摘要
椎弓根螺钉内固定技术是脊柱内固定的核心技术之一,目前已广泛应用于脊柱疾病的后路融合手术。椎弓根螺钉固定的可靠性取决于钉-骨界面的稳定性,但对于骨质疏松(Osteoporosis,OP)患者,由于骨密度(Bone mineral density,BMD)降低、骨小梁稀疏、钉-骨界面不牢固,常面临着螺钉松动或脱出的风险,将会导致脊柱融合内固定失败。随着中国人口老龄化社会的加剧,需要进行脊柱内固定手术的OP患者逐年增多。如何在骨质条件较差的条件下,有效地强化椎弓根内固定的稳定促进脊柱融合,是目前研究的热点。本课题组前期研制了用于腰椎的膨胀式椎弓根螺钉(expandable pedicle screw,EPS),简化了术中强化的操作过程,避免了添加生物材料强化引起的术中及远期并发症,并已用于临床治疗各种腰椎疾病合并OP的患者。但是当患者存在OP时,胸椎椎弓根螺钉内固定也存在松动的风险,且OP引起的椎体骨折常发生在胸腰段脊柱。关于胸椎椎弓根螺钉的强化方面目前研究较少,且临床上应用的EPS无法应用于胸椎。
目的:研制可用于强化稳定的胸椎膨胀式椎弓根螺钉(Thoracic Expandable Pedicle Screw, TEPS),系统评价其强化稳定和耐疲劳的生物力学性能,评价其在OP椎体内稳定机制及取钉钉道的相关参数,为OP条件下胸椎的椎弓根螺钉的强化提供一种安全有效的解决方案。
方法:1.根据胸椎椎弓根的解剖特点研制外径4.5mm TEPS。进行单根螺钉的轴向拔出试验、周期抗屈试验和三点弯曲试验。按美国材料与试验协会F1717-09的标准,对TEPS脊柱内固定系统组件进行了静态拉、压和动态压缩疲劳试验。并与同直径、同长度的普通椎弓根螺钉内固定系统(Spinal Implant New Option Screw,SINOS)对比,系统评价TEPS的强化稳定及耐疲劳性能。2.建立OP绵羊的动物模型。将TEPS和SINOS植入OP绵羊腰椎,饲养4个月后取材。进行在体4个月后的螺钉的轴向拔出试验。应用Micro-CT技术和包含螺钉的硬组织切片技术,研究在OP绵羊椎体内TEPS钉-骨界面以及EPS膨胀缝隙内的组织学情况。应用Micro-CT技术对旋出TEPS后的钉道进行了重建和测量,研究在TEPS取出过程中是否造成椎弓根的破坏。
结果:1.体外的生物力学试验部分:①轴向拔出试验中,以模拟两种不同骨质条件下的Sawbones聚氨酯人工骨试验块为载体,测试了TEPS和SINOS最大轴向拔出力。TEPS与同直径、长度的SINOS相比,最大轴向拔出力分别提高了15.44%和48.83%。②周期抗屈试验中,选用骨量减低的人的新鲜尸体胸椎为载体,测试了TEPS和SINOS周期载荷下的松动率及松动位移,TEPS与同直径、长度的SINOS相比,松动率明显降低、周期载荷下的位移减小。③三点弯曲试验中,膨胀后的TEPS可承受的最大抗折断载荷与同直径、长度的SINOS相比无明显差异。④成组TEPS的静态压缩弯曲、拉伸弯曲和动态压缩弯曲疲劳试验表明,TEPS系统的力学性能可满足胸椎椎弓根内固定系统的要求。2. TEPS植入OP绵羊椎体内4个月后的实验:①TEPS在OP绵羊腰椎4个月后的轴向拔出试验,结果显示TEPS与同直径、长度的SINOS相比,最大轴向拔出力有显著的提高;并且在体4个月组与即刻组相比,最大轴向拔出力亦有进一步提高。②将Micro-CT技术和包含TEPS的硬组织切片技术相结合,分析了在OP的椎体内TEPS的钉-骨界面以及TEPS膨胀段周围的组织学情况。发现TEPS对周围骨质的膨胀加压提高了周围骨质的质量,同时骨小梁逐渐可长入膨胀间隙、增大了骨整合的接触面积、形成了钉-骨绞锁固定的模式,共同实现了TEPS在OP条件下即刻的和长期的稳定。③通过对TEPS在体外膨胀后直径的测量和取出TEPS后钉道的各部位直径的Micro-CT分析测量,发现了TEPS具有植入时有限地膨胀、取钉时可部分回缩的特点。测得的TEPS在椎弓根处的直径为4.72mm,较初始直径增加了4.9%,在安全范围之内。
结论:1. TEPS具有强化稳定的作用,在体内可维持即刻的和长期的稳定。2. TEPS的力学性能和耐疲劳性能可满足胸椎椎弓根内固定系统的要求。3.应用TEPS时只要操作得当是可以安全取出而不破坏椎弓根的。因此,应用TEPS系统内固定,可成为OP条件下强化胸椎椎弓根螺钉稳定的一种解决方案。
Pedicle screw instrumentation is one of the most commonly used and rapidly growing forms of stabilization for spinal fusion. Transpedicular fixation, however, can be very challenging in the osteoporotic (OP) spine as mechanical stability of the pedicle screws is determined by the bone mineral density (BMD). In addition, poor rigidity of the bone-screw contact can lead to loosening of the implant in osteoporotic patients. With rapid advances in the field of spinal surgery and the aging of the population, an increasing proportion of elderly patients undergo surgical treatment for their spinal disorders. To address these issues, we had designed expandable pedicle screws (EPS). Biomechanical studies have demonstrated that the use of EPS significantly improved the fixation strength compared with conventional pedicle screws. Clinical results have indicated that these screws are particularly useful in the OP lumbar spine. However, this kind of screw cannot be used in the OP thoracic spine, and no systemic study has been reported concerning the augmentation of thoracic pedicle screw.
