Change of mechanical properties of Ni-Ti rotary instruments through heat treatment

众所周知，热处理对镍钛旋转仪器的力学性能有很大的影响。本研究的目的是研究热处理对镍钛旋转器械弯曲性能、成形能力和疲劳性能的影响。在400℃(400℃)、450℃(450组)和500℃(500℃)的温度下，对钛合金的修复体进行30分钟的热处理。未经热处理的试件作为对照。采用悬臂弯曲试验评价了热处理引起的弹性变形范围和超弹性变形范围的变化。制作弯曲根管模型。记录准备过程中发生变形和折断的器械。将手术前后的图像进行叠加。测定根尖6 mm内侧和外侧的树脂去除量。旋转弯曲试验以250rpm的速度进行，直到骨折。在距顶端2 mm处，每个标本水平偏转至1.0 mm或1.5 mm。所有试件在每个挠度下都进行了测试。在悬臂弯曲试验中，在弹性范围内，对照组和500组的负荷值高于400组和450组(P&lt；0。05)。在超弹性范围内，对照组的弯曲载荷在所有组中最高(p&lt；0。05)。在根管成形能力方面，与所有热处理组相比，对照组根尖的根管向弯曲的外侧移动更多(p&lt；0。05)。在旋转弯曲试验中，与对照组相比，450组和500组在1.0 mm和1.5 mm挠度下表现出更高的NCF，而400组仅在1.5 mm挠度下表现出更高的NCF(p&lt；0。综上所述，螺纹刀的热处理可以改善其柔韧性和疲劳寿命，使其更有效地制备固化的根管。

It is known that the mechanical behavior of Ni-Ti rotary instruments is strongly affected by heat treatment. The aim of this study was to investigate the effect of heat treatment on bending properties, shaping abilities and fatigue properties of Ni-Ti rotary instruments.Raw Ni. Ti files were heat-treated for 30 minutes at 400(group 400), 450(group 450) and 500(group 500) degrees centigrade. Non heat-treated specimens were used as control. A cantilever-bending test was used to evaluate changes in specimen flexibility both elastic and superelastic deformation range caused by heat treatment. Curved canal models were prepared. Deformed and fractured instruments during preparation were recorded. Pre-and post operative images were superimposed. The amounts of resin removed from both the inner and outer sides of the curvature in the apical 6 mm were determined. The rotational bending test was performed at a rate of 250 rpm until fracture. At 2 mm from the tip, each specimen was deflected horizontally to 1. 0 or 1. 5 mm. All specimens were tested under each deflection. The number of cycles until fracture(NCF) was counted.In the cantilever-bending test, load values of the control group and group 500 were higher than those of groups 400 and 450 at the elastic range(P<0. 05). At the superelastic range, the bending load of the control group was the highest amongst all groups(p<0. 05). Regarding shaping ability, in the control group, root canals at the apex were transported more to the outer side of the curvature compared with those of all heat-treated groups(p<0. 05). In the rotational bending test, as compared to the control group, Groups 450 and 500 showed a higher NCF under 1. 0 and 1. 5 mm deflection, whereas Group 400 showed a higher NCF only under 1. 5 mm deflection(p<0. 05).In conclusion, heat treatment of files might improve their flexibility and fatigue life, making them more effective for preparation of cured canals.