Although the success of using orthodontic miniscrews for primary insertion has been reported in the literature, few studies have followed up on secondary insertions after failure of the first insertion. In this study, we investigated not only the primary but also secondary success rates of miniscrews and considered the risk factors influencing their stability.
Five hundred miniscrews were inserted for orthodontic anchorage in 240 patients. Ninety-eight miniscrews lacked stability; thus, 77 of these were removed and reinserted. We calculated and compared the primary and secondary success rates of insertion. Moreover, we investigated which clinical parameters affected the stability of miniscrews.
The success rate of secondary insertion (44.2%) was significantly lower than that of primary insertion (80.4%). The screw length and jaw receiving the insertion were significantly associated with the stability of miniscrews. The 8.0-mm miniscrews were significantly more stable than the 6.0-mm miniscrews, and the success rate for insertions into the maxilla was significantly higher than that for the mandible.
Secondary insertions lack stability; therefore, clinicians should be aware of the reduced success rate of reinsertion and know the risk factors to avoid failure of secondary insertions.
Orthodontic miniscrews are effective for primary insertions.
Secondary insertions were significantly less successful than primary insertions.
Longer (8.0 mm) miniscrews improved stability compared with shorter (6.0 mm) miniscrews.
Insertions into the maxilla were significantly more successful than into the mandible.
Clinicians should be aware of the reduced success rate of reinsertion.
Anchorage control is one of the most important factors in orthodontic treatment. Although many intraoral and extraoral appliances have been used for anchorage control, the patient’s compliance is indispensable. Recently, the paradigm of anchorage control has shifted toward temporary anchorage devices, including miniscrews and miniplates.
Some reports have indicated that miniplates exhibit more stability than miniscrews, but they require flap surgery for insertion and removal, and the procedure could cause swelling and discomfort. On the other hand, miniscrews can be inserted into the maxillary and mandibular bones, between the roots of teeth, rapidly and easily. Miniscrews have become a popular method for achieving anchorage without requiring compliance from patients. However, some complications related to their use have been reported. A lack of stability has been observed, and some miniscrews were lost before achieving their purpose. Therefore, it is important to understand the success and failure rates when inserting miniscrews and to consider the risk factors associated with the loss of miniscrews.
Many retrospective studies and systematic reviews have reported that the success rate of orthodontic miniscrews exceeds 80%. However, some screws could be lost, with a low failure rate (range, 5%-40%; mean, about 15%). It has been suggested that risk factors associated with the instability of miniscrews could be categorized as host factors, miniscrew factors, and surgical management factors. For example, the host factors associated with failure of miniscrews include age, sex, vertical skeletal relationship, oral hygiene, cortical bone thickness, root proximity, and the jaw receiving the insertion (maxilla or mandible). On the other hand, miniscrew factors include make, shape, diameter, and length of the screws. Finally, surgical management factors include insertion torque, angle, placement height (in the movable mucosa or the attached gingiva), the necessity for predrilling or flap surgery, insertion by a manual or motor-driven method, and latency period (early or delayed orthodontic loading). Although many factors seem to affect success and failure rates, there is little evidence to support their proposed influence. Therefore, more clinical studies are necessary to provide information that will facilitate achieving more predictable results with miniscrews.
Although it is at a low frequency, some screws are lost before achieving their purpose for several reasons. When miniscrews are lost, the options are to reinsert them or to change the treatment plan, including using other appliances for anchorage control. Many previous studies on miniscrews have involved their use in primary insertions. Yet, there are few follow-up studies about the secondary insertion of miniscrews. The purposes of this retrospective study were to investigate the success rate not only of the primary, but also of the secondary insertion, of orthodontic miniscrews, and to consider the risk factors associated with their instability.
Material and methods
The study design and ethical considerations were approved by the ethical committee of Tokyo Medical and Dental University (approval number D2016-029) in Japan.
The subjects consisted of 240 patients (61 men, 179 women; ages, 28.1 ± 9.8 years) who underwent surgery for the insertion of miniscrews for orthodontic edgewise treatment at the Department of Orthodontic Science of Tokyo Medical and Dental University from July 2012 to January 2015. The total number of miniscrews inserted in the subjects was 500.
