Histological evaluation of the effects of initially light and gradually increasing force on orthodontic tooth movement

OBJECTIVE: To investigate histologically the effect of initially light and gradually increasing force on tooth movement in the rat. MATERIALS AND METHODS: Cuboids made of neodymium-iron-boron magnets (experimental groups) or titanium (control group) were bonded to the lingual surface of the right and left maxillary first molars of 18-week-old male Wistar rats. The initial distances between materials were 1.0 mm generating 4.96 gf (experimental group 1) and 1.5 mm generating 2.26 gf (experimental group 2). In three groups, rats were killed 1, 3, 7, 10, or 14 days after treatment. Histological sections were prepared and stained with hematoxylin and eosin or for tartrate-resistant acid phosphatase (TRAP) activity. The number of TRAP-positive osteoclasts was counted, and the relative hyalinized area was measured on the pressure side of periodontal ligament. RESULTS: There were significant differences in the number of osteoclasts among the three groups (P < .05). On days 1 and 3, the numbers of osteoclasts in experimental group 2 were greater than in experimental group 1. There were significant differences in the relative hyalinized area between the control group and experimental group 1 (P < .01) and between experimental groups 1 and 2 (P < .01). On days 1 and 3, the hyalinized area in experimental group 1 was larger than in experimental group 2. CONCLUSION: Initially light and gradually increasing force induced tooth movement without the lag phase and showed smooth recruitment of osteoclasts and inhibition of hyalinization.     

Rapid Palatal Expansion in Mixed Dentition Using a Modified Expander: a Cephalometric Investigation

Abstract

Aim: To investigate light forces for experimental tooth movement.

Method: Light orthodontic forces of 1.2, 3.6, 6.5, and 10 g force (gf) were applied for 14 days to move rat molars, and the effects of the forces on the rate of tooth movement and changes of the periodontium were examined.

Results: In the early period, despite the different levels of force used in each group, there were no significant differences in tooth displacement. From hour 56 to day 14, the tooth displacement in the 1.2 gf group was significantly smaller than that in the other groups and the rate was nearly constant. The rates of tooth displacement in the 3.6, 6.5, and 10 gf groups fluctuated repeatedly, while the orthodontic forces gradually decreased.

Conclusion: Experimental tooth movement in rats, tipping without friction under light forces, were either constant or fluctuated in cycles of several days' duration. This is in contradiction to the three-phases-theory of tooth movement described in previous investigations using heavy forces.     

Studies on reduction of repellent force of rare earth magnets--concerning tooth intrusion]

The purpose of this investigation was to evaluate the sealing effect of the repelling force of the magnets with ferromagnetic stainless steel and also to examine the reduction pattern along with the change of the relative position of the magnets. The Nd-Fe-B magnet as rare earth magnet, and SUSXM 27, YEP-3, SUS 416 as ferromagnetic stainless steel were used in this experiment. The findings were as follows: 1. There was a little decrease of the repelling force of the magnets sealed with ferromagnetic stainless steel. On the other hand, no significant differences in the repelling force sealed with any kind of ferromagnetic stainless steel were found. 2. Direct contact of the repelling force of the phi 4.0 x 1.5 mm magnets sealed with SUSXM 27 of 0.2 mm in thickness was 242 gf. According to relative horizontal 1.2 mm movement keeping direct contact, the vertical and horizontal components of the repelling force were of the same value. 3. The repelling force of the phi 10.0 x 1.8 mm magnets sealed with SUSXM 27 of 0.2 mm in thickness was 815 gf. It showed more than 300 gf of vertical component of the repelling force when the magnets shifted to 3.0 mm horizontally when in contact. 4. It is suggested that the repelling force of the Nd-Fe-B magnets will be clinically useful for the intrusion of molar teeth.     

