In biocreative therapy (C-therapy), torque control

ONLINE ONLY Factors controlling anterior torque with C-implants depend on en-masse retraction without posterior appliances: Biocreative therapy type II technique Sung-Seo Mo, a Seong-Hun Kim, b Sang-Jin Sung, c Kyu-Rhim Chung, d Youn-Sic Chun, e Yoon-Ah Kook, f and Gerald Nelson g Seoul, Korea, and San Francisco, Calif Introduction: Our objective was to evaluate the factors that affect effective torque control during en-masse anterior retraction by using intrusion overlay archwire and partially osseointegrated C-implants as the exclusive sources of anchorage without posterior bonded or banded attachments. Methods: Base models were constructed from a dental study model. No brackets or bands were placed on the posterior maxillary dentition during retraction. Different heights of the anterior retraction hooks to the working segment archwire and different intrusion forces with an overlay archwire placed in the 0.8-mm diameter hole of the C-implant were applied to generate torque on the anterior segment of the teeth. The amount of tooth displacement after finite element analysis was exaggerated 70 times and compared with tooth axis graphs of the central and lateral incisors and the canine. Results: The height of the anterior retraction hook and the amount of intrusion force had a combined effect on the labial crown torque applied to the incisors during en-masse retraction. The difference of anterior retraction hook length highly affected the torque control and also induced a tendency for canine extrusion. Conclusions: Three-dimensional en-masse retraction of the anterior teeth as an independent segment can be accomplished by using partially osseointegrated C-implants as the only source of anchorage, an intrusion overlay archwire, and a retraction hook (biocreative therapy type II technique). (Am J Orthod Dentofacial Orthop 2011;139:e183-e191) a Associate professor, Division of Orthodontics, Department of Dentistry, Catholic University of Korea, Seoul, Korea. b Associate professor, Department of Orthodontics, College of Dentistry, Kyung Hee University, Seoul, Korea. c Associate professor and chairman, Division of Orthodontics, Department of Dentistry, University of Ulsan, College of Medicine, Asan Medical Center, Seoul, Korea. d President, Korean Society of Speedy Orthodontics, Seoul, Korea. e Professor and chairman, Division of Orthodontics, Department of Dentistry, Ehwa Womans University Mokdong Hospital, Seoul, Korea. f Professor and chairman, Division of Orthodontics, Department of Dentistry, Catholic University of Korea, Seoul, Korea. g Clinical professor, Division of Orthodontics, University of California at San Francisco. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Supported by a grant from Kyung Hee University in 2010 (KHU-20100696). Reprint requests to: Seong-Hun Kim, Department of Orthodontics, College of Dentistry, Kyung Hee University, #1 Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Korea; e-mail, bravortho@hanmail.net. Submitted, April 2010; revised and accepted, September 2010. 0889-5406/$36.00 Copyright Ó 2011 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2010.09.023 In biocreative therapy (C-therapy), torque control requires special consideration, since one is retracting the anterior teeth in a segment unattached to the posterior teeth. 1,2 This concept was developed because partially osseointegrated mini-implants or plates can easily endure multi-directional heavy forces even when they support orthodontic archwires. 3-6 In C-therapy, it is possible to retract the anterior segment independently by directly placing the wire into the hole of the mini-implant. 1,2,7,8 When retracting against dental anchorage, or against indirect miniscrew anchorage, actual intrusion vectors on the anterior teeth are hard to achieve without unwanted reactive forces affecting the posterior dental units. With the biocreative approach, true intrusion vectors without side effects are possible, since the osseointegrated C-implant or the C-plate is secure against rotational moments. 1,2 The posterior teeth can be left intact. We reported previous finite element analysis (FEA) studies using the biocreative therapy type I technique, demonstrating 3-dimensional (3D) anterior retraction with gable e183

