# Alternative Pedicle Screw Design via Biomechanical Evaluation
## 1 Introduction  
Spinal disease is a problem that can lower the quality of peoples' lives, and various attempts have been made to solve it. Recent advances in medical technology have made the average life expectancy of humans longer, leading to increased social activities for the older generation. However, frequent and long-lasting activities can cause degenerative spinal disorders, such as lumbar spinal stenosis, herniation of intervertebral discs, and degenerative lumbar diseases [1; 2; 3]. In addition to these, spinal problems also occur in young people with high activity or poor posture. Degenerative lumbar diseases cause instability of the spine due to aging and degeneration of facet joints and intervertebral discs [3]. These are generally caused by osteophytes in the vertebral body and facet joints or spinal stenosis due to herniated intervertebral discs. The instability of the spine can compress the spinal nerves and result in sharp pain from the hips to the legs. Patients suffering from the above-mentioned disease is usually treated via surgical methods using spinal decompression or spinal fusion. Pedicle screws are usually applied in spinal fusion surgery [4; 5; 6]. The spine has many vertebrae, and two or more can be fused or supported by using pedicle screwsand a linking mechanism. Figure 1 shows post-operative radiographs after a lumbar interbody fusion for degenerative disc disease [1]. During surgery, pedicle screws and a linking mechanism are applied to two or more lumbar vertebrae (for example, L4 and L5). pedicle screws are usually used in surgery to correct problems with the spine. The basic idea is to fasten two or more vertebrae together to alleviate spinal distortion, stabilize the spine, and improve its strength. The screws play an essential role in providing additional support and strength through the surgery. Although spinal fusion with pedicle screws is a good way to treat spinal problems, several failure modes and the efforts to overcome them have been reported. Failure can be due to screw loosening and pull-out, as well as fatigue fracturing [4]. Loosening and pull-out are due to failures in the early stages after the surgery, but the fatigue fractures are due to failure after a certain post-operative period due to toggling with cyclic loading and occur at the pedicle screw on the caudal side. If two or three of the factors are compounded, more tragic situations can arise, and so it is necessary to solve the problematic factors at an early stage. Meanwhile, several shapes and characteristics of the pedicle screws have been suggested to address these failure modes. Pedic screws and a linking mechanism are commonly used in spinal fusion surgery. This assembly is secured to the vertebrae and spine by using the pedicle screws. The pedicle screw is invasively implanted into the vertebrae and then becomes osteo-integrated with the bones. Therefore, it is important to avoid failure of this component. From an engineer's point of view, the parts that cannot be disassembled or repaired require more comprehensive design and evaluation. In recent years, with the development of the above-mentioned components and surgery, failure of the screws, other than as a result of fatigue fracturing, rarely occurs. Nevertheless, the first step in the mechanical design of a screw is testing how it withstands static loading. After the static strength test, fatigue testing under repeated loads is usually investigated. The causes of failure of pedicle screws are various and due to factors, such as pedicle fill, bone density, and the shape and mechanical properties of the screw. If the diameter of the screw is too small compared to the pedicle width, it bends too easily and cannot withstand pull-out loading [6]. When the bone density of the vertebrae is low, there is a high probability that the screw will become loose and thus the pull-out strength is also lowered. Moreover, the strength of the screw is weak if it is too long or has an excessively tapered minor diameter.