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 (ø2.7 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 flute 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.
Highlights
The traditional method of inserting bone screws is to drill a pilot hole and use a tapping tool to create a threaded path
For traditional pedicle screw insertion, the first step is to open the superficial cortex of the entry point with a burr and use a pedicle probe to navigate down the isthmus of the pedicle into the vertebral body
The fixation ability of bone screws can be gauged by the insertion torque and pullout strength, which are assessed using methods described in ASTM F543 [3]
Summary
The traditional method of inserting bone screws is to drill a pilot hole and use a tapping tool to create a threaded path. Self-tapping screws are commonly used in trauma and maxillofacial surgery and are increasingly used for pedicle screw insertions because of the simpler insertion technique without the use of a tapping tool and reduced operative time [1,2]. The fixation ability of bone screws can be gauged by the insertion torque and pullout strength, which are assessed using methods described in ASTM F543 [3]. It has been reported [4,5,6] that self-tapping screws have a lower insertion torque and higher pullout strength than conventional screws. 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]
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