Approximately 14% of orthodontic anchor screws (miniscrews) become dislodged regardless of the accuracy of placement. It is therefore important to investigate the factors causing dislodgement. We evaluated the stability of miniscrews after placement to identify factors influencing outcome in orthodontic treatment using miniscrews.
We investigated 120 miniscrews (Dual-top Auto Screw III; Jeil Medical, Seoul, Korea) (diameter, 1.4 mm; length, 6 mm) placed on the buccal or lingual side between the maxillary second premolar and the first molar in women. Patient age and rate and time of screw dislodgement were examined. Insertion torque values and Periotest (Tokyo Dental Industrial, Tokyo, Japan) measurements indicating horizontal and vertical mobility of the inserted screws were compared between groups with and without dislodgement (failure and success groups, respectively).
Mean insertion torque values were 10.7 ± 1.9 N·cm and 8.5 ± 2.1 N·cm in the failure and success groups, respectively. Cortical bone thickness measurements (success group, 1.34 ± 0.35 mm; failure group, 0.99 ± 0.09 mm) were significantly higher, whereas Periotest values at placement (success group, horizontal, 4.9 ± 1.4; vertical, 4.7 ± 1.3; failure group, horizontal, 7.0 ± 0.8; vertical, 7.1 ± 0.9) were significantly lower in the success group than in the failure group.
The Periotest value, together with insertion torque and cortical bone thickness, could serve as an index of initial stability for predicting the outcome of miniscrew placement.
Appropriate insertion torque values obtain good stability after miniscrew placement.
Failure rate significantly increases when cortical bone thickness is ≤1 mm.
Periotest values at placement were significantly higher in the failure group.
Periotest instead of CBCT may give an initial stability index for miniscrew outcome.
In 1983, Creekmore and Eklund placed orthodontic anchor screws (miniscrews) in the anterior nasal spine to intrude maxillary anterior teeth. They reported that the miniscrews would serve as useful orthodontic anchorage devices for these reasons: they are minimally invasive and have no spatial restrictions, they can be inserted into the positions suitable for the treatment, and they are easily removable. Since then, many miniscrew orthodontic anchorage devices have been reported, and predictable treatment without relying on patient cooperation became achievable. Also, this approach enables effective intrusion of molars and front teeth; this was not possible by the conventional edgewise method. For these reasons, miniscrews are now becoming essential devices in today’s orthodontic treatments. However, according to Tomonari et al, the mean rate of screw dislodgement was 13.6% even when miniscrews had been placed accurately. Meanwhile, various studies investigated factors causing screw dislodgement in order to increase the success rate, but obvious factors have not yet been identified as shown in the study of Papageorgiou et al, possibly due to variations in the placement methods used (self-drilling or predrilling) and in screw sizes (length and diameter). Thus, in this study, we aimed to eliminate variables (eg, sex of the patient; type, length, and diameter of miniscrews) to closely examine the factors influencing dislodgement of miniscrews. Based on the notion that correct prediction of outcome immediately after screw placement has a strong impact on the overall outcome of orthodontic treatment, we aimed to identify factors that predict outcome and examine the predictability of the outcome. Atsumi et al reported that the Periotest system (Tokyo Dental Industrial, Tokyo, Japan) is the most commonly used noninvasive quantitative test measuring the mobility of dental implants, and that Periotest measurements after implant placement are useful in the evaluation of primary stability. Çehreli and Arman-Özçırpıcı examined the relationship between Periotest values and the stability of miniscrews in vitro, but close in-vivo examination was not performed. Accordingly, this study was conducted to test the hypothesis that Periotest measurement is a noninvasive and radiation-free means to predict the outcome of miniscrew placement.
