Chinese Journal of Tissue Engineering Research ›› 2013, Vol. 17 ›› Issue (29): 5403-5408.doi: 10.3969/j.issn.2095-4344.2013.29.021
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Zhan Liu1, Xie Shu-juan1, Pan Wei-hong2
Received:
2012-11-18
Revised:
2013-03-25
Online:
2013-07-22
Published:
2013-07-22
Contact:
Pan Wei-hong, Associate professor, Union Hospital, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, Hubei Province, China
pweihong@sina.com
About author:
Zhan Liu★, Master, Affiliated Hospital, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province, China
bravezhanliu_1986@126.com
Supported by:
the Fundamental Research Funds for the Central Universities, No. 2010JC030*
CLC Number:
Zhan Liu, Xie Shu-juan, Pan Wei-hong. Three-dimensional finite element analysis of prosthesis stress variation[J]. Chinese Journal of Tissue Engineering Research, 2013, 17(29): 5403-5408.
2.1 有限元建模方法的进展 随着计算机技术,三维影像技术的发展,建模技术经历了3个阶段。最初,美国国立医学图书馆开展数字化虚拟技术,采用切片、磨片法获得数字模型,即用组织锯片机切割组织块后,分别用800,1 200,2 000目的砂纸磨片,直到达到最终所需的切片厚度。二维模型建立过程复杂,错误率高,且耗费大量人力物力。随着CT技术的发展,研究者们将CT技术与有限元法结合起来,最终获得了牙牙合的三维有限元模型,此方法在整个建模过程中,无创伤和破坏,相对于二维建模法,此法定位准确,空间分辨率高,解决了二维建模技术所存在的影像重叠失真问题。但由于其所建立的三维模型是由水平面上的二维图像叠加而成,其分辨率虽高于二维建模中层与层之间的分辨率,但仍然无法满足科研工作者对高分辨率的严格要求,而Micro-CT分辨率高达几个微米,且其可提供有限元分析接口,利用分析软件,获得弹性模量、泊松比、主应力、Von Mises应力等重要力学参数[3]。因此,Micro-CT越来越广泛的应用于三维有限元法的牙牙合建模之中。 2.2 不同瓷层对烤瓷熔附金属全冠受力分析的影响 烤瓷熔附金属全冠能完美地修复牙的功能及形态,强度高,抗折力强,耐酸耐碱,受到很多患者及临床医生欢迎。然而,在临床工作中,烤瓷熔附金属全冠常因崩瓷等原因而使修复失败。临床上遇到的失败病例,往往涉及到烤瓷材料的生物学匹配,金瓷匹配和色彩学匹配,金瓷匹配是影响烤瓷熔附金属全冠修复成功的关键因素之一。Chowdhuri等[4]的研究表明,在金瓷界面结合处设计良好的接合角度,可消除金瓷界面的残余应力,此研究结果可为设计和制造烤瓷熔附金属全冠提供参考。 