e184 Mo et al Fig 1. Biocreative therapy type II technique: A, intraoral photograph of intrusion overlay NiTi wire application on the segmented archwire with a very short hook (woman, age 22 years); B, after treatment; C, intraoral photograph of intrusion overlay NiTi wire application on the segmented archwire with a long hook (woman, age 26 years); D, after treatment. bends and an anterior retraction hook (ARH). 9 For patients with a deepbite tendency in the anterior segment, we have addressed a weak point of the type I technique by an applying intrusion force with an overlay archwire applied to the anterior segment. We called this the biocreative therapy type II technique. This is an improved method of applying the segmented intrusion arch technique of Burstone. 10 Two common orthodontic biomechanical systems today are the use of a continuous archwire (eg, straightwire appliance) and segmented archwires (Burstone s segmented arch technique). 10-12 Straight-wire appliance techniques are not technique sensitive but have limited applications for some tooth movements. Segmented archwire techniques enable more effective and accurate tooth movements by using mechanically determinate force systems by separating the anterior and posterior segments. These systems require a better understanding of the biomechanics involved. Problems of anchorage loss in segmented arch systems are resolved with temporary skeletal anchorage devices. 13 Chung et al 1,2,7,8 introduced the technique that uses minimum orthodontic hardware and minimizes side effects by replacing the posterior appliance segments with the partially osteointegrated C-implant. This beneficial protocol is only possible if the mini-implant will not loosen in response to the heavy or dynamic forces that would be necessary. The C-implant (sandblasted, large-grit, acid-etched mini-implant) will allow the application of a nickel-titanium (NiTi) reverse curve of Spee overlay archwire, which will apply a moment to the mini-implant but not loosen the screw. 1,2 This intrusion overlay wire produces forces that control both the torque and the vertical position of the incisor segment (Fig 1). The size of overlay NiTi archwire can be changed easily. To date, there are no studies of the factors involved in the control of anterior torque by this technique. Clinical studies and case reports have described the technique. 2 In this study, we constructed a 3D finite element model of the maxillary teeth, periodontal ligament (PDL), and alveolar bone after extracting the first premolars. After placement of the orthodontic mini-implant with the 0.8-mm hole between the second premolar and the first molar, and 8 mm apical to the expected bracket position, we applied an intrusion force using a reversecurved NiTi archwire from a C-implant head to the point between the central incisors to the segmented archwire of the 6 anterior teeth using the mini-implant as a posterior orthodontic tube. We simulated the effect on torque control using different heights of retraction hooks located between the lateral incisor and the canine, and different amounts of intrusion force on the NiTi overlay archwire. MATERIAL AND METHODS For the finite-element model, we obtained the tooth outline forms through the 3D laser scanning of a maxillary right dentition from a dental study model (base model) (model-i21d-400g, Nissin Dental Products, Kyoto, Japan) of an adult with normal occlusion. Using the micro-arch bracket (Tomy, Tokyo, Japan), we aligned and leveled using a broad arch form (Ormco, Glendora, Calif) and referred to previous studies for inclination and February 2011 Vol 139 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

