Abstract
目的
比较不同温度下3种新型热处理镍钛锉在模拟S形根管内的抗疲劳折断性能，为临床应用提供依据。
方法
选取Gold热处理镍钛锉TruNatomy [25 mm长，26号/0.04锥度(尖段)]与ProTaper Gold [25 mm长，25号/0.08锥度(尖段)]作为实验组，M丝技术镍钛锉ProTaper Next [25 mm长，25号/0.06锥度(尖段)]作为对照组，每种镍钛锉各20支。TruNatomy镍钛锉经过Gold热处理使R相变分离，存在完全的中间R相，使柔软度提高；ProTaper Gold镍钛锉为CM丝镍钛锉，用热处理提高相变温度，常温下以马氏体为主，使柔软度提高，表面经Gold热处理产生中间R相；ProTaper Next镍钛锉则采用M丝技术，M丝常温下以奥氏体为主，经过热机械处理引入硬化马氏体，此类马氏体不参与相变，其弹性模量低于奥氏体，从而使镍钛锉柔软度增高。使用双弯金属模拟根管，分别在室温(24 ℃)及加热至65 ℃时检测3种镍钛锉的抗疲劳折断性能，不同温度下的每种镍钛锉各10支(n=10)。对相同温度下不同镍钛锉的疲劳折断圈数(number of cyclic fatigue，NCF)以及折断段长度数据采用单因素方差分析，并进行组间比较，以双侧P < 0.05为差异有统计学意义。对相同镍钛锉不同温度下的NCF以及折断段长度数据采用配对样本t检验，并进行两两比较，检验水准均为双侧α=0.05。用扫描电镜进行折断镍钛锉断口分析。
结果
3种镍钛锉均为在根尖弯曲处首先发生折断，随后在冠方弯曲处发生折断。室温下，根尖段NCF：Tru- Natomy镍钛锉为344.4±96.6，ProTaper Gold镍钛锉为175.0±56.1，ProTaper Next镍钛锉为133.3±39.7，TruNatomy镍钛锉最高(P < 0.05)；冠方段NCF：TruNatomy镍钛锉为618.3±75.3，ProTaper Gold镍钛锉为327.5±111.8，ProTaper Next镍钛锉为376.6±67.9，TruNatomy镍钛锉仍为最高(P < 0.05)。根尖及冠方折断段长度各组间差异无统计学意义(P>0.05)。加热至65 ℃，根尖段NCF：TruNatomy镍钛锉为289.6±65.8，ProTaper Gold镍钛锉为187.5±75.4，ProTaper Next镍钛锉为103.0±38.5，TruNatomy镍钛锉最高(P < 0.05)；冠方段NCF：TruNatomy镍钛锉为454.2±45.4，ProTaper Gold镍钛锉为268.3±31.4，ProTaper Next镍钛锉为283.8±31.7，TruNatomy镍钛锉仍为最高(P < 0.05)。与室温相比，TruNatomy镍钛锉冠方段在65 ℃时的NCF显著下降(P < 0.05)。各组镍钛锉断口均符合典型的疲劳折断断口特征。
结论
Gold热处理镍钛锉在S形弯曲根管中的抗疲劳折断性能优于M丝镍钛锉。
Keywords: 镍钛根管锉, S形根管, 抗疲劳折断性能, 热处理
Abstract
Objective
To compare the cyclic fatigue resistance of nickel-titanium files made by 3 new heat treatment in simulated S-shaped root canals at different temperatures.
