新型PCS椎弓根螺钉的锚定强度生物力学测试和有限元分析研究
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摘要
研究背景
近年来,椎弓根螺钉固定系统在各种脊柱疾患的外科治疗中应用日渐广泛,但部分患者也出现了螺钉松动、脱出等并发症,究其原因,螺钉对骨的把持力不足是导致螺钉松动和轴向脱出的主要因素。目前,椎弓根螺钉与骨界面的结合强度,即螺钉的抗拔出强度已成为学者研究的重点。前期研究已经证实椎弓根皮质下骨质骨密度较骨小梁高,椎弓根螺钉固定纵向负载强度的80%,拔出强度的60%取决于椎弓根而不是椎体,而目前常用的椎弓根螺钉(如CCD螺钉)的螺纹为白上而下螺距一致,椎弓根部分与椎体部分的螺纹并无差别。我们根据中国汉族人种椎弓根的解剖特点及生物力学原理,设计了一套专门适用于中国患者的新型PCS (Part Concentrated Screw)椎弓根内固定系统。螺钉颈部即椎弓根部分改进为螺距更小,更为致密的螺纹,以期增加螺钉与椎弓根骨质的啮合面积,进而增加椎弓根螺钉的把持力,达到增强椎弓根螺钉固定系统稳定性的目的。
目的
本研究拟利用实体生物力学实验比较新型PCS螺钉与常用CCD螺钉的骨质锚定牢固程度,并利用有限元分析方法测量PCS螺钉致密螺纹部分对螺钉锚定能力的贡献比值,为PCS螺钉的下一步开发与应用提供理论依据。
方法
自成年汉族男性尸体获取15个椎体标本(30侧椎弓根),利用骨密度仪测定各标本椎体部分和椎弓根部分的骨密度值。随后将30侧椎弓根随机分为3组,采用Magerl法分别拧入PCSⅠ螺钉、PCSⅡ螺钉和普通CCD螺钉,拧入深度标准为螺钉所有螺纹均旋入骨质,且钉尾钛棒固定部分前表面与椎板紧密接触,利用力矩扳手读出各螺钉的最大拧入扭矩。然后在CSS-22100型材料试验机上进行拔出试验,观察3种不同形状螺纹的椎弓根螺钉的最大轴向拔出力和屈服位移,了解两种的椎弓根螺钉在椎体中的松动生物力学规律。利用UG软件分别建立三种螺钉和骨质的三维模型,在ANSYS有限元分析环境中装配后,模拟拔出过程,利用有限元方法计算螺钉拔除时各个螺纹处骨质所提供的最大把持力值,分析致密部分螺纹对螺钉整体锚定强度的贡献率。
结果
1.各标本椎体部分和椎弓根部分骨密度值分别为0.494±0.058g/cm2和0.783±0.134g/cm2,统计学显示两组间存在显著性差异(p<0.05)。对两组间进行正态性检验,显示两组内各值均来源于各自的正态分布总体,组间各数据无显著性差异,说明各椎体间骨密度差异对实验结果无显著性影响。
2.生物力学试验测定PCSⅠ螺钉、PCSⅡ螺钉的最大轴向拔出力、屈服位移和最大拧入扭矩均显著高于CCD螺钉,采用SNK检验和最小显著差检验分别进行两两比较,结果显示结果均有显著性差异(p<0.05)。PCSⅠ螺钉的最大轴向拔出力和最大拧入扭矩较PCSⅡ螺钉明显提高,结果具有显著性差异。两种PCS螺钉的屈服位移无显著性差异。
3.每一椎弓根骨质螺纹所提供的把持力值是椎体骨质螺纹所提供的3倍以上,椎弓根内骨质所能提供的总把持力值要远远超过椎体内骨质所提供的总把持力,其比重可以占到螺钉所获得总把持力的85%以上,对螺钉总把持力力起到了主要加强作用。
结论
设计新型PCS椎弓根螺钉较普通CCD螺钉具有更高的生物力学锚定强度,椎弓根部分的致密螺纹对螺钉拔除力起到了主要加强作用,其强度提高程度和椎弓根部分螺纹致密程度成正比。本实验证实了PCS螺钉设计的合理性与先进性,为其下一步开发和临床应用奠定了理论基础。
Background
The pedicle screw and rod instrumentation system is one of the most commonly used and rapidly growing forms of stabilization for spinal fusion. Problems associated with pedicle screws include loss of fixation, fatigue, bending failure and improper placement. Bending or breakage is the most common type of hardware failure, which is strongly associated with the purchase strength of screws. Although pure pullout is not the mode of failure in most clinical situations, pullout testing is thought to be a good predictor of pedicle screw fixation strength. Biomechanical studies show that bone mineral density (BMD), pedicle morphology and screw thread area all affect pedicle screw pullout. Trabecular bone at the corticocancellous interface in pedicle is stronger than normal unsupported cancellous bone in the vertebral body. Therefore the pedicle part accounts for more than 80% of the axial purchase strength and more than 60% of the pull-out strength of the whole screw. However, the CCD screw currently used in clinic has the same thread pitch in pedicle as that in vertebral body. Based on these characteristics and the pedicle anatomy features of Chinese Han people, a new set of part concentrated screw (PCS) was designed with concentrated threads in the pedicle part, which was believed to gain more purchase strength with increased thread area in the pedicle of Chinese patients.
Objective
