Abstract: BACKGROUND: Sacral resection has now become the accepted treatment of choice for malignant tumors of the sacrum. There are few biomechanical studies on whether fractures or sacroiliac joint spondylolisthesis will occur after simple sacral resection, and there is no consensus on whether the weight can be fully loaded after subtotal sacral resection and when to rebuild.
OBJECTIVE: To contrast clinical research and analyze Von Mises stress to provide a credible theoretic basis about which level of sacrectomy without spinopelvic reconstruction is acceptable for pelvic stability through the biomechanical testing of intact pelvis and models of pelvis after subdivided sacrectomy.
METHODS: (1) Biomechanical research: Six fresh normal adult human cadaveric L5-pelvis specimens were chosen. Compressive stress loaded on the specimens was increased by 200 N, until 1 000 N, at the speed of 1.4 mm/min. The change of Von Mises stress was measured to the same pelvic specimens on intact sacrum and groups of subdivided sacrectomy. The differences were compared between groups of data. (2) Clinical studies: Totally 15 patients diagnosed with high sacral tumor with tumor resection between January 2012 and June 2019 were enrolled, including 6 males and 9 females with an average age of 46.40±14.94 years. According to preoperative MRI examination, the extent of sacral involvement was determined, and the size of sacral resection was determined. No reconstruction was performed after operation. Postoperative function and complications were recorded.  
RESULTS AND CONCLUSION: (1) Biomechanical research: With the growth of the sacrum resection plane, Von Mises stress had different increases at different test points, particularly by 1/4 S1 to 1/2 S1, which were apparently different with that in other groups (P < 0.05). Compared with group 2/3 S2 and group 1/3 S2, the change of Von Mises stress at point A in group S1-2 was not statistically significant. (2) Clinical results: Among the 15 patients, 4 patients retained the intact S1 vertebral body during the operation (resection of the S1-S2 intervertebral space, as in the biomechanics experiment S1-2 group); sacrum was resected in 3 patients as the group 2/3 S2 during the operation, and sacrum was resected in 2 patients during the operation as group 1/3 S2; and the S1 and S2 vertebrae were kept intact in 6 patients (as resection in the S2-3 group). The mean score of musculoskeletal tumor society was 25.27±3.79. All patients were able to walk, nine without walking aids, six with walking aids, one of them developed residual sacral fracture. (3) With the growth of the sacrum resection plane, Von Mises stress at residual sacrum rapidly rose. When the sacrum was resected by S1-S2 intervertebral space, the stability of the pelvic ring was acceptable without spinopelvic reconstruction. 

Key words: bone, sacrum, tumor, sacroiliac joint, sacral resection, von Mises stress, pelvic ring, intervertebral space