[Biomechanics of scoliosis]

Scoliosis is defined as a three-dimensional deformity of the spine. The most pronounced component of scoliosis is in the frontal plane, comprising the lateral bending of the spine. Rotation of vertebra takes place in the transverse plane. In most cases of idiopathic scoliosis a decrease of thoracic kyphosis in the sagittal plane occurs. A more rare event is the appearance of a junctional kyphosis between the primary and secondary curve. The instrumentation introduced by Harrington dealt mainly with balancing the bending forces in the frontal plane (distraction of the concavity of the curve), along with fusion of the instrumented area. The multisegmental CD instrumentation allowed for the diminution of the lateral curve in the frontal plane, while at the same time "forcing" an increase of thoracic kyphosis in single curves, and restoration of physiological sagittal curves (thoracic kyphosis lumbar lordosis) in double curve scoliosis. The CD method achieved this good by a 90 degrees rotation of the rod towards the concavity of the curve, "changing" the lateral curve into kyphotic curve. In the AO USS (Universal Spine System) correction is achieved by pulling the hooks towards the rod. The procedure ends with the linking of two rods with transverse connectors forming this way a stable framework. The degree of correction achieved with this method is based on the biomechanic inter relation between the spine and the instrumentation system (application of distraction forces, compensatory forces and translocation of the instrumented segment). Post-op decompensation of the spine is usually the result of incorrect hook fixation, inadequate application of forces (distraction and compression) and use of a standard hook pattern for thoracic curves (type III) in other types of scoliosis.