The purpose of the present research was to test the difference in pull-out strength between the two types of PCS screw (PCSI and PCSII) and CCD screw in human spine. Three dimensional finite element analysis (FEA) was employed to study the contribution of the threads in the pedicle part to the pull-out strength of the whole screw.
Methods
Fifteen fresh lumbar vertebrae (L1-L5) from three adult Han male cadavers were harvested and stripped of all soft tissues. The BMD in vertebral body and pedicle of all specimens was measured by dual energy X-ray absorptiometry (DEXA). Each individual vertebra was thawed and prepared for pedicle screw placement. Then the 30 pedicles were randomly assigned to three groups of ten pedicles each. The PCSI、PCSII and CCD screws were inserted by hand using a custom driver without tapering. The screw entry point was selected in accordance with the method of Magrel. Each screw was inserted until the hub of the screw abutted the lamina and all threads were fully contained within the bone. The maximum insertion torque was measured by a torque measuring spanner. The vertebrae, mounted in the testing jig, were placed a variable axis frame attached to the base plate. The hydraulic arm of the CSS-22100 machine was attached to the screw head by a universal joint and aligned so that pullout would be co-axial to the screw. Axial pullout was performed at 2.00 mm/minute displacement. Pullout loads and yield displacement were recorded at 0.1-second intervals until failure occurred. The three dimensional geometries of the screw and the bone with a threaded hole was separately created using the UG software. Then the models were imported into ANSYS and combined to generate the final model. The FEA procedure was performed to study the effect of concentrated threads in the pedicle part on the screw total pull-out strength.
Results
1. The BMD of vertebral body and pedicle of specimen measured by DEXA were 0.494±0.058 g/cm2 and 0.783±0.134 g/cm2 separately, with significant difference between the two area (p<0.05)。Test of normality was performed separately in the data of the two areas. Results showed that all data of the two areas came from a single population separately, with no difference among the data in the same area.
2. The pullout loads, maximum torsion torque and yield displacement of PCSI and PCSII screws were significantly higher than CCD screw. Student-Newman-Keuls test showed that there was significant difference among the three screws (p<0.05). The pullout loads, maximum torsion torque of PCSI were also significantly higher than PCSII, but there is no difference between the two PCS screws in yield displacement.
3. The FEA analysis was successfully performed using UG and ANSYS software. The force taken by each threaded connection in the pedicle was three times more than that in the vertebral body area. Student's t test showed that the thread in the pedicle can resist significantly higher force compared to the thread in the vertebral body (p<0.05). The force taken by the threads in the pedicle accounts for more than 85% of the whole pullout strength of the PCSI and PCSII screw.