Mo et al e185 Table I. Mechanical properties of each material Young s modulus (MPa) Poisson s ratio Periodontal ligament 5.0E-02 0.49 Alveolar bone 2.0E103 0.30 Teeth 2.0E104 0.30 Stainless steel 2.0E105 0.30 Table II. Comparison of ARH length and intrusion force on z-axis displacement Tooth Central incisor Lateral incisor Intrusion force (g) Hook length 70 80 90 1 mm Root apex 2.78E-02 2.69E-02 2.60E-02 Incisal edge 1.89E-03 7.91E-03 1.39E-02 4 mm Root apex 2.70E-02 2.64E-02 2.57E-02 Incisal edge 2.03E-02 2.61E-02 3.21E-02 7 mm Root apex 2.84E-02 2.77E-02 2.70E-02 Incisal edge 3.82E-02 4.42E-02 5.01E-02 10 mm Root apex 2.93E-02 2.86E-02 2.79E-02 Incisal edge 5.63E-02 6.23E-02 6.83E-02 1 mm Root apex 1.91E-02 1.91E-02 1.91E-02 Incisal edge 1.18E-02 8.92E-03 6.08E-03 4 mm Root apex 1.58E-02 1.57E-02 1.57E-02 Incisal edge 1.89E-03 8.32E-04 3.54E-03 7 mm Root apex 1.17E-02 1.16E-02 1.16E-02 Incisal edge 6.24E-03 8.95E-03 1.17E-02 10 mm Root apex 7.46E-03 7.40E-03 7.33E-03 Incisal edge 1.46E-02 1.73E-02 2.00E-02 Canine 1 mm Root apex 1.95E-02 1.89E-02 1.83E-02 Incisal edge 1.52E-02 1.47E-02 1.43E-02 4 mm Root apex 1.40E-02 1.34E-02 1.28E-02 Incisal edge 3.80E-02 3.75E-02 3.70E-02 7 mm Root apex 7.36E-03 6.73E-03 6.10E-03 Incisal edge 5.97E-02 5.92E-02 5.88E-02 10 mm Root apex 5.21E-04 1.09E-04 7.39E-04 Incisal edge 8.07E-02 8.03E-02 7.98E-02 Positive figures mean tooth intrusion; negative figures mean extrusion. Fig 2. Three-dimensional finite element mesh: A and B, lateral views of the maxillary dentition and the PDL; C, lateral views of teeth, PDL, alveolar bone of the maxillary dentition, and C-implant head; D, lateral views of intrusion force application. angulation. 11,12,14 We did not add a curve of Spee or a curve of Wilson (Fig 2, A), and the thickness of the PDL was assumed to be uniform (0.25 mm) (Fig 2, B). 15,16 The alveolar bone crest was constructed to follow the cementoenamel junction curvature 1 mm apical to the cementoenamel junction. 17 The 3D finite element model included 12 teeth, a space for the missing first premolar s periodontal space, and alveolar bone, and was bilaterally symmetrical (Fig 2, C). In the base model, the distance from the incisal edge of the maxillary central incisor to the bracket slot (perpendicular to the occlusal plane) was 4.5 mm, 11 mm labial to the cementoenamel junction, and 11.8 mm to the labial alveolar crest. In the finite element model, teeth, alveolar bones, brackets, periodontal spaces, the C-implant, and the archwire were constructed with fine tetrahedron solid elements; the teeth and brackets were connected without interference, and each tooth contacted the next at the contact point as individual elements (Fig 2, D). In this study, teeth, alveolar bones, and periodontal spaces were assumed to be isoparametric and homogeneous linear elastic bodies; the material properties of the elements were Young s modulus and Poisson s ratio according to previous studies (Table I). 18-20 In the system studies, we construct the American Journal of Orthodontics and Dentofacial Orthopedics February 2011 Vol 139 Issue 2

e186 Mo et al Fig 3. Changes of the axes of the maxillary anterior teeth according to the length of ARH and magnitude of the intrusion force. Solid line, before; dotted line, after the applicatioin of force; B, central incisor;,, lateral incisor; O, mandibular canine end mean midincisal point or cusp tip, upper end mean root apex); IF, intrusion force; ARH, anterior retraction hook. The displacement of teeth was magnified 70 times. The movement of the maxillary central incisor was controlled tipping with a short hook (1 mm). When a longer hook was used, more root movement, and proclination of the anterior teeth, and more extrusion of a canine were observed. x-axis as the in-out direction, the y-axis as the labiolingual direction, and the z-axis as the upper-lower direction, and defined 1x as the left central incisor direction, 1y as the labial direction, 1z as the apical direction, and x-y as the occlusal plane of the teeth. The anterior segmented archwire was modeled by using a 3D beam element (ANSYS beam 4, Swanson Analysis System, Canonsburg, Pa) with the cross section of 0.016 3 0.022-in stainless steel. The archwire hook (0.019 3 0.025-in stainless steel) was set at the midpoint between the lateral incisor bracket and the canine bracket bilaterally. The osseointegration-based C-implant with an 0.8-mm diameter hole on the head part (Cimplant, Seoul, Korea) was placed between the maxillary first molar and the second premolar, and 8 mm apically to the expected bracket position. We assumed that there were no gaps between the bracket and the archwire at the central incisor, lateral incisor, and canine and carried out the nonlinear analysis, allowing the gap element