Severity of Spinal Cord Injury in Adult and Infant Rats after Vertebral Dislocation Depends upon Displacement but not Speed

Spinal cord injury (SCI) is less common in children than in adults, but in children it is generally more severe. Spinal loading conditions (speed and displacement) are also thought to affect SCI severity, but the relationship between these parameters is not well understood. This study aimed to investigate the effects of vertebral speed and displacement on the severity of SCI in infants and adults using a rodent model of vertebral dislocation. Thoracolumbar vertebral dislocation was induced in anaesthetized infant rats (similar to 30 g, 13-15 days postnatal, n = 40) and adult rats (similar to 250 g, n = 57). The 12th thoracic vertebra was secured, whereas the first lumbar vertebra was dislocated laterally. Dislocation speed and magnitude were varied independently and scaled between adults and infants (Adults: 100-250mm/s, 4-10mm; Infants: 40-100mm/s, 1.6-4mm). At 5 h post-injury, rats were euthanized and spinal cords harvested. Spinal cord sections were stained to detect hemorrhage (hematoxylin and eosin) and axonal injury (beta-amyloid precursor protein). For each millimeter increase in vertebral displacement, normalized hemorrhage volume increased by 1.9 x 10(-3) mm(3) (p = 0.028) and normalized area of axonal injury increased by 2.2 x 10(-1) mm(2) (p < 0.001). Normalized hemorrhage volume was 3.3 x 10(-3) mm 3 greater for infants than for adults (p < 0.001). Magnitude of dislocation was found to have a different effect on the normalized area of axonal injury in adults than in infants (p = 0.003). Speed of dislocation was not found to have a significant effect on normalized hemorrhage volume (p = 0.427) or normalized area of axonal injury (p = 0.726) independent of displacement for the range of speeds tested. The findings of this study suggest that both age and amount of spinal motion are key factors in the severity of acute SCI.