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Objective: The aim of this study was to investigate the blomechanical property of lumbosacral reconstructin after subtotal sacrectomy. Methods: Three three-dimensional finite element models of lumbosacral region were established: (1) An intact model (INT); (2) A defective model (DEF) on which subtotal sacrectomy was performed cephalad to the S1 foramina; (3) A reconstructed model (REC). These models were validated by compared with literature. Upright posture was stimulated under a compression load of 925N. A finite element analysis was performed to account for the displacement and stress on the models. The REC model was calculated twice, with the material property of reconstruction instrument set as titanium and stainless steel, respectively. Results: The displacements of anchor point on the L3 vertebrae in INT, DEF and REC model were 6.63 mm, 10.62 mm, 4.29 mm (titanium) and 3.86 mm (stainless steel), respectively. The stress distribution of the instru-ment in REC model showed excessively concentration on caudal spinal rod, which may cause rod failure between spine and ilia. The maximum yon Mise stress of stainless steel instrument was higher than that of titanium instruments (992 MPa vs 655 MPa), and the value of stress of anchor point around sacroiliac joint in REC model were 26.4 MPa with titanium instruments and 23.9 MPa with stainless steel instruments. Conclusion: Lumbosacral reconstruction can significantly increase the stiff-ness of spino-pelvis of the patient who underwent subtotal sacrectomy. However, the rod between L5 and ilia is the weakest region of all the instruments. It is suggested that the bending of rod should be conducted carefully and smoothly to avoid significant stress concentration so as to reduce the risk of rod failure. And stainless steel instrument has higher maximum stress and significantly greater stress shielding effect than titanium instrument, which means stainless steel instruments are of higher risk of rod failure and less favorable for lumboiliac arthrodesis than titanium instruments.