Double pedicle screw instrumentation in the osteoporotic spine: a biomechanical feasibility study

STUDY DESIGN: A biomechanical feasibility test. OBJECTIVE: To assess the overall feasibility, safety, and mechanical effectiveness of an intrapedicular double-screw construct in the thoracolumbar spine. SUMMARY OF BACKGROUND DATA: The bony purchase of the pedicle screw fixation is often not strong enough in elderly patients with osteoporosis. Our hypothesis was that the elliptical cross-section of the pedicle would allow the insertion of 2 smaller diameter pedicle screws resulting in a bony purchase superior to the standard single-screw technique. METHODS: Thirty-six double-screw constructs (5mm diameter AOUSS and 5 mm Schanz screw) and 36 standard single pedicle screws (6mm diameter AOUSS screw) were placed. Screw pullout, multiaxial flexibility, and axial failure load testing was performed. RESULTS: Visual inspection, palpation, and radiograph confirmed that there were no pedicle breaches. In the double-screw group, all but 2 constructs had ideal direction. Pullout strength of the double-screw construct was no different than that of the single-screw construct. However, stiffness increased considerably in all testing modes. Axial load to failure, adjusted for bone mineral density, and dimensional variation, also increased. All differences were statistically significant except for axial rotation that was only marginally significant. CONCLUSIONS: The double-screw construct appears feasible and safe in the thoracolumbar spine. In this study, the new technique demonstrates a mechanical advantage over the standard single-screw technique. Further in vitro cadaveric safety studies with better adapted instrumentation are needed before the technique can be widely recommended.     

骨水泥修复颈椎前路椎体螺钉钉道的生物力学研究

目的 评价聚甲基丙烯酸甲酯(Polymethylmethacrylate,PMMA)修复颈椎前路椎体螺钉钉道的生物力学效果。方法 防腐成人颈椎标本(C_3~7)共30个,分3组:正常骨密度组(A组)、骨质疏松组(B组),骨质疏松修复组(C组),各组10个标本,测试疲劳前后的最大轴向拔出力。A、B对照组椎体二侧钉孔均放置4 mm的椎体螺钉,随机选择一侧行即时最大拔出力实验,另一侧在疲劳实验(2 Hz,20 000次)后行最大拔出力实验。C组(实验前需进行螺钉拔松实验)注入PMMA 0.6~1.0 mL,并拧入螺钉,随机选择一侧进行即时和疲劳后最大拔出力实验。结果 A、B、C组即时拔出力明显高于疲劳后拔出力,差异具有统计学意义(P< 0.05),C组疲劳后与A组即时拔出力比较,差异无统计学意义(P >0.05),但高于B组即时拔出力,差异具有统计学意义(P< 0.05)。即时拔出力C组 >A组 >B组,差异均有统计学意义(P< 0.05)。疲劳后拔出力C组 >A组 >B组,差异均有统计学意义(P< 0.05)。PMMA注射后未发现钉道渗漏。结论 PMMA钉道修复能够明显增强椎体螺钉的即时稳定性和抗疲劳能力, 并且安全有效,适用于螺钉松动和拔出的修复固定。     

新型上胸椎椎弓根钉内固定系统的设计及其生物力学分析

目的测试新型上胸椎椎弓根钉内固定系统的生物力学性能。方法采用国际通用标准ASTM F543-2013及ASTM F1717-2013测试新型上胸椎椎弓根钉内固定系统的生物力学性能,进行单轴钉置入及拔出试验,万向钉压缩试验。并与常用颈胸椎椎弓根钉棒系统(常用组)及强生椎弓根钉棒系统(强生组)进行比较。结果试验组置入力矩低于常用组,拔出力小于常用组,差异有统计学意义(P0.05)。试验组屈服力大于强生组,但小于常用组,差异有统计学意义(P0.05)。试验组屈服刚度大于强生组,弹性位移、屈服位移小于强生组,但与常用组比较差异无统计学意义(P0.05)。试验组动态试验加载力最大值81 N为强生组的屈服力,反复加载后未损坏,故动态测试结果优于强生组。常用组动态试验加载力最大值110.9 N为试验组的屈服力,试验组与常用组反复加载后均未损坏,故动态试验结果接近。结论若上胸椎不稳但胸廓完整,新型上胸椎椎弓根钉内固定系统的生物力学性能可以满足临床的需要,提供即刻稳定性。     

