gms | German Medical Science

Objective: The placement of pedicle screws in spinal instrumentation holds the risk of injury of the dura and the spinal nerves roots. Indeed the number of malplacements could be reduced through the use of image-guidance, yet this operation technique is still afflicted with a considerable morbidity. Therefore, various neurophysiological examination techniques have been described for monitoring, especially of the spinal nerves below the Conus medullaris, but no one of these techniques has enduringly prevailed for routine clinical usage. Direct and indirect transpedicular spinal-nerve stimulation (TSS) by means of stimulation electrodes within the pedicle drill-hole belong to these techniques.

Methods: This study investigated whether TSS can be carried out also directly via the transpedicularly placed guidewire when using a new canulated pedicle screw system (XIA Precision, Stryker). The investigation took place on 20 monosegmental and 5 bisegmental dorsal stabilizations in the area of the middle and lower lumbar vertebrae, for 11 degenerative, 6 traumatic, and 8 tumorous causes. Spontaneous and triggered myogenic EMG-activity (direct: single pulses up to 10 mA; indirect: pulsed 4.7 Hz, 200ms, max 20 mA) were discharged with needle electrodes via the corresponding characteristic muscles of the lower extremities.

Results: Both “train” activity (as a sign for longer lasting traction or compression) and “spike” and “burst” activity (as signs for short-term nerve-root contact) could be found during the decompression of the spinal canal, without external triggering. In the postoperative CT exams, 101 of the 110 screws were seen to be lying entirely intrapedicularly, 6 of the 110 were medially dislocated <2mm, and 3 of the 110 were medially dislocated >2mm. But only one screw malposition with contact to the nerve root L5 was clinically symptomatic. This was not recognized intraoperatively, neither through the direct TSS nor through the indirect TSS.

Conclusions: Radicular complaints in direct causal connection with a screw malposition actually arose postoperatively only in a few cases of a medial breaking-through of the pedicle screw. Because of its limited sensitivity and specificity, the technical and economic expenditure for TSS cannot be justified, in the view of the authors. Some of the reasons for false negative results are: the individual activity threshold of the muscle groups, anesthesia influences and the degree of muscle relaxation, the different tissue resistances, liquid accumulations in the stimulation area, preexisting nerve-root compressions, and preexisting metabolic diseases.