## 4 Discussion
In addition, the results showed that the NYU impactor was similar to the IH impactor for hemicontusion injury. The maximum stresses of 0.32 MPa and 0.79 MPa with the 6.25 mm and 12.5 mm NYU impactors were similar to the 0.21 MPa and 0.80 MPa stresses with the 100 kdyn and 200 kdyn IH impactors with a 2.0 mm impactor tip diameter, respectively. These results were comparable with previous data from a central contusion SCI model [31] showing 0.26 MPa and 0.63 MPa stresses with 6.25 mm and 12.5 mm NYU impactors, and 0.14 MPa and 0.5 MPa stresses with 100 kdyn and 200 kdyn IH impactors with a 2.5 mm impactor tip diameter, respectively. The relationship between the NYU and IH devices also remained for the hemicontusion model as shown in the central contusion model [31], although slightly different values were obtained. The predicted spinal cord displacements showed a trend similar to trends reported elsewhere; spinal cord displacement increased with increasing injury severity or decreasing impact size [11; 12; 15; 38]. Under IH impactor experimental conditions, the FE model showed an impactor displacement of 2.0 mm at an impact force of 200 kdyn with a 1.6 mm diameter impactor, and an impactor displacement of 1.7 mm at an impact force of 100 kdyn with a 1.2 mm diameter impactor. These results are comparable with impactor displacements of 1.6 to 1.8 mm at an impact force of 200 kdyn with a 1.6 mm impactor tip diameter [15]. In addition, [PERSON] et al. [38] showed displacements of 0.57 to 1.56 mm at an impact force of 100 kdyn with a 1.3 mm impactor tip diameter. Furthermore, the stress and strain distributions in the cord, and spinal cord deformations inside the spinal canal, were similar to those in the previous FE model of unilateral injury in the rat, which also showed an impact force of 2.53 N for 1.6 mm displacement with a 1.6 mm diameter impactor [32]. The experimental data demonstrated that the spinal cord displacements at drop heights of 6.25 mm and 12.5 mm for the NYU impactor were 1.8 to 2.3 mm and 2.4 to 2.9 mm, respectively, while our model showed approximate displacements of 1.6 mm and 2.0 mm at drop heights of 6.25 mm and 12.5 mm, respectively. These studies are in agreement with our results, despite some differences in experimental conditions. There are several limitations to our study. Only the 6.25 mm and 12.5 mm cases were considered for the NYU impactor, and 100 kdyn and 200 kdyn, for the IH impactor, due to the low animal survival rates reported with increasing impact forces [17]. [PERSON] et al. [12] showed that with the NYU impactor, all animals survived with an impact height of 6.25 or 12.5 mm. However, an impact force of 300 kdyn resulted in significantly fewer surviving neurons in the spinal cord than impact forces of 100-200 kdyn for the IH device [11]. The model geometries were simplified, as in previous modeling studies of the spinal cord [23; 31; 39]. The impactor tip was modeled with a rounded shape since it can provide more uniform strain with lower peak magnitude, and a substantially enhanced temporal window for the study of cellular injury mechanisms [40]. The homogeneous material properties were used for the white and gray matter in the spinal cord, which could affect the results. The fluid flow was not considered in this study because the inclusion of fluid flow has little effect on impact [39]. Furthermore, the impactor rod was placed 1.0 mm from the midline, although no behavioral differences were observed between the injuries at 1.0 mm to 1.4 mm off the midline [16]. Sensitivity analysis for the impact position on the spinal cord could enhance the confidence of this study. Finally, the results from the FE model should be compared with the data from the in vivo experiments in order to confirm the results of this study. In conclusion, we investigated the effects of impactor size in the hemicontusion SCI model for NYU and IH impactors. The stresses and strains on the ipsilateral side of the spinal cord were dependent on both impact load and impactor tip diameter.