The success rate of infrazygomatic mini-implants placed with and without surgical guides: a historical control study
As a skeletal anchorage, mini-implant has obtained widespread use because its many advantages, such as easy installation and removal, less discomfort and patient cooperation requirement, small size and inexpensiveness [ – ]. Mini-implant can be interradicular or extra-alveolar . As an extra-alveolar skeletal anchorage, infrazygomatic mini-implant has been widely used because its efficiency in the correction of certain malocclusions, especially for the distal movement of the entire maxillary arch [ – ]. It is reported that extractions or orthognathic surgery sometimes could be avoided with the assistance of extra-alveolar mini-implant [ – ]. However, the failure rate of mini-implant placement ranges from 10 to 30% [ – ]. Because orthodontic clinicians often placed a mini-implant by a direct manual method according to their experience and panoramic radiograph, the anatomy of the implantation site and insertion angle usually cannot be judged precisely. In terms of the infrazygomatic mini-implant, proximity to the maxillary sinus is another important characteristic, and sinus perforation will reduce the biting depth and affect its stability . According to the literature, the failure rate of infrazygomatic mini-implant was higher compared to interradicular mini-implants . So as to reduce the risk of implant-to-root contact and sinus perforation, then improve the stability and success rate of mini-implants, many researches have been conducted to explore its appropriate insertion site and angle [ – ]. Furthermore, surgical guides have been developed to achieve three-dimensional (3D) guided placement of mini-implants [ – ]. However, present surgical guide only can be used for the interradicular mini-implants insertion. Infrazygomatic mini-implant should be inserted 1 mm perpendicularly to the cortical bone first, and then placed in an orientation buccal to the roots of molars . Therefore, a double-sleeves surgical guide with an adjustable guide track was fabricated in the current study, and the success rate of mini-implant insertion were evaluated. 

In the historical control study, an experimental treatment (mini-implant insertion guided by a surgical guide) was compared with a previously conducted control treatment (mini-implant insertion by a direct manual method), and it aims at evaluating the benefits of our surgical guide. The study was approved by the Institutional Review Board, and full informed consent was acquired from patients. The orthodontic records were obtained from the files of the Department of Orthodontics. Subjects Group 1 (control group) was retrospectively selected from the records of patients who sought orthodontic treatment from January 2016 to December 2019 and satisfied the inclusion and exclusion criteria for group 2 (surgical guide group) selection. Group 2 was prospectively selected according to the following criteria: permanent dentition; good periodontal condition and oral hygiene maintenance; age 14 to 30 and in good health; complete case records; full informed consent from patients or their parents; normal growth and development. Smokers, drinkers, and those who have a history of systematic diseases and metabolic bone diseases were excluded. Group 1 included 27 patients (mean age, 21.85 ± 4.19 years;11 males,16 females) with infrazygomatic mini-implants insertion between the maxillary first and second molars bilaterally. Group 2 included 31 patients (mean age, 22.87 ± 4.49 years; 13 males, 18 females) with infrazygomatic mini-implants insertion at the same site (Table ). A total of 116 self-drilling mini-implants (control group, n = 54; surgical guide group, n = 62) were placed by the same doctor. Fabrication of the surgical guide CBCT (Cone-beam computed tomography) images (LARGEV, Beijing, China) (100 kV,6 mA,0.25 mm voxel) and digital intraoral scanning images (3Shape, Copenhagen, Denmark) (0.02 mm, 2 min) were obtained first. Differently from the general procedure, two virtual column implants (1.4 mm in diameter and 8 mm in length) were placed at the insertion site. One indicated the position of the mini-implant when it vertically enters 1 mm into the cortical bone. Another determined its final position and direction (Fig. ). Then following design procedure was performed in Geomagic Wrap software (version 2021; Geomagic International, Research Triangle Park, NC) (Fig. ). Characteristically, a movably guide tube was set up, so the guided trajectory transformation can be achieved during the infrazygomatic mini-implant placement procedure. The guide tube was 12 mm in length and 4.5 mm in diameter, which was fit with the screwdriver and can guide the whole insertion procedure. Finally, an acrylic material template will be duplicated by a 3D printing printer (SprintRay Inc, Los Angeles, Calif, USA) (Fig. ). Surgical procedure of mini-implants placement In the control group, the position of the mini-implants was determined according to the orthodontists’ experience and panoramic radiographs. Mini-implants in the control group were inserted via a direct manual method. In the surgical guide group, a two-trajectories surgical guide was used for the infrazygomatic mini-implants placement (Fig. ). After inserting the mini-implant 1 mm into the cortical bone at the initial position, the screwdriver was turned inferiorly and the mini-implant can be inserted fully at the final position. The mini-implant (VectorTAS self-drilling mini-implant, Ormco, Orange, Calif), 1.4 mm in diameter and 8 mm in length, was selected both in the two groups. All the 86 mini-implants were immediately loaded with the same magnitude of retraction force (150 g). And mouthwash containing 0.12% chlorhexidine was used twice daily for two weeks. Evaluation of mini-implant stability After placement, follow-up was taken to check the mobility of mini-implants every 1.5-months interval, and the survival time need to be recorded. The subjects that loss to follow-up would be excluded. If the mini-implant loosen or fell off within 6 months, it would be classified as failure. Conversely, the mini-implant would be counted as success if it kept stable within 6 months. Statistical analysis The data was analyzed using SPSS software (version 26.0; IBM Corp, Chicago, IL). In order to detect the differences of the insertion success rate and the survival time between the control and surgical guide group, Kaplan-Meier survival analysis was performed and statistical significance was defined as at P < 0.05. 

