| Anchorage control is crucial to successful orthodontic treatment. The "anchor"is a subject which resists the orthodontic or orthopedic loading forces. Tooth, group teeth arch, oral muscle and skeleton are "anchors"in orthodontics. There are many ways to strengthen the "anchors" including extraoral bow, Nance bow, TPA, teeth ligating, and intermaxillary pulling as well, unfortunately, these devices have limitations in clinical use.Many reporters reported the use of various implants including non-titanium implants, osseointergration implant, onplant, biodegradable implant, small palate implant and short epithetic implant as orthodontic anchorage where the existing dentition does not provide sufficient anchorage.Kanomi introduced a miniature implant that is a variation of a minibone screw used to fix bone plates in plastic and reconstructive surgery. In his report, a case with sever deepbite caused by overgrown inferior incisor was corrected by this minibone screw. After that, microimplant anchorage developed quickly.Microimplant anchorage has priority over the conventional implants for it can be placed in not only the areas of edentulous retromolar and mid-palate, but also at alveolar segments and even around the root apex. Now it is widely used asorthodontic anchorage in adult in whom tooth movement can not be achieved by conventional anchorage.Many innovations to the shape of the head and root of microimplant were made to improve primary stability, to reduce the stress and strain distribution in the bone interface and to improve gingivaltitis around the neck.However, the orthodontic anchorage microimplant still has limitations, including 1) 75% success rate which is thought to be low in clinic point of view. 2) tipping of the teeth due to single side loading.3) comprimised primary stability with the size of microimplant.As for mesiodistal movement or intrusion of tooth, torque or tipping control is required if orthodontic force is loaded unilaterally, which has side effect on the movement of teeth.Orthodontic force loading on microimplant could be divided into two forces perpendicular to each other, one of which is parallel to long axis of the implant. The parallel force is responsible for the out-pulling of the implant, which is thought to be detrimental to the stability of implant. It is reasonable to adjust the orientation of the loading force to make the force division perpendicular to the long axis of microimplant large, thus a better primary stability for immediate loading is available. However, the increased perpendicular force division increases the stress in the bone interface. It is well known that high stress is one of the most important factors for implant failure. Till now, the physiologic stress in orthodontic microimplant-bone interface is still unknown, but it is recommended that orthodontist use light force in loading of the microimplant.So, we put forward a new orthodontic microimplant—bicortical microimplant. It, 1.2mm in diameter, with slot on two heads, can be inserted in the interradicular regionfrom buccal side to lingual side, with two heads exposed in the oral cavity. Theoretically, the bicortical microimplant has such advantages: 1) two heads act as two anchorage units;2) lateral stress is shared by cortical bone of two sides;3) it has better primary stability due to placement in two cortical bone;4) Moreover, the insertion site is almost at the same level as the center of resistance of the posterior tooth or segment of teeth resulting in a favorable translatory tooth movement.The purpose of this study was to describe a method for precise placement of the new bicortical orthodontic microimplant, to study the potential anchorage of bicortical microimplant in translatory protraction of the fourth premolar in beagle dogs;to find out the difference of stress distribution in the implant-bone interface between bicortical microimplant and monocortical implant.Materials and Methods:To evaluate the method for building up a pricise and repeatable animal model of the new bicortical orthodontic microimplant, 5 adult beagle dogs were enrolled in this study. Bicortical microimplants were placed in the interradicular areas of 3 premolars on one side of the mandible with surgical template (test group), on the other side without surgical template (control group) in 5 dogs. The fabrication and using of the surgical template were described in details. Success rates were calculated and compared with each other. P<0.05 were considered as statistically significant.To study the potential anchorage of bicortical orthodontic microimplant in translatory protraction of the fourth premolar in beagle dogs, 5 dogs were used. After extraction of the third premolars in both arches, the dogs were left for healing for 1 week. A hole perforating the mandible in bucco-lingual direction was drilled onthe interradicular region at the center of resistance of the second premolar (P2) on one side of the mandible with a 1.15 mm pilot drill with continuous irrigation by cooling water using the surgical template described above. The tested bicortical microimplant was then inserted with two heads exposed in the oral cavity. Monocortical microimplant (traditional microimplant), screw type, 1.2mm in diameter, was inserted on the contralateral region on the other side of mandible in a conventional way. Full-cast metal crowns with two clasps on buccal and lingual sides were cemented to the fourth premolars (P4). Two orthodontic springs were connected to the bicortical microimplant and P4, and one spring was connected to the moncortical one and P4 in each dog. The springs were carefully calibrated to generate 50g force in the bicortical side, lOOg force in monocortical side biweekly for 3 months.Clinical and direct implant-tooth