The Investigation Of The Sacral Screw Fixation Techniques Using Finite Element Method

Lumbar spondylolisthesis is the common disease of the orthopedics, while the sacral pedicle fixation technique is widely applied in clinical therapy, because the anterior, middle and posterior three-dimensional space fixation of the vertebra can be achieved. However, the unique anatomy and local adjacent structure of sacrum are complex, it is hard to get the safe, reliable and ideal insertion of the sacral screw. This work investigates the sacral screw fixation techniques using finite element method (FEM), to provide powerful support for clinic in the aspect of theory and practice for selecting the best fixation techniques of sacral screw and directing clinical restoration and reconstruction the stability of spine.First, based on the anatomy structure of sacrum, the safe region of screw fixation is explored, and the factors which influence screw stability, such as the bone density, adjacent structure, screw implantation, fixation strength, are analyzed.Then according to the clinical research, this work carries out a systematical study on the numerical simulation of the load transfer mechanisms with three typical fixation techniques, such as anteromedial sacropromontory method, anterolateral and vertical to the iliac wing ones. For three fixation methods mentioned above, both bonded and contact conditions are employed to demonstrate the interface between the screw and sacrum. By analyzing the stress, displacement and deformation of the sacrum, the effect of lumbosacral fusion and the region broken first during the pullout process are monitored.In term of the above studies, the following results can be concluded: the interface condition plays an important role in transferring the force between the sacrum and the screw. To compare the three fixation techniques of sacral screw, anteromedial sacropromontory method, anterolateral and vertical to the iliac wing ones, the superiority of mechanics is decreasing. Anteromedial sacropromontory fixation technique is the most rigid way of sacral fixation, anterolateral to the iliac wing method is the second, and vertical to the iliac wing is the worst. For the same direction with the screw, the fixation strength of the bicortical fixation is better than the unicortical one.