Biomechanical study on orthodontic tooth movement: changes in biomechanical property of the periodontal tissue in terms of tooth mobility

The magnitude of tooth mobility has been frequently used for evaluating biomechanical response of the periodontal tissue to applied forces. However, tooth mobility during orthodontic tooth movement has not been measured. The purpose of this study was to investigate changes in biomechanical property of the periodontal tissue during canine retraction, in terms of tooth mobility. The upper canines on both sides of ten orthodontic patients were moved in the distal direction for about four weeks with an initial force of 200 gf. An amount of tooth movement and a magnitude of tooth mobility were measured every 3 or 4 days during retraction. A distally directed force up to 500 gf was continuously applied to each canine and tooth mobility was measured with a noncontact type of eddy current displacement sensor. A two-dimensional finite element model was constructed and displacements of the finite element model were calculated with various Young's moduli in loading with a 100 gf force in the distal direction. In comparison with the magnitudes of the tooth mobility, Young's modulus of the periodontal membrane before retraction and the influence of the biomechanical factors on changes in tooth mobility were investigated. The tooth movement curve was divided into three phases; an initial phase, a lag phase and a post-lag phase. The magnitudes of tooth mobility at the initial phase were significantly larger than those before retraction within the range of 250 gf to 500 gf and these magnitudes decreased during the lag phase. The magnitudes of tooth mobility at the post-lag phase significantly increased, within the range of 50 gf to 500 gf, than those before retraction. As a result of curveliniar regression analysis, the tooth mobility curves approximated to delta = AFB, where delta and F denote tooth mobility and force respectively. The coefficients A and B changed according to the phases of tooth movement. An inclination of the tooth mobility curve expressed by a tangent at the 400 gf force was the largest at the initial phase, and this inclination at the 100 gf force was the largest at the post-lag phase. Young's modulus of the periodontal membrane before retraction was determined to be approximately 35 gf/mm2 and Young's modulus of the periodontal membrane was the most important factor on the increase of tooth mobility. Tooth mobility significantly varied associated with tooth movement. It was indicated that biomechanical property of the periodontal tissue changes in response to each phase of tooth movement. In particular, Young's modulus of the periodontal membrane decreased at the post-lag phase of the orthodontic tooth movement.     

Long-term orthodontic tooth movement response to short-term force in the rat.

Both the amount of force applied and the duration of the application affect tooth movement. To study the effect of duration, adult male Sprague-Dawley rats were fitted with orthodontic appliances delivering a 40-gm initial mesial tipping force to the maxillary molars. The animals were divided into two longitudinal groups (I: 1 hour and II: 24 hours; N = 15). Sham-treated control (III) and 14 day (IV) continuous cross-sectional force tooth movement data were also included for comparison (72 rats per group). Extraoral cephalometric radiographs were obtained at appliance placement and at 1, 3, 5, 7, 10, & 14 days. Tooth movement was determined with respect to palatal implants. ANOVA indicated significant differences existed over time in each group (p = 0.0001). Continuous force applied for 14 days generated a classic three-part tooth movement curve. Short-term forces were characterized by initial mesial movement, a distal relapse period (d3-d5), and a late mesial movement period (d7-d14). The results suggest short-term forces of 1 and 24 hours initiate remodeling events, which result in tooth movement 7 to 14 days later and that the minimum effective duration of a 40-gm activation is less than 1 hour in this animal model.     

The effects of pregnancy and fluoride on orthodontic tooth movements in rats.

The present investigation was undertaken in order to study the velocity of orthodontic tooth movement in rats and the effect of the hormonal changes that occur during pregnancy or with fluoride, which is one of several trace elements that affect hard tissue metabolism. Adult, female Sprague-Dawley rats were separated into three groups: non-pregnant, pregnant, and non-pregnant NaF-supplied. All rats were treated with a fixed orthodontic appliance which moved the upper first molars in a buccal direction during 21 days. The appliances delivered an initial force of 150 mN. Repeated intra-oral, standardized radiographs were taken during the experimental period, and at the end of the experiment the maxillae were examined histologically. The velocity of tooth movement was calculated after measurement of the radiographs. The first molars were moved in a buccal direction in all groups. The mean value of the expansion from day 0 to 21 was significantly higher among the pregnant rats (0.64 mm) compared to the non-pregnant, control animals (0.46 mm) while the NaF-supplied rats had a significantly lower expansion (0.22 mm) compared to the control animals. In the histological examination of the pressure sides of the PDL of the upper first molars, the mean value of osteoclasts per microns x 10(-3) increased non-significantly in the group of pregnant rats and decreased significantly among the NaF-supplied animals compared to the non-pregnant control rats. The present experiment in rats indicated that the velocity of orthodontic tooth movement is influenced by hormones as well as trace elements.     