between the archwire and the 0.8-mm diameter hole of the C-implant head. The intrusion force can be applied to 1 point between the central incisors or 2 points between a central incisor and a lateral incisor; in this study, we applied the intrusion force to 1 point between the central incisors. The retraction force was applied via ARH between a lateral incisor and a canine. The retraction force was 150 g between the ARH and the C-implant head, and the lengths of hooks were 1 mm (very short), 4 mm (short), 7 mm (standard), and 10 mm (long). We measured the intrusion forces by using 3 curved NiTi wire sizes (0.016 3 0.022-in, 0.017 3 0.025-in, and 0.019 3 0.025-in) February 2011 Vol 139 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

Mo et al e187 Fig 4. Comparison of the vertical effects (z-axis) of the ARH and intrusion forces in the 3D finite element model (mm); IF, intrusion force; ARH, anterior retraction hook; negative value, mean extrusion; positive value, mean intrusion. When longer ARH and greater intrusion forces were applied, more proclination or less linguoversion of the maxillary central incisor was observed. Accordingly, the intrusion of the maxillary central incisor was increased. (G&H Wire, Franklin, Ind) according to the intraoral condition of patients and obtained 70, 80, and 90 g, respectively. The intrusive force was applied between the right and left central incisors in the 1z direction as in a 1-piece intrusion archwire. The boundary condition for holding the maxillary model was the top of the model base connected to the maxilla. There was significant clearance in the 0.8-mm hole of the C-implant head, so friction for all 3 wires was low. The differences in friction between the wire sizes were not significant. The graphs are labeled with the forces produced as measured on the physical models (Fig 3). The tooth displacements were marked by applying the x, y, and z coordinates at the midpoint of the incisal edges of the central incisor and lateral incisor, the cusp tip of canine, and each tooth s root apex. For the FEA, ANSYS (version 11, Swanson Analysis System), the universal finite element program, was used on a workstation (HP XW6400, Hewlett-Packard, Palo Alto, Calif). RESULTS We observed the tooth displacement pattern on the z-axis (Table II, Figs 3 and 4) based on the movement of the maxillary central incisal edges. The amount of intrusion increased as the intrusion force increased and the length of the hook increased. Because the force system introduces a slight counterclockwise moment to the anterior dental segment, we noticed some canine extrusion, which decreased with a heavier intrusion force and increased with a longer hook arm. For the tooth displacement pattern on the y-axis (Table III, Fig 5), when we applied 70 g of force, the maxillary central incisors tipped lingually with the 1-mm hook group, moved almost bodily in the 4-mm group, American Journal of Orthodontics and Dentofacial Orthopedics February 2011 Vol 139 Issue 2

e188 Mo et al Table III. Comparison of ARH length and intrusion on y-axis displacement Intrusion force (g) Tooth Hook length 70 80 90 Central incisor 1 mm Root apex 1.33E-02 1.74E-02 2.14E-02 Incisal edge 4.31E-02 3.75E-02 3.19E-02 4 mm Root apex 2.45E-02 2.83E-02 3.23E-02 Incisal edge 2.64E-02 2.12E-02 1.59E-02 7 mm Root apex 3.55E-02 3.95E-02 4.34E-02 Incisal edge 1.27E-02 7.39E-03 2.06E-03 10 mm Root apex 4.65E-02 5.05E-02 5.45E-02 Incisal edge 1.82E-03 7.15E-03 1.25E-02 Lateral incisor 1 mm Root apex 1.44E-03 8.40E-04 3.12E-03 Incisal edge 3.98E-02 3.64E-02 3.31E-02 4 mm Root apex 9.81E-03 1.21E-02 1.43E-02 Incisal edge 2.29E-02 1.96E-02 1.63E-02 7 mm Root apex 2.03E-02 2.26E-02 2.48E-02 Incisal edge 6.03E-03 2.71E-03 5.98E-04 10 mm Root apex 3.08E-02 3.30E-02 3.52E-02 Incisal edge 1.13E-02 1.46E-02 1.79E-02 Canine 1 mm Root apex 8.62E-03 7.75E-03 6.88E-03 Incisal edge 4.45E-02 4.22E-02 3.98E-02 4 mm Root apex 1.39E-02 1.30E-02 1.22E-02 Incisal edge 5.42E-02 5.17E-02 4.92E-02 7 mm Root apex 1.88E-02 1.80E-02 1.71E-02 Incisal edge 6.15E-02 5.90E-02 5.66E-02 10 mm Root apex 2.35E-02 2.26E-02 2.17E-02 Incisal edge 6.79E-02 6.54E-02 6.30E-02 Positive figures mean tooth proclination; negative figures mean retraction. and displayed a root-retraction pattern in the 10-mm group. As the intrusion force increased, the amount of coronal retraction decreased, and root retraction increased. The canine crown tipped distally, and this tipping pattern increased as the hook length increased. DISCUSSION In en-masse retraction after the usual extraction of premolars, adjustment of the retraction hook length is recommended to control loss of torque from linguoversion of the anterior teeth during their retraction, and thick wires are recommended to minimize bite deepening and torque loss from vertical bowing of the main archwires. 