After written informed consent was obtained from each patient, the same doctor (Z.K.) inserted or instructed a graduate student to insert all titanium miniscrews (Dualtop; Jeil Medical, Seoul, Korea). We used miniscrews with different diameters (1.4 or 1.6 mm) and lengths (6.0 or 8.0 mm). Before surgery, 3-dimensional computed-tomography images were obtained for all subjects, and the anatomic features (root proximity, cortical bone thickness, and maxillary sinus) were analyzed. Then, we decided on the miniscrew placement site and selected the diameter and length of miniscrews required to avoid injuring roots and to minimize damage to the surrounding tissues. After anesthetic infiltration, all miniscrews were inserted by flapless surgery with the self-drilling procedure. After surgery, analgesics, antibiotics, and 0.12% chlorhexidine mouthwash were prescribed. We confirmed that the miniscrew did not contact the neighboring roots by obtaining periapical x-ray images. Orthodontic treatment started at various times after the surgery. The latency period ranged from immediate loading to 3 months.
If a miniscrew lacked stability, it was removed, and a new miniscrew of the same or a different size (with a change in diameter and length) was reinserted into the same position (ie, insertion between the same pair of teeth as for the first insertion, but the mesiodistal position, height, and insertion angle were changed) or into another position (ie, insertion between a different pair of teeth from those in the first insertion) after 1 to 2 months. For some patients, the treatment plan was changed, including the use of other appliances for anchorage.
We calculated the success and failure rates of primary inserted miniscrews. We defined success by the following 3 criteria: (1) no inflammation of the soft tissues surrounding the miniscrews, (2) no clinically detectable mobility, and (3) anchorage function sustained after 1 year of orthodontic loading. The screws that did not meet these criteria were considered as failures. Then, we calculated the secondary success and failure rates of the reinserted miniscrews. We compared primary and secondary success rates statistically. We collected survival times from insertion to 12 months and plotted a survival curve of the miniscrews to evaluate when screws were lost after primary and secondary insertions. The Kaplan-Meier method and the log-rank test were used to compare the survival curves between primary and secondary insertions.
Moreover, the chi-square test was performed to compare differences in the success rates of primary and secondary insertions according to the classification of each clinical variable. With regard to secondary (reinsertion), we compared the success rate between the insertion in the same and a different position, and between screws of the same and different sizes. With regard to the secondary insertion in the same position, we compared the success rate between the insertion of a new miniscrew 1 month and 2 months after the removal of the first miniscrew.
As for the significant parameters by the chi-square or the Fisher exact test, we also plotted a survival curve of the miniscrews using the Kaplan-Meier method and the log-rank test. All analyses were performed using the Statistical Package for Social Sciences (release 10.0; SPSS, Chicago, Ill). Statistical significance was set at P <0.05.
Over a 1-year period after insertion, the overall primary success rate was 80.4% for all miniscrews (402 of 500 screws), whereas 19.6% of all miniscrews (98 of 500 screws) were considered to have failed. Our success rate was similar to rates in previous studies with a sample size of more than 100 miniscrews inserted at 1 institution ( Table I ).
|Authors||Design||n||Evaluation||Diameter (mm)||Length (mm)||Success rate (%)|
|Berens et al (2006)||Pro||239||P||1.4/1.8/2.0||NA||84.9|
|Wilmes et al (2006)||Pro||133||P||1.2/1.6||5.0-10.0||86.8|
|Kuroda et al (2007)||Pro||216||P||1.3/1.5||6.0-12.0||83.8|
|Moon et al (2008)||Retro||480||P||1.6||8.0||83.8|
|Baek et al (2008)||Retro||107||P/S||2.0||5.0||75.2|
|Motoyoshi et al (2009)||Pro||169||P||1.6||8.0||85.2|
|Bayat and Bauss (2010)||Pro||110||P||2.0||7.0/9.0/11.0||81.8|
|Türköz et al (2011)||Pro||112||P||1.4||7.0||77.7|
|Kim et al (2012)||Retro||429||P||1.6||6.0/8.0||77.4|
In terms of primary insertions, we found that 4 types of screws with different dimensions and the jaw receiving the insertion were significantly associated with success or failure of the primary insertion ( Table II ).