Intermittent 8-hour activation in orthodontic molar movement.

The duration of force application is an important factor, together with the magnitude of force, in stimulating osteoclast recruitment in the periodontal ligament. The purpose of this study was to examine the effect of duration of intermittent versus continuous force application on the amount of molar movement in rats. Seventy-six molars of 38 rats were randomly assigned to continuous, intermittent, and sham-operation groups. A calibrated nickel-titanium coil spring exerting 40 g of initial force was suspended between the maxillary first molars and the incisors for 13 days. In the intermittent group, the coil spring was suspended for 8 hours per day. Tooth movement was measured on radiographs. There was no significant difference in the initial tooth displacement between the continuous-force and intermittent-force groups. Tooth movement at day 7 was significantly (P <.05) less in the intermittent group than in the continuous group. Actual mesial tooth movement and total tooth movement in the intermittent group were 65.9% and 73.7%, respectively, of those in the continuous group. Our results suggest that the amount of tooth movement in response to intermittent force is less than that in response to continuous force, but that the amount of tooth movement achieved by intermittent force exceeds the expected value predicted by the duration (33.3%) of force application.     

Emotional stress- and pain-related behaviors evoked by experimental tooth movement

OBJECTIVE: To investigate by behavioral methods the relationship between emotional stress and pain during experimental tooth movement in rats. MATERIALS AND METHODS: Sixteen male Sprague-Dawley rats (210 to 250 g) were divided into two groups. The experimental group was treated with an active Ti-Ni appliance, and the control group received a passive appliance. A force of 20 gf was delivered by the active appliance between the maxillary first and second molars for 3 days. During this period the rat's behavior was evaluated eight times by means of open-field test and resistance-to-capture test. The specific parameters of animal activity were facial grooming, rearing, and locomotor activity, movement into the center of the open field, and response to capture. RESULTS: Parameters related to stress and pain were higher in the group carrying active appliance, compared to the group with a passive appliance. Statistically significant differences in stress-related behavior between control and experimental groups were found 8 hours after placing the appliance and were most evident on the second day. Pain-related behavior was significantly greater in the experimental group than in the control group at 24 hours. CONCLUSIONS: The increase in emotional stress evoked by orthodontic tooth movement may precede the appearance of periodontal pain.     

Clinical evaluation of the centre of resistance of the upper incisors during retraction

The aim of this study was to evaluate the movement of anterior teeth during retraction with a force applied through the assumed centre of resistance (CRe).Twenty-two subjects with a Class I or II malocclusion were included. Each subject had the two upper first premolars extracted, resulting in a symmetrical extraction space of at least 3 mm between the upper laterals and canines. The force was applied through the assumed CRe, located 9 mm gingival to the lateral tooth bracket. To examine the type of anterior tooth movement, 10 parameters were measured. A Wilcoxon test was used to determine the differences between pre- and post-retraction values, and a Mann-Whitney U-test to determine the mean differences between groups.In spite of the force application through the CRe, tipping of the anterior teeth was observed in 19 subjects and parallel movement in three patients. Consequently, the subjects were divided into two groups according to the location of the centre of rotation (CRo). In group 1 (nine cases), the CRo was located coronal to the root apex, and in group 2 (13 cases), apical to the root apex.Both groups showed a significant decrease in inclination (P < 0.01) and posterior crown movement (P < 0.01 for group 1 and P < 0.001 for group 2) of the anterior teeth. A significant posterior movement of the root apex was observed in group 2 (P < 0.001). Significant differences were found between the groups for anterior tooth inclination (P < 0.05) and root apex movement (P < 0.001). The reasons for these differences could not be conclusively determined.Even though experimental studies provide information regarding CRe location, factors such as bone support, root morphology and incisor inclination should be taken into consideration. The observation of tooth movement occurring during treatment and changes in treatment mechanics would be helpful in obtaining desired tooth movement.     

