Morita Yukiko, Koga Yoshiyuki, Nguyen Tuan Anh, Yoshida Noriaki
Department of Orthodontics and Dentofacial Orthopedics, Nagasaki University Graduate School of Biomedical Sciences, Nagasaki, Japan.
Korean J Orthod. 2020 Jul 25;50(4):268-277. doi: 10.4041/kjod.2020.50.4.268.
This case report demonstrates two different uprighting mechanics separately applied to mesially tipped mandibular first and second molars. The biomechanical considerations for application of these mechanisms are also discussed. For repositioning of the first molar, which was severely tipped and deeply impacted, a novel cantilever mechanics was used. The molar tube was bonded in the buccolingual direction to facilitate insertion of a cantilever from the buccal side. By twisting the distal end of the cantilever, sufficient uprighting moment was generated. The mesial end of the cantilever was hooked over the miniscrew placed between the canine and first premolar, which could prevent exertion of an intrusive force to the anterior portion of the dentition as a side effect. For repositioning of the second molar, an uprighting mechanics using a compression force with two step bends incorporated into a nickel-titanium archwire was employed. This generated an uprighting moment as well as a distal force acting on the tipped second molar to regain the lost space for the first molar and bring it into its normal position. This epoch-making uprighting mechanics could also minimize the extrusion of the molar, thereby preventing occlusal interference by increasing interocclusal clearance between the inferiorly placed two step bends and the antagonist tooth. Consequently, the two step bends could help prevent occlusal interference. After 2 years and 11 months of active treatment, a desirable Class I occlusion was successfully achieved without permanent tooth extraction.
本病例报告展示了分别应用于近中倾斜的下颌第一和第二磨牙的两种不同直立力学机制。还讨论了应用这些机制的生物力学考量。对于严重倾斜且深度阻生的第一磨牙的重新定位，采用了一种新颖的悬臂力学机制。磨牙管沿颊舌向粘结，以便于从颊侧插入悬臂。通过扭转悬臂的远端，产生了足够的直立力矩。悬臂的近端钩在置于尖牙和第一前磨牙之间的微型螺钉上，这可以防止作为副作用对牙列前部施加侵入力。对于第二磨牙的重新定位，采用了一种直立力学机制，该机制利用了结合在镍钛弓丝中的具有两步弯曲的压缩力。这产生了一个直立力矩以及一个作用于倾斜的第二磨牙的远中力，以恢复第一磨牙失去的间隙并使其回到正常位置。这种开创性的直立力学机制还可以最大限度地减少磨牙的伸长，从而通过增加位于下方的两步弯曲与对颌牙之间的咬合间隙来防止咬合干扰。因此，两步弯曲有助于防止咬合干扰。经过2年11个月的积极治疗，成功实现了理想的I类咬合关系，且未拔除恒牙。
