Wonjae Yu

Mechanical properties of self-drilling orthodontic micro-implants with different diameters

OBJECTIVE The hypothesis to be tested is that peak-insertion torque of self-drilling micro-implants of an appropriate diameter correlates with peak-removal torque mechanically. MATERIALS AND METHODS A total of 360 self-drilling micro-implants composed of five different types were used. They (24 of each group) were inserted in three types of artificial bone with the use of a driving torque tester at a speed of 15 rpm. Insertion torque was measured during the placement, while the removal torque was measured within 3 days after insertion. RESULTS Most of the micro-implants in type A sheared before they were completely inserted in 40-pounds per cubic foot bone. The implants in other types were successfully inserted without implant breakage and bone fracture in all bone densities. There was a statistically significant correlation between insertion torque and removal torque (r > or = 0.43543, P = .0001). There were significant differences in insertion and removal torque among the diameters of implants and bone densities with an increasing tendency. The torque loss rates reduced as the diameter of the implant and bone density increased. CONCLUSIONS Micro-implants with a diameter of less than 1.3 mm are unsuitable for insertion into a bone with a density greater than 40 pounds per cubic foot mechanically when one is using a self-drilling technique.

Monitoring of Bone Temperature during Osseous Preparation for Orthodontic Micro-Screw Implants: Effect of Motor Speed and Ressure

The temperature at the surface of the bony recipient site during drilling for orthodontic micro-implant placement was monitored using infrared thermography. The primary objective was to identify proper drilling conditions to allow efficient drilling without raising the bone temperature above the threshold temperature of 44oC to 47oC. Bovine ribs were selected to provide cortical bone of a similar quality to the human mandible. Four drilling conditions combining 2 motor speeds (600 and 1200 rpm) and 2 pressure loads (500 g and 1000 g) were established based on clinical practice. Much care was taken to duplicate an oral environment, although no irrigation was used to allow the infrared radiation to transmit without being hindered by cooling water. Thermal images were recorded using a Thermovision 900 system (Amega, Danderryd, Sweden). The results showed that the temperature rise relys significantly on the drilling speed and pressure. When both the drilling speed and the pressure were low, the cortical bone could not be cut. However, increasing either the speed or the pressure resulted in a temperature increase. Drill speed of 600 rpm at the pressure load 1000g produced more or less the same temperature, 40- 45 o C, as when the drill speed was increased to 1200 rpm while keeping the load at 500g. Yet, a temperature as high as 62.4 o C was recorded when combining the high motor speed and high pressure. Most of the temperature rise took place during the initial 5-10 seconds of drilling, indicating that intermittent irrigation at an interval of 5 seconds or less would be of particular importance to minimize possible thermal trauma.

Effect of surface anodization on stability of orthodontic microimplant

Objective To determine the effect of surface anodization on the interfacial strength between an orthodontic microimplant (MI) and the rabbit tibial bone, particularly in the initial phase after placement. Methods A total of 36 MIs were driven into the tibias of 3 mature rabbits by using the self-drilling method and then removed after 6 weeks. Half the MIs were as-machined (n = 18; machined group), while the remaining had anodized surfaces (n = 18; anodized group). The peak insertion torque (PIT) and the peak removal torque (PRT) values were measured for the 2 groups of MIs. These values were then used to calculate the interfacial shear strength between the MI and cortical bone. Results There were no statistical differences in terms of PIT between the 2 groups. However, mean PRT was significantly greater for the anodized implants (3.79 ± 1.39 Ncm) than for the machined ones (2.05 ± 1.07 Ncm) (p < 0.01). The interfacial strengths, converted from PRT, were calculated at 10.6 MPa and 5.74 MPa for the anodized and machined group implants, respectively. Conclusions Anodization of orthodontic MIs may enhance their early-phase retention capability, thereby ensuring a more reliable source of absolute anchorage.

Biomechanical effectiveness of cortical bone thickness on orthodontic microimplant stability: an evaluation based on the load share between cortical and cancellous bone.