Objective: The aim of this study was to design a thoracic expandable pedicle screw (TEPS), to evaluate its stability and feasibility and to offer a novel strategy for the augmentation of pedicle screw in the OP thoracic spine.
Methods: 1. The TEPS was developed according to the anatomy of the thoracic spine. The Axial pull-out test, the cyclic bending resistance test and the three-point bending test were performed to compare the properties of stabilization of the single TEPS with a standard pedicle screw (SINOS). Static and dynamic mechanical tests of the TEPS assemblies and the SINOS assemblies were employed based on the guidelines of the American Society for Testing and Materials (ASTM) F1717-09 using polyethylene blocks as the test vertebral bodies. 2. The OP sheep model was induced by an ovariectomy and continual injections of methylprednisolone, and was confirmed by dual energy X-ray absorptiometry (DEXA). TEPSs and SINOSs were inserted on the vertebral bodies in OP sheep. After four months, the axial pull-out test was performed to compare the holding strength of these pedicle screws. A high-resolution micro-computed tomography (Micro-CT) was performed for three-dimensional (3D) image reconstruction of the screws and the bone. The same length of regions of interest (ROI) surrounding the anterior part of the TEPS and the SINOS were reconstructed and analyzed in the same threshold. A thorough microscopic analysis was performed on sections of these specimens. The safety and reliability of removal of the TEPS was evaluated by the measurement of trajectory using Micro-CT imaging and a 3D reconstruction.
Results: 1. Biomechanical studies in vitro:①The axial pull-out tests showed that the TEPS can increase pull-out strength by 15.44% and 48.83%, compared with the SINOS, in these two polyurethane test blocks of different densities.②In the cyclic bending resistance tests, the TEPS withstood a greater number of cycles or loads with less displacement before loosening.③In three-point bending test, there was no significant difference in the maximum load between the TEPS and the SINOS.④Furthermore, compressive bending, tensile bending and dynamic compressive bending fatigue tests demonstrated that the TEPS assembly can provide biomechanical stability of the thoracic pedicle screw fixation. 2. Experiments after the four-month insertion on the vertebral bodies in the OP sheep:①All the OP sheep were euthanized and the lumbar spines including TEPSs and SINOSs were harvested. The TEPSs provided a significant increase in maximum axial pullout strength over the SINOSs in the four-month group and this increase was also significantly higher than that of them in an immediate group.②A Micro-CT image reconstruction showed that these 3D parameters were significantly better in the expandable portion of the TEPSs than those in the anterior portion of the SINOSs. The persistent pressure provided by the expanded fins could act as regulators of bone remodeling, leading to increased interfacial BMD. Histological, newly formed bone tissues grew into the groove of the fins. These bone tissues constructed two compartments and increased the contact area of the osteointegration in the screw-bone interface. As shown by the unique 3D structure, compared with SINOS, the bone and the screw grew into each other, which could further improve the pull-out strength of the TEPS.③Though the diameter of the expanded TEPS was indeed larger than the original diameter, expansion was less than that measured by in vitro studies, due to compression of the surrounding trabecular bone. During extraction from the vertebral body to the pedicle, the expanded fins of the TEPS gradually shrunk in diameter. The diameter of the EPS trajectory in the pedicle was 4.72mm, which was a 4.9% increase from the original diameter. Furthermore, the limited expansion indicated that the EPS could be backed-out safely and reliably.
Conclusions: 1. TEPS could provide a significant stabilization in the osteoporotic spine because of its superior biomechanical properties. 2. The mechanical properties of TEPS and the TEPS assembly demonstrated TEPS assembly can provide biomechanical stability of a thoracic pedicle screw fixation. 3. The TEPS could be backed-out safely without fracturing the pedicle. Therefore, the TEPS assembly could be a novel approach for increasing the pedicle screw fixation in the osteoporotic thoracic spine.
引文
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