Study On The Effect Of Sequential Section Of L5/6 Posterior Spinal Structures On Lumbar Mechanical Stability

Purpose: To study the effect of sequential section of L5/6 posterior spinal structures on lumbar mechanical stability using constant displacement testing method by measuring vertebral segmental motion on 6 degrees of freedom.Methods: 14 fresh segments of L4-L7 were selected as spinal specimens from healthy female Small-Tailed Han sheep. AP and LP X-Rays were taken to exclude spinal diseases, Soft issues paraspinal were removed before test. L4 and L7 were embedded by polymethylmethacrylate and the facet joints of L5 and L6 were allocated by CT accurately. The sequences were procedured as follows:Ⅰnormal→Ⅱtotal laminectomy→Ⅲhalf-resection of left facet joint→Ⅳfull-resection of left facet joint→Ⅴhalf-resection of right facet joint→Ⅵfull-resection of right facet joint. Each sequence was tested under the same axial load of 500N under the same physiological range of sheep's spinal motion. Corresponding changes of bending/torsional moment and angular displacement on 6 degrees of freedom were recorded by Instron8874 dynamic multidimensional biomechanical fatigue testing machine. Then the mean slope rate of curves of bending/torsional moment and angular displacement of every sequence were recorded and analyzed. SPSSll.3 statistical package was used for a normal distribution test, a homogeneity of variance test and a repeated measures analysis of variance (ANOVA) with a LSD test. The level of statistical significance was set to 0.05.Results: The resection of posterior ligaments (supraspinous ligament, interspinous ligament and ligamentum flavum) and their affiliated bone structures as welI as left facet joint had some effect on the stability of flexion/extension, but the mechanical instability did not occur. Plus the half-resection of right facet joint, the mechanical instability appeared. The mean stiffness of flexion/extension decreased 32.33% and30.83% than normal respectively. However when both of facet joints were resected, the mean stiffness of flexion/extension decreased 35.59% and 34.80% than normal respectively. The resection of posterior structures of lumbar had little effect on the stability of lateral bending. The resection of posterior ligaments as well as their affiliated bone structures led to the mechanical instability of left/right axial torsion. The mean stiffness decreased 26.13% and 26.20% respectively. Facet joints had important sense on torsional stability, especially on the opposite side. When left facet joint were resected more than half, the right mechanical instability occurred aggressively. The mean stiffness decreased 39.38 % than normal. Plus half-resection of right facet joints, left and right axial torsion destabilized aggressively. The mean stiffness decreased 41.33 % and 47.82% than normal respectively. Furthermore, double joints were full-resected, left axial torsion destabilized aggressively. The mean stiffness decreased 55.33%than normal.Conclusions: The more L5/6 posterior spinal structures were destroyed, the more lumbar mechanical instability occurred on axial rotation and flexion/extension. But the posterior spinal structures have little effect on sustaining lumbar mechanical instability on lateral bending. Maybe intervertebral disc play a vital role on sustaining lumbar mechanical stability on lateral bending. Under conditions of insuring the field of vision, we must carry out microinvasive operation and preserve the integrity of posterior spinal structures as far as possible during performing posterior approach of lumbar.