title: Intraoperative Neurophysiological Changes Induced by Thoracic Pedicle Screws Intentionally Placed Within the Spinal Canal: An Experimental Study in Pigs.
abstract: METHODS
  Experimental study, OBJECTIVES: To document and analyze the neurophysiological changes during spinal cord monitoring when thoracic pedicle screws are intentionally placed within the spinal canal.
  
  
  BACKGROUND
  Although the rate of misplaced screws is relatively high, few patients have neurological impairment. This suggests that a significant degree of medullary displacement and/or compression is necessary to produce neurophysiological changes.
  
  
  METHODS
  The spinal cord of 3 experimental pigs was surgically exposed at 3 different levels (T11, T9, and T6). Two pedicle screws were placed within the spinal canal at each vertebral level under direct vision. One was placed on the lateral edge of the dural sac, causing only a slight cord displacement; a second screw was placed in the middle of the spinal canal, producing marked displacement of the neural structures. During the procedure, neurophysiological monitoring of the spinal cord was performed.
  
  
  RESULTS
  No neurophysiological changes were observed in any screws placed at the lateral edge of the dural sac for 20 minutes after screw placement. When the screws were placed in the center of the spinal canal, neurophysiological changes occurred with a mean latency of 10.1 ± 2.1 minutes, and at 11.6 ± 1.9 minutes there was complete loss of the spinal cord evoked potentials in all cases. After these centered screws were removed, evoked potentials began to recover, with a latency of 9.7 ± 3.0 minutes in 7 of 9 cases.
  
  
  CONCLUSIONS
  Neurophysiological monitoring of the spinal cord does not detect moderate compression even 20 minutes after neural compression. Only thoracic pedicle screws provoking marked displacement of the spinal cord were able to cause delayed neurophysiological changes leading to loss of spinal cord evoked potentials, which in 22% of cases did not recover after the pedicle screw was removed.
authors.affiliation: Division of Pediatric Orthopedics, Hospital Ramón y Cajal, Crta. Colmenar Viejo km 9.100, 28034 Madrid, Spain.
authors.forename: Miguel
authors.initials: M
authors.lastname: Antón-Rodrigálvarez
authors.affiliation: Institute for Research on Musculoskeletal Disorders, Valencia Catholic University, Quevedo 2, 46001 Valencia, Spain. Electronic address: carlos.barrios@ucv.es.
authors.forename: Carlos
authors.initials: C
authors.lastname: Barrios
authors.affiliation: Department of Clinical Neurophysiology, Hospital Ramón y Cajal, Crta. Colmenar Viejo km 9.100, 28034 Madrid, Spain.
authors.forename: Gema
authors.initials: G
authors.lastname: de Blas
authors.affiliation: Division of Pediatric Orthopedics, Hospital Ramón y Cajal, Crta. Colmenar Viejo km 9.100, 2803 Madrid, Spain.
authors.forename: Jesús
authors.initials: J
authors.lastname: Burgos
authors.affiliation: Spinal Surgery Unit, Hospital La Fraternidad-Muprespa, Paseo de la Habana 83-85, 28036 Madrid, Spain.
authors.forename: Eduardo
authors.initials: E
authors.lastname: Hevia
authors.affiliation: Experimental Surgery Unit, Hospital Ramón y Cajal, Crta. Colmenar Viejo km 9.100, 2834 Madrid, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Correa
keywords: Complications; Neurophysiologic monitoring techniques; Pedicle screws