Bioceramics in bone tissue engineering

doi:10.3969/j.issn.2095-4344.2017.22.022

Abstract

Abstract:

BACKGROUND: Bioceramic has similar components compared to human bone tissue and it has shown good ostoconductivity both in vitro and in vivo. Meanwhile, it is biocompatible. So, bioceramics is considered as one of the most promising materials which can be applied to bone tissue engineering.
OBJECTIVE: To summarize the properties of bioceramics and the research progress in experimental studies and clinical applications.
METHODS: PubMed was searched for relevant articles published during 2000 to 2016 with the key words of “bioceramics, hydroxylapatite, calcium phosphate, bioglass, bone tissue engineering” in English.
RESULTS AND CONCLUSION: Bioceramic materials can be divided into two categories: calcium phosphates and bioactive glass. Calcium phosphates have good biocompatibility and osteoconductivity, while the mechanical property is not so satisfying. Bioactive glass is biocompatible and beneficial to the expression of some osteogenic genes, but it is brittle and weak. Some kinds of bioceramics have already been applied to clinical practice. In recent years, calcium phosphates have also been used as coated materials to improve the properties of tissue-engineered scaffolds. Bioceramics combined with synthetic polymers, shows better mechanical performance and biodegradation. Even so, it still has plenty of problems and challenges as a widely used bone repair material in clinical practice.

中国组织工程研究杂志出版内容重点：生物材料；骨生物材料; 口腔生物材料; 纳米材料; 缓释材料; 材料相容性；组织工程

Key words: Dental Porcelain, Apatites, Tissue Engineering

CLC Number:

R318

Lu Chen-pei, Wang Xu-dong, Shen Guo-fang. Bioceramics in bone tissue engineering[J]. Chinese Journal of Tissue Engineering Research, 2017, 21(22): 3576-3582.