Cervical pedicle screw fixation at C6 and C7: A cadaveric study

Background:Cervical pedicle screw fixation is an effective method for treating traumatic and non traumatic injuries. But many studies have reported higher incidence of cervical pedicle penetration, so many research efforts have aimed at improving the accuracy of cervical screw fixation. Most of the anatomical studies on cervical pedicle screw placement previously published focused on the measurements of anatomical parameters, the entry point of pedicle screw is vague. We preliminarily designed a C3, C4 and C5 pedicle screw fixation method that had clear entry point and clinical cases confirmed that this method is feasible and safe. So we did this study of cervical pedicle screw fixation for C6 and C7 vertebrae.Results:In C6, the PW and PH were 6.12 ± 0.78 and 7.48 ± 0.81 mm, respectively. In C7, the PW and PH were 6.85 ± 0.73 and 8.03 ± 0.38 mm, respectively. The LSC was 30.83 ± 0.91 mm. Two E angles were identified, namely E1 and E2 and their values were 89.61 ± 1.24 and 59.71 ± 1.10°, respectively. Meanwhile, F averaged 75.86 ± 1.12°.Conclusion:The intersection of the horizontal line through the midpoint of the transverse process root and vertical line through the intersection of the posterolateral and posterior planes of the isthmus can be used as an entry point for C6 and C7 pedicle screw fixation. The screws should be inserted at 60 or 90° with the posterolateral isthmus in the horizontal plane and at 75° with the posterior isthmus in the sagittal plane. The LSC should not exceed 30 mm.     

Biomechanical evaluation of diagonal fixation in pedicle screw instrumentation.

STUDY DESIGN: Flexibility tests and finite element analyses were performed for the biomechanical evaluation of diagonal transfixation in pedicle screw instrumentation. OBJECTIVE: To assess the biomechanical advantages of diagonal transfixation compared with conventional horizontal transfixation. SUMMARY AND BACKGROUND DATA: A few pedicle screw instrumentation systems allow the use of cross-links in the diagonal direction. Such a diagonal transfixation is anticipated to improve the surgical construct stability, but its biomechanical qualities have not been completely evaluated. METHODS: Flexibility tests were performed on 10 calf lumbar spines (L2-L5). Specimens were subjected to pure moments up to 8.2 Nm in flexion, extension, lateral bending, and extension while the resulting movements of L3 and L4 were measured by a three-dimensional motion analysis system. The tested cases included (1) intact, (2) pedicle screw fixation without transfixation after total removal of the L3-L4 disc, (3) pedicle screw fixation with diagonal transfixation, and (4) pedicle screw fixation with horizontal transfixation. Three-dimensional finite element models of the tested surgical constructs were also developed by use of three-dimensional beam elements to investigate the effect of diagonal transfixation and horizontal transfixation on the construct stability and the corresponding stress changes in the screws. RESULTS: When compared with no transfixation, horizontal transfixation significantly improved the lateral bending and axial rotation stability by 15.7% and 13.9%, respectively, but there was no improvement of stability in flexion and extension. By contrast, diagonal transfixation significantly improved the flexion and extension stability by 12% and 10.7%, respectively, but not the lateral bending and axial rotation stability in comparison with no transfixation. Comparison between horizontal transfixation and diagonal transfixation showed that the stabilizing effect of diagonal transfixation was greater in flexion and extension (13% and 11%, P < 0.01) than that of horizontal transfixation but smaller in lateral bending (11%, P < 0.05) and axial rotation (6.6%, P > 0.1). Finite element model predictions of the motion changes were similar to the changes observed in flexibility tests. In horizontal transfixation, the load changes, compared with no transfixion, were a 0.02% increase in flexion-extension, a 27.5% increase in lateral bending, and a 58% decrease in axial rotation, and the magnitudes of the moments applied on both the right and left pedicle screws were identical. However, when diagonal transfixation was achieved by connecting the left superior screw and the right inferior screw, the loads in the left screw were increased by 11.5% in flexion-extension, 43.6% in lateral bending, and 7.9% in axial rotation, whereas the loads in the right screw were decreased by 10.9% in flexion-extension, increased by 0.06% in lateral bending, and decreased by 18.1% in axial rotation. CONCLUSIONS: The results of this study showed that diagonal transfixation provides more rigid fixation in flexion and extension but less in lateral bending and axial rotation in comparison with horizontal transfixation. Furthermore, greater stresses in the pedicle screws were predicted in the diagonal transfixation model. These limitations of diagonal transfixation should be considered carefully for clinical application.     