measurements were performed biweekly while the dogs were anesthetized. Distances between the central cusp of the first molar (Ml) and the inferior end of the buccal clasp of the metal crown of P4 (M1-P4B), between the central cusp of the first molar (Ml) and the inferior end of the lingual clasp of the metal crown of P4 (M1-P4L), between the central cusp of Ml and the central cusp of P4 (M1-P4), between the cusp of the canine (CA) and the center of the buccal end of the implant (CA-IB), between the canine (CA) and the central cusp of Ml ( CA-M1), were measured.At the end of tooth movement, all the anesthetized animals were sacrificed by air thrombosis administered intravenously. Specimens with bicortical microimplants were immersed in 4% formalin, dehydrated in a graded series of ethanol, and then embedded in methylmethacrylate according the technique reported by Donath and Breuner. Three slices perpendicular to the long axis of the implant were cut using a diamond sawCLeica SP1600. Germany)to 100 um and ground to a thickness of 70ummanually. The slices were stained with Van-Gieson. Bone-to-implant contact (BIC) at the interface of the specimen was calculated on three slices using professional image analysis software (Image-Pro Plus5.0, Media Cybernetics, U.S.A).To study the biomechanics of the new biocortial microimplant, simplified three-dimensional finite-element models were created of a 20mm local mandible with bicortical microimplant (test) and monocortical microimplant (control) embedded in. which resembles the animal models. The coronary cross-section of local mandible was simplified as isosceles trapezoid, 10mm in upper side width, 14mm in low side width, and 30mm in height. The exterior cortical bone was set to 1.6mm. ANSYS 9.0 finite-elememt analysis program (Swanson Analysis System Inc., Houston, PA, USA) was used to generate the solid model, create the mesh of individual elements, and perform the post-processing to calculate the Von-mises stress. The cortical bone and the cancellous bone were modeled as homogenous materials with transverse isotropy. 50 g force in mesio-distal direction was loaded to the both heads of the bicortical microimplant, lOOg force to the head of monocortical microimplant, as in the animal model. Additionally, initial displacement of 0.05mm was set to the model to mimic the situation shortly after the placement of microimplant. The algorithm were performed on an xwfeOOOhp workstation (HP Inc,.Palo Alto, CA, U.S.A)Results:In the* study of evaluating the method for building up a pricise and repeatable animal model of the new bicortical orthodontic microimplant, total 30 bicortical orthodontic microimplants, 1.2mm * 14mm, were placed in the interradicular region of premolars on each side of mandible. No bicortical orthodontic microimplant wasfailed in the test side, vs 4 out of 15 in the control side. Statistical analysis showed significant difference in the comparison of success rate of the two sides (P<0.05).In the study of potential anchorage of bicortical anchorage, obvious mesial movement of P4 was observed in the bicortical microimplant side and obvious rotation of P4 was observed in the monocortical microimplant side. At the end of orthodontic loading, all the P4 showed tipping to some extent. All the bicortical microimplants remained stable during orthodontic loading, while 1 of the monocortical microimplants lost within the first month. Slight mucositis was found at peri-implant mucosal tissues in the buccal side.Changes of M1-P4B and M1-P4L are 2.9±0.21mm, 2.94±0.19 mm respectively in the biocortical microimplant side (test) (P> 0. 05), and 2.33±0.15 mm, 0.53±0.14 mm in the monocortical microimplant side (control) (P < 0. 05);Mesial movement of P4 is 3.98mm in the test side, 1.96mm in the control (P < 0. 05);Displacements of crown and root in each dog in the test side was almost identical within the first two months, but different in the third month;The values of CA-IB were 22.32± 1.35 mm before loading, 22.6±0.87 mm after loading in the test side, and 22.13±1.35 mm, 20.75±l .32 mm respectively in the control (P > 0. 05). The values of CA-M1 were 42.5±2.35 mm before loading, 41.16±2.22 mm after loading in the test side, and43.43±0.99 mm, 42.03±0.71 mm respectively in the control (P > 0. 05).The histological examination didn't find any evidence for infection in the bicortical microimplant-bone interface. The bicortical microimplant showed a certain quantity of BIC varying from 10-25%.Stress distribution in the bone interface was almost similar in the two microimplants, but the maximum stress of biocortical microimplant was smaller than that of monocortical one. Moreover, maximum stress in the bone interface at thecortical bone reached 1782 Mpa in occluso-gingival direction, 1648 Mpa in mesio-distal direction while 0.05 mm initial displacement was taken into consideration. For 0.1mm, it reached 2110 Mpa and 2012Mpa respectively 0Conclusion:1. Surgical template is a choice for precise placement of bicortical microimplant in the interradicular region in an animal model. The rationale of this method can also be applied in clinical situation.2. Bicortical microimplant provides two anchorage units for bodily transition of posterior teeth resulting in faster tooth movement, while conventional one provides one unit, which accounts for tooth rotation.3. Orthodontic force can be shared by bilateral cortical bones, which will decrease the stress on the bone implant interface. Stress caused by initial displacement should be taken into consideration in immediate loading of microimplant.4. bicortical microimplant can function as bilateral orthodontic anchorage for bodily movement of tooth in suitable cases in clinic.
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