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INTRODUCTION
Miniscrews have revolutionized orthodontic treatments, enabling complex tooth movements.1-3 However, even a single failure among several planned miniscrews can necessitate adjustments to a patient’s overall treatment plan. Therefore, the success of miniscrews is critical for orthodontists.
The success rates of miniscrews vary from 77.5% to 95.0%.4,5 Factors influencing these rates include patient age, miniscrew design (including length and diameter), insertion jaw, and operator skill.6-11
Using miniscrews entails several risks, including root damage to adjacent teeth, patient discomfort, inflammation, and edema.12 Root contact is the most significant risk factor for miniscrew failure.13-15 The interradicular distance typically measures between 2.4 mm and 4.9 mm.16 Given that the typical diameter of miniscrews ranges from 1.2 to 1.8 mm, it is challenging to place miniscrews centrally without contacting or being in proximity to the roots when interradicular spaces are narrow.
Advancements in cone-beam computed tomography (CBCT) have enabled orthodontists to accurately evaluate the anatomical limitations of the maxilla and mandible, as well as distances between roots, in three dimensions (3D).16-19 Moreover, the progress in Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) technology has facilitated the precise insertion of miniscrews by utilizing stereolithographic surgical guides based on 3D assessments.9,20-22 However, most existing studies are retrospective with a small number of patients or cadaver studies.9,21,23,24
Therefore, we aimed to assess the effect of a miniscrew surgical guide created using digital and 3D printing technologies on the success rate and root proximity of miniscrews.
MATERIALS AND METHODS
Participants
This retrospective study included patients who received orthodontic treatment using miniscrews as skeletal anchorage at the Department of Orthodontics, Yonsei University Dental Hospital, Seoul, Korea, from March 2018 to March 2021. A total of 224 miniscrews (cylindrical, 1.5 mm diameter, 7.0 mm length; Biomaterials Korea, Seoul, Korea) were placed into the buccal interradicular spaces of 113 patients (64 females, 49 males) by one of two operators; Operator 1: orthodontic specialist or Operator 2: resident. This study was approved by the Institutional Review Board of Yonsei Dental Hospital (CRNo: 2-2018-0028). Written informed consent was obtained from all participants. The entire sample was classified into a manual group (MG) and a surgical guide group (SG) according to the miniscrew placement method.
The inclusion criteria were as follows:
(1) Patients over 18 years of age.
(2) Patients with no missing data related to miniscrew placement or failure.
(3) Patients who underwent CBCT before the start of orthodontic treatment for reasons such as impacted teeth, skeletal discrepancy, asymmetry, or craniofacial deformities.
(4) Patients who had miniscrews implanted using a surgical guide manufactured according to the protocol described below (the SG).
Baseline data included age, sex, jaw of placement, and interradicular distance. Interradicular distances were measured using CBCT axial images taken 4 mm apical to the cemento-enamel junction (CEJ) and defined as the distance between the mid-root portions of two adjacent teeth.
Sample size calculation
A total of 208 samples were determined to provide over 95% power to identify a significant difference, given a 0.25 effect size and a significance level of α = 0.05 (G power version 3.1.9.7; Franz Faul, University of Kiel, Kiel, Germany). Therefore, the inclusion of 224 miniscrews placed using two methods (surgical guide and manual) during the study period was considered sufficient.
Manufacturing of the surgical guide
Intraoral scanning data were acquired using a TRIOS 3 (3 shape, Copenhagen, Denmark; Figure 1A). CBCT (Alphard VEGA; Asahi Roentgen Ind., Kyoto, Japan) was performed with a voxel of 0.30 mm voxel, set at 80 kV, 5.0 mA, and a 17 seconds scanning time (panoramic mode). Afterward, the scan data and CBCT data were merged using the best-fit method by setting only the tooth region in the 3D software (Figure 1B). After determining the placement of the miniscrews (Figure 1C and 1D), the same software company (Cybermed, Daejeon, Korea) was commissioned to produce the surgical guides.23 Finally, the surgical guide was printed using a 3D printer (Eden560v) with Med610 as the base material (Stratasys, Rehovot, Israel; Figure 1E). It had a thickness of 3.0 mm, with a margin of 2.0 mm from the metal sleeves.
