Scoliosis corrective force estimation from the implanted rod deformation using 3D-FEM analysis

Abstract

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.

Risk Factors for Clinically Relevant Loosening of Percutaneous Pedicle Screws

“…Therefore, the strengths and clinical relevance of this study are that it showed that a lower axial trajectory and a high screw pull-out force during rod connection might be crucial risk factors for PPS loosening in clinical situations. A rod reduction device has been reported as the cause for screws to be pulled toward the rigid rod, causing a failure of the screw-bone interface in deformity-correction surgery 23,24) . Our study indicated that this pull-out force toward the rod in the PPS system might be a risk factor for screw pull-out during rod connection, especially at the LIV.…”

Effects of Multilevel Facetectomy and Screw Density on Postoperative Changes in Spinal Rod Contour in Thoracic Adolescent Idiopathic Scoliosis Surgery

“…Destabilization of the posterior spinal segment by releasing soft tissue or facet joints could be important to prevent implant breakage or pedicle fracture during maneuver in more severe curve corrections [18]. However, it is still unclear whether these posterior releases positively affect the TK, especially with a hyphokyphotic thoracic spine [1,9].…”

Identification of optimized rod shapes to guide anatomical spinal reconstruction for adolescent thoracic idiopathic scoliosis

“…Although destabilization of the posterior spinal segment by releasing the soft tissue or facet joints could be important to prevent implant breakage or spinal fracture during maneuvers in curve corrections, it is important for maintenance of fatigue strength to avoid intraoperative notches on the rods, because rod failure would be facilitated by pseudarthrosis . When stress concentrations are introduced in the form of notches, these play a major role in fatigue strength, since rod fractures almost always occur at these initiation points of increased stress .…”