Figures/Tables 2

2.2.1 无机磷酸钙临床上常用的无机磷酸钙包括羟基磷灰石及β-磷酸三钙。羟基磷灰石的主要形式为Ca10(PO4)6(OH)2，其本身是正常骨组织骨盐的主要成分，因此具有极佳的生物相容性。在体外和生物体内实验中，羟基磷灰石都被证明有良好的骨诱导和骨结合能力，同时能够较好地支持成骨细胞的黏附、增殖及分化[3]。但羟基磷灰石在人体中的降解速度十分缓慢，Marcacci等[4]将羟基磷灰石支架植入人体内用于长骨修复，在六七年间的临床随访中，支架没有出现生物降解。相比之下，β-磷酸三钙在具有良好骨诱导性能的同时有良好的可吸收性[5]。与此同时，β-磷酸三钙在降解时可释放钙离子及磷酸盐离子，有助于新骨形成。β-磷酸三钙修复小面积缺损的骨愈合能力明显高于羟基磷灰石，但在修复大面积缺损时，由于β-磷酸三钙的降解速度过快导致缺损部位不能充分的被新骨填充[6]。有学者研究羟基磷灰石及相关无机磷酸钙材料的溶出度，提示其溶解速度顺序：无定型Cap>无定型羟基磷灰石>晶体Cap>晶体羟基磷灰石[7]。因此，研究者们结合羟基磷灰石和磷酸三钙形成双相磷酸钙，针对不同缺损修复大小的要求，通过调整两种材料的复合比例来控制材料降解及骨诱导生成的平衡，达到良好修复缺损的目的。在机械强度方面，正常人体密质骨抗压强度为130-180 MPa，抗张强度为50-151 MPa，弹性模量为12-18 GPa，断裂韧性(断裂韧性:指材料阻止宏观裂纹失稳扩展能力的度量，也是材料抵抗脆性破坏的韧性参数)为6-8 MPa；正常人体松质骨抗压强度为4-12 MPa，弹性模量为0.1-0.5 GPa。单纯羟基磷灰石的抗压强度 > 400 MPa，抗张强度在40 MPa，弹性模量为 100 GPa，断裂韧性<1.0 MPa。无机磷酸钙的抗压强度为20-900 MPa，抗张强度为30-200 MPa，弹性模量为30-103 GPa，断裂韧性<1.0 MPa[8]。可见，单纯生物陶瓷材料脆性较大，其断裂韧性远小于正常密质骨，其抗张抗压强度也不能完全模仿生理骨组织，这导致了单一材料不能适应于承重骨或大面积骨缺损的替代重建。此外材料的多孔性也影响了材料的机械性能[9]。82%- 86%的多孔羟基磷灰石抗张强度为0.21-0.41 MPa，弹性模量为0.83-1.6 GPa[7]。已有研究证明，生物陶瓷材料的硬度和刚度和孔隙数目呈反比。Lane等[10-11]的研究指出，连接空隙的大小至少要达到100 μm直径才能进行有利的营养输送，且不同大小的孔隙(250-350 μm和<8 μm)互存对于羟基磷灰石支架上的骨形态蛋白发生、骨诱导和机械强度有益。Woodard等[12]对多微孔支架和单微孔支架的压力实验证明，单微孔支架的强度和刚性下降了46%，而多微孔支架的强度下降了30%，刚性下降了31%。由此可见，在强度和刚度上，非多孔支架>单微孔支架>多微孔支架。为了解决这一问题，高分子聚合物和生物陶瓷的复合材料应运而生。因高分子聚合物具有与体内骨胶原相似的极性分子结构及优秀的机械性能，从而被运用于改善单纯磷灰石材料的性能。Zhang等[13]的研究证明陶瓷聚合支架材料较松质骨强度低，但其生物可降解性、弹性都较单纯生物陶瓷材料好。此外，在羟基磷灰石材料中加入直径15 nm、长度10-20 μm的纯纳米碳罐也可增加其承重的抗压性能[9]。也有学者通过将羟基磷灰石/磷酸三钙与生物活性玻璃结合，生物活性玻璃充当烧结助剂，增加材料密合度和机械强度[14]。生理性骨组织除主要组成成分羟基磷灰石外，还包含有许多离子，比如碳酸根离子、钠离子、镁离子、锌离子等。为了充分模拟生理骨组织中离子交换情况，近年来，有许多学者将各种离子与替代羟基磷灰石中的部分钙离子，使得离子能在羟基磷灰石支架中缓释，发挥其生理活性功能。例如，Si已被证实在骨组织矿化和基因激活中起重要作用。