Miniscrew implantation procedure
In the MG, the operator assessed the anatomical limitations and interradicular distances of the adjacent roots before implantation. The miniscrew insertion position was determined using both direct and mirror views, and the miniscrew was placed in a position 4–6 mm below the CEJ using a dental probe to mark the location and then inserted at an angle of 30–40 degrees relative to the occlusal plane.7,23,25
In the SG, the fit of the surgical guide was checked before implantation. The miniscrews were then inserted using a driver guided by metal sleeves (Figure 1F).
Primary and secondary outcomes
Primary outcomes
The success of miniscrew placement and mobility were assessed during regular orthodontic appointments at 4-week intervals. Miniscrew placement was considered successful when the mobility of the miniscrew was less than 1 mm without exposure of the screw threads and was clinically stable. Miniscrews were observed for 6 months after placement, and the timing of any early failures was recorded.
Secondary outcomes
Initial stability
Initial stability was evaluated by two variables, the insertion torque (IT) and Periotest value (PTV).26-29 After the miniscrews were placed, the IT was measured using MGT50 (Mark-10, Copiague, NY, USA) and recorded in N cm. The PTV was then measured with a Periotest (Siemens, Bensheim, Germany), ensuring that its head remained parallel to the floor of the clinic and at a right angle to the head of the miniscrew. Both IT and PTV were measured twice immediately following the placement of the miniscrews.
Root proximity
Periapical radiographs were obtained, and root proximity was evaluated according to a previous study.13 The radiographs were retaken if parallelism was not achieved. Root proximity was classified into three categories:
(1) Center between the roots: the miniscrew was completely separated from the root.
(2) Close to the root: the apex of the miniscrew appeared to touch the lamina dura.
(3) Contact with the root: the apex of the miniscrew appeared to have intruded into the periodontal ligament space or made contact with the root.
Statistical analysis
Intra-examiner correlation was considered very good (intraclass correlation coefficient, 0.97). The Shapiro–Wilk test was used to check for normality. Kaplan–Meier survival curves were drawn to visually compare the survival curve of the miniscrews, and the log-rank test was performed to compare the survival distribution between groups. The chi-square test and Fisher’s exact test were used to analyze the correlation between the two groups in terms of proportions. The independent t test and Mann–Whitney U test were employed to assess the statistical difference between continuous variables. Statistical analyses were conducted using SPSS software (version 26; IBM, Armonk, NY, USA) with α = 0.05.
RESULTS
Baseline data
Demographic data and characteristics of the enrolled patients are presented in Table 1. There were no statistically significant differences in age, sex, insertion site, or interradicular distance between the groups. However, there was a statistically significant difference between the two groups in the proportion of jaws in which miniscrews were placed.
Primary outcomes
The Kaplan–Meier survival curves for the two groups are shown in Figure 2. The cumulative survival rate of the SG was significantly higher than that of the MG (P = 0.022, log-rank test).
The success rates according to different qualitative variables are summarized in Table 2. The overall success rates were 79.1% for the MG and 90.5% for the SG, with a statistically significant difference (P = 0.021). Additionally, the success rates between the root proximity categories were significantly different (P = 0.007). Miniscrew placement location (maxilla or mandible, anterior or posterior), sex, and operator (specialist or resident) did not affect the success rate of miniscrew placement.
Secondary outcomes
Chi-square tests were used to assess differences in initial stability variables and root proximity between the MG and SG groups (Table 3). The root center position ratio was 76.8% in the SG and 55.0% in the MG. The root contact rates were 14.7% in the MG and 1.1% in the SG, with this difference being statistically significant (P < 0.001).
A comparison of variables between the success and failure participants within groups is presented in Table 4. Statistical analysis revealed a significant difference in root proximity between the success and failure participants within the MG (P < 0.05).
Regardless of the miniscrew insertion method, the IT, PTV, and root proximity were compared based on the success of the miniscrew, and only root proximity showed a statistically significant difference (P < 0.05; Table 5).
