Midpalatal miniscrew insertion: The accuracy of digital planning and surgical placement

Introduction

The objective of this study was to investigate the accuracy of palatal miniscrew insertion, evaluating the effect of guide fabrication and surgical placement.

Methods

Guided insertion of bilateral paramedian palatal miniscrews was undertaken using Appliance Designer software (3Shape, Copenhagen, Denmark). A resin surgical guide (P Pro Surgical Guide; Straumann AG, Basel, Switzerland) was used. Superimposition of the miniscrew position relative to the digital design was undertaken using bespoke software (Inspect 3D module, OnyxCeph; Image Instruments GmbH, Chemnitz, Germany) to assess surgical inaccuracy. Miniscrew position relative to the surgical guide was also assessed to isolate the effect of planning inaccuracies. Both horizontal and vertical discrepancies were evaluated at both implant locations.

Results

Twenty-seven patients having bilateral palatal insertions were examined. Mean discrepancies were <0.5 mm, both in the horizontal and vertical planes. The mean overall horizontal and vertical discrepancy between the digital design and final miniscrew position on the left side was 0.32 ± 0.15 mm and 0.34 ± 0.17 mm, respectively. The maximum horizontal discrepancy observed was 0.72 mm. No significant differences were observed in relation to the accuracy of mini-implant positioning on the basis of sidedness, either for horizontal ( P = 0.29) or vertical ( P = 0.86) discrepancy.

Conclusions

High levels of accuracy associated with guided insertion of paramedian palatal implants were recorded with mean discrepancies of less than 0.5 mm both in the horizontal and vertical planes. No difference in accuracy was noted between the left and right sides. Very minor levels of inaccuracy associated both with surgical techniques and surgical guide fabrication were recorded.

Highlights

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Guided placement may facilitate more accurate and streamlined palatal mini-implant placement.
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High levels of accuracy associated with guided insertion of paramedian palatal implants were observed with mean discrepancies <0.5 mm.
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Minor inaccuracies were found both with surgical techniques and surgical guide fabrication.

The use of miniscrews in orthodontics has increased in recent years, providing a predictable means of anchorage reinforcement while offering versatility in producing a range of tooth movements in all 3 spatial planes. The definition of success of a temporary skeletal anchorage device (TSAD) is not clear-cut; however, it has been suggested that the presence of a TSAD ≥6 months after placement may represent success. Very promising success rates have been reported for implants placed in the palate. ^, However, miniscrews are prone to failure, with failure rates of up to 13% being representative.

A further problem with TSAD placement is inaccuracy, risking contact with key structures, including adjacent roots and other anatomic boundaries. ^, Furthermore, accurate placement is important to maximize biomechanical advantages and, indeed, to offer optimal levels of stability and bicortical engagement when required. The palatal bone depth and cortical bone thickness are more conducive in the first and second maxillary premolar region, with depth reducing laterally.

Although the popularity of median and paramedian palatal sites for TSAD placement has increased in recent years, research concerning the number and nature of complications is limited, with the invasiveness of procedures inversely related to their acceptance. Guided insertion techniques involving the use of 2-dimensional and 3-dimensional (3D) radiographic imaging coupled with intraoral (IO) scanning have therefore been harnessed as a means of improving the accuracy of miniscrew placement. These approaches have shown encouraging levels of accuracy coupled with the use of lateral cephalometry and cone-beam computed tomography (CBCT). ^,

Our hypothesis for this study was based on the assumption that the level of accuracy of guided miniscrew insertion is negligible overall, with little impact on both guide fabrication and practical use. Therefore, we aimed to evaluate the accuracy of guided palatal miniscrew placement, examining both the errors associated with the fabrication of the surgical guide and the surgical procedures.

Material and methods

Ethical approval was obtained from the Institutional Review Board in Amman, Jordan (reference no. SOB IRB-001). Consecutive patients requiring the indirect placement of palatal miniscrews were included in the study ( Fig 1 , A – D ). An IO scan (TRIOS, 3Shape, Copenhagen, Denmark) and a CBCT image were obtained for each patient whose treatment plan included the use of palatal miniscrews (OrthoLox plus+; Tiger Dental GmbH, Hörbranz, Austria) to construct a palatal appliance. Appliance designer software from 3Shape was used to create stereolithographic (STL) files to fabricate surgical guides for palatal miniscrew insertion (OrthohoLox Plus+, Promedia Medizintechnik Ahnfeldt GmbH, Siegen, Germany) and to create an STL file for the maxilla including the miniscrew heads (digital design [DD]).

