Effects of Vertebral Column Distraction in the Monkey

Experiments were performed to assess the effects of vertebral column distraction on evoked potential responses from multiple recording sites along the conducting pathway in the monkey, and on concurrent blood flows, measured with the radioactive microsphere technique, along the axis of the central nervous system. Linear distractive loads were applied until the amplitude of the evoked response was significantly reduced. In four monkeys, the loads (100 to 150 lb) were sustained, whereas in two monkeys the forces (80 to 110 lb) were relaxed. The earliest response changes were most marked in recordings dependent upon the integrity of the upper cervical dorsal columns or brain stem-lemniscal pathway. The responses returned to control levels with load relaxation, but maintenance of the tractive load produced generalized and progressive response attenuation. At selected periods of significant changes in the evoked potential response, blood flow remained stable except for the late onset of regional ischemia in the middle cervical through upper thoracic spinal cord levels in the animals undergoing sustained loads. These findings indicate that brain-stem or spinal cord dysfunction occurring with both acute and gradual elongation of the spinal canal are the result of excess tensile stress acting on fiber tracts, and the delayed onset of spinal cord ischemia is the probable result of a similar mechanical process acting upon intrinsic spinal cord blood vessels.