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.

Photographs of measurements and measuring devices: A, the insertion torque values were measured using a torque driver; B, the mobility of the miniscrew was measured using the Periotest system; C, the Periotest system.

The study protocol was reviewed and approved by the institutional review board of Aichi Gakuin University, Nagoya, Japan.