Mechanical stability of the fracture at the junction of lumbar vertebral body and pedicle after implantation with different diameters of pedicle screws / 中国组织工程研究

ABSTRACT

BACKGROUND: Most scholars believe that the fracture of the lumbar spine and pedicle is stable, and transvertebral pedicle screw implantation can improve the stability of fractures. However, the diameter of the transvertebral pedicle screw, mechanical stability and safety of the vertebrae still remain controversial. OBJECTIVE: To analyze the effect of pedicle screws of different diameters and pedicle cortex on the mechanical stability of the fractured vertebrae and pedicle by three-dimensional finite element method. METHODS: Based on normal adult L2-L3 CT DICOM data, a mimics software was used to establish a model of the fracture at L2 vertebral body and the pedicle. At the same time, a three-dimensional model of L3 vertebra was created. The L2-L3 model was imported into 3-matic in stl format, and a model of pedicle screws with different diameters (diameter of 6.5 and 6.0 mm, and length of 45 mm) was established. The model was imported into ansys after the material attributes were assigned in the mimics software. A vertical load of 500 N was applied to the upper surface of the L2 vertebral body to simulate the biomechanical performance of a adult with standard body mass after implantation with transvertebral pedicle screw with different diameters under upright condition. RESULTS AND CONCLUSION: (1) After implantation with 6.0 mm screw, the equivalent load on the lower, upper, inner, and outer walls of the pedicle at the junction of the lumbar vertebra and the pedicle was (1.28±0.62), (0.95±0.18), (0.62±0.37), and (0.36±0.16) MPa, respectively. The difference was significant among groups (F=4.298, P < 0.05). (2) After implantation with 6.5 mm screw, the equivalent load on the lower, upper, inner, and outer walls of the pedicle at the junction of the lumbar vertebra and the pedicle was (1.82±0.76), (1.11±0.18), (0.93±0.38), and (0.43±0.14) MPa, respectively. The difference was significant among groups (F=7.034, P < 0.05). (3) The equivalent load on the lower, upper, inner, and outer walls of the 6.5 mm pedicle screw model was significantly higher than that on the 6.0 mm pedicle screw model (P < 0.05). (4) These results imply that the larger the pedicle screw is, the greater the load on the cortical bone at the junction of the lumbar vertebra and the pedicle is, and the stronger the holding force is. The load on the upper, lower, inner and outer walls of the pedicle is positively related to its cortical thickness. The cortical bone of the inferior wall is thickest, the equivalent load it bears is largest, and the cortical bone of the outer wall is thinnest, and it has the smallest equivalent load. The closer the pedicle screw is to the lower medial wall within the pedicle, the stronger the holding force and the better the stability. The closer it is to the upper and outer side walls, the smaller the gripping force and the worse the stability. However, the placement of nails on the upper and outer walls is safer than the placement of nails on the lower inner wall, and the pros and cons need to be weighed in accordance with the experience of the surgeon during placement.