Surface topography and vectors: a new measure for the three dimensional quantification of scoliotic deformity

The monitoring of spinal deformity uses many techniques: clinical history and physical examination for patient status, radiography for precise spinal delineation and Cobb angle, topography to quantify cosmesis and to approximate the Cobb angle. Experience with a system based on Raster photography has shown that adequate correlation with the Cobb angle is achieved, but that the relationship between spinal curvature and cosmetic effect is not simple. A measure was developed to quantify the asymmetry of the back, making it available to statistical analysis, without expressing it in terms of the Cobb angle or referring to trunk balance or rotation. The calculation expresses symmetry about the median saggittal plane (first thoracic vertebra to natal cleft), reflecting the right half onto the left and measuring the three-dimensional displacement between corresponding fixed points on the trunk. Tolerance limits were calculated and correlation with Cobb angles using routine scans was analysed. There were statistically significant correlations between the Cobb angle and all vectors except the middle antero-posterior. All vectors correlated with each other, except again for the middle Z or anteroposterior which correlated only with the middle and lowermost sets. Applied to natural history and to surgical outcome, this new parameter provides a different quantification of back shape which can be used both for patient assessment and monitoring, for the evaluation of the cosmetic (as opposed to the radiological) effect of treatment, and for aetiology and natural history studies.