Finite element analysis of lumbar vertebrae under the pull-out strength of screw in cortical bone trajectory technique

doi:10.12307/2021.340

Abstract

Abstract: BACKGROUND: Pedicle screw fixation is the preferred surgical treatment for clinical treatment of lumbar degenerative disease currently. However, there is a great possibility of screw loosening after the traditional cortical bone trajectory pedicle screw placement scheme, which may cause fixation failure or non-fusion of bone graft.
OBJECTIVE: To compare biomechanical properties of novel cortical bone trajectory screw and cortical bone trajectory screw in lumbar vertebrae under pull-out strength by finite element method.
METHODS: Based on the extracted computed tomography of patients with osteoporosis, the finite element model of L1-L5 full vertebral body functional unit osteoporosis was established. The novel cortical bone trajectory screw and cortical bone trajectory screw were simulated. The stress distribution and maximum stress value of bone around the screw track of the two methods were measured and compared under a certain actual rotation pull-out strength.
RESULTS AND CONCLUSION: (1) Under the same fixation mode, the stress distribution of vertebrae in the novel cortical bone trajectory group and cortical bone trajectory group was significantly different under the same load state. Compared with cortical bone trajectory group, the volume of the stress zone and the stress concentration point were significantly reduced, which could significantly increase the stability of the inserted screw in the novel cortical bone trajectory group. (2) Different screw paths brought different bone mineral densities, which directly affected the mechanical properties of internal fixation screws in lumbar spine. (3) Different shapes and sizes of L1-L5 bone led to different mechanical properties and performance. It can be seen that the novel cortical bone trajectory technology has advantages over the cortical bone trajectory technology. The novel cortical bone trajectory technique is a preferred method for lumbar internal fixation in patients with osteoporosis.

Key words: lumbar spine, cortical bone trajectory, bilateral screw, stress analysis, finite element

CLC Number:

Ren Hangning, Jurat·Matrozi, Paerhati·Rexiti, Luo Huiqing. Finite element analysis of lumbar vertebrae under the pull-out strength of screw in cortical bone trajectory technique[J]. Chinese Journal of Tissue Engineering Research, 2021, 25(36): 5771-5776.

Figures/Tables 5

传统CBT和改良CBT各组两侧钉道的应力均集中分布在植入钉头的由松质骨向皮质骨过渡区一段范围内，离上终板1-5 mm(图2、3中a、c左端与b、d的右端)；钉尾部皮质骨部分(图2、3中a、c右端与b、d左端)则比较稳定，且改良CBT更明显。 L1：传统CBT组最大应力在钉道的螺纹外径配合处，图3左侧的1，2，3点与右侧的4，5，6点位置；改良CBT组钉道骨应力分布则相对比较匀称。 L2：在分布上总体与L1相似，只左侧和右侧钉道钉尾均有少量零星分布应力(图3上部L2栏的中间)，最大应力在顶尖位置(7点)。改良CBT组相比其他4组在分布规律上更加接近4组分布的叠加态，但在与钉尖结合部分(图3下部L2栏两端)相对较少分布，钉尾同样有一定量分布应力(L2栏的中间)，最大应力在19点位置(左侧)和20点所示位置(右侧)。 L3：分布如图2所示，相比L1、L2在分布上大体相同。传统CBT组最大应力在左侧钉道的钉尖与最末一段牙距螺纹外径配合处，a中8，9两点(左侧)，b中10点位置(右侧)。L3的改良CBT组钉尖部分钉道与L2相似，最大应力在21点位置。左侧钉传统CBT组和改良CBT组应力分布面总面积大体相当，右侧钉应力分布面略有差异。 L4和L5：传统CBT组L4和L5应力全段分布比较均匀。改良CBT组中L4和L5两侧尾部钉道(图3下部L4和L5栏中间)应力区分布密集度相比传统CBT组则有减小，L4最大应力在右侧钉头位置(图3的b中12点)，但在左侧钉尾(11点)出现了应力富集点。改良CBT组中L4最大应力位于右侧钉道钉尖(图3中22点)。传统CBT组L5的应力集中点相比其他椎体数量较多，最大应力在钉道的钉尖和最后一节螺距螺纹外径配合处(图3上部L5栏两端)，但在右侧钉中段螺纹外径下侧配合处(16点)也有一处应力富集。改良CBT组L4和L5左右侧钉道周围应力分布相较传统CBT组区别明显。应力分布图中L5钉尖钉道部分与L2相似且更明显，应力分布规律上左右两侧均集中在螺钉中段松质骨结合部分。传统CBT组中各应力集中位置距上终板距离依次为：0.9 mm(12)、1 mm(7)、1.1 mm(2)、1.2 mm(8)、1.3 mm(10)、2.8 mm(13)、2.9 mm(14、15)、3.2 mm(4)、3.5 mm(5)、 3.6 mm(6)、3.7 mm(1)、4 mm(9)、4.7 mm(17)、5.5 mm(3)、 8.5 mm(16)。改良CBT组各应力集中位置距离上终板距离依次为：0.5 mm(23)、0.9 mm(21)、1 mm(18)、1.6 mm(22)、 3.4 mm(19)、4.7 mm(20)。两种方式下钉道内最大应力值统计，以及改良CBT组对比传统CBT组钉道最大应力值、平均值和增长幅度如表1所示。对于生命体，应力大小关系到对细胞的刺激程度，应力越大可能对神经系统影响的程度越大，从结果来看，应力与方案并没有直接联系，而与关节个体差异有关。螺钉只要保证能以较大的外偏角度置钉，就不会损伤到贴附于椎弓根内侧骨壁，特别是内下方骨壁的神经根。结合图4，就神经根受影响的概率而言，只与个体差异和手术的具体实施过程有关，即椎弓根内壁和螺钉牙外径距离以及个体骨关节椎弓根内壁处骨质情况有关。由于个体差异不同，所以依据个体差异的方案提出是非常有必要的。"

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