Objective:To compare the biomechanical differences among modular total sacral prosthesis, integrated total sacral prosthesis and screw-rod system for lumbosacral reconstruction after total sacrectomy by finite element analysis.Methods:Three finite element models of reconstruction after total sacrectomy were established: six-rod plus anterior column, integrated total sacral prosthesis, and modular total sacral prosthesis. A vertical load of 600 N was applied to the L 3 vertebra, and the bilateral acetabula were fixed in all degrees of freedom to restrict their movement, simulating a bipedal standing posture. The maximum stress, stress distribution on the iliac screws, stress distribution on the longitudinal rods, the shift-down displacement of the L 5 vertebra, and the stress direction on the contact surface between the prosthesis and the ilium on all implant components (including prosthesis, screws, and connecting rods) were compared. Results:Finite element analysis results show that the average maximum stress of the six-rod plus anterior column reconstruction on all implant instrumentation was 217.9±10.2 MPa, the integrated total sacral prosthesis reconstruction was 185.7±21.1 MPa, and the modular total sacral prosthesis reconstruction was 157.4±31.2 MPa. The differences were statistically significant ( F=12.357, P<0.001). Among them, the difference between the modular total sacral prosthesis reconstruction and the six-rod plus anterior column reconstruction was statistically significant ( P<0.001), while the difference between the modular total sacral prosthesis reconstruction and the integrated total sacral prosthesis reconstruction was not statistically significant ( P=0.051). The maximum stress on the iliac bone screws and longitudinal connecting rods: for the six-rod plus anterior column reconstruction, it was 157.2 MPa and 105.4 MPa respectively; for the integrated total sacral prosthesis reconstruction, it was 59.2 MPa and 97.8 MPa respectively; for the modular total sacral prosthesis reconstruction, it was 58.4 MPa and 35.6 MPa respectively. The distance of L 5 vertebral body downward displacement: for the six-rod plus anterior column reconstruction, it was 1.05±0.06 mm; for the integrated total sacral prosthesis reconstruction, it was 0.34±0.02 mm; for the modular total sacral prosthesis reconstruction, it was 0.40±0.05 mm. The difference was statistically significant ( F=357.730, P<0.001), among which the differences between the modular total sacral prosthesis reconstruction and the six-rod plus anterior column reconstruction and that between the integrated total sacral prosthesis reconstruction and the six-rod plus anterior column reconstruction were all statistically significant ( P<0.05), while the difference between the modular total sacral prosthesis reconstruction and the integrated total sacral prosthesis reconstruction was not statistically significant ( P=0.145). The stress on the iliac bone contact surface of the integrated total sacral prosthesis was 34.2° and manifested as shear force; the stress on the iliac bone contact surface of the modular total sacral prosthesis was 88.9° and manifested as compressive stress. Conclusions:This modular total sacral prosthesis exhibits lower peak stress compared with the integrated total sacral prosthesis and screw-rod system. The spinal stability of the modular total sacral prosthesis is comparable to that of the integrated total sacral prosthesis and superior to that of the screw-rod system.

Objective:To compare the biomechanical differences among modular total sacral prosthesis, integrated total sacral prosthesis and screw-rod system for lumbosacral reconstruction after total sacrectomy by finite element analysis.Methods:Three finite element models of reconstruction after total sacrectomy were established: six-rod plus anterior column, integrated total sacral prosthesis, and modular total sacral prosthesis. A vertical load of 600 N was applied to the L 3 vertebra, and the bilateral acetabula were fixed in all degrees of freedom to restrict their movement, simulating a bipedal standing posture. The maximum stress, stress distribution on the iliac screws, stress distribution on the longitudinal rods, the shift-down displacement of the L 5 vertebra, and the stress direction on the contact surface between the prosthesis and the ilium on all implant components (including prosthesis, screws, and connecting rods) were compared. Results:Finite element analysis results show that the average maximum stress of the six-rod plus anterior column reconstruction on all implant instrumentation was 217.9±10.2 MPa, the integrated total sacral prosthesis reconstruction was 185.7±21.1 MPa, and the modular total sacral prosthesis reconstruction was 157.4±31.2 MPa. The differences were statistically significant ( F=12.357, P<0.001). Among them, the difference between the modular total sacral prosthesis reconstruction and the six-rod plus anterior column reconstruction was statistically significant ( P<0.001), while the difference between the modular total sacral prosthesis reconstruction and the integrated total sacral prosthesis reconstruction was not statistically significant ( P=0.051). The maximum stress on the iliac bone screws and longitudinal connecting rods: for the six-rod plus anterior column reconstruction, it was 157.2 MPa and 105.4 MPa respectively; for the integrated total sacral prosthesis reconstruction, it was 59.2 MPa and 97.8 MPa respectively; for the modular total sacral prosthesis reconstruction, it was 58.4 MPa and 35.6 MPa respectively. The distance of L 5 vertebral body downward displacement: for the six-rod plus anterior column reconstruction, it was 1.05±0.06 mm; for the integrated total sacral prosthesis reconstruction, it was 0.34±0.02 mm; for the modular total sacral prosthesis reconstruction, it was 0.40±0.05 mm. The difference was statistically significant ( F=357.730, P<0.001), among which the differences between the modular total sacral prosthesis reconstruction and the six-rod plus anterior column reconstruction and that between the integrated total sacral prosthesis reconstruction and the six-rod plus anterior column reconstruction were all statistically significant ( P<0.05), while the difference between the modular total sacral prosthesis reconstruction and the integrated total sacral prosthesis reconstruction was not statistically significant ( P=0.145). The stress on the iliac bone contact surface of the integrated total sacral prosthesis was 34.2° and manifested as shear force; the stress on the iliac bone contact surface of the modular total sacral prosthesis was 88.9° and manifested as compressive stress. Conclusions:This modular total sacral prosthesis exhibits lower peak stress compared with the integrated total sacral prosthesis and screw-rod system. The spinal stability of the modular total sacral prosthesis is comparable to that of the integrated total sacral prosthesis and superior to that of the screw-rod system.