Yukio Kojima

Numerical simulations of canine retraction with T-loop springs based on the updated moment-to-force ratio

The purpose of this study was to develop a new finite element method for simulating long-term tooth movements and to compare the movement process occurring in canine retraction using a T-loop spring having large bends and with that having small bends. Orthodontic tooth movement was assumed to occur in the same manner as the initial tooth movement, which was controlled by the moment-to-force (M/F) ratios acting on the tooth. The M/F ratios were calculated as the reaction forces from the spring ends. For these M/F ratios, the teeth were moved based on the initial tooth movements, which were calculated by using the bilinear elastic model of the periodontal ligament. Repeating these calculations, the teeth were moved step by step while updating the M/F ratio. In the spring with large bends, the canine at first moved bodily, followed by root distal tipping. The bodily movement was quickly achieved, but over a short distance. In the spring with small bends, the canine at first rotated and root mesial tipping occurred, subsequently the canine uprighted and the rotation decreased. After a long time elapsed, the canine moved bodily over a long distance. It was found that the long-term tooth movement produced by the T-loop springs could be simulated by the method proposed in this study. The force system acting on the teeth and the movement type remarkably changed during the long-term tooth movement. The spring with large bends could move the canine bodily faster than that with small bends.

Numeric simulations of en-masse space closure with sliding mechanics

INTRODUCTION En-masse sliding mechanics have been typically used for space closure. Because of friction created at the bracket-wire interface, the force system during tooth movement has not been clarified. METHODS Long-term tooth movements in en-masse sliding mechanics were simulated with the finite element method. RESULTS Tipping of the anterior teeth occurred immediately after application of retraction forces. The force system then changed so that the teeth moved almost bodily, and friction occurred at the bracket-wire interface. Net force transferred to the anterior teeth was approximately one fourth of the applied force. The amount of the mesial force acting on the posterior teeth was the same as that acting on the anterior teeth. Irrespective of the amount of friction, the ratio of movement distances between the posterior and anterior teeth was almost the same. By increasing the applied force or decreasing the frictional coefficient, the teeth moved rapidly, but the tipping angle of the anterior teeth increased because of the elastic deflection of the archwire. CONCLUSIONS Finite element simulation clarified the tooth movement and the force system in en-masse sliding mechanics. Long-term tooth movement could not be predicted from the initial force system. The friction was not detrimental to the anchorage. Increasing the applied force or decreasing the friction for rapid tooth movement might result in tipping of the teeth.

Effects of transpalatal arch on molar movement produced by mesial force: a finite element simulation.

INTRODUCTION The transpalatal arch (TPA), which splints together 2 maxillary molars, has been believed to preserve anchorage. The purpose of this study was to clarify this effect from a mechanical point of view. METHODS The finite element method was used to simulate the movement of anchor teeth subjected to mesial forces with and without a TPA. RESULTS In the initial movement produced by elastic deformation of the periodontal ligament, stress magnitude in the periodontal ligament was not changed by the TPA. In the orthodontic movement produced by bone remodeling, the mesial force tipped the anchor teeth irrespective of the TPA. The tipping angles of anchor teeth with and without the TPA were almost the same. The anchor teeth without the TPA were rotated in the occlusal plane and moved transversely. CONCLUSIONS The TPA had no effect on the initial movement. In the orthodontic movement, the TPA had almost no effect, preserving anchorage for mesial movement. However, the TPA prevented rotational and transverse movements of the anchor teeth. These results are valid when the assumptions used in this calculation are satisfied.

Finite element analysis of the effect of force directions on tooth movement in extraction space closure with miniscrew sliding mechanics

INTRODUCTION Miniscrews placed in bone have been used as orthodontic anchorage in extraction space closure with sliding mechanics. The movement patterns of the teeth depend on the force directions. To move the teeth in a desired pattern, the appropriate direction of force must be selected. The purpose of this article is to clarify the relationship between force directions and movement patterns. METHODS By using the finite element method, orthodontic movements were simulated based on the remodeling law of the alveolar bone. The power arm length and the miniscrew position were varied to change the force directions. RESULTS When the power arm was lengthened, rotation of the entire maxillary dentition decreased. The posterior teeth were effective for preventing rotation of the anterior teeth through an archwire. In cases of a high position of a miniscrew, bodily tooth movement was almost achieved. The vertical component of the force produced intrusion or extrusion of the entire dentition. CONCLUSIONS Within the limits of the method, the mechanical simulations demonstrated the effect of force direction on movement patterns.