生物体内实验表明，含硅的羟基磷灰石颗粒成骨细胞的成骨情况显著优于纯羟基磷灰石[15]。 2.2.2 生物活性玻璃生物活性玻璃在1970年被发现，Hench总结了它在诱导成骨方面的特点，它能够和活体组织紧密结合营造稳定的界面，还可触发一系列生物反应，在自身吸收降解的同时一道组织再生和血管化[16]。在机体内，它通过在生物活性玻璃表面形成碳化羟基磷灰石层使植入物和骨组织形成良好的连接，与此同时，其释放的Si、Na、Ca、磷酸根离子能刺激细胞生成新的组织。临床上最常用的生物活性玻璃为45S5 Bioglass®，其构成主要是Na2O-CaO-SiO2-P2O5。45S5 Bioglass®抗压强度为500 MPa，抗张强度为42 MPa，弹性模量为35 GPa，断裂韧性为0.5-1.0 MPa[7]。它已被证实具有促进成骨细胞黏附、分化和诱导间充质细胞分化为成骨细胞的能力[17]。不仅如此，研究者还发现它具有提高蛋白活性，系促进血管新生的作用[18]。近年来的研究发现，生物活性玻璃通过释放Si、Ca、P等离子促进成骨细胞的有丝分裂和一型胶原纤维的合成，此外，离子的聚集还能够上调某些成骨相关基因的表达[19]。 2.3 生物陶瓷作为骨组织工程支架的体外实验研究很多实验已证实，间充质干细胞、成骨细胞等在羟基磷灰石、磷酸三钙支架、生物玻璃支架上都能保持良好的细胞活性，进行细胞的黏附聚集、增殖、分化等过程。此外，在生物陶瓷材料表面涂布生长因子、骨形成蛋白2、金属离子等也可促进生物陶瓷材料的成骨活性[20-22]。Rahman等[23]将MC3T3-E1细胞定植于多孔连接羟基磷灰石-陶瓷支架，并用成纤维细胞生长因子2和褪黑激素诱导，发现成纤维细胞生长因子2和褪黑激素增强了MC3T3-E1细胞的增殖，上调了OCN和OPN的表达，并且提高了碱性磷酸酶的活性。Chen等[24]将MG63细胞定植于多孔生物玻璃-陶瓷支架上，观测细胞功能及细胞增殖情况，细胞能够在渗透进支架中心生长，复合材料的晶体成分在细胞增殖过程中降解后，黏附的细胞及分泌的胶原修补晶体溶解过程中产生的表面微裂隙，使得支架的强度仍能够维持。在羟基磷灰石/阮藻酸盐复合支架上，人间充质干细胞成骨基因的表达较单纯阮藻酸盐更高，产生的基质及骨矿化数量也较单纯阮藻酸盐支架更高[25]。此外，近年来的许多研究表明，用特定的离子取代羟基磷灰石中的部分离子，形成替代羟基磷灰石，能够更大程度模仿生理骨组织的组成[26-27]。离子取代对材料的性能、晶体结构、形态、溶解度等都有显著影响。有学者分别用Pb2+、Sr2+、Co2+、Zn2+、Fe2+、Cu2+取代羟基磷灰石中一个Ca离子，形成X羟基磷灰石(X为取代钙离子者)，然后测定其细胞相容性，发现除Zn-羟基磷灰石外，所有替代羟基磷灰石都出现细胞凋亡增多的现象。其中Mg-羟基磷灰石、Co-羟基磷灰石的细胞凋亡最甚[28]。Mg-羟基磷灰石支架在新西兰大白兔股骨缺损的模型实验中表现出了良好的骨诱导活性和高材料降解活性，缺损股骨1个月内就可见编织骨形成，3个月后编织骨完全被成熟板层骨取代[29]。 2.4 生物陶瓷作为骨组织工程支架的的体内实验除体外细胞研究，生物陶瓷材料结合3D打技术、激光烧结技术在动物体内实验中也表现出了良好的诱导骨组织再生及血管化的能力。Smeets等[30]运用选择性激光熔融技术制备聚丙交酯/β-磷酸三钙多孔径支架，经体外细胞实验证实其生物相容性和成骨活性后，用于标准大小颌骨及颅顶骨缺损的大鼠模型，实验组大鼠在30 d后出现新骨置换，其骨形成表现优于自体骨修复的对照组。Fernández等[31]用Ca-P生物材料修复大鼠颅顶骨缺损，发现Ca-P在初期以60 μm/d(0.8 mg/d)的速度显著加速编织骨的形成和矿化，组织学检测发现早期骨单位形成，提示早期纤维组织中的血管形成，见图1。