الفهرس 
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Abstract
The stabilizing system of the spine may be divided into three subsystems: the spinal column; the spinal muscles; and the neural control unit. A large number of biomechanical studies of the spinal column have provided insight into the role of the various components of the spinal column in providing spinal stability (Panjabi, 2003). Clinical instability is an important cause of low back pain. Although there is some controversy concerning its definition, it is most widely believed that the loss of normal pattern of spinal motion causes pain and/or neurologic dysfunction (Panjabi, 2003). It is helpful to differentiate between mechanical instability and clinical instability. The former defines inability of the spine to carry spinal loads, while the latter includes the clinical consequences of neurological deficit and/or pain (Panjabi, 2003). The lumbar spine checklist uses several elements, such as biomechanical parameters, neurologic damage and anticipated loading on the spine. A point value system is used to determine clinical stability or instability. If the sum of the points is five or more, then the spine is considered clinically unstable (Mulholland and Sengupta, 2004). The Concept of Microinstability : The first phase of the degenerative cascade, defined as the phase of unstable dysfunction, includes many pathological alterations affecting the constitutive elements of the motor spinal unit. Those alterations will be unavoidably leading, during the years, to spondyloarthrosys . The alterations are related to a dynamic overload of the articulation in the motor spinal unit, especially to the intervertebral disc (responsible for the characteristics of load and torsion resistance) and to the articular processes (the true responsible for the movement). The alterations are generated by an anomalous hypermovement, an overstress of an articulation that is not capable of bearing the applied load. In the phase of unstable dysfunction there are ongoing anatomopathological alterations in absence of vertebral slippage. (Fabrizio Gregori and Roberto Delfini, 2015) Adjacent segment disease is a term with broad meaning since it can describe nearly any abnormal process that develops in the mobile segment next to a spinal fusion. One of the most common findings next to a fused segment was disc degeneration. Listhesis, instability, hypertrophic facet joint arthritis, herniated nucleus pulposus, and stenosis were also reported frequently. Less commonly noted findings included scoliosis and vertebral compression fractures (Park et al., 2004) In recent years, non-fusion stabilization of the lumbar spine has gained more and more popularity. These non-fusion systems intend to maintain or restore the intersegmental motions to magnitudes of the intact spine and have no negative effects on the segments adjacent to the stabilized one (Schnake, Schaeren and Jeanneret, 2006a). The dynamic system is capable of stabilizing an unstable segment sufficiently but allows more motion in the segment than the internal fixator. The adjacent segment does not seem to be influenced by the stiffness of the fixation procedure under the loading conditions (Ross, 2007). Biomechanical alterations may have an impact on the adjacent disc involvement. In vitro experimental studies have confirmed increased mobility in the disc adjacent to a vertebral fusion (Marsol-Puig et al., 2011).Moreover, recent studies have confirmed that dynamic stabilization may provide better surgical alternatives to conventional fusion for treating lumbar degenerative diseases (Qian et al., 2015). For it to succeed, the dynamic system should achieve functional stability and maintain any possible motion between spine segments (Bono, Kadaba and Vaccaro, 2009). The posterior elements of the spine such as lamina, spinous process and the ligaments form a posterior tension band, which is important to the stabilization of the spine. However, surgical removal of these structures leads to potential in- stability. Dynamic stabilization can restore this tension band and provide enough stability to prevent further progression of degeneration (Azzazi and Elhawary, 2010). By preventing further progression of in- stability, dynamic stabilization may also avoid the use of a bone graft and its complications (e.g. wound problems, neurovascular damage, infections, pelvic fractures) (Schnake, Schaeren and Jeanneret, 2006b) HPS™ is a universal system for stabilization of the spine. It allows for multi-segmental fusion with the option of dynamic stabilization of the cranial segment. The aim is to shorten the length of fusion and to thus reduce the risk of degeneration in the adjacent segments. The dynamic coupler controls movement of the spine in all directions. Mobility is ensured for flexion, extension and lateral bending, while translation and axial rotation are reduced to a minimum. (Wilke, Heuer and Schmidt, 2009) The capacity for controlled axial adjustments in length permit changes in the distance between the pedicles of just under 2 mm. This allows the system to cushion axial forces, to reduce the load on facet joints and intervertebral discs and to preserve the physiological center of rotation in the index and adjacent segments. (Wilke, Heuer and Schmidt, 2009) The initial aim was to develop a dynamic coupler that reduces the range of motion in relation to flexion /extension, lateral bending and rotation by about 50 – 70 % based on its spring stiffness. In addition, the change in the distance between the pedicles during movements involving flexion and extension requires a controlled adjustment to the length of the implant by about 2 mm.3 This process preserves the natural center of rotation. The overall range of motion in the spring must simultaneously be limited mechanically to protect the implant from mechanical overloading and to thus achieve fatigue strength. (Wilke, Heuer and Schmidt, 2009; Chamoli, Chen and Diwan, 2014) This in-vitro study compares the kinematic signature of the dynamic DSS/HPS™ coupler with fusion using a pedicle-screw system. It reveals that the dynamic coupler provides adequate stabilization and simultaneously preserves motion. (Wilke, Heuer and Schmidt, 2009).