Pedicle Screws Research Paper

CORTICAL SCREWS vs PEDICLE SCREWS FOR STABILISING THE LUMBAR SPINE

Introduction
Instrumented fusion of the lumbar spine has been common place in spine surgery for the past two decades. Since its inception many advances have been made to make the technique more elegant, less invasive, more effective and biomechanically sound. This had led to percutaneous techniques to stabilize the lumbar spine. For patients who require open surgery, less exposure techniques would be optimal due to the advantages associated with such techniques. The traditional method of placing pedicle screws is through an open operation and leads to exposure of a significant amount of the vertebral elements. This led to the development of cortical screws as an alternative

They used flexion, extension, lateral bending and axial rotation as the spinal mechanics to be tested. They assessed the mechanics in the intact spine, created a burst fracture at L3, and the mechanics were again tested pre and post instrumentation. The instrumented group was further subdivided into two groups, depending on whether they had four or six pedicle screws (ie a screw through the pedicle of the fractured vertebral level). The results showed decreased movement in all mechanical modalities except for axial rotation. They also demonstrated that screws through the fracture vertebrae provided a stiffer construct, and that monoaxial screws resulted in a stiffer construct than polyaxial screws. Anekstein et al achieved similar results with pig spine(10). The tensile strength of the pedicle screw, along with the sizing positioning of the pedicle screw is important in determining the strength of the construct, along with determining if the construct will we strong enough to withstand the von Misses forces applied to it from the biomechanics of the spine(11). Qi et al demonstrated that the larger the pedicle diameter, the greater they were able to improve the distribution of axial pullout stress on the screws, cortical bone and cancellous bone(12). With a 6.5mm screw, load transferred to cortical and cancellous bone was reduced by 47.24% and 34.28%,

placement of pedicle screws in the thoracic and lumbar spine. Accurate screw placement is dependent on the exposure and identification of the posterior elements including the lateral border of the pars interarticularis, the entire transverse process and the caudal and cephalad facet joints(13). The pedicle entry point is at the intersection of the vertical line that forms the extension of the facet joint in line with the bony crest coming from the superior articular facet, and the horizontal line that passes through the middle of the transverse process insertion, or 1-2mm below the joint line(14). In Patel?s retrospective cohort study, they demonstrated statistically significant higher blood loss in patients who underwent open posterior lateral or posterior lumbar interbody fusion, compared to minimally invasive techniques(15). The open technique of placing pedicle screws therefore requires much dissection to expose the posterior vertebral elements and is associated with more blood loss and complications when compared to minimally invasive procedures. Minimally invasive procedures, such as percutaneous pedicle screws, are not available as readily and is more expensive than the open method. A good middle ground would be to provide a technique that offers less dissection, with less complications and blood loss, but achieves the same biomechanical stability achieved by pedicle