Conclusions
The newly designed PCS screws had significantly improved purchased strength compared to the custom CCD screw. The improvement of pullout strength was mainly derived from and directly associated to the density of threads in the pedicle area. The result of current research can serve as foundation for the further development and application of PCS screw.
近年来,椎弓根螺钉固定系统在各种脊柱疾患的外科治疗中应用日渐广泛,但部分患者也出现了螺钉松动、脱出等并发症,究其原因,螺钉对骨的把持力不足是导致螺钉松动和轴向脱出的主要因素。目前,椎弓根螺钉与骨界面的结合强度,即螺钉的抗拔出强度已成为学者研究的重点。前期研究已经证实椎弓根皮质下骨质骨密度较骨小梁高,椎弓根螺钉固定纵向负载强度的80%,拔出强度的60%取决于椎弓根而不是椎体,而目前常用的椎弓根螺钉(如CCD螺钉)的螺纹为白上而下螺距一致,椎弓根部分与椎体部分的螺纹并无差别。我们根据中国汉族人种椎弓根的解剖特点及生物力学原理,设计了一套专门适用于中国患者的新型PCS (Part Concentrated Screw)椎弓根内固定系统。螺钉颈部即椎弓根部分改进为螺距更小,更为致密的螺纹,以期增加螺钉与椎弓根骨质的啮合面积,进而增加椎弓根螺钉的把持力,达到增强椎弓根螺钉固定系统稳定性的目的。
目的
本研究拟利用实体生物力学实验比较新型PCS螺钉与常用CCD螺钉的骨质锚定牢固程度,并利用有限元分析方法测量PCS螺钉致密螺纹部分对螺钉锚定能力的贡献比值,为PCS螺钉的下一步开发与应用提供理论依据。
方法
自成年汉族男性尸体获取15个椎体标本(30侧椎弓根),利用骨密度仪测定各标本椎体部分和椎弓根部分的骨密度值。随后将30侧椎弓根随机分为3组,采用Magerl法分别拧入PCSⅠ螺钉、PCSⅡ螺钉和普通CCD螺钉,拧入深度标准为螺钉所有螺纹均旋入骨质,且钉尾钛棒固定部分前表面与椎板紧密接触,利用力矩扳手读出各螺钉的最大拧入扭矩。然后在CSS-22100型材料试验机上进行拔出试验,观察3种不同形状螺纹的椎弓根螺钉的最大轴向拔出力和屈服位移,了解两种的椎弓根螺钉在椎体中的松动生物力学规律。利用UG软件分别建立三种螺钉和骨质的三维模型,在ANSYS有限元分析环境中装配后,模拟拔出过程,利用有限元方法计算螺钉拔除时各个螺纹处骨质所提供的最大把持力值,分析致密部分螺纹对螺钉整体锚定强度的贡献率。
结果
1.各标本椎体部分和椎弓根部分骨密度值分别为0.494±0.058g/cm2和0.783±0.134g/cm2,统计学显示两组间存在显著性差异(p<0.05)。对两组间进行正态性检验,显示两组内各值均来源于各自的正态分布总体,组间各数据无显著性差异,说明各椎体间骨密度差异对实验结果无显著性影响。
2.生物力学试验测定PCSⅠ螺钉、PCSⅡ螺钉的最大轴向拔出力、屈服位移和最大拧入扭矩均显著高于CCD螺钉,采用SNK检验和最小显著差检验分别进行两两比较,结果显示结果均有显著性差异(p<0.05)。PCSⅠ螺钉的最大轴向拔出力和最大拧入扭矩较PCSⅡ螺钉明显提高,结果具有显著性差异。两种PCS螺钉的屈服位移无显著性差异。
3.每一椎弓根骨质螺纹所提供的把持力值是椎体骨质螺纹所提供的3倍以上,椎弓根内骨质所能提供的总把持力值要远远超过椎体内骨质所提供的总把持力,其比重可以占到螺钉所获得总把持力的85%以上,对螺钉总把持力力起到了主要加强作用。
结论
设计新型PCS椎弓根螺钉较普通CCD螺钉具有更高的生物力学锚定强度,椎弓根部分的致密螺纹对螺钉拔除力起到了主要加强作用,其强度提高程度和椎弓根部分螺纹致密程度成正比。本实验证实了PCS螺钉设计的合理性与先进性,为其下一步开发和临床应用奠定了理论基础。
Background
The pedicle screw and rod instrumentation system is one of the most commonly used and rapidly growing forms of stabilization for spinal fusion. Problems associated with pedicle screws include loss of fixation, fatigue, bending failure and improper placement. Bending or breakage is the most common type of hardware failure, which is strongly associated with the purchase strength of screws. Although pure pullout is not the mode of failure in most clinical situations, pullout testing is thought to be a good predictor of pedicle screw fixation strength. Biomechanical studies show that bone mineral density (BMD), pedicle morphology and screw thread area all affect pedicle screw pullout. Trabecular bone at the corticocancellous interface in pedicle is stronger than normal unsupported cancellous bone in the vertebral body. Therefore the pedicle part accounts for more than 80% of the axial purchase strength and more than 60% of the pull-out strength of the whole screw. However, the CCD screw currently used in clinic has the same thread pitch in pedicle as that in vertebral body. Based on these characteristics and the pedicle anatomy features of Chinese Han people, a new set of part concentrated screw (PCS) was designed with concentrated threads in the pedicle part, which was believed to gain more purchase strength with increased thread area in the pedicle of Chinese patients.
