## 4 Discussion  
In this study, the influence of impactor tip diameters on stresses, strains, and displacement in the spinal cord were investigated and quantified for NYU and IH impactors. The stress and strains on the ipsilateral side of the spinal cord occurred with a 1.6 mm diameter impactor with the 12.5 mm NYU impactor and 200 kdyn IH impactor. In addition, the larger impactor tip resulted in increased stress and strain to the contralateral side, which is consistent with [PERSON]'s study [12], showing that a larger impactor tip produced lesions that spread to the side contralateral to the injury; thus, the impactor tip diameter was reduced from 2.5 to 2.0 mm. In previous experimental studies, an impactor tip diameter ranging from 0.8 mm to 2.0 mm was used for the hemicontusion SCI models; however, producing consistent ipsilateral injury was difficult in some cases because of problems related to mechanical impact [11; 12; 15; 16; 17; 20; 22]. [PERSON] et al. [16] reported that some of the experiments failed to produce severe or sustained functional deficits, and damage in the spinal cord was placed too far laterally because the impactor tip was hitting the vertebral column or a lateral movement of the cord occurred during the experiment. Thus, they reported a modified model with a 22.5\({}^{\circ}\) rotation to more directly impact (perpendicularly) the spinal cord without so much lateral displacement [16]. The spinal cord FE model with a rotation could resolve the problem related to mechanical impact with a smaller impactor tip, such as less lateral movement, and showed more uniform stress and strain in the cord. The results from the present study showed that the impactor tip diameter of 0.8 mm resulted in higher penetration into tissue but lower stresses and strains within the cord, because the cord laterally shifted and the force missed the spinal cord. Impactor tip diameters from 1.2 to 1.6 mm are appropriate for the cervical hemicontusion SCI rat model without damaging the contralateral side of the spinal cord. The NYU impactor at the drop height of 6.25 mm showed less stress and strain than that with the drop height of 12.5 mm, regardless of the impactor diameter, as the experimental study by [PERSON] et al. [37] showed that the 25.0 mm drop height produced greater damage than the 12.5 mm drop height. The NYU impactor at the drop height of 12.5 mm and the IH impactor at the force of 200 kdyn resulted in dramatically higher stress and strain, especially at the ipsilateral or confused side of the spinal cord. The 12.5 mm case resulted in 1.4-3.2 times higher stress than the 6.25 mm case, while the 200 kdyn case resulted in 1.3-4.1 times higher stress than the 100 kdyn case. Previous in vivo studies have reported that the graded ipsilateral tissue sparing, including white and gray matter damage, was directly related to injury severity in the hemicontusion SCI model using IH [11] and NYU [12] impactors. [PERSON] et al. [11] showed that ipsilateral white matter area was reduced by approximately 10% when the impact force was increased by 100 kdyn for the IH impactor. [PERSON] et al. [9] presented that the white and gray matter area at the epicenter of injury was reduced by 1.7-2.2 and 1.2-2.0 times for moderate and severe unilateral SCI, respectively. [PERSON] et al. [12] reported that ipsilateral tissue damage at the lesion center was two times higher in the 12.5 mm group, compared to in the 6.25 mm group. They also found that anatomical damage was isolated to the spinal cord ipsilateral to the injury since less than 2% of the contralateral damage was detected. The FE model showed that stress and strain in the ipsilateral side were significantly higher than in the contralateral side of the spinal cord, which showed similar trends in the severity of injury. Thus, the FE model could be considered to mimic the histological outcomes in the in vivo studies, but a more accurate representation of the spinal cord tissues is required to properly correlate the simulation results with the experimental findings [33].