New method for intraoperative determination of proper screw insertion or screw malposition

Object. Inadequate imaging techniques may lead to misjudgment of screw positioning when applying transpedicular instrumentation; this can create potential risks of major vessel and nerve damage. in this article the authors present a new method to determine screw malpositioning intraoperatively. Methods. The authors retrospectively evaluated pre- and postoperative plain radiographs of 97 spinal segments: in which screws had been placed in 41 patients suffering from thoracolumbar injury who had previously undergone transpedicular screw fixation. They developed a new mathematical equation with which they determine the distance ratios of two screw tips in the same segment by comparing the distance between the pedicles on preoperative radiographs with these on postoperative radiographs. Subsequently, the results are compared with postoperative computerized tomogaphy findings to determine which screws are in the correct position and which are penetrating the medial or lateral cortex of the pedicle. It was found that the ratio range of correctly placed screw tips was 46 +/- 10% (mean +/- standard deviation) in the thoracic region and 60 +/- 9% in the lumbar region (ranges 43-50% and 57-63%, respectively, 95% confidence intervals). Higher ratios (higher percentages) than these values indicated extreme closeness of screw tips and therefore medial malpositioning. Lower ratios (lower percentages) indicated lateral malpositioning. Conclusions. This proposed method may provide intraoperative determination of correct screw positioning or malpositioning. This method allows surgeons to replace the malpositioned screw, and, consequently, early resolution of neurovascular injuries is made possible. Additionally, repositioning of the screw correctly will avoid rigidity failure of the fixation device.