|Parameter||Success (%)||Failure (%)||Total (%)||Probability|
|Male||112 (85.5)||19 (14.5)||131 (100.0)|
|Female||290 (78.6)||79 (21.4)||369 (100.0)|
|<20||96 (77.4)||28 (22.6)||124 (100.0)|
|≥20 to <30||169 (79.7)||43 (20.3)||212 (100.0)|
|≥30||137 (83.5)||27 (16.5)||164 (100.0)|
|Screw length (mm)||0.02|
|6.0||185 (76.1)||58 (23.9)||243 (100.0)|
|8.0||217 (84.4)||40 (15.6)||257 (100.0)|
|Screw diameter (mm)||0.72|
|1.4||250 (80.9)||59 (19.1)||309 (100.0)|
|1.6||152 (79.6)||39 (20.4)||191 (100.0)|
|Screw size (diameter × length [mm])||0.03|
|1.4 × 6.0||112 (74.2)||39 (25.8)||151 (100.0)|
|1.6 × 6.0||73 (79.4)||19 (20.6)||92 (100.0)|
|1.4 × 8.0||138 (87.3)||20 (12.7)||158 (100.0)|
|1.6 × 8.0||79 (79.8)||20 (20.2)||99 (100.0)|
|Maxilla||336 (82.6)||71 (17.4)||407 (100.0)|
|Mandible||66 (71.0)||27 (29.0)||93 (100.0)|
|Maxillary incisor||7 (77.8)||2 (22.2)||9 (100.0)|
|Maxillary premolar||36 (81.8)||8 (18.2)||44 (100.0)|
|Maxillary molar||293 (82.8)||61 (17.2)||354 (100.0)|
|Mandibular premolar||38 (74.5)||13 (25.5)||51 (100.0)|
|Mandibular molar||28 (66.7)||14 (33.3)||42 (100.0)|
|Total||402 (80.4)||98 (19.6)||500 (100.0)|
The success rate when using 8.0-mm screws (84.4%) was significantly higher than that with 6.0-mm screws (76.1%). Moreover, the success rate of inserting miniscrews into the maxilla (82.6%) was significantly higher than when inserting them into the mandible (71.0%). On the other hand, sex, age, screw diameter, and insertion area showed no significant associations with the success or failure of primary insertions.
Among the 98 failures of primary insertions, reinsertion was attempted for 77 screws. The secondary success rate was 44.2% for all reinserted miniscrews (34 of 77 screws). Secondary insertions tended to lack stability. In terms of the location and size of the screws at the secondary insertion, 59 of 77 screws were inserted into the same place, whereas the place for insertion was changed for 18 screws ( Table III ). Same-sized screws were reinserted for 30 patients, and the size (diameter or length) was changed for 47 patients. For secondary insertions ( Table III ), the success rate for 8.0-mm screws (65.6%) was significantly higher than for 6.0-mm screws (28.9%). There were no significant differences in the success rates of reinsertion between the same (45.8%) and different (38.9%) positions, or between using the same (43.3%) and different (44.7%) sizes of miniscrews. With regard to the latency period of insertion in the same position after removal of the first miniscrew, there were no significant differences in the success rates between reinsertion after 1 month (9 of 20; 45.0%) and reinsertion after 2 months (18 of 39; 46.2%).
|Parameter||Success (%)||Failure (%)||Total (%)||Probability|
|Male||7 (41.2)||10 (58.5)||17 (100.0)|
|Female||27 (45.0)||33 (55.0)||60 (100.0)|
|<20||9 (45.0)||11 (55.0)||20 (100.0)|
|≥20 to <30||16 (42.1)||22 (57.9)||38 (100.0)|
|≥30||9 (47.4)||10 (52.6)||19 (100.0)|
|Screw length (mm)||0.01|
|6.0||13 (28.9)||32 (71.1)||45 (100.0)|
|8.0||21 (65.6)||11 (34.4)||32 (100.0)|
|Screw diameter (mm)||0.61|
|1.4||23 (41.8)||32 (58.2)||55 (100.0)|
|1.6||11 (50.0)||11 (50.0)||22 (100.0)|
|Screw size (diameter × length [mm])||0.01|
|1.4 × 6.0||11 (31.4)||24 (68.6)||35 (100.0)|
|1.6 × 6.0||2 (20.0)||8 (80.0)||10 (100.0)|
|1.4 × 8.0||12 (60.0)||8 (40.0)||20 (100.0)|
|1.6 × 8.0||9 (75.0)||3 (25.0)||12 (100.0)|
|Maxilla||27 (46.6)||31 (53.4)||58 (100.0)|
|Mandible||7 (36.8)||12 (63.2)||19 (100.0)|
|Maxillary incisor||1 (50.0)||1 (50.0)||2 (100.0)|
|Maxillary premolar||3 (42.9)||4 (57.1)||7 (100.0)|
|Maxillary molar||23 (46.9)||26 (56.5)||49 (100.0)|
|Mandibular premolar||3 (42.9)||4 (57.1)||7 (100.0)|
|Mandibular molar||4 (33.3)||8 (66.7)||12 (100.0)|
|Same||27 (45.8)||32 (54.2)||59 (100.0)|
|Different||7 (38.9)||11 (61.1)||18 (100.0)|
|Reinserted screw size||0.91|
|Same||13 (43.3)||17 (56.7)||30 (100.0)|
|Different||21 (44.7)||26 (55.3)||47 (100.0)|
|Total||34 (44.2)||43 (55.8)||77 (100.0)|