当然,应力集中和金瓷厚度差距过大是造成瓷体折裂的主要原因。临床上常因牙体缺损、不规则、畸形牙的存在以及牙齿严重错位等原因,致使修复体瓷层和金属基底层厚度差距过大,而瓷层的过薄或过厚均会造成瓷体折裂。增加瓷层结构厚度,避免瓷断裂导致金属烤瓷冠修复失败,一直是临床医生和和口腔工艺学的关注热点之一[5-6]。 瓷粉属于脆性材料,其抗压强度高,但其拉应力强度低。据测,当压力达50 kg时,瓷粉的抗拉强度为600-800 kg/cm2,瓷粉就从金属内冠脱落。如何增强瓷层的拉应力强度,减少瓷剥脱现象的发生,主要取决于瓷层的厚度。目前,如何界定瓷层过薄或过厚尚未形成定论。Tsai等[7]在测试不同瓷层厚度的抗折强度时发现,从0.4-1.6 mm厚瓷层的抗折裂强度逐渐增强,1.6 mm时达到高峰,1.6-2.4 mm瓷层厚度抗折裂强度相当。Harrington等[8]的研究指出,瓷层过厚是指3.5 mm以上。2005年翟丽云等[9]的研究报告也证实了这一点。其研究指出,当固定义齿瓷层厚达3.5 mm时,烧结时有瓷裂出现,瓷厚达4 mm时,所有标本烧结均出现瓷崩,故认为瓷厚在3.5 mm以下为宜。目前,比较公认的说法是瓷厚度不低于 0.85 mm为宜。以牙合力分析看,0.4 mm厚的瓷层足以用于烤瓷熔附金属全冠的制作。从临床意义上论,由于烤瓷熔附金属全冠在咀嚼过程中的受力是一个动态的过程,应该根据患者的牙合力峰值及牙周健康状况选择适宜瓷层厚度。只要能够承受牙体受力,其瓷层的厚度都应该认为是科学合理的。因此,烤瓷熔附金属全冠瓷层厚度的选择较广[10-12]。 2.3 不同金属基底冠厚度对烤瓷熔附金属全冠受力分析的影响 烤瓷熔附金属全冠是由金属和陶瓷材料两相烧结而形成的板层复合材料,其受力状况与其各组分的力学性质和组分的多少有明显的关系。金属烤瓷用陶材,抗压强度高,但抗拉强度弱,如何使之获得最大的抗拉强度,金属基底冠的设计也至关重要[13]。在临床上要求金属基底冠表面无尖锐棱角、锐边,各轴面呈流线形,以免出现应力集中,破坏金瓷结合。 为了增强金瓷匹配,考虑到烤瓷合金的铸造流动性、强度等材料性能,一般要求贵金属基底厚度为0.3-0.5 mm,非贵金属基底最低厚度为0.5 mm。在牙体预备的过程中,要为瓷层提供适当的空间,保持金瓷的结合强度及美观。当金属基底冠过薄时,其机械强度降低,金属和瓷表面的拉应力逐渐增大,增大的拉应力可致使修复后崩瓷。当金属基底冠厚度增加时,压应力峰值与Von-Mises应力峰值增大,而拉应力峰值降低。当基底冠增大为0.4 mm及以上时,由于金属厚度增大,金属对瓷的加强作用也随之加大,烤瓷冠不易变形。 李芸等[14]的研究报告表明,当金属基底冠的厚度增加到0.6-0.8 mm时,瓷上的应力峰值,Mises应力峰值较0.4 mm时大幅度增大,这在临床上表现为瓷的碎裂,所以不能选择厚度为0.6 mm以上的基底冠设计。苏萍等[15]的研究显示,当基底冠的厚度为0.2 mm时,瓷层的耐压力最小,基底冠的厚度为 1.0 mm时,瓷层耐压力最大,即在允许范围内,金属基底冠越厚,瓷层耐压力越强。李芸等[16]利用三维有限元法分析基底冠厚度对烤瓷应力的影响时,得出如下结论:在备牙量一定时,金属基底冠厚度为 0.4 mm时,能最大限度减少瓷剥脱和碎裂现象。 Colpani等[17]对不同金属基底冠与瓷层的接合,通过计算机辅助设计与制造技术进行比较测定,得出的结论是不同金属基底冠材料与瓷层的结合能力差异无显著性意义。此结果表明,无论是贵金属还是非贵金属,其与瓷层的结合能力并无太大差异。当然,全瓷冠在修复牙体硬组织缺损时,其在咬合状态下的载荷能力远大于烤瓷熔附金属全冠,这是毋庸置疑的。Martins[18]、Durand等[19]的研究也证实了这一点。 2.4 不同根管预备技术及器械对根管壁应力的影响 根管预备是根管治疗中的关键步骤。