Mechanical characterization of the injured spinal cord after lateral spinal hemisection injury in the rat

The glial scar formed at the site of traumatic spinal cord injury (SCI) has been classically hypothesized to be a potent physical and biochemical barrier to nerve regeneration. One longstanding hypothesis is that the scar acts as a physical barrier due to its increased stiffness in comparison to uninjured spinal cord tissue. However, the information regarding the mechanical properties of the glial scar in the current literature is mostly anecdotal and not well quantified. We monitored the mechanical relaxation behavior of injured rat spinal cord tissue at the site of mid-thoracic spinal hemisection 2 weeks and 8 weeks post-injury using a microindentation test method. Elastic moduli were calculated and a modified standard linear model (mSLM) was fit to the data to estimate the relaxation time constant and viscosity. The SLM was modified to account for a spectrum of relaxation times, a phenomenon common to biological tissues, by incorporating a stretched exponential term. Injured tissue exhibited significantly lower stiffness and elastic modulus in comparison to uninjured control tissue, and the results from the model parameters indicated that the relaxation time constant and viscosity of injured tissue were significantly higher than controls. This study presents direct micromechanical measurements of injured spinal cord tissue post-injury. The results of this study show that the injured spinal tissue displays complex viscoelastic behavior, likely indicating changes in tissue permeability and diffusivity.