Comparison of the biomechanical properties between variable diameter screws and conventional pedicle screws in L4 vertebrae with different osteoporosis grading-finite element analysis

English Keywords:Finite element analysis  Osteoporosis grading  Variable diameter all-cortical bone threaded screws  Modified cortical bone trajectory  Biomechanics

English Abstract:

  【Abstract】 Objectives: To evaluate the difference in biomechanical properties of lumbar spine between traditional trajectory(TT) and modified cortical bone trajectory(MCBT) pedicle screws in different osteoporotic conditions using finite element analysis. Methods：　CT scanning was used to obtain lumbar spinal tomographic images of osteoporotic patients, and then the L4 lumbar spine structure was reconstructed in three dimensions and validated. Two nail channels were constructed by varying the bone density as osteopenia(A1), moderate osteoporosis(A2), and severe osteoporosis(A3), respectively. One was a conventional double-threaded pedicle screw used in TT with a diameter of 6mm and an overall length of 45mm(TT group), the other was a variable-diameter, all-cortical bone threaded pedicle screw used in MCBT with a diameter of 4-5.5mm and an overall length of 45mm(MCBT group). The effects of the two designs on screw pullout resistance, stability(load-displacement ratio of upward, downward, left and right working conditions, defined as the ratio of load value to displacement when screw tail end displaced 0.1mm upward, downward, left and right towards its vertical axis) and lumbar spine mobility(when forward flexion, backward extension, lateral bending and axial rotation) were analyzed under different osteoporotic conditions. Results: In terms of axial pullout force, the MCBT group significantly improved 93.5%, 96.5%, and 98.5% than the TT group in A1, A2, and A3, respectively. In terms of stability, at upward, downward, left, and right working conditions, the load-displacement ratios of the screws in the MCBT group were improved in comparison with the TT group in A1, A2, and A3, respectively, by 42.2%, 40.8%, 41.7%; 49%, 49%, 51.5%; 82.4%, 81.5%, 85.2%; 73.1%, 70.8%, 72.5%. In terms of lumbar spine mobility, at forward flexion, under A1, A2, and A3, the TT group increased by 25%, 26.6% and 28.7% compared to the MCBT group, respectively; at backward extension, the TT group increased by 24.5% and 22.9% compared to the MCBT group under A1 and A2, respectively, whereas under A3, the difference was not significant(P>0.05); at lateral bending and axial rotation, the MCBT group had a slightly slight increase compared to the TT group under A1, A2, and A3(P>0.05). Conclusions: Under different degrees of osteoporosis grading, MCBT technique was inferior in stability to TT technique only in spinal axial rotation and lateral flexion, while it outperformed TT technique in axial pull-out force, screw stability, and spinal stability during forward flexion and backward extension.