## 1 Introduction  
An advent of pedicle screws for spinal and chiropractic treatments were 1950s by [PERSON] [1]. However, problems including bending, fracture, loosening, detachment, and movement of pedicle screws from the bone have been constantly raised due to the insufficient fixation strength of the interface between the screw and bone. This is because the selection of pedicle screw design and surgical method fail to consider the bone density of various patients. There are various reasons for pedicle screw fixation failure such as sudden shock, fatigue, insufficiency of surgical techniques, and stability of pedicle screws [2; 3; 4; 5; 6]. Screw loosening occurs between 0.6% and 11%, and screw fracture due to stress concentration occurs between 2.6% and 60%. This causes serious postoperative complications, and surgery is subsequently needed. Specifically, in the cases of older patients or patients with osteoporosis, fixation failure and loosening of the pedicle in the body frequently occur [7; 8; 9]. The fixation failure ratio of the pedicle screw is approximately 0.8-1.7% due to decreases in and loosening of the fixation strength at the interface between the pedicle screw and bone. Additionally, the fixation failure leads to decreases in the stress and fracture load capacity as well as stress shielding. Therefore, this also causes the micro bone fracture of verteb due to decreases in bone mineral density [7; 8; 10]. Thus, it is important to avoid screw loosening from the viewpoint of biomechanics and to secure mechanical stability in the vertebrae. Hence, it is necessary to improve the pedicle screw design to increase the pullout force. Furthermore, the pedicle screw design should be selected by considering a patient's condition and bone density, and postoperative stability of a patient should be improved through appropriate surgical methods. The design variables that affect the pullout force including outer diameter, conical angle, and core diameter vary widely. The results of previous studies indicate that increases in the outer diameter and decreases in the pitch increase the contact area between the bones which have low density (0.16 g/cc) to high density (0.32 g/cc) and the screw, and thereby improve the pullout force [11; 12]. Increases in outer diameter, however, are extremely limited due to the vertebrae size and thickness based on patients. Furthermore, the pitch variation is extremely limited because the torque decreases when the pitch of screw decreases and bone damage accordingly increases. The conical angle is another variable that contributes to increased pullout stiffness and improves the stability of the inserted screw when compared to the cylindrical type [12; 13; 14]. Furthermore, the core diameter reduces in the case of a screw with the same outer diameter, and the thread height increases. Additionally, the pullout force increase due to increase of the contact area between bone and screw increases [15; 16; 17]. The influence of screw design variables on pullout force is actively examined. Nevertheless, there is a paucity of studies on the correlation between the pullout force and each variable that influences the pullout force. Specifically, while inserting a screw into a patient, it is necessary to consider the pullout force as well as bone damage after insertion and failure [18]. Previous studies focused on improving the pullout force based on the screw design with the exception of bone damage. Thus, it is difficult to predict that bone damage occurs due to torque and external force occurs when a screw that exhibits high pullout strength is inserted into patients with various bone densities. Thus, it is necessary to increase the pullout force by improving the pedicle screw design and also to simultaneously analyze bone damage and fracture behavior of the bone when a screw is inserted into the human body [19; 20]. In this study, a pedicle screw with core diameter and conical angle as variables of screw design that contribute to pullout force improvement was designed and manufactured.