Title:
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Computational investigation of anterior cervical spine stabilisation
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Anterior cervical interbody fusion with anterior plate stabilisation has consistently been reported to obtain excellent results in treatment of numerous spinal disorders. Although several authors have examined anterior fixation techniques in animal and cadaveric studies, a considerable discrepancy of results makes the comparison of separate reports extremely difficult. The finite element method can supplement experimental research in understanding the clinical biomechanics of the cervical spine, with advantages such as absolute repeatability and reproducibility. In this study the effects of different anterior stabilisation systems on the kinematic responses of a cervical spinal unit were determined in all loading conditions expected to occur during daily activities. A geometrically accurate, three-dimensional finite element model of the human lower cervical spine was developed and validated against experimental data, and then modified to simulate a surgically treated injuiy, to investigate the relative stability conferred by three commercially available anterior fixation plates, as compared to the intact spinal unit and unplated fusion procedure. The additional support of a fixator resulted in stability of the spinal segment similar to or greater than that of the intact state, but also caused a significant reduction of stress in the anterior part of the bone graft, suggesting the possibility of a stress shielding effect which may result in a failure of fusion. An attempt was then made to address that deficiency and two solutions were suggested: an innovative less-rigid plate design, and a biodegradable composite as a constmction material; their influence on the stress distribution in the bone graft as well as the overall stability provided was investigated. Both solutions offered sufficient stabilisation of the injured spine and a considerably elevated stress level in the interbody bone graft. The biodegradable composite plate was evidently superior, with the most physiologically accurate kinematical behaviour of all investigated systems, and a uniform stress distribution across the bone graft, similar to that found in the uninstrumented fusion. Results of the presented study confirm that the formulated design assumptions and employed numerical approach form a promising prospect for the future development of spinal stabilisation devices.
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