Effects of pulsed electromagnetic field vibration on tooth movement induced by magnetic and mechanical forces: a preliminary study

BACKGROUND: This study was designed to determine whether or not high-frequency and low-magnitude vibration affects orthodontic tooth movement caused by magnetic or/and mechanical forces. METHODS: Forty-four 7-week-old Wistar rats were randomly divided into four groups, with each group further divided into experimental and control subgroups. Neodymium-Iron-Boron (Nd-Fe-B) magnets and Sentalloy closed coil springs were placed between maxillary or mandibular first molars and incisors to activate tooth movement. The animals of experimental subgroups were exposed to the vibration induced by pulsed electromagnetic fields (PEMF) whilst the control subgroups were under normal atmosphere. The experiment lasted for 14 days and all of the animals were sacrificed for examination. The changes in the space between the molar and incisor were measured to indicate the amount of tooth movement. RESULTS: The coil springs, either with sham or active magnets, move molar much more than magnets alone, regardless of absence or presence of PEMF (p < 0.001). Under PEMF, the coil spring moved significantly more amount of tooth movement than that of coil-magnet combination (p < 0.01), as did the magnets compared to sham magnets (p < 0.019). Under a non-PEMF scenario, there was no significant difference in tooth movement between coil spring and coil-magnets combination, nor was there difference between magnets and sham magnets. CONCLUSIONS: It is suggested that the PEMF-induced vibration may enhance the effect of mechanical and magnetic forces on tooth movement.     

牙周膜牵张成骨快速移动牙齿的组织学观察

目的 观察牙周膜牵张成骨快速移动牙齿的牙周组织学变化。方法 将6只杂种犬随机分为2w，4w，6w三组（每组二只）。拔除两侧第二前磨牙，并随机选择其中一侧为实验侧，另一侧为常规橡皮圈加力方法对照侧。动物基牙预备：拔牙凿骨减阻；粘结自制牵张装置；实验组以2次／天的频率、0．15fnm／次的速率加力，2w后固定保持。对照组以传统橡皮链对移动牙施加100g力值，加力2w后，固定保持。测量支抗牙、移动牙移动距离：分别于第2、4、6w各宰杀二只动物，观察实验侧与对照侧的组织学变化。结果 ①实验侧移动牙加力2w平均向远中移动3．78mm，对照侧移动牙平均向远中移动1．09mm（P〈0．01），实验侧支抗牙平均向近中移动0．42mm，对照侧支抗牙平均向近中移动0．40mm（P〉0．01）。②组织学观察得出：2w组，实验侧与对照侧相比，实验侧牙周膜显著增宽，牙周膜内成纤维细胞增殖，新生板层状牙槽骨明显增多，与牵张作用力方向一致；4w组，旧间隔骨已改建完成，牙周膜宽度基本恢复正常；6如组两种方法作用下移动牙牙周组织己无明显差异。结论 牙周膜牵张成骨可以快速移动牙齿；牙周组织不会出现不可复性损害。     

Biomechanical study on orthodontic tooth movement by means of numerical simulation. Effects of principal stresses in periodontal membrane

The effect of biomechanical factors on tooth movement has not been clarified in a quantitative manner. This study was designed to investigate the stresses affecting tooth movement, using a numerical simulation. The influence of decrease in Young's modulus of the periodontal membrane on canine retraction was also examined through the simulation. A two-dimensional finite element model was constructed based on the average anatomic morphology of Japanese canine. A numerical simulation program based on the finite element method was developed for the orthodontic tooth movement. The stresses in the periodontal membrane were evaluated. The principal stress of which absolute is larger was selected as a reference stress. Each nodal point at the alveolar bone-periodontal membrane interface was repositioned in the direction of reference stresses, in response to discrepancy between assumed thresholds and the reference stresses. Moment to force (M/F) ratios at the bracket position of this model were examined for evaluating force conditions. Simulation of tooth movement were executed under three force conditions with different M/F ratios for distal movements and two force conditions for vertical movements. Three types of canine retraction, tipping movement, bodily movement and root movement, were displayed with the numerical simulation. Extrusion and intrusion were also displayed. Analytic movements of the canine were close to the actual tooth movements that have been reported, utilizing the principal stresses with the thresholds of the maximum and minimum principal stresses being about +0.4 gf/mm2 and -0.4 gf/mm2. The decrease in the Young's modulus of the periodontal membrane changed bodily movement to tipping movement under the same force conditions. These results indicate that the value and the direction of the principal stress in the periodontal membrane are key determinants of tooth movement and this numerical simulation is useful for investigating the influence of the biomechanical factors on tooth movement.     

