Study Of Immediately Loaded Micro-implant's Anchorage Stability In Hyperthyroid Rat Model

ObjectivesTo study the anchorage ability and bone-implant contact ratio of immediately loaded micro-implants by established hyperthyroid rats models, thus providing theoretical and practical base for the design of micro-implant anchorage in clinical orthodontic.Material and methods20 healthy female adult Wistar rats, weighting 200±8 g, were divided into hyperthyroid group and control group randomly. Levothyroxine sodium tablets were administered orally 60μg/(100g·d) to hyperthyroid group and normal saline 1ml/(100g·d) to control group. After 28 days, blood was taken from the rats'angular vein. Measured the serum T3, T4 level in order to make sure whether the hyperthyroid rats model was established. After the model was established, 2 implants were placed into the left tibia of each rat. Then, with the help of X-ray, observed the implants'positions in tibia and the height of bone margin, checked the implants'mobility degree, measured the distance between the implants, and gave the implants a constant load of 100g with Ti-Ni coil springs immediately. Before the rats were sacrificed, labelled the bone formation with tetracycline hydrochloride on the 14th and the 13th days and labelled the bone formation with calcein on the 4th and the 3rd days. Sacrificed the rats after the implants were given constant load for 30 days, collected their alveolar bones and bilateral tibias. Maked a correlation analysis after the bone mineral density (BMD) in alveolar bones and right tibias was determined. For the left tibia with implants, after an X-ray measurement, observed the implants'position in tibia and the bone changes around the implants, examined the implants'mobility degree, measured the distance between the implants. Then prepared undecalcified grinding slices and conducted fluorescence observation. After that, calculated the bone-implant contact rateo after toluidine blue staining. Analyzed the experimental data statistically.Results1 General condition: The symptom expression of hyperthyroid rats accorded with the clinical traits of hyperthyroidism. There was no loosening and shedding in micro-implant anchorage in the two groups and no infection was detected in tissues around the micro-implants. After the implant surgery, X-ray film suggested that the implants'position in tibia was favorable and the two implants nearly paralleled to each other. At the end of implant loading, X-ray suggested no spot around the implants and no obvious change in the implants'height in tibias.2 Model establishment: 28days after ig, the serum T3, T4 level in hyperthyroid group and control group showed statistical difference (P<0.01). Compared with rats in control group, hyperthyroid rats'serum T3, T4 level increased significantly that the rats were in hyperthyroid state, which suggested the successful establishment of the model.3 Bone mineral density: The alveolar bones and tibias density correlation coefficient in hyperthyroid rats r=0.879 (P<0.01), showed a significant positive correlation. The alveolar bones and tibias density correlation coefficient in control group r=0.915 (P<0.01), showed a significant positive correlation.4 Moving distance measurement: The distance between micro-implant anchorage at different stages showed statistical difference (P<0.01). Micro-implant anchorage displacement occurred in both groups. Micro-implant anchorage displacement in hyperthyroid group was bigger than that in control group (P<0.05).5 Fluorescence observation: Obvious fluorescence double markers could be seen in control group, and fluorescence concentrated on the surface of bone trabecula and bone-implant interface. Compared with control group, hyperthyroid group showed a stronger fluorescence marker, and distance of double label (DLL) widened significantly (P<0.01). Markers concentrated mainly on the surface of bone trabecula and bone-implant interface and part of the bone trabecula surface showed lacunas. The mineral appositional rate (MAR) in hyperthyroid group was higher than that of control group (P<0.01).6 Bone-implant contact ratio (BIC) calculation: BIC of hyperthyroid group was higher than that of the control group(P<0.01).Conclusions1 Findings in hyperthyroid group and control group were consistent with each other. Micro-implants showed stability, no loosening or abscission, and better biocompatibility indicated the feasibility of immediate loading of micro-implant anchorage when it was in hyperthyroid.2 Neither of the micro-implants in hyperthyroid group and control group was absolute anchorages. Displacement in all micro-implants occurred after loading. Micro-implant anchorages in hyperthyroid group and control group varied in strength. As far as displacement was concerned, when it was in hyperthyroid, the displacement of micro-implant enlarged.3 BIC of hyperthyroid group was higher than that of the control group. Thyroid hormone could increas the mass of bone around implant in the early implantation.