Back shape measurement using video rasterstereography and three-dimensional reconstruction of spinal shape.

Video rasterstereography is a method for back surface measurement comprising automatic back surface reconstruction and shape analysis. It is particularly appropriate for the examination of scoliosis. In this application shape analysis includes model-based calculations of vertebral rotation (determined from surface rotation) and of the spinal midline in three dimensions. The results are delivered in quasi-real time (computing time < 5 min). The aim of the present study was to validate the method by comparison of rasterstereographic and radiographic data. Anteroposterior radiographs and rasterstereographs (478) of 113 scoliosis patients were analysed, each pair taken on the same day. Matching the radiographic midline of the spine to its rasterstereographic equivalent, the deviations between the two curves are properly expressed by their root mean square (r.m.s.) deviation. A r.m.s. deviation in the order of 4 mm was found. Similarly, the r.m.s. deviation of vertebral rotation from surface rotation was about 3°. No systematic difference of vertebral and surface rotation, as reported by other authors, could be found. This may be attributed to our method of data evaluation, consisting of a sophisticated analysis of surface curvature and shape asymmetry. These mathematical procedures are made possible by the high sampling density and resolution of video rasterstereography. Conventional scoliosis parameters (e.g. Cobb angle, apical rotation, apex height, etc.) can be estimated with limited accuracy from the reconstructed midline. The relevant standard deviations are given.