الفهرس 
Anatomy of Motion SegmentOnly 14 pages are availabe for public view
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Abstract
The functional spinal unit (FSU), or the motion segment, is the smallest segment of the spine that exhibits biomechanical characteristics similar to those of the entire spine. It consists of two adjacent vertebrae, the disc in between and the connecting ligamentous tissues; the behavior of the FSU is dependent upon, among other things, the physical properties of its component, such as the intervertebral disc, ligaments and articulating surfaces. Because the spine may be considered as a structure composed of multiple FSU`s connected in series, its total behavior may be approximated as a composite of the behavior of the individual FSU`s constituting the spine.
Spinal fusion is designed to eliminate painful motion, remove the pathological process, and allow decompression of the neural elements with subsequent stabilization of the affected spine segments.
Spinal fusion is usually successful in treating the patient`s symptoms, however it can result in increased motion and stress in the segment adjacent to the fused level, which may initiate and/or accelerate the degenerative disease process in adjacent segments.
Degenerative changes of the adjacent segments ranges from degenerative arthritis of the facet joint, spinal canal stenosis, and sever disc degeneration to degenerative spondylolisthesis and they are called Adjacent Segment Disease (ASD).
ASD is a concern to both patients and surgeons, and is a potential cause of further spinal surgery, As ASD is raised, careful consideration of fusion devices and new devices are proposed.
Motion preservation strategies provide an alternative approach to fusion surgery and can be looked to as a means of avoiding ASD, but it also represents the ideal solution to the previously fused patient who has already developed adjacent segment degeneration.
As there are strict inclusion criteria as well as contraindications to motion preservation procedures, spinal fusion still has its own indications.
ASD is thought to be a cause of failed back surgery syndrome, so a combination of symptoms description, clinical examination, and radiological findings are needed to enable differentiation.
Once ASD is diagnosed, an adequate cause of conservative therapy must be completed prior to surgery, and proper patient selection with proper surgical technique is vital to good patient outcomes.
The surgical treatment option of ASD varies from motion elimination strategies or motion preservation strategies, it is important for the surgeon to understand the biomechanical principles of the spine so that a proper strategy can be devised to address the pathological entity.
ASD may require revision surgery and extension of the fusion to the affected adjacent levels, if there`s contraindication to motion preservation, As when adjacent segment degeneration has rapidly developed next to a one or two-level fusion the prospect of adding another level or two of further fusion is quite bleak for the ((new)) adjacent segments. Motion preservation may be the solution of stopping further progression of the adjacent segment degenerative cascade.
B-Motion preservation strategy:-
Motion can be preserved by replacing the nucleus, replacing the whole disc, or using dynamic stabilization systems. The most important parameters in achieving a good result were patient selection, and strict adherence to traditional inclusion and exclusion criteria.
The clear advantage of disc arthroplasty is its ability to restore and maintain the motion segmental anatomy and function.
-The dynamic stabilization system is less invasive compared with fusion procedures and it allows the treatment of degenerative lumbar disease in a segment-by-segment manner.