应用正畸移动装置在牵引成骨新成骨中移动牙的动物模型的建立

目的探讨应用自制正畸移动装置在牵引成骨新成骨中移动牙的方法的可行性,建立动物模型,为临床应用提供客观理论依据。方法将六只成年（12～15月龄）雄性小尾寒羊（绵羊）的下颌两侧随机分组,随机选择一侧为实验组,另一侧为对照组。实验组按照骨成熟期分为一周组、两周组、三周组、四周组（非成熟骨组）和半年组、一年组（成熟骨组）。对照组分为常规对照和空白对照。非成熟组对侧为常规对照,成熟骨组对侧为空白对照。实验组在下颌第一双尖牙的近中牙槽嵴位置行手术骨皮质切开,放置牵引成骨器,术后5天开始牵引成骨,每12小时牵引加力一次,1．2mm／d,牵引6天,在第一双尖牙近中形成7．2mm新生骨段;分别在一周、两周、三周、四周和半年、一年后,在第一双尖牙近中约2cm位置牙槽嵴顶植入种植钉作为支抗,使用正畸轻力移动牙齿的方法应用镍钛拉簧牵引第一双尖牙向近中移动,力值约100g（0．98N）,3周后第一双尖牙移入牵引成骨骨段。常规对照组,使用同样正畸方法牵引第一双尖牙向近中移动3周。结果实验组牙齿顺利移动进入牵引成骨骨段,松动度Ⅰ度,局部牙龈红肿、轻度炎症,X线检查示无X尖周病变、无明显牙根吸收。结论使用正畸力在牵引成骨新成骨中移动牙齿的方法切实可行,本研究所建立的动物模型可以应用于后续的研究中。     

The effects of samarium-cobalt magnets and pulsed electromagnetic fields on tooth movement

The purpose of this study was to determine whether the application of either samarium cobalt magnets or pulsed electromagnetic fields could increase the rate and amount of orthodontic tooth movement observed in guinea pigs. In addition, the objective was to evaluate the effect of a magnetic field on bony physiology and metabolism and to monitor for possible systemic side effects. Fifteen grams of laterally directed orthodontic force were applied to move the maxillary central incisors of a sample of 18 young male Hartley guinea pigs divided into three groups: group 1, an orthodontic coil spring was used to move the incisors; group 2, a pair of samarium-cobalt magnets provided the tooth moving force; and group 3, a coil spring was used in combination with a pulsed electromagnetic field. The results showed that both the static magnetic field produced by the samarium-cobalt magnets and the pulsed electromagnetic field used in combination with the coil spring were successful in increasing the rate of tooth movement over that produced by the coil springs alone. The mechanism producing this effect appears to have involved a reduction in the “lag” phase often seen in orthodontic tooth movement. Both magnetically stimulated groups also showed increases in both the organization and amount of new bone deposited in the area of tension between the orthodontically moved maxillary incisors. (AM J ORTHOD DENTOFAC ORTHOP 1995;107:578-88.)     

Administration of osteocalcin accelerates orthodontic tooth movement induced by a closed coil spring in rats.

