Abstract: BACKGROUND: The scarcity of bone trabecular structures caused by osteoporosis is not enough to maintain screw holding power, which often leads to the failure of internal fixation surgery. The screw holding power is often increased by increasing the diameter and length of screws, changing the surface coating of screws, and reinforcing the nail track with bone cement. The cement reinforced cortical bone track nailing technique and the modified cortical bone track nailing technique using a new type of variable diameter screw have been proven to have good fixation effects, and now the related mechanical properties of the two need to be analyzed and compared.
OBJECTIVE: Finite element analysis was used to compare the mechanical properties of bone cement-strengthened cortical bone trajectory nailing technology, cortical bone trajectory nailing technology, and modified cortical bone trajectory nailing technology using a new variable-diameter total cortical bone thread screw in lumbar spine internal fixation surgery. 
METHODS: Based on the CT scan data processing of three osteoporotic vertebrae, the L4 lumbar spine model was constructed, and the innovative variable-diameter all-cortical bone screw was applied in the modified cortical bone nailing technique, with the screw having a total length of 45 mm and a variable diameter of 5.5-4.3 mm as the modified cortical bone trajectory screw group. It was compared with the standard cortical bone trajectory screw group (5.5 mm in diameter and 40 mm in length) and the bone cement-augmented cortical bone trajectory screw group (5.5 mm in diameter, 40 mm in length, and 1 mm in diameter in the center opening). The fixation strength of the screws in each group was compared by measuring the axial withdrawal force, screw stability (upper, lower, left, and right load displacement ratio), and vertebral body mobility.
RESULTS AND CONCLUSION: (1) Axial pullout strength: The bone cement-augmented cortical bone trajectory screw group showed greater axial pullout strength than the modified cortical bone trajectory screw group (P=0.024 6). Additionally, both the bone cement-augmented cortical bone trajectory screw group and the modified cortical bone trajectory screw group exhibited significantly higher axial pullout strength compared with the standard cortical bone trajectory screw group (P=0.000 1 and P=0.002 64, respectively). (2) Screw stability: Under inferior loading conditions, the load-displacement ratio of the screws was highest in the bone cement-augmented cortical bone trajectory screw group, followed by the standard cortical bone trajectory screw group and the modified cortical bone trajectory screw group (all P < 0.05). The modified cortical bone trajectory screw group showed a higher stability compared with the standard cortical bone trajectory screw group (P < 0.05). (3) Vertebral mobility: Under five different loading conditions, the bone cement-augmented cortical bone trajectory screw group exhibited less vertebral mobility than the modified cortical bone trajectory screw group, and no significant difference was detected (P > 0.05). Furthermore, both the bone cement-augmented cortical bone trajectory screw group and the modified cortical bone trajectory screw group showed less vertebral mobility compared with the standard cortical bone trajectory screw group. (4) There were no statistically significant differences between the bone cement-augmented cortical bone trajectory screw group and the modified cortical bone trajectory screw group in terms of load-displacement ratio and lumbar mobility (P > 0.05). (5) This suggests that the bone cement-augmented cortical bone trajectory screw technique offers superior biomechanical performance compared with the novel variable-diameter full-cortical bone screw. Additionally, the bone cement-augmented cortical bone trajectory screw technique may be a more suitable choice for internal fixation surgery in clinical patients with osteoporosis.

Key words: modified cortical bone trajectory, lumbar internal fixation, finite element analysis, bone cement, variable diameter full cortical bone screw, biomechanics