Abstract: BACKGROUND: In osteoporotic patients, cancellous bone is insufficient to maintain screw grip and the failure rate of internal spinal fixation is high. Traditional trajectory and cortical bone trajectory screw placement techniques are still insufficient. The modified cortical bone trajectory technique has been proven to have good performance, but requires screws that match its characteristics.
OBJECTIVE: To compare the performance of three new variable-diameter all-cortical bone threaded screws with nonvariable-diameter all-cortical bone threaded screws in modified cortical bone track placement technique by finite element analysis.
METHODS: Based on the CT data of an adult osteoporosis wet body specimen (male, 63 years old), the L1-L5 lumbar vertebral and sacral model was established, and three new variable-diameter all-cortical bone thread screws (single thread screw, 45 mm in overall length, 5.5 mm in diameter for thicker rods, 4.35 mm in diameter for finer rods, and uniformly variable diameter at two-thirds, one-half, and one-third of the variable-diameter segments of 8 mm in length, respectively), were used in a modified cortical bone-track nailing technique, respectively. With the non-variable diameter full cortical bone threaded screw (single threaded screw, 45 mm in overall length, and 5.5 mm in diameter) in the lumbar spine L4-L5 nailed and modeled. The un-nailed control group and the four nailed model groups received finite element analysis comparison. The mechanical properties of the screws in each group were compared under the modified cortical bone trajectory technique by calculating the range of motion of the lumbar vertebrae, internal fixation system stress, L4-L5 vertebral body stress, and L4-L5 intervertebral disc stress. 
RESULTS AND CONCLUSION: Under flexion, extension, lateral bending, and axial rotation working conditions, (1) the overall range of motion of the lumbar vertebrae was reduced by 14.16%, 23.75%, 8.44%, and 7.17% in the two-thirds group compared to the control group. The non-variable-diameter group was reduced by 14.45%, 23.97%, 8.52%, and 7.07%, respectively, compared with the control group. The three new variable-diameter screws had similar levels to the non- variable-diameter screws. (2) In the two-thirds group compared with the non-reducing screw group, the internal fixation system stress decreased by 3.94%, increased by 20.60%, increased by 1.16%, and decreased by 8.59%, respectively. In the one-half group compared with the non-reducing screw group, the internal fixation system stress increased by 2.06%, decreased by 0.05%, 0.14%, and 5.98%, respectively. In the one-third group compared with the non-reducing screw group, the internal fixation system stress increased by 4.16%, 13.96%, 5.07%, and decreased by 4.19%, respectively. (3) The vertebral body stresses in the four nailed model groups were elevated compared with the non-nailed group, and there was no significant increase in the three reducer screws group compared with the non-variable-diameter group. (4) The stress of the fixed segment of the intervertebral disc in the four nailed model groups was significantly reduced compared with the un-nailed group, and was basically the same in the three variable-diameter screw groups compared with the non-variable-diameter group. It is indicated that in the modified cortical bone trajectory technique, the new variable-diameter screws can provide a fixation effect that is not weaker than that of the non-variable-diameter screws. 

Key words: modified cortical bone trajectory, variable-diameter screw, internal fixation, lumbar vertebrae, osteoporosis, finite element analysis