0–90°) was measured between the implant axis and bone surface line on p-1. c A mesiodistal virtual plane (p-2) was set through the implant body axis in the axial slice at the entry of the implant body in the preoperative simulation. d Mesiodistal bone surface inclination Y (0–90°) was measured between the implant axis and bone surface line on p-2

Other information obtained from an electronic medical database for regular implant treatments with reference to the previous reports [11] was as follows: patient’s age at implant placement surgery; sex; surgical guide fabrication method (SCT/DCT/MSCT); number of remaining teeth, coronal teeth (number of remaining teeth with coronal structure and dummy teeth in fixed partial dentures), and teeth with metal restorations (including zirconia restorations); number of placed implants; dentition defect type (Kennedy classification Class I/II/III/IV/complete edentulism); number of fixation pins used during the implant placement surgery; shape of the implant body (straight/tapered); implant width (narrow platform [NP]/regular platform [RP]/wide platform [WP]); implant length; implant location (anterior/posterior, maxillary/mandibular); whether or not immediate implant placement was performed; whether or not bone augmentation was performed; and distance between guide sleeve bottom to bone surface (4.0/5.5/6.0 mm).

Preoperative planning and measurement of 3D deviation between preoperative simulation and actual placement of the implant body (surrogate endpoints)

The planning software program, coDiagnostiXⓇ, was used to measure the 3D deviations (mm) at the entry and tip of the implant body between the preoperative simulation and actual placement position by the SCT and MSCT methods. The measurement protocol described by Monaco et al. (2020) was applied to this study [12]. The DICOM data of the pre- and postoperative CT images were assessed using coDiagnostiXⓇ, which were adjusted to the same CT threshold and then automatically and accurately superimposed with reference to the characteristic anatomical morphology on CT images using a software function. The pseudo-implant body was accurately placed on the actually placed implant body on the postoperative CT images (Fig. 2a). The 3D positional deviations and distances between the preoperative simulation and actual placement position of the implant bodies were measured automatically using the “Treatment Evaluation” function. The X (mesiodistal axis), Y (buccolingual axis), and Z deviations (depth axis) were automatically measured at the position of entry and tip of the planned and placed implant bodies, and the 3D deviation (3D = \(\sqrt}^+}^+}^}\)) was automatically calculated (Fig. 2b).

Fig. 2

Measurement method of the 3D deviation between preoperative simulation and placed implant body position with coDiagnostiXⓇ. a A pseudo-implant body was placed based on the actual implant shadow on the CT image after implant placement. b Automatic calculated 3D deviation at the entry and tip between the planned implant body and actual implant body placement position. (Red: Placed implant body; Blue: Planned implant body). 3D, three-dimensional; CT, computed tomography

In the DCT method, the planning software program, Nobel ClinicianⓇ, was used to measure the 3D deviations at the entry and tip of the implant body between the preoperative simulation and actual placement position. The measurement protocol described by Verhamme et al. (2015) was applied to this study [13]. Superimposition of the pre- and postoperative DICOM data and pseudo-implant body placement were performed using the same method as that used for coDiagnostiXⓇ (Fig. 3a). In contrast to the method used with coDiagnostiXⓇ, the 3D positional deviations and distances between the actually placed implant body and preoperative simulation were measured manually. The X, Y, and Z deviations at the position of entry and tip between the planned and placed implant bodies were measured with a distance measuring tool, and the 3D deviation was calculated manually using the same formula (Fig. 3b–d).

Fig. 3

Measurement method of 3D deviation between preoperative simulation and placed implant body position with Nobel ClinicianⓇ. a A pseudo-implant body (I-1) was placed based on the actual implant shadow on the CT image after implant placement. b A postoperative pseudo-implant body (I-1) on the cross-sectional slice through the planned implant body (I-2) axis. c Vertices of implant bodies of b were enlarged. 2D deviation of the depth (Z) at the tip between the preoperative and postoperative pseudo-implant body was manually measured using the distance measurement tool. d The axial slice which was contacted to tip of planned implant body (I-2) and rectangular to planned implant (I-2) axis, red line of b. The mesiodistal 2D deviation (X) and buccolingual 2D deviation (Y) at the tip between the preoperative and postoperative pseudo-implant body were manually measured using the distance measurement tool. 3D, three-dimensional; 2D, two-dimensional; CT, computed tomography

Measurements of the 3D deviations between the preoperative simulation and actual placement position of the implant bodies were performed by two examiners (T. M. and Y. K.) independently, while they were not informed about the applied surgical guide fabrication methods. The average of the deviation values measured by the two examiners was adopted as the 3D deviation of each implant.

Statistical analysis

The Kruskal–Wallis and chi-square tests were used to compare the observation factors among the SCT, DCT, and MSCT methods. Since there were no missing data among all observation factors of patients, statistical correction was not needed.

The inter-rater reliability in the 3D deviation measurements of the planned and placed implant bodies between the two examiners was assessed using the Intraclass correlation coefficient (ICC) for each planning software program.

The Steel–Dwass test was used to compare the median 3D deviation between the planned and placed implant bodies among the SCT, DCT, and MSCT methods. Kruskal–Wallis test, Mann–Whitney U test, or Spearman’s rank correlation coefficient was used to analyze the relationships between the predictor variables and the 3D deviations between the planned and placed implant positions at the entry and tip of the implant body between each surgical guide fabrication method.

To evaluate the accuracy of implant placement position using each surgical guide fabrication method, generalized estimating equations (GEEs) were used to identify the risk factors for large 3D deviations at the entry and tip of the implant body between the preoperative simulation and actually placed position. GEEs were implemented using the forced entry method.

Statistical analyses were performed using SPSS for Windows (version 25 for SPSS, IBM, Japan). Steel–Dwass test was performed using (JMP version 11, SAS, Japan). The level of significance was set at p < 0.05.