Self-drilling mini-implants are being used more frequently as an orthodontic anchorage, but the placement torque of self-drilling mini-implants can easily become excessive in the thick, mandibular cortical bone. The purpose of this study is to examine a safe self-drilling placement technique that provides adequate placement torque for orthodontic mini-implants. The mini-implants were placed using self-drilling and pre-drilling methods into the ribs of pigs. Specimens were classified into two groups, thin and thick, with cortical bone thicknesses of 1.2 ± 0.02 and 2.0 ± 0.03 mm, respectively, and used to model the human maxillary and the mandibular bones. The peak mini-implant placement torque value was measured and the surrounding cortical bone was observed histologically. In the mandible model, the torque in the self-drilling and pre-drilling groups exceeded 10 N cm, except in one case which had a 1.3 mm diameter pilot hole. Histology revealed cracks in the surrounding cortical bone in the groups whose torque value was 10 N cm or more. Therefore, when using the self-drilling technique to place a 1.6 mm diameter mini-implant in the mandibular alveolar bone, it is preferable to drill a 1.3 mm diameter pilot hole first.
Orthodontic treatments have been advanced by the use of mini-implants. Many animal experiments have concluded that mini-implants are highly stable after placement and can be used as an effective orthodontic anchorage, but mini-implants are occasionally removed because they become mobile during treatment. Many researchers have investigated the risk factors for mini-implant failure in an attempt to improve the success rate. They reported that the initial stability of a mini-implant is related to its diameter, length, and design, as well as the bone quality and quantity. Motoyoshi et al. evaluated the initial stability of mini-implants by measuring the placement torque when tightening, and found that the placement torque recommended for successful implantation was 5–10 N cm. In recent years, self-drilling mini-implants that are inserted into the bone without a pilot hole have often been used as orthodontic anchorage. Self-drilling mini-implants have many advantages, such as simple placement, decreased discomfort after implantation, and reduced treatment time. It is thought that the placement torque of self-drilling mini-implants can easily become excessive in the thick, mandibular cortical bone, which can cause the mini-implant to loosen and fracture. Therefore, it is important to determine the placement technique that provides adequate placement torque without over-torque. In the present study, self-drilling mini-implants were fixed by self-drilling or pre-drilling into the rib bones of pigs, representing the maxilla and mandible of humans. The authors measured the peak mini-implant placement torque value and histologically examined the mini-implant surrounding the cortical bone to identify the placement technique with adequate placement torque.
Materials and methods
Self-drilling mini-implants (Biodent Co., Ltd., Tokyo, Japan), 1.6 mm in diameter (spearhead 1.3 mm) and 8.0 mm long ( Fig. 1 ), were used. The mini-implants were placed into pig rib bones, which were stripped and cut into sections 1.0 cm in length. The cortical bone thickness of the specimens was measured using digital vernier callipers and classified into thin (1.2 ± 0.02 mm) or thick (2.0 ± 0.03 mm). Bones in the thin group served as a model of the human maxillary bone and those in the thick group as a model of the human mandibular bone. Specimens were kept moist by soaking them in physiological saline. The bone surface was fixed with mini-vise (Sakai Machine Tool Co., Ltd., Osaka, Japan) so that it was parallel to the floor.
The operator inserted the mini-implants using a protractor to confirm placement perpendicular to the bone surface ( Fig. 2 ). Two manners of placement were used: self-drilling and pre-drilling. In the self-drilling group, the mini-implants were inserted perpendicular to the bone surface without a pilot hole to a depth of 8.0 mm using a micro motor (Implanter, Dentsply, Tochigi, Japan) set at 50 rpm. In the pre-drilling group, a pilot hole was drilled with a bone twist drill (Fujikoshi Co., Ltd., Toyama, Japan) perpendicular to the bone surface while flushing with saline. The mini-implants were inserted into the pilot holes to a depth of 8.0 mm using an implanter at 50 rpm. Drills with a length of 8.0 mm and diameters of 1.0 and 1.3 mm were used to make the pilot holes in the maxillary model. Drills with a length of 8.0 mm and diameters of 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, and 1.3 mm were used to make the pilot holes in the mandibular model, and since over-torquing and cracks are more likely in the stiff mandibular bone, the torque and the state of the bone surrounding the mini-implants were examined in greater detail.
The peak mini-implant placement torque value during final tightening of the mini-implant, up to 8.0 mm in depth, was measured using a digital torque tester (DIS-RL05, Sugisaki Keiki Co. Ltd., Ibaraki, Japan). To improve measurement precision, the digital torque tester was fixed with a fixation device ( Fig. 3 ). One operator performed all drilling procedures and measured the torque of the mini-implants to eliminate operator bias. Each torque measurement was repeated 10 times and the average was taken.
After measuring the torque, the mini-implant was removed from the specimen. The surface of the specimen was ground with waterproof grinding paper (GRID1000, Riken Corundum Co. Ltd., Saitama, Japan). Each specimen was washed thoroughly in physiological saline and cleaned ultrasonically. After 24 h of natural drying, the surface of the specimen was observed using field-emission scanning electron microscopy (Miniscope TM-1000, Hitachi Science Systems, Ibaraki, Japan). The Scheffe test was used to compare groups with the same levels of thickness. A t test was used to evaluate the impact of the cortical bone thickness. All statistical analyses were carried out using the SPSS software (SPSS Japan, Inc., Tokyo, Japan).
For bones with a thickness of 1.2 mm, the average placement torques in both groups (self-drilling and pre-drilling) with respect to the differing pilot hole diameters (1.0 and 1.3 mm) were 8.1, 7.1 and 4.2 N cm, respectively ( Fig. 4 ). The torque of the pre-drilling group that used a 1.3 mm diameter pilot hole was significantly smaller than that of the self-drilling group ( P < 0.05).
For bones with a thickness of 2.0 mm, the average placement torques in both groups (self-drilling and pre-drilling) with respect to the differing pilot hole diameters (0.7, 0.8, 0.9, 1.0, 1.1, 1.2 and 1.3 mm) were 13.2, 16.1, 17.7, 16.4, 16.3, 12.0, 11.6 and 6.4 N cm, respectively ( Fig. 5 ). The torque of the self-drilling and pre-drilling groups, other than those with a 1.3 mm diameter pilot hole, exceeded 10 N cm. The torque of the pre-drilling group with a 1.3 mm pilot hole was significantly less than that of both the self-drilling group and the other pre-drilling groups. The torques of the pre-drilling groups with pilot holes of 1.1 and diameters of 1.2 mm were significantly less than those of the other drill sizes ( P < 0.05).