INTRODUCTION The aim of this study was to determine the appropriate range of cortical bone thickness (CBT) for supporting an orthodontic microimplant. METHODS Analysis of an orthodontic microimplant subjected to a horizontal force of 2N was performed using a nonlinear finite element method. The peak stresses in the cortical bone of 6 bone specimens (6 base models) with CBT of 0.5, 0.75, 1.0, 1.5, 2.0, and 3.0 mm, respectively, were analyzed. Assuming that the biomechanical effectiveness of cortical and cancellous bone is determined by the portion of the orthodontic force that each bone component takes up, we defined the ratios of the orthodontic force divided between the cortical and cancellous bone as load share ratios (LSR): ie, LSRcortical and LSRcancellous. Along with the base models, imaginary models created by removal of the cancellous bone from the base model bone specimens were analyzed in parallel; the imaginary models were designed so that the cortical bone alone took up all of the orthodontic force. By comparing the peak stresses in the imaginary and base models, the ratios of orthodontic force taken up by the cancellous and cortical bone (LSRcancellous and LSRcortical) were calculated. RESULTS The highest stress concentration occurred near the fulcrum where the orthodontic microimplant, undergoing tipping, presses the cortical bone surface in the direction of the force. Overall, the increase in CBT resulted in a decrease of the peak stress in the cortical bone. The decrease of stress, however, was not significant when the CBT was > 2.0 mm. LSR analysis showed that the cancellous bone has a substantial role in resisting the orthodontic force in cases of CBT ≤1.0 mm. Its role, however, declined rapidly with an increase of CBT and virtually disappeared at CBT values > 2.0 mm. LSRcortical was approximately 95% (LSRcancellous was 5%) at CBT = 1.5 mm and almost 100% at CBT = 2.0 mm, indicating that virtually all of the orthodontic force is transmitted to the cortical bone at CBT values of 2.0 mm or above. These results collectively demonstrated that CBT > 2.0 mm is biomechanically redundant. CONCLUSIONS From the biomechanical perspective, CBT values of 1.0 to 2.0 mm might be appropriate for orthodontic microimplant treatment.

Dynamic simulation of the self-tapping insertion process of orthodontic microimplants into cortical bone with a 3-dimensional finite element method

INTRODUCTION The aim of this study was to evaluate the stress state in the cortical bone around an orthodontic microimplant during and after the insertion surgery. METHODS The self-tapping insertion of an orthodontic microimplant into 1-mm-thick cortical bone containing a predrilled hole was simulated by using a 3-dimensional finite element method. The entire insertion surgery was replicated by a total of 3601 calculation steps: ie, the first 3600 dynamic steps analyzing the insertion process and an additional static step for analyzing the residual stress state after insertion. Four microimplants were experimentally inserted into rabbit tibiae to measure the insertion torques and compare them with the finite element analysis results. RESULTS Reasonable agreement was observed between the experimentally measured and the finite element calculated torques, confirming the validity of our finite element simulation, which showed that high stresses can develop in the interfacial bone during microimplant insertion. Hoop stresses above the ultimate tensile strength and radial stresses above the ultimate compressive strength of cortical bone developed in the bone. Furthermore, residual radial stresses higher than the critical threshold stress to trigger pathologic bone resorption were observed after insertion. These high insertion-related stresses implied that it is not the orthodontic force or the timing of its application, but the insertion conditions that can determine the bones response to the microimplant and its clinical prognosis. CONCLUSIONS This in-vitro finite element analysis showed that, during the self-tapping insertion of orthodontic microimplants, stresses high enough to fracture cortical bone can develop. After the self-tapping insertion, the radial stresses calculated at the interfacial bone were higher than the threshold value to trigger pathologic bone resorption.

