Assessment Of Three-Dimensional Scoliotic Deformity By Rasterstereography

AbstractAnalysis of three -dimensional back shape enables the calculation of the so- calledsymmetry line, which is closely related to the line of the spinous processes on theback surface. A comparative photogrammetric measurement proved a mean deviation inthe order of the palpation error. Furthermore, the angle of the surface normal on thesymmetry line proved to be equal to the angle of vertebral rotation within the limitsof radiographic accuracy. These findings enable a three -dimensional reconstruction ofthe spinal midline. Scoliotic deformity may thus be determined solely from surfacedata. In frontal projection the mean error of the reconstructed midline is about 4 mmas compared to standard radiographs.1. IntroductionAssessment of lateral asymmetry of the trunk and, in particular, of the back shapeis generally one of the first steps in the assessment of scoliotic deformity. Howev-er, a quantitative relation between back shape asymmetry and spinal deformity is dif-ficult to establish. Yet it is very desirable to find such a connection in order toreduce the use of x rays for screening and follow -up of scoliosis and other spinaldeformities.In recent years, Turner -Smith et al.' proposed an objective method for the assessmentof spinal shape using the ISIS surface measurement system. According to this methodthe relation between back surface and spinal shape is established via the spinousprocesses, the tips of which may be determined as bony landmarks on the surface. Inthe case of scoliosis, the centre of the vertebral body is laterally displaced withrespect to the spinous process in consequence of vertebral rotation. Thus, the coor-dinates of the vetrebral centres, and hence the three -dimensional shape of the spinalmidline, may be calculated from the coordinates of the surface landmarks, the angleof vertebral rotation and the length of the spinous processes (or, more exactly thedistance between the tip of the process and the centre of the vertebral body). Ifthese three data areknown, the lateral and sagittal displacements of the spinal mid -line with respect to the superficial line of the processes may be calculated. Thatis, the three -dimensional shape of the spinal midline can be assessed from these da-ta. The following formulae may be used to calculate the displacement of the midline:lateral displacement: