Clinic of Orthodontics and Pediatric Dentistry, Center of Dental Medicine, University of Zurich, Switzerland.
Department of Dental Biomaterials, School of Dentistry, National and Kapodistrian University of Athens, Greece.
Eur J Orthod. 2020 Jan 27;42(1):72-77. doi: 10.1093/ejo/cjz024.
Although ceramic brackets have been extensively used for decades in orthodontics there is not till today any study focusing on the possible deterioration of mechanical properties after in vivo ageing.
To determine whether the mechanical properties of alumina orthodontic brackets change after intraoral ageing thereby assessing the validity of a theoretical model established for the performance of ceramics in wet environments.
Two alumina brackets, one single crystal (Radiance, American Orthodontics, Sheboygan, WI) and one polycrystalline (Clarity, 3M, St. Paul, MN) were included in this study. Ten brackets for each group were collected from different patients after a minimum of 3-month intraoral exposure, whereas as-received brackets of the same manufacturers were used as controls. The specimens were subjected to Raman spectroscopy and were then embedded in epoxy resin and metallographic ground and polished. The mechanical properties of four groups (radiance control: RAC, radiance-retrieved RAR, clarity control: CLC and clarity-retrieved CLR) were determined using instrumented indentation testing according to ISO 14577-2002. The mechanical properties tested were Martens hardness (HM), indentation modulus (EIT), the ratio of elastic to total work, commonly known as elastic index (ηIT), and fracture toughness (KIC). The numerical results were statistically analysed employing two-way analysis of variance (ANOVA) and Tukey multiple comparison test at a = 0.05.
Raman analysis revealed that both brackets are made of a-Al2O3 (corundum). No statistically significant differences were found for HM (N/mm2): RAC = 7249 (1507), RAR = 6926 (1144), CLC = 8052 (1360), CLR = 7390 (2393), or for EIT (GPa): RAC = 141 (27), RAR = 139 (23), CLC = 139 (28), CLR = 131 (47). However, significant differences were identified between the two alumina brackets tested for ηIT (%): RAC = 55.7 (4.2), RAR = 54.0 (3.5), CLC = 62.5 (4.4), CLR = 61.8 (4.7), while KIC was measured only for the polycrystalline bracket (Clarity) because of the complicated fractured pattern of the single-crystal bracket. Both brackets share equal HM and EIT before and after orthodontic intraoral ageing.
Whereas the study assessed the changes after intraoral exposure per theoretical model, which describes the reduction of critical stress to induce fracture after wetting, long-term intraoral ageing could have induced more pronounced effects.
CONCLUSIONS/IMPLICATIONS: The results of this study indicate that 3 months of intraoral ageing do not change the mechanical properties of single-crystal and polycrystalline orthodontic brackets tested, thus indicating that the Griffith theory may not be applied to the case of manufactured ceramic brackets owing possibly to internal defects.
尽管陶瓷托槽在正畸学中已经广泛使用了几十年，但直到今天，仍没有任何研究关注体内老化后机械性能可能的恶化。
确定氧化铝正畸托槽在体内老化后机械性能是否发生变化，从而评估为湿环境中陶瓷性能建立的理论模型的有效性。
本研究纳入了一种单晶氧化铝托槽（Radiance，美国 Orthodontics，Sheboygan，WI）和一种多晶氧化铝托槽（Clarity，3M，St. Paul，MN）。从不同患者的口腔内至少 3 个月的暴露后收集每组 10 个托槽，而相同制造商的原始托槽用作对照。对标本进行拉曼光谱分析，然后嵌入环氧树脂中并进行金相研磨和抛光。根据 ISO 14577-2002 标准，使用仪器压痕测试法确定 4 组（Radiance 对照：RAC、回收的 Radiance：RAR、Clarity 对照：CLC 和回收的 Clarity：CLR）的机械性能。测试的机械性能有马氏体硬度（HM）、压痕模量（EIT）、弹性与总功的比值，通常称为弹性指数（ηIT）和断裂韧性（KIC）。采用方差分析（ANOVA）和 Tukey 多重比较检验（α=0.05）对数值结果进行统计学分析。
拉曼分析表明，这两种托槽均由 α-Al2O3（刚玉）制成。在 HM（N/mm2）方面没有发现统计学差异：RAC=7249（1507），RAR=6926（1144），CLC=8052（1360），CLR=7390（2393），或在 EIT（GPa）方面没有发现统计学差异：RAC=141（27），RAR=139（23），CLC=139（28），CLR=131（47）。然而，在 ηIT（%）方面，两种氧化铝托槽之间存在显著差异：RAC=55.7（4.2），RAR=54.0（3.5），CLC=62.5（4.4），CLR=61.8（4.7），而由于单晶托槽的断裂模式复杂，仅对多晶托槽（Clarity）进行了 KIC 测量。在正畸口腔内老化前后，两种托槽的 HM 和 EIT 相同。
虽然该研究根据描述湿化后引发断裂的临界应力降低的理论模型评估了体内暴露后的变化，但长期的口腔内老化可能会产生更明显的影响。
结论/意义：本研究结果表明，3 个月的口腔内老化不会改变测试的单晶和多晶正畸托槽的机械性能，这表明 Griffith 理论可能不适用于制造陶瓷托槽的情况，因为可能存在内部缺陷。
