| Summary: | 碩士 === 國立臺灣大學 === 臨床牙醫學研究所 === 93 === Abstract
With more psychosocial and facial concerns, there is an increasing demand for orthodontic treatment to improve facial esthetics, speech, and chewing function. The success of orthodontic treatments relies on the precision of tooth movement with proper anchorage control. Recently, implant anchors such as miniscrews and miniplates have been used for absolute anchorage during orthodontic treatment. However, the potential problem of damaging the adjacent roots while mini-implants placement is a major concern. Therefore, we use animal experiments simulating orthodontic tooth movement to evaluate the risks of placing the miniscrews.
Eighty-four miniscrews were surgically placed in maxillary and mandibular alveolar bone at each side of 6 adult beagle dogs with metabolic bone labeling at three-week intervals. In experiment 1, the miniscrews were placed, retained for different time points and then removed in mandibular area with or without hitting the adjacent teeth. In experiment 2, a NiTi coil for orthodontic force (150g) was used to pull the maxillary fourth premolar (PM4) and the canine towards each other for 6 months after a miniscrew was placed in the furcation of PM3 with or without intentional root damaging. The insertion torque, clinical measurements every three weeks, the removal torque, and the histological findings were analyzed.
In the animal study, we demonstrated that (1) the values of insertion torque were significantly higher in the mandible than those in the maxilla. The miniscrews contacting with the roots showed a significantly higher insertion torque than those without contact. (2) For removal torque: there was a significant difference of the removal torque measurements for the following variables: the mobility of miniscrew, the state of root contact, and the duration of miniscrew in place (3 weeks vs. 24 weeks). (3) The miniscrews contacting with the root was at greater risks for failures. (4) From histological findings: the failed miniscrews were surrounded with more soft tissue with inflammation. When more inflammation was present, it seems that the adjacent roots experienced more surface resorption. However, some of the inflammation induced root resorption could be repaired with surface deposition of mineralized tissue, likely the cellular cementum, or being partially filled with alveolar bone, both labeled with fluorophore markers.
The other purpose of this study was to measure the removal torque of the immediately loaded miniscrews in orthodontic patients after clinical usage and to find the possible factors associated with this value. From twenty patients with malocclusions, 35 miniscrews were removed and the removal torque was measured with a torque gauge. Removal torque values were subjected to statistical analysis for possible association of different clinical characteristics. The removal-torque value of 75% miniscrews was above 0.5kgcm. These values were significantly higher in the mandible than those in the maxilla. However, we could not detect a significant association between the removal-torque value and the following variables: age, gender, screw length, healing time and time in function. For uprighting tipped molars using anchorage from miniscrew, excessive torque in the counterclockwise direction may loosen it. From our measurement, the miniscrews should be able to sustain an uprighting moment in order for a more precise tooth movement.
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