Biomechanical Analysis of Pedicle Screw Instrumentation in a Thoracic Scoliosis Spine Finite Element Model

ABSTRACT

In this study, the important kinematic and post-operative changes in the spinal column and rib cage after corrective surgery of scoliosis using the pedicle screw and rod derotation method was determined using a finite element (FE) scoliosis spine model, and the optimized instrumented levels for scoliosis correction was determined. A three dimensional FE scoliosis model was reconstructed by translation and rotation of a normal spinal column model using geometric mapping with x-ray and CT. By changing the fusion levels in the calculations, postoperative changes like Cobb angle, apical vertebrae axial rotation (AVAR), thoracic kyphosis, and rib hump were qualitatively analyzed. In the analysis of operative kinematics, the decrease in Cobb angle was most prominent in distraction than in deroation. Applying only the rod derotation was not effective in decreasing the Cobb angle and just caused an increase in the AVAR and rib hump. In the operative simulation, co-action of distraction and translation during rod insertion had a major impact on decreasing the Cobb angle and maintaining kyphosis. For the rod rotation, a decrease in the Cobb angle was obtained; however, this was accompanied by a simultaneous increase in the AVAR and rib hump. When the most extended instrumentation range with a 60o rod rotation was used the Cobb angle was decreased in half, but an increase in the rib hump and AVAR also occurred. The optimum fusion level was found to be one level less than the inflection position of the thoracic spine curvature.

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