21,22 But improvement of bite deepening and control of torque loss can be obtained with restriction in a patient having enmasse retraction. C-therapy, which is the subject of this study, has advantages that can minimize unwanted tooth movement in the posterior teeth and maintain the occlusal relationship of the posterior area and good oral hygiene by minimally bonding braces to molars and premolars. On the other hand, more effort to control anterior tooth movement is needed with C-therapy. 1,2,7,8 Unlike mechanical locked mini-implants, 23,24 the C-implant used in C-therapy can resist the rotational force and be removed easily because of its partial osteointegration. 3-6 With this feature, biocreative therapy type II mechanics can be used after completion of anterior intrusion and decrowding. 1,9 Instead of an intrusion arch, a 0.016 3 0.022-in stainless steel utility archwire is placed from the anterior segment into the implant tube, and distinct gable bends are used to generate an anterior torque moment on the anterior segment of the teeth to provide bodily movement during en-masse retraction. Biocreative therapy type II can be the other method to control the anterior teeth. 2 This technique allows application of bodily retraction and early intrusion even in a patient with deepbite. Since the length of the ARH affects the quality of the tooth-movement pattern, the ARH can be adjusted to fit the goals of retraction. That is, we can use the 1-mm ARH for controlled tipping in flared incisors, the 4-mm ARH for bodily movement, and the 7-mm or 10-mm ARH for root retraction. A longer ARH allows a little more extrusion of the canine during retraction. Biocreative therapy type II shows features of the 3-piece intrusion archwire. Biocreative therapy type II February 2011 Vol 139 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

Mo et al e189 Fig 5. Comparison of the sagittal effects (y-axis) of the ARH and intrusion forces in the 3D finite element mode (mm); IF, intrusion force; ARH, anterior retraction hook; negative value, mean lingual or posterior movement; positive value, mean labial or anterior movement. When longer ARH and greater intrusion force were applied, less linguoversion and further proclination of the central incisor crown were observed. can control various patterns of tooth movement through the combination of intrusion force, retraction force, and length of the ARH. In this study, intrusion forces of 70, 80, and 90 g generated by 0.016 3 0.022-in, 0.017 3 0.025-in, and 0.019 3 0.025-in reverse-curved NiTi wires for patients various intraoral conditions were applied, retraction forces were fixed to 150 g, and the length of the ARH was 1, 4, 7, or 10 mm. The toothmovement patterns for the anterior 6-tooth segment were evaluated in these conditions. A recent study showed the center of resistance to be located 13.5 mm posteriorly and 9 mm superiorly from the center of the archwire, similar to the estimation of Melsen et al. 25-28 In another study, the center of resistance of the 6 maxillary anterior teeth is known to be located 13.5 mm apically and 14 mm posteriorly from the central incisal edge. 22 If retraction and intrusion force are applied to the 1-mm ARH, the result is controlled lingual tipping of the segment (Fig 6, A) because the clockwise moment that is equal to the magnitude of the retraction force multiplied by the perpendicular distance of the line of action of the force to the center of resistance exceeds the counterclockwise moment that is equal to the magnitude of the intrusion force multiplied by the perpendicular distance of the line of action of the force to a center of resistance. When a retraction force is