## 1 Introduction  
Most human spinal cord injuries (SCIs) occur in the cervical region [1]. However, several experimental contusion SCI models have been developed to study injury in the thoracic spinal cord [2; 3; 4; 5; 6; 7; 8] due to their reliability and reproducibility [9]. Thus, how findings in experimental injury models in the thoracic region are applicable to human cervical SCIs is questionable because the functional deficits and symptoms are significantly different for these two injury levels [10; 11]. Contusion injuries damage both white and gray matter at either the cervical or thoracic region. However, their functional consequences are more related to white matter [12]. Therefore, some attempts have been made to characterize confusion injury models in the cervical spinal cord in rats or mice [13; 14; 15; 10], including cervical hemicontusion injuries [16; 17; 18; 9; 11]. Reportedly, cervical hemicontusion injuries produce forelimb deficits on the ipsilateral side of the injury while sparing the function of the limbs on the contralateral side of the injury [16; 17; 9; 12], allowing for the efficacy of experimental therapeutics for upper limb functional recovery to be evaluated [18]Several devices are available for confusion injuries including the Ohio State University (OSU) device [2], the New York University (NYU) impactor [3], the Infinite Horizon (IH) impactor [5], and the Leica Impact One Stereotaxic Impactor [19]. The IH impactor has been utilized in numerous studies [11; 15; 16; 17; 20], and the NYU impactor or weight drop device has been used in several studies for cervical hemicontusion SCI models with some modifications [9; 12; 21]. [PERSON] et al. [18] modified the OSU device for a lateralized cervical contusion in rats. To produce unilateral cervical injuries, some of the device parameters required adjustments, including of the impactor tip diameter and off-center distances, since these devices were originally developed for thoracic injuries. [PERSON] et al. [12] performed unilateral cervical injuries with a modified 2.0 mm impactor using the 6.25 and 12.5 mm weight-drop contusion model. The experimental studies that utilized the IH impactor for the hemicontusion injury rat model used impact forces ranging from 100 kdyn to 395 kdyn with impactor tips 0.8 mm to 1.6 mm in diameter [11; 15; 16; 20; 22]. Although the effects of contusion force on the functional and behavioral outcomes were adequately described in previous experimental studies [11; 12], the size of the impactor tip also had significant effects on the extent of the lesion on the contralateral side in the hemicontusion rat model [12].  
Due to the limitations associated with experimental pre-clinical research, finite element (FE) analysis has been used to investigate the relationship between spinal cord tissue mechanics and neurological injury [23; 24; 25; 26; 27; 28; 29; 30]. The FE model of the rat spinal cord was developed to simulate the weight-drop contusion model, including the off-center impact on the spinal cord [23], and to investigate strains in the cervical region during contusion and dislocation injuries [28]. Moreover, the impact depth was shown to be the most important factor in determining the injury severity in rats [30]. Recently, the correlation between impact parameters in the NYU and IH impactors was investigated using the mid-thoracic spinal cord rat model [31]. In addition, a non-human primate unilateral cervical contusion injury model was developed by [PERSON] et al. [32], and a murine lateral cervical contusion model was presented by [PERSON] et al. [33]. However, studies regarding the effects of impactor size on the spinal cord for the hemicontusion model are limited. In this study, we investigated the effects of impactor size on the biomechanical characteristics of the spinal cord in a hemicontusion injury model for the NYU and IH impactors using the FE model of the rat spinal cord in the cervical region.