The effect of local administration of osteocalcin (OC) on experimental tooth movement was examined in the rat. The maxillary first molar was first moved mesially with an initial tipping force of 30 g with a closed-coil spring anchored to the incisor for 10 days (n = 48). Three experimental groups (n = 8) were injected with purified rat OC at doses of 0.1, 1, and 10 micrograms, respectively. The injection into the palatal bifurcation site of the first molar was repeated daily. The control groups (n = 8) were injected with rat serum albumin (10 micrograms), phosphate buffered saline (PBS), or were not injected. Tooth movement was evaluated daily by measuring the inter-cuspal distance between the first and the second molars on a precise plaster model. The cumulative tooth movement (mm) in the 1-microgram OC-injected groups was significantly more than that in all of the control groups on day 9. The rate of tooth movement (mm/day) showed periodical elevation, with high values on days 1, 4, 7, and 9. Acceleration of tooth movement by OC was significant in the early experimental period. Subsequently, acceleration of early tooth movement by OC was histologically evaluated (n = 40). Each of four animals from the control (PBS, n = 20) and the experimental (1 microgram OC, n = 20) groups was killed daily up to 5 days. A significantly larger number of osteoclasts accumulated on the mesial alveolar bone surface in the 1-microgram OC-injected group on day 3 than that observed in control group. These results suggest that administration of OC accelerates orthodontic tooth movement due to enhancement of osteoclastogenesis on the pressure side, primarily in the early experimental period.     

大鼠正畸牙移动过程中TRPV1和CGRP在三叉神经节中的表达变化

目的建立SD大鼠正畸牙移动模型,研究牙移动过程中,三叉神经节(trigeminal ganglion,TG)中瞬时受体电位香草酸亚型1（transient receptor potential vanilloid 1,TRPV1）及降钙素基因相关肽(calcitonin gene related peptide,CGRP)的表达位置及表达量的变化规律,进而探讨其在正畸疼痛中的作用机制。方法将66只SD大鼠随机分为空白对照组（6只）、假手术组（6只）和实验组（54只）。建立正畸牙移动模型,实验组大鼠施加50 g力后,分别于4、8 h、1 d（1 d组按力值大小分为1 d-30 g、1 d-50 g、1 d-80 g 3个亚组）、3、5、7、14 d,随机处死6只,切取三叉神经节,采用免疫荧光技术分别检测大鼠三叉神经节中TRPV1及CGRP的表达位置及表达量变化。采用SPSS16.0软件包对数据进行统计学分析。结果免疫荧光染色显示,TRPV1及CGRP主要表达于小型及中型神经元。随着加力时间的延长,三叉神经节中TRPV1免疫反应阳性（TRPV1-IR）神经元百分比及CGRP免疫反应阳性（CGRP-IR）神经元百分比增加,1~3 d相继达到高峰,之后逐渐降低,回落至初始水平,且两者随加力力值增大而呈现增高趋势。结论大鼠正畸牙移动过程中,三叉神经节内TRPV1及CGRP的表达随加力时间及加力力值改变呈现规律性变化,提示TRPV1及CGRP可能在正畸疼痛的发生机制中具有重要作用。     

Effects of Low-Level Laser Therapy and Orthodontic Tooth Movement on Dental Pulps in Rats

Objectives:To describe the microscopic pulpal reactions resulting from orthodontically induced tooth movement associated with low-level laser therapy (LLLT) in rats.Materials and Methods:Forty-five young male Wistar rats were randomly assigned to three groups. In group I (n = 20), the maxillary right first molars were submitted to orthodontic movement with placement of a coil spring. In group II (n = 20), the teeth were submitted to orthodontic movement plus LLLT at 4 seconds per point (buccal, palatal, and mesial) with a GaAlAs diode laser source (830 nm, 100 mW, 18 J/cm²). Group III (n = 5) served as a control (no orthodontic movement or LLLT). Groups I and II were divided into four subgroups according to the time elapsed between the start of tooth movement and sacrifice (12 hours, 24 hours, 3 days, and 7 days).Results:Up until the 3-day period, the specimens in group I presented a thicker odontoblastic layer, no cell-free zone of Weil, pulp core with differentiated mesenchymal and defense cells, and a high concentration of blood vessels. In group II, at the 12- and 24-hour time points, the odontoblastic layer was disorganized and the cell-free zone of Weil was absent, presenting undifferentiated cells, intensive vascularization with congested capillaries, and scarce defense cells in the cell-rich zone. In groups I and II, pulpal responses to the stimuli were more intense in the area underneath the region of application of the force or force/laser.Conclusions:The orthodontic-induced tooth movement and LLLT association showed reversible hyperemia as a tissue response to the stimulus. LLLT leads to a faster repair of the pulpal tissue due to orthodontic movement.     