不同的根管预备技术影响根管壁的应力分布。临床上常用的根管预备方法有常规法和逐步后退法。常规法在根管粗大,较直的情况下采用比较合适,若根管细小且有弯曲,用常规法预备可能出现根尖偏移等情况。而逐步后退法的根管成形作用较好,能较好保持原根管形态。 洪瑾等[20]在已建立下颌第一磨牙有限元模型的基础之上,分别模拟常规法和逐步后退法修改模型,分别对其进行垂直加载和侧方加载,发现两种模型应力变化趋势相似,与原始模型亦相似。垂直加载时可见两种模型的最大应力自冠方向根方逐渐降低,但在髓室底根管口的位置,最大应力又出现一个峰值,随后应力逐渐下降,至牙根中1/3时应力明显下降,至牙根尖1/3应力下降至0.3 MPa以下。侧方加载时应力变化情况与垂直加载时相似,侧方应力大于垂直应力。 不同的根管预备技术及材料将影响根管壁的应力分布。张娜等[21]通过建立三维有限元模型模拟K锉、ProTable锉和ProFile锉进行根管预备,研究发现ProTable锉预备根管时根管壁所受应力最大,K锉预备根管时根管壁所受应力最小,ProFile锉居中。Grande等[22]的研究显示,纤维桩比传统的金属桩具有更大的抗弯强度、耐疲劳能力以及抗渗透能力。且有研究证实,选择与牙本质弹性模量相近的纤维桩直接修复薄弱的根管,有利于保护牙根[23]。 在根管预备过程中,都以彻底去除根管感染物质为原则。但为了便于根管冲洗及充填,往往会过多地切削根管壁,导致牙根强度下降,易引发压根折裂。牙本质的过度切削,是发生根折的生物学基础。Rundquist等[24]的研究表明,当扩锉锥度增加时,应用三维有限元法分析锥度分别为0.04,0.06, 0.12 mm/mm的器械,在充填过程中牙根所受应力减小,而在咀嚼过程中,牙根所受应力增加。 根管预备后,根管中心线常发生偏移,衡量根管预备效果的重要标准之一,是测定根管预备后的偏移情况[25],磨牙承担着重要的咀嚼功能,临床上医生一般会尽量保留此牙。然而,磨牙的压根数目,根管形态非常复杂[26]。近年来,学者们常用非创伤性方法来评价根管预备效果,如数字放射线影像等,但它们不能三维成像。序列切片技术、光学显微镜观察在根管预备后常须切割牙齿,以致造成样本破坏[27]。而利用三维有限元法建立三维立体模型既能精确模拟根管预备后结构,同时也能保存完整模型,使三维评估根管效果得以实现[28]。Vahid等[29]的研究表明,Protaper根管预备后,根管的弯曲度降低,根管中心线发生偏移。Aguiar以根管中心定位率的方法提示,当根管预备到30#时,根管中心偏移风险增大,即便是镍钛器械也是如此[30]。 2.5 不同充填方式对根管壁应力的影响 根管充填是在经过严格的根管预备和根管消毒后,对根管进行严密充填,促进牙髓炎及根尖周炎的愈合,提高根管的封闭性,防止微渗漏。Telli等[31]通过三维有限元模型研究发现,无论采用何种加压方式,根管壁牙本质所受的最大Von-Mises应力仍远低于牙本质所最常报告的抗拉强度(50-100 N/mm2),因此,在充填过程中,垂直加压力量或侧方加压力量并不能使牙根折裂。但是若侧方充填器械锥度过大,且根管预备时牙体组织丧失过多,会导致牙折。应用三维有限元分析法对器械进行生物力学分析和优化设计,具有无创伤、无重复、观察指标明确等优势,可以减少根折发生率和临床操作的失误。 当充填椭圆形根管时,由于其根管形态非常特殊,使得根管充填不能达到理想的状态[32-38]。临床上常用的充填方法为热牙胶充填法和冷牙胶测压法,其中,冷牙胶测压法是各种充填方法加以综合比较后的金标准[39]。但是大量研究证实,热牙胶充填更加严实,三维充填密度更大,根尖更为密闭,操作更加省时[40]。"
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