Abstract: BACKGROUND: In the past, the correction of ankylosing kyphosis often focused on the correction of local deformities, ignoring the reconstruction of sagittal balance.  
OBJECTIVE: To establish a three-dimensional finite element model of ankylosing spondylitis kyphosis by using computer-aided software, design a personalized osteotomy scheme based on the reconstruction of sagittal balance of spine by hilum hip axis method, simulate the operation of vertebral column resection (VCR), calculate and analyze its biomechanical characteristics, and compare with two kinds of sagittal balance models without reconstruction.
METHODS:  CT data of an inpatient from the Sixth Affiliated Hospital of Xinjiang Medical University were collected. According to 301 classification, kyphosis was identified as IIIA. The CT data were imported into computer modeling software to establish a three-dimensional finite element model of ankylosing kyphosis. Three different surgical models were designed, which were VCR30°, VCR32.2° and VCR40°, in which VCR32.2° was used to measure the pre osteotomy angle of L3, which was 32.2° using the measurement method of hilum hip axis. In addition, two non-reconstructed spinal sagittal plane balance models were constructed, in which VCR30° was set as the osteotomy angle 30° and VCR40° was set as the osteotomy angle 40° to simulate VCR in the three models. The spinal displacement, screw rod system, titanium cage and osteotomy surface of the three models were calculated by finite element method.  
RESULTS AND CONCLUSION: (1) The order of total displacement and displacement of 24 vertebral bodies was VCR40° > VCR30° > VCR32.2°; VCR40° was significantly higher than VCR32.2°; VCR30° and VCR32.2° had no significant displacement difference due to the small difference of osteotomy angle. (2) The stress distribution trend of pedicle screw, titanium rod, titanium cage and osteotomy contact surface: no matter titanium rod or titanium cage, VCR40° was higher than the other two models and the average, and the difference was significant. VCR32.3° had the smallest stress in the three models, and was lower than the average value. The equivalent stress distribution of each segment was uniform, and there was no stress concentration. (3) Results confirmed that reconstruction of sagittal balance of spine based on hilum hip axis method can increase postoperative spinal stability, and lower stress distribution of internal fixation, which is a reasonable and scientific surgical design.

Key words: ankylosing kyphosis, vertebral column resection, finite element analysis, sagittal balance,  biomechanics, model, deformity, pelvic parameters