Abstract: BACKGROUND: As kyphosis is mainly manifested as sagittal deformity. The angle of osteotomy is one of the key problems to be solved before operation. Insufficient and excessive correction of angle may cause secondary imbalance of sagittal spine.  
OBJECTIVE: To establish a three-dimensional finite element model of kyphotomy for ankylosing spondylitis by using computer-aided software, design a personalized surgical scheme based on pelvic sagittal parameters, restore sagittal balance, and analyze biomechanical characteristics.
METHODS:  CT data from C1 to sacrococcyx of a hospitalized patient from the Sixth Affiliated Hospital of Xinjiang Medical University were collected. According to 301 classification, kyphosis was determined as IIIA type. CT data were imported into computer modeling software to establish a three-dimensional finite element model of ankylosing spondylitis kyphosis. The values of pelvic incidence and pelvic tilt were measured, and the angle of theoretical pelvic tilt was calculated by pelvic incidence. Two different osteotomy models were designed. In model 2, the pre-osteotomy angle of L3 measured by hilar hip axis measurement was 32.2°. In model 1, and the osteotomy angle was set at 40° under the sagittal plane balance without reconstruction. The cancellous osteotomy was simulated on the two models; and the finite element calculation was carried out on the nail rod system and osteotomy surface of the model after osteotomy.  
RESULTS AND CONCLUSION: (1) Finite element analysis results showed that in the condition of spinal flexion, except S1 segment, the screw stress of model 1 of the other five fixed segments was higher than that of model 2, and the stress of titanium rod and bone connecting surface was also the same. (2) In the condition of spinal extension, due to too large osteotomy angle, the patient’s center of gravity moved backward, resulting in more obvious stress difference between the two. The largest distance was L4 segment screw, and the difference was 149.69 MPa between model 1 and model 2. (3) The stress diagram of osteotomy surface showed that the stress mainly concentrated in the rear of the osteotomy surface, while the stress in the front was generally less than 
12 MPa. (4) It is a reasonable and scientific surgical plan to restore sagittal balance of ankylosing kyphosis based on pelvic sagittal parameters, which can ensure the orthopedic effect and reduce the stress distribution.

Key words: ankylosing spondylitis, pelvic, sagittal parameters, biomechanics, sagittal balance