Group 1 (control group) was retrospectively selected from the records of patients who sought orthodontic treatment from January 2016 to December 2019 and satisfied the inclusion and exclusion criteria for group 2 (surgical guide group) selection. Group 2 was prospectively selected according to the following criteria: permanent dentition; good periodontal condition and oral hygiene maintenance; age 14 to 30 and in good health; complete case records; full informed consent from patients or their parents; normal growth and development. Smokers, drinkers, and those who have a history of systematic diseases and metabolic bone diseases were excluded. Group 1 included 27 patients (mean age, 21.85 ± 4.19 years;11 males,16 females) with infrazygomatic mini-implants insertion between the maxillary first and second molars bilaterally. Group 2 included 31 patients (mean age, 22.87 ± 4.49 years; 13 males, 18 females) with infrazygomatic mini-implants insertion at the same site (Table ). A total of 116 self-drilling mini-implants (control group, n = 54; surgical guide group, n = 62) were placed by the same doctor. 

CBCT (Cone-beam computed tomography) images (LARGEV, Beijing, China) (100 kV,6 mA,0.25 mm voxel) and digital intraoral scanning images (3Shape, Copenhagen, Denmark) (0.02 mm, 2 min) were obtained first. Differently from the general procedure, two virtual column implants (1.4 mm in diameter and 8 mm in length) were placed at the insertion site. One indicated the position of the mini-implant when it vertically enters 1 mm into the cortical bone. Another determined its final position and direction (Fig. ). Then following design procedure was performed in Geomagic Wrap software (version 2021; Geomagic International, Research Triangle Park, NC) (Fig. ). Characteristically, a movably guide tube was set up, so the guided trajectory transformation can be achieved during the infrazygomatic mini-implant placement procedure. The guide tube was 12 mm in length and 4.5 mm in diameter, which was fit with the screwdriver and can guide the whole insertion procedure. Finally, an acrylic material template will be duplicated by a 3D printing printer (SprintRay Inc, Los Angeles, Calif, USA) (Fig. ). 

In the control group, the position of the mini-implants was determined according to the orthodontists’ experience and panoramic radiographs. Mini-implants in the control group were inserted via a direct manual method. In the surgical guide group, a two-trajectories surgical guide was used for the infrazygomatic mini-implants placement (Fig. ). After inserting the mini-implant 1 mm into the cortical bone at the initial position, the screwdriver was turned inferiorly and the mini-implant can be inserted fully at the final position. The mini-implant (VectorTAS self-drilling mini-implant, Ormco, Orange, Calif), 1.4 mm in diameter and 8 mm in length, was selected both in the two groups. All the 86 mini-implants were immediately loaded with the same magnitude of retraction force (150 g). And mouthwash containing 0.12% chlorhexidine was used twice daily for two weeks. 

After placement, follow-up was taken to check the mobility of mini-implants every 1.5-months interval, and the survival time need to be recorded. The subjects that loss to follow-up would be excluded. If the mini-implant loosen or fell off within 6 months, it would be classified as failure. Conversely, the mini-implant would be counted as success if it kept stable within 6 months. 