The Easy CT DICOM Viewer module within Blueskye software (version 4.8.3, Blueskye plan, Libertyville, IL) was used to create an STL file of the maxilla (including bone and teeth) and maxillary teeth (crowns and roots) within the CBCT image.

The virtual dataset for each patient, including IO scan and CBCT-extracted STL files, was imported into Appliance Designer software (3Shape). CBCT-extracted STL files were then aligned with the maxillary STL file (IO scan) using the “add align model icon.” STL files, including TSADs (OrthoLox Plus+) of all available diameters and lengths, were already saved using the Appliance Designer software.

The surgical guide was developed from a 3 mm-thick shell covering palatal mucosa and maxillary teeth. Appropriately sized TSADs were selected and virtually aligned and sited to provide optimal bony support remote from the roots. TSADs were positioned at a 90° angle to the mucosal surface, with the TSAD shoulder resting on the mucosa. Once the position of the miniscrews was determined, a shell was designed incorporating the insertion guide, miniscrew heads, and the connectors. Three copies of the shell were then created; to make the surgical guide, to create a model with the screw heads, and to fabricate the connectors. To finalize the design of the guide, the “add modify model” icon was used to remove the miniscrew head and connectors. The “create combine model” icon allowed subtraction of the surgical guide from the maxillary STL file. This deleted any part of the surgical guide that might interfere with the fit of the printed surgical guide. The STL file of the guide was then exported and printed using an autoclavable resin material (P Pro Surgical Guide; Straumann AG, Basel, Switzerland) with the Rapid Shape 3D P 20+ printer. An STL file of the miniscrew heads within the maxillary model (DD) involved the “add modify model” icon to remove the connectors and the surgical guide. The “create combine models” icon was then used to combine the miniscrew head with the maxillary STL file.

In the in vitro study, maxillary models were created for 27 patients and used for the insertion of 2 palatal TSADs using the same surgical template as used clinically. To negate any effect of the hardness of the acrylic on the accuracy of the miniscrew placement, a hole was drilled at the miniscrew site. The miniscrews were then attached to the screwdriver and positioned using the drilling template. The TSADs were stabilized with self-cured acrylic resin before being removed from the screwdrivers and the drilling template. The model with the TSADs was scanned (model [MO] scan) using the TRIOS 3Shape scanner. To investigate the error associated with TSAD insertion clinically, superimposition of the DD with the IO scan was conducted using the “Inspect 3D” module in the OnyxCeph software (Image Instruments GmbH, Chemnitz, Germany). For the in vitro study, superimposition of the DD with the MO scan was performed in the “Inspect 3D” module. In addition, the superimposition of the IO scan with the MO scan was performed to identify significant differences between in vivo and in vitro data. The Inspect 3D module allowed for precise quantification of differences in the 3D datasets compared with the aligned reference model. The software employed the iterative closest point algorithm for model registration, using the median palatine raphe to serve as a stable anatomic reference structure for the superimposition. Both horizontal and vertical discrepancies were evaluated at both TSADs. The threshold values for color changes were set to 0.2 mm to detect noticeable dimensional deviations. These color changes enabled the identification of the biggest discrepancies and helped in understanding positional deviations. The assessment was carried out using the distance reference as the preferred measurement technique by 1 examiner (V.K.). The results of the measurements were visually documented and stored for later analysis and evaluation.

Statistical analysis

Descriptive statistics were calculated and plotted, highlighting the magnitude of error associated with each step of guided miniscrew placement. The analysis included (1) miniscrew position relative to the surgical guide by comparing the DD and MO scans, (2) in vivo evaluation of overall error using the surgical guide by comparing the DD and MO scans, and (3) miniscrew insertion inaccuracy associated with surgical guides related to in vivo and in vitro factors by comparing the DD and MO scans.

Statistical differences between the accuracy of placement based on left and right sides were evaluated using paired t tests. All analyses were conducted in Stata software (version 17; StataCorp, College Station, Tex).

Results

In total, 27 patients having bilateral palatal TSAD insertion (54 TSADs) in paramedian palatal sites using the digitally designed surgical guide were examined in vitro and in vivo.

Some degree of inaccuracy was observed with miniscrew insertion, with mean discrepancies being <0.5 mm both in the horizontal and vertical planes ( Fig 2 ; Table ). The mean overall horizontal and vertical discrepancy between the DD and final TSAD position (DD − final position IO) on the left side was 0.32 ± 0.15 mm and 0.34 ± 0.17 mm, respectively. The maximum horizontal discrepancy observed was 0.72 mm. No significant differences were identified in relation to the accuracy of miniscrew positioning on the basis of sidedness either for horizontal ( P = 0.29) or vertical ( P = 0.86) discrepancy ( Table ).