Analysis of Cobb angle measurements in scoliosis.

The measurement of the magnitude of the deformity in idiopathic scoliosis is usually performed by either the Cobb method or the Ferguson method. In this study an error analysis of the Cobb method is given. The projection error is 3° and is more or less the same in moderate and severe scoliosis. The observation error is 3-5° in mild scoliosis and may increase to 10° or more in severe scoliosis. This observation error is strongly related to axial rotation of the vertebrae. Progression of the deformity in idiopathic scoliosis, either by increasing lateral deviation and/or increasing axial rotation is the indication to treatment. Because the Cobb angle is mainly influenced by the lateral deviation and less by the axial rotation, it is recommended to use, in addition to the Cobb method, an accurate method for measuring the axial rotation.