# Alternative Pedicle Screw Design via Biomechanical Evaluation
## 2 Alternative Pedicle Screw Design  
The spine plays an essential role in supporting the human body, and the characteristics of vertebrae depend on age, race, and gender. Their appearance varies depending on their location (i.e., in the cervical, thoracic, and lumbar spine). The length, diameter, pitch, and type of screw are important and must be determined thoroughly. Furthermore, the linking mechanism between the screw and vertebrae is equally important [11] but is not covered in this work. The fusion structures among the vertebrae, screws, and links should be able to support varying loads on the spine. In general, the movement of the human body causes bending and tensile forces to be applied to the fusion structures, and so the inserted pedicle screws should be strong enough to withstand bending forces and should not be pulled out easily by tensile forces. In addition, stress concentration can damage the fusion structures; therefore, stress due to applying a load must be distributed throughout the screw volumes. Over the past few decades, a single thread has been applied most often because of its simplicity (Figure 2a). A single-threaded screw has a head, shank, and tapered thread with one type of pitch and lead. This screw has a much shorter lead compared to its full inserting depth and so it should be tightened repeatedly during the surgery. In general, if the screw is repeatedly tightened, a lot of shear stress is repeatedly applied to the cross-section between the head and the shank and thus shear fracture is likely to occur. The repeated tightening also causes discomfort to the surgeon during the surgery. When the double screw in Figure 2b is applied, the operation time is shortened and the convenience to the surgeon is improved because of the insertion depth (i.e., the lead of the screw is twice per rotation). The double thread has a longer pitch and twice the lead of the single screw, thereby reducing the number of turns during an operation. However, this design is likely to be vulnerable to bending or tensile loading on the spine during normal human activity because the longer pitch results in a smaller cross-section of the screw. The ease of screw insertion means that it can be easily unfastened by human motion, which is a disadvantage that lowers the stability of the fused structure after surgery. Figure 2: Various pedicle screws: (**a**) single, (**b**) double, (**c**), and the proposed thread. In a previous study, dual pedicle screws were proposed with single and double threads. More recently, quadruple threads were also applied to this dual screw [7]. Although this design is used clinically with few problems, there are reports that the pull-out strength of the dual screw is weaker than that of a screw with a single thread [12]. If the taper from one thread to the other is not designed to run smoothly, stress can become concentrated under loading, resulting in lower bending or pull-out strength, and can lead to fatigue failure [5]. In addition, there have been many studies focusing on the structural stability of various pedicle screw design. This study shows a meaningful outcome, such as insertion torque and pullout stiffness for each design using cadaveric thoracolumbar spines [13]. However, the researchers found it difficult to produce a detailed design and evaluation methodology for a double screw with quadruple threads. In addition, in the analysis of the conventional dual screw, the stress concentration on the vertebrae and the screw was not analyzed in detail. Therefore, for future screw development, it is necessary to propose a design and evaluation platform that can analyze the stress on the vertebrae and screws together, the vertebrae only, and the screws only. The platform should enable us to carry out tests and analyses concurrently. For this paper, an alternative pedicle screw (Figure 2c) was designed and evaluated via the design platform. This was an improved hybrid screw compared to the commercially available screws [7].