Rasterstereographic analysis of axial back surface rotation in standing versus forward bending posture in idiopathic scoliosis

The forward bending test according to Adams and rib hump quantification by scoliometer are common clinical examination techniques in idiopathic scoliosis, although precise data about the change of axial surface rotation in forward bending posture are not available. In a pilot study the influence of leg length inequalities on the back shape of five normal subjects was clarified. Then 91 patients with idiopathic scoliosis with Cobb-angles between 20° and 82° were examined by rasterstereography, a 3D back surface analysis system. The axial back surface rotation in standing posture was compared with that in forward bending posture and additionally with a scoliometer measurement in forward bending posture. The changes of back shape in forward bending posture were correlated with the Cobb-angle, the level of the apex of the scoliotic primary curve and the age of the patient. Averaged over all patients, the back surface rotation amplitude increased from 23.1° in standing to 26.3° in forward bending posture. The standard deviation of this difference was high (6.1°). The correlation of back surface rotation amplitude in standing with that in forward bending posture was poor (R2=0.41) as was the correlation of back surface rotation in standing posture with the scoliometer in forward bending posture measured rotation (R2=0.35). No significant correlation could be found between the change of back shape in forward bending and the degree of deformity (R2=0.07), likewise no correlation with the height of the apex of the scoliosis (R2=0.005) and the age of the patient (R2=0.001). Before forward bending test leg length inequalities have to be compensated accurately. Compared to the standing posture, forward bending changes back surface rotation. However, this change varies greatly between patients, and is independent of the type and degree of scoliosis. Furthermore remarkable differences were found between scoliometer measurement of the rib hump and rasterstereographic measurement of the vertebral rotation. Therefore the forward bending test and the identification of idiopathic scoliosis rotation by scoliometer can be markedly different compared to rasterstereographic surface measurement in the standing posture