To evaluate the biomechanical stability of oblique lumbar inter-body fusion stand-alone (OLIF SA) under different bone mineral density (BMD) conditions and analyze the effect of different BMD conditions on cage subsidence.
The finite element model of the L2-5 was reconstructed and verified via computed tomography scan images(M0). The stand-alone model of L4/5 segments OLIF was created based on the validation model. By changing bone mineral density, OLIF SA was established in the normal bone mineral density group (M1), osteopenia group (M2), and osteoporosis group(M3), respectively. A 500N vertical axial preload was imposed on the superior surface of L2 and a 10 N·m moment was applied on the L2 superior surface along the radial direction to simulate 6 different physiological motions: flexion, extension, left lateral bending, right lateral bending, left rotation and right rotation.
The normal ROM of L4-L5 of flexion, extension, left lateral bending, right lateral bending, left torsion and right torsion were 5.37°, 2.27°, 2.9°, 2.79°, 2.72° and 2.75° , respectively. Compared with the literature data, the L4/5 segment ROM of the finite element model is similar to the previous research results under different motion states, which proves the effectiveness of the model. Compared with the normal model, the ROM of the fusion segment was significantly reduced and the maximum stress of the upper and lower end-plates was significantly increased in all motion modes. Compared with the M1 model, the maximum relative increases of ROM, cephalical end-plate stress (lower end plate of L4) and tail end-plate stress (upper end plate of L5) of the M2 model in L4/5 segment are 39.1%, 9.9% and 10.7%, respectively, and the maximum increases of the above parameters in the M3 model are 100%, 28.9% and 31.6%, respectively. The maximum stress of the tail end-plate of the M3 model during flexion is 54.617 MPa, which is very close to the yield stress of the lamellar bone (60 MPa).
For patients with normal bone mass, OLIF SA may provide adequate stability. With the increase of the degree of osteoporosis, the maximum stress on the upper and lower end-plates of the fusion segment increased significantly, thus increasing the potential risk of implant subsidence.The tail endplate is more easily damaged than the head endplate. OLIF SA could not provide sufficient stability for patients with osteoporosis.

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