Some aspects of spine biomechanics and their clinical implications in idiopathic scoliosis]

The spine with it's physiological curves is designed to transfer loads to the pelvis and the lower limbs. The complex interplay of forces in the spine can be described by breaking down these forces into their basic components. These forces have a direction and can therefore treated as vectors in three axis, related to the anatomic planes: frontal, sagittal and transverse the static spine forces responsible for a correct posture create tension. Tension is defined as the relation of the vector force to the surface area to which the force is applied. Tension, depending an the direction of the force applied can be normal or tangent. Gravitational force is transferred by the vertebral bodies. Posterior elements of the spine (the lamina, the processes and ligaments) are stabilizing elements. The articular processes bear loads only when lateral bending of the spine. The complex nature of the scoliotic deformity usually leads toa decompensated spine even prior to surgery. Although considerable correction of the curve can be achieved using modern instrumentation systems, the spine can be restored. Lach of decompensation or decompensation of the spine is a major problem among many patients treated surgically with systems based on hooks and rods. Such decompensation in the frontal plane can be a result of correction beyond the compensatory possibilities of the lumbar spine, inadequate placing of hooks and incorrectly applied distraction forces. Overlooking the proximal junctional kyphosis (between the two proximal thoracic curves or overlooking the distal junctional kyphosis (between the lumbar and thoracic curve) can also lead to decompensation of the spine in the sagittal plane.