Methods
Gold heat-treated nickel-titanium files TruNatomy (25 mm, tip size 26#/0.04) and ProTaper Gold (25 mm, tip size 25#/0.08) were selected as the experimental group, M wire technique nickel-titanium file ProTaper Next (25 mm, tip size 25#/0.06) was selected as the control group. It was speculated that the Gold technique used in TruNatomy nickel-titanium file was R phase separation technique, which included a complete intermediate R-phase, increasing its flexibility. ProTaper Gold was a CM wire nickel-titanium file and the increased phase transformation temperature by heat treatment introduced martensite at room temperature, while it underwent gold heat treatment on the surface, generating an intermediate R phase during phase transformation, providing hyperelastic. ProTaper Next used M wire technique, M wire included austenite at room temperature, where heat mechanical processing introduced hardened martensite, which was incapable of participating phase transformation. Because of the lower elastic modulus of hardened martensite than austenite, the flexibility of the file was increased. Twenty instruments of each nickel-titanium file were submitted to the cyclic fatigue test by using a simulated canal with double curvatures at room tem-perature (24 ℃) and 65 ℃, 10 instruments of each nickel-titanium file were selected at each temperature (n=10). At the same temperature, the number of cyclic fatigue (NCF) and fragment length were analyzed by using One-Way analysis of variance at a significance level of P < 0.05. NCF and fragment length of the same nickel-titanium file at room temperature and 65 ℃ were compared by paired sample t test and the significance level was α=0.05. Fractured surfaces were analyzed by using scanning electron microscope.
Results
In double-curved canals, all the failure of the files due to cyclic fatigue was first seen in the apical curvature before the coronal curvature. At room temperature, in the apical curvature, NCF of TruNatomy was 344.4±96.6, ProTaper Gold was 175.0±56.1, ProTaper Next was 133.3±39.7, NCF of Tru Natomy was the highest (P < 0.05). In the coronal curvature, NCF of TruNatomy was 618.3± 75.3, ProTaper Gold was 327.5±111.8, ProTaper Next was 376.6±67.9, NCF of TruNatomy was also the highest (P < 0.05). There was no significant difference among the apical and coronal fragment length of the 3 nickel-titanium files (P>0.05). At 65 ℃, in the apical curvature, NCF of TruNatomy was 289.6±65.8, ProTaper Gold was 187.5±75.4, ProTaper Next was 103.0±38.5, NCF