Optimal asymmetric thread for orthodontic microimplants: Laboratory and clinical evaluation

OBJECTIVES To investigate the performance of microimplants incorporating a newly designed asymmetric thread. MATERIALS AND METHODS Three microimplants were compared. The control group comprised microimplants with the original v-shaped thread. The two experimental groups (Taper 1.0 and Taper 1.25) comprised prototype microimplants constructed with the new asymmetric thread; the Taper 1.25 specimens had a 1.25-mm-long and sharper tip, while the Taper 1.0 and control groups had a less sharp 1-mm tip. Two specially designed artificial bone blocks mimicking soft (maxillary) and hard (mandibular) bone were used to evaluate the microimplant insertion characteristics and postinsertion lateral stability. The peak insertion torque, insertion time, Periotest value (PTV), and torsional strength were measured. Then the microimplants were evaluated clinically over a 3-month period. RESULTS Significant differences in peak insertion torque, insertion time, and PTV were observed and favored the experimental groups. Although statistically insignificant, the clinical success rate was also higher in the Taper 1.25 experimental group than in the control group (87.2% vs 75.6%). CONCLUSIONS The better performances of the experimental microimplant, under both laboratory and clinical conditions (although statistically insignificant in the latter), demonstrate the superiority of the new asymmetric thread.

Effects of bracket slot size during en-masse retraction of the six maxillary anterior teeth using an induction-heating typodont simulation system

Objective To investigate how bracket slot size affects the direction of maxillary anterior tooth movement when en-masse retraction is performed in sliding mechanics using an induction-heating typodont simulation system. Methods An induction-heating typodont simulation system was designed based on the Calorific Machine system. The typodont included metal anterior and resin posterior teeth embedded in a sticky wax arch. Three bracket slot groups (0.018, 0.020, and 0.022 inch [in]) were tested. A retraction force of 250 g was applied in the posterior-superior direction. Results In the anteroposterior direction, the cusp tip of the canine in the 0.020-in slot group moved more distally than in the 0.018-in slot group. In the vertical direction, all six anterior teeth were intruded in the 0.018-in slot group and extruded in the 0.020- and 0.022-in slot groups. The lateral incisor was significantly extruded in the 0.020- and 0.022-in slot groups. Significant differences in the crown linguoversion were found between the 0.018- and 0.020-in slot groups and 0.018- and 0.022-in slot groups for the central incisor and between the 0.018- and 0.022-in slot groups and 0.020- and 0.022-in slot groups for the canine. In the 0.018-in slot group, all anterior teeth showed crown mesial angulation. Significant differences were found between the 0.018- and 0.022-in slot groups for the lateral incisor and between the 0.018- and 0.020-in slot groups and 0.018- and 0.022-in slot groups for the canine. Conclusions Use of 0.018-in slot brackets was effective for preventing extrusion and crown linguoversion of anterior teeth in sliding mechanics.

Torque Measures as Orthodontic Microimplants Stability Indicators

Objectives: To evaluate the effectiveness of multiple torque measures in describing the stability/prognosis of Orthodontic Microimplants (OMIs) and to find the most reliable one to perform from those reported in the literature. Methods: A total of 84 OMIs (Dentos Inc, Daegu, South Korea, 7mm in length) that had the same design except the diameter were divided into 3 equal groups of 28 (SH1312, SH1413 and SH1514). They were inserted and then removed from custom-made rigid polyurethane foam using a surgical engine and contra-angle handpiece. Multiple torque measures then were analysed and compared according to the relation between the OMI diameter and torque values. The correlation between Maximum Removal Torque (MRT) - which was taken as a reference - and other variables was tested. All statistical tests were performed at P <0.05 level of significance. Results: All torque measures except one (Torque Ratio, TR) showed statistically significant differences between the 3 OMIs groups with the SH1514 group having comparatively the largest mean torque values then SH1413, and then SH1312 group. The correlation to MRT was significant with only TR, and although it was statistically not significant; the correlation between MRT and Maximum Insertion Torque (MIT) was increasing with the diameter increase. Conclusion: All of the tested measures showed the same idea at the end from statistical view and that considering any of them is feasible with no superiority of one measure over the other.