膨胀式脊柱内固定系统椎弓根螺钉翻修作用的生物力学研究

目的：评价自行设计的膨胀式脊柱同定系统（expansive spinal fixation system，ESFS）的椎弓根螺钉对椎弓根螺钉固定失败后的翻修作用。方法：将30个深低温冰冻的正常成人腰椎体标本随机分为A、B、C三组．每组10个椎体（20侧椎弓根）。各组标本每个椎体的两侧椎弓根均先拧入直径6．0mm、长45mm的CD-Ⅱ螺钉．行螺钉拔出试验，记录螺钉的最大旋入力矩和最大轴向拔出力。然后将CD-Ⅱ螺钉拔出，各组标本每一椎体随机经一侧椎弓根原钉道拧入直径7．0mm、长45mm的ESFS螺钉；A组另一侧椎弓根拧入直径7．0mm、长45mm的CD-Ⅱ螺钉，B组另一侧拧入直径7．0mm、长45mm的TSRH螺钉．C组另一侧拧入直径7．0mm、长45mm的GSS螺钉。分别测试螺钉最大旋入力矩和最大轴向拔出力。结果：A、B、C三组的ESFS螺钉最大轴向拔出力分别为6mm CD-Ⅱ螺钉的113％、110％和112％，而直径7．0mm、长45mm的CD-Ⅱ螺钉、TSRH螺钉和GSS螺钉的最大轴向拔出力分别只有6mm CD-Ⅱ螺钉的80％、82％和88％，各组ESFS螺钉最大轴向拔出力明显高于其它三种螺钉。差异有显著性（P〈0．01）。各组各螺钉最大旋入力矩之间差异无显著性（P〉0．05）。结论：ESFS螺钉具有很好的椎弓根锚固作用及翻修作用。     

聚甲基丙烯酸甲酯强化和修复椎弓根螺钉的生物力学研究

目的 评价聚甲基丙烯酸甲酯（ｐｏｌｙｍｅｔｈｙｌｍｅｔｈａｃｒｙｌａｔｅ,ＰＭＭＡ）强化和修复椎弓根螺钉固定的生物力学效果。方法 ８具成人新鲜腰椎骨Ｌ３－５,随机选择一侧椎弓根放置直径为６．０ｍｍ的ＣＣＤ（ｃｏｍｐａｃｔＣＤ）椎弓根螺钉,另一侧以直径为３．５ｍｍ的钻头导孔,均不穿透椎体前皮质。在材料试验机上进行椎弓根螺钉拔出实验,拔出速率为５ｍｍ／ｍｉｎ。然后沿椎弓根孔道注入ＰＭＭＡ粉和水按体积１     

椎弓根螺钉设计与其生物力学稳定性的研究进展

<正>椎弓根螺钉内固定术是治疗脊柱疾病的主要方式之一[1],而螺钉松动是椎弓根螺钉内固定术的常见并发症[2],骨质疏松是引起螺钉松动的重要因素之一,其发生率高达60%[3]。有研究报道,当胸腰椎的骨密度时,椎弓根螺钉的生物力学稳定性明显下降[4]。测试椎弓根螺钉的生物力学稳定性指标主要有抗拔出力、疲劳强度、刚度、拧入扭矩[5-9]等。有研究表明,用聚甲基丙     

Design and biomechanical study of a modified pedicle screw

Objective： In pedicle screw fixation, the heads of monoaxial screws need to be directed in the same straight line to accommodate the rod placement by backing out during operation, which decreases the insertional torque and internal fixation strength. While polyaxial screws facilitate the assembly of the connecting rod, but its ball-in-cup locking mechanism reduces the static compressive bending yield strength as compared with monoaxial screws. Our study aimed to assess the mechanical performance of a modified pedicle screw.
Methods： In this study, the tail of the screw body of the modified pedicle screw was designed to be a cylindershaped structure that well matched the inner wall of the screw head and the screw head only rotated around the cyclinder. Monoaxial screws, modified screws and polyaxial screws were respectively assembled into 3 groups ofvertebrectomy models simulated by ultra high molecular weight polyethylene （UHMWPE） blocks. This model was developed according to a standard for destructive mechanical testing published by the American Society for Testing Materials （ASTM F1717-04）. Each screw design had 6 subgroups, including 3 for static tension, load compression and torsion tests, and the rest for dynamic compression tests. In dynamic tests, the cyclic loads were 25%, 50%, and 75% of the compressive bending ultimate loads respectively. Yield load, yield ultimate load, yield stiffness, torsional stiffness, cycles to failure and modes of failure for the 3 types of screws were recorded. The results of modified screws were compared with those ofmonoaxial and polyaxial screws.
Results： In static tests, results of bending stiffness, yield load, yield torque and torsional stiffness indicated no significant differences between the modified and monoaxial screws （P〉0.05）, but both differed significantly from those ofpolyaxial screws （P〈0.05）. In dynamic compression tests, both modified and monoaxial screws showed failures that occurred at the insertion point of screw body into the UHMWPE block, while the polyaxial screw group showed screw body swung up and down the screw head because of loosening of the ball-in-cup mechanism.
Conclusions： The modified screw is well-designed and biomechanically improved. And it can provide sufficient stability for segment fixation as monoaxial screws.     

