The Experiment On The Stability Of Mini-screws Under Different Orthodontic Force And Different Loading Time

Objective:The purpose of this study was to investigate the stability of mini-screws of under different orthodontic force and different loading time, and provide evidence for clinical application of orthodontic mini-screws.Method:For 3 adult dogs,60 titanium mini-screws were installed on each side of maxilla and mandible. Then 3-dimentional CT was taken for each dog to examine the places of the mini-screws. The mini-screws were loaded by using NiTi-coil springs with 200g or 350g constant force immediately, and at week 4 respectively. Control groups were unloaded. At week 12, the displacement of the mini-screws of experimental group to control group were measured respectively (beginning and ending) and other items were also measured.Result:1. All the fifty-seven mini-screws in this study remained stable without clinically detectable mobility or loosening except for 1 missing and 2 loosening; The gingival around the mini-screws was observed to be not detectable infection, one or two were hyperplasia and there was ulcer under some nickel-titanium coil spring.2. The displacement of immediately loading and loaded 200g is 0.29±0.04mm, and that of immediately loading and loaded 350g is 0.37±0.06mm; The displacement of 4 week loading and loaded 200g is 0.21±0.03mm, and that of 4 week loading and loaded 350g is 0.28±0.05mm. The displacement of immediately loading is more than that of 4 week loading. There was significant difference in displacement of mini-screws between immediately loading and 4 weeks later loading group (P<0.05)3. The displacement of immediately loading and loaded 200g is 0.29±0.04mm, and that of 4 week loading and loaded 200g is 0.21±0.03mm; The displacement of immediately loading and loaded 350g is 0.37±0.06mm, and that of 4 week loading and loaded 350g is 0.28±0.05mm. The displacement of loaded 350g is more than that of loaded 200g. There was significant difference in displacement of mini-screws between loaded 200g and loaded 350g group (P<0.05)Conclusion:1.Most of mini-crews can remained stable except for one or two missing or loosening according to the orthodontic loading in force 200g and 350g suggested that the mini-screws is an perfect anchorage for clinical application.2.mini-screws are a stable anchorage but do not remain absolutely stationary like osseointegration. They might move according to the orthodontic loading.To prevent mini-screws hitting any vital organs because of displacement, it is recommended that they be placed in a non-tooth-bearing area that has no major nerves or blood vessel pathways or in a tooth-bearing area allowing 2 mm of safety clearance between the mini-screws and dental root.3. The displacement of immediately loading is more than that of 4 week loading. It is better for force exertion after 4 weeks for less displacement of mini-screws.4. CT is a rapid, accurate but uneconomic method for the accurate placement of mini-screws.