Ghorbanyjavadpour Fataneh, Kazemi Parisa, Moradinezhad Mehrnaz, Rakhshan Vahid
Orthodontics Department, School of Dentistry, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.
Orthodontics Department, School of Dentistry, lIam University of Medical Science, lIam, Iran.
Int Orthod. 2019 Dec;17(4):758-768. doi: 10.1016/j.ortho.2019.08.013. Epub 2019 Sep 4.
Initial stability of miniscrews is an important factor in their success as orthodontic anchorages. One of the factors affecting this stability is the stresses exerted to the bone by the screw. Since the distribution and extent of stresses and strains produced during insertion or removal of miniscrews had not been measured before, this study used finite element analysis (FEA) to measure these parameters in tapered versus cylindrical screws with or without pilot sockets.
An FEA model of maxilla, pilot hole, and tapered/cylindrical miniscrews were created from 875 CT scan data. The bone cortex was considered 2mm thick. The cancellous bone was reconstructed below the cortical bone. Miniscrews were modelled on the basis of commercial titanium tapered and cylindrical miniscrews (1.6mm wide, 8mm long). The diameter and length of the guiding hole were considered to be 1.1 and 1.5mm, respectively. The miniscrews were inserted (and removed) between the maxillary second premolar and first molar. Stress/strain produced in the bones or screws were measured.
During screw insertion, in all setups, the highest stress existed within both the bone and screw, when the screw was in the cortical bone; after insertion into the cancellous bone, the stress suddenly dropped. In cylindrical screws, the highest amount of stress was distributed around the neck which was used for screw driving. In tapered screws, the stress was mostly distributed around the front one-third of the screw. During screw removal, the results of four setups were rather similar with stresses concentrated around screw necks, in the depth of the screw hole, and around the bone surface. The greatest bone stress during insertion was caused by the pilot-less tapered screw (10.18MPa) and the lowest stress was exerted by a pilot-less cylindrical screw (0.74MPa).
Most of the stress and strain is tolerated by the cortical bone and not the cancellous one. Using cylindrical miniscrews might be more bone-friendly. However, all cases had stresses below tolerable thresholds, and hence are safe.
微螺钉的初始稳定性是其作为正畸支抗成功的重要因素。影响这种稳定性的因素之一是螺钉对骨施加的应力。由于在微螺钉植入或取出过程中产生的应力和应变的分布及程度此前尚未被测量，本研究采用有限元分析（FEA）来测量有或无引导孔的锥形与圆柱形螺钉的这些参数。
利用875份CT扫描数据创建了上颌骨、引导孔及锥形/圆柱形微螺钉的有限元分析模型。骨皮质被视为2毫米厚。松质骨在皮质骨下方重建。微螺钉基于商用钛制锥形和圆柱形微螺钉（宽1.6毫米，长8毫米）进行建模。引导孔的直径和长度分别被视为1.1毫米和1.5毫米。微螺钉在上颌第二前磨牙和第一磨牙之间植入（及取出）。测量骨或螺钉中产生的应力/应变。
在螺钉植入过程中，在所有设置下，当螺钉位于皮质骨内时，骨和螺钉内的应力最高；插入松质骨后，应力突然下降。在圆柱形螺钉中，最高应力分布在用于拧动螺钉的颈部周围。在锥形螺钉中，应力大多分布在螺钉前三分之一周围。在螺钉取出过程中，四种设置的结果相当相似，应力集中在螺钉颈部周围、螺钉孔深处及骨表面周围。植入过程中最大的骨应力由无引导孔的锥形螺钉引起（10.18兆帕），而无引导孔的圆柱形螺钉施加的应力最低（0.74兆帕）。
大部分应力和应变由皮质骨而非松质骨承受。使用圆柱形微螺钉可能对骨更友好。然而，所有情况的应力均低于可耐受阈值，因此是安全的。