Duration of orthodontic force affecting initial response of nitric oxide synthase in rat periodontal ligaments

Clinical experience suggests that successful orthodontic tooth movement can be produced with a threshold for force duration at about 6 hours, however, the changes in the periodontal ligaments (PDL) during this period is still unclear. Nitric oxide (NO) is a marker of signal transduction relating to bone remodeling. The aim of this study was to observe the initial response of NO synthase (NOS) when PDL equilibrium would be broken against light continuous orthodontic force. Rat maxillary first molars were moved mesially with 2 gf Titanium-Nickel closed coil springs for 1, 3 and 6 hours. The number of NADPH-diaphorase positive cells in PDL was counted for investigating NOS activity. At the control group, NOS activity in the distal area of the PDL was significantly higher than that of the mesial area (P<0.001). The activity of mesial area increased at 1-hour group (P<0.01), while the activity of distal area dropped down at 3- and 6-hour groups (P 3-hour<0.05, P 6-hour<0.001), compared with the control group. These results suggest that 1-3 hours would be the threshold of force duration for tooth movement with light continuous force.     

双膜透明矫治器移动兔下颌中切牙的效果评价

目的:应用双膜透明矫治器对兔下颌中切牙进行正畸移动,评价双膜透明矫治器移动牙的效果。方法:选用6只新西兰大白兔,实验组为双膜透明矫治器加力组（3只）,设计戴用3步矫治器,每一步牙移动0.33 mm,先戴用薄膜透明矫治器2 d,再戴用厚膜透明矫治器2 d;对照组为常规厚度膜片透明矫治器加力组（3只）,设计戴用3步矫治器,每一步牙移动0.20 mm,每步矫治器戴4 d。在每组兔分别戴用第1、2和3步矫治器后制取模型,测量牙移动数据。采用SPSS 19.0软件包对数据进行统计学分析。结果:2组牙移动表达的精确度从牙冠切端向龈端逐渐降低,实验组的精确度随矫治时间增加呈下降趋势,而对照组的精确度随矫治时间增加无明显变化。实验组牙冠切端、中端和龈端牙移动速率均显著大于对照组（P<0.001）。结论:兔下颌中切牙在透明矫治器的作用下产生倾斜移动。采用双膜透明矫治器矫治,牙移动速率加快,但牙移动表达的精确度降低。     

The effects of TBC3214, a selective endothelin ETA receptor antagonist, on orthodontic tooth movement in rats

Many chemical messengers are involved in the process of alveolar bone and periodontal ligament remodelling during orthodontic tooth movement. Among them is probably endothelin-1 (ET-1). Its role in this process has been partly explained using tezosentan, which affects endothelin A (ET(A)) and endothelin B (ET(B)) receptors. Tezosentan enhances orthodontic tooth movement. The aim of this study was to determine the possible effects of a highly selective ET(A) antagonist on orthodontic tooth movement in rats. Thirty male Wistar rats, 11-12 weeks of age, divided into three equal groups. In group I, a closed-coil spring was used and they were treated daily with 15 mg/kg body weight of TBC3214, a highly selective ET(A) antagonist. A closed-coil spring was also used in group II and the animals were treated daily with a placebo. Group III were treated daily with a placebo. The coil spring delivered a force of 25 cN and was attached between the upper left first molar and upper left incisor. The distance between the teeth was measured with a digital calliper (accuracy +/- 0.01 mm) on days 0, 7, 14, 21, 24, 32, 37, and 40. The differences in the distance between the teeth were calculated to determine the amount of tooth movement. Statistical analysis was performed using two-way analysis of variance, Bonferroni's correction, and paired t-tests. The distance between the upper left first molar and the upper left incisor decreased in groups I and II. In group I, tooth movement was significantly less on days 32 and 37 (P < 0.01) and on day 40 (P < 0.001) compared with group II. In group III, the distance between the teeth increased during the study (P < 0.001). In animals treated daily with TBC3214, tooth movement was significantly less compared with the animals treated with a placebo. It is concluded that ET-1, which is the predominant form of endothelin isopeptides, is involved in orthodontic tooth movement in rats, probably by enhancing bone resorption via ET(A) receptors.