Kinematics of the scoliotic spine as related to the normal spine

A coupling between the lateral flexion and axial rotation as a result of the geometric arrangement of the motion segments is well known in a normal spine. The kinematic behavior of idiopathic scoliotic spines has been analyzed by means of a biomechanical model study and a radiologic study. The anteroposterior and lateral flexion radiographs of 40 patients with progressive adolescent idiopathic scoliosis were studied. In five of these patients, anteroposterior radiographs were also made with the spine in a ventrally flexed position. The kinematic behavior of a nonpathologic spine was examined by means of a three-dimensional, nonlinear geometric mathematical model of the spine. The frontal plane inclination of the facet joints in conjunction with the vertebral orientation in the sagittal plane influence the kinematic behavior in the normal spine. In a scoliotic spine, there is an axially rotated position and, in most cases, a dorsal inclination (lordotic) of the motion segments. Nevertheless, the direction of the axial rotation during lateral flexion does not differ from the direction of the axial rotation during lateral flexion in a normal spine. The existing axial rotation in idiopathic scoliosis cannot be explained on the basis of spinal kinematics. In contrast to normal spines, in scoliotic spines exists a coupling between ventral flexion or extension and axial rotation. This may be essential in the management of idiopathic scoliosis.