| Objectives:The purpose of this study was to evaluate the association between lumbar spine facet joint orientation and tropism, sagittal spinopelvic alignment, and rotational deformity, identified by radiographic and computed tomography (CT) measurements, in degenerative lumbar scoliosis (DLS).Develop modified finite element models to simulate degenerative lumbar scoliosis (DLS) based on the normal lumbar spine model.To investigate the facet joint force of the degenerative lumbar scoliosis via finite element study. By researching the above models, provide a reference and method to understand the development of DLS and the forming mechanism from the view point of biomechanics.Methods:Standing whole-spine sagittal radiographs and CT scans, including the pelvis, were performed and analyzed in 60 DLS patients (16 males,44 females; mean age 65 years). Cobb angle, pelvic incidence (PI) and lumbar lordosis (LL) were measured on standing lumbar radiographs. Facet joint orientation on both sides at L3/4, L4/5, and L5/S1 was determined from transverse-plane CT. Facet joint tropism was defined as a difference in symmetry of more than 10 degrees between the orientations of the facet joints. DLS Using DICOM data of lumbar spine from CT scanning, set up FEMs of human normal lumbar spine. We developed a finite element model of three patterns of scoliotic curves (10-20°,20-30°,>30° Cobb’s angle). Analyze the biomechanical behaviors of L1-L5 level FEMs with professional FEA software.Given 400N axial load or 400N axial load combined with 5N.m、 10 N.m or 15 N.m moment on the superior end plate of L1 vertebra. The stress on the facet joint was calculated on both sides (right and left) of the each vertebra. Compare and analyze the stress distribution and contact forces between facet joints of the three models.Observe whether there are an increase and transformation of stress and asymmetry in contact forces between facet joints in the development of DLS to better understand the development of DLS and the biomechanical forming mechanism.Results:There were significant differences in the incidence of facet joint tropism between the two groups at L3/4, L4/5, and L5/S1 (P= 0.011, P= 0.043, and P= 0.004, respectively). LL was significantly smaller in type Ⅱ DLS (P= 0.049). Facet joint orientation, pelvic incidence, and Cobb angle did not differ between groups. No significant correlation between LL and PI was observed in either group. This study provides a reliability analysis of rotational deformity in patients with DLS. The results revealed that asymmetric responses of the facet joint forces exist in various postures and that such effect is amplified with larger curve. After the formation of DLS,the deformation of the facet joints due to compression keep itself out of capability of limitting rotation and blocking listhesis. In the lateral bending curves, the compressive facet joint stress on the concave side was greater than the tensile stress on the convex side. when the segmental rotation of the vertebra, there was significantly higher tensile stress on the convex side, and lower compressive stress on the concave side.Conclusions:We observed a significant relationship between facet joint tropism and rotational deformity in patients with DLS. Furthermore, the different types of DLS demonstrated significant differences in LL that may induce spinal symptoms. Left and right side of the facet joint on the evolution of DLS have an important role. DLS make the facet joints carrying asymmetric loading. As the Cobb’s angle was between 10°to 20°, the convex side of the facet joints suffered larger force.When the Cobb’s angle was increased from 20°to 30°, the concave side of the facet joints suffered larger force.In the DLS,Lumbar stress line tends convex side. With the eccentric compressive stress,the vertebral wedging deformation appears on the concave side and facet sompressive deformation appears on the convex side,and it decreases in the effect of the facet joints of the convex side to limit the vertebral rotation and listhesis due to the shear forces act when loaded. Asymmetric loading on facet joint contact forces maked the lumbar spine lost its balance, eventually leading to degenerative lumbar scoliosis. |
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