Zinelis Spiros, Eliades Theodore, Pandis Nikolaos, Eliades George, Bourauel Christoph
Department of Biomaterials, School of Dentistry, University of Athens, Athens, Greece.
Am J Orthod Dentofacial Orthop. 2007 Jul;132(1):84-9. doi: 10.1016/j.ajodo.2005.11.039.
The aim of this study was to characterize intraorally fractured nickel-titanium (Ni-Ti) archwires, determine the type of fracture, assess changes in the alloy's hardness and structure, and propose a mechanism of failure.
Eleven Ni-Ti SE 200 and 19 copper-Ni-Ti (both, Ormco, Glendora, Calif) intraorally fractured archwires were collected. The location of fracture (anterior or posterior), wire type, cross section, and period of service before fracture were recorded. The retrieved wires and brand-, type-, and size-matched specimens of unused wires were subjected to scanning electron microscopy to assess the fracture type and morphological variation of fracture site of retrieved specimens, and to Vickers hardness (HV200) testing to investigate the hardness of as-received and in-vivo fractured specimens. Fracture site distribution was statistically analyzed with the chi-square test (alpha = 0.05), whereas the results of the hardness testing were analyzed with 2-way ANOVA with state (control vs in-vivo fractured) and composition (Ni-Ti SE vs copper-Ni-Ti) serving as discriminating variables and the Student-Newman-Keuls test at the 95% confidence level.
The fracture site distribution showed a preferential location at the midspan between the premolar and the molar, suggesting that masticatory forces and complex loading during engagement of the wire to the bracket slot and potential intraoral aging might account for fracture incidence. All retrieved wires had the distinct features of brittle fracture without plastic deformation or crack propagation, whereas no increase in hardness was observed for the retrieved specimens.
Most fractures sites were in the posterior region of the arch, probably because of the high-magnitude masticatory forces. Brittle fracture without plastic deformation was observed in most Ni-Ti wires regardless of archwire composition. There was no increase in the hardness of the intraorally exposed specimens regardless of wire type. This contradicts previous in-vitro studies and rules out hydrogen embrittlement as the cause of fracture.
本研究旨在对口腔内折断的镍钛（Ni-Ti）弓丝进行特性分析，确定骨折类型，评估合金硬度和结构的变化，并提出失效机制。
收集了11根口腔内折断的Ni-Ti SE 200和19根铜镍钛（均为奥美科公司，加利福尼亚州格伦多拉市）弓丝。记录骨折位置（前部或后部）、弓丝类型、横截面以及骨折前的使用期限。将回收的弓丝和未使用的弓丝的品牌、类型和尺寸匹配的样本进行扫描电子显微镜检查，以评估回收样本的骨折类型和骨折部位的形态变化，并进行维氏硬度（HV200）测试，以研究接收时和体内骨折样本的硬度。骨折部位分布采用卡方检验进行统计分析（α = 0.05），而硬度测试结果采用双向方差分析，以状态（对照与体内骨折）和成分（Ni-Ti SE与铜镍钛）作为区分变量，并在95%置信水平下进行Student-Newman-Keuls检验。
骨折部位分布显示，骨折优先发生在前磨牙和磨牙之间的中跨处，这表明咀嚼力、弓丝与托槽槽沟啮合过程中的复杂载荷以及潜在的口腔内老化可能是骨折发生率的原因。所有回收的弓丝均具有脆性骨折的明显特征，无塑性变形或裂纹扩展，而回收样本的硬度未观察到增加。
大多数骨折部位位于牙弓后部，可能是由于咀嚼力较大。无论弓丝成分如何，大多数Ni-Ti弓丝均观察到无塑性变形的脆性骨折。无论弓丝类型如何，口腔内暴露样本的硬度均未增加。这与先前的体外研究相矛盾，并排除了氢脆作为骨折原因的可能性。