of TruNatomy was the highest (P < 0.05). In the coronal curvature, NCF of TruNatomy was 454.2±45.4, ProTaper Gold was 268.3±31.4, ProTaper Next was 283.8±31.7, NCF of TruNatomy was also the highest (P < 0.05). The apical fragment length of ProTaper Next was the highest (P < 0.05), and there was no significant difference among coronal fragment length of the 3 nickel-titanium files (P>0.05). Compared with room temperature, at 65 ℃, in the coronal curvature, NCF of TruNatomy decreased significantly (P < 0.05). The fractured surfaces of the three nickel-titanium files demonstrated typical cyclic fatigue.
Conclusion
Gold heat-treated nickel-titanium file had better cyclic fatigue resistance than M wire nickel-titanium file in S-shaped root canals.
Keywords: Nickel-titanium endodontic files, S-shaped canal, Cyclic fatigue resistance, Heat treatment
S形根管是临床根管治疗中常见的疑难情况。Zheng等[1]对299个下颌切牙进行研究，结果显示，S形根管发生率为41.8%。此外，三根的下颌第一磨牙远舌根管S形的发生率高达60%[2]。S形根管由于有两个角度不同的弯曲，连续旋转镍钛锉进行根管预备时，镍钛锉对根管壁的应力响应较为复杂，其内部应力分布亦呈现多个应力集中区，因此，易于发生镍钛锉折断[3]。
近年来，热处理镍钛锉逐渐成为临床应用的主流镍钛锉，因其柔软顺应性好，根管成形效果优越，所以对S形根管预备也多选用该类镍钛锉。但是，对各种热处理镍钛锉用于S形根管的抗疲劳折断性能对比研究较少，仅见Topuolu等[4]对S形根管的研究，发现CM丝镍钛锉优于M丝镍钛锉，CM丝和M丝是镍钛锉常用金属丝，CM丝常温下主要为马氏体相，M丝常温下主要为奥氏体相，而更新型的Gold热处理镍钛锉，在S形根管内抗疲劳折断性能相关研究则更少。此外，热处理镍钛锉的相变温度为60 ℃左右[5]，当镍钛锉在根管内旋转与根管壁摩擦产热，根管内温度上升到60 ℃以上时，可能导致热处理镍钛锉构成相的变化，从而导致抗疲劳折断性能变化。也有报道使用加热次氯酸钠进行根管冲洗发现可增加消毒效果[6]，但同时温度上升会导致镍钛锉抗疲劳折断性能变化，因此，有必要研究在高温下镍钛锉的抗疲劳折断性能。
本研究选取新型热处理镍钛锉为研究对象，使用S形根管模型研究不同温度下镍钛锉的抗疲劳折断性能，以便为S形根管预备的临床治疗镍钛锉选择提供理论依据。
1. 资料与方法
1.1. 实验分组
选取3种镍钛锉，实验组分别为Gold热处理镍钛锉TruNatomy[25 mm长，26号/0.04锥度(尖段)，登士柏公司，美国]及ProTaper Gold[25 mm长，25号/0.08锥度(尖段)，登士柏公司，美国]，对照组为M丝技术镍钛锉ProTaper Next[25 mm长，25号/0.06锥度(尖段)，登士柏公司，美国]，每种镍钛锉各20支，在体视显微镜(SZ760，奥特公司，中国)40倍下观察，确认锉刃无明显缺损。
1.2. 抗疲劳折断性能实验
将减速手机(16 ∶ 1 ENDO-MATE DT，NSK公司，日本)固定于自制S形金属模拟根管旁，使用口腔科手术显微镜(OPMI Sensera，Carl Zeiss公司，德国)放大10倍确认镍钛锉尖与S形金属模拟根管末端平齐。将数码相机垂直于S形金属模拟根管照相, 使用ImageJ2x软件测量, 确认镍钛锉的根尖弯曲角度70°，弯曲半径2 mm，最大弯曲点位于距根尖2 mm处，冠方弯曲角度60°，弯曲半径5 mm，最大弯曲点位于距根尖8 mm处(图 1)。
图 1.
S形金属模拟根管用于测试镍钛锉抗疲劳折断性能
Metal simulated S-shaped root canal was used to test the cyclic fatigue resistance of nickel-titanium files
将减速手机连接于扭矩控制马达，TruNatomy镍钛锉转速为500 r/min，ProTaper Gold镍钛锉、ProTaper Next镍钛锉转速为300 r/min, 扭矩均设为5.2 N·cm。S形金属模拟根管内涂布润滑油(联合利华公司，中国), 开启马达, 镍钛锉旋转至折断, 使用摄像设备拍摄整个过程, 回放录像, 记录折断时间(s)。
热处理镍钛锉的相变温度为60 ℃左右[5]，当镍钛锉在根管内旋转摩擦产热，或者使用加热次氯酸钠冲洗液[6]，可能使根管内温度上升到60 ℃以上，导致热处理镍钛锉构成相的变化，从而导致抗疲劳折断性能变化，因此，本研究设置在室温(24 ℃)及用加热罩(BW001，井瓷公司，中国)加热至65 ℃时分别检测各镍钛锉的抗疲劳折断性能，不同温度条件下的每种镍钛锉各10支(n=10)。
1.3. 镍钛锉抗疲劳折断性能的评价指标
1.3.1. 疲劳折断圈数(number of cyclic fatigue，NCF)
由折断时间(s)乘以每秒旋转圈数得到。
1.3.2. 折断段长度
口腔科手术显微镜(10倍)下使用数显卡尺(精度为0.01 mm, 111-101，景如公司，中国)测量镍钛锉根尖方及冠方折断段长度(mm)。
1.3.3. 折断断口分析
将镍钛锉剩余段交替浸泡于无水乙醇和蒸馏水中, 超声振荡清洗各3次，每次20 min，自然干燥。室温下使用导电胶布把镍钛锉剩余段固定于扫描电镜(SU-8010, Hitachi公司, 日本)样品台上，使用扫描电镜在不同放大倍数下观察、拍摄断口形态。用ImageJ2x软件测量各镍钛锉冠方及根尖断面的核心金属面积，即横截面的内切圆面积，以扫描电镜图的标尺作为参照。
1.4. 统计学分析
使用SPSS 24.0统计软件，用单样本K-S检验对相同温度下各镍钛锉的NCF及折断段长度数据进行正态性检验, 组间比较使用单因素方差分析, 组间两两比较采用SNK法, 以双侧P < 0.05为差异有统计学意义, 并进行多重检验修正。
对室温以及加热条件下的同一镍钛锉的NCF以及折断段长度数据采用配对样本t检验进行两两比较，检验水准为双侧α=0.05。
2. 结果
2.1. NCF及折断长度
各温度下3种镍钛锉均为在根尖弯曲处首先发生折断，随后在冠方弯曲处发生折断(表 1、2)。室温下，TruNatomy镍钛锉的根尖段NCF及冠方段NCF最高(P < 0.05)，根尖及冠方折断段长度各组间差异无统计学意义(P>0.05)。加热至65 ℃后，TruNatomy镍钛锉的根尖段NCF及冠方段NCF最高(P < 0.05)。ProTaper Next镍钛锉的根尖折断段长度最高(P < 0.05)，冠方折断段长度各组间差异无统计学意义(P>0.05)。加热至65 ℃后，3种镍钛锉根尖段与冠方段的NCF均下降，其中Tru-Natomy镍钛锉冠方段的NCF显著下降(P < 0.05)。各组镍钛锉根尖及冠方段折断段长度随温度上升无改变(P>0.05)。
表 1.
3种镍钛锉的根尖段NCF和折断段长度
Comparison of apical NCF and fragment length of the three nickel-titanium files
| Nickel-titanium files | Apical NCF，x±s | Apical fragment length/mm，x±s | |||||||
| 24 ℃ | 65 ℃ | t | P | 24 ℃ | 65 ℃ | t | P | ||
| * P < 0.05，TruNatomy vs. ProTaper Gold and ProTaper Next; # P < 0.05，ProTaper Next vs. TruNatomy and ProTaper Gold. NCF, number of cyclic fatigue. | |||||||||
| TruNatomy | 344.4±96.6* | 289.6±65.5* | 0.903 | 0.408 | 2.3±0.5 | 2.2±0.6 | 0.328 | 0.752 | |
| ProTaper Gold | 175.0±56.1 | 187.5±75.4 | 0.286 | 0.783 | 2.0±0.4 | 2.1±0.3 | -0.447 | 0.671 | |
| ProTaper Next | 133.3±39.7 | 103.0±38.5 | 1.283 | 0.233 | 2.2±0.6 | 2.9±0.3# | -2.015 | 0.075 | |
| F | 12.894 | 10.753 | 0.256 | 5.106 | |||||
| P | 0.001 | 0.003 | 0.779 | 0.027 | |||||
表 2.
3种镍钛锉的冠方段NCF和折断段长度
Comparison of coronal NCF and fragment length of the three nickel-titanium files
| Nickel-titanium files | Coronal NCF，x±s | Coronal fragment length/mm，x±s | |||||||
| 24 ℃ | 65 ℃ | t | P | 24 ℃ | 65 ℃ | t | P | ||
| * P < 0.05，TruNatomy vs. ProTaper Gold and ProTaper Next; # P < 0.05，TruNatomy at 24 ℃ vs. TruNatomy at 65 ℃. NCF, number of cyclic fatigue. | |||||||||
| TruNatomy | 618.3±75.3*# | 454.2±45.4* | 3.810 | 0.007 | 4.8±0.3 | 4.8±0.2 | -0.078 | 0.940 | |
| ProTaper Gold | 327.5±111.8 | 268.3±31.4 | 1.259 | 0.244 | 4.5±0.4 | 4.8±0.2 | -1.170 | 0.286 | |
| ProTaper Next | 376.7±67.9 | 283.8±31.7 | 2.459 | 0.057 | 4.6±0.4 | 4.5±0.5 | 0.266 | 0.796 | |
| F | 14.034 | 36.102 | 0.424 | 0.786 | |||||
| P | 0.002 | 0.001 | 0.665 | 0.480 | |||||
2.2. 断口分析
3种镍钛锉断口均可见裂纹源区、裂纹扩展区、瞬间折断区(韧窝区)，符合典型的疲劳折断断口特征，在裂纹扩展区可见典型的海滩状疲劳条纹，在瞬间折断区可见大量韧窝结构，属于韧性折断(图 2)。计算得到的各镍钛锉断面的核心金属面积见表 3。
图 2.
