| Since the introduction of the transpedicular screw system by Boucher, the applications of this system in the treatment of degenerative disorders and unstable fractures, tumors of the spine have become very popular in the last two decades. The advantages of pedicle screw fixation are dependent on their ability to retain bony purchase until the fusion mass is stable. The bone-screw interface is a major determinant in the stability of spinal instrumentation systems. Loosening and failure of the screws are among the most common complications reported, especially for osteoporosis. Besides that, during the operations, because surgeons can not successfully insert screws into proper position at the first time, the turning back of the screws will be necessary. This also reduces the holding strength. Revision is often necessary. Increasing the diameter and/or length of the pedicle screw appears to provide the best solution. However, increasing screw diameter may not always be possible because of anatomical constraints. There is an increased risk of pedicle fracture with possible neural injury if larger screws are used. The use of longer screws increases the risk of anterior body penetration with possible vascular or visceral injury. Besides that, to enhance fixation of salvagescrews in cases of severe bone loss, some surgeons have chosen to fill the void with a variety of materials, including corticocancellous bone, polymethyl-methacrylate (PMMA). However, PMMA is not frequently used in spine surgery because of the potential danger if leakage into the spinal canal were to occur. Immediate risks resulting from leakage into the spinal canal are the result of the exothermic reaction present in the curing process of PMMA, whereas long-term risks are secondary to a nonresorbable foreign body in the spinal canal. So it has been a highlight on how to increase the bone-screw interface strength. Our goal in the current study was to discuss a new kind of expansive pedicle screw and evaluate the mechanical properties of the expansive pedicle screw as a "rescue" revision screw.This study contains three sections: 1) design and produce expansive pedicle screw. The expansive pedicle screw design has a barrel shape. The anterior half of the screw is split lengthwise by two perpendicular grooves to form four anterior fins. A smaller gage screw is inserted into the threaded interior of the expansion screw and opens the fins concentrically as it is advanced. This system increases the diameter of the expanding screw tip by approximately 2.5 mm. The diameter of the posterior portion of the screw does not change to prevent fracture of the pedicle during expansion of the screw. 2) Pullout tests, turning-back tests were performed to compare the holding strength of an expansive pedicle screw with conventional screws: USS, CDH and Tenor. Revision tests were performed to evaluate the mechanical properties of the expansive pedicle screw as a "rescue" revision screw. 3) A fatigue simulation using perpendicular loading up to 1,500,000 cycles was carried out. And 3-point bend tests were also performed to evaluated the static strength of expansivepedicle screws.The diameters of expanding screw tips measured by radiographs ranged from lmm to 2.5mm. Mean diameter is 1.6mm. After expansive pedicle screws were pulled out, the diameters ranged from 1.5mm to 2.5mm. Mean diameter is 1.9mm.The highest turning-back torque, pull-out force and energy absorption were seen in expansive pedicle screws, with significantly higher values than the other three pedicle screws. Even in group of lower bone density, the pullout force of expansive pedicle screws is higher than control screws in group of higher bone density.No failures occurred after 1,500,000 cycles. In no case did either the control or expansive pedicle screws fail. The results indicated a system fatigue strength at 1.5 million cycles of 200~250N, which was identical to that determined for the 6.5-mm control screws. There was no predisposition to failure of the expansive screw or an... |
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