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With the aging population, osteoporosis, which leads to poor fusion, has become a common challenge for lumbar surgery. In addition, most people with osteoporosis are elderly individuals with poor surgical tolerance, and poor bone quality can also weaken the stability of internal fixation. This study compared the fixation strength of the bilateral traditional trajectory screw structure (TT-TT), the bilateral cortical bone trajectory screw structure (CBT-CBT), and the hybrid CBT-TT (CBT screws at the cranial level and TT screws at the caudal level) structure under different bone mineral density conditions. A finite element (FE) analysis study. Above all, we established a healthy adult lumbar spine model. Second, under normal and osteoporotic conditions, three transforaminal lumbar interbody fusion (TLIF) models were established: bilateral traditional trajectory (TT-TT) screw fixation, bilateral cortical bone trajectory (CBT-CBT) screw fixation, and hybrid cortical bone trajectory screw and traditional trajectory screw (CBT-TT) fixation. Finally, a 500-N compression load with a torque of 10 N/m was applied to simulate flexion, extension, lateral bending, and axial rotation. We compared the range of motion (ROM), adjacent disc stress, cage stress, and posterior fixation stress of the different fusion models. Under different bone mineral density conditions, the range of motion of the fusion segment was significantly reduced. Compared to normal bone conditions, the ROM of the L4–L5 segment, the stress of the adjacent intervertebral disc, the surface stress of the cage, and the maximum stress of the posterior fixation system were all increased in osteoporosis. Under most loads, the ROM and surface stress of the cage and the maximum stress of the posterior fixation system of the TT-TT structure are the lowest under normal bone mineral density conditions. However, under osteoporotic conditions, the fixation strength of the CBT-CBT and CBT-TT structures are higher than that of the TT-TT structures under certain load conditions. At the same time, the surface stress of the intervertebral fusion cage and the maximum stress of the posterior fixation system for the two structures are lower than those of the TT-TT structure. Under normal bone mineral density conditions, transforaminal lumbar interbody fusion combined with TT-TT fixation provides the best biomechanictability. However, under osteoporotic conditions, CBT-CBT and CBT-TT structures have higher fixed strength compared to TT-TT structures. The hybrid CBT-TT structure exhibits advantages in minimal trauma and fixation strength. Therefore, this seems to be an alternative fixation method for patients with osteoporosis and degenerative spinal diseases. This study provides biomechanical support for the clinical application of hybrid CBT-TT structure for osteoporotic patients undergoing TLIF surgery. [ABSTRACT FROM AUTHOR]

• Analysis of tri-cortical pedicle screw fixation was preformed on thoracic vertebrae for the first time by computed tomography followed by three dimensional finite element analysis. • Tri-cortical pedicle screw fixation in thoracic vertebrae can provide better stability and strength than the uni-cortical and bi-cortical techniques. • Tri-cortical pedicle screw internal fixation can be used in the osteoporotic thoracic vertebral body to improve the anchoring strength and reduce the risk of loosening. The technique of tri-cortical pedicle screw (TCPS) has been used to improve the anchoring strength in the sacral vertebrae. However, no studies have reported their application in the thoracic vertebrae. Our research is aimed to assess the stability and strength of the TCPS in thoracic vertebrae under osteoporotic condition by three dimensional (3D) finite element method on the basis of medical image reconstruction using computed tomography (CT), and verifying its effectiveness in clinical application. The 3D finite element models were constructed using Mimcs to transfer two dimensional CT images into 3D models by marching cubes algorithm of six-partition. Six physiological activities were simulated in 3D finite element models. Compared with the strength and stability of the uni-cortical pedicle screw (UCPS) and bi-cortical pedicle screw (BCPS), the effectiveness of TCPS was assessed. The stress distribution and maximum stress were measured to evaluate the strength. The maximum displacement and the range of motion were analysed to assessed the stability. Four geometrically accurate and nonlinear T7–T9 finite element models were constructed successfully by 3D finite element method based on the CT images. Three kinds of internal fixation methods in the osteoporotic thoracic vertebral body can improved the maximum stress, decrease the maximum displacement and range of motion in six physiological activities. The range of motion and maximum displacement of TCPS decreased more significantly than that of UCPS and BCPS. The maximum von Mises stress of TCPS was minimum and UCPS was maximum under the condition of extension, right lateral bending, left rotation and right rotation. Effectively, TCPS can provide better stability and strength than that of UCPS and BCPS techniques in the osteoporotic thoracic vertebrae. In practice, the technique of TCPS can be applied in the osteoporotic thoracic vertebral body to enhance the griping strength of the screws and reduce the risk of pedicle screw loosening. However, further cadaver experiments and more biomechanical analysis are necessary to confirmed our findings. [ABSTRACT FROM AUTHOR]

