###### Abstract  
Self-tapping screws are commonly used in trauma and maxillofacial surgery and are increasingly used for pedicle screw insertions. In order to evaluate how the quantity and length of cutting flutes on self-tapping pedicle screws affect the insertion torque and pullout strength, eight different self-tapping pedicle screw designs were evaluated. All screws had a threaded length of 35 mm and featured variations in the number of leads, as well as the length and quantity of cutting flutes. Five samples of each design were inserted into pre-drilled, untapped holes (@2.7 mm, length 35 mm) in sawbone blocks of density 20 PCF. The insertion torque and pullout strength were measured according to ASTM F543. The results showed that screws with a longer cutting flutes of 9.5 mm had a lower mean maximum insertion torque than screws with shorter 2.9 mm cutting flutes. Pedicle screws with a double-lead thread design had a greater insertion torque than their single-lead counterparts, and the use of three cutting flutes produced a lower torque than two cutting flutes. The results demonstrated a greater pullout strength in screws with a single-lead thread rather than a double-lead, three cutting flutes instead of two, and a longer length for the cutting flute. In conclusion, to provide immediate stability and reduce the surgical insertion time, a single-lead, self-tapping pedicle screw incorporating three long cutting flutes is recommended because of the significantly greater pullout strength. This design could also reduce the risk of implant loosening in comparison to double-lead, self-tapping pedicle screw designs. self-tapping screws; pedicle screws; cutting flute; insertion torque; pullout strength Article  
Cutting Flute and Thread Design on Self-Tapping Pedicle Screws Influence the Insertion Torque and Pullout Strength April 202212022120221202212022120221202212022120221202212022120221202212022122022120221220222120222120222120222122022212202221220222212222222222222222222222222222222222222222  
of the bone screw could provide surgeons with the feeling of the screw driving into bone and provide a higher holding power for a more stable fixation [1]. Although screws rarely fail by pulling out of the bone in clinical applications, pullout testing is still considered a good predictor of screw fixation capability [7]. Previous studies have reported that the thread pitch, cutting flute length, and the total number of flutes can have a considerable influence on the insertion torque and pullout strength of self-tapping bone screws inserted into cortical and cancellous bone [8; 9; 10]. Although a number of manufacturers claim their screw designs are self-tapping, most commercially available pedicle screws with a self-tapping design still state that a pre-tapped hole is required. [PERSON] et al. [11] showed that a pre-tapping technique could increase the pullout force of self-tapping pedicle screws inserted into solid rigid polyurethane foam, but this finding is not consistent with similar research [9; 10]. Pedicle bone acts as a strong bridge between the dorsal spinal elements and the vertebral body, consisting of a strong shell of cortical bone and a dense core of cancellous bone [12]. Since the bone material modulus impacts the self-tapping screw design, the material properties of spine pedicle may cause the performance of self-tapping design to be different from the trauma bone screw used in long bone insertion. The insertion process of pedicle screws is different from the insertion of a small bone screw into long bone shaft, and the influence of flute design may not be comparable for small cortical/cancellous bone screws. In most commercial pedicle screws, the conical core often narrows towards the distal aspect of the screw, and hence screws with short flutes (one to two cutting pitches) may not provide adequate fixation in cases where a pre-drilled pilot hole is used.