applied to the 4-mm ARH, the clockwise and counterclockwise moments are neutralized, and bodily movement occurs (Fig 6, B). When retraction forces are applied to hooks with arms longer than 4 mm, the sum of the moments rotates the segment counterclockwise so that incisor torque and intrusion are increased, and the canines extrude (Fig 6, C). In the C-therapy type II technique, 1 to 4 mm ARH are recommended for anterior retraction in a patient who needs maxillary first premolar extractions. Although the retraction forces were uniform in this study, a force similar to that of the 3-piece intrusion archwire of Burstone 10 will occur if the retraction forces are decreased; American Journal of Orthodontics and Dentofacial Orthopedics February 2011 Vol 139 Issue 2

e190 Mo et al of temporary skeletal anchorage devices. When using this technique, one must consider the center of resistance of the anterior segment to retraction in each patient. Root length, bone levels, pretreatment incisor inclination, and close monitoring of the effects of force application are all important considerations. Fig 6. Schematic representation of the biocreative therapy type II technique. A black dot indicates the center of resistance (CR). Dotted lines indicate intrusion force (blue line) and retraction force (red line). Solid arrows express the moments (force times the distance between force and CR) that originated from 2 forces. A, 1-mm hook (short): when the intrusion force and its moment (blue) are constant, the clockwise moments generated from the distance between the red dotted line and the CR are greater, so that the group of 6 anterior teeth inclines lingually. B, 4-mm hook: the distance between the red dotted line and the CR is shorter than for the 1-mm hook, and the decreased clockwise moment is neutralized with a counterclockwise moment, so that the group of 6 anterior teeth translates. C, 7 and 10 mm hooks (long): the retraction force nearly passes by the CR, and a clockwise moment is not generated as a result, so that the group of 6 anterior teeth flares, and a canine extrudes. this might be suited for intrusion of the anterior segment. Therefore, further study seems necessary. The technique described here resulted from several years of experience and observation of the clinical application CONCLUSIONS Based on the findings of this study, we concluded the following. 1. Finite element studies demonstrated that variations of the height of the ARH and the amount of intrusion force produced measurable effects on the inclination and vertical position of the incisors during en-masse retraction. 2. With a 70-g intrusion force and a 1-mm high hook, the maxillary central incisors displaced lingually in a controlled tipping pattern. Increasing the hook height to 4 mm produced almost bodily movement, and, in the 10-mm group, root retraction was produced ahead of the crowns. As intrusion force increased, the amount of coronal retraction decreased, and root retraction increased. Higher intrusion forces and longer retraction hooks also caused increased incisor intrusion and canine extrusion. 3. Three-dimension controlled en-masse retraction of the 6 anterior teeth as an independent unit can be accomplished by using partially osseointegrated C-implants as the only source of anchorage, a NiTi intrusion overlay archwire, and a retraction hook (biocreative therapy type II technique). We thank Ki-Joon Lee, Department of Orthodontics, Youn-Sei University, for FEM model construction and Jin-Kyung Lee, Division of Orthodontics, Department of Dentistry, Catholic University of Korea, Yoido St. Mary s Hospital, for editing the manuscript. REFERENCES 1. Chung KR, Kim SH, Kook YA, Sohn JH. Anterior torque control using partial-osseointegrated mini-implants: biocreative therapy type I technique. World J Orthod 2008;9:95-104. 2. Chung KR, Kim SH, Kook YA, Choo HR. Anterior torque control using partial osseointegration based mini-implants: biocreative therapy type II technique. World J Orthod 2008;9:105-13. 3. Kim SH, Cho JH, Chung KR, Kook YA, Nelson G. Removal torque values of surface-treated mini-implants after loading. Am J Orthod Dentofacial Orthop 2008;134:36-43. 4. Jeon MS, Kang YG, Mo SS, Lee KH, Kook YA, Kim SH. Effects of surface treatment on the osseointegration potential of orthodontic mini-implant. Korean J Orthod 2008;38:328-36. 5. Kim SH, Lee SJ, Cho IS, Kim SK, Kim TW. Rotational resistance of surface-treated mini-implants. Angle Orthod 2009;79: 899-907. February 2011 Vol 139 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

