| Purpose:When the upper anterior teeth are retracted,the crown of the upper anterior teeth tend to lingually incline and the roots tend to incline labially.In order to avoid the anterior teeth being too upright or lingually inclined,the method of arch wire with positive torque is often adopted.In this study,three-dimensional finite element models of complete maxillary dentition,periodontal ligament,alveolar bone,traction hook,bracket,straight wire plus positive torque and T-loop arch wire with positive torque were established to stimulate the sliding and closure curves commonly used in clinical practice,and to analyze the initial displacement of upper anterior teeth and the stress distribution of periodontal ligament when two closure methods applied different numerical positive torque to upper anterior teeth.We attempt to provide a theoretical basis and reference for choosing the method of closing the gap and controlling the positive torque of the upper anterior teethMaterials and Methods:In experiment 1,high resolution CT images of maxilla and upper dentition of volunteers were obtained by cone beam CT scanning.A series of computer-aided design software and finite element analysis software were used to establish the the model of three-dimensional finite element study of maxillary complete dentition,periodontal ligament,alveolar bone,traction hook,bracket,straight wire sliding closure and T-loop curve closure.In experiment 2,two kinds of closure methods were divided into sliding closure group(group A)and closing T-loop group(group B).The effective torque of the upper anterior teeth of the two groups was set to 0,1,3,5 and 7,corresponding to group A,group A0,group A1,group A2,group A3 and group A4;group B was divided into group B0,group B1,group B2,group B3 and group B4,a total of 10 working conditions were set.The displacement trend of central incisor and lateral incisor in three-dimensional direction and the equivalent stress of periodontal ligament were studied by finite element analysis software under each working condition.Two kinds of extraction space closure methods were applied to apply the same torque to the upper anterior teeth,then we analyse the initial displacement of the central incisor and the lateral incisor and the change of periodontal membrane stress.When different torques are applied by the same kind of closure method,then we analyse the initial displacement of the central incisor and the lateral incisor and the change of periodontal membrane stress.The study tried to is explorethe optimal torque force value suitable for the upper anterior teeth torque control,which help us select the appropriate torque control to provide a reference.Results:1.A three-dimensional finite element model including complete maxillary dentition,periodontal ligament,alveolar bone,traction hook,bracket,square wire and T-loop closed curved arch wire was established.2.Group A applied 150g tension,as sagittal changes,central incisor A0,A1 group showed crown lingual movement and root labial movement.Compared with the AO group,Al group showed less lingual movement.When loading +3° torque(A2 Group),the initial displacement of the central incisor changes from crown lingual movement and root labial movement to bodily movement.The middle incisor showed bodily movement.As the torque force increases,the incisors in the A3 and A4 groups showed crown labial movement and root lingual movement,the labial tendency of the A4 group increased.The displacement of the five groups' crown was greater than the displacement of the root,and the displacement of the A4 group was significantly larger than the other four groups.The displacement was compared as follows A4>A3>A0>A1>A2.The displacement trend of the lateral incisors A0,A1,A3,and A4 was basically the same as that of the central incisors.The A2 group showed the tendency of crown labial and root lingual movement.The displacements of the five groups were increased compared with the central incisors.The displacement difference between the central incisor and the lateral incisor crown is the smallest when loading +3° torque(A2 group),and the maximum when loading +7° torque(A4 group).As for vertically changes,the central incisor AO group showed elongation movement,so does the A1 group,the two groups as a whole showed elongation movement,the elongation trend decreased as the torque increased.The root and the crown of the A2,A3,A4 group is oriented to the root,and the tooth gradually moves from the extension to the depression.The displacement of the crown is A4>A3>A2,and the displacement of the root is A2>A3>A4.The displacement trend of the lateral incisors is basically the same as that of the central incisors.The difference is that in the A1 group,the initial displacement of the crown roots is lingual movement,and the displacement of the five groups is larger than that of the central incisors.In the horizontal direction,the central incisors in A0,Al,A2,and A3 all showed the proximal movement of the root and the distal movement of the crown.When the torque continues to increase to +7°(group A4),A4 showed the distal movement of the root and the proximal movement of the crown;The displacement trend of the lateral incisor is basically the same as that of the central incisor.When the torque continues to increase to +5°(group A3),the lateral incisor showed the distal movement of the root and the proximal movement of the crow,and the displacements of the five groups are increased compared with the central incisors.3.The T-loop on both sides of the B group is opened 1mm,the crown of central incisor B0,B1,B2 showed crown lingual movement and root labial movement.The displacement of the crown is greater than the displacement of the root.The displacements were compared as follows B0>B1>B2,the inclination trend of lingual movement is decreasing.When the torque force value is increased to +5°,the roots of the B3 group showed the lingual movement,the teeth tend to move as a whole.The B4 group crown showed crown labial movement and root lingual movementand the teeth show a tendency of labial movement.The displacement is B4>B0>B1>B2>B3.The displacement trend of the lateral incisors is basically the same as that of the central incisors.Only the lateral incisors of the B3 group showed crown labial movement and root lingual movement.The displacement of lateral incisorsis significantly larger than the central incisor.The displacement difference between the central incisor and the lateral incisor crown is the smallest when loading +5° torque(B3 group),and the maximum when loading +7° torque(B4 group).As for vertical changes,the initial displacement of the roots of the BO and B1 teeth of the middle incisors is negative,the teeth move in the direction of the crown.The displacement B0>B1,and the elongation trend decreases.When the torque increases to +3°(B2 group),the initial displacement of the roots and the crown was positive,the teeth showed intrusion,and the displacement trends of the B2,B3 and B4 groups were consistent.The displacement B4>B3>B2,and the trend of intrusion was more and more obvious.The teeth of the lateral incisors BO showed an elongation trend,and the other four groups showed a trend of intrusion.The displacement of the five groups was larger than that of the central incisors.In the horizontal direction,the middle group of the incisors showed the proximal movement of the root and the distal movement of the crown,and the displacement of the crown was greater than the displacement of the root.The displacement decreased with the increase of the torque force(B0>B1>B2>B3>B4),and the tendency of inclining distally is gradually reduced.The displacement of the lateral incisors is consistent with that of the central incisors,but the displacement is larger than that of the central incisors.4.When the sliding method is loaded with +3° torque and the closing method is applied +5° torque,the tensile stress and compressive stress of the periodontal ligament of the central incisor are minimal,and the equivalent stress of the periodontal ligament is in the lingual side of the middle incisor with average distribution.Conclusion:1.When the extraction space is closed by applying 150g pulling force on both sides of 0.018 inch × 0.025 inch stainless steel square wire,the effective torque of +3° applied to the upper front teeth is most beneficial to the bodily movement of the upper central incisor.The torque is slightly less than +3°,which is effective for the bodily movement of the lateral incisors.2.When the T-loop closure method is opened 1mm on both sides of the 0.018 inch ×0.025 stainless steel square wire,the application of +5° effective torque on the upper front teeth is most beneficial to the bodily movement of the upper central incisor.The effective torque is slightly less than+5°,which is suitable for the overall movement of the lateral incisors.3.The two closure method will produce intrusion and crown proximally movement after applying the positive torque on the upper anterior teeth.4.The lateral incisors are not completely synchronized with the movement of the central incisors,and the lateral incisors are more sensitive to torque.5,Two closure method loading the same torque on the upper front teeth will produce different movement trends.6.The same kind of closure method in the upper front teeth loading different torque will produce different movement trends. |
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