• This is the first study to compare the biomechanical advantages between the new pedicle screw-plate orthopedic internal fixation system and traditional pedicle screw-rod orthopedic internal fixation system by the finite element method. • The novel pedicle screw-plate internal fixation system has lower profile in orthopedics. • The novel pedicle screw-plate internal fixation system has higher strength when comparing with the traditional pedicle screw-rod orthopedic internal fixation system. • The novel spine pedicle screw-plate system can be used as a new internal fixation and provide better comfort for patients. The traditional pedicle screw-rod internal fixation system has been widely used for thoracic diseases in clinical practice, but its high profile increases the damage to soft tissue, leading to long-term intractable back stiffness. The purpose of this study is to compare biomechanical advantages between the new spine pedicle screw-plate internal fixation system and traditional pedicle screw-rod internal fixation system using finite element analysis. Based on computed tomography (CT), four three-dimensional finite element models of T7-T9 were constructed. The downward concentrated force of 150 N and the moment of 5 Nm was applied to the models to simulate six physiological activities, including flexion, extension, left and right lateral bending, left and right axial torsion. The maximum displacement, range of motion (ROM) and maximum stress of the two models in six physiological activities, was measured to evaluate the biomechanical advantages of the novel pedicle screw-plate internal fixation system. The novel pedicle screw-plate internal fixation system has a lower profile than the traditional pedicle screw-rod internal fixation system. With regards to the stability, the maximum displacement of the models of two internal fixation systems decreased by 56.2%–91.4% under the six motion status when comparing with the unstable model. Meanwhile, the ROM remained unchanged between the two models of internal fixation systems besides the left lateral bending. However, there is no significant difference in the ROM between the models of the two internal fixation systems in left lateral bending motion (P = 0.203). In terms of the strength, the maximum stress in the model with the new pedicle screw-plate internal fixation system was higher than that of model with the traditional pedicle screw-rod internal fixation system in every motion status but left and right lateral bending motion. The novel pedicle screw-plate internal fixation system has lower profile in orthopedics and higher strength, However, it has no disadvantage when comparing with the traditional pedicle screw-rod internal fixation system in terms of the stability. In summary, we suggest that the novel spine pedicle screw-plate system can be used as a new internal fixation and provide better comfort for patients. [ABSTRACT FROM AUTHOR]

• Finite element method was used to compare the internal fixation methods of lumbar spondylolysis. • Two types of lumbar spondylolysis internal fixation systems: pedicle screw-U shape rod (PSUSR) and pedicle screw-vertebral plate hook (PSVPH) internal fixation systems. • Based on CT images, four models are established by threshold segmentation and contour line reconstruction algorithm. • After finite element analysis, the PSVPH internal fixation system has better biomechanical properties. Internal fixation surgeries are currently the most effective treatments for lumbar spondylolysis, but the optimal fixation method is still on debate. This study was designed to compare the biomechanical characteristics of two fixation methods for lumbar spondylolysis, the pedicle screw-U shape rod (PSUSR) internal fixation system, and the pedicle screw-vertebral plate hook (PSVPH) internal fixation system, through three-dimensional finite element analysis, expecting to provide clinical guidance. Four finite element models (A, B, C, D) of L4-S1 vertebral body of a female patient were reconstructed by CT image segmentation. (A: intact model. B: spondylolysis model. C: spondylolysis model with PSUSR internal fixation. D: spondylolysis model with PSVPH internal fixation). Six physiological motion states were simulated by applying 500N concentrated force and 10Nm moment load to four models. The biomechanical advantages of the two internal fixation systems were evaluated by comparing the range of motion (ROM), maximum stress, maximum strain, and maximum displacement of the models. Compared to model B, the ROM decreased by 35.7%-57.1% in model C and 39.7%-64.8% in model D. The maximum displacements of model C and D both decreased. The maximum stresses in both vertebral and the internal fixation system are greater in model C than those in model D. The maximum stress and strain reduction of L5-S1 intervertebral disc in model D was greater than that in model C. Model D restores the articular cartilage stresses to the normal levels of model A. The maximum stress and maximum displacement of the bone graft in model C are greater than those in model D. The PSVPH internal fixation system has better biomechanical properties than PSUSR internal fixation system in several mechanical comparisons. Experimental results suggest that PSVPH internal fixation system can effectively treat lumbar spondylolysis while preserving segmental mobility, and can be the treatment of choice. [ABSTRACT FROM AUTHOR]