## 3 Results  
The von-Mises stress in the cord increased as the impact height or force increased for both impactors, and the impactor tip diameter of 1.6 mm resulted in the highest stress in the cord on the right (ipsilateral) side the spinal cord. By contrast, the von-Mises stresses were decreased with larger impactors at a drop height of 6.25 mm for the NYU impactor and an impact force of 100 kdyn for the IH impactor. The stresses on the right (ipsilateral) side were significantly higher than on the left (contralateral) side of the spinal cord (\(p<0.05\)) (Figure 3a). On the contralateral side, the von-Mises stresses were slightly increased with the larger impactor diameters, regardless of which impactor  
Figure 1: (**A**) Axial view of the cervical spinal cord in the rat finite element (FE) model. (**B**) Sagittal view of the spinal cord model with an impactor.  
Figure 2: Spinal cord with four different impactors which were moved 1.0 mm to the right side of the midline, which was approximately centered over the right side of the spinal cord.  
(NYU or IH) was used (Figure 3a). For the NYU impactor, the stresses in the cord were 0.55 MPa, 0.38 MPa, 0.35 MPa, and 0.32 MPa at the drop heights of 6.25 mm, and 0.79 MPa, 0.93 MPa, 1.12 MPa, and 0.79 MPa at the drop height of 12.5 mm, with 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm diameters, respectively. Under experimental conditions for the IH impactor, the stresses were 0.57 MPa, 0.37 MPa, 0.29 MPa, and 0.21 MPa with 100 kdyn and 0.77 MPa, 0.97 MPa, 1.19 MPa, and 0.8 MP with 200 kdyn impact forces, with 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm diameters, respectively (Figure 3a). The stress distributions in the transverse views at the maximum compression are shown in Figure 4. Similarly, the maximum principal strains peaked at the 1.6 mm diameter impactor on the ipsilateral side with the 12.5 mm NYU impactor and 200 kdyn IH impactor, and decreased with the larger impactor diameter under low impact loading conditions (Figure 3b). A significant difference was observed between the right and left side of the spinal cord (\(p<0.05\)) (Figure 3b). Higher strains were mostly located on the ipsilateral side, regardless of the load and impactor tip diameter (Figure 5).  
The maximum spinal cord displacement was gradually reduced as the impactor tip diameter increased, regardless of which impactor was used (Figure 6). The smaller impactor tip diameters caused higher displacement, but the spinal cord was likely to shift laterally. The maximum displacements were 2.3 mm, 2.2 mm, 2.1 mm, and 1.9 mm for impactor tip diameters of 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm, respectively. There was no significant difference in the spinal cord displacement according to the impactor tip diameter (\(p=0.10\)). Based on the results, the 6.25 mm and 12.5 mm conditions of the NYU impactor were similar to the 100 kdyn and 200 kdyn conditions of the IH impactor, respectively.  
Figure 3: (**A**) Peak von-Mises stress and (**B**) maximum principal strain in the right and left side of the spinal cord for the New York University (NYU) and Infinite Horizon (IH) impactors at impactor tip diameters of 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm.  
Figure 4: Stress distributions in the transverse view at the maximum compression for the NYU and IH impactors at impactor tip diameters of 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm.  
Figure 5: Strain distributions in the transverse view at the maximum compression for the NYU and IH impactors at impactor tip diameters of 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm.  
Figure 6: The maximum displacement of the spinal cord for the NYU and IH impactors at impactor tip diameters of 0.8 mm, 1.2 mm, 1.6 mm, and 2.0 mm.