Scoliosis is a deformity characterized by coronal, sagittal and axial rotation of the spine. Surgical instrumentation (metal pins and rods) and eventual fusion are required in severe cases. Assessment of the correction requires enough accuracy to allow rational proactive planning of individual interventions or implant design. Conventional 2-D radiography and newer 3-D CT scanning do not provide this accuracy. A new stereophotogrammetric analysis and 3-D visualization allow accurate assessment of the scoliotic spine during instrumentation. Stereophoto pairs taken at each stage of the operation and robust statistical techniques are used to compute 3-D transformations of the vertebrae between stages. These determine rotation, translation, goodness of fit, and overall spinal contour. A polygonal model of the spine using a commercial 3-D modeling package is used to produce an animation sequence of the transformation. The visualizations have provided some important observations. Correction of the scoliosis is achieved largely through vertebral translation and coronal plane rotation, contrary to claims that large axial rotations are required. The animations provide valuable qualitative information for surgeons assessing the results of scoliotic correction. A detailed study of derotation provided by different instrumentation systems and the assessment of hook position patterns is underway.
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