经伤椎单节段椎弓根螺钉固定治疗胸腰椎爆裂骨折的生物力学研究

目的 通过生物力学测试验证经伤椎单节段固定脊柱胸腰椎爆裂骨折(AO分型A3.1型)的稳定性. 方法 选取6具T10～L2的人脊柱标本,通过落锤实验机建立AO分型A3.1型T12爆裂骨折模型,在伤椎及其相邻椎体上置入椎弓根螺钉,建立单节段椎弓根螺钉固定(MSPI)模型,每具标本均进行完整、骨折及MSPI固定3种状态的生物力学测试,分析MSPI固定与完整标本的稳定性差异. 结果 MSPI固定组的前屈、后伸、左侧弯及右侧弯时的活动度平均分别为0.14°±0.02°、0.07°±0.02°、0.73°±0.22°和0.74°±0.13°明显小于骨折组0.84°±0.10°、1.94°±0.30°、2.33 °±0.56°和2.54°±0.44°及完整组0.46°±0.05°、1.01°±0 23°、1.34°±0.24°和1.30°±0.31°,差异均有统计学意义(P＜0.05).当扭转相同角度时,MPSI固定组所需的扭力(3103.00±213.11)N与完整组(2654.75±122.90)N相当,差异无统计学意义(P＞0.05),大于骨折组(1519.60±100.82)N,差异有统计学意义(P＜0.05). 结论 治疗AOA3.1型胸腰椎爆裂骨折时,MSPI能重建脊柱的稳定性.     

聚甲基丙烯酸甲酯强化对骨质疏松椎弓根螺钉固定的生物力学作用

目的探讨聚甲基丙烯酸甲酯 (polymethylmethacrylate,PMMA)骨水泥强化椎弓根螺钉的方法和评价 PMMA强化骨质疏松椎弓根螺钉后的生物力学性质。方法 6具新鲜老年女性胸腰段骨质疏松脊柱标本 (T10～ L5),使用双能 X线骨密度吸收仪测试每个椎体的骨密度,随机取 16个椎体 (32侧椎弓根 ),一侧椎弓根拧入 CCD螺钉,测量最大旋入力偶矩后拔出螺钉作为正常对照组,用 PMMA骨水泥强化椎弓根螺钉作为修复固定组,行螺钉拔出试验;另一侧经导孔直接强化椎弓根螺钉后拔出作为强化固定组,记录三组螺钉的最大轴向拔出力。结果椎体平均骨密度为 (0.445± 0.019)g/cm2;螺钉最大旋入力偶矩为（ 0.525± 0.104） Nm;正常对照组螺钉最大轴向拔出力为 (271.5± 57.3)N;修复固定组为 (765.9± 130.7)N;强化固定组为 (845.7± 105.0)N。 PMMA骨水泥强化或修复骨质疏松椎弓根螺钉后最大抗压力明显高于强化前,差异有非常显著性意义 (P< 0.01)。结论 PMMA骨水泥强化骨质疏松椎弓根螺钉能显著增加螺钉在椎体内的稳固性。     

通用型脊柱内固定系统椎弓根螺钉翻修作用的生物力学研究

目的测试自行设计的通用型脊柱内固定系统(generalspinesystem,GSS)椎弓根螺钉以及SOCON、TSRH和Diapason螺钉置入椎体的拔出力及旋入力矩,评价GSS螺钉的翻修作用。方法将36个正常成人腰椎椎体标本随机分为三组,每组12个椎体(24侧椎弓根)。各组标本每个椎体的每侧椎弓根均先拧入CCD螺钉(直径6.0mm,长45mm),行螺钉拔出试验,测试并记录螺钉的最大旋入力矩和最大轴向拔出力。然后各组标本每个椎体均随机选择一侧椎弓根经原钉道拧入GSS螺钉(直径6.5mm,长45mm);第一组另一侧拧入SOCON螺钉(直径7.0mm,长45mm),第二组另一侧拧入TSRH螺钉(直径7.0mm,长45mm),第三组另一侧拧入Diapason螺钉(直径6.7mm,长45mm)。分别测试螺钉最大旋入力矩及最大轴向拔出力。结果第一组GSS螺钉最大轴向拔出力为CCD螺钉的114%,SOCON螺钉为CCD螺钉的108%;GSS螺钉最大轴向拔出力大于SOCON螺钉,差异无显著性(P>0.05);GSS螺钉最大旋入力矩小于SOCON螺钉,差异无显著性(P>0.05)。第二组GSS螺钉最大轴向拔出力为CCD螺钉的127%,TSRH螺钉为CCD螺钉的64%;GSS螺钉最大轴向拔出力大于TSRH螺钉,差异有显著性(P<0.01);GSS螺钉最大旋入力矩大于TSRH螺钉,差异有显著性(P<0.01)。第三组GSS螺钉最大轴向拔出力为CCD螺钉的122%,Diapason螺钉为CCD螺钉的8