| Objective(1) To measure the L5 isthmus portion of lumbar vertebra, get anatomic data and analyze it.(2) To make pathologic defect models of isthmus external 1/2 and internal 1/2(3) To compare and analyze the anti-fatigue features of these two models(4) To make finite element model on the defective isthmus external 1/2(5) To analyze and solve the transmit and distribution features of breaking stress on the isthmus portion of lumbar vertebraMethods(1) Take 12 fresh lumbar vertebra specimens. All the specimens are taken from fresh adult male corpse died from accident, age unidentified, without lumbar vertebra disease. Dissect L5 centrum, remove soft tissue and maintain only the bone part. Embedding with PMMA to fix the specimen (dental base acrylic resin powder, PMMA, self-cures. Manufactured by shanghai orthodontics material factory). Store them in-20℃freezer. Use electric vernier caliper to measure the width of lumbar vertebra isthmus. Each specimen should be measured twice so as to ensure the accuracy. Compare the left and right isthmus side width and analyze them.(2) Divide these 12 specimens randomly into 2 groups. Use rongeur to remove respectively the external 1/2 isthmus and the internal 1/2 isthmus bone and then make pathological defect models. Use electric vernier caliper to measure the width of the remaining isthmus. Compare the width of the external 1/2 isthmus defect model and the internal 1/2 isthmus defect model.(3) Fix the specimen on spine 3D movement test machine, put the external 1/2 isthmus defect model and the internal 1/2 isthmus defect model on mechanic test machine MTS-858 Mini Bionix (MTS, Minneapolis, MN, USA). Give each of them compressed load, with 0.5 Hz frequency and 60-600 N periodic sine waves. Keep spraying physiological saline onto the specimen throughout the test to keep them moisture. Record the breakage times and analyze them.(4) Take a young male fresh corpse, use spiral CT to scan each layer of the lumbar vertebra specimen along axial direction, each layer space is 1mm. Input the CT image into computer, use MIMICS 11.1 to collect matrix of the contour line data, transform them into dat files and then put them into ANSYS 10.0 to build up model.(5) Use pre-treatment software Mimics 11.1 and Simpleware3.1 to make L4 vertebra isthmus external 1/2 defect model, and then lead it in to finite element system with ANSYS 10.0, and then apply force onto the upper centrum surface. Apply 5N* meter load, ante flexion, rear protraction and side bend, and then enter the Ansys solver to analyze and solve the transmit and distribution features of breaking stress on the isthmus portion of lumbar vertebraResult(1) The width of the normal L5 left isthmus portion of lumbar vertebra is 15.06±0.83, the right width is 14.85±0.72. The difference between the left width and the right width is P=0.52>0.05, no statistic value.(2) The isthmus width of the external 1/2 isthmus defect model is 5.24±0.41, the isthmus width of the internal 1/2 isthmus defect model is 5.15±0.34. The difference between the external and the internal isthmus width is P=0.57>0.05, no statistic value.(3) The breakage times of the external 1/2 isthmus defect model are 25160±1019, while the internal 1/2 isthmus defect model are 18782±3126. The difference between them is P=0.006<0.05, has statistic value.(4) Build up a detail 3D model for the waist, including bone structure and soft tissue. The bone structure include lumbar vertebra movement segment L4-L5; the soft tissue include inter vertebral disk, gristle, and main ligament and muscle on the waist. The whole model include 5668 8-nodeSolid 45 units,8236 shell units and 239 connecting units, consisting of 5462 surfaces,20811 lines and 81221 nodal points.(5) The result of finite element experiment shows:in the rear protraction rotation, the left side joint has the maximum stress; its dynamic direction forms a 45 degree angle with the X axis. In the ante flexion rotation, the stress is 164n; it forms an approximate 45 degree angle with the X axis. Side bend rotation has the minimum stress; it forms an approximate 15 degree angle with the X axis.Conclusion(1) The anti-fatigue features of the external 1/2 lumbar vertebra isthmus defect model is better than the internal 1/2. The internal 1/2 bears more loads. The external 1/2 is easier to become fatigue and fracture.(2) According to the stress distribution map of the L4 external 1/2 isthmus portion of lumbar vertebra, the articular process and vertebral pedicle neighboring part exist high stress zone. The result shows that rear protraction plus right rotation have the maximum stress, which has distinctive effect on articular process distribution. While the spinous process is not the rotation center, the result shows that there is no concentration trend of stress distribution. The result also shows that when the isthmus defection reaches 1/2, the stress of the remaining isthmus decreases notably, the rear protraction should be avoided in a period of time.(3) There is no obvious stress change to the anterior longitudinal ligament, the posterior longitudinal ligament, interspinal ligaments or other ligament structures. |
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