Mo et al e191 6. Mo SS, Kim SH, Kook YA, Jeong DM, Chung KR, Nelson G. Resistance to immediate orthodontic loading of surface-treated mini-implants. Angle Orthod 2010;80:123-9. 7. Chung KR, Nelson G, Kim SH, Kook YA. Severe bidentoalveolar protrusion treated with orthodontic microimplant-dependent enmasse retraction. Am J Orthod Dentofacial Orthop 2007;132: 105-15. 8. Kim SH, Hwang YS, Ferreira A, Chung KR. Analysis of temporary skeletal anchorage devices used for en-masse retraction: a preliminary study. Am J Orthod Dentofacial Orthop 2009;136: 268-76. 9. Mo SS, Kim SH, Sung SJ, Chung KR, Chun YS, Kook YA, et al. Factors controlling anterior torque during temporary skeletal anchorage devices dependent en-masse retraction without posterior appliances: biocreative therapy type I technique. Am J Orthod Dentofacial Orthop 2011 (in press). 10. Burstone CJ. The segmental arch approach to space closure. Am J Orthod 1982;82:361-78. 11. Andrews LF. Straight wire, the concept and appliance. San Diego: L. A. Wells; 1989. 12. Germane N, Bentley BE Jr, Isaacson RJ. Three biologic variables modifying faciolingual tooth angulation by straight-wire appliances. Am J Orthod Dentofacial Orthop 1989;96:312-9. 13. Bae SM, Park HS, Kyung HM, Kwon OW, Sung JH. Clinical application of micro-implant anchorage. J Clin Orthod 2002;36:298-302. 14. Park CK, Yang WS. A three-dimensional finite element analysis on the location of center of resistance during intrusion of upper anterior teeth. Korean J Orthod 1997;27:259-72. 15. Coolidge E. The thickness of the human periodontal membrane. J Am Dent Assoc 1937;24:1260-7. 16. Kronfeld R. Histologic study of the influence of function on the human periodontal membrane. J Am Dent Assoc 1931;18: 1242-74. 17. Block PL. Restorative margins and periodontal health: a new look at an old perspective. J Prosthet Dent 1987;57:683-9. 18. Tanne K, Sakuda M, Burstone CJ. Three-dimensional finite element analysis for stress in the periodontal tissue by orthodontic forces. Am J Orthod Dentofacial Orthop 1987;92:499-505. 19. Zeigler A, Keilig L, Kawarizadeh A, J ager A, Bourauel C. Numerical simulation of the biomechanical behaviour of multi-root teeth. Eur J Orthod 2005;27:333-9. 20. Poppe M, Bourauel C, J ager A. Determination of the elasticity parameters of the human periodontal ligament and the location of the center of resistance of single-rooted teeth: a study of autopsy specimens and their conversion into finite element models. J Orofac Orthop 2002;63:358-70. 21. Sia S, Shibazaki T, Koga Y, Yoshida N. Experimental determination of optimal force system required for control of anterior tooth movement in sliding mechanics. Am J Orthod Dentofacial Orthop 2009;135:36-41. 22. Chung KR, Jeong DM, Park HJ, Kim SH, Nelson G. Severe bidentoalveolar protrusion with lingual biocreative therapy using palatal miniplate. Korean J Orthod 2010;40:276-87. 23. Jung MH, Kim TW. Biomechanical considerations in treatment with miniscrew anchorage. Part 1: the sagittal plane. J Clin Orthod 2008;42:79-83. 24. Cha JY, Yu HS, Hwang CJ. The validation of periotest values for the evaluation of orthodontic mini-implants stability. Korean J Orthod 2010;40:167-75. 25. Chung GM, Sung SJ, Lee KJ, Chun YS, Mo SS. Finite-element investigation of the center of resistance of the maxillary dentition. Korean J Orthod 2009;39:83-94. 26. Sung SJ, Jang GW, Chun YS, Moon YS. Effective en-masse retraction design with orthodontic mini-implant anchorage: a finite element analysis. Am J Orthod Dentofacial Orthop 2010;137:648-57. 27. Melsen B, Fotis V, Burstone CJ. Vertical force considerations in differential space closure. J Clin Orthod 1990;24:678-83. 28. Sung SJ, Kim IT, Kook YA, Chun YS, Kim SH, Mo SS. Finite-element analysis of the shift in center of resistance of the maxillary dentition in relation to alveolar bone loss. Korean J Orthod 2009;39:278-88. American Journal of Orthodontics and Dentofacial Orthopedics February 2011 Vol 139 Issue 2