Abstract:

BACKGROUND: Implanted site and direction of miniscrew implants, post-implantation movement of implants, contact between tooth and implant after tooth movement all can cause root damage.
OBJECTIVE: To investigate the changes of the root surface, periodontal tissue and osteoprotegerin expression in the periodontal tissue at periods of time after miniscrew implant loading.
METHODS: Two beagle dogs were selected. For each dog, miniscrews were implanted into the labial alveolar bone near the roots of the maxillary second, third, and fourth premolars and the maxillary second, third, and fourth premolars and first molar. Fourteen miniscrew implants were in one dog, including six in the maxillary and eight in the mandible; two miniscrews of the maxillary acted as control group with no loading, and the rest miniscrews were as experimental group under orthodontics force. Two weeks after miniscrew implantation, 150 g force was loaded between two miniscrews with nickel-titanium closed-coil springs. The dogs were sacrificed after miniscrews loading 4 and 8 weeks respectively, the teeth were cut completely with alveolar bone tissue and decalcified to make the histological specimen. The histological changes of the root and periodontal tissue were observed by hematoxylin-eosin staining and expression of osteoprotegrin in the periodontal ligament was also inspected by using immunohistochemistry. 
RESULTS AND CONCLUSION: When the miniscrews were adjacent to the roots, absorption lacuna occurred in the alveolar bone; after horizontal force was loaded opposite to the roots, alveolar resorption became much more animate. When the miniscrews contacted with the pericementum, some areas of the cement absorbed seriously and even reached the dentin; when sustaining pressure from the miniscrews, the whole layer of the cement absorbed and the dentin was exposed and began to be absorbed. The osteoprotegrin in the periodontal ligament of control group expressed sharply and its expression was notable at 8 weeks; while the osteoprotegrin of experimental group weakly expressed at 8 weeks, and there was significant divergence between control group and experimental group at 8 weeks (P < 0.01). The roots and periodontal tissue were all damaged variously after miniscrew implantation adjacent to the roots. When horizontal orthodontic force was loaded opposite to the roots, the expression of osteoprotegrin was influenced slightly; as the loading period prolonged, pressure dramatically restrained the expression of osteoprotegrin. When discovering the miniscrew implant adjacent to roors, the miniscrew implant anchorage should not be used to load orthodontical force opposite to the roots but ought to be removed in order to leave the root repair autonomously and avoid further damage.

Key words: tooth, tooth root, orthodontic anchorage procedures