Reliability of the Cobb angle index derived by traditional and computer assisted methods.

The reliability of the traditional Cobb angle index, which at present exhibits a wide range of reported values, was determined in this study and amounted to a within subject error of +/-4.3 degrees and a between subject error of +/-4.9 degrees. An attempt to pinpoint the source of this variability by investigating the influence of choice of end vertebrae proved inconclusive. The reliability of a computer assisted Cobb angle index, developed to improve measurement accuracy, was also examined and an estimated within subject error of +/-1.5 degrees and a between subject error of +/-1.6 degrees obtained. A comparison of the traditional and computer assisted methods indicates a high correlation between the Cobb angle derived using the two methods when measuring like curves and an improved reliability obtained by the computer method. Such an improvement is worth implementing clinically but of course the availability of a computer and digitizer and a little extra time for an operator to input the X-ray coordinates may be a barrier to its general use.