Assessment Of Segmental Flexibility In Main Thoracic Curves In Severe Adolescent Idiopathic Scoliosis And Finite Element Analysis Of Bidirectional Loading

[Background]Adolescent Idiopathic Scoliosis represents a spinal 3D deformity, and its etiology is unknown. Preoperative assessment of the spinal flexibility is critical to determine the most appropriate treatment procedure. Only the changes of Cobb angle in coronal plane are evaluated in the current clinical assessment of spinal stiffness. Displacement in coronal plane often accompany with vertebral rotation, especially for the severe scoliosis. However, few studies have searched the rotatoty variation when assessing the curve flexibility. With the progress in segmental fixation and osteotomy technology, knowledge on spinal segmental characteristics is useful to template a surgical intervention. However, few morphological description and flexibility studies have been reported about the single segment of spine. Flexibility describes the mathematical ratio between the displacement of the spine and the force vector that was used to generate this motion. Current assessment methods is based on the vertebral displacement observed during motion of the patient, but fail to take the orthopedic force into consideration. As a result, the biomechanical properties of the scoliosis cannot be obtained in these tests.[Objectives]The aim of this study is to assess the "rotation flexibility" of the severe idiopathic scoliosis, and compare the evaluation effect among Supine bending radiograph, Suspension traction radiograph, and Fulcrum bending radiograph, and determine the best methods. The purpose of the second part of this study is to measure vertebral body and intervertebral disc wedging angle in main thoracic curves of severe idiopathic scoliosis, and describe their distributions. Meanwhile, assess the flexibility of the intervertebral discs, and analyze their distributions. With the finite element simulation research, we want to compare the corrective effect between the axial and lateral load, and explore the optimal combination of the bidirectional loads to provide scientific basis for optimization of flexibility assessment methods and understanding the biomechanical properties of scoliosis.[Methods]1. Patients with severe adolescent idiopathic scoliosis were prospectively studied with standing posteroanterior and lateral, supine bending, suspension traction and fulcrum bending radiographs before surgery; and standing PA and lateral after surgery. Cobb angle, apical vertebral body-rib ratio(AVB-R) and apical Perdriollerotation were observed. Statistical analysis was performed with one-way Analysis of Variance(ANOVA). Correlations between postoperative AVB-R and AVB-R in supine bending, suspension traction and fulcrum bending radiographs were assessed utilizing the Linear Regression. 2. The total extent of bony and disc wedging was calculated. The degree of vertebral and disc wedging was measured on the PA radiographs by drawing a line across the superior and inferior endplates of each vertebra in the curve. Bony angle was measured between the two lines along inferior and superior endplates in the vertebral body. Disc angle was measured between lines along the inferior endplate of the upper and the superior endplate of the lower vertebra in a segment. Further, their distributions were analyzed. The difference of wedging between the disc and bony was analyzed by two-tail t test. Least Significant Difference Analysis was used to compare distribution of the variable. 3. Finite element model was used to compare the corrective effect between the axial and lateral load. With this model, multiple loading conditions were set to simulate the assessment method using Push traction film. The Cobb angle in the main thoracic curves, T9 vertebra deviation from the sacral midline and T9 vertebral rotation were compared.[Results]1. 12 patients with severe scoliosis were included for study. There were trends towards increased flexibility in fulcrum bending versus suspension traction, and suspension traction versus supine bending, but there were no significant differences due to the limited sample size. And all were were significantly lower than postoperative correction(33.96±12.52,35.60±16.11,41.59±17.13vs57.30±15.05,P<0.05). Compared with the preoperative AVB-R, postoperative demonstrated a significantly better correction than fulcrum bending(0.48±0.31vs0.01±0.36,P<0.001), and suspension traction corrected more AVB-R than fulcrum bending(0.33±0.23vs0.01±0.36, P=0.012). Supine bending, suspension traction and postoperative were not significantly different from each other. For the correction of apical Perdriolle rotation, suspension was significantly larger than supine bending(9.33±4.52vs4.17±3.19, P=0.021), and larger than fulcrum bending with no statistically significant. Measurement of Perdriolle rotation was hampered by the blocking of pedicle screws, so this study did not assess the correction of postoperative Perdriolle rotation. Linear regression analysis showed moderate togood correlation between postoperative AVB-R and AVB-R in the flexibility evaluation methods. 2. 20 patients with severe scoliosis were included for study. The disc wedging in suspension traction was significantly less than PA film(22.25±12.9vs39.49±10.88,P<0.05), but no differences were found between vertebral wedging in suspension traction and PA film. The changes of Cobb angle and intervertebral disc wedge deformation between suspension traction and PA were not significantly different from each other. 4 patients were excluded due to their mild vertebral wedging. The main thoracic curve contains at least apical and the adjacent vertebrae two above and two below the apical in all patients in both suspension traction and PA film. The end vertebraes were located three above or three below the apical in most patients, however, no significant differences were found between U3 and U4, L3 and L4. The vertebral wedging decreased from the apical towards the end. The apical and the adjacent vertebraes accounted for 67.44±8.05% of the total vertebral wedging deformities. The disc wedging significantly decreased as upwards and downwards from the apical. At every level, the values were significantly smaller on the traction views. The disc wedging demonstrated no significant difference between U1 and U2, L1 and L2, furthermore, the four wedging accounted for 75.47±9.25% of the total disc wedging deformities. The levels close to the apical discs(U2 and L2) had the smallest extent of limited correctability. From U4 to L4, segment disc flexibility was a "W"- shaped distribution. The disc wedging was significantly greater than the vertebral body wedging(39.49±10.88vs31.02±9.56,0.013) in patients with severe scoliosis. Patients were divided into stiff group(n=12) and non stiff group(n=8), according to the flexibility index was whether larger than 30%. The disc angle in the non stiff group was significantly larger than the vertebral angle(40.13±4.67vs26.62±6.99,P<0.001), however, similar result was not found in the stiff group. 3. The established finite element model showed good geometric similarity with PA and push traction radiograph. The Cobb angle in the main thoracic curves, T9 vertebra deviation from the sacral midline and T9 vertebral rotation showed a negative correlation with the axial and lateral loading using the Sperman analysis. The traction force obtained a better orthopedic rate than the pushing pressure in the same magnitude. The Cobb angle in the main thoracic curves, T9 vertebra deviation from the sacral midline and T9 vertebral rotation were all smaller when a tractionforce was loaded. The model showed a minimal Cobb angle when loaded with 1/2 weight traction force and 1/2 weight pushing pressure. However, it showed minimal T9 vertebra deviation from the sacral midline and T9 vertebral rotation when loaded with 1/2 weight traction force and 1/4 weight pushing pressure. The displacement of the whole spine in the coronal plane was corrected for the best with the 1/2 weight traction force and 1/2 weight pushing pressure.[Conclusions]Although fulcrum bending radiographs suggested greater preoperative curve flexibility than the suspension traction radiographs, this may be caused by the measurement errors due to the better derotation in suspension traction radiographs. As showed in this study, suspension traction radiographs may provide a better estimate of the flexibility of the rotational deformity. Segmental analysis of the severe idiopathic scoliosis revealed significant apical tethering in both vertebral body and disc wedging, and the relative wedging changed significantly according to the rigid of the main thoracic curves. The axial loading showed the better correction than the lateral in coronal plane and rotation with the finite element model. 1/2 weight loading in both traction and push was the best loading condition.