3种镍钛锉折断的断口形态
Fractured surfaces of the three nickel-titanium files
A1, B1, C1, fractured surfaces of TruNatomy, ProTaper Gold, ProTaper Next' s coronal section, respectively; A2, B2, C2, fractured surfaces of TruNatomy, ProTaper Gold, ProTaper Next' s apical section, respectively; Crack origins (red arrow), crack propagation areas (white line area) and dimple areas (residual area of cross section) were identified (×150); D, typical crack origins (black arrow) and crack propagation areas (white arrow) of ProTaper Next (×3 000); E, crack propagation areas of ProTaper Next, beach-like fatigue stripes (yellow arrows) were identified (×3 000).
表 3.
3种镍钛锉的核心金属面积
Center-core area of the three nickel-titanium files
| Nickel-titanium files | Coronal center-core area/mm2 | Apical center-core area/mm2 |
| TruNatomy | 0.081 | 0.039 |
| ProTaper Gold | 0.173 | 0.064 |
| ProTaper Next | 0.122 | 0.030 |
3. 讨论
镍钛合金的机械性能使根管治疗锉可以更好地适应复杂根管弯曲，相比不锈钢锉[7]顺应性更好、磨损更少[8], 但是，旋转镍钛锉仍有较高的折断风险[9]。旋转镍钛锉断裂的原因包括根管解剖结构的变化，如根管弯曲度[10]。同时，镍钛锉尺寸、锥度、合金成分、制造方法、弹性和硬度也会影响它的临床表现[11-12]。另外，镍钛锉的横断面几何形状和凹槽设计也会对镍钛锉的力学行为产生重大影响[11-12]。
根管弯曲度是可能导致镍钛锉疲劳折断的最重要变量之一[10]。在临床条件下，同一根管通路中可能出现两个弯曲，这种几何形状被称为“S”形。针对根管弯曲程度的影像学研究表明，双弯曲(S形)根管发生率较高[1-2]，这是根管治疗的难点[3]，也是使用镍钛旋转锉最具挑战性的临床情境之一。目前关于S形根管内镍钛锉机械性能的相关信息很少。
本研究结果显示，镍钛锉根尖段与冠方段相比，抗疲劳折断性能较低。镍钛锉总是在根尖段先发生断裂，这是由于对同一支镍钛锉而言，根尖段的直径小于冠方段。以往的研究[13]显示，随着镍钛锉直径增加，NCF增加，与本研究结果一致。S形根管根尖段折断镍钛锉的取出极为困难，提示临床工作要警惕并积极识别S形根管，对其预备要选取全新的新型镍钛锉，谨防镍钛锉折断。
关于NCF的比较，有研究显示Gold热处理技术的ProTaper Gold镍钛锉、WaveOne Gold镍钛锉的抗疲劳折断性能优于ProTaper Next镍钛锉(M丝)[3, 14]，原因可能是Gold热处理技术可以产生双阶段特定相变行为，即马氏体向R相的相变与R相向奥氏体的后续相变，产生中间相R相，由于R相变需要的剪切应力是马氏体相变的1/10，使Gold镍钛锉柔软度提高[15]。本研究中，Gold热处理技术的TruNatomy镍钛锉室温下在根尖段和冠方段的NCF显著高于ProTaper Next镍钛锉，亦验证了Gold热处理技术的优势。同时，与同为Gold热处理技术的ProTaper Gold镍钛锉相比，TruNatomy镍钛锉也表现优越，可能的原因在于核心金属面积不同，TruNatomy镍钛锉的尖段锥度减小到0.04，相比ProTaper Gold镍钛锉的0.08，TruNatomy镍钛锉的核心金属面积降低近一半(表 3)。镍钛锉的横截面积及核心金属面积也与镍钛锉的抗疲劳折断性能息息相关[16-17]，有研究显示，当镍钛锉的核心金属面积较小时，抗疲劳折断性能提高[18]，因此，本研究中，TruNatomy镍钛锉因其优越的金属弹性以及较小的核心金属面积，使其抗疲劳折断性能提升。
