Abstract
Objectives
To determine the limits of spinal displacement before the onset of neurophysiological changes during spinal surgery. Assessing if the type of force applied or the section of the adjacent nerve roots increases the tolerance to displacement.
Methods
Experimental study in 21 domestic pigs. Three groups were established according to the displacing force applied to the cord: separation (group 1, n=7), root stump pull (group2, n=7) and torque (group3, n=7). Successive records of cord-to-cord motor evoked potential were obtained. The displacing force was removed immediately when neurophysiological changes observed. The experiment was repeated after sectioning the adjacent nerve roots.
Results
The diameter of the dura in the study area was 7.2 ± 1 mm. Group 1: evoked potential changes appeared with displacement of 10.1 ± 1.6 mm with roots unharmed and 15.3 ± 4.7 mm (p <0.01) with section of four adjacent roots. Group 2: evoked potential disturbance at 17.5 ± 4.7 mm, which increased to 23.5 ± 2.1 mm (p <0.05) after cutting the two contralateral roots. Group 3: cord allowed torque of 95.3° ± 9.2 increasing to 112.4 ° ± 7.1 ° if the contralateral roots were cut. Except in two cases in group 3 (torsion), the potentials were normalized immediately after releasing the deforming force.
Discussion
This experimental study shows that it is possible to surgically displace the medulla a distance superior to the diameter of the dura without detecting neurophysiological changes. The limits of cord displacement may be increased by the section of the adjacent nerve roots and if the tensile force is applied by traction of the root stumps. These findings support the neurological safety of spine deformity correction by isolated posterior approach, obviating the morbidity related to an additional anterior procedure.
