Currently a treatment for spinal cord injury (SCI) remains elusive to clinicians and researchers. This is, in part, due to variation between primary injury mechanisms and diversity of mechanical impact factors such as impact velocity, depth, force, and acceleration. This research examines both the individual and combined effects of impact velocity and depth on the cervical spinal cord and also aims to understand the contribution of the energy applied, not only the impact factors.

In this study, contusion spinal cord injuries were induced in 54 male, Sprague-Dawley rats at impact speeds of 8 mm/s, 80 mm/s, or 800 mm/s with displacements of 0.9 mm or 1.5 mm. Animals recovered for seven days followed by behavioural assessment and examination of the spinal cord tissue for demyelination and tissue sparing at 1 mm intervals ±3 mm rostrocaudally to the epicentre. In parallel, a finite element model of the rat spinal cord was used to examine the resulting maximum principal strains in the spinal cord during impact.

Impact depth was a consistent factor in qualifying axonal damage in the spinal cord, tissue sparing, and resulting behavioural deficit. Increased impact velocity resulted in significantly different impact energies and measureable outcomes at the 1.5 mm impact depth, but not the 0.9 mm impact depth, identifying threshold interactions between the two factors. The difference of injury severity to velocity at different impact depths identifies the existence of threshold interactions between the two impact factors.

Linear correlation analysis with finite element analysis (FEA) strain showed significant (p ≪ 0.001) correlations with axonal damage in the ventral (R² = 0.86) and lateral (R² = 0.74) regions of the spinal cord and with white matter (R² = 0.90) and grey matter (R² = 0.76) sparing. Non-parametric correlation analysis identified strong correlations between grey and white matter strain with open field behavioural scores (p = 0.005, r_s = -0.94).

The results shown by this work extend the research identifying significant correlation between maximum principal strain and neurologic tissue damage. Furthermore, a relationship between the impact depth and velocity of injury demonstrated a more rate sensitive response of the spinal cord at the 1.5 mm impact depth than at the 0.9 mm impact depth.