Material and methods
Totals of 50 and 70 miniscrews (Dual-top Auto Screw III; Jeil Medical, Seoul, Korea) placed on the buccal and lingual sides between the right or left maxillary second premolar and the corresponding first molar, respectively, were examined in 60 female Japanese patients (mean age, 25.4 ± 10.5 years), who had given their informed consent to participate in this study. Benson et al reported that black patients have increased cortical bone mass compared with white patients, indicating differences in racial backgrounds. Thus, we examined Japanese women from the same racial background who satisfied the following criteria: requiring absolute anchorage involving the maxillary first molar, no periodontal disease with alveolar bone loss, no facial asymmetry, no cleft lip or palate, no impacted or missing teeth in the measurement site, no diagnosed systemic disease, no craniofacial dysmorphology, and not taking medication on a regular basis. The size of the screws was 1.4 mm in diameter and 6 mm in length. The self-drilling method was used to insert the screw into the position approximately 5 mm above the tooth cervix at a 45° angle against the alveolar bone, so that the screw was placed on the cortical bone approximately 6 mm from the alveolar crest. The same dentist (T.W.) performed the procedure in all patients. Cone-beam computed tomography (CBCT) and x-ray examination were used to confirm whether the inserted miniscrew was in contact with the root of the tooth. The maxillary first molar was fixed to a miniscrew inserted between the maxillary second premolar and first molar with a piece of ligature wire, and this was used as indirect anchorage in all patients. The inserted miniscrews were loaded 3 months after placement.
Test items were the rate of screw dislodgement (failure rate), time to screw dislodgement (time of failure), thickness of the cortical bone at the site of screw placement, and mobility (Periotest value) of the miniscrew immediately after placement. Screw failure was defined as the dislodgement of a miniscrew at any time during active treatment.
The cortical bone thickness on the buccal or lingual side between the maxillary second premolar and first molar, where a miniscrew was inserted, was measured on CBCT (3DeXam; KaVo, Baden-Wurttemberg, Germany) images on the coronal plane in each patient. Because the screw was placed approximately 6 mm from the alveolar crest, 2 positions were used for thickness measurements (5 and 7 mm from the alveolar crest), and the average measurement was considered as the cortical bone thickness. The insertion torque values were measured using a torque driver (Kanon Indicaor disk-type torque driver with a set pointer N1.2-DPSK; Nakamura Manufacturing, Matsudo, Japan: N cm) after screw placement ( Fig , A ). The mobility of the miniscrew was measured using the Periotest system. The original setting for Periotest values (PT values) were in the range of −8 to +50, where 0 indicates normal physiologic mobility, and −8 is the highest measurable stability. In this study, we added 8 to the raw measurements so that the higher Periotest values simply indicated the higher degree of screw mobility. The Periotest device was held at the horizontal or vertical position in relation to the long axis of the miniscrew, and 5 measurements were taken at each position ( Fig , B and C ) to obtain the average of each position.
The study protocol was reviewed and approved by the institutional review board of Aichi Gakuin University, Nagoya, Japan.
Data were analyzed by unpaired t test for statistical significance ( P <0.05). StatView statistical analysis software (SAS, Cary, NC) was used.
The overall failure rate was 14.2%; 17 of 120 miniscrews were dislodged during treatment ( Table I ). The failure rate was significantly higher on the buccal side (22.0%, 11/50) than on the lingual side (8.6%, 6/70).
|Success||Failure||Total||Failure rate (%)||P value|
The mean time to failure was 120.6 days after placement ( Table II ). More precisely, 8.3% (10 screws) of the failures occurred earlier than 3 months after placement, 1.7% (2 screws) of the failures occurred 3 to 6 months after placement, and 4.2% (5 screws) of the failures occurred 6 to 12 months after placement. More than half of the dislodgement incidents took place at an early stage (<3 months after placement).
|Failures (n)||Failure number/failure group (%)||Failure rate (%) (number/total insertion screws)|
The mean cortical bone thickness was 1.29 ± 0.35 mm ( Table III ). The cortical bone thickness was significantly lower on the buccal side (1.21 ± 0.34 mm) than on the lingual side (1.37 ± 0.37 mm). The cortical bone thickness was significantly higher in the success group (1.34 ± 0.35 mm) than in the failure group (0.99 ± 0.09 mm). On the buccal side, the mean cortical bone thickness was 1.26 ± 0.34 mm in the success group but significantly lower in the failure group at 0.92 ± 0.08 mm. Similarly, on the lingual side, the mean cortical bone thickness was 1.42 ± 0.37 mm in the success group but significantly lower in the failure group at 1.06 ± 0.12 mm.
|Success group||1.34 ∗||0.35|
|Buccal total||1.21 †||0.34|
The overall mean insertion torque was 8.8 ± 2.2 N·cm ( Table IV ). Mean insertion torque was 8.5 ± 2.1 N·cm in the success group but significantly higher at 10.7 ± 1.9 N·cm in the failure group. The mean insertion torque values were 8.7 ± 2.2 N·cm on the buccal side and 8.8 ± 2.3 N·cm on the lingual side; the difference between the sides was not statistically significant.