Distal movement of the mandibular dentition with temporary skeletal anchorage devices to correct a Class III malocclusion

This case report describes the treatment of an 18-year-old man with a skeletal Class III pattern and a full-step Class III malocclusion. The orthodontic treatment included distal movement of the mandibular dentition with temporary skeletal anchorage devices. The total active treatment time was 30 months. His occlusion and facial appearance were significantly improved by the orthodontic treatment. Posttreatment records 2 years later showed excellent results with good occlusion and facial balance.

A finite element simulation of initial movement, orthodontic movement, and the centre of resistance of the maxillary teeth connected with an archwire

The purpose of this article is to simulate long-term movement of maxillary teeth connected with an archwire and to clarify the difference between the initial tooth movement and the long-term orthodontic movement. Initial tooth movement was calculated based on the elastic deformation of the periodontal ligament. Orthodontic tooth movement was simulated based on the bone remodeling law of the alveolar bone, while consequentially updating the force system. In the initial tooth movement, all teeth tipped individually due to an elastic deflection of the archwire. In the long-term movement, the maxillary teeth moved as one united body, as if the archwire were a rigid material. Difference of both movement patterns was due to the change in force system during tooth movement. The long-term movement could not be predicted from the initial tooth movement. Movement pattern and location of the centre of resistance in the long-term movement were almost the same as those in the initial tooth movement as calculated by assuming the archwire to be a rigid material.

Esthetic orthodontic treatment with a double J retractor and temporary anchorage devices

This clinical article reports an esthetic treatment option for managing a Class II malocclusion in an adult. The patient, a woman aged 24 years 2 months, had crowding and a convex profile. She was treated with maxillary first premolar extractions, a double J retractor, and temporary skeletal anchorage devices in the maxillary arch. Posttreatment records after 2 years showed excellent results with good occlusion and long-term stability.

Diffusion of Hydrogen Near an Elasto-Plastically Deformed Crack Tip

The mechanism of hydrogen embrittlemnt(HE) of high strength steels at room temperature has been discussed, with no unified consensus on understanding of its phenomenon itself nor the method of its quantitative evaluation.

Temperature variation of foil strain gage by ambient air condition.

The temperature variations of strain gages, caused by the power dissipated in the gage are calculated by the three-dimensional finite element method. When gages are bonded to materials with lower thermal diffusivities, such as plastic, the rise in the temperature of the gages depends not only upon the area of the grid of the gage, which is often used to characterize the heat-dissipation characteristics of a strain gage, but upon the length and the width of the filament, and the number of filaments. When gages are bonded to materials with higher thermal diffusivities such as steel or alumimum alloy, most of the temperature rise is produced in the base of the gage, so as to depend upon the heat flux per unit area of filament and the thickness of the base. Temperature variations of strain gages caused by ambient-temperature variations with time are also calculated using an axis-symmetrical model. In various conditions of a gage installation, the effective factors which govern temperature variation were clarified.

Prediction of optimal bending angles of a running loop to achieve bodily protraction of a molar using the finite element method

Objective The purpose of this study was to predict the optimal bending angles of a running loop for bodily protraction of the mandibular first molars and to clarify the mechanics of molar tipping and rotation. Methods A three-dimensional finite element model was developed for predicting tooth movement, and a mechanical model based on the beam theory was constructed for clarifying force systems. Results When a running loop without bends was used, the molar tipped mesially by 9.6° and rotated counterclockwise by 5.4°. These angles were almost similar to those predicted by the beam theory. When the amount of tip-back and toe-in angles were 11.5° and 9.9°, respectively, bodily movement of the molar was achieved. When the bend angles were increased to 14.2° and 18.7°, the molar tipped distally by 4.9° and rotated clockwise by 1.5°. Conclusions Bodily movement of a mandibular first molar was achieved during protraction by controlling the tip-back and toe-in angles with the use of a running loop. The beam theory was effective for understanding the mechanics of molar tipping and rotation, as well as for predicting the optimal bending angles.