fax: +81117066405The total number of words of the main text excluding references: 3171The number of words of the abstract: 245The number of figures: 7The number of table: 1 ABSTRACT Background: Scoliosis is a serious disease in which a human spine is abnormally deformed in three dimensions with vertebral rotation. Surgical treatment is attained when the scoliotic spine is corrected into its normal shape by implant rods and screws fixed into the vertebrae. The three-dimensional corrective forces acting at the screws deformed the implant rod during the surgical treatment of scoliosis. The objective of this study was to propose a method to analyze the three-dimensional forces acting at the rod using the changes of implant rod geometry before and after the surgical treatment.Methods: An inverse method based on Finite Element Analysis is proposed. The geometries of implant rod before and after the surgical treatment were measured three-dimensionally. The implant rod before the surgical treatment was reconstructed using an elasto-plastic finite element model. The three-dimensional forces were applied iteratively to the rod through the screws such that the rod is deformed the same after the surgical treatment of scoliosis. Findings:The maximum force acting at the screw of each patient ranged from 198 N to 439 N. The magnitude of forces were clinically acceptable. The maximum forces occurred at the lowest fixation level of vertebra of each patient.Interpretation: The three-dimensional forces distribution that deformed the rod can be evaluated using the changes of implant geometry. Although the current clinical cases are still few, this study demonstrated the feasibility of measuring the forces that deformed the implant rod after the surgical treatment of scoliosis.
Background: Adolescent idiopathic scoliosis is a complex spinal pathology characterized as a threedimensional spine deformity combined with vertebral rotation. Various surgical techniques for correction of severe scoliotic deformity have evolved and became more advanced in applying the corrective forces. The objective of this study was to investigate the relationship between corrective forces acting on deformed rods and degree of scoliosis correction. Methods: Implant rod geometries of six adolescent idiopathic scoliosis patients were measured before and after surgery. An elasto-plastic finite element model of the implant rod before surgery was reconstructed for each patient. An inverse method based on Finite Element Analysis was used to apply forces to the implant rod model such that it was deformed the same after surgery. Relationship between the magnitude of corrective forces and degree of correction expressed as change of Cobb angle was evaluated. The effects of screw configuration on the corrective forces were also investigated. Findings: Corrective forces acting on rods and degree of correction were not correlated. Increase in number of implant screws tended to decrease the magnitude of corrective forces but did not provide higher degree of correction. Although greater correction was achieved with higher screw density, the forces increased at some level. Interpretation: The biomechanics of scoliosis correction is not only dependent to the corrective forces acting on implant rods but also associated with various parameters such as screw placement configuration and spine stiffness. Considering the magnitude of forces, increasing screw density is not guaranteed as the safest surgical strategy
PURPOSE:To analyze the changes of implant rod angle of curvature during surgery and establish its influence on sagittal correction of scoliosis deformity. STUDY DESIGN:A retrospective analysis of the preoperative and postoperative implant rod geometry and angle of curvature was conducted.PATIENT SAMPLE: Twenty adolescent idiopathic scoliosis patients underwent surgical operation. Average age at the time of operation was 14 years. OUTCOME MEASURES:The preoperative and postoperative implant rod angle of curvature expressed in degrees was obtained for each patient. METHODS:Two implant rods were attached to the concave and convex side of the spinal deformity. The preoperative implant rod geometry was measured before surgical implantation.The postoperative implant rod geometry after surgery was measured by Computed Tomography scanner. The implant rod angle of curvature at the sagittal plane was obtained from the implant rod geometry. The angle of curvature between the implant rod extreme ends was measured before implantation and after surgery. The sagittal curvature between the corresponding spinal levels of healthy adolescents obtained by previous studies was compared to the implant rod angle of curvature to evaluate the sagittal curve correction. The difference between the postoperative implant rod angle of curvature and normal spine sagittal curvature of the corresponding instrumented level was used to evaluate over or under correction of the sagittal deformity.Page | 3 RESULTS:The implant rods at the concave side of deformity of all patients were significantly deformed after surgery. The average degree of rod deformation Δθ at the concave and convex side was 15.8 deg. and 1.6 deg., respectively. The average preoperative and postoperative implant rod angle of curvature at the concave side was 33.6 deg. and 17.8 degrees, respectively. The average preoperative and postoperative implant rod angle of curvature at the convex side was 25.5 degrees and 23.9 degrees, respectively. A significant relationship was found between the degree of rod deformation and preoperative implant rod angle of curvature (r = 0.60, p < 0.005). The implant rods at the convex side of all patients did not have significant deformation. The results indicate that the postoperative sagittal outcome could be predicted from the initial rod shape. CONCLUSIONS:Changes in implant rod angle of curvature may lead to over or under correction of the sagittal curve. Rod deformation at the concave side suggests that corrective forces acting on that side are higher than the convex side.
BackgroundImprovement of material property in spinal instrumentation has brought better deformity correction in scoliosis surgery in recent years. The increase of mechanical strength in instruments directly means the increase of force, which acts on bone-implant interface during scoliosis surgery. However, the actual correction force during the correction maneuver and safety margin of pull out force on each screw were not well known. In the present study, estimated corrective forces and pull out forces were analyzed using a novel method based on Finite Element Analysis (FEA).MethodsTwenty adolescent idiopathic scoliosis patients (1 boy and 19 girls) who underwent reconstructive scoliosis surgery between June 2009 and Jun 2011 were included in this study. Scoliosis correction was performed with 6mm diameter titanium rod (Ti6Al7Nb) using the simultaneous double rod rotation technique (SDRRT) in all cases. The pre-maneuver and post-maneuver rod geometry was collected from intraoperative tracing and postoperative 3D-CT images, and 3D-FEA was performed with ANSYS. Cobb angle of major curve, correction rate and thoracic kyphosis were measured on X-ray images.ResultsAverage age at surgery was 14.8, and average fusion length was 8.9 segments. Major curve was corrected from 63.1 to 18.1 degrees in average and correction rate was 71.4%. Rod geometry showed significant change on the concave side. Curvature of the rod on concave and convex sides decreased from 33.6 to 17.8 degrees, and from 25.9 to 23.8 degrees, respectively. Estimated pull out forces at apical vertebrae were 160.0N in the concave side screw and 35.6N in the convex side screw. Estimated push in force at LIV and UIV were 305.1N in the concave side screw and 86.4N in the convex side screw.ConclusionsCorrective force during scoliosis surgery was demonstrated to be about four times greater in the concave side than in convex side. Averaged pull out and push in force fell below previously reported safety margin. Therefore, the SDRRT maneuver was safe for correcting moderate magnitude curves. To prevent implant breakage or pedicle fracture during the maneuver in a severe curve correction, mobilization of spinal segment by releasing soft tissue or facet joint could be more important than using a stronger correction maneuver with a rigid implant.
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