Abstract: BACKGROUND: The incidence of high lumbar disc herniation in elderly patients with osteoporosis is increasing. The changes in lumbar overall mechanical strength and biomechanical properties of adjacent vertebrae after conventional posterior approach decompression and interbody fusion have not been clarified. Finite element analysis has great value in the field of biomechanics because of its advantages of non-invasiveness, high repeatability and accuracy.  
OBJECTIVE: To explore the establishment of a finite element model of senile osteoporotic L3/4 lamina decompression using finite element analysis and intervertebral fusion spine, and to evaluate the biomechanical status of the internal fixation complex of bones under bending motion in elderly patients with osteoporosis.
METHODS: Mimics 21.0 was used to extract the DICOM data of the spine CT to establish the 3D bony structure of the lumbar spine (T12-L5), which was imported into Geomagic wrap 2017. The L3/4 full lamina decompression model was established after redrawing the mesh, deleting the nails, cutting the model, filling the cavity, detecting and editing the contour line, constructing curved surfaces and grids, fitting curved surfaces and other operations. Solidworks 2017 was used to construct pedicle screws, connecting rods, and intervertebral fusion cages, which were assembled into the L3/4 full lamina decompression model. Intervertebral discs and articular process cartilage and other structures were established through operations such as stretching, isometric surfaces, moving and copying entities. ANSYS Workbench 17.0 was utilized for material assignment, simulating spinal ligaments, meshing, applying force and limiting boundary conditions. A complete osteoporotic L3/4 laminar decompression and intervertebral fusion spine finite element model was established. The stress, strain and displacement cloud diagram of the L3/4 lamina decompression and intervertebral fusion full lumbar spine finite element model were observed under simulated bending conditions.  
RESULTS AND CONCLUSION: (1) In terms of the stress cloud diagram: The T12-L1 vertebra had the highest average stress value; L2 dropped by 24%; L3 dropped by 55%, and L4-5 dropped by about 80%. The stress concentration in the L4/5 articular process area was the highest, followed by L2/3, and lighter in L1/2 and T12/L1. The stress concentration at the junction of the screw and the connecting rod was obvious, followed by the screw at the entrance and exit of the pedicle. (2) In terms of strain and displacement cloud map: The strain degree of L4/5 and L2/3 articular process was the highest; the strain degree of T12/L1 and L1/2 was the second, and the L3/4 segmental fusion cage, pedicle screw and connecting rod had no any visible deformation. The intervertebral discs of all segments showed great deformation. (3) It is concluded that the cooperative operation of multiple software can successfully construct a finite element model of the spine with L3/4 lamina decompression and intervertebral fusion in elderly patients with osteoporosis. Elderly patients after lumbar spine surgery can tolerate forward flexion, confirming that L3/4 laminar decompression and intervertebral fusion can maintain the shape of the spine and ensure the stability of the spine, but it is necessary to be alert to the occurrence of thoracolumbar stress fractures and adjacent spondylopathy.

Key words: umbar spine model, osteoporosis, laminar decompression, intervertebral fusion, finite element model, biomechanics