Abstract
BackgroundBack surface topography has gained acceptance in recent decades. At the same time, the motivation to use this technique has increased. From the view of the patient, the cosmetic aspect has played and still plays a major role as it provides a comprehensive documentation of cosmetic impairment. From the view of the medical practitioner, the aspect of reducing X-ray exposures in diagnosis and follow-up has been dominant and still prevails. Meanwhile, new aspects have emerged: due to the consequent three-dimensional view of the scoliotic condition, treatment success can be visualized convincingly. Clinical diagnosis is supported by information otherwise not supplied by X-rays, such as when functional examinations and diagnostic tests are recorded.MethodsLike rasterstereography, most techniques of actual back surface measurement refer to photogrammetry and the triangulation method. However, with respect to the particular clinical application, a wide spectrum of implementations exists. Applications in a clinic require high accuracy of measurement in a short time and comprehensive analysis providing data to be used to supplement and compare with radiographic data. This is exemplified by rasterstereography; the procedures of surface analysis and localization of landmarks using curvatures and the reconstruction of the spinal midline will be described.Orthopaedic relevanceBased on rasterstereographic analysis, different geometrical measures that characterize the back surface are given and underlying skeletal structures described. Furthermore, in analogy to radiological projection, a 3-D reconstruction of the spinal midline is visualized by a frontal and lateral projection, allowing comparison with pertinent X-rays.ConclusionsSurface topography and, in particular, rasterstereography provide reliable and consistent results that may be used to reduce X-ray exposure. Unfortunately, the correlation of shape parameters with the radiological Cobb angle is poor. However, the wealth of additional applications substantially enhances the spectrum of clinical value.
Highlights
Back surface topography has gained acceptance in recent decades
The wealth of additional applications substantially enhances the spectrum of clinical value
The technical solution in recording a surface point with high accuracy is based on the methods of photogrammetry, which in turn are based on the geometrical method of triangulation
Summary
Photogrammetry Using the term “measurement of back surface” illustrates that the back is recorded as a whole instead of as a small number of points that have been marked prior to the measurement. The deformation of the projected line seen in the camera – in combination with the calibration data – allows for 3-D reconstruction of the sectioned surface points by methods of triangulation. Besides the anatomical landmarks detected as singular points, the so-called symmetry line of the back can be extracted from the curvature maps [41] 2) The surface normals along the symmetry line indicate the vertebral rotation at the particular level. A mean rms deviation of 3° between vertebral rotation and surface rotation was found This figure is composed in roughly equal parts by the uncertainties in reconstructing the symmetry line and by different errors in the radiological measurement of vertebral rotation. The authors conclude that rasterstereography should be used as the technique of choice for recording the progression of scoliosis during the long-term followup, reducing the patient’s radiation exposure by approximately 50%
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