Suba 等[32]在20例缺牙患者上比较了上颌窦提升术后分别植入自体骨粉和β-磷酸三钙后的骨形成情况，术后6个月双侧上颌窦底骨组织学检测结果显示，植入β-磷酸三钙的一侧的骨形成情况与对照侧无明显差异，新生骨密度与对照侧相当。β-磷酸三钙具有良好的骨诱导、骨结合和骨再生能力，可形成适合牙种植的骨床。除了能够支持细胞的增殖分化外，生物陶瓷材料还能在成骨基因的表达、骨组织血管化中起到促进作用。Meagher等[33]将羟基磷灰石/胶原支架在小鼠动物模型中皮下异位植入，12周的观察显示，支架上的血管密度、基质沉积及矿化情况与支架中羟基磷灰石体积比有剂量依赖关系。 2.5 生物陶瓷支架在骨缺损重建的现有临床应用生物陶瓷在骨组织工程中的各项研究都显示出了其在骨修复重建领域广阔的前景，与此同时，生物陶瓷也已部分运用于临床，为患者带来福音。在骨科中，Marcacci等[4]利用3D 打印技术，将患者体内分离的骨髓基质干细胞置于羟基磷灰石支架上，用以修复大面积胫骨缺损，在长达6.5年的随访中，赝复体与宿主骨组织结合良好，患者肢体功能大部分恢复。生物陶瓷结合3D打印技术和计算机辅助设计在颅面部个性化定制缺损重建上也有许多运用。Li等[34]运用计算机辅助设计及快速成型技术，以纳米羟基磷灰石/聚酰胺为材料，对髁突切除术后面部外形不良的患者进行个性化下颌骨髁突修复，最终恢复面部外观和稳定的颞下颌关节功能。Engstrand等[35]运用β-磷酸三钙等生物陶瓷材料结合计算机辅助技术，制成8 mm×8 mm× 4 mm大小的方块，用钛网连接形成马赛克样式的植入体，用以修复颅颞骨大面积缺损，24个月随访显示颅骨缺损处新骨形成。利摩日大学医院神经外科及颌面外科学部以8例患者为临床研究对象，运用立体平板印刷技术，以羟基磷灰石为生物材料定制赝复体，修复颅面部复杂性骨缺损(缺损面积25 cm2)，术后随访12个月未发现并发症，患者对外形效果满意[36-37]。在耳鼻咽喉科学方面，Huang等[38]对14例因鼓室硬化置换生物陶瓷人工听小骨的患者随访发现，术后3-24个月内语音频率及音听阈均值有明显改善。此外生物陶瓷还运用于眼眶的修复[39]。 2.6 生物活性陶瓷作为骨组织工程支架表面涂层的体内外实验及部分临床应用生物活性陶瓷不仅可作为主体材料在细胞黏附增殖、骨组织形成、缺损重建中起到重要作用，还可通过等离子喷射、溶胶-凝胶法、电沉积法等形成表面涂层，改善其他材料的生物活性。许多研究表明，在钛植入物表面覆盖Ca-P涂层可改善植入物的生物功能[40-42]。Orsini等[43]比较了上颌骨后部的羟基磷灰石涂层钛种植体和单纯钛种植体，发现羟基磷灰石涂层的钛种植体平均骨-种植体接触大于对照组，证明种植体表面的涂层能够缩短种植体骨整合时间，有利于减小微小移动，缩短治疗时间。Johansson等[44]将羟基磷灰石纳米微粒涂层的聚醚支架植入12只兔股骨缺损模型中，发现骨-植入物的接触面面积大于无涂层的对照组面积，植入物螺纹处的骨修复也较对照组多，证明羟基磷灰石涂层能显著改善聚醚支架的成骨性能。此外，羟基磷灰石用作金属支架涂层，可用于改善金属合金表面的抗腐蚀性[45]，Zhang等[46]将Ca-P和BGCC/Ca-P分别涂层至纯镁支架上，分析其微结构、形态、矿化能力等发现，两种涂层都能显著加速骨诱导矿化的成核现象且BGCC/Ca-P涂层的镁支架降解速率比纯镁支架降低10倍。提示BGCC/Ca-P能抵抗镁金属表面的腐蚀，增强骨-支架表面的连接。钛表面的晶体羟基磷灰石涂层还能增加细胞的反应活性[47]。临床上，羟基磷灰石还用作全髋关节置换体表面涂层，Capellpo等[48]在316例羟基磷灰石涂层全髋关节置换患者术后的临床随访显示，羟基磷灰石涂层的髋关节修复体促进了骨组织的内向生长和表面生长，并且不伴有骨质溶解现象。 "

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