| Objective: There is currently controversy over the length of pedicle screw placement for thoracolumbar burst fractures.This article explores the biomechanical impact of pedicle screw placement on the treatment of thoracolumbar burst fractures through finite element analysis.Method: This article applies the medical finite element method to construct a T12-L2 spinal model of healthy volunteers using CT images of the thoracic and lumbar vertebrae.The three-dimensional finite element software Minicis,Geomagic,and Solidworks were used to validate the effectiveness of the model through in vitro experiments and previous finite element models.Apply the "V" osteotomy method to remove some of the bone in the anterior and middle columns of the injured vertebra,simulating the collapse of the anterior and middle columns after a fracture.And after applying normal vertebral osteotomy,simulate the state after fracture reduction.According to different forms of internal fixation,starting from the absence of screws in the injured vertebra,the length of the injured vertebra increases by 5mm step by step until the pedicle screw breaks through the anterior cortex of the vertebral body,with a screw length of 50 mm.From this,10 models of different lengths of injured vertebra screws are constructed,and a total of 11 models of internal fixation for spinal burst fractures are added,to compare the role of the length of injured vertebra screws in thoracolumbar burst fractures.Import the model into Ansys analysis software,fix the lower surface of the L2 vertebral body,and then apply500 N simulated gravity to the upper surface of the T12 vertebral body.Apply a torque of10 Nm according to the coordinate axis to simulate forward bending,backward extension,lateral bending and torsion.Measure the displacement and activity of the fixed segment,the displacement and activity of the injured vertebral body,the displacement and activity of the injured vertebral nail,longitudinal rod The equivalent stress of the pedicle screw adjacent to the vertebral body and the injured vertebral body,as well as the stress on the endplate and articular cartilage on L1 and L2,comprehensively evaluate the effect of the length of the injured vertebral body screw placement on the stability of percutaneous nail treatment for thoracolumbar burst fractures,and define its appropriate length.Result: 1.A normal vertebral spine model of T12-L2 was successfully established and its effectiveness was verified.2.Construct 11 models of thoracolumbar burst fractures with internal fixation of vertebral screws with different injuries.3.As the length of the injured vertebral nail increases(0,5,10,15,20,25,30,35,40,45,50)mm,the results of the changes in internal fixation segment mobility are as follows: 2.1anterior flexion(8.62,6.07,6.04,6,3.98,3.44,3.18,2.98,2.94,2.92,2.89)°,posterior extension(1.95,1.04,1.03,1.01,0.31,0.12,0.03,0.04,0.02,0.05,0.06)°,Lateral flexion(5.4,3.72,3.69,3.66,2.39,2.06,1.91,1.8,1.79,1.77,1.75)°,torsion(5.78,3.83,3.8,3.77,2.51,2.18,2.04,1.92,1.9,1.88,1.87))° Gravity(5.29,3.56,3.53,3.5,2.15,1.79,1.6,1.47,1.47,1.43,1.41)°,2.2 Changes in vertebral activity are as follows: Forward flexion(8.08,5.52,5.48,5.44,4.63,4.39,4.23,2.5,2.12,1.83,2.28)°,Extension(2.53,1.61,1.6,1.59,1.31,1.23,1.19,0.66,0.55,0.43,0.6)°,lateral flexion(5.34,3.62,3.59,3.57,3.08,2.91,2.78,1.67,1.4,1.24,1.51)°,torsion(5.76,3.82,3.79,3.76,3.13,2.99,2.93,1.88,1.59,1.47,1.64)°,gravity(5.31,3.57,3.55,3.52,2.97,2.81,2.71,1.57,1.34,1.13,1.44)°.2.3 In terms of anterior flexion and internal motion,there was a statistically significant difference in motion between 30 mm and 50 mm internal fixation segments,while there was no statistically significant difference in motion between 35 mm,40mm,and 45 mm internal fixation segments and 50 mm internal fixation segments;The difference in vertebral activity between 35 mm,40mm and 45 mm was statistically significant,while the difference in vertebral activity between 50 mm and 45 mm was not statistically significant.2.4The displacement and motion axis activity of the internal fixation segment and the injured vertebra,the stress of the injured vertebra screw,longitudinal rod,pedicle screw near the injured vertebra,and the injured vertebra,as well as the stress on the upper endplate and small joint,are shown in the results table.4.Based on the above results,18 trend charts of activity were drawn.Through the analysis of the trend chart,it was found that as the length of the injured vertebral nail increased,the activity of the internal fixation segment decreased under different activity states.After the length of the injured vertebral nail exceeded 35 mm,the lateral view of the vertebral body showed that the overall activity of the internal fixation did not change much after the length of the inserted screw exceeded 1/2 of the anterior and posterior diameter of the vertebral body.The range of motion of the injured vertebral body also decreased.When the length of the injured vertebral nail was 45 mm,the lateral view of the vertebral body showed that the length of the screw exceeded 3/4 of the anterior and posterior diameter of the vertebral body,and the range of motion tended to stabilize;In terms of the effect on the equivalent stress of pedicle screws and longitudinal rods,as the length of the screw increases,the stress of the injured vertebral nail increases,while the stress of the longitudinal rod and the upper vertebral body pedicle screw decreases significantly.The stress of the lower vertebral pedicle screw and the injured vertebral body have little change;The impact on the transmission of internal fixation force is that as the length of the injured vertebral nail increases,the force on the upper endplate of the injured vertebral body significantly increases,and the force on the small joint of the injured vertebral body significantly decreases.However,there is little change in the upper endplate force and the lower joint force of the injured vertebra.5.As the length of the injured vertebra increases,the overall stability of internal fixation and the injured vertebra further increases.For the overall stability of internal fixation,the length of the injured vertebra nail should exceed 1/2 of the anterior and posterior diameter of the vertebra.For the injured vertebra,the length of the injured vertebra nail should exceed 3/4 of the anterior and posterior diameter of the vertebra for good stability.In order to achieve good stability,the length of the injured vertebral pedicle screw should exceed 3/4 of the anteroposterior diameter of the vertebral body.Therefore,it is recommended to define a pedicle screw with a length exceeding 3/4of the vertebral body as a long screw.After the placement of long screws on the injured vertebra,the stress on the injured vertebra increases,and due to the reduced mobility,the stress on the longitudinal rod and upper vertebral pedicle screws is reduced,resulting in a more balanced stress in the internal fixation system.In the overall mechanical transmission of internal fixation,the transfer of force from the posterior column of the spine to the anterior middle column of the vertebral body is more in line with the transmission of biomechanical force after the placement of long screws in the injured vertebra.Conclusion: Percutaneous pedicle screw placement through the injured vertebra can significantly increase the stability of the internal fixation system for thoracolumbar burst fractures.As the length of the percutaneous pedicle screw increases,its stability and safety increase simultaneously.To achieve the best fixation effect,the length of the injured vertebral screw should be at least 45 mm,and in the lateral view of the vertebral body,the length of the injured vertebral screw should be at least 3/4 of the anteroposterior diameter of the injured vertebral body. |
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