The data was analyzed using SPSS software (version 26.0; IBM Corp, Chicago, IL). In order to detect the differences of the insertion success rate and the survival time between the control and surgical guide group, Kaplan-Meier survival analysis was performed and statistical significance was defined as at P < 0.05. 

After implantation, 2 mini-implants out of 62 failed and the success rate was 96.8% in the surgical guide group. In the historical control group, 11 mini-implants out of 54 failed and the success rate was 79.6%. There was statistically significant difference in the success rate and survival time between the two groups ( P = 0.003) (Table ; Fig. ). 

With the wide application of infrazygomatic mini-implants in orthodontics, researches about its appropriate insertion site and angle were conducted by many scholars. Liou et al.  suggested that the infrazygomatic mini-implant insertion should be at 14 to 16 mm above the maxillary occlusal plane and the maxillary first molar, and at an angle of 55° to 70° to the maxillary occlusal plane. Liu et al.  also indicated that the region between the maxillary first and second molars should be the first choice for the infrazygomatic mini-implants insertion. Al Amri et al.  recommended that placing an infrazygomatic mini-implant at a 45° insertion angle was greatest. However, the failure rate of infrazygomatic mini-implants is still at a relatively high level because its higher implantation site and difficult insertion method, as well as proximity to the maxillary sinus. According to the literature , the failure rate of infrazygomatic mini-implants was 23.1%, which is slightly higher than 13.5% reported for interradicular mini-implants . Mohammed et al.  reported that infrazygomatic crest region was the least successful insertion site, with an overall failure rate of 16.4%, higher than the buccal interradicular sites and palatal sites. Additionally, it is inevitably to penetrate the maxillary sinus during implantation process , causing decrease in the intraosseous length of mini-implants and ultimately the dislodgement. According to the literature, it is reported that 78.3% of mini-implants inserted in the IZC penetrated into the maxillary sinus . Viwattanatipa et al.  also suggested that placement at the high level had a greater risk than placement at the medium level. Because mini-implant usually be placed through a direct manual method, and it cannot be positioned precisely relative to tooth roots and available bone. To provide stable anchorage for tooth movement, various surgical guides have been fabricated to achieve the accurate placement of mini-implant. Conventional surgical wire guide technique was performed using a brass wire as marker , the insertion direction cannot be precisely controlled, and accurate placement of mini-implants still did not be achieved. Kim et al.  firstly fabricated a three-dimensional surgical guide for mini-implant placement based on CBCT images, and it allowed precise pilot drilling and accurate placement of the mini-implant. Bae et al.  integrated the digital model images with CBCT images for surgical guide fabrication, and it showed that 20% of the mini-implants in the manual method group caused root damage, but no root damage occurred in the surgical guide group. Surgical guide for palatal mini-implant placement was also been reported, and it may provide sufficient accuracy for palatal mini-implant placement . Generally speaking, mini-implant was inserted more precisely and safely when a surgical guide was used. However, all of the surgical guides developed to date cannot be applied for the infrazygomatic mini-implant insertion, because infrazygomatic mini-implant should be inserted by another distinctive implantation method, and a surgical guide with an adjustable guide trajectory is needed to be made. In our study, the guide part was designed as a double-sleeves device and the guide tube was movable. Therefore, transformation of the insertion angle and full guidance of the insertion procedure can be achieved. Furthermore, it showed a success rate of 79.6% and 96.8% respectively in the control group and the surgical guide group. And a statistically significant improvement in the success rate of mini-implant placement and survival time was found when using the guide. In the study conducted by Miyazawa et al. , the success rate of mini-implants inserted under the guidance of a surgical guide was 90.9%. Al-Suleiman et al.  reported that the success rate of mini-implants placed with a 3D surgical guide was 95%, while the success rate of those placed with a 2D wire guide was only 75%. The failure rate of mini-implant in this study appears to be in the range of those of previous researches. Within the limits of this historical control study, it is the first attempt to investigate the clinical application of a 3D surgical guide which allows for transformation of guide trajectory. What’s more, it can be concluded that the success rate of infrazygomatic mini-implant insertion was significantly improved when the two-trajectories surgical guide was used. However, the sample size was needed to be enlarged and further studies for evaluating the accuracy and stability were also necessary. 

The two-trajectories surgical guide is advantageous for the insertion of infrazygomatic mini-implant. Both the success rate and stability of infrazygomatic mini-implant were improved significantly with the use of the guide. But further studies with enlarged sample are needed to evaluate the accuracy and stability of the mini-implant placement. 

