| ObjectiveSince Kawata( 1977 ) reported the research of PtCo magnet on orthodontics, magnetc force research on orthodontics become a hot topic in recent twenty years. As a new orthodontic force , magnetic force has the quality of distinctive direction. Magnetic field can promote the rebuild of periodontal tissues and accelarate tooth moving. But previous magnetic force wasnt large enough in long distance , so most researchers focus on the reaction of tissue in magnetic field, biologic effect and inventing removable magnetic appliance, there's no fixed magnetic appliance reported.The N48 NdFeB is invented in few years,its property is the highest in all NdFeB magnets, if we make it into magnetic bracket, it may become effective force instead of spring and elastics to accelarate tooth moving, reduce the appliance in orthodontic treatment and decrease the cost of orthodontic treatment . but NdFeB is likely to be errosive, it cant be used in oral for a long time, further more, it is unknown whether the errosive product is harmful to patients health.To test the magnetic and mechanical property of the N48 NdFeB magnets which is similar to brackets in shape and size, to evaluate the biocompatibility of NdFeB magnets with different erosion - risistance coating, in order to evaluate the possibility of using NdFeB magnets on fixed magnetic appliance in clinic.Methods1. Quality and Magnetic force testingRaw materials of N48 NdFeB were cut into 2×3 ×4mm3 by digital wire e-lectric discharge machine. Residual flux density( Br),coercive force( Hc),maximal magnetic energy product BH(max) were tested. Curves of demagnetization and magnetic energy product were recorded . Changes of attracting force and repulsive force related to the air gaps between pair of magnets were tested with u-niversal fatigue test system in varied among the range of 0 - 10mm. Curves of magnetic force related to air gaps between magnets were recorded.2. Integrated force testing1/8,3/16,1/4,3/8 inch elastics(Ormco 3.5 ounce) were placed around each pairs of magnetized magnets. The main features of integrated force and e-lastic force were compared.3 . The evaluation of biocompatibilitySteep samples of Bra,Ni,Ti,Ag,Cu and M into DMEM for 5 days to make testing infusions. Gingival blast cells of rat cultivated with the infusions in 96 -wellflat -bottomed plates, detecting the reduced AlamarBlueTM. Gingival cells blast cells of rat were cultivated with 10% infusions in a 6 - well flat - bottomed plate, observed by scan electrion telescope。 Cells of rat were cultivated with 5% and 10% infusions, analysis the expression of mTOR and P70s6k by Western blotting .Results1. magnetic quality and magnetic forceThe residual flux density (Br)of N48 NdFeB magnets was 13. 73kGs, its coercive force ( Hc ) was 13. 00kOe, maximal magnetic energy product [BH(max))]was 46. 13MGsOe。 Its quality was outstanding than previous magnet (PtCo,SmCo,AlNiCo). The surface flux density was 3380Gs(3. 38T) 。 The flux intensity decreased to 100Gs in 7mm from the surface of magnets. The attractive forces were 456 -40g when the air gaps were 0 -5mm. The repulsive forces were 288 -36g when the air gaps were 0 -5mm.2. Feature of integrated forceThe integrated force were relative stable and the value were relative invaria-bleness compared with magnetic force and elastic force. This was the result ofmagnetic force plus elastic force, Integrated force with 1/8,3/16, 1/4,3/8 inch elastic force produced 192,163,138,86g forces respectively within invariable range. Statistics analysis showed that there were significant difference between integrated forces and elastic forces at 3 ,6,9mm extension.3. Influence of diferrent coating magnets to cells biocompatibility3.1 Influence of diferrent coating magnets to cell activityThe percentage of reduced AlamarBlue identify the activity of the cell. The percentage of reduced AlamarBlue in 5% brackets, titanium - nitride magnets groups were 74.56 ± 1.60%,76.54 ±3.82% ,it was 79.16 ±3.17% in control group(P> 0.05) ; The percentage of reduced AlamarBlue in 10% bracket, titanium - nitride magnet groups were 76. 00 ± 2. 17% ,,72. 86 ± 2. 47% , it was 77.76 ± 2. 76% in control group (P > 0. 05). The percentage of reduced AlamarBlue in other groups had significant difference with control groups ( P < 0.05).3. 2 Observation by scanning electron telescope in low and high magnificationThe cells of bracket and titanium - nitride magnet groups had well surface and integrity membrane, high magnification scanning showed more and longer microvillus on the surface and fringe of the cells. The microvillus on the surface and fringe of of Cu and Ni group cells were relative less and short. The cell surface of bare magnet group had coarse and incontinuity membranein in low magnification scanning, high magnification scanning showed there are only a few short microvillus on the surface and fringe of cell.3.3 Average gray value of mTOR and P70s6k expression in the cells cultured diferrent coating magnetsIn 5%solution cultured cell,the expression of mTOR in bracket group was77.1 ±2.79 ,and 77. 2 ±2. 67 in titanium - nitride group , there were no significant difference (P >0.05) , but there were significant differences compared bracket group with nickle group(55.6 ±2.34) , copper group( 18.4 ±2.15) , bared magnet group(53. 5 ± 2. 26) respectively ( P <0. 01). In 10% solution cultured cell,the expression of mTOR in bracket group was 77. 0 ±2. 85 ,and81.2 ±2.60 in titanium - nitride group , there were no significant difference (P> 0.05) , but there were significant differences compared bracket group to nick-le group(40. 5 ± 1. 94) , copper group(15. 0 ± 2. 01) , bared magnet group (24.7 ±2.10) respectively (P<0.01).In 5%solution cultured cell,the expression of P70s6k in bracket group was 83.2 ±4.16,and 85.3 ±2.34 in titanium - nitride group , there were no significant difference ( P > 0. 05 ) , but there were significant differences compared bracket group to nickle group (62. 4 ± 2. 73 ) , copper group (33. 7 ±2.61), bared magnet group(48. 9 ± 3. 27 ) respectively (P < 0. 01). In 10% solution cultured cell,the expression of P70s6k in bracket group was 76. 0 ± 1. 5 ,and 75. 5 ± 2.65 in titanium - nitride group , there were no significant difference ( P>0.05) , but there were significant differences compared bracket group to nickle group(51. 4 ±2. 51) , copper group(29. 2 ±2. 13) , bared magnet group (40.4 ±1.75) respectively (P<0.01).Conclusion1. The property of N48 NdFeB was outstanding, it could be ideal orthodotic force to to replace classical force, such as spring and elastic chains, to move teeth rapidly cut down orthodontic cost.2. Combination of magnetic attractive force with elastic forces of 1/8, 3/ 16,1/4,3/8 inch elastics could reach 86 - 192g super - long working range (minimal working range was 11. 25mm, maximal working range was 17. 2 -20mm). The relative stable invariableness orthodontic force showed the potential of integrated force.3. NdFeB magnets coated with Ti have well biocompatibility. Ti coating is a good way of avoiding NdFeB magnets erosion in fixed appliance treatment. NdFeB magnets coated nickel (Ni) have a little cytotoxicity. NdFeB magnets coated silver (Ag) ,copper(Cu)and bare magnets (M )have certain toxicity .4. AlamarBlue method is a simple,rapid,exact and effective method for selecting materials with good biocompatibility.5. Scanning electric telescope is a direct and exact measure for judging material's biocompatibility.
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