Patient selection

Patients in need of orthodontic treatment with a miniscrew-supported palatal appliance were selected from the Section of Orthodontics of the Department of Medicine, Surgery, and Health Sciences of the University of Trieste. The inclusion criteria were the following:

  • Indication for a TAD-supported palatal orthodontic device, either with 2, 3, or 4 TADs, including, but not limited to, distalization or mesialization requiring total anchorage, orthopedic palatal expansion in post-pubertal patients, and orthopedic treatment of Class III malocclusions in prepubertal or pubertal patients [23,24,25,26,27,28].

  • Indication for a guided surgical procedure and a digital workflow, including, but not limited to, subjects with anterior crowding, impacted teeth, narrow palate, thick mucosa, and cases where parallelism between the miniscrews is fundamental, such as cases with 4 palatal TADs.

No restrictions were placed regarding the age or gender of the patients considered.

Patients were excluded if they had any systemic disease affecting bone metabolism, syndromes or craniofacial malformations, pathologic processes in the maxilla, use of drugs affecting bone metabolism, compromised immune defense, bleeding disorders, or inadequate oral hygiene [29].

Digital planning

A 1-visit protocol was applied (i.e., insertion of the miniscrews and the orthodontic device in the same appointment), following the REPLICA System®’s planning and insertion protocol (Fig. 1) [6]. In this case, the initial records used are a cone-beam computed tomography (CBCT) (My Ray HyperionX9) and a digital impression of the patient’s upper arch and palate (CS3600, Carestream Dental), that are matched and superimposed with the software Viewbox (dHAL Software, Kifissia, Greece). The same miniscrews (BENEfit®, psm medical solutions) that will be used for the clinical procedure are then selected from a virtual library and positioned based on the bone availability and the future device. Following the guidelines reported in the literature, the correct position of the miniscrews is in the anterior paramedian region, at a 4–5 mm distance from the palatal midline, between the second and third palatal ruga and considering adequate parallelism among the screws and maintaining enough distance from anterior teeth roots. In the posterior palatal region, the premolar and molar areas can be used [7, 8, 30, 31]. The miniscrews are planned in a way to perforate both the palatal and the lower nasal cortical bone (bicortical insertion). Successively, the surgical guide is virtually designed, and guiding pillars and analogs are positioned. The final planning step involves a digital model with holes (named file 1) for the actual analogs and the finalization of the surgical guide.

Fig. 1
figure 1

Digital planning of bicortical insertion of two paramedian miniscrews using the REPLICA System®. A, B virtual miniscrews position on the superimposition of cone-beam computed tomography and digital impression. C coronal view of the position of the virtual miniscrews on the digital model

Laboratory procedure (step 2)

The laboratory step begins with the digital design of the orthodontic device with the software Appliance Designer TM (3Shape A/S, Copenhagen, Denmark). The final step involves the prototyping of the model, the digital impression (CS 3600, Carestream Dental) of the prototype with scan bodies (named file 2), the prototyping of the surgical guide, the sintherization of the orthodontic device, and the fitting of such device on the prototype.

Surgical procedure (step 3)

After anesthetization with local infiltrative anesthesia, the surgical guide is positioned to check perfect correspondence and stability. The miniscrews used are BENEfit® Orthodontic Screws (PSM Medical Solutions) 2 mm diameter, 9–11 mm length. The procedure was performed with a manually turned unit connected to a contra-angled handpiece (NSK dental). Before that, a pilot hole was performed with a drill equipped with a drill stop calibrated based on the CBCT to perforate only the palatal cortical bone. After positioning the miniscrews, PEEK scan bodies (BENEfit ® system, PSM) are fixed on the screws to acquire a digital impression of their actual position (named file 3). The last step involved the fitting of the orthodontic device on the inserted miniscrews.

Software analysis

Software analysis was performed with Geomagic Design X (version 2019.0.2). The three files in STL (Standard Triangle Language) format analyzed for each patient were (Fig. 2):

  • File 1: the digital plan of the virtual position of the miniscrews (model with holes).

  • File 2: digital impression with scan bodies of the 3D prototype for fitting of the orthodontic device.

  • File 3: digital impression with scan bodies of the post-insertion position of the miniscrews after the surgical procedure.

Fig. 2
figure 2

The three files in STL format were analyzed with Geomagic Design X software (Geomagic Design X- version 2019.0.2). A digital planning of the miniscrews’ position (blue); B scanning of the 3D model with scan bodies for the design and fitting of the orthodontic device (green); C post-insertion digital impression with scan bodies (yellow)

The three files were uploaded and superimposed first roughly with the point-to-point function, choosing the mesiobuccal cusps of the upper first molars and the mesial angle of the incisal edges of the central incisors. A fine superimposition was then performed with an Iterative Closest Point (ICP) algorithm. Once superimposed, an automatic shape recognition function divided each mesh into recognizable and well-defined geometric shapes. At this point, each mesh was visualized singularly, by “switching off” the view of the other two. The following procedure was performed consecutively for the three meshes. The longitudinal axis of the guiding holes (for file 1) and the scan bodies (for files 2 and 3) was drawn with the function “model_add vector_find axis of the cylinder”, selecting the automatically recognized cylinders.

Once the axes were traced, angular measurements between the digital plan and the laboratory model, the digital plan, and the post-insertion position and between the laboratory model and the post-insertion position were performed with the function “measure angle_ between two vectors” (Fig. 3).

Fig. 3
figure 3

A view of all three files is “switched on” once all the axes are identified; B angular deviations between vectors are calculated

Error analysis

A subset of 30 randomly chosen measurements was repeated after a 2-week interval by the same investigator. The calibration of the investigator was assessed with the intraclass correlation coefficient (ICC). The ICC for inter-rater reliability was between good and excellent, being 0.93 (0.86–0.97).

Power analysis

The power analysis found that a sample size of 45 achieve 80% of power to detect a mean of paired differences of 1.5 with a known standard deviation of differences of 3.4 and with a significance level (alpha) of 0.05. Data were acquired from a previous pilot study (unpublished data). A priori sample size required was calculated with G*Power (version 3.1.9.7).

Statistical analysis

Statistical analysis was performed using the statistical software package SPSS version 26.0 (SPSS Inc., USA). Descriptive statistics were performed and reported as median, IQR range, and range. Mean values ± standard deviations were also reported for uniformity with existing literature. Failure of normality assumption was verified with the Shapiro–Wilk test to yield significant results, thus a non-parametric test for related samples was carried out to test the null hypothesis that there are no significant differences between the deviations among the three operative steps. A Friedman test was performed to compare the deviations among the three operative steps. The significance of the differences in deviations between the left and right sides was tested with the Mann–Whitney U test. Significance was set at p < 0.05.

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