Abstract: BACKGROUND: In patients undergoing interbody fusion surgery, the proportion of patients with osteoporosis is increasing. Osteoporosis has an impact on the fixation effect of screw rod system, such as extraction of pedicle screw, failure of interbody fusion surgery, and so on. The use of bone cement screw channel augmentation surgery has effectively improved this problem.
OBJECTIVE: To compare biomechanical changes of lumbar spine strengthened with different bone cement nail channels under the condition of low bone mineral density by finite element method. 
METHODS: A finite element model of L4-5 vertebral body functional unit was established according to the extracted CT images. All vertebral body bone materials were set as osteoporotic state. Models of non-cemented screw channel augmentation, all-bone cement screw channel augmentation, L4 vertebral body bone cement screw channel augmentation, unilateral bone cement screw channel augmentation, and cross bone cement screw channel augmentation were established. The lower surface of the L5 segment was fixed. The upper surface of the L4 segment was subjected to a vertical downward load of 500 N and a moment of 7.5 N•m. The von-Mises stresses of the screw rod system, vertebral body screw channel and bone cement mass were detected. 
RESULTS AND CONCLUSION: (1) Compared with the non-cemented screw channel fixation group, the effect of the bone cement augmentation groups on the von-Mises stress of the screw-rod system was very small. (2) The von-Mises stress of the bone-cemented screw channel in each group was generally smaller than that of the non-cemented screw channel group; the more significant one was the all-bone cement screw channel augmentation group. All-bone cement screw channel augmentation group had a similar effect to the cross bone cement augmentation group. The decrease of von-Mises stress in the unilateral bone cement screw channel augmentation group was lower than that of the cross bone cement augmentation group. The L4 vertebral bone cement screw channel augmentation group had no obvious effect on the reduction of the screw channel von-Mises stress, and the screw channel von-Mises stress was higher than that of the non-cemented screw channel fixation group during scoliosis and rotation. (3) Under various motion states, the von-Mises stress of bone cement in all-bone cement screw channel augmentation group was the minimum value. The von-Mises stress of bone cement in the unilateral bone cement screw channel augmentation group and the cross bone cement screw channel augmentation group was slightly greater than that of the all-bone cement screw channel augmentation group. (4) These results showed that the bone cement screw channel augmentation technology effectively reduces the stress concentration effect of the screw rod fixation system on the vertebral screw channel, optimizes the stress distribution of the internal fixation system, and reduces the risk of screw loosening relative to the vertebral body. The effects of different bone cement screw channel augmentation methods are different, but the overall trend is the reduction of screw channel stress. Among them, the most effective fixation methods are all-bone cement screw channel augmentation and cross bone cement screw channel augmentation.  

Key words: lumbar vertebra, stress analysis, finite element, osteoporosis, screw channel augmentation, bone cement, stability