本研究结果显示，在温度加热至65 ℃后，3种镍钛锉的NCF均下降，其中TruNatomy镍钛锉冠方段的NCF显著下降(P < 0.05)。根据目前文献[19-21]中TruNatomy镍钛锉的相变温度及曲线，TruNatomy镍钛锉的相变温度为25.9 ℃，在室温24 ℃下，Tru-Natomy镍钛锉由奥氏体、马氏体及R相组成，当温度升高后，奥氏体成分升高，NCF下降，与本研究结果相符。同时在其差示扫描量热仪的加热曲线上观察到两个独立的吸热峰[21]，说明其R相变抽离充分，存在完全的中间R相，因此，其表现出超弹性，抗疲劳折断性能优越。ProTaper Gold镍钛锉为表面经过Gold热处理的CM丝镍钛锉，相变温度一般为50~52 ℃[15]，同时在其差示扫描量热仪的加热曲线上观察到两个重叠的吸热峰，表明在相变过程中产生了中间R相，因此，在常温24 ℃下，ProTaper Gold镍钛锉主要以马氏体为主，当温度升高后，奥氏体成分升高，马氏体成分下降，使镍钛锉弹性下降，但同时由于存在R相变，R相的弹性模量不仅低于奥氏体相，还低于马氏体相，因此，温度升高至65 ℃，相较于常温24 ℃，ProTaper Gold镍钛锉的NCF下降差异无统计学意义。ProTaper Next镍钛锉为M丝镍钛锉，其相变温度一般为43~50 ℃[22]。在室温下，ProTaper Next镍钛锉含有奥氏体相和大量加工硬化马氏体结构[23]，因此，可保持超弹性，当温度升高至65 ℃时，奥氏体成分相应升高，由于奥氏体的高弹性模量，导致镍钛锉弹性下降，但因为ProTaper Next镍钛锉中的大量加工硬化马氏体不随温度变化发生相变，因而其抗疲劳折断性能下降，但ProTaper Next镍钛锉在常温24 ℃和65 ℃的NCF差异无统计学意义。
本研究中，镍钛锉折断长度方面，室温下根尖及冠方折断段长度各组间差异无统计学意义(P>0.05)，加热至65 ℃，根尖折断段长度在ProTaper Next镍钛锉最高(P < 0.05)，冠方折断段长度各组间差异无统计学意义(P>0.05)，同时，温度的上升未导致镍钛锉折断段长度变化(P>0.05)。Kaval等[24]的研究结果显示，折断段长度的差异可能是由于镍钛锉不同的金属特性和横断面设计。本研究显示，ProTaper Gold镍钛锉的横截面为圆三角形，TruNatomy镍钛锉、ProTaper Next镍钛锉的横截面为矩形(图 2)，当镍钛锉在模拟金属根管内旋转时，最大弯曲点的位置不完全相同，因此，虽然折断段长度不一致，但是均位于弯曲最大位置附近。
综上所述，不同的特殊热处理和加工方式对镍钛锉的力学性能产生了重要影响。同时，镍钛锉的锥度、横断面形状、环境温度等因素也对其抗疲劳折断性能产生影响。本研究提示在预备S形根管时，镍钛锉中的疲劳积累迅速，镍钛锉根尖段在使用很短时间后会发生断裂。根据本研究的结果，在复杂弯曲根管和怀疑存在S形弯曲的根管内使用镍钛锉时，应严格限制使用时间及次数。
Funding Statement
北京市自然科学基金(L242135)和国家重点研发计划政府间国际科技创新合作项目(2024YFE0107100)
Supported by Beijing Natural Science Foundation (L242135) and National Key Research and Development Program for Government-to-Government International Scientific and Technological Cooperation (2024YFE0107100)
Footnotes
利益冲突 所有作者均声明不存在利益冲突。
作者贡献声明 陈文新：设计研究方案，收集、分析、整理数据，撰写论文；侯晓玫：提出研究思路，设计研究方案，总体把关和审定论文。
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