Abstract: BACKGROUND: Anterior subtotal corpectomy, decompression and fusion is a conventional method to treat cervical degenerative diseases. A titanium cage is an important implant to maintain the stability of the cervical spine after subtotal corpectomy. In recent years, many patients have complications such as titanium cage sinking, which are highly controversial.
OBJECTIVE: To investigate the internal biomechanical relationship between the tilt angle of the titanium cage and postoperative titanium cage subsidence after anterior subtotal cervical corpectomy, decompression and fusion.
METHODS: A three-dimensional finite element model of the C4-C6 segment was established by CT images of a normal human cervical spine, in which the anterior subtotal resection, decompression and fusion of the C5 vertebral body were simulated, and titanium cages with different tilt angles (-6° to -1° negative angle, that is, the front edge of titanium cage is shorter than the rear edge of titanium cage; 1° to 6° positive angle, that is, the front edge of titanium cage is longer than the rear edge of titanium cage) were placed. After setting the boundary conditions, preloads of 50, 100 and 150 N were applied respectively on the C4 vertebral body. The stress value of each contact point between the titanium cage and C4 lower-end plate and C6 lower-end plate (seven stress contact points on the contact surface of titanium mesh) was recorded and statistical analysis was conducted. 
RESULTS AND CONCLUSION: (1) The tilt angles of the titanium cage of the positive angle group and negative angle group under 50, 100 and 150 N stress respectively were found by Mann Whitn test, with P < 0.05, which was statistically significant. The dispersion coefficients of the positive angle group were smaller than those of the negative angle group under 50, 100 and 150 N stress conditions. (2) Under 50, 100 and 150 N stress conditions, the Wilcoxon sign rank test in the positive angle group of titanium cage tilt angle found that when the angle was set to 1° to 5°, the difference was not statistically significant (P > 0.05). However, when the tilt angle of the titanium cage was set to 6°, the difference was statistically significant (P < 0.05). (3) Under 50, 100 and 150 N stress conditions, the Wilcoxon sign rank test in the negative angle group of titanium cage tilt angle found that when the tilt angle was set to -1° to-6°, the difference was not statistically significant (P > 0.05). (4) It is concluded that in the sagittal position, the titanium cage with a positive tilt angle is more stable than with a negative tilt angle, which is more suitable for clinical use. The tilt angle of the titanium cage is relatively stable in the range of 1° to 5°. When the tilt angle is 6°, the stability starts to decline, which is easy to cause complications of titanium cage sinking after surgery. It is more suitable to select the titanium cage with a tilt angle of 1° to 5° according to the clinical situation during surgery to improve the efficacy.

Key words: cervical vertebra, anterior subtotal cervical corpectomy, tilt angle of titanium cage, finite element analysis, biomechanics