Objective
The purpose of the present research was to test the difference in pull-out strength between the two types of PCS screw (PCSI and PCSII) and CCD screw in human spine. Three dimensional finite element analysis (FEA) was employed to study the contribution of the threads in the pedicle part to the pull-out strength of the whole screw.
Methods
Fifteen fresh lumbar vertebrae (L1-L5) from three adult Han male cadavers were harvested and stripped of all soft tissues. The BMD in vertebral body and pedicle of all specimens was measured by dual energy X-ray absorptiometry (DEXA). Each individual vertebra was thawed and prepared for pedicle screw placement. Then the 30 pedicles were randomly assigned to three groups of ten pedicles each. The PCSI、PCSII and CCD screws were inserted by hand using a custom driver without tapering. The screw entry point was selected in accordance with the method of Magrel. Each screw was inserted until the hub of the screw abutted the lamina and all threads were fully contained within the bone. The maximum insertion torque was measured by a torque measuring spanner. The vertebrae, mounted in the testing jig, were placed a variable axis frame attached to the base plate. The hydraulic arm of the CSS-22100 machine was attached to the screw head by a universal joint and aligned so that pullout would be co-axial to the screw. Axial pullout was performed at 2.00 mm/minute displacement. Pullout loads and yield displacement were recorded at 0.1-second intervals until failure occurred. The three dimensional geometries of the screw and the bone with a threaded hole was separately created using the UG software. Then the models were imported into ANSYS and combined to generate the final model. The FEA procedure was performed to study the effect of concentrated threads in the pedicle part on the screw total pull-out strength.
Results
1. The BMD of vertebral body and pedicle of specimen measured by DEXA were 0.494±0.058 g/cm2 and 0.783±0.134 g/cm2 separately, with significant difference between the two area (p<0.05)。Test of normality was performed separately in the data of the two areas. Results showed that all data of the two areas came from a single population separately, with no difference among the data in the same area.
2. The pullout loads, maximum torsion torque and yield displacement of PCSI and PCSII screws were significantly higher than CCD screw. Student-Newman-Keuls test showed that there was significant difference among the three screws (p<0.05). The pullout loads, maximum torsion torque of PCSI were also significantly higher than PCSII, but there is no difference between the two PCS screws in yield displacement.
3. The FEA analysis was successfully performed using UG and ANSYS software. The force taken by each threaded connection in the pedicle was three times more than that in the vertebral body area. Student's t test showed that the thread in the pedicle can resist significantly higher force compared to the thread in the vertebral body (p<0.05). The force taken by the threads in the pedicle accounts for more than 85% of the whole pullout strength of the PCSI and PCSII screw.
Conclusions
The newly designed PCS screws had significantly improved purchased strength compared to the custom CCD screw. The improvement of pullout strength was mainly derived from and directly associated to the density of threads in the pedicle area. The result of current research can serve as foundation for the further development and application of PCS screw.
引文
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[48]崔新刚,张佐伦,刘建营等.棘突定位法在胸腰椎椎弓根螺钉内固定中的应用.中国脊柱脊髓杂志.2004,14(7)：429-431.
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[50]刘达,吴子祥,高明暄等.改进螺钉设计提高骨质疏松条件下椎弓根螺钉稳定性的研究进展.临床骨科杂志.2009,12(5)：571-574.
[51]Halvorson TL, Kelley LA, Thomas KA, et al. Effects of bone mineral density on pedicle screw fixation. Spine (Phila Pa 1976).1994,19(21):2415-2420.
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[54]Hirano T, Hasegawa K, Takahashi HE, et al. Structural characteristics of the pedicle and its role in screw stability. Spine (Phila Pa 1976).1997,22(21): 2504-2509,2510.
[55]郭世跋.骨科临床解剖学.2002,山东科学技术出版社.济南.1079
[56]Zhang QH, Tan SH, Chou SM. Effects of bone materials on the screw pull-out strength in human spine. Med Eng Phys.2006,28(8):795-801.
[57]Conrad BP, Cordista AG, Horodyski M, et al. Biomechanical evaluation of the pullout strength of cervical screws. J Spinal Disord Tech.2005,18(6):506-510.
[58]徐又佳,王以进.不同直径锥形椎弓根螺钉对腰椎骨折复位固定效果的动态生物力学研究.中华实验外科杂志.1994,11(4)：234-235.
[59]Krag MH, Beynnon BD, Pope MH, et al. An internal fixator for posterior application to short segments of the thoracic, lumbar, or lumbosacral spine. Design and testing. Clin Orthop Relat Res.1986, (203):75-98.
[60]Steffee AD, Biscup RS, Sitkowski DJ. Segmental spine plates with pedicle screw fixation. A new internal fixation device for disorders of the lumbar and thoracolumbar spine. Clin Orthop Relat Res.1986, (203):45-53.
[61]侯树勋,史亚民.国人下胸椎及腰椎椎弓根形态学特点及其临床意义.中华骨科杂志.1994,14(4)：222-225.
[62]王东,孙彤,郑文彬等.腰椎椎弓根螺钉植入前的X线测量.重庆医学.2007,36(15)：1511-1515.
[63]Zindrick MR, Wiltse LL, Doornik A, et al. Analysis of the morphometric characteristics of the thoracic and lumbar pedicles. Spine (Phila Pa 1976).1987, 12(2):160-166.
[64]李志军,温树正,汪建威等.椎弓根骨质的CT断面测量及其临床意义.中国临床解剖学杂志.2003,21(1)：37-40.
[65]Zdeblick TA, Kunz DN, Cooke ME, et al. Pedicle screw pullout strength. Correlation with insertional torque. Spine (Phila Pa 1976).1993,18(12): 1673-1676.
[66]Cleek TM, Reynolds KJ, Hearn TC. Effect of screw torque level on cortical bone pullout strength. J Orthop Trauma.2007,21(2):117-123.
[67]Okuyama K, Abe E, Suzuki T, et al. Can insertional torque predict screw loosening and related failures? An in vivo study of pedicle screw fixation augmenting posterior lumbar interbody fusion. Spine (Phila Pa 1976).2000,25(7): 858-864.
[68]Brekelmans WA, Poort HW, Slooff TJ. A new method to analyse the mechanical behaviour of skeletal parts. Acta Orthop Scand.1972,43(5):301-317.
[69]Zhang QH, Tan SH, Chou SM. Investigation of fixation screw pull-out strength on human spine. J Biomech.2004,37(4):479-485.
[70]Chatzistergos PE, Magnissalis EA, Kourkoulis SK. A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model. Med Eng Phys.2010,32(2): 145-154.