DISCUSSION
The Kaplan–Meier survival curves for the SG and MG showed a statistically significant difference, indicating that the miniscrew survival rate between the two groups differed significantly. However, this was a retrospective study that investigated miniscrew failure based solely on past treatment records, and not all patients were contacted or visited the hospital immediately after screw dislodgement, which may have resulted in inaccurate survival curve analysis.
The miniscrew insertion method (manual or surgical guide) and root proximity significantly impacted the success rate of miniscrews.
The success rates for the MG and SG groups were 79.1% and 90.5%, respectively. A systematic review of miniscrew placement success rates reported an average success rate of 86.5%.4 Additionally, previous studies have reported success rates ranging from 77.5% to 95.0%.4,5 Our study’s success rate falls within this range. Two factors may explain the higher success rate in the SG group. First, the SG group had a significantly higher number of miniscrews placed in the maxilla, which may have affected the success rate. While this study found no statistically significant difference in the success rate of miniscrews between the maxilla (87.0%) and mandible (80.2%), it is generally reported that the miniscrew success rate in the mandible is lower than that in the maxilla because of the thick cortical bone, narrow attached gingiva, and accessibility to screwdrivers. Second, the root contact rate was significantly lower in the SG group (1.1%) compared to the MG group (14.7%) (P < 0.05), which likely contributed to the higher success rate in the SG group. Contact between the miniscrew and root is a major risk factor for screw failure.30,31 This is further supported by our finding that root proximity was significantly different between the success and failure groups (P < 0.05; Table 5). The 3D printed surgical guide produced using the method described in this study significantly increased the accuracy of miniscrew placement, even in narrow interradicular spaces. By guiding the insertion process, the surgical guide ensures the miniscrews are placed in a position that does not contact adjacent tooth roots, directly contributing to a higher success rate of miniscrew placement.
When comparing the variables associated with success and failure miniscrews within each group, a significant difference in root proximity was found only in the MG group (P < 0.05; Table 4), with no significant difference in the SG group. The use of a surgical guide allows for strategic design, such as intentionally placing miniscrews close to the roots of the posterior teeth when posterior dentition movement is necessary. In such cases, root proximity decreases with orthodontic teeth movement, suggesting that root proximity does not significantly impact the success rate in the SG group. Regardless of orthodontic biomechanics, miniscrews should ideally be placed in consistent locations to allow for accurate comparisons within and between groups. However, this is considered a limitation of our retrospective study.
The initial stability values of miniscrews in the two groups, measured by IT and PTV, were not significantly different, and both values were within the mean range of those previously reported.27,29,32 Furthermore, IT and PTV did not differ based on the success of the miniscrews or their proximity to the root. When using a surgical guide, miniscrews are planned to be placed in areas of good bone quality and quantity based on CBCT images during the guide fabrication process. Consequently, owing to the strong initial stability between the bone and miniscrews, the PTV may be relatively low. However, the PTV of the successful miniscrew group was larger than that of the failed group, although this difference was not statistically significant. Therefore, while IT and PTV may be used as indicators of the initial stability of miniscrews, they may not reliably predict miniscrew success.
This study was conducted at a single university center, which limits the generalizability of the results to surgical guides for miniscrews produced under the same protocol. Additionally, the proximity of the miniscrews to the roots was evaluated in a two-dimensional manner using periapical radiographs, which is less accurate compared to 3D assessments using CBCT scans. As shown in Table 5, many miniscrews were successful despite being classified as being in contact with the root. This discrepancy suggests that contrary to the results of the periapical radiographs, the miniscrews may not have been in actual contact with the roots. Such errors have also been reported in previous studies and highlight the limitations of using periapical radiographs to evaluate miniscrew proximity to the roots.33 However, it would be unethical to perform additional CBCT scans solely to evaluate the placement of miniscrews. Periapical radiographs were taken to minimize errors and ensure alignment parallel to the long axis of the miniscrew and were retaken if the angle was improper.
CONCLUSIONS
Root proximity and the utilization of surgical guides have the most direct impact on the success rate of miniscrews. Root proximity can be effectively reduced by using surgical guides. Therefore, the use of a miniscrew surgical guide is recommended to increase the success rate of miniscrews as stable anchorage devices, particularly in cases with narrow interradicular space.
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