255. EMG STIMULATION THRESHOLDS FOR DORSAL SCREWS DEPENDS ON THE DISTANCE FROM THE SCREW TO THE SPINAL CORE, BUT NOT TO THE INTEGRITY OF THE PEDICULAR CORTICAL: A CLINICAL AND EXPERIMENTAL STUDY

Abstract

Purpose of the study: Neurophysiological monitoring during pedicular screw insertion has been used to verity bone integrity of instrumented pedicles. The purpose of this study was to determine, experimentally, whether the EMG thresholds after stimulation of dorsal pedicular screws depend on the distance between the nerve structures and the screw, or on the interposition of different tissues.

Material and methods: EMG thresholds were recorded after stimulation of 18 VPD in fivde pigs, by varying the distance between the screws and the spinal cord (2, 6, 10 mm). The thresholds were recorded after rupture of the median pedicular cortical and after interposition of different tissues (blood, muscle, fat and bone) between the screws and the spinal cord. In four patients with a hemivertebra, four pedicular screws sere stimulated at insertion, just after resection of the hemivertebra.

Results: The average intensity of the EMG thresholds was 5.60±1.90mA when the screws were in contact with the dural sac. When the distance was 2 mm, the average threshold reached ±3.42 mA, at 6 mm 13.59±6.27 mA and at 10mm, 15.86±5.83 mA (p< 0.05). Rupture of the median pedicular cortical and interposition of different biological tissues in experimental animals did not modify the stimulation thresholds of the dorsal pedicle screws. In the four operated patients with resection of a hemivertebra, the EMG stimulation thresholds exhibited a wide spread but did not provide any evidence for a significant change related to interposition of different tissues. The impedance of the bone material was higher than muscle or adipose tissue. In these patients, the distance from the screw to the spinal cord was not correlated with a modification in the stimulation threshold.

Discussion: Further clinical study is needed to better understand the stimulation role of the EMG in the implantation of pedicular screws, considering that this technique does not determine pedicle rupture.

Conclusion: In experimental animals, the electrical impedance appears to depend on the distance between the screw and the nerve structures but not on the integrity of the median pedicular cortical. Response to intensity does not appear to be related to the type of interpositioned tissue.